Shuttle Main Engines
Oxidizer from the external
tank enters the orbiter at the orbiter/external tank umbilical disconnect
and then the orbiter's main propulsion system liquid oxygen feed
line. There it branches out into three parallel paths, one to each
engine. In each branch, a liquid oxygen prevalve must be opened
to permit flow to the low-pressure oxidizer turbopump.
The LPOT is an axial-flow pump driven by a six-stage turbine
powered by liquid oxygen. It boosts the liquid oxygen's pressure
from 100 psia to 422 psia. The flow from the LPOT is supplied
to the high-pressure oxidizer turbopump. During engine operation,
the pressure boost permits the HPOT to operate at high speeds
without cavitating. The LPOT operates at approximately 5,150 rpm.
The LPOT, which is approximately 18 by 18 inches, is connected
to the vehicle propellant ducting and supported in a fixed position
by the orbiter structure.
The HPOT consists of two single-stage centrifugal pumps (a main
pump and a preburner pump) mounted on a common shaft and driven
by a two-stage, hot-gas turbine. The main pump boosts the liquid
oxygen's pressure from 422 psia to 4,300 psia while operating
at approximately 28,120 rpm. The HPOT discharge flow splits into
several paths, one of which is routed to drive the LPOT turbine.
Another path is routed to and through the main oxidizer valve
and enters into the main combustion chamber. Another small flow
path is tapped off and sent to the oxidizer heat exchanger. The
liquid oxygen flows through an anti-flood valve that prevents
it from entering the heat exchanger until sufficient heat is present
to convert the liquid oxygen to gas. The heat exchanger utilizes
the heat contained in the discharge gases from the HPOT turbine
to convert the liquid oxygen to gas. The gas is sent to a manifold
and is then routed to the external tank to pressurize the liquid
oxygen tank. Another path enters the HPOT second-stage preburner
pump to boost the liquid oxygen's pressure from 4,300 psia to
7,420 psia. It passes through the oxidizer preburner oxidizer
valve into the oxidizer preburner and through the fuel preburner
oxidizer valve into the fuel preburner. The HPOT is approximately
24 by 36 inches. It is attached by flanges to the hot-gas manifold.
Fuel enters the orbiter at the liquid hydrogen feed line disconnect
valve, then flows into the orbiter gaseous hydrogen feed line
manifold and branches out into three parallel paths to each engine.
In each liquid hydrogen branch, a prevalve permits liquid hydrogen
to flow to the low-pressure fuel turbopump when the prevalve is
The LPFT is an axial-flow pump driven by a two-stage turbine
powered by gaseous hydrogen. It boosts the pressure of the liquid
hydrogen from 30 psia to 276 psia and supplies it to the high-pressure
fuel turbopump. During engine operation, the pressure boost provided
by the LPFT permits the HPFT to operate at high speeds without
cavitating. The LPFT operates at approximately 16,185 rpm. The
LPFT is approximately 18 by 24 inches. It is connected to the
vehicle propellant ducting and is supported in a fixed position
by the orbiter structure 180 degrees from the LPOT.
The HPFT is a three-stage centrifugal pump driven by a two-stage,
hot-gas turbine. It boosts the pressure of the liquid hydrogen
from 276 psia to 6,515 psia. The HPFT operates at approximately
35,360 rpm. The discharge flow from the turbopump is routed to
and through the main valve and then splits into three flow paths.
One path is through the jacket of the main combustion chamber,
where the hydrogen is used to cool the chamber walls. It is then
routed from the main combustion chamber to the LPFT, where it
is used to drive the LPFT turbine. A small portion of the flow
from the LPFT is then directed to a common manifold from all three
engines to form a single path to the external tank to maintain
liquid hydrogen tank pressurization. The remaining hydrogen passes
between the inner and outer walls to cool the hot-gas manifold
and is discharged into the main combustion chamber. The second
hydrogen flow path from the main fuel valve is through the engine
nozzle (to cool the nozzle). It then joins the third flow path
from the chamber coolant valve. The combined flow is then directed
to the fuel and oxidizer preburners. The HPFT is approximately
22 by 44 inches. It is attached by flanges to the hot-gas manifold.
The oxidizer and fuel preburners are welded to the hot-gas manifold.
The fuel and oxidizer enter the preburners and are mixed so that
efficient combustion can occur. The augmented spark igniter is
a small combination chamber located in the center of the injector
of each preburner. The two dual-redundant spark igniters, which
are activated by the engine controller, are used during the engine
start sequence to initiate combustion in each preburner. They
are turned off after approximately three seconds because the combustion
process is then self-sustaining. The preburners produce the fuel-rich
hot gas that passes through the turbines to generate the power
to operate the high-pressure turbopumps. The oxidizer preburner's
outflow drives a turbine that is connected to the HPOT and the
oxidizer preburner pump. The fuel preburner's outflow drives a
turbine that is connected to the HPFT.
