The Space Shuttle Main Engine

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I originally wrote this essay in 1979. Since then the Space Shuttle has come and gone, but its hydrogen-burning main engine is still the best chemically-fueled rocket engine in existence. As such it can play an important role in extending our human presence into the space out to and including the moon.

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The Space Shuttle Main Engine (SSME) is a whole new breed of liquid-bipropellant rocket engine. At first glance you might think that it was a direct descendant of the J-2, the engine used to propel the Saturn upper stages. Both engines generate thrust by burning hydrogen and oxygen. And both engines are manufactured by Rockwell International’s Rocketdyne Division. But there are fundamental differences between the two machines, difference that reflect the changes in the technology of rocket-making and changes in space travel policy between 1960, when the J-2 contract was let out, and 1972, when the SSME’s development was contracted.

 

SSME

J-2

Thrust

   

Sea Level

1,668,080 Newtons

not applicable

Vacuum

2,090,660 Nt

1,022,686 Nt

Throttlability

50% – 109%

90% – 100%

Chamber Pressure

202 atmospheres

53 atmospheres

Area Ratio

77.5

27.5

Specific Impulse

   

Sea Level

363.2 seconds

not applicable

Vacuum

455.2 seconds

425 seconds

Mixture Ratio

   

Oxidizer:Fuel

6:1

5.5:1

Length

4.24 meters

3.38 meters

Greatest Diameter

2.67 meters

2.04 meters

Mass

3014 kilograms

1584 kilograms

Life

450 minutes

62.5 minutes

 

55 starts

30 starts (max.)

 

55 flights

1 flight

Propellant Flux

466.3 kilograms/sec

250 kilograms/sec

Production Cost

$30,000,000

$1,500,000 (approx.)

TABLE ONE: SSME vs. J-2

Twelve years of progress. The SSME generates slightly over 100% more vacuum thrust than the J-2 did but requires only 86% more propellant flow to do it.

Important to the goals of the Space Shuttle program is the SSME’s refurbishability. The SSME is the first rocket engine specifically designed for easy, routine maintenance like that given by the airlines to their jet engines. The J-2 was never intended to be used more than once; it was part of a booster that was discarded after one flight. The Space Shuttle, on the other hand, will be bringing its three SSMEs safely back to Earth to be cleaned up, checked out, repaired if necessary, and reused. Each SSME is meant to propel fifty-five Shuttle flights before it becomes unusable. That reflects NASA’s shift from an ammunition policy of using expendable, "no-deposit, no-return" boosters to the transportation policy of using fully recyclable spaceships.

Advances in rocket-making technology are reflected in the SSME’s use of the staged-combustion process. Combined with high combustion pressure, the staged-combustion process gives the SSME the high efficiency and wide-range throttlability that NASA wanted for the Space Shuttle. To use the staged-combustion process an engine must have two combustion chambers connected through a hot-gas manifold. In the first combustion chamber, called the preburner, all of the fuel is mixed with a little of the oxidizer and ignited to create a moderately-hot, fuel-rich gas. In the main combustion chamber the remainder of the oxidizer is sprayed into the gas and lit off. The SSME varies the process slightly in using two preburners and in interposing a high-pressure propellant turbopump between each preburner and the hot-gas manifold: the engine uses its preburned fuel to generate the power needed to run the pumps.

That contrasts with the usual practice in liquid-propellant engines, the use of the gas-generator process. In the J-2, for example, a small amount of propellant was bled from the main feedlines and burned in a gas generator to create a fuel-rich gas. That gas passed through the turbines that ran the propellant pumps and exhausted into space, contributing nothing to the engine’s thrust or specific impulse. Part of the purpose in using high combustion pressure in the SSME is to get maximum specific impulse from the propellants (and the SSME does get 98 - 99% of the theoretical maximum). The use of the gas-generator process in the SSME would have defeated that purpose because the engine would have diverted so much propellant to run the high-pressure pumps that it would have ended up with less specific impulse than the J-2 had.

