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© 1993 by Donald F. Robertson.



This article may be distributed at will, but only if it is not changed in any way, and only if the author's name, the copyright notice, the name of the journal it first appeared in, and this notice remain attached. In addition, this article may not be sold for money, or published for sale in any way, without the author's prior written permission.

This article originally appeared in substantially different form in Space and Communications.



Donald F. Robertson

Setting aside the Space Shuttle's many operational failings, the Orbiter's main engines are a technological wonder. Only superlatives can be used to describe them. They burn with unprecedented temperatures -- over six thousand degrees Fahrenheit -- while handling fuel more than four hundred degrees below zero. The pressures and power densities are fully three times higher than any engine used in the Apollo project to land people on Earth's moon. In many ways, the Space Shuttle Main Engines may be the United States' supreme technological achievement.

Nobody but the Japanese is even trying to duplicate the feat. And they have taken twenty years and many failures to achieve a similar, but non-reusable, engine on a much smaller scale. Since the trend today is to back away from performance in favor of safety margins and reliability, and since deep space propulsion is slowly moving away from chemical rockets toward higher energy systems, it is possible that the SSMEs will remain the most efficient large chemical rocket engines ever built.

The SSMEs also are the most reliable of today's rocket engines. In 168 engine flights, there have been no critical failures at all, just one early shut-down in flight caused by a sensor problem, and only four shut-downs on the pad, according to Rocketdyne. Challenger was destroyed by a Solid Rocket Booster leak. This main engine reliability has been achieved despite engines being re-use as many as fifteen times.

Including test failures, these figures equate to a total reliability of 0.9991 in over 500,000 seconds of operation, according to Dick McMillion, Rocketdyne's Director of Advanced Chemical Propulsion. That reliability, and the engine's operating experience, "has not been approached by any other rocket engine except the [Apollo] F-1," said McMillion. The SSMEs have twice the total experience of all the F-1 Saturn-V first stage engines ever fired in the Apollo project. The F-1 had a reliability of .998 with 250,000 seconds of operation.

None of this means that the SSMEs have no faults. Individual SSMEs are expensive, costing about $40 million each, according to Rocketdyne. This is only slightly less than the entire cost of a Delta launch. Their very complexity makes them delicate creatures, requiring crews of costly engineers to tinker them into reliable operation. Report after report has argued that operating the SSMEs remains a risky proposition; their high reliability record may reflect more on Rocketdyne than on the engines themselves. NASA considers them one of the most likely routes to another catastrophic Space Shuttle failure.

Nonetheless, Rocketdyne officials believe that the SSME's very high thrust-to-weight ratio makes these already-existing engines ideal for Single-Stage-to-Orbit applications. Boeing has chosen the engine for their Space Lifter stage-and-a-half design for the Air Force.

By burning non-stop from the ground almost into orbit, McMillion said, "The SSME has been flying that Single-Stage-to- Orbit type mission all its life, since 1981. Being a fully reusable engine, and having the highest performance of any engine in the world, the SSME has all the attributes necessary for an SSTO engine. The thing that has changed over the years is that the aircraft structures, now, have become quite a bit lighter, which enables an SSTO-type vehicle" utilizing the Space Shuttle Main Engine's performance.

Rocketdyne also argues that it would take a long and expensive development program to duplicate the SSME's already- existing capabilities. "While there are other concepts for SSTO propulsion systems, they are only on paper," according to Terry Murphy, Rockwell's Program Manager for Advanced Propulsion Systems. "It takes a long time to get to a system that is this mature, one that you would be willing to put onto" SSTO vehicles.

Asked about the European Ariane-V engine, Murphy said, "The Vulcan is a good engine, but, when you start getting up into [Single-Stage-to-Orbit] types of missions, specific impulse is everything. The Vulcan just does not have the specific impulse," since it is not a staged-combustion engine.

