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Power Reactant Storage and Distribution

Cryogenic hydrogen and oxygen are stored in a supercritical condition in double-walled, thermally insulated spherical tanks with a vacuum annulus between the inner pressure vessel and outer shell of the tank. Each tank has heaters to add energy to the reactants during depletion to control pressure. Each tank is capable of measuring quantity remaining.

The tanks are grouped in sets consisting of one hydrogen and one oxygen tank. The number of tank sets installed depends on the specific mission requirement. Up to five tank sets can be installed. The five tank sets are all installed in the midfuselage under the payload bay liner.

The oxygen tanks are identical and consist of inner pressure vessels of Inconel 718 and outer shells of aluminum 2219. The inner vessel is 33.43 inches in diameter and the outer shell is 36.8 inches in diameter. Each tank has a volume of 11.2 cubic feet and stores 781 pounds of oxygen. The dry weight of each tank is 201 pounds. The initial temperature of the stored oxygen is minus 285 F. Maximum fill time is 45 minutes.

The hydrogen tanks also are identical. Both the inner pressure vessel and the outer shell are constructed of aluminum 2219. The inner vessel's diameter is 41.51 inches and the outer shell's is 45.5 inches. The volume of each tank is 21.39 cubic feet, and each stores 92 pounds of hydrogen. Each tank weighs 216 pounds dry. The initial storage temperature is minus 420 F. Maximum fill time is 45 minutes.

The inner pressure vessels are kept supercold by minimizing conductive, convective and radiant heat transfer. Twelve low-conductive supports suspend the inner vessel within the outer shell. Radiant heat transfer is reduced by a shield between the inner vessel and outer shell (hydrogen tanks only), and convective heat transfer is minimized by maintaining a vacuum between the vessel and shell. A vacuum ion pump maintains the required vacuum level and is also used as a vacuum gauge to determine the vacuum's integrity.

Each hydrogen tank has one heater probe with two elements, while each oxygen tank has two heater probes with two elements on each probe. As the reactants are depleted, the heaters add heat energy to maintain a constant pressure in the tanks. The heaters operate in manual and automatic modes. The oxygen tank and hydrogen tank switches (auto, on, off) for tanks 1, 2 and 3 are located on panel R1; switches for the oxygen and hydrogen tank 4 heaters are on panel A11. When a heater switch is positioned to auto, the heater is controlled by a tank heater controller. Each heater controller receives a signal from a tank pressure sensor. If pressure in a tank is equal to or below a specific pressure and the controller sends a low pressure signal to the heater logic and the heater is powered on, the pressure bands are 200 to 206 psia; hydrogen tanks 3 and 4, 217 to 223 psia; oxygen tanks 1 and 2, 805 to 817 psia; and oxygen tanks 3 and 4, 834 to 846 psia. When the pressure of hydrogen tanks 1 and 2 is 220 to 226 psia, hydrogen tanks 3 and 4 is 237 to 243 psia, oxygen tanks 1 and 2 is 840 to 852 psia, and oxygen tanks 3 and 4 is 869 to 881 psia, the respective controller sends a high pressure signal to the heater logic, and the heater involved is turned off.

Dual-mode heater operation is available for pairs of oxygen and hydrogen tanks. If the heaters of both tanks 1 and 2 or tanks 3 and 4 are placed in the automatic mode, the tank heater logic is interconnected. In this case, the heater controllers of both tanks must send a low pressure signal to the heater logic before the heaters will turn on. Once the heaters are on, a high pressure signal from either tank will turn off the heaters in both tanks.

In the manual mode, the flight crew controls the heaters by using the on/off positions for each heater switch on panel R1 or A11. High or low pressure in each tank is shown on the CRT display or the gauges on panel O2. The specific tank is selected by setting the rotary switch on panel O2.

Before lift-off, the oxygen and hydrogen tank 1 and 2 heater switches are set on auto. After SRB separation, all the hydrogen and oxygen tank 1 and 2 heater switches are positioned to auto, and the tank 3 and 4 heaters remain off. On orbit, the tank 3 and 4 heater switches are positioned to auto. Because the tank 3 and 4 heater controller pressure limits are higher than those of tanks 1 and 2, tanks 3 and 4 supply the reactants to the fuel cells. For entry, the tank 3 and 4 heater switches are set to off, and tanks 1 and 2 supply the reactants to the fuel cells.

