Reference

Aerospace terms and definitions

Aerospace and defense terms: launch and propulsion, aircraft performance and hypersonics, rotorcraft and eVTOL, uncrewed systems, orbits and satellite communications, certification, and the programs, contracts and export rules behind them. These are general reference definitions, not tied to any one report.

358 terms · Updated 17 Oct 2026

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3GPP (3rd Generation Partnership Project): The partnership of standards bodies that writes the technical specifications for mobile networks such as 4G LTE and 5G, issued in numbered releases.
14 CFR (Title 14 of the Code of Federal Regulations, Federal Aviation Regulations): Title 14 of the US Code of Federal Regulations, which holds the federal rules on aviation and commercial space. Its parts are cited by number, such as Part 107 for small drones.

A

Adaptive Acquisition Framework: The set of acquisition pathways the US Defense Department uses, from urgent needs and middle-tier rapid programs to major weapon systems and software, each with its own rules.
Advanced air mobility (AAM): A broad term for new air transport using electric or hybrid aircraft, often with vertical lift, for passengers and cargo over short and regional distances. It includes air taxis, regional routes and drone delivery.
Advisory circular: FAA guidance that describes acceptable, usually voluntary, ways to meet a regulation.
Affordable mass: A defense strategy of fielding large numbers of cheaper, often uncrewed, systems instead of a few expensive ones, to overwhelm defenses and absorb losses.
AFRL (Air Force Research Laboratory): The research laboratory of the US Air Force and Space Force, covering aerospace vehicles, propulsion, materials, sensors and directed energy.
Air operator certificate (AOC, air carrier certificate): Approval for a company to carry passengers or cargo commercially, covering its procedures, crews and maintenance.
Air-breathing propulsion (air-breathing): Engines that take oxygen from the surrounding air instead of carrying it, such as jet engines, ramjets and scramjets. Not carrying oxidizer raises efficiency but limits them to the atmosphere.
Airworthiness: The condition of an aircraft that conforms to its approved design and is safe to operate. Regulators grant it through type, production and individual airworthiness certificates.
Airworthiness certificate (standard airworthiness certificate): Approval that an individual aircraft conforms to its type design and is safe to fly. Each aircraft carries its own.
American Security Drone Act: A US law, enacted in the 2024 National Defense Authorization Act, on drones from covered foreign entities, chiefly Chinese manufacturers. It bars federal agencies and federally funded projects from buying or operating them. Its federal-funds restrictions took effect in December 2025.
Ammonium perchlorate: The standard oxidizer in solid rocket propellant, mixed with aluminum powder and a rubbery binder. Few plants make it, so its supply limits how fast missile and motor production can grow.
Antenna gain (gain): How strongly an antenna concentrates radiated power in a given direction, compared with a reference antenna, in decibels.
ARP4761 (SAE ARP4761): An SAE guideline for safety assessment of civil aircraft systems, using methods such as fault tree analysis to show that failure probabilities meet their targets.
Artemis Accords: Non-binding principles for civil space exploration, led by the US, covering transparency, interoperability, sharing of scientific data and avoiding harmful interference. Countries join by signing.
Artemis program (Artemis): NASA's program to return astronauts to the Moon and build a sustained presence there, with commercial and international partners. Its missions are numbered, as in Artemis II and Artemis III.
AS9100: The aerospace and defense quality management standard, built on ISO 9001 with added requirements. Most aerospace buyers require suppliers to be certified to it.
Assured access to space: The US policy goal of keeping at least two independent launch vehicles able to carry national security payloads, so one failure does not ground them all.
Attritable (attritable aircraft, attritable systems): Cheap enough that losing some in combat is acceptable, yet designed to be reused if they survive. Attritable aircraft sit between expendable munitions and costly crewed jets.
Autorotation: A helicopter's ability to descend safely without engine power, with upward airflow keeping the rotor spinning so it can cushion the landing. Many eVTOLs cannot autorotate and rely on redundancy instead.

B

Ballistic coefficient: A measure of how well a falling body pushes through drag: its mass divided by its drag coefficient times frontal area. A low value means the vehicle slows high in the atmosphere, where heating is gentler.
Ballistic recovery parachute (whole-airframe parachute, ballistic parachute): A parachute deployed by a small rocket that lowers an entire aircraft to the ground in an emergency. It is common on light aircraft and some eVTOL designs.
Beam steering (null steering, beamforming): Pointing an antenna's beam in a chosen direction by adjusting the phase of its elements or its physical shape. Null steering does the reverse, placing a blind spot toward a jammer.
Beamwidth: The angular width of an antenna's main beam, usually measured where power falls to half. Narrow beams reach further but must be pointed more precisely.
Beyond visual line of sight (BVLOS): Flying a drone where the pilot cannot see it directly. Long-range delivery and inspection depend on it; in the US it needs a waiver or exemption until a dedicated rule, Part 108, takes effect.
Bilateral aviation safety agreement (BASA, bilateral airworthiness agreement): An agreement under which two countries accept each other's aircraft certification findings, so a design approved in one can be validated more easily in the other.
Blue UAS (Blue UAS List, Blue UAS Cleared List, Blue sUAS): A US defense list of drones and drone components vetted for cybersecurity and NDAA compliance, which military units can buy with fewer approvals. The Defense Innovation Unit created it in 2020; the Defense Contract Management Agency took over its management in December 2025.
Boil-off: The gradual loss of cryogenic propellant as heat leaks into tanks and turns liquid into gas that must be vented. It limits how long cryogenic stages and depots can wait in orbit.
Boost-glide vehicle (hypersonic glide vehicle, HGV, boost-glide): A hypersonic weapon or test vehicle that a rocket boosts to high speed and altitude before it glides unpowered toward its target, maneuvering on the way. It flies lower than a ballistic missile, which makes it harder to track.
Boostback, entry and landing burns (boostback burn, entry burn, landing burn): Engine firings a reusable booster makes after stage separation. The boostback burn reverses its course and the entry burn slows it before the dense lower atmosphere. The landing burn brings it to a stop just above the ground or platform.
Booster (first stage, strap-on booster): The first stage of a rocket, or a smaller motor strapped to its side, that gives most of the thrust at liftoff. On reusable rockets the booster is the part usually recovered and flown again.
Boundary layer (boundary-layer heating): The thin layer of air next to a vehicle's surface, slowed by friction. Its behavior sets skin drag and heating; turbulent boundary layers heat a surface far more than smooth, laminar ones.
Bow shock: The curved shock wave that forms ahead of a blunt body moving faster than sound. Air passing through it is compressed and heated, and at reentry speeds it becomes ionized.
Breguet range equation (Breguet equation): The standard formula for an aircraft's cruise range. Range grows with speed, lift-to-drag ratio and engine efficiency, and with the logarithm of the ratio of starting to ending weight.
Brushless DC motor (BLDC, brushless motor): An electric motor that switches current electronically instead of through brushes, making it efficient and durable. Nearly all small drones use brushless motors with permanent magnets.
Bureau of Industry and Security (BIS): The US Commerce Department agency that administers the EAR and keeps lists of restricted foreign parties, such as the Entity List.

