Think-Dash

Technology Readiness: A pair of advanced-technology development projects at NASA’s Marshall Space Flight Center illustrates how the agency is making the transition from work that had focused on the Bush administration’s back-to-the-Moon exploration approach to a more open-ended drive to advance technology readiness levels. Efforts behind the technology work that was once directed at the Constellation program’s goal of planting a human outpost on the Moon have been shifted to support NASA’s latest plans to push beyond low Earth orbit with human crews on an as-yet-uncharted route to Mars.

At Marshall, technology work is now directed toward the U.S. space policy hammered out between the Obama administration and Congress in the past two years. That policy cedes to the private sector human and cargo transport to low Earth orbit, beefs up research into enabling technologies for deep-space exploration, and starts work on a heavy-lift rocket that will send humans on lunar flybys before carrying them to deeper targets such as Apollo-like lunar sorties, asteroids, perhaps a Martian moon and eventually the surface of Mars.
Credit: FRANK MORRING, JR./AW&ST

When the configuration for the heavy-lift Space Launch System (SLS) was finally cleared by the White House in September, Marshall already was working on a critical technology that could help the big new rocket evolve to the 135-metric-ton capability—composite cryogenic propellant tanks. That same month the agency awarded Boeing a $24 million contract to build two composite liquid-hydrogen tanks, with a goal of saving 30% on weight and 25% on cost compared to aluminum tanks. The cost savings would come from out-of-autoclave manufacturing, which avoids the cost of building pressure vessels large enough to bake big propellant tanks and relies instead on unpressurized ovens that can heat the tank material to 350F (AW&ST Oct. 3, p. 54).

Marshall engineers have experience with trying to contain liquid hydrogen in a composite tank, and it isn’t good. The X-33 suborbital reusable launch vehicle testbed project foundered when a composite tank failed during testing at Marshall. The engineers hope the use of thinner composite plies and a fluted-core configuration will prevent the microcracking that causes leaks, but they concede it will be difficult.

“Out-of-autoclave materials today don’t have quite the performance that autoclave materials do, and we’re asking those materials to perform at that same high level,” says John Vickers, manager of the composite cryotank technology demonstration project. “So that’s a technology challenge.”

Boeing will build a 2.4-meter-dia. (8-ft.) and a 5.5-meter-dia. liquid hydrogen tank for testing at Marshall. The larger tank design is scalable to a 10-meter diameter, and could help the SLS achieve its 130-metric-ton performance goal after initial flights with aluminum tanks, Vickers suggests. The project is also working closely with other launch vehicle “stakeholders,” including the commercial launch companies that might one day use the technology.

During the Constellation years, Marshall worked with the Energy Department on nuclear-power technology for a lunar outpost. While Los Alamos and other national labs handled the radioactive material, NASA experts used heating elements to simulate nuclear fuel and concentrated on power systems to generate electricity on the Moon.

That work continues, but it has expanded to encompass another technology goal of the new policy—advanced in-space propulsion. To test the way different materials react with the hydrogen at high temperature and pressure, a nuclear-thermal rocket environmental simulator flows gaseous hydrogen over heating elements that mimic different nuclear-fuel configurations.

“Before you move into any nuclear testing, you have a good feel for how those elements might behave,” says project engineer Dave Houts.



Frank Morring, Jr./AW&ST

The setup (see photo) includes a mass spectrometer and optical pyrometers that monitor temperatures and materials performance during runs. The work pre-screening simulated nuclear elements could help humans use radioactive fuel to reach Mars and other distant destinations faster, reducing time in the dangerous space-radiation environment.

“Because you can use hydrogen as a propellant, which has a very low molecular weight, a nuclear thermal rocket allows you to get a very high specific impulse even at reasonable material temperatures,” Houts says. “We should be able to able to achieve a 900-second specific impulse or better, so you’re roughly twice that of a chemical engine. We think we can go up from there using some of the advanced materials, advanced cycles and advanced geometries in the actual system.”