Appendix:Glossary of atmospheric reentry

Over the decades since the 1950s, a rich technical jargon has grown around the engineering of vehicles designed to enter planetary atmospheres. Definition of the jargon is prerequisite to meaningful discussion about and.

0-9

 * 3 Degrees of Freedom (3-DoF) trajectory simulation : a trajectory simulation based upon three spatial coordinates, e.g. X,Y,Z and their corresponding velocities. A 3-DoF simulation models an entry vehicle's center-of-mass trajectory. Total angle-of-attack and bank angle are user input parameters.


 * 6 Degree of Freedom (6-DoF) trajectory simulation : a 3-DoF simulation augmented by a model of the vehicle's orientation based upon pitch, yaw and roll angles along with Euler angles. A 6-DoF simulation includes angle-of-attack and bank angle within the model (center-of-mass location, control surface orientation and RCS are the user input parameters).  6-DoF simulation of a hypersonic entry vehicle is much more complicated than classical 6-DoF simulation of a low speed, low flying airplane, e.g. the atmosphere can not be assumed to be fixed in inertial space over a flat Earth.

A

 * aerobraking : when the free molecular gas of a planet's upper atmosphere is used to reshape the orbit of a spacecraft. Heating due to aerobraking is normally insignificant thus requiring no special thermal protection. The term aerobraking is often used incorrectly by people outside the aerospace industry.


 * aerocapture : when a planet's atmosphere is used to dissipate the kinetic energy of a spacecraft entering from a heliocentric orbit and which then skips out of the atmosphere into an elliptical orbit around said planet. Aerocapture helps enable . However protecting against heat soak resulting from aerocapture is technically challenging for aerocapture to a gas giant planet such as Neptune. Also, contrary to science fiction, it is more energy efficient to rocket brake into a Jupiter-centered orbit rather than aerocapture (this is an unusual case and due to Jupiter's very strong gravity). Manned aerocapture into an Earth centered orbit may be impractical due to increased radiation exposure from the van Allen belts. A well posed aerocapture entry state has two possible trajectory solutions to one capture orbit apoapsis (maximum orbital altitude):
 * Overshoot trajectory : A "lift-down" trajectory (the bank angle points the lift vector towards the ground).
 * Undershoot trajectory : A "lift-up" trajectory (the bank angle points the lift vector towards the zenith).


 * aerodynamic roll angle : An angle around the entry vehicle's principal axis and between the plane containing the lift vector and a body axis orthogonal to the principal axis. Aerodynamic roll angle is used with total angle-of-attack and typically used with entry vehicles and missiles.


 * aeroshell : The outer structure of an entry vehicle, RV, or RB that defines its aerodynamics.


 * angle of attack : The angle between an entry vehicle's principal axis (axis-of-symmetry) and the free stream velocity vector. There are two forms of angle-of-attack.  The more common form used with airplanes restricts angle-of-attack to a plane defined by the vehicle's principal axis and lift vector (pitch plane). Angle-of-attack is often called alpha.


 * atmospheric entry : the transition from the vacuum of space to the atmosphere of any planet or other celestial body. The term is not used for landing on bodies which have no atmosphere such as the Moon.


 * atmospheric re-entry : the return to an atmosphere previously left for space. Often the word atmospheric is dropped and the term re-entry (or reentry) is taken to mean atmospheric re-entry in context.


 * augered-in : after EDL (Entry, Decent and Landing) or TPS (Thermal Protection System) failure, the entry vehicle is said to have augered-in. This expression for undesired high speed surface impact dates back to World War II fighter pilots and was also used by test pilots at Edwards Air Force Base, the most well known being  Brigadier General Chuck Yeager.

B

 * ballistic coefficient : For an entry vehicle, the ballistic coefficient is the entry mass divided by the product of its aerodynamic area times its drag coefficient. This definition is typically used by NASA.  For an RV or RB, the ballistic coefficient is the entry weight divided by the product of its aerodynamic area times its drag coefficient.  This definition is typically used by the USAF.  In some literature it is defined as the inverse of this, as in the product of the aerodynamic area times its drag coefficient divided by the mass of the vehicle. Ballistic coefficient is sometimes called beta or ballistic number.


