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Shuttle Era Testing

Engine firing in test stand.
Liquid oxygen cooling test in the RETF (2/15/1989).

After the Apollo Program, researchers used the Rocket Engine Test Facility (RETF) for a number of research programs that contributed to the development of propulsion systems for the space shuttle and advanced engine concepts.


In 1972, President Richard Nixon announced that the nation would focus on the development of a reusable shuttle for future space travel rather than expendable launch vehicles. NASA reorganized its resources to complete this mandate. The RETF’s low costs and flexibility proved invaluable for studying concepts that could be applied to the shuttle’s liquid hydrogen-fueled main engine. The RETF analyzed the durability issues for these enginesthat were expected to complete dozens of missions without failing.

The RETF continued to play an important role in propulsion technology development during the 1980s and the first half of the 1990s. The facility was significantly upgraded with the addition of a liquid hydrogen vaporizer and a second test stand (Test Stand B) which could test engines in simulated altitude conditions. The RETF tested liquid oxygen-cooled engines, high-area-ratio nozzles for shuttle-launched stages, and a low-cost rocket engine developed by the TRW Space and Technology Group.


Low Cycle Thermal Fatigue

The RETF conducted an extensive program in the 1970s to study low-cycle thermal fatigue for the reusable shuttle engines. Low-cycle thermal fatigue is the structural weaknesses of metals caused by exposure to hot gases during combustion. To facilitate the studies, RETF scientist Richard Quentmeyer came up with the efficient and cost effective idea of a reusable “plug nozzle.” These water-cooled plug nozzles, which simulated the interior of the combustion chamber, were constructed of various metal alloys and featured a number of different cooling passage designs.

NASA engineers lined the nozzles with various coatings to determine how quickly they would crack when subjected to cyclic thermal stresses.The engine was fired, shut off, and fired again in the RETF to replicate multiple uses. In some cases an engine was fired more than 300 times in one evening. Testing stopped when the engine began to leak. The results showed that most lining materials hardened and became brittle—a phenomenon that Quentmeyer termed “thermal ratcheting.” The tests verified that the original copper-silver-zirconium alloy was the best option, but the engine’s thrust chamber would have to be regularly replaced.

The RETF was also used to study different thermal barriers to protect the combustion chambers against the high temperatures produced by hydrogen combustion. This led to experimentation with high-aspect-ratio cooling channels in rocket engine liners. Lewis researchers found that the use of more than 400 cooling passages, each about ten-thousandths of an inch thick, reduced the temperatures within the chamber from 1,000°- F to between 400°F and 600°F and thus dramatically extended engine life. High-aspect-ratio cooling channels are used in all combustion-chamber designs for modern rockets today.


Liquid Oxygen Cooling

Since the beginning of rocketry, the question of how to cool a rocket engine has always been a concern, particularly for booster engines. In the late 1970s and 1980s, engineers at the RETF tested the first liquid oxygen-cooled engines built by NASA and explored the problems of using this unique cooling concept. Up until this point, NASA researchers were reluctant to use the cryogenic liquid oxygen to cool the engine because they feared that it could cause cracks in the engine that would be flooded with oxygen and likely to explode.

Between 1986 and 1990, Lewis researchers used the RETF to experiment with liquid oxygen cooling on five different combustion chambers with identical coolant passages. After four years of testing, they determined that liquid oxygen was a workable cooling agent and that the leaking of liquid oxygen into the engine did not affect the integrity of the combustion chamber. This technology has yet to be incorporated into American rocket engines.


Coaxial Pintel Injector

During the early 1990s, NASA Lewis worked with Thompson-Ramo-Wooldridge (TRW Inc.) to demonstrate the feasibility of operating a Coaxial Pintle Injector Rocket Engine in the RETF. The experimental engine had been previously tested with liquid oxygen and propane and liquid oxygen and RP-1 combinations, but never using liquid oxygen and liquid hydrogen as the propellant. The purpose of this effort was to demonstrate technology that would significantly reduce the cost of launching payloads into space. TRW Inc. provided the engine and NASA provided the hardware, piping, instrumentation, computers, software, and operations personnel to complete the testing at the RETF.

The first two phases of the program tested a 16,000-pound thrust engine, first with liquid hydrogen and liquid oxygen, then kerosene and liquid oxygen. The third phase, which built on the first two, tested a 40,000-pound thrust liquid hydrogen-fueled engine. The four-year program successfully demonstrated the pintle injector’s combustion stability and durability.


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