Flying the Shuttle - Entry

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Note  This article refers to the SpaceShuttle in the FGAddon repository.

This mission phase can directly be started using --aircraft=SpaceShuttle-entry on the command line.

What are we trying to do?

The entry phase of a Shuttle mission lasts from the so-called entry interface at about 400.000 ft to the terminal area energy management (TAEM) interface at 85.000 ft. During that time, the Shuttle decelerates from orbital speed (Mach 26) to supersonic speed (Mach 2.5) and changes from a ballistic flight trajectory to an aerodynamical glide. The deceleration is provided by atmospheric friction which converts the tremendous energy of the propellant burned during launch and used to reach orbit into heat. The goals during entry are hence (in decreasing order of importance):

  • manage the heat load such as to prevent destruction of the orbiter
  • manage the deceleration rate such as to prevent destruction of the orbiter and TAEM interface is reached close to the landing site
  • manage the ground track such that it points towards the landing site

Some theory

The total energy in the friction heat is sufficient to destroy the orbiter (as the Columbia disaster has demonstrated). It is therefore mandatory that most of the energy never reaches the orbiter. This is accomplished by a blunt-body entry - for a streamlined hypersonic body, the shockwave of compressed air is attached to the surface, but for a blunt body it detatches, containing most of the energy, and only a small fraction of a few percent are radiated to the actual spacecraft structure. This remaining heat flow still leads to temperatures close to 3000 F at the nose and leading wing edges, but can be managed by the thermal protection system.

It is hence mandatory that the airstream during an entry never reaches the weakly protected upper fuselage and that the shockwave remains always detatched from the orbiter.

This is accomplished by maintaining a high AoA, i.e. during the hot entry phase, the Shuttle is flown in what is technically close to a stall condition - pitched up at 40 degrees.

Pitch up attitude of the Space Shuttle on entry


As the lift/drag curves of the Shuttle show, at hypersonic speeds, even in that regime, the glide ratio is still about 0.8, so the shuttle will produce lift even at such high AoA.

Lift/Drag of the Space Shuttle


The density of the ambient atmosphere largely sets the deceleration force and thermal load, and the altitude determines the atmosphere density, hence to manage the deceleration force and thermal load, the vertical speed needs to be controlled. As soon as the transition to aerodynamical flight is made, aerodynamical lift provides the means to do so.

As lift builds up, the descent of the Shuttle will slow down, come to a halt and, if no further action is taken, will turn into a rise, i.e. the orbiter is able to 'bounce' off the atmosphere. This is different from the entry trajectory of a non-lifting body (such as the Vostok-1 spacecraft modeled in FG) which undergoes a so-called ballistic entry, i.e. it will penetrate the atmosphere till drag eventually stops it. Ballistic entries consequently are much harder both in peak g and thermal loads. The sinkrate when reaching the upper atmosphere essentially determines how deeply the Shuttle will penetrate at near-orbital velocity and how hard the deceleration will be. On a normal de-orbit, sinkrates are well within acceptable limits, but entry trajectories for scenarios like a three engine failure during launch may not be survivable.

Once the initial vertical descent comes to a halt, the vertical velocity can be actively controlled to manage heat and deceleration force - if the shuttle is steered upward, heat load and deceleration decrease, if downward heat load and deceleration increase. In a normal airplane, one would use the elevator to control AoA and simply steer down. In the Shuttle, this is not an option, because, as explained above, the AoA is fixed by the need to have thermal protection. The solution is to roll the orbiter to a high bank angle (up to 70 degrees) to reduce lift.

High bank roll of the Space Shuttle on entry

The combination of 40 degree upward pitch and 70 degree roll is something not usually experienced by pilots. In such a confguration, the lift now acts sideways, i.e. the orbiter changes course. This may be desirable to steer the trajectory towards a landing site, or it may not. In the second case, the roll needs to be reversed periodically ('roll reversal') to steer the ground track into an S-shape around the desired trajectory.

Using a combination of high bank angle, low bank angle and roll reversals, direction and deceleration rate are managed to steer the orbiter to the landing site.

High bank roll of the Space Shuttle on entry

Note that the heat load is proportional to the dynamical pressure qbar times the velocity relative to the air, whereas the structural load is proportional to qbar. Thermal management is thus most important at the initial high-velocity phase in which the deceleration force is modest, and only when thermal management is over, trajectory deceleration control becomes important. For the same reason, the pitch angle can be gradually reduced with Mach number, ending in a just 14 degree pitch at Mach 2.5 where the TAEM interface is reached.

Since aerodynamical forces push the shuttle into a low AoA configuration, a high pitch angle, once lost, is not easily recoverable. It is best established outside the atmosphere where qbar is low and kept during entry, only to be relaxed in the final phase.

How it feels in FG

Entry preparations begin in orbit - it is important to work through the entry preparation checklists, in particular payload bay door and ET umbilical doors need to be closed or the Shuttle will have incomplete thermal protection and burn up. APUs need to be running to provide hydraulic power for the aerodynamical control surfaces.

Another important aspect is CoG management - during entry, the CoG should ideally be at 66% of the vehicle length - check this in the propellant dialog. There is about a 1 % margin around this value in which the orbiter gets progressively more difficult to manage, but for instance an entry with full OMS tanks is not survivable, the elevons can not hold the Shuttle in such a tail-heavy configuration.