 * ballistic entry : when an entry vehicle has only drag with no apparent lift. An axisymmetric entry vehicle would have no apparent lift if its angle-of-attack time averaged out to zero, e.g. sinusoid angle-of-attack centered or trimmed about zero lift.


 * bank angle : The angle around the velocity vector (not the principal axis) and between the plane containing the lift vector and the plane containing the zenith axis. Bank angle and roll angle are often confused.


 * bluntness ratio : the ratio of a sphere-cone's nose radius divided by the base radius. Most American interplanetary entry vehicles and the Mk-6 RV have a bluntness ratio of 1:2. The Mk-6 had a half-angle of 12.5°.  The Mk-6 was launched by the Titan II ICBM and carried the largest nuclear warhead of any RV, i.e. the W-53.  The Mk-6 RV was also the design ancestor of almost all American interplanetary entry vehicles, e.g. Pioneer Venus, Galileo Probe, etc.


 * bondline material : typically is RTV-560 (room-temperature vulcanizing adhesives were originally developed by the General Electric Corp. for use on RVs). A typical maximum allowed bondline temperature is 250 °C.

C

 * Conservative design : a process that begins by bracketing the set of operating parameters for an entry vehicle where mission success is possible, i.e. "What's the worst case scenario that still leads to mission success?" The entry vehicle is then designed to survive within that envelope of operating parameters.

D

 * Dynamic aerodynamic stability : occurs when a vehicle can return to its trim flight condition after being perturbed off trim (see dihedral and relaxed stability). A vehicle with static stability can still be unstable if it lacks dynamic stability.  Dynamic stability is a conseqeunce of both the vehicle's aerodynamics and inertial properties.  Dynamic stability is best determined through 6-DoF trajectory simulation based upon wind tunnel or ballistic range data.


 * Dynamic pressure : one half of the local density of the atmosphere times the atmosphere relative velocity squared. Dynamic pressure is typically referred to as q (see Max Q for more information).

E

 * Entry angle, flight-path angle or velocity angle : different names for the angle of the velocity vector to the local horizon. Entry angle is typically referred to as gamma. There are two types of gamma, i.e. gamma relative to the atmosphere and gamma relative to inertial space. If no distinction is made then assume gamma, velocity and azimuth are atmosphere relative.


 * Entry corridor : the angular range between the overshoot and undershoot angles.


 * Entry, Descent and Landing (EDL) : the process of getting an entry vehicle from orbit to a planet's surface. EDL includes parachute deployment (descent) and planet surface landing, e.g. braking rockets, air bags, etc.


 * Entry vehicle : typically a non-military vehicle travelling from the near vacuum of space into the atmosphere of a planet. An entry vehicle is used by National Aeronautics and Space Administration (NASA) and civilian space agencies for purposes of space exploration.

F

 * Fay–Riddell equation : a relatively compact closed form equation used to model the convective and catalytic heat flux at the stagnation point of an aeroshell.  The Fay-Riddell equation is remarkably accurate and sometimes used to validate modern computational fluid dynamics (CFD) solutions.


 * Free molecular gas : a gas so tenuous that it can be modelled as a collection of tiny particles. The transition from a free molecular gas to a continuum gas can be determined by its Knudsen number. An entry vehicle's trajectory is said to be Keplerian or Kepler if the atmospheric density is so tenuous that it does not significantly affect the trajectory.

H

 * Half-angle : one-half of the angle formed by the sphere-cone, or the angle from the sphere-cone's axis of symmetry to the frustum. The half-angle of a modern RV or RB is typically around 11°. The half-angle of a non-military entry vehicle is typically 45° or greater but never greater than 70°.


 * Heat pulse : the interval along the trajectory where the heat flux rises from insignificance, reaches its peak value and then descends back into insignificant. Heat pulse plotted as a function of time typically has a bell curve shape. The heat pulse for Mars Pathfinder lasted about 100 seconds.  For the Galileo Probe, the heat pulse lasted about 70 seconds.