The staged-combustion process also gives the SSME its wide-range throttlability, 50 - 109% of rated thrust compared to the J-2's 90 - 100% of rated thrust. Throttling a rocket engine is not a simple matter of closing propellant valves. No rocket engine, or at least none that’s meant to last for any length of time, can be throttled back so far that its combustion can become unstable. The SSME, by bringing one of its propellants into the main combustion chamber as a gas, creates a flame so stable that the engine can be cut back to half thrust safely. This gives the Space Shuttle engines that can be run at full thrust for lift-off power and can be throttled back later to limit the spaceship’s acceleration to three gees.

SKETCH ONE: Propellant Flow Schematic

Basic organization of the SSME and the flow of propellants through it. Numbers refer to Table Two. Letters refer to; A) low-pressure turbopumps, B) high-pressure turbopumps, C) preburners, D) hot-gas manifold, E) main combustion chamber, F) nozzle, G) main fuel valve, H) main oxidizer valve, I) preburner oxidizer valves, J) pogo suppressor, and K) External Tank pressurization feeds. (Copied from Rockwell International Corporation diagram.)

The Propellant Flow Schematic represents an SSME running at rated thrust. It will give you an idea of how the staged-combustion process is accomplished in practice. Starting on the fuel side of the engine, we can follow each propellant on its path to the main combustion chamber:

Liquid hydrogen flows from the fuel compartment of the Space Shuttle’s External Tank and enters the engine through the Orbiter prevalves. Two turbopumps boost the fuel’s pressure from a little over two atmospheres to nearly 420 atmospheres. The fuel splits into two streams just past the main fuel valve.

 

Temperature

Pressure

Mass Flux

(1)

20.55 Kelvin

2.04 atmosphere

66.77 kg/sec

(2)

51.66

419.83

13.79

(3)

51.66

419.83

52.98

(4)

958.9

351.15

65.09

(5)

867.8

225.15

65.09

(6)

91.11

6.8

400.57

(7)

814.44

357.34

26.54

(8)

735

225.15

26.54

(9)

104.44

263.29

360.6

(10)

3590

200.87

466.3

TABLE TWO: Engine Conditions

Temperature, pressure, and the flow rate of propellants at selected places in the engine.

About twenty percent of the fuel is led through the main combustion chamber’s liner to cool it. From there it goes to drive the low-pressure fuel turbopump and then a small amount, 320 grams per second, is bled off to pressurize the External Tank’s fuel compartment. Finally the stream is split and sent through the liners of the preburners and of the hot-gas manifold to cool them before it is fed directly into the main injector.

The remaining eighty percent of the fuel flows into the preburners after about a third of it cools the nozzle. A small amount of liquid oxygen sprays into each preburner, in an oxidizer:fuel mixture ratio of about ½:1, and the mixture is lit off by a pilot flame from twin spark igniters. The hydrogen-rich steam that results spins the turbines that run the high-pressure fuel and oxidizer pumps, then flows through the hot-gas manifold into the main injector.

Liquid oxygen from the External Tank’s oxidizer compartment enters the low-pressure oxidizer turbopump at 6.8 atmospheres and is bumped up to 31 atmospheres so that it can enter the high-pressure oxidizer turbopump without cavitation. After the high-pressure turbopump raises its pressure to 315 atmospheres the flow splits into four streams.

The largest stream goes directly through the main oxidizer valve to the main injector. The next largest stream drives the low-pressure oxidizer turbopump and then joins the pump’s output to be recycled through the high-pressure turbopump. The third stream is raised to a pressure of 520 atmospheres, the highest pressure reached in the SSME, by a small booster pump at the base of the high-pressure turbopump before it flows into the two preburners. The smallest stream flows through a heat exchanger that warms it into a gas, which is supplied to the pogo suppressor (a kind of hydraulic shock absorber that damps out oscillations in the main oxygen flow), and into the External Tank’s oxidizer compartment for pressurization.

 

Fuel-side

Oxidizer-side

 

A) Low-pressure

     

Inlet Pressure

2.04 atmosphere

6.8 atmosphere

 

Outlet Pressure

16 atmos.

31.08 atmos.

 

Temperature

20.55 Kelvin

91.11 Kelvin

 

Flow Rate

66.77 kg/sec

400.57 kg/sec

 

Pressurization

axial-flow

axial-flow

 

Speed

14,700 rpm

5150 rpm

 

Power

2400 hp

1470 hp

 
 

1788 kw

1095 kw

 
       

B) High-pressure

     

Inlet Pressure

16 atmos.

