Appendix A Glossary


Device that measures the acceleration of a rocket or space vehicle, used in inertial guidance systems. In an integrating accelerometer, the acceleration is integrated over time to give accurate velocity and position readouts.

Attitude control system, the system of small thrusters that allow a spacecraft to control its attitude in the vacuum of space.


The technique of using a planet's atmosphere to decelerate a spacecraft by aerodynamic friction, rather than having to use onboard propellants.


An "inside-out" rocket engine, incorporating a typically truncated linear or annular "ramp" that is exposed to the open atmosphere. The exhaust gases are expelled into the region between the ramp and the atmosphere, resulting in continuous altitude compensation (see separate entry), optimum expansion, and increased efficiency at any altitude or pressure.

Altitude compensation

The automatic and continuous optimum expansion of the exhaust gases exiting a rocket nozzle in reaction to changes in ambient pressure conditions. Conventional nozzles allow optimum expansion at only one ambient pressure, resulting in overexpansion (see separate entry) at lower altitudes and underexpansion (see separate entry) at higher altitudes.

Ambient pressure

The local prevailing atmospheric pressure.


The point in a ballistic trajectory halfway around the world from the launch site.


The point of highest altitude in an elliptical orbit or suborbital trajectory. APT

Aerial propellant transfer. Area ratio

The ratio of a rocket nozzle's exit plane area to its nozzle throat area. Aspect ratio

Wingspan (b) divided by average chord (c), also equal to the square of the span divided by the wing planiform area (S). Stubby wings have a low aspect ratio.


The orientation in space of a flight vehicle relative to some reference frame. Ballistic coefficient

A determination of how much heating takes place during atmospheric reentry.


A high-thrust rocket stage used to help lift heavy payloads into orbit or boost spacecraft further into space, usually dropping off after use. Many modern launchers use solid rocket boosters in the initial stage of flight.

Bespilotniy Orbitalniy Raketoplan, the Russian acronym for unpiloted orbital rocketplane.

Boundary layer

The thin layer of fluid (gas or liquid) between a vehicle surface and the free stream. The molecules nearest the surface adhere to the surface and those adjacent to the free stream move with it. A boundary layer may be laminar or turbulent, and may be assumed to be viscous (gluey) or inviscid (frictionless).

Carbon vane

A jet vane made of carbon, placed in the rocket engine exhaust stream for steering control.


The straight-line distance between a wing's leading and trailing edges. CM

The Apollo Command Module; also, the Orion Crew Module. Combustion chamber

The enclosed area in a thrust chamber (see separate entry) in which rocket propel-lants are ignited and burned before accelerating through a convergent-divergent throat and expanding through a nozzle.

Congreve, Sir William (1772-1828)

A British lieutenant colonel who developed working solid-fuel one- and two-stage rockets, time fuses, and parachute systems for the Royal Army. Congreve's rockets are immortalized in The Star Spangled Banner, the national anthem of the United States.

Command and Service Module (Apollo); also, Crew and Service Module (Orion). Cryogenic propellant

An extremely low temperature liquid rocket fuel or oxidizer. Examples are liquid hydrogen (-435 to -423°F) and liquid oxygen (-362 to -297°F).

De Laval nozzle

Conventional convergent-divergent rocket nozzle, named for the Swedish engineer Carl de Laval (1845-1913). The shape accelerates exhaust gases to supersonic speeds at the nozzle exit.

The theoretical change in velocity, or AV, expected from a rocket burn in the absence of performance losses due to gravity, atmospheric drag, back pressure, etc. Specific impulse (see separate entry) and mass ratio (see separate entry) are the main factors that determine AV.

Drag loss

The reduction in velocity increment (see separate entry) affecting launch vehicles operating in an atmosphere, due to aerodynamic drag.

Dynamic pressure

The pressure q felt by a flight vehicle due to the velocity v of onrushing air molecules with density p


The ratio e of the distance between the foci (2c), to the length of the major axis (2a) in an ellipse, a hyperbola, or a parabola (see separate entries).


Earth Departure Stage, part of the Ares V launch vehicle.


An oval-shaped curve, such that the distance from one focus to any point on the curve and back to the other focus remains constant. All ellipses have eccentricities

(see separate entry) between 0 and 1, and all closed orbits are ellipses, with the parent body occupying one of the foci.