C

CAAC (Civil Aviation Administration of China): China's civil aviation regulator, responsible for aircraft certification, airlines and airspace use.
Captive fairing: A payload fairing that stays attached to the first stage and opens to release the upper stage, instead of separating and falling away. The design lets the fairing return and be reused with the booster.
Certification basis: The set of regulations and special conditions a specific design must meet to earn its type certificate, agreed between the applicant and the regulator.
Chamber pressure: The pressure inside a rocket engine's combustion chamber. Higher pressure packs more thrust into a smaller, lighter engine and improves efficiency, especially at sea level.
Cislunar space (cislunar): The region between Earth and the Moon, including lunar orbits. Interest in it is growing for exploration, commerce and military awareness.
CMMC (Cybersecurity Maturity Model Certification): The US Defense Department program requiring contractors to meet, and prove, cybersecurity standards for the information they handle, at Levels 1 to 3. It began phasing into contracts on 10 November 2025.
Coking: Solid carbon deposits left when hydrocarbon fuel overheats in engine passages or cooling channels. Coking clogs those channels, which limits how hot kerosene-fueled engines can run and how often they can be reused.
Cold-gas thruster: A simple thruster that releases stored compressed gas, such as nitrogen, through a nozzle without burning it. It gives little thrust but is reliable for small attitude adjustments.
Collaborative Combat Aircraft (CCA): The US Air Force program to buy uncrewed, autonomous combat aircraft that fly with crewed fighters. It runs in increments; in June 2026 the Air Force ordered both Increment 1 designs, the FQ-42A and FQ-44A, into production.
Combat radius: How far a military aircraft can fly out, carry out its mission and return without refueling. It is less than half its one-way range because of time on station and fuel reserves.
Command and control (C2): The authority, communications and systems commanders use to direct forces. For drones it also means the data link that carries control commands to the aircraft.
Commerce Control List (CCL): The EAR list of controlled dual-use items, each identified by an export control classification number that determines which destinations need a license.
Commercial Lunar Payload Services (CLPS): A NASA program that buys delivery of science instruments and technology to the Moon on commercial landers, accepting more risk for lower cost than traditional missions.
Commercial solutions opening: A US defense solicitation that invites companies to propose commercial technologies against broad problem statements, often leading to other transaction agreements.
Common operating picture: A single shared display of friendly forces, threats and other data, built from many sensors, so that every user sees the same situation.
Compound helicopter: A helicopter that adds wings, extra propellers or both to share the load at high speed, letting it fly faster than a conventional helicopter.
Conjunction (conjunction assessment, collision avoidance): A predicted close approach between two objects in orbit. Operators assess the collision probability and may fire thrusters to move away; large constellations make many thousands of such maneuvers.
Continued safe flight and landing (CSFL): A certification requirement that an aircraft can keep flying and land safely after specified failures, such as losing a motor or battery. It replaces autorotation as the safety case for many eVTOLs.
Continuing resolution: A stopgap US law that funds agencies at prior-year levels when Congress has not passed annual appropriations. It usually bars new programs, called new starts, and production increases, which delays defense work.
Contract ceiling (ceiling value): The maximum value a contract allows over its life if all options and orders are used. Actual spending is often far lower, so ceilings overstate likely revenue.
Control moment gyroscope (CMG): A spinning wheel on a tilting mount that produces large turning forces when its axis is tilted. Large spacecraft, such as space stations, use it to point quickly without propellant.
Controlled reception pattern antenna (CRPA): A GPS antenna array that electronically places nulls toward jamming sources while keeping reception from satellites, making navigation harder to jam.
Controlled unclassified information (CUI): Sensitive US government information that is not classified but must be protected under set rules, such as export-controlled technical data. Contractors who handle it generally need CMMC Level 2 or higher.
CORSIA (Carbon Offsetting and Reduction Scheme for International Aviation)In Sustainability: The International Civil Aviation Organization's global scheme, adopted in 2016, under which airlines offset growth in international aviation emissions or cut them with eligible lower-carbon fuels. Its lifecycle values combine a core value with an ILUC value, amortized over 25 years against an 89 g CO₂e/MJ jet-fuel baseline.
COSPAR (Committee on Space Research): The international Committee on Space Research, which sets the planetary protection guidelines most space agencies follow.
Cost per kilogram (price per kilogram): Launch price divided by the payload mass a vehicle can deliver to a given orbit. It is the standard way to compare launch costs, though it flatters large rockets that often fly partly empty.
Cost per seat-mile (cost per available seat-mile, CASM): An aircraft's operating cost divided by the number of seats times the miles flown. It lets operators compare aircraft of different sizes.
Cost-plus contract (cost-reimbursement contract, cost-plus-fee): A contract that repays a contractor's allowable costs plus a fee. It shifts cost risk to the government and is common for risky development work.
Counter-UAS (C-UAS, counter-drone): Systems that detect, track and defeat hostile drones, using jamming, interceptors, guns, lasers or high-power microwaves.
Critical density (cutoff density): The electron density at which a plasma's natural frequency equals a given radio frequency, so the plasma blocks that signal. Higher frequencies need denser plasma to be blocked.
Critical mineral (critical material)In Finance: A mineral or material considered essential to economic or national security whose supply is at risk of disruption. Governments publish official lists, such as the US Geological Survey critical minerals list.
Cryogenic propellant (cryogenic propellants): A propellant stored as a very cold liquid, such as liquid oxygen, methane or hydrogen. Cryogenics give high performance but warm up and boil off over time, which limits long missions.
Cyclic and collective pitch (cyclic, collective): The two main helicopter rotor controls. Collective changes the pitch of all blades together to climb or descend; cyclic varies pitch around each rotation to tilt the rotor and steer.

D

DARPA (Defense Advanced Research Projects Agency): The US Defense Department agency that funds high-risk, high-payoff research, often through short, focused programs with industry and universities.
Debye length: The distance over which a plasma screens out an electric field. It sets the thickness of the sheath between a plasma and a surface.
Decibel (dB, dBA, A-weighted decibel): A logarithmic unit for ratios of power or intensity: +3 dB is ≈2× the power and +10 dB is 10×. A-weighted decibels (dBA) adjust sound levels to match human hearing and are used for aircraft noise.
Dedicated launch: A launch bought by a single customer, who sets the orbit and launch date. It costs more per kilogram than a rideshare but avoids waiting for, or compromising with, other payloads.
Defense Autonomous Warfare Group (DAWG): A US defense organization formed in 2025 within Special Operations Command to scale up autonomous and uncrewed systems. It took over the Replicator initiative's portfolio in November 2025.
Defense Innovation Unit (DIU): A US Defense Department organization that helps the military adopt commercial technology quickly, often through other transaction agreements.
Defense Production Act Title III (DPA Title III, Defense Production Act): A US authority that lets the Defense Department fund, lend to or guarantee domestic production of materials and components critical to national defense.
Delta-v (Δv, delta v): The total change in velocity a spacecraft can achieve with its propellant, used as the budget for planning a mission. Reaching low Earth orbit from the ground takes ≈9.4 km/s once gravity and drag losses are counted.
Deorbit (deorbit burn): To lower a spacecraft's orbit until it reenters the atmosphere, usually by firing an engine against the direction of travel. Satellites are deorbited at end of life to limit debris.
Design Assurance Level (DAL, DAL A): A rating from A to E for how severe the effect of a software or hardware failure would be, from catastrophic (A) to none (E). Level A demands the most rigorous development and verification.
DFARS (Defense Federal Acquisition Regulation Supplement): The US Defense Department's supplement to the Federal Acquisition Regulation, adding defense-specific contract rules. Its clauses restrict sourcing of specialty metals, certain magnets and some Chinese-made components.
Direct operating cost (DOC): The costs of flying an aircraft that vary with its use, such as energy or fuel, crew, maintenance and fees, usually given per flight hour or per seat-mile. Definitions differ on whether ownership costs are included.
Direct-to-device (direct-to-cell, D2D, direct-to-handset, direct-to-phone): Satellite service that connects directly to ordinary mobile phones, without a dish or special terminal. It needs very large antennas in orbit and spectrum shared with or leased from mobile operators.
Directorate of Defense Trade Controls (DDTC): The US State Department office that administers ITAR, including registrations, licenses and enforcement.
Disk loading: Aircraft weight divided by the total area swept by its rotors. Low disk loading means efficient, quiet hover; high disk loading means more power and stronger downwash.
Distributed electric propulsion (DEP): Spreading many electric motors and propellers across an airframe. It adds redundancy and can improve lift and efficiency, but multiplies parts and possible failure points.
DO-178C (RTCA DO-178C): The standard for developing and verifying airborne software, used by the FAA and EASA. The rigor required scales with how severe a software failure would be.
DO-254 (RTCA DO-254): The standard for designing airborne electronic hardware, such as programmable chips, with rigor scaled to the effect of a failure.
DO-311A (RTCA DO-311A): An RTCA standard for the performance and safety of rechargeable lithium batteries installed in aircraft, including tests that a cell fire stays contained.
DoD Directive 3000.09 (Directive 3000.09): The US Defense Department policy on autonomy in weapon systems, last updated in 2023. It requires designs that allow appropriate levels of human judgment over the use of force and sets senior review before development and fielding.
Doppler shift (Doppler): The change in a signal's frequency when transmitter and receiver move relative to each other. Fast low-orbit satellites cause large shifts that receivers must correct.
Downwash and outwash (downwash, outwash): Downwash is the air a hovering rotor drives downward; outwash is that air spreading out along the ground. Strong flows can blow debris and knock people over, so vertiport design sets caution areas around the pad.
Drone ship (autonomous spaceport drone ship, landing barge, landing platform vessel): An ocean platform, usually uncrewed, on which a rocket booster lands downrange of the launch site. Landing at sea saves the propellant the booster would need to fly back.
Dual-mode ramjet (DMRJ, dual-mode scramjet): An engine that runs as a ramjet at lower speeds and as a scramjet at higher ones, using the same flow path.
Dual-use (dual-use technology, dual-use goods): Describes goods, software and technology with both civilian and military uses, such as drones, machine tools and some chips. They are controlled under regimes such as the EAR and the Wassenaar Arrangement.
Dutch roll: A coupled side-to-side oscillation in which an aircraft yaws and rolls out of phase, its nose tracing a figure eight. Most aircraft damp it naturally or with yaw dampers; poor damping is a certification issue.
Dynamic pressure (max q, max Q): The pressure from a vehicle's motion through air, rising with air density and the square of speed. Rockets throttle down near the point of highest dynamic pressure, called max q, to limit structural loads.