31.08 atmos.

(315.04) atmos.

Outlet Pressure

419.83 atmos.

315.04 atmos.

(520.54) atmos.

Temperature

51.66 Kelvin

104.44 Kelvin

(114.44) Kelvin

Flow Rate

66.77 kg/sec

483.08 kg/sec

(39.1) kg/sec

Pressurization

centrifugal

centrifugal

centrifugal

Speed

35,000 rpm

29,057 rpm

(29,057) rpm

Power

62,270 hp

20,977 hp

(1472) hp

 

46,404 kw

15,632 kw

(1097) kw

TABLE THREE: SSME Tubopumps

These are tough machines. The heaviest, the high-pressure fuel turbopump, ponders 326 kilograms and processes power at 191 horsepower per kilogram. Figures in parentheses are values for the oxidizer booster pump.

The main injector consists of 600 doubly-hollow injector posts projecting from the colander-like bottom of the oxidizer manifold, through a secondary plate that divides the injector into two compartments (the upper for the vapor from the hot-gas manifold and the lower for the hydrogen coolant from the manifold’s liner), and into holes in the primary plate that forms the top of the main combustion chamber. Hot fuel flows into each injector post through slots in its outer sleeve and enters the combustion chamber surrounding a core of oxygen. The whole thing is lit off by the pilot flame from a pair of augments spark igniters.

Digression on the fine art of rocket-making:

Conditions in the main combustion chamber are somewhat less than comfortable, the flame being more than hot enough to boil iron. And although the J-2 ran at the same temperature, the SSME uses nearly four times the pressure. The heat flux into the chamber wall is so much greater than that in the J-2 that a brazed-tubing liner like the J-2's simply would not work. Rocketdyne had to come up with something new.

The liner is a ruddy, hourglass-shaped cylinder of NARloy-Z, an alloy of copper, silver, and zirconium developed just for this application. The coolant channels are 390 slots milled into the exterior surface of the cylinder. A technician fills the channels with wax and the rest of the liner is electroplated over them, first a layer of copper and then a thick layer of nickel. When the wax has been melted and flushed out of the channels the liner is ready to have the coolant inlet and outlet manifolds welded onto it and the structural jacket of Inconel 718 welded around it. What this procedure creates is a perfectly smooth liner that promotes the kind of fast, efficient heat flow to the coolant that the SSME needs.

The key to making the liner is that electroplating process. Rocketdyne isn’t just "chroming" the NARloy-Z. Those layers of metal must be tight and tough because the hydrogen coolant is going to be crammed into the channels at 420 atmospheres. As it had to do with other technologies, such as electrical discharge machining and electron-beam welding, Rocketdyne had to make substantial advances in the art of electroplating before it could create the SSME.

End of Digression.

The combustion process represented in the Propellant Flow Schematic, carried out at any thrust level, is called Mainstage and it is one of six operating phases programmed into the SSME controller, a computerized electronic package that checks out the engine, operates it, and monitors its performance. Mounted on the engine next to the main combustion chamber and connected directly to the engine’s sensors and controls, the controller performs seven basic functions: 1) it reads the engine’s temperature, flow, and pressure sensors and it generates the signals that operate the engine’s valves, actuators, and spark igniters, 2) it verifies the engine’s flight readiness, 3) it starts and shuts down the engine, 4) it provides closed-loop control of thrust and propellant mixture ratio (Mainstage), 5) it monitors engine performance limits, 6) it responds to commands from the Shuttle Orbiter and transmits engine-status, performance, and maintenance data to the Orbiter’s computers, and 7) it provides on-board engine checkout for maintenance.

 

 

SKETCH TWO: Controller Organization

Digital computer has program storage capacity of 16,384 data and instruction words (17 bits each). Power supply electronics draws power from a bus outside the controller. (Copied from Rockwell International Corporation diagram.)

