Space Shuttle main engines (
SSMEs) are reusable liquid-fuel rocket engines built by Rocketdyne. Each
Space ShuttleThe Space Shuttle, part of the Space Transportation System , is a spacecraft operated by NASA for orbital human spaceflight missions. It began operations in the 1980s and is scheduled to be retired from service in 2010 after 134 launches...
ascent to orbit is propelled by three of the fourteen SSME engines currently used by the
NASAThe National Aeronautics and Space Administration is an agency of the United States government, responsible for the nation's public space program. NASA was established by the National Aeronautics and Space Act on July 29, 1958, replacing its predecessor, the National Advisory Committee for...
Space Shuttle program. After each flight, the three SSMEs are removed from the
Space Shuttle orbiterThe Space Shuttle orbiters are the orbital spacecraft of the Space Shuttle program operated by NASA, the space agency of the United States. Each orbiter is a reusable winged "spaceplane", a mixture of rocket, spacecraft, and aircraft...
, inspected and refurbished in preparation for reuse on a subsequent flight. The SSME is also designated as the
RS-24 for engineering purposes.
Introduction
The Space Shuttle main engines burn
liquid hydrogenLiquid hydrogen is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecular H
2 form....
and
liquid oxygenLiquid oxygen is a form of the element oxygen. It has a pale blue color and is strongly paramagnetic and can be suspended between the poles of a powerful horse shoe magnet...
from the
Space Shuttle external tankA Space Shuttle External Tank is the component of the Space Shuttle launch vehicle that contains the liquid hydrogen fuel and liquid oxygen oxidizer. During lift-off and ascent it supplies the fuel and oxidizer under pressure to the three space shuttle main engines in the orbiter...
. They are used for propulsion during its ascent, in addition to the two more powerful
solid rocket boostersThe Space Shuttle Solid Rocket Boosters are the pair of large solid rockets used by the space shuttle during the first two minutes of powered flight. Together they provide about 83% of liftoff thrust for the Space Shuttle. They are located on either side of the orange external propellant tank...
and sometimes the
Orbital Maneuvering SystemThe Space Shuttle Orbital Maneuvering System, or OMS , is a system of rocket engines used on the space shuttle orbiter for orbital injection and modifying its orbit. It consists of two "packs" at the back of the Shuttle, the large lumps on either side of the vertical stabilizer...
. Each engine can generate almost 1.8 meganewtons (MN) or 400,000
lbfThe pound-force or simply pound is a unit of force.- Definitions :The pound-force is approximately equal to the gravitational force exerted on a mass of one avoirdupois pound on the surface of Earth...
of
thrustThrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a proportional but opposite force on that system.-Examples:...
at liftoff. The engines are capable of generating a
specific impulseSpecific impulse is a way to describe the efficiency of rocket and jet engines. It represents the impulse per unit of propellant. The higher the specific impulse, the less propellant is needed to gain a given amount of momentum...
(
Isp) of 453 seconds in a vacuum, or 363 seconds at sea level (exhaust velocities of 4440 m/s and 3560 m/s respectively). Overall, a space shuttle main engine weighs approximately 3.2 t (7,000
lbThe pound or pound-mass is a unit of mass used in the imperial, United States customary and other systems of measurement...
). The engines are removed after every flight and taken to the Space Shuttle Main Engine Processing Facility (SSMEPF) for inspection and replacement of any necessary components.
The Space Shuttle's
rocket engineA rocket engine or simply "rocket" is a jet engine[Rocket Propulsion Elements; 7th edition- chapter 1] that uses only propellant mass for forming its high speed propulsive jet. Rocket engines are reaction engines and obtain thrust in accordance with Newton's third law...
s are capable of operating at extreme temperatures. The liquid hydrogen fuel is stored at −253 degrees
CelsiusCelsius is a temperature scale that is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...
(−423 degrees
FahrenheitFahrenheit is the temperature scale proposed in 1724 by, and named after, the physicist Daniel Gabriel Fahrenheit . Today, the scale has been replaced by the Celsius scale in most countries; it is still in use for non-scientific purposes in the United States and a few other nations, such as...
). However, when burned with liquid oxygen, the temperature in the combustion chamber reaches 3,300 °C (6,000 °F), higher than the
boiling pointThe boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....
of
ironIron is a metallic chemical element with the symbol Fe and atomic number 26. Iron is a group 8 and period 4 element and is therefore classified as a transition metal. Iron and iron alloys are by far the most common metals and the most common ferromagnetic materials in everyday use...
. The main engines collectively consume 3,917 liters (1,035 gallons) of propellant per second. If the main engines pumped water instead of liquid oxygen and liquid hydrogen, an average-sized
swimming poolA swimming pool, swimming bath, wading pool, or simply a pool, is an artificially enclosed body of water intended for swimming or water-based recreation. There are many standard sizes; the largest and deepest is the Olympic size...
could be drained in 25 seconds.
