NASA Tests Environmentally Friendly Rocket Fuel

Two years of collaboration between Stanford University, Palo Alto, Calif., and NASA's Ames Research Center, Moffett Field, Calif., have led to the development of a non-toxic, easily handled fuel made from a substance similar to what is used in common candles. The by-products of combustion of the new fuel are carbon dioxide and water; unlike conventional rocket fuel that produces aluminum oxide and acidic gasses, such as hydrogen chloride.

"There is great cost in making, handling and transporting traditional solid rocket fuels, but the new paraffin-based fuel is less expensive, non-toxic and non-hazardous," said Greg Zilliac of Ames. "Because the fuel is very stable and environmentally friendly, a hybrid rocket could be fueled at the launch site rather than at the factory, thereby saving money," he added.

The main goal of the NASA test program is to determine if the promising results of earlier bench-top experiments conducted at Stanford will scale up to the combustion chamber conditions required for space launch operational systems. "The NASA combustion tests have been very promising and indicate the burn rate for the larger-scale apparatus is as high as that achieved in the small-scale Stanford tests," Zilliac continued. "This new fuel could significantly impact the future of space transportation," he said.

"A hybrid rocket equivalent to the Space Shuttle's solid rockets would be about the same diameter, but would be somewhat longer," said Stanford University Professor Brian Cantwell. "Hybrid rockets, using the paraffin-based fuel, can be throttled over a wide range, including shut-down and restart. That's one reason why they could be considered as possible replacements for the Shuttle's current solid rocket boosters that cannot be shut off after they are lit," he said. "One design concept being considered is a new hybrid booster rocket that is able to fly back to the launch site for recharging," he added.

A hybrid rocket uses a liquefied oxidizer that is gasified before being injected into the combustion chamber containing the solid fuel. Upon ignition, a flame develops over the fuel surface causing the solid to evaporate, thereby sustaining the combustion. Because current hybrid fuels, other than paraffin-based fuels, cannot sustain a high combustion rate, they have found only limited application and are not commercially viable for space applications. Tests at Stanford and Ames have shown the new paraffin-based fuel has a burn rate that is three times greater than that of other hybrid fuels.

Scientists are testing the new fuel at the Ames Hybrid Combustion Facility. The first successful test in the series took place on Sept. 24, 2001. The heavy-duty test chamber can accommodate pressures up to 60 atmospheres.

The first phase of the program included approximately 40 runs. A new combustion chamber with sapphire windows will soon be installed to allow researchers to observe the combustion process using optical instruments. Scientists will study the underlying physical processes that produce the fuel's high performance.

NASA engineers will conduct roughly 200 test runs during the lifetime of the project. A maximum of one test will take place per day, each lasting 20 seconds or less. The concept of a fast-burning, low-cost, paraffin-based fuel was originally conceived by Dr. Arif Karabeyoglu of Stanford, Dr. David Altman, president of Space Propulsion Group Inc., Menlo Park, Calif. and Cantwell. Karabeyoglu developed the theory in his doctoral thesis that was partially supported by Stanford and NASA. He leads the Stanford contribution to the fuel research.