In this standard equation, the center of the ellipse lies at the origin of the Cartesian coordinate plane. The ends of the major and minor axes are located a and b units from the y and x axes, respectively, and each focus lies on the x-axis at a distance c = -Ja2 - b2 from the origin.


The particular range of altitudes and airspeeds characteristic of a given aircraft. Spaceplanes will clearly require much larger flight envelopes than any other aircraft.

Escape velocity

The speed required to permanently escape from a gravitating body in space. It is always equal to the square root of two times the circular orbital velocity. In the following expression, g refers to the local acceleration of gravity at the spacecraft's location, and r is the radial distance from planet's center to the spacecraft.

Exhaust velocity

The speed (c) that exhaust products depart the nozzle exit plane, a critical factor in determining specific impulse (/ ) and rocket performance. It can be found by taking the ratio of thrust (F) to propellant mass flow rate ( m).

Flight and fuel management. Film cooling

A method of cooling a rocket thrust chamber from the inside, using a thin film of propellants injected against or very near the chamber walls. As this film vaporizes, it forms a cool layer next to the chamber walls, providing effective cooling.

Free fall

The state of weightlessness characterized by a body falling freely in a gravitational field. Since gravity fields pervade all space, orbiting objects are in free fall around the central body.

Free stream

The stream of fluid moving relative to a vehicle operating in air or water, and typically isolated from the vehicle by a boundary layer (see separate entry).


Solid, liquid, or gaseous propellant (see separate entry) that produces useful heat or energy when burned with an oxidizer (see separate entry). Examples are ethyl alcohol, liquid hydrogen, kerosene, methane, HTPB, and powdered aluminum.


A launch vehicle service tower, used to provide access to launch team workers, electrical connections, and propellant lines.

Gas generator

A device used to generate and deliver hot pressurized gases or steam to other parts of a rocket propulsion system. These gases are typically used to pressurize propel-lant tanks or spin the turbine of a turbopump.

Geosynchronous earth orbit, an orbit at an altitude of 22,300 miles in which a satellite seems to hover over one location on the ground, typically used by communications and meteorological satellites.


An inertial force that mimics weight and is experienced by astronauts during periods of high acceleration or deceleration.

Gimbal mount

A pair of connected swiveling frames hinged on mutually orthogonal axes, allowing a thrust chamber to point in any direction, thereby enabling thrust vector control. Gimbal mounts are also used to support gyroscopes and gyroscopic flight instruments, which maintain fixed attitudes in space while the structure of the vehicle itself tilts or rotates.


Gross lift-off weight; also, gross light-off weight. Goddard, Robert H. (1882-1945)

An American physics professor and pioneer rocket scientist. On March 16, 1926, Goddard flew the first liquid-fueled rocket from a farm near Auburn, MA, before moving his base of operations to Roswell, NM. He also conducted experiments proving rockets work in a vacuum (circa 1912), invented an early form of bazooka (1918), pioneered the use of gyroscopes and jet vanes in rockets (1932), and used film cooling and gimbal mounts (see separate entry) in his designs, among numerous other contributions.

Gravitational parameter

A constant (m) used in orbital calculations around a given gravitating body, equal either to the product of Newton's universal gravitational constant (G) and the mass (M) of the central body, or to the product of the surface acceleration of gravity (g) and the square of the radius (r) of that body.

Gravitational acceleration

The change in velocity with respect to time experienced by objects falling in gravity fields, designated by lower case g, and varying from place to place in the universe. On the surface of Earth, the standard acceleration of gravity is ge = 32. 174 ft/s2 = 9.80665 m/s2

Gravitational constant

Newton's universal gravitational constant (G), not to be confused with the standard acceleration of gravity (ge) at Earth's surface.

G = 6.6726 x 10-11 N m2/kg2 = 3.4389 x 10-8 ft3/slug sec2

Gravity gradient

The magnitude and direction of a local gravitational field.

Gravity loss

The reduction in velocity increment affecting space launch vehicles, due to gravity. Two methods of reducing gravity loss are to accelerate to orbital speed as quickly as possible, or to generate aerodynamic lift in winged vehicles.

Gravity turn

A trajectory followed by a launch vehicle, designed to minimize gravity loss. By gradually turning a launch vehicle sideways during launch, two things happen as it accelerates: (1) Earth's surface begins to fall away from the rocket's flight path due to curvature and (2) centrifugal forces begin to counteract the centripetal forces of gravity.