With special respect to the controller and its engine, a typical Space Shuttle cycle will go something like this:

The Space Shuttle is assembled on its launch pad and is ready to fly. The controller in each of the three SSMEs performs "Checkout Phase", verifying that the engine is ready to be fired, that all sensors are reading their proper values. It is during this phase that specific firing instructions and data for this mission will be fed into the controller’s computer. At the completion of Checkout Phase the Orbiter’s on-board computers initiate "Start Preparation Phase".

Before the SSME can be started it must be filled with propellant as far as the main propellant valves and the preburner oxidizer valves and then allowed to chill down. The Orbiter’s computers accomplish that by opening the Orbiter prevalves to allow the propellants into the engine and then cycling the engine through four purge sequences to remove the propellant vapors that form when liquid hydrogen and liquid oxygen hit the warm metal. When the proper thermal conditions are achieved and all other prestart criteria met to the controller’s satisfaction the controller signals "engine ready" and the Orbiter’s computers give a start command to the controller.

"Start Phase" is quick and straightforward. The controller opens the main propellant valves and the preburner oxidizer valves, lights off the preburners, and then lights off the main combustion chamber. It is important that the preburners ignite before the main combustion chamber does. If the combustion chamber were to light off first, the resulting flashback would ruin the engine and cancel the flight. Once combustion chamber ignition has been verified the controller builds up thrust to rated power level.

All three of the Shuttle’s SSME’s are now running and their thrust is starting to tip the Shuttle over. At the peak of that motion the two Solid Rocket Boosters (SRBs) light off, balance the Shuttle, and, with the SSMEs, lift the Shuttle into the Florida sky at half a gee.

The controller is now in "Mainstage Phase", monitoring engine performance fifty times a second and controlling propellant mixture ratio and engine thrust according to its instructions. It accomplishes that control primarily by manipulation of the preburner oxidizer valves. That manipulation controls the power going into the high-pressure turbopumps and thus the rate at which each propellant is crammed into the engine. After the first 30 – 35 seconds of flight the controller throttles the engine back to 65% of rated thrust while the Shuttle goes through "Max-Q", maximum aerodynamic loading. Thirty seconds later it brings the engine back up to full thrust.

The SRBs burn out 125 seconds into the flight. The Shuttle’s acceleration decreases rapidly, then reincreases slightly when the 82-tonne SRB casings are dropped. The SSMEs continue to burn at full thrust and the Shuttle’s acceleration increases again until it reaches three gees. When that happens the controller throttles its engine back to keep the Shuttle at a three-gee acceleration until the engines are shut down.

In "Shutdown Phase" each controller reduces its engine’s thrust to minimum power level, 65% of rated thrust, and then closes the propellant valves. It then queries its sensors to verify that the engine has shut down.

The Shuttle is now coasting at slightly less than orbital speed. Before it can use its Orbital Maneuvering System to make a final orbital insertion it must drop its External Tank and before it can do that it must empty the tank of all remaining propellant and propellant vapors. If it were to release the tank unemptied, the propellant and vapor venting through the open feedlines at the tank’s base would cause the tank to spin and strike the Orbiter. The controller performs the propellant dump by opening the SSME valves in sequence to allow one propellant and then the other to vent into space through the engine.

When the External Tank has been dropped the controller goes into "Post-Shutdown Phase". It closes all propellant valves, deenergizes all solenoid and torque-motor valves, and then it shuts itself off. The only part of the SSME still operating at that point is a small electric heater that keeps the controller from freezing.

The Orbital Maneuvering System’s thrust makes up less than 200 meters per second of delta-vee to bring the Orbiter up to full orbital speed. For from a few hours to a week the crew will perform experiments, deploy satellites and planetary probes, and retrieve others. When the mission is complete the crew will use the Orbital Maneuvering System to kick the Orbiter out of orbit and onto a reentry path.

Once the Orbiter has returned to Earth it spends an hour in the landing area while crew and passengers leave and ground crews give it a preliminary "safing". This safing ensures that the Orbiter if fit to be towed to the Orbiter Processing Facility. Preliminary safing of the SSMEs is a simple visual inspection. Exposed portions of the engines are examined for any structural damage that might compromise the engines’ safety during Orbiter towing.