E

EAR (Export Administration Regulations): The US Commerce Department rules that control exports of commercial and dual-use items, software and technology. Items are classified on the Commerce Control List.
EASA (European Union Aviation Safety Agency): The EU agency responsible for civil aviation safety, including type certification of aircraft, engines and parts.
EBITDAIn Finance: Earnings before interest, taxes, depreciation and amortisation, a rough measure of operating earnings before financing and non-cash charges. It is not defined under GAAP and ignores capital spending and working capital needs.
ECCN (export control classification number): A five-character code that places an item on the Commerce Control List, such as 2B001 for certain machine tools. It determines which license requirements apply.
Electric-pump-fed engine (electric pump-fed): A rocket engine whose propellant pumps are driven by battery-powered electric motors instead of a turbine. It simplifies small engines, but battery mass rules it out for large ones.
Electromagnetic pulse (EMP): A short, intense burst of electromagnetic energy, from a nuclear detonation or a dedicated device, that can induce damaging currents in electronics.
Electronic speed controller (ESC): The circuit that drives an electric motor on a drone, switching current to set its speed on commands from the flight controller.
Electronic warfare (EW): Military use of the electromagnetic spectrum to attack, protect or exploit, including jamming, spoofing and intercepting enemy signals.
Endothermic fuel: A jet fuel that absorbs heat by breaking down chemically as it warms, so it can cool engine and airframe parts before it burns. High-Mach vehicles need it because their heat loads exceed what ordinary fuel can absorb.
Energetics: The science and industry of explosives, propellants and pyrotechnics used in munitions and rocket motors. Western supply depends on a small number of plants.
Engine-out capability (engine-out tolerance): The ability of a vehicle with several engines to complete its mission if one engine fails. It needs spare thrust and flight software that can adjust to the loss.
Engineering Brief 105A (EB 105A): FAA guidance for designing vertiports, issued in December 2024. It covers landing area sizes, approach paths, markings, lighting and charging infrastructure; following it is voluntary.
Entry into service: The point at which an aircraft type begins regular commercial operation with a customer, after certification and delivery.
Environmental assessment (EA, tiered environmental assessment): A NEPA review that decides whether a federal action may significantly affect the environment. If not, it ends with a finding of no significant impact; if so, a full environmental impact statement is needed.
Environmental impact statement (EIS): The most detailed NEPA review, required when a federal action may significantly affect the environment. It takes longer and involves more public comment than an environmental assessment.
eSTOL (electric short takeoff and landing): An electric aircraft designed to take off and land on very short strips, often using propellers blowing over the wing for extra lift. It trades vertical flight for better range and payload than an eVTOL.
eVTOL (electric vertical takeoff and landing): An electric aircraft that takes off and lands vertically, usually with several battery-powered rotors. eVTOLs are being developed as air taxis, cargo carriers and military vehicles. Battery energy limits their range and payload.
eVTOL Integration Pilot Program (eIPP): A US program created under a June 2025 executive order that pairs state, local and tribal governments with eVTOL and other advanced air mobility companies. Its projects fly early operations and generate data for future FAA rules.
Expendable launch vehicle (expendable, ELV): A rocket, or part of one, used for a single flight and not recovered. Flying a reusable booster in expendable mode lifts more payload, because no propellant is held back for landing.
Experimental amateur-built (amateur-built, 51 percent rule): An FAA category for aircraft whose owner built the major portion, at least 51%, for education or recreation. Such aircraft cannot be used commercially.
Extremely improbable: The certification target for catastrophic failure conditions on large aircraft: no more likely than about once per billion flight hours. Regulators apply similar targets to eVTOLs carrying passengers.

F

FAA (Federal Aviation Administration): The US agency that regulates civil aviation and commercial space transportation, including aircraft certification, pilots, airspace and launch licensing.
FAA Office of Commercial Space Transportation (FAA AST): The FAA office that licenses commercial launches and reentries in the US and oversees their public safety.
Fault tree analysis: A top-down method that maps how combinations of component failures could cause a hazard, used to estimate how likely it is.
FCC Covered List (Covered List): A US Federal Communications Commission list of communications equipment and services judged to pose national security risks. Listed equipment cannot get new FCC equipment authorizations, which most radio devices need to be sold in the US. In December 2025 the FCC added foreign-made drones and drone critical components.
Feeder link (gateway link): The radio link between a satellite or high-altitude platform and a ground gateway that connects it to the internet or a core network. Users connect on separate service links.
Fiber-optic guided drone (fiber-optic drone): A drone controlled through a thin optical fiber that unspools behind it, instead of by radio. It cannot be jammed, but the spool limits its range.
Figure of merit (FoM, hover efficiency): A rotor's hover efficiency: the ratio of the ideal power predicted by momentum theory to the power actually needed. Good helicopter rotors reach ≈0.7–0.8.
Final approach and takeoff area (FATO): The area around the landing pad over which an aircraft completes its approach to a hover or landing and begins takeoff. In FAA vertiport guidance it is at least twice the rotor diameter across.
Finding of no significant impact (FONSI, mitigated FONSI): A NEPA decision that an action will not significantly affect the environment, sometimes on condition of mitigation measures, so no full impact statement is required.
Finite-difference time-domain (FDTD): A computer method for simulating how electromagnetic waves travel and interact with objects, by stepping Maxwell's equations through space and time.
Firm-fixed-price contract (firm fixed price, FFP): A contract with a set price the contractor receives regardless of its actual costs. The contractor keeps any savings but absorbs overruns, which can cause large losses on development work.
Fission surface power: A small nuclear reactor designed to power a base on the Moon or Mars, including through the ≈14-day lunar night when solar power is unavailable.
Fixed-base operator (FBO): A business at an airport that serves private and charter aircraft with fuel, parking, hangars and passenger lounges. Such sites are early candidates for eVTOL operations.
Flight controller: The onboard computer that keeps a drone stable and follows commands or a flight plan, using data from its sensors.
Flight envelope (envelope expansion): The range of speeds, altitudes and loads within which an aircraft can fly safely. Flight testing expands the envelope step by step until it covers what certification requires.
Fly-by-wire: A flight-control system in which pilot inputs go to computers that move the control surfaces, instead of through cables or rods. It allows stability augmentation and is essential for aircraft that are unstable or hard to fly by hand.
Foreign direct product rule (FDPR): An EAR provision that extends US export controls to foreign-made items produced with certain US technology, software or equipment.
Foreign military sales (FMS): The US government program for selling defense equipment and services to allied governments, with the Defense Department managing the purchase on the buyer's behalf.
FPV drone (first-person view, FPV): A small drone flown by a pilot wearing goggles that show its live camera view, as if sitting on board. Cheap FPV drones carrying explosives have become a mass weapon in the war in Ukraine.
Free cash flowIn Finance: Cash generated by operations minus capital expenditures. It is the cash left over that can repay debt, pay dividends, buy back shares or be reinvested.
Free-space path loss: The weakening of a radio signal as it spreads out over distance, growing with distance and frequency. It is the largest loss in most satellite links.
Frequency selective surface (FSS): A patterned surface that passes some radio frequencies and blocks others. It is used in radomes and reflectors to shield sensors from unwanted bands.
Full-flow staged combustion (FFSC): A staged-combustion cycle with two preburners, one fuel-rich and one oxygen-rich, so all propellant passes through the turbines. It allows very high chamber pressure with cooler turbines, at the cost of complexity.
Function and reliability testing (F&R testing, function-and-reliability testing): Flight testing before type certification that shows an aircraft and its systems work reliably in service-like use. US rules set a minimum number of flight hours, commonly 150.