Apart from the three main engines, the orbiter has 44 smaller rockets around its surface, which are part of the
Orbital Maneuvering SystemThe Space Shuttle Orbital Maneuvering System, or OMS , is a system of rocket engines used on the space shuttle orbiter for orbital injection and modifying its orbit. It consists of two "packs" at the back of the Shuttle, the large lumps on either side of the vertical stabilizer...
and
Reaction Control SystemA reaction control system, abbreviated RCS, is a subsystem of a spacecraft. Its purpose is attitude control and steering. An RCS system is capable of providing small amounts of thrust in any desired direction or combination of directions. An RCS is also capable of providing torque to allow control...
, used to provide steering, pointing, and altitude adjustment capability while in orbit.
The engines perform as follows:
FuelFuel is any material that is burned or altered to obtain energy and to heat or to move object. Fuel releases its energy either through a chemical reaction means, such as combustion, or nuclear means, such as nuclear fission or nuclear fusion...
and oxidizer from the external tank enters the orbiter at the orbiter/external tank
umbilicalAn umbilical cable or umbilical is a cable which supplies required consumables to an apparatus. It is named for its similar function to an umbilical cord...
disconnect and then the orbiter's main propulsion system feed lines. There the fuel and oxidizer each branch out into three parallel paths, to each engine. In each branch, prevalves must be opened to permit flow to the low-pressure fuel or oxidizer
turbopumpAs the name suggests, a turbopump comprises basically two main components: a rotodynamic pump and a driving turbine, both mounted on the same shaft....
.
Oxidizer system
The Low Pressure Oxidizer Turbopump (LPOTP) is an axial-flow pump driven by a six-stage
turbineA turbine is a rotary engine that extracts energy from a fluid or air flow and converts it into useful work.The simplest turbines have one moving part, a rotor assembly, which is a shaft or drum, with blades attached. Moving fluid acts on the blades, or the blades react to the flow, so that they...
powered by liquid oxygen. It boosts the liquid oxygen's pressure from 0.7 to 2.9 MPa (100 to 420 psia). The flow from the LPOTP is supplied to the High-Pressure Oxidizer Turbopump (HPOTP). During engine operation, the pressure boost permits the High Pressure Oxidizer Turbine to operate at high speeds without
cavitatingCavitation is the formation of vapour bubbles of a flowing liquid in a region where the pressure of the liquid falls below its vapor pressure. Cavitation is usually divided into two classes of behavior: inertial cavitation, and noninertial cavitation. Inertial cavitation is the process where a...
. The LPOTP operates at approximately 5,150 rpm. The LPOTP, which measures approximately 450 by 450 mm (18 by 18
inchAn inch is the name of a unit of length in a number of different systems, including Imperial units, and United States customary units. There are 36 inches in a yard and 12 inches in a foot...
es), is connected to the vehicle propellant ducting and supported in a fixed position by the orbiter structure.
The HPOTP 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 2.9 to 30 MPa (420 to 4,300 psi) while operating at approximately 28,120 rpm. The HPOTP discharge flow splits into several paths, one of which is routed to drive the LPOTP turbine. Another path is routed to and through the main oxidizer
valveA valve is a device that regulates the flow of a fluid by opening, closing, or partially obstructing various passageways. Valves are technically pipe fittings, but are usually discussed as a separate category....
and enters into the main
combustionCombustion or burning is a complex sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat or both heat and light in the form of either a glow or flames, appearance of light flickering.Direct combustion by atmospheric oxygen is a reaction...
chamber. Another small flow path is tapped off and sent to the oxidizer
heat exchangerA heat exchanger is a device built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact. They are widely used in space heating, refrigeration, air conditioning, power plants, chemical plants,...
. 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 HPOTP 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 30 to 51 MPa (4,300 psia to 7,400 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 HPOTP measures approximately 600 by 900 mm (24 by 36 inches). It is attached by flanges to the hot-gas manifold.
The HPOTP turbine and HPOTP 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.
Hydrogen fuel system
FuelFuel is any material that is burned or altered to obtain energy and to heat or to move object. Fuel releases its energy either through a chemical reaction means, such as combustion, or nuclear means, such as nuclear fission or nuclear fusion...
enters the orbiter at the liquid hydrogen feed line disconnect valve, then flows into the orbiter liquid 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 open.
The Low Pressure Fuel Turbopump (LPFTP) 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 to 276 psia (0.2 to 1.9 MPa) and supplies it to the High-Pressure Fuel Turbopump (HPFTP). During engine operation, the pressure boost provided by the LPFTP permits the HPFTP to operate at high speeds without cavitating. The LPFTP operates at approximately 16,185 rpm. The LPFTP is approximately 450 by 600 mm (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 LPOTP.
