Katerina Swanson
The Space Shuttle external tank (ET) is an integral component of the Space Shuttle launch vehicle, supplying fuel and oxidizer to the three RS-25 main engines during lift-off and ascent. The ET is jettisoned just over 10 seconds after the main engine cut-off and re-enters the Earth's atmosphere, breaking up before impact in the ocean. The separation of the external tank from the Space Shuttle is a crucial step in the launch process, as it involves intricate timing and coordination to ensure a successful and safe mission. This process involves the activation of specific switches and systems, and it is important to monitor the perigee counter to ensure a smooth separation and transition into orbit.
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What You'll Learn
Dropping the external tank
The process of dropping the ET typically occurs just before the shuttle reaches orbit. Here are the key steps involved:
- SRB Separation: At around 150,000 feet altitude, the SRBs separate from the shuttle. While they initially appear to fall back, they continue to rise due to remaining vertical velocity. This separation results in a significant drop in acceleration.
- Final Preparations: Ensure that all valves for fuel and oxidizer are in the correct position. Switch to an inertial frame on the adi attitude and manoeuvre the shuttle to the correct ignition position.
- Confirm Burn Sequence: Turn on the OMS Arm/Press switches, and 15 seconds before the burn, confirm the burn sequence by pressing EXE on the keypad.
- Monitor Burn Results: After the burn, monitor the new apogee and perigee values to ensure they align with expectations.
- Drop the External Tank: Once the shuttle is close to orbital velocity, reduce thrust as the apogee reaches the desired value. Cut the thrust once the apogee is achieved, and drop the external tank.
- Post-Separation: The Reaction Control System (RCS) will activate automatically to separate the orbiter from the tank. The orbital manoeuvring system (OMS) provides the final push into orbit. Ensure any remaining rotation of the orbiter is nulled.
It is worth noting that the ET is not reused and breaks up before impacting the Indian or Pacific Ocean, away from shipping lanes. Additionally, after ET separation, a propellant dump should occur to release leftover liquid oxygen and hydrogen.
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The role of the orbital manoeuvring system (OMS)
The Orbital Maneuvering System (OMS) is an integral part of the space shuttle's functionality. The OMS consists of two pods, each with a single AJ10-190 engine, mounted on the orbiter's aft fuselage, on either side of the vertical stabilizer. These OMS/RCS pods contain the Orbiter's aft set of reaction control system (RCS) engines.
The OMS engines burn monomethylhydrazine (MMH) as fuel, oxidised with MON-3 (mixed oxides of nitrogen, 3% nitric acid). The propellants are stored in tanks within the OMS/RCS pod. The OMS engine produces 26.7 kilonewtons (6,000 lbf) of thrust with a specific impulse (Isp) of 316 seconds. The oxidizer-to-fuel ratio is 1.65-to-1, and the chamber pressure of the engine is 8.6 bar. Each engine has a dry weight of 118kg (260lb) and can be reused for 100 missions, with a total of 1,000 starts and 15 hours of burn time.
The OMS plays a crucial role in the final push into orbit. After the external tank (ET) separation, the RCS thrusters in the forward and aft RCS pods provide attitude hold, and the RCS manoeuvres the shuttle away from the ET. The OMS engines are then used to raise the orbiter to a predetermined elliptical orbit. During this OMS-1 thrusting period, vehicle attitude is maintained by gimbaling (swiveling) the OMS engines. The OMS-2 thrusting period occurs near the apogee of the orbit and is used to circularize the predetermined orbit. The OMS engines are also used to deorbit, with target data computed by the ground control and loaded into onboard computers.
The OMS is controlled via the cathode ray tube keyboard, with the ability to modify target data if necessary. OMS Arm/Press switches are turned on to allow the opening of injection valves for fuel and oxidizer.
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The role of the reaction control system (RCS)
The Reaction Control System (RCS) is a crucial component of spacecraft and rockets, responsible for controlling their orientation and position in space. It consists of thrusters strategically placed to provide small amounts of thrust in different directions, enabling precise maneuvers and adjustments. The RCS thrusters can use various propellants, including gases like nitrogen or hydrazine, and liquids such as monopropellant or bipropellant.