G

g-load (g-force): The acceleration a vehicle and its occupants feel, in multiples of standard gravity. In a 9 g turn everything weighs nine times its normal weight.
Gas-generator cycle (gas generator cycle): A rocket engine cycle that burns a small share of propellant to drive the turbopumps and dumps that exhaust overboard. It is simpler than staged combustion but wastes some propellant, lowering efficiency.
Geostationary orbit (GEO, geostationary): A circular orbit 35,786 km above the equator in which a satellite circles once a day and appears fixed in the sky. Three satellites can cover most of the globe, but the distance adds ≈0.24 s of delay each way between two ground points.
Geostationary transfer orbit (GTO): An elongated orbit that rockets use to deliver satellites toward geostationary orbit; the satellite then raises its own low point to reach the final circular orbit. Payload to it is a standard rocket performance figure.
GNSS (global navigation satellite system, satellite navigation): The general term for satellite navigation systems, including GPS, Galileo, GLONASS and BeiDou. Receivers compute position from the timing of signals from several satellites.
GNSS-denied navigation (GPS-denied navigation, visual-inertial navigation): Navigating when satellite positioning is jammed, spoofed or unavailable, using inertial sensors, cameras, terrain matching or other references instead.
Golden Dome (Golden Dome for America, Iron Dome for America): A US homeland missile-defense initiative launched by executive order in January 2025, first titled Iron Dome for America and renamed the next month. It aims to defend against ballistic, hypersonic and cruise missiles, including with space-based sensors and interceptors.
Ground control station (GCS): The equipment an operator uses to fly a drone and receive its data, from a handheld controller to a trailer of consoles.
Ground station (ground segment): A site with antennas and equipment that communicates with satellites. The ground segment as a whole also includes networks, control centers and software.
Ground support equipment (GSE): The equipment on the ground that prepares, fuels, powers and launches a vehicle, such as propellant lines, umbilical connections, transporters and launch mounts.
Gyroscopic precession: The effect by which a force applied to a spinning rotor takes effect 90° later in its rotation. Rotor designers must account for it, and some aircraft use it deliberately for control.

H

Handover: Passing a user's connection from one cell, beam or satellite to another as either side moves. Low Earth orbit satellites move fast, so handovers happen every few minutes.
HAPS (high-altitude platform station, high-altitude platform system, high-altitude pseudo-satellite): An aircraft, airship or balloon that holds position in the stratosphere, ≈20 km up, to provide communications, sensing or navigation over a region. The ITU defines high-altitude platform stations as operating at 20–50 km. Solar power and weather limit their payload and endurance.
HIBS (HAPS as IMT base stations): A high-altitude platform acting as a mobile-network base station that serves ordinary phones. WRC-23 identified bands for it worldwide at 694–960 MHz, 1,710–1,885 MHz and 2,500–2,690 MHz, with conditions to protect existing services.
High-enthalpy test facility (high-enthalpy ground testing, shock tunnel, arc jet): A ground facility, such as a shock tunnel or arc jet, that reproduces the extreme heat and flow conditions of hypersonic flight. Test runs last seconds or less, so flight tests are still needed.
High-power microwave (HPM): A weapon that emits intense bursts of microwave energy to disrupt or burn out electronics. It can disable many drones at once.
High-throughput satellite (HTS): A communications satellite that reuses frequencies across many narrow spot beams, giving far more total capacity than older wide-beam designs.
Hot-fire and static-fire tests (hot-fire test, hot fire, static-fire test, static fire): Ground tests that fire rocket engines while the hardware is held down. A hot-fire usually tests an engine or stage on a test stand; a static fire tests the fully assembled vehicle on its launch pad before flight.
Human Landing System (HLS): NASA's program buying commercial landers to carry astronauts between lunar orbit and the Moon's surface for Artemis missions.
Human-in-the-loop and human-on-the-loop (human-in-the-loop, human-on-the-loop): Two levels of human control over autonomous weapons. In the loop, a person must approve each engagement; on the loop, the system acts on its own while a person monitors and can intervene.
Hydrolox (LOX/LH2): Rocket propellant made of liquid hydrogen and liquid oxygen. It gives the highest specific impulse of common propellants, but hydrogen is bulky and must be kept at ≈20 K, so tanks are large and leaks are frequent.
Hypersonic (hypersonics, hypersonic flight): Flight at roughly Mach 5 and above. At these speeds heating, air chemistry and shock waves dominate vehicle design, and materials must survive extreme temperatures.

I

ICAO (International Civil Aviation Organization)In Sustainability: The United Nations agency for international civil aviation, based in Montreal. It runs CORSIA and publishes its default lifecycle values.
IDIQ (indefinite delivery, indefinite quantity, indefinite-delivery contract): A contract that sets terms and a maximum value but leaves quantities and timing open, with work ordered later through task or delivery orders. Its ceiling is not money the contractor is guaranteed to receive.
In-situ resource utilization (ISRU): Making useful products, such as oxygen, water or propellant, from materials found on the Moon, Mars or asteroids, to cut what must be launched from Earth.
In-space manufacturing (in-orbit manufacturing, microgravity manufacturing): Making materials or products in orbit to exploit microgravity or vacuum, such as certain drug crystals, optical fibers or tissues. It depends on regular, affordable return flights to bring products back.
Inertial measurement unit (IMU): A sensor package of accelerometers and gyroscopes that measures a vehicle's acceleration and rotation. It is the core of autopilots and of navigation without satellite positioning.
Insertion loss: The signal power lost when a component is placed in a transmission path, given in decibels.
Intelligence, surveillance and reconnaissance (ISR): Collecting and processing information about an adversary or area with sensors on aircraft, satellites and other platforms. Many drones and high-altitude platforms are bought mainly for it.
International Lunar Research Station (ILRS): A China-led program, with Russia and other partners, to build a research base near the Moon's south pole in the 2030s.
Interstage: The structure that joins two rocket stages and houses the upper-stage engine until separation. It carries loads between the stages and usually stays with the lower stage.
ITAR (International Traffic in Arms Regulations): The US State Department rules that control exports of defense articles, services and technical data on the US Munitions List. Sharing controlled data with a foreign person, even inside the US, can count as an export.
ITU (International Telecommunication Union, ITU-R): The UN agency that coordinates use of radio spectrum and satellite orbits worldwide. Satellite operators file with it to secure frequencies and orbital positions.

J

Jamming and spoofing (jamming, spoofing, anti-jam, anti-spoofing): Jamming drowns out a signal with noise; spoofing feeds a receiver false signals, such as fake satellite navigation data, so it computes the wrong position. Both are routine against drones.
JARUS (Joint Authorities for Rulemaking on Unmanned Systems): A group of national aviation authorities that drafts shared rules and guidance for drones and other uncrewed systems, which members may adopt.
JP-7: A low-volatility, heat-stable jet fuel developed for the SR-71 reconnaissance aircraft. It resists boiling and coking at high temperatures but is expensive and hard to ignite.

K

Kerolox (RP-1, LOX/kerosene): Rocket propellant made of refined kerosene, called RP-1, and liquid oxygen. It is dense and easy to store but leaves carbon deposits in engines, known as coking.
Kick stage (kick-stage): A small extra stage that gives a payload a final push after the main rocket has done its work. It is used to circularize an orbit, change orbits, or deorbit a spacecraft that has no engine of its own.
Knot: A speed of one nautical mile per hour, ≈1.852 km/h. Aviation and shipping use it worldwide.
Küchemann limit (Küchemann barrier): An empirical ceiling on the best lift-to-drag ratio achievable at a given Mach number M, often written as (L/D)max ≈ 4(M + 3)/M. At Mach 6 it gives ≈6, which caps the range of hypersonic aircraft.