The HPFTP is a three-stage centrifugal pump driven by a two-stage, hot-gas turbine. It boosts the pressure of the liquid hydrogen from 1.9 to 45 MPa (276 to 6,515 psia). The HPFTP 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 LPFTP, where it is used to drive the LPFTP turbine. A small portion of the flow from the LPFTP 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 HPFTP is approximately 550 by 1100 mm (22 by 44 inches). It is attached by flanges to the hot-gas manifold.
Pre-burners and thrust control system
The oxidizer and fuel preburners are welded to the hot-
gasThis page is about the physical properties of gas as a state of matter. For the uses of gases, and other meanings, see Gas .A gas is one of four states of matter. Near absolute zero, a substance exists as a solid...
manifoldA manifold, in systems for moving fluids or gases is a junction of pipes or channels, typically bringing one into many or many into one.-Applications:*Heated-manifold direct-injection die casting for zinc die casting....
. The fuel and oxidizer enter the preburners and are mixed so that efficient combustion can occur. The augmented
sparkAn electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. The phenomenon was first described by Vasily V. Petrov, a Russian scientist who discovered...
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 HPOTP and the oxidizer preburner pump. The fuel preburner's outflow drives a turbine that is connected to the HPFTP.
The speed of the HPOTP and HPFTP 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, HPOTP and HPFTP 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.
Cooling control system
A
coolantA coolant is a fluid which flows through a device to prevent its overheating, transferring the heat produced by the device to other devices that use or dissipate it. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, and chemically inert, neither causing nor...
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 % open before engine start. During engine operation, it will be 100 % open for throttle settings of 100 to 109 % for maximum cooling. For throttle settings between 65 to 100 %, its position will range from 66.4 to 100 % open for reduced cooling.
Combustion chamber and nozzle
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 liquid hydrogen flowing through
brazedBrazing is a metal-joining process whereby a filler metal or alloy is heated to melting temperature above and distributed between two or more close-fitting parts by capillary action. The filler metal is brought slightly above its melting temperature while protected by a suitable atmosphere or flux...
stainless steel tube-wall coolant passages. The nozzle assembly is a bell-shaped extension bolted to the main combustion chamber. The nozzle is 2.9 m (113 inches) long, and the outside diameter of the exit is 2.4 m (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 is necessary because of the exposure 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 and screening.
For a nozzle able to run at sea level, the SSME nozzle has an unusually large expansion ratio (about 77) for the chamber pressure. A nozzle that large would normally undergo flow separation of the jet from the nozzle which would cause control difficulties and could even mechanically damage the vehicle. Instead the Rocketdyne engineers varied the angle of the nozzle, reducing it near the exit. This raises the pressure just around the rim to between 4.6 and 5.7 psi, and prevents flow separation. The inner part of the flow is at much lower pressure, around 2 psi or less.
Main valves
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 actuation system.
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.
Gimbal
The
gimbal250px|thumb|right|Illustration of a simple two-axis gimbal set.A gimbal is a pivoted support that allows the rotation of an object about a single axis. A set of two gimbals, one mounted on the other with pivot axes orthogonal, may be used to allow an object mounted on the innermost gimbal to...
bearingA bearing is a device to allow constrained relative motion between two or more parts, typically rotation or linear movement. Bearings may be classified broadly according to the motions they allow and according to their principle of operation as well as by the directions of applied loads they can...
is bolted to the main injector and dome assembly and is the thrust interface between the engine and orbiter. The bearing assembly is approximately 290 by 360 mm (11.3 by 14 inches).
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 LPFTP to the HPFTP is insulated to prevent the formation of liquid air.
SSME thrust specifications
SSME thrust (or power level) can be throttled between 67 to 109% of rated thrust. Current launches use 104.5%, with 106 or 109% available for
abort contingenciesA Space Shuttle abort is an emergency procedure due to equipment failure on NASA's Space Shuttle, most commonly during ascent. A main engine failure is a typical abort scenario. There are fewer abort options during reentry and descent...
. Thrust can be specified as sea level or vacuum thrust. Vacuum thrust will be higher due to the absence of atmospheric effects.
- 100% thrust (sea level / vacuum): 1670 kN / 2090 kN (375,000 lbf / 470,000 lbf)
- 104.5% thrust (sea level / vacuum): 1750 kN / 2170 kN (393,800 lbf / 488,800 lbf)
- 109% thrust (sea level / vacuum): 1860 kN / 2280 kN (417,300 lbf / 513,250 lbf)
Specifying power levels over 100% may seem confusing, but there is a logic behind it. The 100% level does not mean the maximum physical power level attainable. Rather it is a specification, decided on early during SSME development, for the "normal" rated power level. Later studies indicated the engine could operate safely at levels above 100%, which is now the norm. Maintaining the original relationship of power level to physical thrust helps reduce confusion. It creates an unvarying fixed relationship, so that test data, or operational data from past or future missions can be easily compared. If each time the power level was increased, that value was made 100%, then all previous data and documentation would either require changing, or cross-checking against what physical thrust corresponded to 100% power level on that date.
