ablation	Thermal process where the surface melts or vaporizes at high temperature, thereby absorbing heat created aerodynamically.
ablator	Surface material that will melt or vaporize to absorb heat.
active cooling	Process whereby a heat-conductive fluid circulates between a hot and cool region, drawing off heat.
ADP	Advanced Development Projects--a Lockheed group located in California.
AF or USAF	United States Air Force.
AFB	Air Force Base.
AFFTC	Air Force Flight Test Center.
AFSC	Air Force Systems Command, an Air Force major command during the period of this narrative.
analog	In the context of this book, a computer in a
computer	simulator that solves equations of motion using analogous electrical circuits; that is, it expresses data in terms of measurable quantities, such as voltages, rather than by numbers as a digital computer does.
AOA	Angle of Attack: direction of relative wind with respect to an aircraft's longitudinal axis.
Apollo	NASA program to land a human on the moon and return to earth.
ARC	NASA Ames Research Center.
ASD	Aeronautical Systems Division (Air Force).
aspect ratio	The ratio of squared airfoil length (span) to total airfoil area or of airfoil length to its mean chord (distance from leading to trailing edge). Thus, an airfoil of high aspect ratio is relatively long with a relatively short chord, whereas one of low aspect ratio is comparatively short and stubby.
attitude	The position or orientation of an aircraft or spacecraft with relation to its axes and some reference line or plane.
ballistic	Adjective describing the path of a body launched into a path where it is subject only to the forces of gravity and drag.
Ballistic	Weight divided by the drag coefficient times the Coefficient frontal area.
bank angle	Angle between the plane of an aircraft's wings and the horizon
boat-tail	Shape of the rear of a vehicle whose cross section decreases from the center to the aft end.
C-130	Four-engine, turboprop-powered transport airplane.
capsule	A self-contained, symmetrical container capable of safely entering the earth's atmosphere from orbital or higher speeds.
CD	Drag coefficient. A non-dimensional parameter for measuring drag.
c.g.	Center of gravity--an imaginary location within an object that identifies its center of mass.
ceramic tiles	Small blocks of rigid material (primarily silica) attached to the outside of a gliding re-entry vehicle that prevent the heat generated by re-entry speeds from reaching the vehicle structure.
chase planes	Aircraft used to fly close to research airplanes for purposes of providing the research pilot with an additional set of eyes for safety purposes.
chord	The straight-line distance from the leading to the trailing edge of an airfoil such as a wing.
CL	Lift coefficient. A non-dimensional parameter for measuring lift.
CLS/W	Lift coefficient divided by wing loading. A non-dimensional parameter that allows the glide performance of several aircraft to be compared at the same airspeed.
control laws	The relationship between the pilot's commands and the actual control surface (aileron, elevon, etc.) movements produced by a flight control system.
cross range	The distance that can be achieved by a re-entry vehicle (as it enters the atmosphere) in a direction perpendicular to that of the initial entry path.
damp	To slow down.
decouple mode	An entry concept that uses a different deceleration method for entry than for landing.
delta wing	A wing that has a triangular shape when viewed from above.
digital	Adjective describing a mechanism, such as a computer, that expresses data in discrete, numerical digits.
dihedral	Effect on lifting bodies of sideslip, producing roll.
DoD	Department of Defense.
doublet	An aircraft control movement from neutral to a deflected position that is held, then returned in the opposite direction back to the original neutral position.
drag	A force that resists motion and is produced by friction with the atmosphere.
Dutch roll	A complex oscillating motion of an aircraft involving rolling, yawing, and sideslipping--so-named from the resemblance to the characteristic rhythm of an ice skater.
Dyna-Soar	Short for Dynamic Soaring. Name of a boost-glide research program that was canceled in 1963 before its first flight. The aircraft designation was X-20A.
effective dihedral	An aircraft aerodynamic characteristic that makes the airplane roll (rotate around the longitudinal axis) when a sideslip or side gust is encountered.
eyeballs-in	A descriptive term used to identify the direction of a force due to acceleration.
F-104	Air Force century series fighter built by Lockheed and used as a chase and research airplane at the Flight Research Center for many years.
FDL	The Air Force Flight Dynamics Laboratory located at the Wright-Patterson Air Force Base in Dayton, Ohio.
FDL-7	Seventh re-entry design created at the FDL.
FDL-8	Eighth re-entry design created at the FDL.
fineness ratio	The ratio of body length to body width of an aerodynamic shape.
flight cards	A type of check list in card form used by pilots and other crew members to track events in a planned flight test.
flight path	The path of a moving object, usually measured in the vertical plane relative to the horizon.

