Space Shuttle

Space Shuttle

Space Shuttle Atlantis in orbit
Space Shuttle cockpit (April 2015)
Type	Spacecraft
Author(s)	Thorsten Renk, Richard Harrison
FDM	/JSBSim, /JSBSim, /JSBSim, /JSBSim, /JSBSim, /JSBSim, /JSBSim, /JSBSim
--aircraft=	/SpaceShuttle SpaceShuttle-orbit SpaceShuttle-launch SpaceShuttle-entry SpaceShuttle-approach SpaceShuttle-TAEM /SpaceShuttle SpaceShuttle-orbit SpaceShuttle-launch SpaceShuttle-entry SpaceShuttle-approach SpaceShuttle-TAEM /SpaceShuttle SpaceShuttle-orbit SpaceShuttle-launch SpaceShuttle-entry SpaceShuttle-approach SpaceShuttle-TAEM /SpaceShuttle SpaceShuttle-orbit SpaceShuttle-launch SpaceShuttle-entry SpaceShuttle-approach SpaceShuttle-TAEM /SpaceShuttle SpaceShuttle-orbit SpaceShuttle-launch SpaceShuttle-entry SpaceShuttle-approach SpaceShuttle-TAEM /SpaceShuttle SpaceShuttle-orbit SpaceShuttle-launch SpaceShuttle-entry SpaceShuttle-approach SpaceShuttle-TAEM /SpaceShuttle SpaceShuttle-orbit SpaceShuttle-launch SpaceShuttle-entry SpaceShuttle-approach SpaceShuttle-TAEM /SpaceShuttle SpaceShuttle-orbit SpaceShuttle-launch SpaceShuttle-entry SpaceShuttle-approach SpaceShuttle-TAEM /SpaceShuttle SpaceShuttle-orbit SpaceShuttle-launch SpaceShuttle-entry SpaceShuttle-approach SpaceShuttle-TAEM /SpaceShuttle SpaceShuttle-orbit SpaceShuttle-launch SpaceShuttle-entry SpaceShuttle-approach SpaceShuttle-TAEM /SpaceShuttle SpaceShuttle-orbit SpaceShuttle-launch SpaceShuttle-entry SpaceShuttle-approach SpaceShuttle-TAEM
Status	Beta
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The NASA Space Shuttle was the world's first operational space plane capable of reaching orbit. It was operated from 1981 to 2011 on a total of 135 missions during which two orbiters, Challenger and Columbia, were lost in accidents.

The Shuttle launch system components include the Orbiter Vehicle (OV), a pair of solid rocket boosters (SRBs) and the external tank (ET) containing the liquid hydrogen and oxygen fuel for the engines of the orbiter. Out of these, only the external tank is expendable, the SRBs splash into the sea shortly after launch and are recovered, and the orbiter itself returns to a landing site where it lands like an airplane.

The mixture of a rocket-like launch, a spacecraft-like near ballistic early atmospheric phase and an airplane like approach and landing makes the Space Shuttle a truly unique flying experience.

Project Aim

The aim of the Shuttle Project is to create a highly realistic simulation of the capabilities of the Space Shuttle in FlightGear. While most of the time the real Shuttle is under the control of automatic guidance systems, there are fallback modes to control the spacecraft manually, the so-called CSS (control stick steering) modes, and it is these modes we primarily try to implement.

In addition to the real avionics and control modes, the idea is also to provide various 'educational' modes and instruments such as to explore and appreciate certain aspects of a Shuttle mission more.

The NASA technical reports server supplies a large base of wind tunnel and in-situ performance data of both the mated launch vehicle and the orbiter, and the aerodynamics of the simulated shuttle is based on these documents. The authoritative source for procedures for trajectory management, instrumentation, limits and emergency procedures is the Space Shuttle Crew Operations Manual and currently a normal mission, i.e. ascent, orbital insertion, de-orbit, entry, terminal area energy management and landing can be flown largely 'by the book', i.e. following the real procedure for CSS. As of May 2015, this does not yet hold for emergency procedures.

Limit and failure modeling

The project contains code to simulate the various structural and aerodynamical limits as well as component failures based on sections 4 and 6 of the Space Shuttle crew manual.

The general philosophy on limit modeling is that they can be treated dependent on a user setting as 'soft', 'hard' and 'realistic'. Where applicable, warnings when the state of the orbiter is getting dangerously close to a limit are called out in addition to a recommendation how to deal with the situation. Dependent on the trajectory of the orbiter, there may or may not be sufficient time to redeem the situation.

soft

Limit violations are called out, but their violation has no consequences for aerodynamics or component failures.

hard

Any limit violation immediately ends the simulation.

realistic

In reality, components do not necessarily fail immediately if used outside their design specs. This option applies a probabilistic failure model in which the chance for a component to fail grows with the degree of limit violation. The failure may or may not be immediately visible, e.g. too much qbar upon ascent may damage the heat shield, but this may not be apparent (unless specifically checked) until the heat shield fails upon atmospheric entry.