Gravity well

A geometrical representation of a gravitational field, with the gravitating body at the bottom and the slopes of the sides representing the gravity gradient.


Gross takeoff weight.


An instrument that uses a spinning mass to maintain a fixed orientation in space, attached by a gimbal mount (see separate entry) to its case, and used in attitude and directional control instrumentation and accelerometers.

Heat exchanger

A device consisting of a coiled tube, past which hot turbine exhaust gases (already heated in a gas generator) are passed, in order to transfer heat into a cold liquid propellant or a pressurizing gas carried inside the coil. Heat exchangers can also remove heat from an airflow.

Heat transfer

The conveyance of heat from one body or system to another. Heat may be transferred by conduction, convection, or radiation. Understanding these phenomena is important in keeping a space vehicle at the proper temperature in all flight regimes.


Horizontal takeoff horizontal landing, the flight mode of an advanced spaceplane. See also VLHA.


Hydroxyl-terminated polybutadiene, a solid rocket fuel also known as rubber. Hyperbola

An open-ended two-branch curve in which the eccentricity (e = c/a) (see separate entry) is greater than 1. Corresponding hyperbolic orbits have specific energies (see separate entry) greater than 0 and result in escape from the central gravitating body.

The formula for the hyperbola is similar to that for an ellipse. Here, the distance from the origin to each focus is c = ->Ja2 + b2 measured along the x-axis. Hyperbolas can be drawn by first constructing linear asymptotes with equations y = bx/a and y = -bx/a, The curves are then drawn through the points (a,0) and (-a,0), and the foci are located at (c,0) and (-c,0). The two curves will be symmetrical to both the x and y axes, and one of the foci will correspond to the center of a gravitating body, with the curve corresponding to the path of an escaping spacecraft.

Hyperbolic trajectory

An escape orbit followed by an object near a gravitating body. A hyperbolic trajectory has specific energy (see separate entry) greater than 0 and eccentricity (see separate entry) greater than 1. An object on a hyperbolic trajectory relative to a planet may, nevertheless, remain in an elliptical orbit around the Sun.

Hypergolic propellant

A fuel or an oxidizer that spontaneously ignites on contact with the other. Examples are MMH, UDMH, and N2O4.

Ideal exhaust velocity

The theoretically maximum velocity of exhaust products from a rocket thrust chamber, given a mean molecular weight of propellants, combustion chamber pressure and temperature, and ambient pressure.


The total impulse (I) is the product of a constant thrust (F) and the burn-time (t), or a variable thrust integrated over that time interval. Compare to specific impulse (see separate entry).

Inertial guidance

A self-contained system by which a space vehicle is controlled by reference to an inertial frame, using a system of three gyroscopes and three accelerometers mounted on a stabilized gimbal-supported platform. The accelerometers measure the acceleration of the vehicle in three axes, while the gyroscopes provide signals keeping the platform properly aligned. Velocity and position are continuously integrated (calculated) from accelerometer readouts.

Interplanetary travel

Space travel within the solar system.

Interstellar travel

Space travel beyond the solar system.

Jet vane

A movable gyroscopically controlled plane, made of carbon or some other resilient material, placed in the exhaust stream of a rocket nozzle to steer the vehicle.

Kepler's laws

Three laws of planetary motion formulated by Johannes Kepler in the early seventeenth century, using prior detailed observations by Tycho Brahe:

1. A planet moves in an elliptical orbit about the Sun, with the Sun at one focus.

2. A line drawn from the Sun to a planet sweeps out equal areas in equal times.

3. The square of a planet's orbital period is proportional to the cube of its semimajor axis.

Launch abort system, to be used on the Orion Ares launch vehicles. Launch vehicle

A rocket used to launch a spacecraft into orbit, typically consisting of two or more liquid or solid propellant stages. In the future, launch vehicles and spacecraft will be integrated into a reusable single-stage-to-orbit vehicle.

Lift-to-drag ratio, the ratio of aerodynamic lift to atmospheric drag. An airplane with L/D = 20 would provide 20 pounds of lift at the cost of only 1 pound of drag, giving it an excellent mechanical advantage.

Low-Earth orbit, encompassing altitudes from 100 to 500 miles above the surface, or 200-800 km.