fly-by-wire	A flight control concept that uses only electrical signals between the pilot's stick and the control surfaces.
FRC	The NASA Flight Research Center located at Edwards, California. From 1954 to 1959, the designation of this organization was the NACA and then the NASA High Speed Flight Station. In 1976, it became the NASA Hugh L. Dryden Flight Research Center.
frontal area	The area of an object as projected onto a plane perpendicular to the flight direction.
gain	Sensitivity with respect to flight controls or a stability augmentation system.
HYPER III	A light-weight, unpiloted vehicle built by the NASA FRC and patterned after the FDL-7 shape.
hypersonic	Characterized by speeds of Mach 5 or greater.
jack points	Designated points marked on the underside of an aircraft wing to push upward with a hydraulic jack for the purpose of calibrating strain gages inside the wing structure.
LaRC	The NASA Langley Research Center located in Hampton, Virginia.
L/D	Lift to drag ratio.
Lift	A force on an object produced by aerodynamic reaction with the atmosphere as the object moves; it acts perpendicularly to the flight direction.
limit cycle	A run-away oscillation of an aircraft control surface that occurs when the sensitivity (gain) of the automatic stabilization system is too high.
lower flap control horn	A small mechanical arm attached to a lifting body lower flap control surface to which an actuator control rod is attached.
LOX	Liquid OXygen.
Mach number	The ratio of an object's speed to that of sound. Mach 1 is the speed of sound at a given altitude and temperature; Mach 2 is twice the speed of sound; and so forth.
Mercury	First U.S. manned space capsule program.
MLRV	Manned Lifting Re-entry Vehicle. An early NASA Langley Research Center lifting body design.
moment	A tendency to cause rotation about a point or axis, as of a control surface about its hinge.
MOU	Memorandum of Understanding--usually a simple document with signatures stating the agreed-upon responsibilities between two or more organizations.
MSL	Mean Sea Level.
NACA	National Advisory Committee for Aeronautics.
NASA	National Aeronautics and Space Administration.
neutral longitudinal stability	A flight condition in which an aircraft that is disturbed in pitch continues to rotate away from the initial angle of attack at a constant angular rate without returning.
non-receding, charring ablator	A type of ablator (see above) that maintains its external dimensions while melting or vaporizing.
nose-wheel	The point in an aircraft take-off maneuver at which rotation the pilot commands the aircraft to rotate its nose upwards, increasing lift so as to depart the ground.
notch filter	An electronic filter in an aircraft's automatic control system to remove or obstruct unwanted frequencies within a narrow band to prevent them from causing problems with the system.
on-the-street	The time when an agency advertises (in a request for proposals) that a new job or contract is planned.
overdrive	Slang term used to describe the 14 percent increase in thrust that was available on the X-24B rocket engine as compared with that used on previous lifting bodies.
PILOT	PIloted LOw speed Test. Early name for the X-24A program.
PIO	Pilot Induced Oscillation--a situation in flight in which a pilot causes an aircraft to deviate from the intended path of flight by making excessive control inputs.
pitch	Angular displacement of a vehicle such as an aircraft about the lateral axis (i.e., nose up or nose down).
plow horse	The author's term for chubby lifting bodies that are capable of carrying large payloads but have shorter hypersonic cross ranges than race horses (which see).
Pregnant Guppy	A C-97 cargo airplane modified to carry an oversized cargo.
PRIME	Precision Recovery Including Maneuvering Entry. Early designation for the SV-SD or X-23 program.
projected area	The area of an object as projected onto a horizontal plane parallel with the flight direction.
PSTS	Propulsion System Test Stand.
race horse	The author's term for streamlined, slender lifting bodies with smaller payload capacity than the plow horses (which see) but with very high hypersonic cross ranges.
radiative	A type of cooling that radiates heat away from a cooling hot surface.
ramjet	A type of jet engine without any mechanical compressor, comprised of a specially shaped, open tube into which the air necessary for combustion is forced and then compressed by the forward motion of the aircraft.
rate limited	Term indicating the maximum angular rate at which an actuator can drive an aircraft control surface.
Real Stuff	Term (derived from Tom Wolfe's The Right Stuff) for people who create and service aircraft or spacecraft for experimental flights rather than fly them.
retrofire	Short-term rocket ignition with the thrust pointed in the direction of flight so as to reduce the speed of an orbiting object and to initiate entry.