Component failure is modeled gradually where applicable - while a tire can only blow or not blow, an airfoil or a thruster for instance may lose a certain percentage of its efficiency.

The mated launch vehicle

At liftoff, thrust for the shuttle is provided by its three main engines (SSMEs) and the two SRBs. The assembled launch configuration has a height of 184.2 ft (56.1 m) and a mass of about 4,470,000 lb or 2.030 tons (in addition to payload), over 90% of this being propellant. The main engines would at this point be incapable of lifting the launch stack.

The SRBs burn an ammonium perchlorate composite fuel with a relatively low ISP of 268 s in vacuum, supplying 2,800,000 lbf of liftoff thrust each, this is supplemented by the SSME burning liquid hydrogen/oxygen with an ISP of 455 s, supplying an additional total liftoff thrust of 1,180,000 lbf. At liftoff, the shuttle hence reaches a thrust/weight ratio over 1.6, i.e. it leaves the launch pad rapidly.

Control during ascent is provided by thrust vectoring of both the SRB and SSME nozzles. The real-world CSS scheme is a 'stick controls rates' scheme which for stick to neutral does 'attitude hold' which makes it possible to control the launch trajectory very precisely.

The Solid Rocket Boosters

Each SRB weighs about 1,300,000 lb, out of which 1,100,000 is propellant weight. The propellant of the SRBs is shaped to provide a high liftoff thrust, followed by a thrust reduction during the phase of the highest dynamical pressure (max. qbar). The actual thrust as a function of time is fairly complicated:

Thrust characteristics of the Space Shuttle Solid Rocket Boosters

The distribution is faithfully modeled in FG and the definitions to match the real thrust characteristics is taken from the JSBSim code repository

The SRBs can not be throttled, once ignited, they provide thrust as explained above. SRB ignition takes place some three seconds after main engine ignition, and once they ramp up to full thrust, the shuttle has no choice but to leave the launch pad. For thrust vectoring, SRB nozzles can be gimbaled up to 8 deg in both pitch and yaw axes, a roll moment is created by gimbaling the two SRBs in opposite directions.

As of May 2015, SRB separation happens automatically once the thrust drops below some threshold to avoid having to drag dead weight, but there is no provision to manually separate. The SRBs are pushed away from the remaining launch vehicle by separation motor burns. These (including the separation animation with still burning SRBs) are modeled in FG, however due to technical issues with the submodel code at high velocities, thrust of the separation motors in the sim is set larger than in reality to provide the same visual separation dynamics.

The SRBs are implemented as ballistic submodels, i.e. they follow a correct trajectory and ascent with the shuttle, however since (unlike the shuttle) they are not accelerating, they visually fall behind quite quickly.

The Main Engines

The three main engines (SSMEs) are used during ascent and burn propellant from the ET. They are mounted in a triangular configuration at the stern, tilted by 13 degrees with respect to the spacecraft main axis and can be gimbaled by 10.5 degrees in the pitch and by 8.5 degrees in the yaw axis. The reason for the tilted arrangement is to have a sensible CoG of the OV together with the ET during the later ascent stages. The heavy oxygen is stored forward in the ET, leading to a fairly forward CoG for the mated vehicle such that the SSMEs can be vectored through the CoG. This assembly is faithfully modeled in FG.

The engines can be throttled between 67 and 109% of rated power, this is necessary to keep the launch vehicle within structural limits during the high qbar phase in the atmosphere and later close to MECO as the propellant in the ET is almost depleted. Thrust increases during ascent as the exhaust gases do no longer have to push against an atmosphere. Both liftoff and vacuum thrust of the modeled engines are in agreement with published values.

Since the SSME's are mounted much closer to each other than the SRBs, the Shuttle loses significant yaw and roll maneuverability after SRB separation. However as the spacecraft is nearly out of the atmosphere by then, no such maneuverability reserves are actually needed.

In FG, the throttle controls all three SSMEs during ascent. Engines ignite once throttle is moved above 67%, this triggers the SRB ignition. If the throttle is moved below 67%, the engines will stop, however they will restart once throttle is moved again up as long as fuel is available in the ET.

The propellant for the SSMEs is carried in the ET. The tank has a liftoff weight of approximately 1,680,000 lb (760 tons) and a dry weight of about 66,000 lb. The ET is the only expendable component of the launch stack, it is dropped after MECO upon almost reaching orbit and then the shuttle uses the OMS to attain orbit while the tank re-enters the atmosphere half an orbit later and breaks up during entry.

External tank separation

In FG, the tank is normally separated using d. This is vetoed if the Shuttle has unsafe yaw, pitch or roll motion in which case the RCS should be used to stabilize the orbiter before ET separation. If an emergency separation needs to be performed, Control+d overrides the veto. At separation, a translational RCS burn will automatically push the shuttle away from the tank.