Launch escape system, used on the Apollo Saturn launch vehicles. Lift

An aerodynamic force caused by a difference in pressure above and below a wing. It is found by the algebraic product of three terms: the coefficient of lift (CL), the dynamic pressure (q) (see separate entry), and the wing area (S).

Lifting body

A flight vehicle that develops lift by the shape of its fuselage. LM

Lunar module, the Apollo two-stage Lunar lander, pronounced "lem." LSAM

Lunar surface access module, the Orion two-stage Lunar lander. Mach number

The ratio of vehicle speed to the local speed of sound; this depends upon air temperature and density.


A measure of the amount of material in a body, expressed in slugs or kilograms. A body's mass stays constant throughout the known universe, in contrast to weight.

Mass flow rate

A quantity used to specify propellant flow rate in a liquid rocket engine, expressed in slugs or kilograms per second, or weight flow rate divided by the standard acceleration of gravity.

di ge

Mass ratio

The ratio R of the total initial mass M (or weight W)ofa launch vehicle and propel-lants just before engine ignition, to the final mass m (or weight w) just after end-of-burn cutoff. The higher the mass ratio, the more change in velocity (AV) that can be achieved at any specific impulse (see separate entry).

Max q

Maximum dynamic pressure (q), which is equal to one-half the air density "rho" (p) times the velocity squared, or 'Vpv2. During space launch, dynamic pressure increases exponentially with airspeed while decreasing linearly with air density. Thus, there is typically one point during ascent that experiences "max q."

Mono-methyl hydrazine, a hypergolic fuel used in many spacecraft attitude control thrusters.


The product (p) of the mass (m) and velocity (v) of any object:

Momentum thrust

That portion of a rocket's thrust due to the product of mass flow rate m and exhaust velocity c. The other component of thrust is called pressure thrust (see separate entry).


The US National Aeronautics and Space Administration, organised in 1958 from the former NACA, the National Advisory Committee for Aeronautics.


The US National Aero-Space Plane, under study in the late 1980s and 1990s. Newton's laws of motion

Three laws formulated by Sir Isaac Newton during the seventeenth century:

1. A body at rest remains at rest and a body in unaccelerated motion continues in a straight line unless acted upon by an external force.

2. A force (F) imparted to a body gives that body an acceleration (a) inversely proportional to its mass (m). This law may be expressed mathematically by the equation F = ma.

3. For every action there is an equal and opposite reaction.

Newton's law of universal gravitation

A mathematical principle discovered by Sir Isaac Newton, stating that every mass (M) attracts every other mass (m) in the universe with a force (F) directly proportional to the product of the two masses and inversely proportional to the square of the distance, or radius, (r) between their centers. The constant of proportionality (G) is Newton's gravitational constant.

Oberth, Hermann (1894-1989)

A German rocket pioneer (born in Transylvania) who independently worked out the mathematical details of rocket-powered spaceflight in his 1923 book, The Rocket to Interplanetary Space.

Orbital propellant transfer.

Optimum expansion

The condition that exists when the divergent section of a thrust chamber exactly matches the natural expansion of the exhaust gases at a given ambient pressure. This results in an exhaust stream parallel to the thrust chamber axis, yielding peak efficiency. As a rocket climbs into orbit, its engines typically experience regions of overexpansion, optimum expansion, and underexpansion - in that order - unless they are altitude compensated.


The elliptical, parabolic, or hyperbolic path followed by a satellite under the influence of a gravitating body in space.


An oxygen-containing compound required for combustion of a fuel. Examples are ordinary air, liquid air, liquid oxygen (LOX or LO2) , nitrogen tetroxide (N2O4), hydrogen peroxide (H2O2), nitrous oxide (N2O), and ammonium perchlorate

(NH4QO4) Overexpansion

Condition affecting rocket nozzles that are too large for a given ambient pressure at low altitude, typically causing separation of exhaust gases from nozzle walls.


An open-ended single curve with an eccentricity of 1, corresponding to a trajectory with specific energy equal to 0. A parabolic orbit would eventually result in the body reaching zero speed at infinite distance.

Referring to this standard equation and Cartesian coordinates, the curve is symmetrical to the y-axis and opens upward or downward depending on the sign of a. It will be shifted k units from the x-axis. Interchanging x and y results in a left or right opening parabola.