Inside the Orbiter Processing Facility the ground crews jack up the Orbiter, level it, and place access stands around it. The engines are purged to remove any trace of propellant vapors and are dried to remove any moisture that may have settled on them. At the same time the engine heat shields are taken off to give maintenance crews access to the powerheads. (Those heat shields, fastened to the upper end of each SSME’s nozzle, are needed to protect the engines’ powerheads during the Space Shuttle’s ascent. The thermal radiation emitted by the engines’ exhaust plume is so intense that temperatures on the rear of the Orbiter go as high as 1490 Kelvin). When protective covers have been installed on the engines maintenance can begin.

Routine maintenance consists of automatic checkout and a complete external inspection of the engine. To those procedures have been added a heat exchanger leak test and turbopump torque tests since those specific components have been found to be sources of potential trouble.

Each engine’s controller performs automatic checkout. It’s pretty much the same as the preflight verification; the controller manipulates the engine in a preprogrammed pattern, reads its sensors, and compares the readings with the ideals stored in its memory. If the controller finds a discrepant reading, it identifies the Line Replaceable Unit responsible for the reading and notifies the maintenance crew that corrective maintenance is needed.

At the same time computers in the flight center will be performing an in-depth data analysis on each engine. Using checkout and flight performance data that the controller fed to the Orbiter’s flight recorder, the computer looks for discrepancies from ideal values. Then the computer calls up the engine’s "dossier", comprising data from previous checkouts and flights, and compares it with the current set of data to look for trends, to determine how the engine is evolving with use. For example, there will always be a discrepancy between what a sensor should read and what is does read. The computer will analyze how that discrepancy changes from flight to flight. The hope is that potential failures can be spotted well enough in advance that the appropriate part can be replaced before it fails and causes serious trouble.

External inspection of the engine will be needed after every flight to find faults that cannot be detected by the automatic checkout system. Inspectors will look for broken or deformed parts, parts knocked out of alignment, foreign matter, excessive erosion or overheating of parts exposed to combustion products, and any other damage that might affect engine performance.

The heat exchanger leak test has become an important item on the maintenance agenda since a heat exchanger failed during an engine test in late 1978. The heat exchanger is a coil of metal tubing that sits below the oxidizer-side preburner and bathes in the hot, hydrogen-rich gas from the preburner. Liquid oxygen flowing into the heat exchanger becomes a gas and flows up into the External Tank’s oxidizer compartment to pressurize it. If the heat exchanger were to break, gas from the preburner could flow up the line into the oxidizer compartment and cause an explosion that could lead to the loss of a Space Shuttle.

Four strategies are used to prevent such an occurrence. First, the coil is made twelve times tougher than theory says it should be. Second, the pressure in the tubing is kept sufficiently greater than the pressure in the hot-gas manifold at that point that hydrogen won’t be able to enter the line if a leak occurs. Third, if the pressure in the heat exchanger drops below a set value for any reason, the controller will shut down the engine and the Shuttle will either go into an abort sequence or try to make orbit on its other two SSMEs. And fourth, every flight will be followed by a leak test in the hope that a weakening heat exchanger will give itself away in time to be replaced before a failure can occur.

The turbopump torque tests, while not as vital as the heat exchanger leak test, are also important. Not surprisingly the high-pressure turbopumps have been a source of problems throughout the development program. Of those problems, only the blade cracking in the high-pressure fuel turbopump remains to cause concern. In tests the turbine blades have failed after the pump has run 3000 – 4000 seconds (considering that each blade, roughly the size of your thumbnail, transmits about 620 horsepower to the pump, perhaps we should be astonished that they last so long). That’s enough time for six to eight Shuttle flights but it’s not nearly as long as NASA and Rockwell want the turbopumps to last. Rocketdyne’s engineers are working to resolve the problem but until they succeed there remains the possibility of an inflight turbine failure and engine shutdown, there remains the desire to keep a close watch on how the turbopumps are evolving. Hence, torque tests after every flight.

Once maintenance is completed the crews close out the engine compartment and reinstall the engine heat shields. As far as the SSMEs are concerned the Orbiter is ready to be towed to the Vertical Assembly Building to be prepared for its next flight.