The HPOT turbine and HPOT pumps are mounted on a common shaft.
Mixing of the fuel-rich hot gas in the turbine section and the
liquid oxygen in the main pump could create a hazard. To prevent
this, the two sections are separated by a cavity that is continuously
purged by the MPS engine helium supply during engine operation.
Two seals minimize leakage into the cavity. One seal is located
between the turbine section and the cavity, and the other is between
the pump section and cavity. Loss of helium pressure in this cavity
results in an automatic engine shutdown.
The speed of the HPOT and HPFT turbines depends on the position
of the corresponding oxidizer and fuel preburner oxidizer valves.
These valves are positioned by the engine controller, which uses
them to throttle the flow of liquid oxygen to the preburners and,
thus, control engine thrust. The oxidizer and fuel preburner oxidizer
valves increase or decrease the liquid oxygen flow, thus increasing
or decreasing preburner chamber pressure, HPOT and HPFT turbine
speed, and liquid oxygen and gaseous hydrogen flow into the main
combustion chamber, which increases or decreases engine thrust,
thus throttling the engine. The oxidizer and fuel preburner valves
operate together to throttle the engine and maintain a constant
6-1 propellant mixture ratio.
The main oxidizer valve and the main fuel valve control the flow
of liquid oxygen and liquid hydrogen into the engine and are controlled
by each engine controller. When an engine is operating, the main
valves are fully open.
A coolant control valve is mounted on the combustion chamber
coolant bypass duct of each engine. The engine controller regulates
the amount of gaseous hydrogen allowed to bypass the nozzle coolant
loop, thus controlling its temperature. The chamber coolant valve
is 100 percent open before engine start. During engine operation,
it will be 100 percent open for throttle settings of 100 to 109
percent for minimum cooling. For throttle settings between 65
to 100 percent, its position will range from 66.4 to 100 percent
open for maximum cooling.
Each engine main combustion chamber receives fuel-rich hot gas
from a hot-gas manifold cooling circuit. The gaseous hydrogen
and liquid oxygen enter the chamber at the injector, which mixes
the propellants. A small augmented spark igniter chamber is located
in the center of the injector. The dual-redundant igniter is used
during the engine start sequence to initiate combustion. The igniters
are turned off after approximately three seconds because the combustion
process is self-sustaining. The main injector and dome assembly
is welded to the hot-gas manifold. The main combustion chamber
also is bolted to the hot-gas manifold.
The inner surface of each combustion chamber, as well as the
inner surface of each nozzle, is cooled by gaseous hydrogen flowing
through coolant passages. The nozzle assembly is a bell-shaped
extension bolted to the main combustion chamber. The nozzle is
113 inches long, and the outside diameter of the exit is 94 inches.
A support ring welded to the forward end of the nozzle is the
engine attach point to the orbiter-supplied heat shield. Thermal
protection for the nozzles is necessary because of the exposure
that portions of the nozzles experience during the launch, ascent,
on-orbit and entry phases of a mission. The insulation consists
of four layers of metallic batting covered with a metallic foil
The five propellant valves on each engine (oxidizer preburner
oxidizer, fuel preburner oxidizer, main oxidizer, main fuel, and
chamber coolant) are hydraulically actuated and controlled by
electrical signals from the engine controller. They can be fully
closed by using the MPS engine helium supply system as a backup
The low-pressure oxygen and low-pressure fuel turbopumps are
mounted 180 degrees apart on the orbiter's aft fuselage thrust
structure. The lines from the low-pressure turbopumps to the high-pressure
turbopumps contain flexible bellows that enable the low-pressure
turbopumps to remain stationary while the rest of the engine is
gimbaled for thrust vector control. The liquid hydrogen line from
the LPFT to the HPFT is insulated to prevent the formation of
The main oxidizer valve and fuel bleed valve are used after shutdown.
The main oxidizer valve is opened during a propellant dump to
allow residual liquid oxygen to be dumped overboard through the
engine, and the fuel bleed valve is opened to allow residual liquid
hydrogen to be dumped through the liquid hydrogen fill and drain
valves overboard. After the dump is completed, the valves close
and remain closed for the remainder of the mission.
The gimbal bearing is bolted to the main injector and dome assembly
and is the thrust interface between the engine and orbiter. The
bearing assembly is approximately 11.3 by 14 inches.
Overall, a space shuttle main engine weighs approximately 7,000