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Nine months of testing proves integrity of tank manufacturing process boosts the confidence in using composite fuel tanks for reusable space transportation systems.
Engineers from Northrop Grumman Corporation and NASA’s Marshall Space Flight Center, have proven that a new type of cryogenic fuel tank made from composite materials has the structural integrity to withstand the mechanical and thermal stresses associated with repeated fueling and simulated launch cycles.
The nine-month, 40-cycle series of tests, concluded last month, is part of Northrop Grumman’s ongoing partnership with NASA to help mature space technologies required to develop safe, affordable and reusable space transportation systems. The test program began in November 2003.
“”These tests prove that it’s possible to build a lightweight fuel tank that’s not only a safe, reliable container for liquid hydrogen,”” said Drew Smith, NASA’s cryotanks project leader, “”but also a durable, reusable component that can also help us reduce the costs associated with acquiring and operating a reusable launch vehicle.”” Composite cryogenic fuel tanks also offer a 10 to 25 percent weight savings over conventional aluminium fuel tanks, he added, which could enable larger payloads in the future.
Liquid hydrogen is an essential but highly volatile fuel used in the combustion process that propels rockets. It must be stored and used at -423 degrees Fahrenheit, a temperature that causes most materials to become quite brittle. Liquid hydrogen also has an extremely fine molecular structure, which allows it to seep through the tiniest of holes.
The composite tank used for the tests was a 6-foot-diameter, 15-foot-long tank produced by Northrop Grumman as part of NASA’s Next Generation Launch Technology program. The tank was fabricated and cryo-structurally tested at the agency’s Marshall Space Flight Center.
A Northrop Grumman source informed NetComposites that the composite cryogenic fuel tanks were constructed out of a composite sandwich structure with graphite/epoxy facesheets and a perforated non-metallic honeycomb core.
“”Each cycle in our test program consisted of filling the tank with liquid hydrogen, pressurizing to an internal pressure of 113 pounds per square inch, then subjecting it an axial load to simulate the stresses experienced by a rocket during launch,”” explained Tod Palm, Northrop Grumman’s cryogenic tank project leader. “”Nine months and 40 cycles of testing and monitoring the composite test tank for leaks has given us the confidence that this type of cryogenic fuel tank can be safely and repeatedly launched, recovered and reused for next-generation space missions.”” An axial load is applied along the vertical axis of the launch vehicle.
Much of the team’s success in the test program, added Palm, can be attributed to key technical advances made by Northrop Grumman and NASA in designing and constructing the composite tank. The tank is approximately one quarter of the projected size (27.5 feet in diameter x 80 feet long) of a fuel tank envisioned for some reusable launch vehicle concepts.
The composite tank technology demonstrated has potential applications not only as cryogenic fuel tanks for Earth-launched space vehicles, but also as on-orbit storage of cryogenic fuels such as liquid hydrogen or liquid oxygen. This orbiting “”fuel depot”” would be used to fuel space vehicles traveling from low-Earth orbit to the moon, Mars or beyond.
Northrop Grumman’s work for NASA on the cryogenic fuel tanks was done as part of a three-year series of Next Generation Launch Technology contract options that began in June 2001. The contracts, collectively worth approximately $30 million, included work on permeation-resistant composite cryotanks, development and refinement of new manufacturing processes that will allow the company to build large composite tanks without an autoclave; and design and engineering development of conformal fuel tanks appropriate for use on a single-stage-to-orbit vehicle.
Lessons learned from these contracts are expected to help NASA continue to mature technologies needed to develop and launch space systems envisioned under Project Constellation, a core part of the Vision for Space Exploration.
The use of composite tanks eliminates many current engineering costs associated with mating aluminum cryogenic fuel tanks to composite launch vehicle structures. Aluminum and composite materials expand and contract at significantly differently rates when exposed to extremely hot or cold temperatures. This “”mismatch”” in the materials’ coefficients of thermal expansion creates mechanical stress at joints where aluminum structures are attached to composite structures. Using composite materials for fuel tanks would effectively eliminate the need for specialized engineering solutions at critical joints between fuel tanks and the launch vehicle.
Northrop Grumman is using a half-tank composite demonstration structure as a test bed to evaluate non-destructive (NDE) techniques for detecting and identifying manufacturing defects in large composite structures. The tank was produced with areas of intentional, known defects to evaluate how well quality control processes can detect and locate known or unknown defects.
The company is using two NDE techniques to examine the workmanship of the half-tank: thermography and laser shearography. Both techniques will be evaluated for potential future use in manufacturing large reusable composite fuel tanks.
Northrop Grumman has delivered the first composite fuel-tank structure to be made without using an autoclave, demonstrating the viability of the new process for making large, reusable fuel tanks. The photo shows the 10.5-foot-diameter half tank ready for inspection prior to its delivery to NASA’s Marshall Space Flight Center. The new composites manufacturing process offers the promise of fuel tanks 10 to 25 percent lighter than comparably sized single-use aluminium tank, which could lead to heavier payloads and lower operating costs for reusable space vehicles.
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