SSME power level affects engine reliability. Studies indicate the probability of an engine failure increases rapidly with power levels over 104.5%, which is why those are retained for contingency use only.
The SSME after the Shuttle era
Originally, the SSME was to see service in the post-Shuttle era as the main engines for the unmanned
Ares VThe Ares V is the cargo launch component of the upcoming Constellation program, which will replace the Space Shuttle after its retirement in 2010...
cargo-launch vehicle and as a second-stage engine for the manned-rated
Ares IAres I is the crew launch vehicle being developed by NASA as a component of the Constellation Program. The name "Ares" refers to the Greek deity Ares, who is identified with the Roman god Mars...
crew-launch vehicle. Although the use of the SSME seemed good on paper, as it would use current Shuttle technology after the Shuttle's retirement in 2010, it had several drawbacks:
- It would not be reusable, as they would be permanently attached to the discarded stage(s).
- It would have to undergo a flight-readiness firing (FRF) before installation – the so-called "Main Engine Test" that NASA conducted with each new Orbiter and prior to the STS-26
STS-26 was the 26th Space Shuttle mission and the seventh flight for Discovery, launched from Kennedy Space Center, Florida. It was the "Return to Flight" mission, being the first mission after the Space Shuttle Challenger disaster...
flight.
- It would be expensive, time-consuming, and weight-intensive to convert the ground-started SSME to an air-started version for the Ares I second stage.
With several design changes to the Ares I and Ares V rockets, the SSME will be replaced with a single
J-2XRocketdyne's J-2 was America's largest production liquid hydrogen fueled rocket engine before the Space Shuttle main engines , and is being revived in support of NASA's return to the Moon....
engine for the Ares I second stage. The Ares V will use six modified
RS-68The Pratt & Whitney Rocketdyne RS-68 is a liquid-fuel rocket engine that burns LH2 with LOX. It is the largest hydrogen-fueled engine in the world. Development of the engine started in the 1990s with the goal of producing a simpler, less-costly, heavy-lift engine for the Delta IV launch system...
engines (which is based on both the SSME and Apollo-era J-2 engine) for its core stage. Hence the SSMEs will be retired along with the Shuttle fleet. If, however, the
DIRECTDIRECT is a proposed alternative Shuttle-Derived Launch Vehicle architecture supporting NASA's Vision for Space Exploration, which would replace the space agency's planned Ares I and Ares V rockets with another family of launch vehicles named "Jupiter"....
Jupiter family replaces the Ares rockets due to delays and cost overruns, the core stage will use 3 or 4 SSMEs.
Specifications
Specifications as listed in the Encyclopedia Astronautix
- Engine Model: SSME
- Manufacturer Name: RS-25
- Other Designations: RS-24
- Designer: Rocketdyne
- Developed in: 1972
- Propellants: Lox/LH2
- Thrust(vac): 2,278,000 N (512,114 lbf)
- Thrust(sl): 1,817,400 N (408,568 lbf)
- Isp: 453 sec
- Isp (sea level): 363 sec
- Burn time: 480 sec
- Mass Engine: 3,177 kg (7,004 lb)
- Diameter: 1.63 m (5.36 ft)
- Length: 4.24 m (13.92 ft)
- Chambers: 1
- Chamber Pressure: 204.08 bar
- Area Ratio: 77.50
- Oxidizer to Fuel Ratio: 6.00
- Thrust to Weight Ratio: 73.12
- Country: USA. Status: In production
- First Flight: 1981
Other Specifications as previously listed on Wikipedia:
- Design altitude =
- Nozzle Mach number = 5.05 (calculated)
- Throat area =
- Nozzle area =
- Chamber pressure = at 100% power
- Exit pressure = (calculated)
- Burn time = 520 seconds
- Vacuum Isp = 452.5 seconds
- Vacuum thrust per engine = at 104.5% of design thrust
See also
- MPTA-098
The Main Propulsion Test Article was built by Rockwell International as a testbed for the definitive propulsion and fuel delivery systems for the U.S...
- the SSME test article used in Shuttle development
- Space Shuttle Solid Rocket Booster
The Space Shuttle Solid Rocket Boosters are the pair of large solid rockets used by the space shuttle during the first two minutes of powered flight. Together they provide about 83% of liftoff thrust for the Space Shuttle. They are located on either side of the orange external propellant tank...
s used to provide additional power during ascent