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External Tank

The external tank is attached to the orbiter at one forward and two aft attach points. At the two aft attach points are the two external tank/orbiter umbilicals for the fluid, gas, signal and electrical power connections between the orbiter and the external tank. Each external tank umbilical plate mates with a corresponding umbilical plate on the orbiter. The umbilical plates help maintain alignment of the various connecting components. The corresponding umbilical plates are bolted together; and when external tank separation is commanded, the bolts are severed by pyrotechnics.

At the forward end of each external tank propellant tank is a vent and relief valve that can be opened by GSE-supplied helium before launch for venting or by excessive tank pressure for relief. The vent function is available only before launch; after lift-off only the relief function is operable. The liquid oxygen tank relieves at an ullage pressure of 25 psig, while the liquid hydrogen tank relieves at an ullage pressure of 38 psi. The flight crew has no control over the position of the vent and relief valves before launch or during ascent. Normal range of the tank ullage pressure of the liquid hydrogen tank during ascent is 32 to 39 psia. During prelaunch activities, the liquid hydrogen tank is pressurized to 44.1 psi to meet the start requirement of the main engine LPFT. The liquid oxygen and liquid hydrogen tanks' ullage pressures are monitored on the panel F7 eng manf LO2 and LH2 meters as well as on a cathode ray tube display.

In addition to the vent and relief valve, the liquid oxygen tank has a tumble vent valve that is opened during the external tank separation sequence. The thrust force provided by opening the valve imparts an angular velocity to the external tank to assist in the separation maneuver and provide more positive control of the external tank's re-entry aerodynamics.

There are eight propellant depletion sensors. Four of them sense fuel depletion and four sense oxidizer depletion. The oxidizer depletion sensors are mounted in the external tank's liquid oxygen feed line manifold downstream of the tank. The fuel depletion sensors are located in the liquid hydrogen tank. During prelaunch activities, the launch processing system tests each propellant depletion sensor. If any are found to be in a failed condition, the LPS sets a flag in the computer's SSME operational sequence, sequence logic that will instruct the computer to ignore the output of the failed sensor or sensors. During main engine thrusting, the computer constantly computes the instantaneous mass of the vehicle, which constantly decreases due to propellant usage from the external tank. When the computed vehicle mass matches a predetermined initialized-loaded value, the computer arms the propellant depletion sensors. After this time, if any two of the good fuel depletion sensors (those not flagged before launch) or any two of the good oxidizer depletion sensors indicate a dry condition, the computers command main engine cutoff. This type of MECO is a backup to the nominal MECO, which is based on vehicle velocity. The oxidizer sensors sense propellant depletion before the fuel sensors to ensure that all depletion cutoffs are fuel-rich since an oxidizer-rich cutoff can cause burning and severe erosion of engine components. To ensure that the oxidizer sensors sense depletion first, a plus 700-pound bias is included in the amount of liquid hydrogen loaded in the external tank. This amount is in excess of that dictated by the 6-1 ratio of oxidizer to fuel. The position of the oxidizer propellant depletion sensors allows the maximum amount of oxidizer to be consumed in the engines and allows sufficient time to cut off the engines before the oxidizer turbopumps cavitate (run dry).

Four ullage pressure transducers are located at the top end of each propellant tank (liquid oxygen and liquid hydrogen). One of the four is considered a spare and is normally off-line. Before launch, GSE normally checks out the four transducers; and if one of the three active transducers is determined to be bad, it can be taken off-line and the output of the spare transducer selected. The flight crew can also perform this operation after lift-off via the computer keyboard; however, because of the time involved from lift-off to MECO, this would probably be impractical. The three active ullage pressure sensors provide outputs for CRT display and control of ullage pressure within their particular propellant tanks. For CRT display, computer processing selects the middle value output of the three transducers and displays this single value. For ullage pressure control, all three outputs are used.

The external tank/orbiter aft umbilicals have five propellant disconnects: two for the liquid oxygen tank and three for the liquid hydrogen tank. One of the liquid oxygen propellant umbilicals carries liquid oxygen and the other carries gaseous oxygen. The liquid hydrogen tank has two disconnects that carry liquid hydrogen and one that carries gaseous hydrogen. The external tank liquid hydrogen recirculation disconnect is the smaller of the two disconnects that carry liquid hydrogen and is used only during the liquid hydrogen chill-down sequence before launch.

In addition, the external tank/orbiter umbilicals contain two electrical umbilicals, each made of many smaller electrical cables. These cables carry electrical power from the orbiter to the external tank and the two solid rocket boosters and bring telemetry back to the orbiter from the SRBs and external tank. The operational instrumentation telemetry that comes back from the SRBs is conditioned, digitized and multiplexed in the SRBs themselves. The external tank OI measurements that return to the orbiter are raw transducer outputs and must be processed within the orbiter telemetry system.

The external tank's liquid oxygen tank is serviced at the launch pad before prelaunch from ground support equipment through the starboard T-0 umbilical of the orbiter, the MPS outboard and inboard fill and drain valves, the MPS 17-inch liquid oxygen line, and the orbiter/external tank 17-inch umbilical disconnect valves. Once the liquid oxygen is loaded and ready for main engine ignition, the liquid oxygen tank's vent and relief valve is closed, and the tank is pressurized to 21 psig by GSE-supplied helium. During SSME thrusting, liquid oxygen flows out of the external tank through the orbiter/external tank umbilical into the orbiter MPS and to each SSME. Pressurization in the tank is maintained by gaseous oxygen tapped from the three main engines and supplied to the liquid oxygen tank through the orbiter/external tank gaseous oxygen umbilical.

The external tank's liquid hydrogen tank is serviced before launch from GSE at the launch pad similarly to the liquid oxygen tank but through the port T-0 umbilical and port orbiter/external tank umbilical. When the liquid hydrogen is loaded and ready for main engine ignition, the liquid hydrogen tank's vent and relief valve is closed, and the tank is pressurized to 42.5 psia by GSE-supplied helium.

Approximately 45 minutes after loading starts, three electrically powered liquid hydrogen pumps in the orbiter begin to circulate the liquid hydrogen in the external tank through the three SSMEs and back to the external tank through a special recirculation umbilical. This recirculation chills down the liquid hydrogen lines between the external tank and the high-pressure fuel turbopump in the SSMEs so that the path is free of any gaseous hydrogen bubbles and is at the proper temperature for engine start. Recirculation ends approximately six seconds before engine start. During engine thrusting, liquid hydrogen flows from the external tank and through the orbiter/external tank liquid hydrogen umbilical into the orbiter MPS and to the main engines. Tank pressurization is maintained by gaseous hydrogen tapped from the three SSMEs and supplied to the liquid hydrogen tank through the orbiter/external tank gaseous hydrogen umbilical.

Curator: Kim Dismukes | Responsible NASA Official: John Ira Petty | Updated: 04/07/2002
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