L

Launch cadence (cadence, flight rate): How often a launch vehicle or launch site flies, usually counted in launches per year. Cadence depends on production rate, refurbishment time, pad turnaround and licensing limits.
Launch complex (Space Launch Complex, launch pad): A site with one or more launch pads plus the propellant storage, integration buildings and support equipment needed to prepare and fly rockets. US sites are numbered, as in Launch Complex 39A or Space Launch Complex 40. A vehicle with only one pad stops flying if that pad is damaged.
Launch vehicle (launcher): A rocket that carries payloads such as satellites, cargo or crew from Earth's surface to space. Most are built in two or more stages that drop away as their propellant runs out. Launch vehicles are compared by the mass they can lift to a given orbit.
Leading edge: The front edge of a wing, fin or inlet. On hypersonic vehicles leading edges take the most heat, so their sharpness trades aerodynamic efficiency against temperature.
Lethal autonomous weapon system (LAWS): A weapon that can select and engage targets without further human input once activated. Its regulation is debated internationally, including under the Convention on Certain Conventional Weapons.
Life cycle assessment (LCA)In Sustainability: The standardized method, set out in ISO 14040 and 14044, for totaling the environmental impacts of a product across its life cycle. It can be run on an attributional or consequential basis.
Lift-plus-cruise (lift-and-cruise, lift+cruise): An eVTOL layout with one set of rotors for vertical lift and a separate propeller for forward flight on a wing. The lift rotors are dead weight in cruise, but the design avoids tilting mechanisms.
Lift-to-drag ratio (L/D, glide ratio): The ratio of the lift a vehicle produces to its drag, a key measure of aerodynamic efficiency. It sets glide distance and, with engine efficiency, cruise range. It falls sharply at hypersonic speeds.
Link budget: An accounting of every gain and loss along a radio link, from transmitter power and antenna gain to path loss and noise. It shows whether the link works with enough margin.
Liquid oxygen (LOX): Oxygen cooled to a liquid at ≈90 K (≈−183 °C). It is the oxidizer in most liquid-fueled rockets.
Loitering munition (one-way attack drone, kamikaze drone): A weapon that flies to an area, circles while searching for a target, then dives into it. It blends a drone with a missile and is usually expendable.
Low Earth orbit (LEO): Orbits from ≈160 km up to 2,000 km altitude, where satellites circle Earth in ≈90–130 minutes. The short distance means little signal delay, but each satellite sees a small area, so coverage needs many.
Low observability (low observable, stealth): Design features that make a vehicle hard to detect by radar, infrared, acoustic or visual sensors, such as shaping, absorbent materials and controlled emissions.
Low-altitude economy: A Chinese policy term for commercial activity in low airspace, generally below 1,000 m, including drone delivery, eVTOL passenger flights and aerial services. Beijing treats it as a strategic emerging industry.
Low-rate initial production (LRIP, low-rate production): The first, limited production run of a defense system, used to prove the production line and supply test articles before full-rate production is approved.
Loyal wingman: An uncrewed combat aircraft that flies alongside a crewed fighter and takes direction from its pilot, extending sensors and weapons and taking on the riskiest tasks.
Lunar Gateway (Gateway): A small space station planned for lunar orbit as a staging point for Artemis missions, built with international partners. Its funding has been contested in US budget debates.

M

Mach number (Mach): Speed expressed as a multiple of the local speed of sound. Sound travels slower in cold air, so the same Mach number means a lower true speed at high altitude.
MACH-TB (Multi-Service Advanced Capability Hypersonics Test Bed): A US Defense Department program, run by its Test Resource Management Center, that raises the number and pace of affordable hypersonic flight tests using commercial vehicles and test services.
Magnetic window: A blackout-mitigation concept that applies a strong magnetic field near an antenna to change how the surrounding plasma behaves, so radio signals can get through.
Manifest: A launch provider's list of contracted missions and their planned order. A full manifest signals demand; a slipping one signals delays.
Maximum takeoff weight (MTOW, gross takeoff weight, maximum takeoff mass): The heaviest weight at which an aircraft is certified to take off, including fuel, payload and crew. It is the main size measure in aviation rules and fees.
Means of compliance (MoC): The accepted methods an applicant uses to show that a design meets a regulation, such as specific tests, analyses or industry standards.
Medium Earth orbit (MEO): Orbits between low Earth orbit and geostationary altitude, roughly 2,000–35,786 km. Navigation constellations such as GPS and Galileo fly here, at ≈20,000–23,000 km.
Medium-lift, heavy-lift and super heavy-lift (small-lift, medium-lift, heavy-lift, super heavy-lift, super heavy lift launch vehicle, SHLLV): Size classes for launch vehicles, based on the mass they can place in low Earth orbit (LEO). The commonly cited NASA thresholds are below. See the table
Megaconstellation (satellite constellation, constellation): A network of hundreds to thousands of satellites, usually in low Earth orbit, working together to give continuous coverage. Such fleets need frequent launches both to deploy and to replace satellites as they wear out.
Metasurface: A thin layer of engineered, repeating structures smaller than the wavelength, which shapes radio waves or light in ways ordinary materials cannot.
Methalox (LOX/methane): Rocket propellant made of liquid methane and liquid oxygen. Methane burns cleaner than kerosene, leaving less soot in engines, which suits reuse, and in principle it can be made on Mars.
Microgravity: The near-weightless condition inside an orbiting spacecraft, which is in continuous free fall. It changes how fluids, crystals and cells behave, which is the basis for research and manufacturing in orbit.
Middle tier of acquisition (middle-tier acquisition): A US defense acquisition path for rapid prototyping or rapid fielding, meant to deliver capability to users within five years without the full traditional process.
Mishap investigation: A formal inquiry into a launch or flight failure, led by the operator and overseen in the US by the FAA. Flights usually resume only after the regulator accepts the findings and corrective actions.
Mission assurance: The engineering, testing and independent review that raise confidence a mission will succeed. Government customers demand more of it for their most valuable payloads, which adds cost and time.
Mode transition: The handoff in a combined-cycle engine from turbine power to ramjet or scramjet power, usually between ≈Mach 2 and Mach 4. Thrust tends to dip during it, so the vehicle needs margin to keep accelerating.
Model-based systems engineering (MBSE): Designing and managing a complex system with linked digital models instead of separate documents, so requirements, design and tests stay consistent.
More-electric aircraft (MEA): A design approach that replaces hydraulic, pneumatic and mechanical aircraft systems with electrical ones. It raises demand for compact generators, motors and power electronics on board.
Multirotor (multicopter): An aircraft lifted and controlled by several fixed-pitch rotors whose speeds are varied independently. It is mechanically simple and hovers well but is inefficient in forward flight.

N

National Security Space Launch (NSSL): The US Space Force program that buys launches for the most important military and intelligence satellites. Rockets must be certified, and the program keeps more than one provider to assure access to space.
Nautical mile (nmi): A distance of 1,852 m by international definition, used in aviation and shipping because it relates to minutes of latitude.
NDAA (National Defense Authorization Act): The National Defense Authorization Act, the annual US law that sets defense policy and authorizes spending levels. It often carries sourcing bans, such as limits on Chinese-made drones, magnets and electronics.
NDAA-compliant (NDAA compliant): Describes drones and components free of parts from manufacturers or countries barred by US defense authorization laws, chiefly Chinese suppliers. Compliance is a precondition for many government purchases.
NEPA (National Environmental Policy Act): The US National Environmental Policy Act, which requires federal agencies to assess environmental effects before major actions, such as licensing a launch site or a higher flight rate.
Noise temperature: A measure of the electrical noise a receiver or antenna adds, given as an equivalent temperature in kelvin. Lower is better; hot, lossy elements such as plasma raise it.
Non-recurring engineering (NRE): The one-time cost of designing, developing and tooling a product before production. It is spread over the units built, so low volumes mean high unit costs.
Non-terrestrial network (NTN): The 3GPP term for parts of a mobile network carried by satellites or high-altitude platforms instead of ground towers. Standards from Release 17 onward let phones and base stations work through them.
NSSL Phase 3 Lane 1 and Lane 2 (NSSL Phase 3, Lane 1, Lane 2): The two tracks of the current NSSL procurement. Lane 1 buys launches for less demanding missions and is open to newer providers. Lane 2 covers the most demanding missions, requires full certification, and spans orders placed in fiscal years 2025–2029.
Nuclear thermal propulsion (NTP): A rocket engine that heats hydrogen in a nuclear reactor and expels it through a nozzle. It offers about twice the specific impulse of the best chemical engines, which could shorten trips to Mars.

