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
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 285º 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 420º 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 425º F to
plus 475º 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 349º F. A signal also is sent to the computers,
where software checks the limit; and if the temperature is at or
above 349º 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
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
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.