The cryo oxygen htr assy temp meter on panel O2, in conjunction with the rotary switch tk1 1-2, tk2 1-2, tk3 1-2, tk4 1-2, selects one of the two heaters in each tank and permits the temperature of the heater element to be displayed. The range of the display is from minus 425 F to plus 475 F. The temperature sensor in each heater also is hard-wired directly to the yellow O 2 heater temp caution and warning light on panel F7. This light is illuminated if the temperature is at or above 349 F. A signal also is sent to the computers, where software checks the limit; and if the temperature is at or above 349 F, the backup C/W alarm light on panel F7 is illuminated. This signal also is transmitted to the CRT and telemetry.

Two current level detectors are built into the circuit of each oxygen tank heater to interrupt power in case of electrical shorts. The second detector is redundant. Each detector is divided into A and B detectors. One monitors the heater A current and the other monitors the heater B current. The detectors are powered by circuit breakers on panels O14, O15, O16 and ML86B and are identified as cryo O2 htr tk1, 2, 3, 4 snsr 1, 2. The detectors monitor the current in and out of a heater. If the current difference is 0.9 amp or greater for 1.5 milliseconds, a trip signal is sent to the heater logic to remove power from the heaters regardless of the heater switch position. If one element of a heater causes a ''trip-out,'' power to both elements is removed. The O 2 tk 1, 2, 3 heaters reset/test switches on panel R1 and the O 2 tk 4/5 reset/test switch on panel A11 can be used to reapply power to that heater by positioning them to reset. The test position will cause a 1.4-amp delta current to flow through all four detectors of a specified oxygen tank, causing them to trip out. During on-orbit operations, the flight crew will be alerted to a current level detector trip-out by an SM alert on panel F7 and on the CRT.

Each oxygen and hydrogen tank has a quantity sensor powered by a circuit breaker. These are identified on panel O13 as cryo qty O 2 (or H2) tk1 and tk2 and on panel ML86B as cryo qty O2 (or H2) tk3 and tk4. Data from the quantity sensors is sent to panel O2, where the tk1, tk2, tk3, tk4 rotary switch is used to select the tank for display on the cryo O2 (or H2) qty meters. The range of the meters is zero to 100 percent. The data is also sent to the CRT.

There are two tank pressure sensors for each oxygen and hydrogen tank. One sensor transmits its data to the tank heater controllers and to the yellow O2 or H2 press C/W light on panel F7, which is illuminated if oxygen tank pressure is below 540 psia or above 985 psia or if hydrogen tank pressure is below 153 psia or above 293.8 psia. The signal also is transmitted to the CRT and to panel O2, where the tk1, tk2, tk3, tk4 rotary switch is used to select the tank for display on the cryo O 2 (or H 2 ) press meter. The data also goes to the SM alert, backup C/W alarm light on panel F7 and to telemetry. The range of the oxygen meter is zero to 1,200 psia. The hydrogen meter's range is zero to 400 psia.

The oxygen and hydrogen fluid temperature sensors transmit data to the CRT and telemetry.

Each tank set (one hydrogen and one oxygen tank) has a hydrogen/oxygen control box that contains the electrical logic for the hydrogen and oxygen heaters and controllers. The control box is located on cold plates in the midbody under the payload bay envelope.

The reactants from the tanks flow through two relief valve/filter package modules and valve modules and then to the fuel cells through a common manifold. Oxygen is supplied to the manifold from the tank at a pressure of 815 to 881 psia, and hydrogen is supplied at a pressure of 200 to 243 psia. The pressure of the reactants will be essentially the same at the fuel cell interface as it is in the tanks since only a small decrease in pressure occurs in the manifolds.

The relief valve/filter package module contains the tank relief valve and a 12-micron filter. The filter removes contaminants that could affect the performance of components within the power reactant storage and distribution subsystem and fuel cells. The valve relieves excessive pressure that builds up in the tank, and a manifold valve relieves pressure in the manifold lines. The oxygen tank relief valve relieves at 1,005 psia, and the hydrogen tank relief valve relieves at 310 psia.

The reactants flow from the relief valve/filter packages through four reactant valve modules: two hydrogen (hydrogen valve modules 1 and 2) and two oxygen (oxygen modules 1 and 2). Each valve module contains a check valve for each cryogenic tank line to prevent the reactants from flowing from one tank to another tank in the event of a tank leak. This prevents a total loss of reactants. The oxygen valve modules also contain the environmental control and life support system atmosphere pressure control system 1 and 2 oxygen supply. Each module also contains a manifold valve and fuel cell reactant valves.

