America's Space Shuttle: Reaction Control System NASA Astronaut Training Manual (RCS 2102A)
National Aeronautics and Space Administration (NASA), World Spaceflight News
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Space Shuttle Reaction Control System Training Manual
RCS 2102A
Mission Operations Directorate
Space Flight Training Division
Current as of January 1995 * Originally published November 1992
National Aeronautics and Space Administration * Lyndon B. Johnson Space Center * Houston, Texas
Section 1: Introduction
The reaction control system (RCS) is located in three separate modules in the vehicle: forward, left, and right. The forward module is contained in the nose area, forward of the cockpit windows. The left and right modules are collocated with the orbital maneuvering system (OMS) in the left and right OMS pods, near the tail of the vehicle.
The RCS provides propulsive forces from a collection of jet thrusters in order to control the motion of the Space Shuttle vehicle. Each jet is permanently fixed to fire in a particular direction, either up, down, left, right, forward, or aft. The selective firing of individual jets (or specific combinations of jets) will provide the Orbiter with rotational movement (about an axis) or translational movement (along an axis). Refer to figure 1-2 for jet fire directions.
Rotational movement is primarily utilized for attitude control. RCS attitude control is used during orbital conditions, except during OMS burns when OMS gimballing provides attitude control. (The RCS can assist the OMS with attitude control if OMS gimballing is inadequate due to failures). During ascent, the RCS can assist the main propulsion system (MPS) during off-nominal situations with additional roll control. During entry, the RCS provides attitude control until the flight control surfaces become effective.
Translational movement is primarily utilized for velocity changes. Rendezvous and proximity operations on orbit may require a combination of both rotation and translation.
The RCS has a total of 44 jets. There are 38 primary jets and 6 vernier jets. Each primary jet is rated at 870 lb of thrust and each vernier jet is rated at 24 lb of thrust. The vernier jets are only used on orbit for fine attitude control. The forward RCS has 14 primary jets and 2 verniers while each aft module has 12 primary jets and 2 verniers.
The RCS jets use monomethylhydrazine (MMH) as the fuel and nitrogen tetroxide (N2O4) as the oxidizer. (Note: the OMS uses the same type of propellants.) These propellants are hypergolic, which means that ignition occurs spontaneously upon fuel-oxidizer contact, thus eliminating the need for an ignition device. Although this adds reliability to the system, the volatile, corrosive, and poisonous nature of these propellants adds an operational risk, should any leakage occur.
Both propellants remain liquid at the temperatures normally experienced during a mission. However, there is a possibility of propellant freezing on orbit during long periods of RCS inactivity. Electrical heaters are utilized during these low activity periods to prevent the freezing of any propellant.
Each RCS module contains a collection of jets, a fuel tank, an oxidizer tank, and two helium tanks, along with associated feedlines, manifolds, and other supporting equipment. Propellant flow (fuel and oxidizer) to the jets is normally maintained by pressurizing the propellant tanks with helium. Refer to figure 1-3 for equipment location.
There are two major plumbing differences between the forward and aft RCS modules, as noted in figure 1-4. One difference is the redundancy in the aft RCS 3/4/5 tank isolation valves and the lack of the same redundancy in the forward.
The other difference is the crossfeed lines between the left and right OMS pods and the lack of crossfeed lines in the forward RCS. The two crossfeed lines (one fuel and one oxidizer) are common to both OMS and RCS (figure 1-5). Depending upon the configuration of the OMS and RCS crossfeed valves, RCS propellant from one pod can be used to feed jets in the other pod (RCS to RCS crossfeed), or OMS propellant can be used to feed jets in either or both, aft modules (OMS to RCS interconnect).
RCS propellant should never be interconnected to the OMS engines because the RCS tanks are too small to support the increased propellant flow of the larger OMS engines. Subsequent propellant tank damage would occur.
Nominal usage of the RCS is depicted in figures 1-6,1-7, and 1-8 and includes control during the following time periods:
• Between main engine cutoff (MECO) and the OMS 1 (orbital insertion) burn which includes the external tank (ET) separation maneuver.
• Between the OMS 1 and OMS 2 (orbital circularization) burn.
• On-orbit (post-OMS 2 but before deorbit burn) when not in free drift or executing OMS burns.
Post-deorbit burn until the flight control surfaces become effective. The RCS jets are used simultaneously with the flight control system (PCS) until the flight control surfaces are fully effective. When a flight control surface becomes effective, the associated RCS jets are deactivated. All RCS jets are deactivated by Mach 1.
The RCS is also used in the following off-nominal cases:
• Single engine roll control (figure 1-9).
In the event two main engines fail, the remaining main engine does not have sufficient gimballing authority to control the rolling moment of the Orbiter/ET stack. The RCS is commanded to assist the single main engine with roll control. An OMS to RCS interconnect is automatically provided in order to conserve RCS propellant during this period of high RCS activity.
Gimbal failures during OMS burns (figure 1-10).
If failures cause the lack of gimbal authority, the OMS engine(s) may not adequately control vehicle roll. The RCS can be commanded to assist the OMS (RCS wraparound). Although an interconnect is possible, it is not a requirement for RCS wraparound.
Note: Some on-orbit burns are nominally planned for a single engine burn. Single engine OMS burns usually require RCS wraparound to provide roll control.
OMS burn completion (figure 1-11).
In the event that both OMS engines fail during a burn, the RCS (forward and/or aft) can be used in the translational mode to complete the burn. The figure shows the use of the four aft firing jets for a + X completion. Use of jets in other directions (-X, + Y, etc.) would require significant attitude changes before starting the translational burn. The RCS may be interconnected to the OMS if adequate OMS propellant is available; otherwise, RCS propellant can be used to complete the burn.
Abort dumps (figure 1-12).