Most modern rocket engines pre-burn fuel, then use the resulting gas to run turbo-pumps. The turbo-pumps, in turn, pump the fuel and oxidizer into the combustion chamber. Most engines then dump the exhaust from the turbo-pumps overboard. "A staged- combustion engine exhausts the fuel-rich gas from the turbo-pumps into the main injector," instead of to the atmosphere, said Murphy. There, this gas "is burned with additional oxygen before exiting the main combustion chamber," contributing to the overall thrust. "This results in a much higher efficiency than if the turbo-pump exhaust is dumped overboard, as in the Vulcan and most competing high-energy engines."

The only exception is the new Japanese LE-7 engine under test for their new launch vehicle. This is a staged-combustion engine like the SSME. However, it is not designed to be re-used, which essentially eliminates it from the SSTO competition. The LE-7 also has too low a thrust, according to Murphy, although McDonnell Douglas' DC-X and other SSTO-demonstrator projects are looking for an engine smaller than the SSME.

Murphy added, however, "I would say that the Japanese are next closest" to a working SSTO engine, after the United States. The Russians have demonstrated a staged-combustion engine, but "it uses a kerosene-based fuel, which offers far too low a performance for an SSTO vehicle," said Murphy. "Other engine concepts, such as tri-propellant [engines], have not progressed beyond the design phase, and remain to be proven."

An improvement program is slowly addressing some of the Space Shuttle Main Engine's problems by simplifying and reducing the part-counts of many of the engine's components. New computerized controllers are now beginning to fly. Rocketdyne is conducting a "Block-1" re-design of part of the engine for flight in 1995, according to Sally Stohler, Rocketdyne's Director of SSME Marketing. Block-1 changes include a new Pratt & Whitney oxidizer turbo-pump, and changing the three-duct hot gas fuel manifold power head to a two-duct design.

The hot gas manifold power head tubes transfer fuel-rich turbine exhaust from the pre-burner to the main combustion chamber. Computational Fluid Dynamics analysis showed that the two-duct design reduced pressure gradients within the system, and eliminated pressure variations, said Murphy. These changes reduced the pressure and temperature extremes inside the engine.

Block-2 changes, intended to fly in early 1997, include a new fuel turbo-pump, and widening the thrust chamber throat. The latter change again reduces the total pressure differentials within the engine, reducing stress, which should extend engine life. Rocketdyne is looking beyond these first two block changes, to new low-cost advanced-technology components still in the conceptual stage, but the company declined to talk about these ideas in detail.

SSMEs may be the best engines flying, but reducing engine weight may still prove essential for true Single-Stage-to-Orbit flight. Stohler said it may be possible to reduce the weight of the nozzle with new materials. However, she added, "The original challenge of the Space Shuttle Main Engine was keeping the weight very, very low. Without introducing more advanced technology or simplifying the engine, it would be tough to take more weight out." An SSME currently weighs about 7,000 pounds [3,200 kilograms].

Nonetheless, Rocketdyne argues that, if a new large launch vehicle is developed, it would be cheaper to continue incremental improvements to an existing engine than to develop something new from scratch. Asked if there were any limiting factors on how long SSMEs could be kept flying, McMillion said that upgrades will continue, and "we are going to be marketing this engine for the foreseeable future." He expects no problems keeping the vendor base in line.

The Rocketdyne officials declined to comment on whether Rockwell, the owner of Rocketdyne, is designing a new launch vehicle to use the SSME. However, a Congressional source critical of the SSME pointedly told Space and Communications that they were not.

McDonnell Douglas' DC-X Single-Stage-to-Orbit (SSTO) project has argued that both the SSME and the new Space Transportation Main Engine are far too large, even for the operational DC-Y. NASA's STME is likely to be canceled if Congress declines to fund the Air Force's Space Lifter project.

For a smaller, demonstration SSTO like the DC-Y, a new rocket mid-way between Pratt & Whitney's RL-10 and the SSME or STME would need to be developed, possibly from the old Apollo J-2s. The Rocketdyne officials agreed, also pointing out that Shuttle engines will throttle only to sixty-five percent power. A vertically landing vehicle like the DC-Y design needs engines that can throttle almost to zero. For large SSTO vehicles that land horizontally on a runway, Rocketdyne believes the SSMEs remain ideal.