O

Obligated funding: Money the government has legally committed to a contract, as opposed to its ceiling or announced value. It is the firmest measure of contract revenue to come.
Office of Strategic Capital (OSC): A US Defense Department office, created in 2022, that makes loans and loan guarantees to companies producing critical technologies.
One engine inoperative (OEI): The condition of a multi-engine aircraft flying with one engine failed. Certification requires defined performance in this state, such as continuing to climb or landing safely.
Optical intersatellite link (laser crosslink, optical crosslink, laser communications terminal): A laser link that carries data directly between satellites, avoiding ground relays. It offers high data rates and is hard to intercept, but needs very precise pointing.
Orbital data center (orbital compute, space-based data center): A proposed computing facility in orbit, powered by solar arrays and cooled by radiating heat into space. Backers cite abundant solar power; launch cost, heat rejection and maintenance are the main obstacles.
Orbital debris (space debris, space junk): Defunct satellites, spent stages and fragments left in orbit. At orbital speeds even small pieces can damage spacecraft, so operators track debris and maneuver to avoid it.
Orbital inclination (inclination): The angle between an orbit's plane and Earth's equator. Changing it in space costs a lot of propellant, so launch-site latitude and rocket performance limit which inclinations are reachable.
Orbital propellant transfer (in-orbit refueling, on-orbit refueling, cryogenic propellant transfer, ship-to-ship transfer): Moving rocket propellant from one spacecraft to another in orbit, so a vehicle can refill after launch and continue to the Moon or beyond. Doing it with cryogenic propellants, which slowly boil off, is far harder than with storable ones.
Orbital transfer vehicle (OTV, space tug): A spacecraft that moves payloads from the orbit a rocket drops them in to the orbit they actually need. It lets satellites on one rideshare reach different final orbits.
Orion: NASA's crew capsule for deep-space missions, launched on the Space Launch System. A European-built service module supplies its power and propulsion.
Other transaction authority (OTA, other transaction agreement): Legal authority for US defense agencies to sign agreements outside standard procurement rules, mainly for prototypes and research. It is faster and suits commercial firms, and a successful prototype can move to follow-on production without a new competition.
Outer Space Treaty: The 1967 UN treaty that underpins space law. It bars national claims of sovereignty over celestial bodies and weapons of mass destruction in orbit, and makes states responsible for their companies' space activities.

P

Part 36: The FAA rules setting the noise limits and test procedures aircraft must meet for type certification.
Part 103 (ultralight): The FAA rule for ultralight vehicles: single-seat recreational aircraft that need no pilot certificate or registration. A powered ultralight must weigh under 254 lb empty, carry at most 5 US gal of fuel and fly no faster than 55 knots.
Part 107: The FAA rule for routine commercial flights of small drones under 55 lb, flown within the operator's line of sight. Flights outside its limits need a waiver.
Part 108: The FAA's proposed rule for routine beyond-visual-line-of-sight drone operations, replacing case-by-case waivers with permits and certificates. It was proposed in August 2025; as of October 2026 the final rule had not been published.
Part 450: The FAA's consolidated rule for licensing commercial launch and reentry, in effect since March 2021. It sets performance-based requirements and lets one license cover many missions and sites; older licenses had to move to it by March 2026.
Parts 23, 25, 27 and 29 (Part 23, Part 25, Part 27, Part 29): The FAA airworthiness standards for the main aircraft types, with matching EASA specifications. New aircraft such as powered-lift often draw on several of them under special-class rules. See the table
Payload: What a launch vehicle or aircraft carries to perform its mission, such as satellites, cargo, crew, sensors or weapons. Rocket payload capacity is quoted as mass to a named orbit, for example kilograms to low Earth orbit.
Payload fairing (fairing): The protective shell at the top of a rocket that shields the payload from air pressure and heating during the climb through the atmosphere. It is jettisoned once the rocket is high enough. Its inner diameter limits how large a payload can fly.
Payload user guide (payload user's guide): A launch provider's public technical document listing payload mass to each orbit, fairing dimensions, and the loads, vibration and environment a payload must survive.
Phased array (phased-array antenna, electronically scanned array, AESA): An antenna made of many small elements whose signal timing is adjusted electronically to steer the beam without moving parts. Radars, satellite terminals and satellites use it.
Planetary protection: Practices that keep spacecraft from contaminating other worlds with Earth life, and Earth with material returned from space.
Plasma antenna: An antenna that uses a column of ionized gas instead of metal as its conductor. It can be switched off to become nearly invisible to radio waves and retuned electronically, but it is less efficient than metal.
Plasma frequency: The natural oscillation frequency of electrons in a plasma, set by electron density. Radio waves below it are reflected or absorbed; waves above it pass through.
Plasma sheath: The layer of ionized gas that surrounds a vehicle at hypersonic or reentry speeds. It reflects and absorbs radio waves, causing communications blackout.
Powered-lift (powered lift): A category of aircraft that takes off and lands vertically using engine-driven lift but relies on a wing in cruise, such as tiltrotors and many eVTOLs. US regulators set out pilot and operating rules for it in a 2024 final rule.
Production certificate: Approval for a manufacturer to build copies of a certified design under an approved quality system, without the regulator inspecting each one from scratch.
Program of record: A defense program with its own line in the budget and an approved acquisition plan, giving it multi-year funding. Becoming one is a key milestone for a defense startup's product.
Proliferated LEO (pLEO, proliferated low Earth orbit): A military space architecture that spreads missions across many cheaper low Earth orbit satellites instead of a few large ones, so losing some does not break the system.
Proliferated Warfighter Space Architecture (PWSA): The US Space Development Agency's low Earth orbit constellation, with a transport layer for military data relay and a tracking layer for missile warning and tracking. It is fielded in tranches about two years apart.
Propagation delay (signal delay): The time a signal takes to travel between two points at the speed of light. It sets a floor on latency: under 0.1 ms each way to a platform 20 km up and ≈120 ms each way to a geostationary satellite.
Propellant densification (propellant subcooling, subcooled propellant, densified propellant): Chilling propellants below their normal boiling point so more mass fits in the same tank. It adds performance but tightens the fueling timeline before launch.
Propellant depot (orbital depot): A spacecraft that stores propellant in orbit for other vehicles to draw on. Tanker flights fill it; departing vehicles refuel from it.
Pseudolite: A ground- or air-based transmitter that broadcasts signals like those of navigation satellites, to add to or replace satellite positioning in a local area.
PX4 and ArduPilot (PX4, ArduPilot): Two widely used open-source autopilot software projects for drones. Many drone makers build on them instead of writing flight software from scratch.

Q

Qualification and acceptance testing (qualification testing, acceptance testing): Qualification testing proves a design survives conditions beyond what it will see in service, often using dedicated test units. Acceptance testing checks each flight unit at milder levels before delivery.
Quenchant injection (electrophilic injection): A blackout-mitigation method that sprays water or electron-capturing chemicals into the plasma around a reentry vehicle, cutting electron density so radio signals can pass.

