Introduction to Hypersonic Flight

During much of the early reentry period the space shuttle travels at ¹hypersonic speed which, for the purposes of calculating heat transfer and thermodynamic properties, is distinctly different from ²supersonic and ²subsonic speeds.  Hypersonic flight differs from super sonic flight in that it has a separate distinct region between the shock wave and the ³boundary layer known as the ⁴shock layer.  In the shock layer kinetic energy is turned into heat so temperature, pressure and or density may change by a factor of 2 or more.  This happens when air molecules pass through the shock wave and are excited to higher vibrational and chemical energy modes.  As more energy is absorbed by the air molecules the nitrogen and oxygen molecules begin to disassociate creating ionized particles or forming an ⁵ionized plasma.  The major sources of increased heating during supersonic and hypersonic flight are skin and fluid friction as the atmospheric gasses pass over the surface of the aircraft and compression of the gas molecules as they pass through the boundary of the shock wave.

Sonic Shock Waves and Boundary Layer Flow

Shock wave formation:

Fig. C1 represents the regions that form around the space shuttle when it is in hypersonic flight.  Although properties of the gas  may change radically from one side of the shock wave to the other, the gasses must still obey thermodynamic laws and specifically equilibrium by maintaining the energy budget across the shock wave and boundary layer.

Fig. C1

	6Example  Eqn. C1
	If it is assumed that the gas is   thermally perfect, the following equation can be used,

It should also be noted the many things about super sonic shock waves are very predictable and can easily be calculated.  Many properties of the fluid flow on either side of the shock wave can be calculated accurately, see Eqn. C1, assuming that you know the current state of the atmosphere at the altitude the calculations are being done for.  The angle that the shock wave comes off the skin of the object that is creating it can also be calculated.  The basic parameters needed for the calculation are the Mach number, the aerodynamic properties of the atmosphere and the basic shape of the objects leading edge.

Laminar flow:

By definition the gasses in the boundary layer are ⁷incompressible and move over the body of the orbiter with ⁸laminar flow.  (This is opposed to turbulent flow which results in excessive heating during shuttle reentry).  The laminar flow helps to keep the superheated gasses from coming into prolonged direct contact with the thermal tiles on the windward surface of the shuttle.  In this way the boundary layer separates the wall of the shuttle from the super heated ionized plasma and actually works to protect the vehicle from the effects of reentry heating.

Turbulent flow:
At a point during reentry, usually around Mach 8, the laminar flow trips to turbulent which dramatically increases the heating of the orbiters skin and tiles.  This is why the Time Vs. Temperature chart, Fig. C2, for reentry shows a violent upswing in temperature at the 1300 sec. mark while the heating appears to be decreasing.  Turbulent flow may occur sooner during reentry primarily if the flow is tripped artificially by the edge of a surface on the orbiter, a rough wing surface or by meteorological conditions.  When this happens, as it did on STS-73, (cause unknown),  it increases the heat of reentry tremendously and typically causes damage to the TPS that requires greater than usual effort to repair before the next flight.  Turbulent flow caused by a defect on the surface of the orbiter will most likely be confined to a small area aft of the surface anomaly that tripped the flow.  Excessive turbulent flow has never placed an orbiter or crew in danger during reentry.

Fig. C2

Missing tiles and turbulent flow:

Fig. C3 is a good representation of how fluid in the boundary layer flows over gaps in the TPS without letting super heated plasma get near the skin of the space shuttle.  This is not to say that missing tiles are not a reason for concern.  Missing tiles hurt the shuttle's performance and increase the probability of having a burn through of the outer skin.  Obviously a very small number of missing tiles has almost no effect but as the number of lost tiles increases so do the shuttle's problems.  This is why the reasons for the loss of the tiles must always be determined.  The following animation is only meant to show that a small number of missing tiles spread out over a large area will not cause the loss of a shuttle.