In addition to routine maintenance one engine is subjected to periodic maintenance after every fourth flight. That involves a complete inspection of internal engine parts (performed with fiberscopes fed through special access ports in the engine) and a complete leak test of the engine’s fluid systems. As with routine maintenance, this procedure is meant to keep track of how the engine is evolving and to make sure that it is not developing problems.

If a problem is found, then corrective maintenance will be performed based on the Line Replacement Unit (LRU) system. LRUs are those components specially designed to be replaced in the Orbiter Processing Facility and comprise those parts of the engine that are most likely to develop problems. The time required to replace, inspect, and reverify an LRU (including leak checks and functional checks) ranges from 2.3 hours for an accelerometer or a flow meter to 26.7 hours for a high-pressure oxidizer turbopump. Replacement of the engine itself would take 11.5 hours but that will be avoided if at all possible because engine removal would interfere with other systems in the Orbiter that a simple LRU replacement would leave unaffected.

After the Orbiter has been towed to the Vertical Assembly Building assembly crews will jack it up, attach erection slings, and retract the landing gear. Cranes will hoist the Orbiter, rotate it to the vertical, and carry it into the Shuttle integration cell where it will be mated to an External Tank/SRB set. The SSMEs are now back where they started, ready to barge another Space Shuttle mission into low Earth orbit.

But the Space Shuttle is not the sole application of the SSME; it’s only the beginning. Aerospace designers are already looking forward to the second generation of reusable spaceships. They’re already designing single-stage-to-orbit shuttles, heavy-lift launch vehicles, interorbital tugs, and lunar landers, most or all of them SSME-driven. They’re already preparing the technological foundation for the creation of Humanity’s first space civilization.

When the SSME contract was given to Rockwell in 1972 no one suspected that two and a half years later Gerard O’Neill would propose the founding of a civilization in space by the end of this century. Any thought that such a thing could be achieved with available technology would have been dismissed as very bad science fiction. Yet one of the first conclusions reached by the participants in the 1975 Ames Summer Design Study cosponsored by NASA and the American Society for Engineering Education was that the technology being developed for the Space Shuttle was adequate for the task of space colonization and in some ways superior to more exotic technology. An example of the latter is the finding that the SSME will be better than the NERVA nuclear rocket for barging the necessary payloads around Geolunar Space.

 

SSME

NERVA

Vacuum Thrust

2,090,660 Newtons

333,615 Newtons

Chamber Pressure

202 atmospheres

30.62 atmospheres

Area Ratio

77.5

100

Specific Impulse

455.2 seconds

825 seconds

Propellant (by mass)

85.71% Oxygen

100% Hydrogen

 

14.29% Hydrogen

 

Mass

3014 kilograms

15,646 kg (with shield)

Life

7.5 hours

10 hours

 

55 starts

60 cycles

Propellant Flux

466.3 kg/sec

41.22 kg/sec

TABLE FOUR: SSME vs. NERVA

Nuclear engines are more efficient than chemical ones, but for a long time to come the SSME will be the cheaper because it can replenish most of its propellant from material available in space.

At first the NERVA’s 825 seconds of specific impulse would seem to be a clear preference over the SSME’s 455 seconds. And the NERVA’s 10-hour service life certainly seems better than the SSME’s 7.5 hours. The SSME 12.6-tonne lesser mass and its nonradioactivity seem to be its only significant advantages. But for a long time to come the SSME will have another advantage that the NERVA and Dumbo, the NERVA’s more powerful cousin, will not have; it will be able to replenish nearly 86% of its propellant in space at very little cost.

One of the first consequences of space manufacturing using lunar regolith as raw material will be the production of oxygen as waste. Easily liquified by pumping it through a radiator shielded from the sun, that oxygen can be used to replenish the oxidizer tanks of SSME-driven tugs. According to figures derived for the 1975 Summer Study partial refueling of spacetugs with such "waste" oxygen will give the SSME an effective specific impulse of 721 seconds.