After separation, the ET will approximately co-orbit with the OV, i.e. unless the Shuttle ignites the OMS engines, the tank will be visible for a long time, slowly drifting off, and it is quite possible to use the Shuttle's RCS engines to do a visual inspection of the tank.

The ET seen from the Shuttle

A note on aerodynamics of the mated vehicle

With the ET and SRBs attached, the launch stack has quite different aerodynamical characteristics than the OV alone, for instance the stack is more yaw-stable than the orbiter and its pitching moment as function of alpha and rolling moment as function of beta are very different. Where such data could be obtained from wind tunnel tests with the mated stack, it has been used in the simulation.

As in reality, the simulated shuttle has an automated downward elevon deflection schedule with Mach number upon ascent to provide further load relief for the wings (with corresponding aerodynamical forces acting).

In general though, aerodynamical effects are subleading, the ascent dynamics is dominated by the thruster forces and the flight control systems have a large margin to compensate for them.

CSS DAP schemes for ascent

During ascent, the stick controls thrust vectoring for both SSMEs and SRBs. The following two DAP schemes are available:

Thrust vectoring

This is the real CSS ascent mode for the shuttle in which stick motion controls rate, stick to neutral commands an attitude hold. Internally a PID controller vectors the thrusters and uses the stick input as a bias for the error. This is a very stable scheme and can be easily used to achieve high precision in controlling ascent speed or orbital inclination.

Thrust vectoring (gimbal)

This is an educational scheme in which the stick motion directly controls the engine gimbal, i.e. the pilot needs to do the task of the PID controller himself. To make things somewhat easier, the engines are automatically vectored through the stack's CoG, i.e. outside the atmosphere stick neutral corresponds to zero moments acting on the stack. In the atmosphere, the control input hence needs to compensate for aerodynamical forces. Launch in this scheme is fairly rough and it is not possible to reach high precision, but it is possible to fly into orbit and gain a first-hand experience of the forces acting on the stack.

m switches between the ascent DAPs. Control+m switches from the ascent to the orbital DAP modes (do not use an orbital DAP for ascent control unless you know very well what you're doing).

Ascent structural and aerodynamical limits

The following structural and aerodynamical limits need to be observed during ascent:

• Dynamical pressure qbar < 819 lb/sqf (modeled)

This is a structural limit for the orbiter and mated stack, in actual operations the orbiter should be kept below 650 lb/sqf.

• Wing bending moment CBW between -0.019 and 0.019 (not modeled)

At max qbar, the wing bending moment is a function of Mach number and AoA. Since Mach number is close to 1.4 in this phase of the flight, this limit basically translates into alpha between -8 degrees and 2 degrees. This can only be achieved if the orbiter is in inverted flight.

• Translational accelerations Nx between 0 and 3.11 g (modeled), Ny between -0.18 and 0.18 g (not modeled) and Nz between -0.06 and 0.73 g (not modeled).

These are structural limits of the mated stack to acceleration rather than aerodynamical forces. Especially the Nx (acceleration along the orbiter axis, i.e. main engine thrust) is important and requires to throttle down the SSMEs towards the end of the burn time.

• Late ascent trajectory may not drop below 265.000 ft (modeled)

This is a heat load limit for the external tank insulation, if the thermal protection of the ET fails, it will explode.

The Shuttle in orbit

For maneuvering in orbit, the OV is equipped with three RCS thruster clusters and the two OMS engines. The propellant for these systems is monomethylhydrazine (MMH) oxydized with dinitrogen tetroxide, resulting in a specific impulse of 312 s. This is an hypergolic fuel combination (i.e. ignites automatically). OMS and RCS tanks have an interconnect valve, however only the RCS can be fired from the OMS propellant reserves, not vice versa (currently not modeled).

The OMS engines are located at the rear of the spacecraft in pods attached to the fuselage. Two of the RCS clusters are attached to the OMS pods, one is located at the spacecraft nose.

The Orbital Maneuvering System engines

The two OMS engines provide a thrust of 6,000 lb and, using the propellant reserves of 7,773 lb of nitrogen tetrozide and 4,718 lb of MMH can induce a total velocity change of about 1000 ft/sec if all propellant is spent. Typically half of this is used to push the OV into a proper orbit after ET separation and for the de-orbit burn, the rest is available for orbital maneuvers such as inclination adjustments.

Once in orbit, in FG throttle control is transferred to both OMS engines. They can be throttled from zero to 100% of nominal thrust and are automatically vectored by the flight controls through the CoG of the orbiter. The real shuttle has a DAP for thrust vectoring of the OMS engines as well as the option of using a single engine with partial thrust vectoring, neither option is currently modeled.