Fig. C3

Because the boundary layer flow is both laminar and incompressible it creates a natural barrier between the exposed skin of the orbiter and the ionized plasma in the shock layer.  This means that any gaps in the TPS may be passed over with most of the fluid flow crossing over the gap and not entering it, although this does depends on the size of the gap.  The trailing edge of the gap will cause the flow to become turbulent just aft of the opening.  If the flow is already turbulent then more of the plasma may enter the gap and contact the skin although not as much as the typically smooth and  uniform areas around the defect.

The Wing Leading Edge and RCC Material

RCC Panels:

The RCC material on the orbiters protects the surface underneath through ablation.  Ablation means that tiny particles of carbon are coming off the material during reentry.  During every reentry the RCC material loses some mass and after so many flights must be replaced.  One issue throughout the space shuttle program has been the formation of pinholes in the material.  These holes were found to be the result of impurities within the Reinforced Carbon Carbon material.  When the holes are found during post flight inspection, the panels are repaired or replaced.  These openings have never resulted in hot plasma entering the shuttles wing.

Stagnation heating:

The point right at dead center front of the shock wave is called the stagnation point, see Fig. C5.  This is where the highest temperatures occur and they are known as stagnation temperatures and or stagnation heating.  As long as the orbiter is reentering the atmosphere with the blunt nose forward and the body at the correct Angle of Attack (AOA) the hypersonic shockwave is simply pushed along by the shuttle some distance in front of it.  This keeps the hot gasses from coming into direct contact with the shuttles skin and TPS material.  If the shuttles attitude changes so that an edge with a shape other than the blunt nose faces forward, such as a wing tip,  the shockwave may touch the shuttle subjecting it to stagnation heating and subsequent damage.  The areas of the shuttle that have stagnation points are the nose, the leading edges of the wings and the leading edge of the tail.

Plasma Formation

Angle of Attack

Angle of attack plays the most significant role in how plasma forms around the shuttle and where heat is concentrated.  The shuttle's extreme Angle of Attack of 40° is difficult to maintain but helps to slow the rate of descent more than any other method.  Fig. C7 shows how the hypersonic boundary layer affects the shuttle during two different reentry scenarios.  One is the standard Angle of Attack while the other shows what would happen without AOA control.

Fig. C7

Communications:

Another problem caused by hypersonic flight and ionized plasma was the10 minute blackout period that spacecraft traditionally went through during the early part of reentry. The ionized plasma that formed underneath a spacecraft as the result of hypersonic flight made transmitting radio communications down through the atmosphere impossible, (the communication signal simply could not pass through the electrically charged field created by the plasma), see Fig. C8 for a diagram of the areas blocked by plasma and the subsequent direction of signal transmission.  This problem was solved with the completion of the Tracking and Data Relay Satellite (TDRS) array which allowed reentering space craft to beam signals upward back into space and then relay them back down to mission control.   For sending communications the shuttle relies on four hemispherical shaped S-Band PM (Phase Modulation) antennas mounted to the body of the orbiter on the forward fuselage and located 90° apart at the upper left and upper right, lower left and lower right, Fig. C9A and C9B are photos of the actual S-Band antennas located at the four quadrants for transmitting to the TDRS system.

Fig. C8

Fig. C9A & C9B
S-Band Antennas

For the purpose of transmitting data to the ground only there are two S-Band FM (Frequency Modulation) antennas located on the forward fuselage on the top and bottom of the orbiter.  For receiving voice and data communications as well as return link or communicating back down to Mission Control there are four S-Band PM (Phase Modulation) antennas located on the forward fuselage with one at each of the four quadrants.  All of the communication systems on the orbiter have more than one redundant backup so that except in the event of a catastrophic structural failure of the orbiter there should never be a loss of communication with the shuttle.  Fig. C10 and Fig. C11 show the location of all S-Band, L-Band and Ku-Band antennas on the space shuttle.  All antennas mounted to the body of the orbiter are covered with TPS material.

Fig. C10	Fig. C11

Fig. C12 shows how the shuttle transmits back up to the TDRS system and then relays the signal back down to mission control.

Fig. C12