That portion of a loaded space vehicle that pays to enter space: the spacecraft atop a launch vehicle or passengers and cargo within a spaceship.

Payload fraction

The fraction of a space launch vehicle, by mass or weight, which is taken up by the payload. The economic goal is to maximize the payload fraction, which conflicts with the engineering requirement to maximize the mass ratio (see separate entry).


A solid rocket that air-launched the X-43 hypersonic scramjet from the belly of a B-52B in 2004.

Performance parameters

Four parameters critical for airplane performance: power loading (weight/power), wing loading (weight/wing surface area), drag coefficient, and maximum lift-to-drag ratio (L/D)max.


The lowest point in an elliptical orbit around Earth.

Pogo effect

A longitudinal oscillation in a rocket induced by propellant flow instabilities.


A force per unit area, expressed in pounds per square inch (psi) or pounds per square foot (psf). In the SI system, pressure is expressed in newtons per square meter, called pascals.

Pressure thrust

That portion of a rocket's thrust due to the difference between the ambient pressure (PA) and nozzle exit pressure (PE) acting on the nozzle exit area (AE), the other portion being the momentum thrust (see separate entry).


A combustible substance, a fuel or an oxidizer, burned in a rocket engine and used to propel a space vehicle.


A simple air-breathing jet engine, with no moving parts, that utilizes ram air pressure in place of a turbine and compressor to sustain combustion and produce thrust.

Reaction control system. Regenerative cooling

A method of cooling a rocket engine, in which one of the propellants is circulated through a cooling jacket in the thrust chamber (see separate entry) before being introduced to the combustion chamber (see separate entry).


A device that accelerates by means of the conservation of momentum and Newton's third law. The momentum of the exhaust gases in one direction equals the momentum of the rocket in the other direction.

Rocket equation

The fundamental equation of rocket flight, relating the effective exhaust velocity (c) of the expelled gases and the mass ratio (M/m) (see separate entry) of the rocket to the achievable velocity increment (AV):

DV = c ln(R) = /spge ln (M / m) In exponential form this becomes

An Austrian-German rocket engineer, designer of the Silbervogel antipodal bomber, and author of Raketenflugtechnik (Rocketplane Engineering).


Any body that orbits another, usually more massive, body.

Saturn V

The eight-piece 363-ft Apollo launch vehicle and Moonship, consisting of three main rocket stages, the two-piece Lunar Module, the combined Command and Service Modules, and the Launch Escape System. The five F-1 engines of the first stage produced a total of 7 V million pounds of thrust to lift the 6V million pound vehicle off the launch pad. The only piece that returned to Earth was the three-man conical Command Module.


Supersonic combustion ramjet, in which combustion is maintained in a supersonic airflow within the engine. Unlike ramjets, scramjets have no diffuser and are typically integrated with the airframe.

Shock diamond

A visible shock wave in a rocket exhaust stream, which may appear as a row of luminous stationary spots.

Shock wave

A thin region where pressure, density, and temperature drastically change as a result of supersonic flow. It may also be produced by an explosion, lightning, or rocket engine.


A unit of mass in the US customary system of measurement, which has a weight of 32.174 pounds at the surface of Earth.

Slush hydrogen

A mixture of liquid and solid hydrogen, with a 16% higher density and 18% higher heat capacity than liquid hydrogen. Its main advantage is to reduce the gross takeoff weight of any hydrogen fueled spacecraft because of smaller fuel tank requirements. For this reason, it was selected for the NASP project in the mid-1990s.

Specific energy

The sum of the kinetic and potential energies per unit mass of a space vehicle, expressed as e=v2 - m

2 r where v is the velocity of the spacecraft relative to a gravitating body, m is the gravitational parameter (see separate entry) for the central body in question, and r is the distance of the vehicle from the center of the gravitating body.

Specific impulse

The ratio of thrust to fuel consumption, or the time in seconds that one pound of onboard propellant will provide one pound of thrust. This can be expressed as the thrust force (F) in pounds divided by the propellant weight flow rate (w = mge) in pounds per second; as the total impulse I = Ft divided by the total weight (w) of propellant consumed; or as the ratio of effective exhaust velocity (c) (see separate entry) to the standard acceleration of gravity (ge) on Earth.