 * Heat flux : the thermal power per unit area experienced by a TPS. The modern preferred units for heat flux are watts per square centimeter (W/cm²).  The obsolete units used by the American aerospace industry are BTU/ft²-sec.  Because 1.0 BTU/ft²-sec equals 1.13489 W/cm², the  numerical quantity is almost the same thus making W/cm² preferable to W/m² (the standard SI unit).  The total heat flux experienced by an aeroshell undergoing hypersonic entry can have up to three components:
 * Convective heat flux : simply heat convected from the hot shock layer gas to the cooler aeroshell wall.
 * Catalytic heat flux: produced by dissociated gas species in the shocklayer gas recombining into less reactive molecules on the aeroshell wall thus releasing heat.
 * Radiative heat flux : comes from the intense light radiating from the shock layer which is in a state of chemical non-equilibrium due to passing through the shock wave. As a function of time from entry, radiative heat flux normally reaches its peak value before the convective heat flux reaches its peak value (this can be used as a simple test of a heating model's validity).


 * heat load : time-integrated heat flux. The modern preferred units for heat load are joules per square centimeter (joule/cm²).


 * heat soak : the component of heat load that actually penetrates the TPS and entry vehicle structure.


 * hypersonic : when an entry vehicle, RV, or RB has a supersonic free-stream velocity that creates a shock wave processing atmospheric gas into chemical dissociation, e.g. molecular nitrogen breaks down into atomic nitrogen. The term hypersonics can also have a special meaning referring to the engineering of vehicles that cruise or glide at hypersonic velocity or employ a supersonic combustion ramjet or scramjet.

I

 * Inertial Measurement Unit (IMU): used to measure a vehicle's acceleration and orientation with respect to inertial space. The Delco Corp. (AC Spark Plug) was an early developer of ICBM IMUs.

L

 * lifting entry : occurs when an entry-vehicle has lift and drag. Both American and Russian manned entry vehicles use lifting entry to reduce deceleration loading on the crew and improve crossrange.


 * lift-over-drag ratio (L/D) : the ratio of the coefficient-of-lift divided by the coefficient-of-drag. The L/D of an entry vehicle undergoing ballistic entry is by definition zero.  An entry vehicle's designed geometry is often a consequence of an L/D requirement.  L/D is of extreme importance in designing a manned entry vehicle since the peak deceleration is a function of L/D, e.g. an Apollo-CM can not be used for a return from Mars entry because the peak deceleration is too large due to the Apollo-CM having too low of an L/D.

M

 * Mach number : a dimensionless number derived from the free stream relative velocity of an object divided by the free stream speed-of-sound of the medium it is traveling through. Speed-of-sound is proportional to the square root of the absolute temperature of the gas.  An extremely hot gas hitting an object at high velocity could have a relatively low Mach number, e.g. the gas from a commercial plasma cutter flowing at orbital velocity (7.8 km/s) could have a Mach number less than three.  An entry vehicle is probably hypersonic if its Mach number is greater than six and certainly hypersonic if the Mach number is greater than nine (being hypersonic depends upon whether the shock layer has undergone chemical dissociation).

N

 * Newtonian impact theory : a method for modelling the aerodynamics of blunt entry vehicles at Mach numbers that are normally hypersonic. Newtonian impact theory assumes the existence of a shockwave and ignores the effects of viscosity (Newtonian impact theory should not be confused with Newtonian viscosity).  Newtonian impact theory enables closed form solutions for simple aeroshell geometries and was a preferred modelling method prior to the development of computational fluid dynamics (CFD).  Newtonian impact theory is still extremely useful for the preliminary design of entry vehicles.

O

 * Outer mold line (OML) : an aeroshell's outer surface.


 * Overshoot angle : the maximum allowed entry angle for an entry-vehicle.

R

 * Reaction control system (RCS) : the system of small rocket thrusters that reorient a spacecraft with respect to the inertial frame of reference (inertial space).


