Space Shuttle: Difference between revisions

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.

The [http://ntrs.nasa.gov  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 authoritive source for procedures for trajectory management, instrumentation, limits and energency procedures is the [https://www.nasa.gov/centers/johnson/pdf/390651main_shuttle_crew_operations_manual.pdf 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.

 

<b>soft:</b> Limit violations are called out, but their violation has no consequences for aerodynamics or component failures.

<b>hard:</b> Any limit violation immediately ends the simulation.

<b>realistic:</b> 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.

[[File:SRB thrust.png|400px|Thrust characteristics of the Space Shuttle Solid Rocket Boosters]]

[[File:Shuttle-ETsep01.jpg|600px| External tank separation]]

In FG, the tank is normally separated using {{Key press|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, {{Key press|Control}} + {{Key press|d}} overrides the veto. At separation, a translational RCS burn will automatically push the shuttle away from the tank.

[[File:Shuttle orbit02.jpg|600px| The ET seen from the Shuttle]]

<b>Thrust vectoring:</b> 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.

<b>Thrust vectoring (gimbal):</b> 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.

{{Key press|m}} switches between the ascent DAPs. {{Key press|Control}} + {{Key press|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).

* dynamical pressure qbar < 819 lb/sqf (modeled)

* wing bending moment CBW between -0.019 and 0.019 (not modeled)

* 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).

* late ascent trajectory may not drop below 265.000 ft (modeled)

[[File:Shuttle orbit04.jpg|600px| OMS burn for orbital insertion]]

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, {{Key press|m}} corresponds to switching from THC to RHC and back, {{Key press|Shift}} + {{Key press|m}} switches between the different DAPs and {{Key press|Control}} + {{Key press|m}} is the override switch to aerodynamical controls. The HUD will display the currently selected mode for clarity.

<b>RCS rotation:</b> 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.

<b>RCS DAP-A:</b> A precision 'stick controlls 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.

<b>RCS DAP-B:</b> As DAP-A, but more permissive in terms of deadbands, trades less strictly stabilized attitude against reduced propellant consumption.

<b>RCS ROT TAIL ONLY:</b> A 'stick controls thrust' scheme in which the nose module is not used. This causes significant mode mixing.

<b>RCS ROT NOSE ONLY:</b> 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).

<b>RS ROT ENTRY:</b> 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 agressive 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!