OMS burn for orbital insertion

The Reaction Control System

The RCS system consists of three modules, one forward at the nose and two at the OMS pods. The forward module contains 14 primary and 2 secondary thrusters, each aft module carries 12 primary and two secondary thrusters. Propellant reserves in each module are 1,477 lb of oxidizer and 928 lb of MMH. Each primary thruster has 870 lb of thrust with an ISP of 289 s, the secondary vernier thrusters produce a mere 24 lb each with an ISP of 228 s. Due to geometric constraints, the thrusters are not aligned with the main spacecraft axes or in the same plane (for instance, there is no purely downward firing nose thruster, as its nozzle would have to fire through the heat shield).

As of May 2015, this assembly has been simplified in FG as follows: Thrusters are located at their correct position (nose and aft pods), resulting in approximately correct yaw, pitch and roll moments when fired. Each basic direction in which a thruster can fire from a module is modeled as a single effective thruster (for instance, no thruster can fire backward through the shuttle from the nose module, pod thrusters can't fire towards each other) with the combined thrust of all real thrusters at the module into that direction, resulting in a total of 13 individually controlled RCS thrusters and RCS-generated control moments very close to the real assembly.

RCS DAP schemes

The real Space Shuttle has a multitude of (partially mission-specific) DAP schemes, each with different gains and deadbands, which control the thruster firing pattern in response to the controllers. A fair selection of these is implemented in FG. In the real Shuttle cockpit, there is both a rotational hand controller (RHC) and a translational hand controller (THC) to initiate either rotations of the shuttle or translational accelerations (e.g. for approach and docking). In FG, m corresponds to switching from THC to RHC and back, ⇧ Shift+m switches between the different DAPs and Control+m is the override switch to aerodynamical controls. The HUD will display the currently selected mode for clarity.

Due to the geometry of the thruster arrangement, there is significant mode mixing. For instance, a lateral translation firing nose and right pod thruster with the same thrust would also induce a yaw motion (since the modules do not have the same distance to the CoG) and a roll (since they are not in the CoG plane and in fact not even in the same plane). In most implemented modes, the FCS logic takes care of most of these effects by firing additional thruster to cancel the unwanted motion, however in some modes this is not easily possible and mode mixing has to be anticipated and accounted for manually. This is in fact the same as in the real Shuttle.

RCS rotation

This is a simple scheme in which the stick motion controls thrust, i.e. angular acceleration. Stick to neutral commands no thrust, i.e. the Shuttle will continue its current rotation.

RCS DAP-A

A precision 'stick controls rate' scheme in which stick to neutral commands an attitude hold. The mode has fairly strict deadbands and steep gains and hence uses comparatively much propellant to stabilize attitude.

RCS DAP-B

As DAP-A, but more permissive in terms of deadbands, trades less strictly stabilized attitude against reduced propellant consumption.

RCS ROT TAIL ONLY

A 'stick controls thrust' scheme in which the nose module is not used. This causes significant mode mixing.

RCS ROT NOSE ONLY

A 'stick controls thrust' scheme in which the OMS pod modules are not used. This causes significant mode mixing and has no roll control available (it would have some very limited roll control in reality).

RS ROT ENTRY

A 'stick controls rates' DAP designed for entering the atmosphere which enforces a 'no sideslip' attitude in which the nose module is not used. This has very strict deadbands and aggressive gains to combat the yaw instability of the Shuttle upon entry, significant mode mixing and is very propellant-consuming. Do not use in orbit and only activate at the entry interface once the shuttle has the correct attitude!

Glossary of acronyms

AoA	Angle of Attack
CoG	Center of Gravity
CSS	Control stick steering
DAP	Digital autopilot
ET	External tank
ISP	Specific impulse
MECO	Main Engine Cutoff
MMH	monomethylhydrazine (a propellant)
OV	Orbiter vehicle
OMS	Orbital Maneuvering System
RCS	Reaction Control System
RHC	Rotational Hand Controller
SRB	Solid rocket booster
SSME	Space Shuttle main engine
TAEM	Terminal Area Energy Management
THC	Translational Hand Controller

Latest development snapshot

The latest development version (possibly unstable) is found in a dedicated repository on SourceForge. You can download the latest snapshot from http://sourceforge.net/p/fgspaceshuttledev/code/ci/development/tarball. Stable updates are pushed to FGAddon periodically.

Gallery

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  Shuttle Launch

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  Shuttle Launch

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  Shuttle Launch

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  Launch smoke trail

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  SRB separation

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  Accelerating to orbital speed

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  Shuttle 3d cockpit

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  External tank separation

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  A view of Earth after reaching orbit

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  The ET seen from the Shuttle

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  The orbiter high over Africa

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  Orbital insertion burn at sunset

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  On final approach into Vandenberg

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  Pre-flare

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  Flare

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  Touchdown in Vandenberg