Each fuel cell reactant valve consists of two valves-one for hydrogen and one for oxygen. The valves are controlled by the fuel cell 1, 2, 3 reac open/close switches on panel R1. When the switch is positioned to open, the hydrogen and oxygen reactant valves for that fuel cell are opened, and reactants are allowed to flow from the manifold into the fuel cell. When the switch is positioned to close, the hydrogen and oxygen reactant valves for that fuel cell are closed, isolating the reactants from the fuel cell and rendering that fuel cell inoperative. Each fuel cell reac switch on panel R1 also has a talkback indicator. The corresponding talkback indicator indicates op when both valves are open and cl when either valve is closed.

Because it is critical to have reactants available to the fuel cells, the red fuel cell reac light on panel F7 is illuminated when any fuel cell reactant valve is closed, a caution/warning tone is sounded, and the computers sense the closed valve, which causes the backup C/W alarm light on panel F7 to be illuminated, an SM alert to occur, and the data to be displayed on the CRT. This alerts the flight crew that the fuel cell will be inoperative within approximately 20 seconds for a hydrogen valve closure and 130 seconds for an oxygen valve closure.

Each H2 and O2 manifold 1, 2 open/close switch on panel R1 controls the respective hydrogen and oxygen manifold valve. When the two hydrogen and two oxygen manifold valves are in the close position, fuel cell 1 receives reactants from cryogenic tank set 1, fuel cell 2 receives reactants from cryogenic tank set 2, and fuel cell 3 receives reactants from cryogenic tank sets 3 and 4. ECLSS atmosphere pressure control system 1 receives oxygen from oxygen tank 1, and system 2 receives oxygen from oxygen tank 2. When each H 2 and O 2 manifold 1, 2 open/close switch is positioned to close, the respective talkback indicator associated with each switch indicates cl.

With the H 2 and/or O2 manifold 1 open/close switch positioned to open, cryogenic tanks 1 and 2 supply hydrogen to fuel cells 1 and 3, and oxygen cryogenic tanks 1 and 3 supply oxygen to fuel cells 1 and 3 as well as to ECLSS atmosphere pressure control system 1. The talkback indicator associated with each switch indicates op.

When the H 2 and/or O2 manifold 2 open/close switch is positioned to open, hydrogen cryogenic tanks 2 and 3/4/5 supply hydrogen to fuel cells 2 and 3, and oxygen cryogenic tanks 2 and 3/4/5 supply oxygen to fuel cells 2 and 3 as well as to ECLSS atmosphere pressure control system 2. The talkback indicator associated with each switch indicates op.

With the H 2 and O2 manifold 1 and 2 switches positioned to op, all hydrogen cryogenic tanks are supplying hydrogen to all three fuel cells, and all oxygen cryogenic tanks are supplying oxygen to all three fuel cells as well as to ECLSS atmosphere pressure control systems 1 and 2.

The manifold relief valves are a built-in safety device in the event a manifold valve and fuel cell reactant valves are closed because of a malfunction. The reactants trapped in the manifold lines would be warmed up by the internal heat of the orbiter and overpressurize. The manifold relief valve will open at 290 psi for hydrogen and 975 psi for oxygen to relieve pressure and allow the trapped reactants to flow back to their tanks.

Two pressure sensors located in the respective hydrogen and oxygen valve modules transmit data to the CRT. This data is also sent to the systems management computer, where its lower limit is checked; and if the respective hydrogen and oxygen manifold pressures are below 150 psia and 200 psia, respectively, an SM alert will occur.

If cryogenic tank set 5 is added to an orbiter, the displays and controls associated with controlling the tank set will be added to panel A15.

During prelaunch operations, the onboard fuel cell reactants (oxygen and hydrogen) are supplied by ground support equipment to assure a full load of onboard reactants before lift-off. At T minus two minutes 35 seconds, the GSE filling operation is terminated. The GSE supply pressure is 300 to 320 psia for hydrogen and 1,000 to 1,020 psia for oxygen, which is higher than the onboard PRSD pressures. The GSE supply valves close automatically to transfer to onboard reactants.


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