Solid rocket booster. SSTO

Single-stage-to-orbit. Stage

A section of a rocket that drops off when its propellants are expended, thereby lightening the load and increasing the overall mass ratio (see separate entry) of the launch vehicle.


An aerodynamically shaped component or compartment, mounted along a spaceplane fuselage, often used for carrying extra propellants.


A ballistic trajectory followed by a space vehicle possessing insufficient velocity or energy to attain orbit.


A force (F) delivered by a rocket thrust chamber, expressed in pounds or newtons, and resulting in an acceleration of a spacecraft or launch vehicle in accordance with Sir Isaac Newton's laws of motion. Thrust force comprises the sum of a momentum thrust and a pressure thrust component:

where m is propellant mass flow rate (slugs or kg/s), c is exhaust velocity (ft or m/s), PE is nozzle exit pressure (psi or pascals), PA is ambient pressure (psi or pascals), and AE is nozzle exit area (in.2 or m2 ). Because PA decreases with altitude, thrust always reaches a peak value in vacuum conditions.

Thrust cell

One of many small thrust chambers (see separate entry) of low area ratio (see separate entry) in an aerospike engine; these emit their exhaust products onto an annular or linear ramp.

Thrust chamber

The combustion chamber (see separate entry), throat, and nozzle of a conventional rocket engine.

Thrust mount

A typically steel tubular structure designed to transmit thrust forces from the rocket engine to the space vehicle itself.

Thrust vector control

Means by which a gimbaled thrust chamber can vector its thrust axis, thereby controlling the direction of flight.

Trans-lunar insertion, the maneuver used to insert a spacecraft on a trajectory for the Moon.

Thermal protection system. Trajectory

A path through space governed by the laws of Sir Isaac Newton. Tsiolkovskiy, Konstantin E. (1857-1935)

Russian rocket scientist, the "father of cosmonautics." A self-educated mathematics and physics teacher, he was the first to work out the theoretical details of rocket-powered spaceflight, including the use of liquid propellants, jet vanes, and staging.




A turbine-driven pump, which in rockets is used to deliver liquid propellants to the combustion chamber.


Unsymmetrical di-methyl hydrazine, a toxic and corrosive hypergolic fuel used in the reaction control systems of many spacecraft.


The intentional empty space in a liquid propellant tank. Underexpansion

A condition affecting rocket nozzles that are too small for a given ambient pressure at high altitude, typically causing ballooning of exhaust gases behind the nozzle.


A physical quantity having both magnitude and direction, represented graphically as an arrow. Examples include velocity, thrust, drag, and acceleration.


A vector quantity specifying both the speed and direction of motion. Velocity increment

The theoretical delta-V (AV), or change in velocity, either gained or lost by a space vehicle due to a rocket impulse. The actual AV may be less than this, due to gravity, drag, and other losses. Delta-V is directly proportional to exhaust velocity (c) and logarithmically proportional to mass ratio (M/m).

Vernier engines

Small rocket engines used to fine-tune the final velocity of a launch vehicle. Viscous interaction

A phenomenon whereby a thickening boundary layer may interact with an inviscid flowfield at supersonic speeds. As high-speed heating occurs, the boundary layer expands due to increased viscosity and decreased density, causing it to interact with inviscid regions farther from the vehicle and severely increasing drag. Lift, stability, and heat transfer are also affected by viscous interaction.


Vertical landing in a horizontal attitude, also vertical lift-off in a horizontal attitude, describing Harrier-like operations for ultra-advanced spaceplanes. This flight mode operates within HTHL.


Vertical takeoff horizontal landing, as in the Space Shuttle.


Vertical takeoff vertical landing. Compare HTHL and VLHA. Weight

The force of gravity that pulls on a mass, different on every gravitating body in the universe.

Wing loading

The pressure borne by a wing, determined by dividing gross vehicle weight by the wing area. Airplanes experience typical wing loadings of anywhere from 15 (Cessna 172) to 100 (C-5 Galaxy) pounds per square foot.

Working mass

The fluid material that is accelerated through and exhausted from a jet or rocket engine to provide propulsion.

Von Braun, Wernher (1912-1977)

A German-American rocket scientist, motivated by the quest for manned spaceflight. An early member of the German Society for Spaceship Travel, he designed the V-2 rocket and later helped NASA develop its Saturn V launch vehicle for the Apollo program.

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