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→RCS DAP schemes
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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. | 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. | ||
The Shuttle has four different control pushbuttons (implemented in the menu) to control the basic way the orbital DAP works. These are AUTO, INRTL, LVLH and FREE. | |||
If AUTO is selected, the RCS is controlled by the on-board flight software (specifically either the pointing and tracking routines available on the UNIV PTG display or the automatic burn attitude maneuvering routines available on the MNVR display). In this mode, stick control input is not used. Note that if an automatic maneuver program is selected, the controls need to be switched to AUTO prior to the start of the program. If this is not done, a SEL AUTO warning message is created. | |||
In INRTL (inertial), the stick controls roll rates and the Shuttle holds inertial altitude for stick to neutral. The orbiting Shuttle in this mode thus has an apparent slow attitude drift with respect to the horizon. | |||
In contrast, LVLH (local vertical, local horizon) commands an attitude hold with respect to the local horizon, i.e. the Shuttle appears not to change attitude relative to Earth. Again in this scheme, the stick controls rates. | |||
The following DAPs are available for INRTL and LVLH: | |||
; RCS DAP-A | ; RCS DAP-A | ||
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; RCS DAP-B | ; RCS DAP-B | ||
: As DAP-A, but more permissive in terms of deadbands, trades less strictly stabilized attitude against reduced propellant consumption. | : As DAP-A, but more permissive in terms of deadbands, trades less strictly stabilized attitude against reduced propellant consumption. | ||
; RCS DAP-A VERNIER | |||
: A 'stick controls rate' scheme in which the Vernier thrusters are used to maneuver the Shuttle. The Verniers are not very powerful and moreover fire in an awkward geometry, so there is significant mode mixing into translations when using them and the response of the Shuttle is very slow - the mode should mainly be used for automatic attitude hold as it is very propellant-friendly. | |||
; RCS TRANS ATT HLD | |||
: A translational DAP in which 'attitude hold' is commanded for all rotation channels. This makes this mode very stable and controllable at the expense of an increased propellant consumption - use e.g. for a precision approach to a docking. | |||
; RCS TRANS LOW-Z ATT HLD | |||
: No upward-firing thrusters are used in this mode to avoid plume impingement on a satellite or docking target. For this reason, forward and backward firing jets are used simultaneously which are both angled slightly upward. For -Z-translations, this causes a 12 times higher fuel consumption. For weak thrust attitude control works well, for strong thrust the controller is, without using upward-pointing thrusters, unable to completely control the pitching motion. | |||
Finally, FREE puts the orbiter into free drift. Stick to neutral then commands all RCS jets off, and stick movements control angular acceleration. The following DAPs are available for this control: | |||
; 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 ROT TAIL ONLY | ; RCS ROT TAIL ONLY | ||
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; RCS ROT NOSE ONLY | ; 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 very limited roll control (the roll moment only comes from the position difference between left-mounted and right-mounted upward and downward firing thrusters) | : A 'stick controls thrust' scheme in which the OMS pod modules are not used. This causes significant mode mixing and has very limited roll control (the roll moment only comes from the position difference between left-mounted and right-mounted upward and downward firing thrusters) | ||
; RCS translation | ; RCS translation | ||
: A translational DAP in which the stick controls translational thrust along the spacecraft x, y and z axes. Stick to idle commands no thrust, but the Shuttle will of course retain its relative velocity to a fix point until counter-thrust is used. RCS translation can be used for emergency de-orbit burns if the OMS is not available. Limited compensation is done for cross-coupling to rotational modes. | : A translational DAP in which the stick controls translational thrust along the spacecraft x, y and z axes. Stick to idle commands no thrust, but the Shuttle will of course retain its relative velocity to a fix point until counter-thrust is used. RCS translation can be used for emergency de-orbit burns if the OMS is not available. Limited compensation is done for cross-coupling to rotational modes. | ||
; RCS TRANS LOW-Z | ; RCS TRANS LOW-Z | ||
: To prevent thruster plume impingement on a docking target, say the ISS, in this mode all upward-firing thrusters are inhibited. To provide the deceleration force for a docking (which is needed in -Z direction), foreward and backward firing thrusters are used simultaneously - since they point about 10 degrees upward, this provides a downward acceleration without upward plume at the expense of 12 times higher than normal propellant consumption. There is strong cross-coupling to a pitching motion. | : To prevent thruster plume impingement on a docking target, say the ISS, in this mode all upward-firing thrusters are inhibited. To provide the deceleration force for a docking (which is needed in -Z direction), foreward and backward firing thrusters are used simultaneously - since they point about 10 degrees upward, this provides a downward acceleration without upward plume at the expense of 12 times higher than normal propellant consumption. There is strong cross-coupling to a pitching motion. | ||
The following DAPs are available for re-entry (OPS 304): | |||
; RCS 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! During entry, the DAP will gradually transfer control to the 'Aerodynamical' DAP - at qbar of 10 lb/sqft the roll axis, at 40 lb/sqft the pitch axis and at around Mach 3.5 the yaw axis. | |||
; Aerojet | |||
: The Aerojet DAP is close to the real entry DAP used by the Shuttle. Its RCS part works similar to RCS ROT ENTRY, but control is not transferred to to the Aerodynamical DAP but to the atmosphere part of Aerojet (see below) which employs the same rate control routines as the RCS part. The scheme also supports an automatic AoA control scheme in which the pilot only has to manage the roll axis during entry, which makes this the most easy to fly DAP for entry and atmospheric flight. | |||
For precision control, the keyboard is a more suitable input device than a joystick or a mouse since exact nulling of rates is somewhat easier with keystrokes. | For precision control, the keyboard is a more suitable input device than a joystick or a mouse since exact nulling of rates is somewhat easier with keystrokes. | ||
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|Cut thrust | |Cut thrust | ||
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=== Spacewalk === | === Spacewalk === | ||