R

Radiation efficiency: The share of power fed into an antenna that is actually radiated, rather than lost as heat. Metal antennas exceed 99%; lossy conductors such as plasma radiate less.
Radio Attenuation Measurement program (RAM-C, RAM program): A NASA flight program of the 1960s that flew instrumented reentry vehicles to measure plasma density and radio blackout. Its data remain a standard reference in blackout research.
Radio frequency bands (L-band, S-band, C-band, X-band, Ku-band, K-band, Ka-band, V-band, VHF, UHF): Letter names for ranges of radio frequencies. Higher bands carry more data and allow smaller antennas but suffer more from rain and atmospheric loss. The ranges below follow the IEEE convention; satellite usage can differ slightly, for example satellite C-band starts at ≈3.7 GHz. See the table
Radome: A weatherproof cover over an antenna or radar that lets radio waves pass with little loss. On hypersonic vehicles radomes must also survive extreme heat.
Rain fade (rain attenuation): Weakening of radio signals by rain, which grows sharply at higher frequencies such as Ku- and Ka-band. Satellite links carry extra margin to ride through it.
Ramjet: An air-breathing engine with no major moving parts that uses the vehicle's own speed to compress incoming air. It cannot work from a standstill and needs a booster to reach ≈Mach 2–3 first. Combustion happens at subsonic speed.
Reaction wheel (momentum wheel): A flywheel inside a spacecraft that is spun faster or slower to turn the spacecraft the opposite way, letting it point without using propellant. Most are driven by permanent-magnet motors.
Reconfigurable antenna (frequency-agile antenna, reconfigurable aperture): An antenna whose frequency, pattern or polarization can be changed in use, letting one aperture do several jobs.
Reconfigurable intelligent surface (RIS): A metasurface whose elements can be switched electronically to redirect radio signals, for example around obstacles in a wireless network.
Record of decision (ROD): The formal document in which a US agency states its decision after an environmental review, and the reasons for it.
Reentry (re-entry, atmospheric entry): The return of a spacecraft or rocket stage from space through the atmosphere. Air compressed ahead of the vehicle causes intense heating and, at high speed, forms a layer of ionized gas around it.
Reentry blackout (communications blackout, radio blackout, plasma blackout): The loss of radio contact with a vehicle during reentry or hypersonic flight, because the plasma sheath blocks signals below a critical frequency. It typically lasts minutes. Higher radio frequencies, vehicle shaping and injected chemicals can shorten it.
Reentry capsule (return capsule): A small spacecraft built to survive reentry and bring cargo or crew back to Earth, usually slowed by a heat shield and parachutes. Uncrewed capsules return experiments and products made in orbit.
Refurbishment: The work done between flights of reusable rocket hardware: inspecting engines, replacing worn parts and retesting. Its cost and duration decide how much reuse actually saves.
Regenerative cooling: Cooling a rocket engine by running cold propellant through channels in the chamber and nozzle walls before it is burned. The heat it picks up goes back into combustion instead of being wasted.
Remote ID (Remote Identification): A requirement for most drones flown in the US to broadcast their identity and location in flight, so authorities and the public can identify them. It is set out in 14 CFR Part 89.
Replicator (Replicator initiative): A US Defense Department initiative announced in 2023 to field thousands of cheap, attritable autonomous systems within about two years. In November 2025 its portfolio moved to the Defense Autonomous Warfare Group.
Rescue Agreement, Liability Convention and Registration Convention (Rescue Agreement, Liability Convention, Registration Convention): UN treaties that build on the Outer Space Treaty. The 1968 Rescue Agreement covers aid to astronauts, the 1972 Liability Convention makes launching states liable for damage, and the 1975 Registration Convention requires registering space objects.
Responsive launch (tactically responsive space): The ability to launch a satellite on short notice, in days or hours rather than months. Militaries want it to replace lost satellites quickly or respond to a crisis.
Restartable engine (engine relight, relight, in-space restart): An engine that can shut down and fire again in flight. Upper stages need restarts to reach high or multiple orbits; reusable boosters need them to land.
Return to flight: The first launch after a failure or grounding, once investigators and regulators accept the fixes. Its timing shows how long a vehicle or pad was out of service.
Return to launch site (RTLS): A booster recovery method in which the first stage turns around after separation and flies back to land near its launch pad. It costs more propellant than landing at sea, which reduces payload.
Reusable launch vehicle (reusable rocket, reusability, RLV): A rocket built so that major parts, usually the first stage and sometimes the fairing, can be recovered and flown again. Reuse spreads build cost over many flights, but flown hardware still needs inspection and refurbishment. Fully reusable designs also recover the upper stage.
Reynolds number: A dimensionless ratio of inertial to viscous forces in a flow. Small aircraft and subscale models fly at low Reynolds numbers, so their aerodynamics may not scale up cleanly.
Rideshare: A launch in which many small satellites from different customers share one rocket and split its cost. It is cheap per kilogram, but each customer accepts the shared orbit and schedule.
Rotorcraft: An aircraft that gets its lift from rotating blades, such as a helicopter or gyroplane. US rules certify rotorcraft separately from airplanes.

S

SC-VTOL (Special Condition VTOL): EASA's special condition setting airworthiness requirements for small vertical-takeoff aircraft with distributed lift and thrust, such as eVTOLs. Aircraft in commercial passenger service must achieve continued safe flight and landing after failures.
Scramjet (supersonic combustion ramjet): A ramjet in which air stays supersonic through the combustor. It is the leading air-breathing option above ≈Mach 5–6, but keeping a flame lit in supersonic flow is very hard.
Sensor fusion: Combining data from several sensors, such as radar, cameras and radio receivers, into one picture more accurate than any single sensor gives.
Service ceiling: The highest altitude at which an aircraft can still climb at a small set rate, commonly 100 ft per minute. It marks the practical top of its flight envelope.
SFAR No. 120 (Part 194, Special Federal Aviation Regulation No. 120, powered-lift SFAR): The FAA's special rule for powered-lift pilot certification, training and operations, codified as 14 CFR Part 194. Published in November 2024, it took effect in January 2025 and runs for 10 years.
Smallsat (small satellite): A small satellite, typically light enough to share a rocket with many others. Definitions vary widely: NASA often uses 180 kg as the upper limit, while some industry counts go up to 1,200 kg.
Solid rocket motor (SRM, solid rocket booster, SRB): A rocket motor whose fuel and oxidizer are mixed into a rubbery solid cast inside the casing. It is simple, storable and powerful, but cannot be shut down or throttled once lit. Missiles and strap-on boosters use them widely.
Space Act Agreement: A flexible NASA agreement, outside standard procurement rules, used to partner with companies on technology development and commercial services.
Space Development Agency (SDA): A US Space Force agency that fields proliferated low Earth orbit constellations quickly, in roughly two-year tranches, starting with data transport and missile tracking layers.
Space Launch System (SLS): NASA's super heavy-lift rocket for the Artemis program, built largely from Space Shuttle-derived engines and boosters. It is expendable, and auditors have criticized its cost per launch.
Space Systems Command (SSC): The US Space Force field command that develops and buys space systems and launch services, including National Security Space Launch.
Space traffic management: The tracking, rules and coordination needed to keep objects in orbit from colliding as satellite numbers grow.
Spacecraft bus (satellite bus): The standard platform of a satellite that supplies power, propulsion, pointing, thermal control and communications. The mission's instruments, called the payload, are mounted on it.
Special class (21.17(b), special-class aircraft): A US type certification path under 14 CFR 21.17(b) for aircraft that do not fit existing airworthiness standards, such as powered-lift. The FAA writes tailored criteria for each design, drawn from existing rules.
Special conditions: Additional safety standards a regulator issues when a design has novel features that existing rules do not cover, such as electric propulsion.
Specific impulse (Isp): A measure of rocket engine efficiency: thrust produced per unit of propellant used per second, given in seconds. Higher is better. Hydrogen engines reach ≈450 s in vacuum; kerosene and methane engines sit lower, ≈300–380 s.
Spot beam: A narrow antenna beam covering a small area on the ground. Many spot beams let a satellite reuse the same frequencies in different places, multiplying capacity.
Stage (staging): A self-contained section of a rocket with its own engines and propellant tanks. Dropping a stage once its propellant is spent sheds dead mass, so the remaining stages can reach higher speeds.
Staged combustion (oxidizer-rich staged combustion, oxygen-rich staged combustion, ORSC, fuel-rich staged combustion): A rocket engine cycle that routes the turbopump exhaust into the main combustion chamber instead of dumping it. No propellant is wasted and chamber pressure can be high, but the hot, oxygen-rich or fuel-rich gas is hard on hardware.
Stagnation temperature (total temperature): The temperature air reaches when brought to rest against a vehicle's surface, such as its nose or leading edges. It rises steeply with speed, reaching ≈1,000 °C at Mach 5 in the stratosphere.
Stall speed: The lowest speed at which a wing produces enough lift to hold an aircraft in level flight. Certification and ultralight rules often set limits on it.
Standard atmosphere (1976 US Standard Atmosphere, International Standard Atmosphere, US Standard Atmosphere): A reference model of how air temperature, pressure and density change with altitude. Engineers use it to compare aircraft performance on a common basis.
Station-keeping: Small, regular corrections that hold a satellite or high-altitude platform at its assigned position. On geostationary satellites the propellant for station-keeping often sets working life.
Sun-synchronous orbit (SSO): A near-polar low Earth orbit whose plane turns with the Sun, so the satellite passes over each place at the same local time. Earth-observation satellites favor it for consistent lighting.
Sustainable aviation fuel (SAF)In Sustainability: Jet fuel made from non-fossil sources, such as HEFA from fats and oils. Because it draws on the same lipid feedstocks as HVO, the two compete directly for limited residues.
SWaP-C (SWaP, size, weight, power and cost): Size, weight, power and cost: the four constraints that decide whether a sensor, computer or radio fits on a platform. Small drones and satellites are especially limited by them.
Swashplate: The mechanical linkage that changes the pitch of helicopter rotor blades as they turn, letting the pilot tilt and adjust the rotor's thrust.