RCS plays a vital role in maintaining stability and control during critical maneuvers. For instance, when docking with another spacecraft, re-entering the Earth's atmosphere, or avoiding space debris, the RCS ensures the spacecraft remains stable and on course. The system achieves this by using small thrusters that expel pressurized gas or liquid in a specific direction to generate the required thrust.
The thrusters in an RCS are typically managed by a computer system that receives data from sensors like gyroscopes and accelerometers. These sensors provide real-time information about the spacecraft's position and orientation, enabling the computer to calculate and execute the necessary thrust adjustments for the desired maneuver. This automated process ensures the spacecraft can respond quickly and accurately to any changes in its trajectory or orientation.
In the context of the FlightGear space shuttle simulation, the RCS plays a key role in separating the orbiter from the external tank. After releasing the external tank, the RCS automatically activates in translational mode, ensuring the orbiter separates safely from the tank. Additionally, the RCS helps nullify any remaining rotation of the orbiter, demonstrating its role in maintaining stability and control even after significant maneuvers.
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The launch phase
The first two minutes of the flight pass through the atmosphere, during which the SRBs provide most of the thrust. The SRBs cannot be throttled, so once they are on, they burn until they are spent. At an altitude of about 150,000 ft, the SRBs are disconnected, and the shuttle accelerates using only the main engines fed from the ET. The ET is the "backbone" of the shuttle during launch, providing structural support for attachment with the SRBs and the orbiter. It is also the largest and heaviest element of the Space Shuttle. The ET is jettisoned just over 10 seconds after the main engine cut-off and re-enters the Earth's atmosphere.
During the launch phase, heaters are turned on for the APU fuel tank and lines, and for H2O boiler systems. The FES feedlines heaters are also turned on to avoid water freezing. The heaters will slightly warm up the tanks to increase the pressure to its nominal value. The final preparations for the burn include checking that all the correct valves for fuel and oxidizer OMS feedings are in the correct position. The OMS Arm/Press switches are turned on, and 15 seconds before the burn, EXE is pressed on the keypad to confirm the burn sequence.
Once the shuttle is close to orbital velocity, thrust should be reduced as soon as the perigee comes above zero. Cut the thrust once the apogee reaches the desired value and drop the external tank. The RCS will fire automatically in translational mode to separate the orbiter from the tank. The final push into orbit is done by the orbital maneuvering system (OMS).
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The role of the external tank (ET)
The External Tank (ET) is an integral component of the Space Shuttle launch vehicle. It is the "'gas tank'" for the Orbiter, supplying propellants to the Space Shuttle Main Engines (SSMEs). The ET is the largest element of the Space Shuttle and when loaded, it is also the heaviest.
The ET is comprised of three major components: an oxygen tank in the forward position, a hydrogen tank in the aft position, and a collar-like intertank that connects the two propellant tanks. The intertank houses instrumentation and processing equipment and provides the attachment structure for the forward end of the solid rocket boosters.
The ET serves as the "backbone" of the shuttle during launch, providing structural support for attachment with the solid rocket boosters and the orbiter. It is connected to the solid rocket boosters and the orbiter through multiple attachment points and umbilicals, which also carry fluids, gases, electrical signals, and power between the tank and the orbiter.
Over the years, NASA worked to reduce the weight of the ET to increase overall efficiency and cargo-carrying capability. The Super Lightweight Tank (SLWT), for example, used an aluminium-lithium alloy that provided a significant reduction in weight, contributing to a 50% performance increase required for the shuttle to reach the International Space Station.
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Frequently asked questions
The external fuel tank, also known as the ET, will be jettisoned just over 10 seconds after the main engine cut-off (MECO). The Reaction Control System (RCS) will fire automatically in translational mode to separate the orbiter from the tank.
The ET is the "backbone" of the shuttle during launch, providing structural support and attachment for the Solid Rocket Boosters (SRBs) and the orbiter. It also supplies propellant for the Space Shuttle Main Engines (SSMEs) and fuel and oxidizer under pressure to the three RS-25 main engines in the orbiter.
One of the first things to do is the propellant dump (liquid oxygen and hydrogen). This should start automatically but can be configured manually.