In fact, the situation is even more heavily weighted in favor of the SSME than that figure indicates. Given that liquid oxygen is cheaply available in space, the major part of the propellant cost for any mission will be the cost of barging liquid hydrogen into low Earth orbit, about 200 dollars per kilogram with SSME-driven tankers, a third of that with Dumbo-driven tankers. If a nuclear rocket is to compete with the SSME on the basis of hydrogen use, it will have to develop an effective specific impulse of at least 1430 seconds. Even the discovery of water on the moon would not necessarily make the nuclear engine cheaper to operate since it would likely reduce the cost of propellants so much that other cost factors would become important determinants of the more economical engine.

One of those factors is longevity. The NERVA’s 10-hour service life is deceptive. An SSME will actually be able to perform more missions than a NERVA will because, although it must shove around a greater mass of propellant for a given mission, its thrust is so much harder than a NERVA’s that the SSME will not have to burn as long. Less frequent engine replacement along with safer and less expensive maintenance and repair will be another "plus" for the SSME.

The upshot of that is that the Space Shuttle Main Engine looks like it has a long and spectacular career ahead of it. If all goes as planned, that career will begin on 1979 Nov 09 when the Space Shuttle makes its first orbital flight.

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SOURCES

1) Altseimer, J.H., G.F. Mader, and J.J. Stewart, "Operating Characteristics and Requirements for the NERVA Flight Engine," Journal of Spacecraft and Rockets, Volume 8, Number 7, 1971 July, pp. 766 – 773.

2) Anonymous, "Space Settlements, a Design Study," NASA, Washington, DC, SP-413, 1977.

3) Anonymous, "Space Shuttle Engines Stress Low Cost, Maintenance, Reuse," SAE Journal of Automotive Engineering, Volume 80, Number 10, 1972 Oct., pp. 21 – 28.

4) Anonymous, "Space Shuttle Main Engine Summary," Rockwell International Corporation/Rocketdyne Division, Canoga Park, CA, report prepared for NASA.

5) Anonymous, "Space Shuttle Technical Manual, SSME Description and Operation," Rockwell International Corporation/Rocketdyne Division, Canoga Park, CA, E41000/RSS-8559-1-1-1, 1977 Aug 01.

6) Dyer, H.L., and C.F. Warner, "Performance of a Staged Combustion Rocket Motor," Journal of Spacecraft and Rockets, Volume 9, Number 3, 1972 March, pp. 215 – 217.

7) Johnson, J. and H. Colbo, "Update on Development of the Space Shuttle Main Engine (SSME)," Rockwell International Corporation/Rocketdyne Division, Canoga Park, CA, 78-1001, presented at AIAA/SAE 14th Joint Propulsion Conference, 1978 July 25 – 27.

8) Parkinson, Gerald, "Engines for the Space Shuttle," American Machinist, 1975 Feb 01, pp. 36 – 39.

9) Schwinghamer, R.J., "Materials and Processes for Shuttle Engine, External Tank, and Solid Rocket Booster," NASA, Washington, DC, TN D-8511, 1977 June.

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2013 Addendum

In the period 1981 Apr 12 – 2011 Jul 21 the Space Shuttle made 135 flights without a major mishap in its Main Engines. That fact tells us that, although the Space Shuttle no longer flies, its Main Engines can still offer benefits to human spaceflight. In particular it can provide primary propulsion in the space around the smaller bodies of the solar system. Consider the obvious example:

Though it doesn’t generate enough thrust to lift a spaceship off Earth without help from stronger boosters, in the softer gravity of the moon an SSME could both land a ship and launch it again. With its two megaNewtons of thrust it could support the weight of a 1224.5-tonne spaceship against the lunar gravity. Of course a lunar lander based on one SSME would have to ponder less mass than that, but we still have the possibility of creating a craft with a substantial cargo capacity.

Imagine a spherical liquid-hydrogen tank with a spherical liquid-oxygen tank suspended at its center. That tank rests on top of the engine, landing gear, and support structure like a grapefruit held in an open hand. The command and service modules and the cargo pods sit on top of the tank or hang from its side. Thus we have a relatively simple craft that may serve as the workhorse of our initial spacefaring civilization, a craft that will carry cargoes and passengers from orbit to landings on the moon, Mercury, Mars, and the moons of the outer planets and thence back to orbit. Propelled by the Space Shuttle Main Engine, such craft will serve Humanity for decades to come, until someone devises a cheaper engine with greater thrust and specific impulse.

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