Reynolds number	A nondimensional parameter representing the ratio of momentum forces to viscous forces about a body in fluid flow, as in the atmosphere; named for English scientist Osborne Reynolds (1842-1912); among other applications the ratio is vital to the use of wind tunnels for scale-model testing, as it provides a basis for extrapolating the test data to full-sized test vehicles.
Right Stuff	A term first coined by Tom Wolfe in his book of the same title. It refers to pilots and astronauts who fly experimental aircraft or spacecraft.
Rogallo Wing	A wing-like parachute design that enables the parachuting object to move forward as well as descend.
roll	Rotational or oscillatory movement of an aircraft or similar body about its longitudinal axis.
Roll Reversal	An adverse aircraft design condition in which an aircraft rolls in the opposite direction from that commanded by the pilot or control surfaces.
Rotation Speed	The minimum speed at which a pilot can rotate the aircraft nose upward (lift the nose wheel off the runway) during a take-off roll.
RPV	Remotely Piloted Vehicle--a vehicle controlled through radio links by a pilot not physically in the vehicle.
RTD	Research and Technology Development--an Air Force Organization.
SAMSO	Space and Missile Systems Organization--an Air Force organization, part of AFSC during the period covered by this narrative.

SAS	Stability Augmentation System--electronic control components designed to augment the stability of an airplane.
second generation vehicle	A vehicle that has benefited from the previous generation design, development and testing of a similar vehicle.
self-adaptive	Adjective describing a flight control concept that samples, then alters, internal electronic signals to compensate for changing flight conditions.
semi-ballistic	Adjective describing a state in which an object is subject to a small lifting force in addition to the predominant forces of drag and gravity.
Shuttle	The winged vehicle developed by NASA and its contractors to serve as a Space Transportation System to carry cargo to and from earth orbit.
side-arm	A two- or three-axis control stick mounted on the controller side of the cockpit and operated by a pilot's wrist movements.
sideslip	A sideways movement of an aircraft away from the initial flight path.
simulator	A partial aircraft cockpit connected to an electronic computer; it allows a pilot to replicate to a significant degree the flight of an airplane.
Skunk Works	Popular term for a small, highly efficient design and fabrication organization capable of creating innovative prototype aircraft in a short period of time. The Lockheed Advanced Development Projects group was the first organization to use the term "Skunk Works" officially to describe its organization.
span	The distance from tip to tip or root to tip of an airfoil such as an airplane's wing.
spiral stability	A natural aircraft characteristic that allows the vehicle either to remain in level flight or to return thereto when upset in roll or bank angle.
Sputnik 1	The first man-made object to be placed in earth orbit (by the Soviet Union on 4 October 1957).
strain gage	An instrument used to measure the strain or distortion in a member or test specimen (such as an aircraft structural part) that is subjected to a force.
strakes	Wing-like appendages at the aft end of an aircraft that provide lift or added stability; also long, flat surfaces attached to the exterior of an aircraft's skin and aligned with the local free-stream conditions.
SV-5	Basic configuration of a re-entry vehicle that led to the SV-5P (X-24A) and SV-SD (PRIME).
SV-5J	Jet-powered version of the SV-5 configuration. Two were built but neither was flown.
swashplate	A mechanical plate with a universal joint giving it freedom to pivot in any direction about one point. Multiple attachment points for control rods in the plane of the plate allowed flexibility for different controls in the M2-F1 lifting body.
test-bed	A conventional aircraft that has been equipped aircraft with some newly designed internal or external components for in-flight testing.
Thor-Delta	A two-stage rocket using a Thor 1st stage and a Delta 2nd stage.
triply	Concept of using three parallel components to redundant accomplish a single function, with automatic de-selection of any faulty component.
tufts	Short segments of yarn or string taped to an aerodynamic surface to allow airflow characteristics to be observed directly or photographed.
volumetric efficiency	The ratio of total volume to the surface area of a three-dimensional shape. A sphere has the highest volumetric efficiency of any shape.
wedge angle	The angle of the aft control surfaces relative to the flight direction. Large angles produce shuttlecock-like stability.
wetted skin	The total exposed surface area of any shape. In an area aircraft, this is all skin area exposed to the outside airstream.
wing loading	Vehicle weight divided by the area of the wing.
X-24C	A follow-on proposal to the X-24B to test advanced air-breathing propulsion.
yaw	Motion of an aircraft or similar vehicle about the vertical axis (i.e., nose left or right).
Y-plane	Lateral (left to right) axis of an aircraft or flight vehicle.

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