T

Technical Standard Order (TSO): An FAA minimum performance standard for specific parts and equipment, such as batteries or avionics. Meeting one lets a part be approved once and installed in many aircraft.
Technology readiness level (TRL): A 1–9 scale, developed at NASA, for how mature a technology is, from basic principles observed to proven in operation. Agencies and investors use it to judge development risk. See the table
Telemetry, tracking and command (TT&C, telemetry, tracking and control): The radio links and ground systems used to monitor a spacecraft's health, measure its position and send it commands.
Thermal protection system (TPS, heat shield): The materials that shield a vehicle from heating during reentry or hypersonic flight, such as tiles, blankets and heat shields. Ablative types char and erode to carry heat away; reusable types must survive many flights.
Throttling (deep throttling, throttle range): Varying an engine's thrust during flight. Deep throttling, to a small fraction of full thrust, is needed for propulsive landing and for gentle lunar descent.
Thrust gap: A speed range in which neither engine of a combined-cycle system gives enough thrust: the turbine is running out of performance and the ramjet is not yet efficient.
Thrust vectoring (vectored thrust): Steering by changing the direction of engine or rotor thrust, instead of relying only on control surfaces. Tiltrotors, many eVTOLs, some fighters and most rockets use it.
Tiltrotor (proprotor, prop-rotor, tilt rotor): An aircraft with rotors at its wingtips that tilt upward to hover like a helicopter and forward to cruise like an airplane. It is faster than a helicopter but mechanically complex.
Tip speed (tip Mach number, rotor tip speed): The speed of a rotor or propeller blade's tip. Higher tip speeds give more thrust but more noise, and near the speed of sound shock waves form, so quiet designs run slower tips.
Touchdown and lift-off area (TLOF, touchdown and liftoff area): The load-bearing central pad at a heliport or vertiport where an aircraft touches down and lifts off. In FAA vertiport guidance it is at least one rotor diameter across.
Tower catch (booster catch, chopstick arms, chopsticks): A recovery method in which arms on the launch tower grab a returning booster in mid-air instead of letting it land on legs. Dropping the landing legs saves mass and can speed turnaround, but a failed catch puts the tower at risk.
Trans-lunar injection (TLI): The engine burn that sends a spacecraft from Earth orbit onto a path to the Moon. Payload to it measures a rocket's usefulness for lunar missions.
Transition: The flight phase in which a VTOL aircraft shifts between rotor-borne hover and wing-borne cruise. Lift, control and power needs change rapidly, which makes it a focus of testing.
Traveling-wave tube (TWT): A vacuum tube that amplifies microwave signals by passing them alongside an electron beam. Satellites and radars use them in high-power transmitters, with permanent magnets focusing the beam.
True and calibrated airspeed (true airspeed, calibrated airspeed): Calibrated airspeed is the instrument reading corrected for installation errors; it governs how the aircraft handles. True airspeed is the actual speed through the air, which exceeds calibrated airspeed at altitude.
Turbine-based combined cycle (TBCC): A propulsion system that pairs a turbine engine for takeoff and low speeds with a ramjet or scramjet for high speeds. It could enable reusable, runway-launched hypersonic aircraft, but handing off between the two engines remains a major engineering challenge.
Turbofan: A jet engine in which a large fan pushes some air around the engine core, called bypass air. It is quieter and more fuel efficient than a turbojet at subsonic speeds.
Turbojet: A jet engine that passes all of its intake air through the compressor, combustor and turbine. It is simple and suits small, fast, short-lived vehicles such as aerial targets and cruise missiles.
Turboshaft: A gas-turbine engine that delivers power through a rotating shaft instead of jet thrust. It drives the rotors of helicopters and tiltrotors.
Type certificate (type certification): A regulator's approval that an aircraft, engine or propeller design meets its airworthiness standards. It is the main hurdle for a new design; building and flying it commercially need further approvals.
Type Inspection Authorization (TIA): FAA approval to begin official certification flight tests with FAA pilots, issued once a design is mature and its compliance data are largely complete. It marks the final phase of type certification.
Type rating: A pilot qualification for a specific aircraft type, required for large, jet-powered or otherwise complex aircraft, including powered-lift.

U

UAS groups 1–5 (DoD UAS groups): The US military's five size classes for uncrewed aircraft, set by weight, normal operating altitude and speed. See the table
Uncrewed aerial vehicle (UAV, unmanned aerial vehicle, drone): An aircraft flown with no pilot on board, either remotely or autonomously. Sizes range from palm-sized quadcopters to airliner-sized surveillance aircraft.
Uncrewed aircraft system (UAS, unmanned aircraft system, sUAS, small unmanned aircraft system): An uncrewed aircraft together with everything needed to operate it: the ground control station, data links and support equipment. Regulators and militaries use the term because the aircraft alone is not the whole system.
UnicornIn Finance: A privately held startup valued at $1 billion or more. The valuation usually comes from the price paid in its latest funding round rather than from trading on a public market.
Unstart (inlet unstart): A sudden breakdown of supersonic flow in an engine inlet, in which the shock wave is pushed out of the inlet and thrust collapses. It can cause violent yawing and was a known hazard on Mach 3 aircraft.
Upmass and downmass (upmass, downmass): Upmass is the mass carried from the ground to orbit; downmass is the mass returned safely to Earth. Downmass capacity is much scarcer, which matters for returning experiments and products made in orbit.
Upper stage (second stage): A rocket stage that ignites after the first stage separates and carries the payload the rest of the way to orbit. It often needs an engine that can restart in space to reach high or multiple orbits. Most upper stages are still discarded after one flight.
Urban air mobility (UAM): The part of advanced air mobility focused on short passenger and cargo flights within cities, such as air taxis between vertiports.
US Munitions List (USML, United States Munitions List): The list of defense articles and services controlled under ITAR, organized in 21 categories, such as Category IV for launch vehicles and missiles and Category XI for military electronics.
Useful load: The weight an aircraft can carry beyond its empty weight: crew, passengers, cargo and usable fuel.

V

Vacuum-optimized engine (vacuum engine): An engine with a large nozzle designed for the near-vacuum of space, where it gives more thrust and efficiency. The same nozzle would perform poorly at sea level, so first stages use shorter sea-level nozzles.
ValuationIn Finance: An estimate of what a company or asset is worth, based on what it owns today and the cash it is expected to generate in future. Common methods are discounted cash flow, comparisons with similar companies, and the value of net assets.
Vertiport: A site where VTOL aircraft such as eVTOLs take off, land, charge and board passengers, on the ground, on rooftops or at airports. In the US, FAA Engineering Brief 105A gives voluntary design guidance.
VTOL (vertical takeoff and landing): Able to take off and land vertically, without a runway. The term covers helicopters, tiltrotors, eVTOLs, some jets and many drones.

W

Wassenaar Arrangement (Wassenaar): A multilateral export-control regime in which participating states agree on lists of conventional arms and dual-use goods to control, then apply them in national rules.
Waverider: A hypersonic vehicle shape designed to ride on its own shock wave, using the high-pressure air beneath it for lift. It improves the lift-to-drag ratio at hypersonic speeds.
World Radiocommunication Conference (WRC): The ITU treaty conference, held every three to four years, that revises the international Radio Regulations, including spectrum for satellites and new services such as high-altitude platforms.