 * Re-entry vehicle (RV): a munition delivered by an Intercontinental Ballistic Missile (ICBM) of the United States Air Force (USAF) or the military of another country. The photographic film return capsule (now obsolete) for a low earth orbit satellite or military reconnaissance satellite was also called an RV for Recovery Vehicle.  In the aerospace industry, the term "reentry vehicle" or "RV" normally refers to an ICBM munition.


 * Re-entry body (RB) : a munition delivered by a Submarine Launched Ballistic Missile (SLBM) of the United States Navy (USN). An RB or RV could in theory be the same device but never are (the USAF and USN never use the same design).  The container inside an RB or RV holding the military payload (thermonuclear explosive) is called a "bomb can" or a "physics package".

S

 * Shock layer : the gas layer processed by the hypersonic shock wave located between the shock wave and aeroshell.


 * Side-slip angle : used with the common form of angle-of-attack (used with airplanes). While pitch roll and yaw are relative to the horizon, angle of attack and side-slip angle are relative to the local wind direction or the velocity vector. Side slip angle is in the plane orthogonal to the pitch plane, but not necessarily in the yaw plane. Side slip angle is often called beta.


 * Skip reentry : aerocapture to a suborbital ellipse having an apoapsis that is just outside of the atmosphere.


 * Spallation : if spallation occurs, TPS failure can be exacerbated. Spallation is where chunks of TPS material are torn away from the outer wall of the TPS.  This can happen if the maximum allowed dynamic pressure is exceeded.


 * Sphere-cone : a conical aeroshell with a spherical nose. The outer surfaces are tangential along the line of contact (ring) connecting the spherical nose to the cone or frustum.


 * Stagnation point : a region of stagnant flow on the leading edge of an aeroshell (there is also at least one stagnation point in the vehicle's wake). For a sphere-cone undergoing ballistic entry, the main stagnation point is on the outer surface of the spherical nose that intersects with the sphere-cone's axis-of-symmetry.  For an aeroshell undergoing hypersonic heating due to a convection dominated heat flux from a laminar flow, the stagnation point is almost always the hottest point on the entry vehicle's outer wall.  Therefore, conservative design for a TPS is normally based upon conditions at the stagnation point. However, for a radiation dominated heat flux or a turbulent flow, the stagnation point might not be the hottest point on the entry vehicle (this situation can occur during high speed return from the Moon or Mars and is difficult to model)


 * Static aerodynamic stability : occurs when the center-of-mass (also called center-of-gravity) is upstream from its aerodynamic center.
 * Subsonic cap : the region of high temperature gas near the stagnation point where the local flow is subsonic.

T

 * Thermal Protection System (TPS) : the atmospheric entry system or material used to protect an aeroshell's payload from heating due to hypersonic entry into the atmosphere. The outer layer of material on the aeroshell is also called TPS, TPS material or TPS layer.  A famous TPS material is Teflon which was originally developed by the DuPont Corp. and used on RVs.


 * Total angle-of-attack : is the less common form of angle-of-attack typically used with entry vehicles and missiles and not restricted to the pitch plane. Total angle-of-attack is sometimes called resultant angle-of-attack.


 * TPS failure : occurs when the temperature of the material bonding the TPS layer to the aeroshell's structure exceeds the maximum allowed for the bonding material.


 * TPS (Monolithic) : a TPS is "monolithic" if it is made from a single slab of material, e.g. silica foam or balsa wood saturated with resin. An alternative to a monolithic TPS is a TPS based upon tiles such as used by the Space Shuttle or the Huygens probe. A TPS method sometimes called "monolithic" is a honeycomb of composite material with each cell injected with an ablating TPS material made of resin mixed with cork dust and other substances (this arguably represents a misuse of the word "monolithic").  Honeycomb based TPS is normally very expensive to manufacture and can have voids in individual cells (this possibility requires expensive inspection methods).


 * Trim angle-of-attack : is an angle-of-attack where an aircraft or entry vehicle's pitching moment is equal to zero, e.g. a pilot could let go of the control stick and the aircraft's attitude would remain unchanged.

U

 * Undershoot angle : the minimum allowed entry angle for an entry-vehicle