Non-autoclave manufacturing process proven viable for making large, reusable fuel tanks

The viability of a reusable launch vehicle got a major technological boost recently when Northrop Grumman Corporation (NYSE:NOC) delivered the first composite fuel-tank structure to be made without using an autoclave. The new manufacturing process, which offers the promise of fuel tanks 10 to 25 percent lighter than comparably sized single-use aluminum tanks, would enable heavier payloads and lower operating costs for reusable space vehicles.

An autoclave is a pressurized oven used to heat, shape and cure composite structures. To date, one of the biggest barriers to producing large composite structures has been the cost of building a suitable autoclave, which is generally proportional to the size of the structure.

Northrop Grumman Integrated Systems delivered one-half of a 10.5-foot diameter composite fuel tank to NASA’s Marshall Space Flight Center, Huntsville, Ala., on Oct. 10. The half-tank was produced as part of a contract option under NASA’s Next Generation Launch Technology program, an effort to develop and mature technologies required for a next-generation reusable launch vehicle.

"Producing the half-tank taught us a lot about this non-autoclave process and convinced us that we could scale it up to produce significantly larger and very reliable composite fuel tanks," said Tod Palm, Northrop Grumman’s cryogenic fuel tank project leader.

NASA’s vision for a reusable launch vehicle calls for a tank up to 27.5 feet in diameter and 80 feet long. The tanks would be used to store liquid hydrogen, an essential but highly volatile fuel used in the combustion process that propels rockets.

According to Palm, the next phase of tank development will involve refinement of non-destructive techniques for evaluating the quality of the half-tank manufacturing process.

"This tank is not a structural test article but rather a demonstration structure, so we included in its construction an area of intentional, known defects called a standards panel," he explained. "We’d like to see how good our quality-control processes are at detecting and identifying these defects. We’d also like to see how good our processes are at finding any unknown defects."

Northrop Grumman will use two types of non-destructive evaluation techniques to examine the workmanship of the half-tank: thermography and laser shearography. The company will evaluate both techniques for potential future use in manufacturing large reusable composite fuel tanks.

Thermography flashes infrared energy on a composite structure, then uses a heat-sensing video camera to create a thermal "map" of the structure. By analyzing consecutive thermography images, engineers can trace the flow of heat through the part. Defects in the composite material will show up as visual "irregularities."

Laser shearography involves scanning the surface of the composite part with a laser, imparting a mechanical load or strain on the part, and then scanning the surface again. Any discontinuities or defects in the composite material, such as an improper bond between the face sheet of the tank wall and its internal honeycomb structure, will show up as a "bubble" or other visual distortion.

Northrop Grumman’s work on cryogenic fuel tanks is part of a larger initiative by the company to help NASA and the Department of Defense achieve the goal of affordable, reusable access to space. Leveraging its legacy expertise in large-scale aircraft systems integration; precision strike; avionics; and autonomous, unmanned surveillance platforms, the company has been working closely with NASA since May 2001 to define operational requirements, concepts and critical enabling technologies for a reusable, two-stage-to-orbit launch vehicle. Northrop Grumman’s work includes not only cryogenic fuel tanks, but also integrated vehicle health management systems and autonomous flight management systems to increase launch vehicle safety and reliability. The company is also blending aircraft science with rocket science to help the Department of Defense define concepts and critical enabling technologies for hypersonic cruise vehicles capable of conducting precision strikes from the continental U.S. to anywhere in the world in two hours or less.

Northrop Grumman Integrated Systems is a premier aerospace and defense systems integration enterprise. Headquartered in El Segundo, Calif., it designs, develops, produces and supports network-enabled integrated systems and subsystems for government and civil customers worldwide. Integrated Systems delivers best-value solutions, products and services that support military and homeland defense missions in the areas of intelligence, surveillance and reconnaissance; space access; battle management command and control; and integrated strike warfare.

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