Futuristic Single-Stage-to-Orbit rockets may be beyond the United States' means, but Rocketdyne is actively marketing the SSME to many of the companies competing in United States Air Force's Space Lifter competition. Boeing chose the Shuttle engine for a partially reusable stage-and-a-half design. Rockwell said that there are other customers for the engine who wish to remain confidential.

Vince Caluori, Boeing's Space Lifter Program Manager, said that his company is trying to address three issues with their Space Lifter design: low development cost, high mission and dispatch reliability, and low cost per flight. The Space Shuttle Main Engine is relevant to all three.

The Boeing Space Lifter -- which Caluori called his "Ariane- killer" -- is a conservative design, generally avoiding new technology. "This vehicle is straight-forward as can be, we've got an aluminum tank, no exotica, no fancy materials," said Caluori. It is intended to place 30,000 pounds [13,600 kilograms] in low Earth orbit or 10,000 pounds [4,500 kilograms] in geostationary orbit

The Boeing rocket is a very conventional stage-and-a-half design, similar to the Atlas. It is propelled by two recoverable "engine modules," each containing two engines, mounted at the base of rocket. Like the Atlas, the engine modules feed from the central fuel tank, but unlike the Atlas, the central core does not ignite until the engine modules shut down almost in orbit. A number of different, specialized cores and upper stages are assembled in different configurations to vary the vehicle's performance. The cores and upper stages are expendable.

"Using the same booster, tank module, and engine modules, widely differing payload ranges are served by changing the expendable upper stage," said Caluori. "Delta- and Atlas-class payloads would use a simple low-energy upper stage, while Titan- class payloads use a cryogenic stage."

In designing their Space Lifter, the first issue Boeing addressed was low development cost. Caluori said, "We have to deliver a system that the country can afford, and the country has spoken clearly that we cannot afford ten or twelve billion dollars." Boeing thinks they can develop their Space Lifter for under $5 billion. The company intends to achieve that by avoiding development of a new engine and by addressing the entire payload range with a single vehicle.

Once the decision was made to go with an existing engine, Boeing believed there was only one real choice. "The best existing engine in the whole world, and the only reusable engine, is the SSME," said Caluori. Sounding remarkably like the salesman, rather than the customer, Caluori said, "It has the highest performance, the best thrust-to-weight. At reasonable power levels it has proven to be extremely reliable, it is a continuing program, it is throttleable -- it has all the features you would like to have." Caluori admitted, "There is no question we would like to have a more robust engine," but he believes Rocketdyne's on-going improvements should address that issue. In addition, "Engine integration is always the big issue on a fairly conventional rocket," said Caluori. With SSME's, "I've got a known factor in the engine," which should reduce development risk.

The second issue involved reliability. Boeing believes that propulsion lies at the root of most launch vehicle failures. The company looked at their experience with commercial aircraft -- the company's principle product. Modern commercial airliners have only two engines, which means that in an emergency one engine must be able to power the entire airplane. The engines are vastly over-powered and "idle along" at well below their rated power during normal operation: the engines are not stressed.

Applying the same philosophy to a launch vehicle led Boeing toward very large engines, that is, toward SSMEs. Their vehicle uses four SSMEs -- two in each booster -- each operating at seventy-five percent power. This has three advantages: rocket engines operating a low power are far less likely to fail at all; when they do fail, they "almost assuredly" will not do so catastrophically since the engines are operating at relatively low pressure; and after a non-catastrophic failure the launcher can continue its mission on the three remaining engines, now operating at one-hundred percent power. "For the first time," said Caluori, "we are talking about a vehicle that is going to cruise to space at `cruise power'," resulting in "an incredible increase in reliability."

