Chair Force Engineer

Tuesday, November 11, 2008

The Shuttle Legacy

When Project Apollo was shut down, the most tragic aspect of it was all of the useful technologies that were lost as tooling was destroyed and experts were reassigned to other programs. The mighty F-1 engine was relegated to museums, ceding the kerosene-engine market to the Russians. The demise of the J-2 engine has led to an expensive development program for the new J-2X that will be used on the Ares launchers. Even the exact formulation and processes for creating the Apollo capsule's ablative heat shield were lost to time, complicating the effort to develop the Orion heat shield.

As the Space Shuttle winds down, it appears that the same mistake is not being repeated, at least not on the same scale. In taking stock of the program's technical accomplishments, many of them are being preserved or leveraged for the Ares and Orion systems. Of the ones being discarded, they have served as lessons for ways that a reusable launch vehicle should not be built.

When NASA transitioned from Saturn to Shuttle, significant propulsion elements had to be re-developed. While Space Shuttle Main Engine owes a lot to the J-2 program, it is a much bigger engine with higher specific impulse and thrust, a more complex staged combustion cycle, and built-in reusability. The solid rockets were a massive undertaking in many ways, eclipsing any solid rocket with flight history up to that point in time.

Some elements of the propulsion system will remain relevant for the Ares generation. The shuttle solid rocket boosters will be leveraged for the boosters on the Ares rockets. While the new boosters are a leap beyond the current SRB, it's not as far of a leap as the original SRB was when compared to its predecessors. Additionally, the shuttle's maneuvering engines are being re-used for Orion. This may be the only element of the shuttle system that is reused with no changes.

For other shuttle developments, they will best serve as lessons learned in the development of equivalent systems. Perhaps the shuttle's most remarkable achievement was its main engines. Nevertheless, the SSME's taught us a lot of the ways not to design an engine for producibility or reliability. The high chamber pressure and staged combustion cycle ensured high performance, but required lots of ground support equipment. Thousands of tiny welded tubes in the nozzle and chamber for cooling? It was state of the art for the 70's, but channel-wall cooling is the preferred method nowadays. The RS-68 benefited from the lessons of SSME, sacrificing specific impulse in favor of producibility. Its gas-generator cycle, lower chamber pressure, and channel-wall chamber with ablative nozzle make for a much cheaper engine. Its only drawbacks when compared to SSME are specific impulse (which can be increased with a redesigned injector and regen-cooled nozzle) and lack of reusability. In fact, a channel-wall nozzle was planned as a shuttle upgrade until the program's 2010 retirement was announced.

At the same time, a lot of the shuttle's pioneering achievements in the field of re-usability are being discarded as dead-ends which taught us how not to build a reusable launcher. Case in point is the shuttle's thermal protection system. While the blankets will likely find use on the cooler surfaces of a future reusable launcher, the other heat shield materials will likely be dismissed. The ceramic tiles still are remarkable, but they form a complex system that is difficult to maintain. Reinforced carbon-carbon had incredible abilities to stand up to high temperatures on the shuttle's nose cap and leading edges, but they were too brittle to reliably ensure safe reentry. A future reusable launcher will likely be a "fluffier" design along the lines of X-33, which can get by using a robust, metallic thermal protection system.

Overall, NASA and the industry are taking a wiser approach to the end of the shuttle program than was taken at the end of Apollo. Many critical technologies are being reused, albeit in expendable rockets. The clear succession from Shuttle to Ares is mainly in the field of propulsion, where breakthroughs during the shuttle's development have reduced the risk for Ares. The enduring challenge from the shuttle program is to learn the correct lessons from the reusability concepts that proved so difficult to implement on the operational shuttle.

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