Forward RCS fuel should be dumped for safety reasons, the forward RCS will not be used any more (usually it is burned up by firing left and right-firing thrusters at the same time - Control+f toggles this). Excess OMS fuel should be burned by orienting the Shuttle perpendicular to the motion vector (i.e. a normal or anti-normal burn) which will change the entry trajectory very little. In the unlikely case that there is excess aft RCS fuel, this can also be burned by a normal burn in translational mode.

If there is insufficient RCS fuel in one or both of the rear pods, pre-entry preparations are a good time to set up RCS to RCS or OMS to RCS crossfeeding.

The next thing is to activate the guidance computer. The details of what is supported depend on the version of the Shuttle:

De-orbit and entry planning in the Shuttle devel version

For a real Shuttle mission, mission control would take care of trajectory planning, and the Shuttle crew would just execute the plan. In FG, this option is not available and you have to do the planning yourself. As of September 2015, several planning and guidance instruments have been added to give the Shuttle the required capability.

A viable entry trajectory based on maintaining 70 m/s sink throughout the entry is part of the vertical trajectory guidance. This trajectory has a length of 4100 miles and it is possible to deviate both into the direction of a shorter and of a longer trajectory by changing sink.

This means that ideally the entry interface (EI) needs to be 4100 miles from landing site. This can be accomplished by using the range lines of the entry guidance computer.

The entry interface location is likewise shown as soon as the orbit intersects the lower atmosphere. Thus, in order to manage the de-orbit, make sure the projected groundtrack is close to the site, then start the de-orbit burn some 10.000 to 12.000 miles ahead. As soon as the EI is shown, burn as long as it moves across the range line and you'll have an entry trajectory with a specified length.

However, what is crucially important is also the perigee. If it is too high (> 70 km) atmospheric capture will be slow or might not happen at all - this will lead to a very long trajectory. If it is too low (< 20 km), sink into the lower atmosphere will be rapid, reducing range quite a bit and making the trajectory difficult to fly (the Shuttle will however take a lot of punishment, which is required for the abort trajectories specifically).

Here's an example of a good de-orbit burn stared from a 320 km orbit 10.000 miles to range to a perigee of 40 km.

De-orbit planning for the Space Shuttle

Pitch up and wait for the atmosphere to grab the shuttle. This will be felt initially by a very slow drift of the attitude, trying to reduce pitch. Apply thrusters to keep the nose up. The thrust level needed to hold the 40 degrees will increase with increasing qbar, and eventually the controls will revert to aerodynamical surfaces for roll (qbar = 10 psf) and pitch (qbar = 40 psf). Using the RCS ROT ENTRY DAP steering characteristics changes quite drastically - initially it is probably easier to make minute thruster adjustments with the keyboard, in the later phase a stick or mouse is a much better option. With the Aerojet DAP, the change will hardly be felt as the rate controller logic adapts automatically to all changes in qbar and Mach number.

Once the Shuttle is under aerodynamical control, watch descent rate slow and reverse. Once the rate comes back up to about -50 m/s, you can initiate the first careful high-bank roll. Do it gently in order not to lose the AoA if you're flying by hand! If you're maneuvering with automatic pitch axis control, aim for about 6 deg/s roll rate. Watch the response of nose cone temperature and acceleration and the slow drift in course. The descent rate will fall again, don't let it fall too fast, or you'll get too hot. Allow for some lag, get a feeling for how the trajectory responds to what you're doing. This is actually piloting, and you can influence a lot of what is happening here. With the Aerojet DAP flight characteristics are very stable and the vehicle holds attitude and AoA automatically, which means you can use roll to control sinkrate very precisely - flying by hand, that may not work so well. Once deviation to target azimuth exceeds 10 degrees, do a roll reversal.

In the devel version, you can open the vertical trajectory planner (lower left). This is a plot of range (x-axis) vs. speed (y-axis) and gives you an instant feedback how you are doing with regard to the reference trajectory. If you're above the line, you need to slow down, i.e. aim for a higher sinkrate, if you're below the trajectory, you need to reduce drag, i.e. reduce sinkrate. If you stay on the trajectory, you should be delivered to TAEM interface 60 miles to site.

De-orbit planning for the Space Shuttle

Monitor Mach number and altitude decrease, reduce pitch angle later in the flight as commanded by the guidance computer. Around Mach 3.5, you should finally get the rudder back with RCS jets switched completely off, at which point the Shuttle definitely feels like an aircraft. It can now actually change course and turn, although still sluggishly. Steer the course towards the landing site if you're close. Aim for TAEM interface of 85.000 ft, Mach 2.5, around 60 miles before the runway. Don't try to brake too fast, as the manual has it:

It is better to arrive at TAEM interface with too much energy than without wings.

(NASA has a sense of humor...)

Going home - a tutorial

(WIP)


Entry tutorial 1
Entry tutorial 2
Entry tutorial 3
Entry tutorial 4
Entry tutorial 5
Entry tutorial 6
Entry tutorial 7
Entry tutorial 8

Further reading

NASA human space flight page on Shuttle entry

Wikipedia article on atmospheric entry