The dispatch reliability of an airliner is directly related to the amount of avionics redundancy. The same is true of a space launch system. Boeing added four complete strings of avionics to their Space Lifter so that a failure could be tolerated on the ground without giving up the flight. This also is based on experience with Boeing's Inertial Upper Stage. Caluori said that the IUS is the first and only United States automated rocket with completely redundant avionics, and he claimed that redundancy has saved at least three missions after the first avionics string failed.

Most launch delays are caused by last minute launch pad repairs to the avionics. Having four complete avionics strings allows most maintenance to be deferred until after a mission has flown and landed, avoiding work on the launch pad.

The third issue was low cost per flight. That is achieved by recovering all of the expensive hardware: the SSMEs, the avionics, the electrical power systems, auxiliary propulsion, and the thrust vectoring system. These systems are packaged together in very simple, oval-shaped ballistic recovery modules. The recovery modules fly almost all the way to orbit, then the ballistic module separates from the core vehicle using standard explosive bolts. Prior to reentry, each module is oriented with nitrogen thrusters. Large parachutes open to gently place the engine modules into the ocean for recovery. An inflatable "door" covers the engines. Successfully recovered pods are reused "as is" after minor refurbishment, simply attached to a new core stage.

Rocketdyne's Sally Stohler said she thinks the Space Shuttle Main Engines are still capable of the original fifty-five re- flight goal. Because of its current low flight rate, the Shuttle program is aiming for only thirty reuses. Boeing is planning only ten to twenty re-uses for economic reasons.

Asked if end-of-life engines that have flown too often to be safe on the human Shuttle might be sold cheap to Boeing or another customer for automated flight, Stohler said, Yes. But since NASA owns the engines that would be up to the space agency. Caluori was unimpressed: "We [Boeing] don't want any old engines since we would not throw them away." Stohler added that all of the Shuttle flight engines are still being flown; none are even close to their end-of-life. Rocketdyne has essentially completed all new engine builds, and no further engines are planned for the life of the Shuttle program. The new upgrades will be retrofitted later to the existing engines.

Boeing's bottom line? "We think we can launch for $50 million dollars per flight," said Caluori. "$50 million gives the reliability we have, which nobody can match with any of those [Russian or Chinese] vehicles. It looks very attractive, especially if we double-manifest," or carry two satellites at a time, as on Europe's Ariane. Referring to Ariane, Caluori said, "I think we've got something that will hurt."

Boeing has spent $10 to $15 million of both company and government money testing the recovery system, since that is the only part of the launch vehicle that is not conventional. Caluori thinks Boeing's chances of winning the Space Lifter competition "are excellent. I think we've got a really good reception from our customers on this." In proof, Caluori sited Department of Defense's continued willingness to fund Boeing's SSME recovery tests during massive overall cuts in military spending.

In the absence of a new Space Lifter launch vehicle, Rocketdyne sees an on-going market for refurbished Space Shuttle engines. Whether the United States develops a new launch vehicle, or continues with the existing fleet, "The SSME is going to play a major role," said McMillion. "The Space Transportation System will certainly keep flying for a long time. And then if something does happen in these other arenas, the SSME is certain to be a player there."

Something had better happen in another arena, according to Boeing's Caluori. "We think our space program is in a dead spiral, we need [a new launch vehicle] as soon as possible, and we certainly need it to maintain any kind of international competitiveness. We're losing this [commercial launch vehicle] battle left and right. With the Russians and Chinese coming on as strong as they are, how [long] are we going to sit back" and wait for some new technology before getting started on a real, working vehicle. "We don't have the money, and we don't have the time" to wait for exotic technology.

Once more sounding more like the salesman than the customer, Boeing's Caluori was barely able to contain his anger when discussing the way the Space Shuttle Main Engine is treated in the press and on Capital Hill. Because of its association with the Shuttle, the SSME "is perceived in a terrible light. We have invested maybe five billion dollars in this engine. It is a remarkable, incredible, absolutely world-class engine. And we are willing to walk past it because some people have some poor perceptions of it.

"I can't stand to see that as a taxpayer, let alone as an engineer."


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