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The greening of rocketry

Published by Sigurd De Keyser on Wed Oct 27, 2004 10:32 am
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By Alan Boyle MSNBC.com: SpaceShipOne used an innovative “rubber-and-laughing-gas” hybrid rocket engine in its history-making spaceflights — primarily because of the safety associated with the fuel (hydroxy-terminated polybutadiene, or HTPB, a common ingredient in tire rubber) and the oxidizer (nitrous oxide, which is the “laughing gas” once widely used as an anesthetic).

Unlike most conventional rocket fuels, the chemicals can be transported without fear of explosion, and if something had gone wrong in flight, SpaceShipOne’s pilots could have shut down the engine merely by closing the valve on the nitrous oxide supply.

But how do such engines score on the environmental scale? That’s the question that was on the mind of Cosmic Log reader Mark Sean.

“I’ve opened inquiries at both the EPA and the OAR [the Environmental Protection Agency and its Office of Air and Radiation], but have not gotten any substantial response, either confirming or denying the safety of burning HTPB’s and nitrous oxide,” Sean wrote.

Environmental clearance was one of the elements required for SpaceShipOne to get its launch license from the Federal Aviation Administration. In the rocket industry, the HTPB/nitrous oxide system is considered a relatively benign way to launch a spacecraft. For specifics, I turned to the aRocket discussion list — and one of the experts who responded to my inquiry was Korey Kline, propulsion designer and director of research and development for Florida-based Environmental Aeroscience Corp.

EAC was one of the two companies in the engine competition for SpaceShipOne, and although California-based SpaceDev won out, Kline’s company still played a supporting role in the rocket plane’s success — and Kline is a big proponent of hybrid engines that use nitrous oxide, or N2O.

“The N2O hybrids have relatively clean exhaust products, second only to LOX [liquid oxygen]/hydrogen!” Kline wrote. “The oxidizer-to-fuel ratio is typically 6 to 1, so the bulk of the exhaust gases are superheated nitrogen from the N2O. As you know, the air we breathe is typically 78 percent inert nitrogen.”

Even when small amounts of metals are added to the HTPB-based fuel, the exhaust contains less carbon dioxide and carbon monoxide than traditional liquid oxygen/kerosene propellants, Kline said. The only other substantial chemicals in Kline’s list of exhaust products are hydrogen and water — plus aluminum oxide, for the metallized fuel.

Other rocket enthusiasts concurred with Kline’s assessment. “Even if SpaceShipTwo became downright popular, the exhaust products are a tiny fraction of what a single NASA shuttle launch produces,” Jerry Irvine observed.

Bill Claybaugh, a former business adviser at NASA, noted that nitrous oxide “is a greenhouse gas more than 300 times more ‘efficient’ than CO2 — leakage and spillage of the oxidizer is probably the greatest environmental effect.”

However, he added, the total environmental damage done by suborbital tourist flights would still be “close to unmeasurable” when compared with other industrial greenhouse-gas sources.

Looking ahead, SpaceShipOne designer Burt Rutan and his team intend to stick with hybrid rocket engines for Virgin Galactic’s future fleet of SpaceShipTwo planes. During a presentation at this month’s Space Frontier Conference, Rutan hinted that EAC might be back in the running to provide the SpaceShipTwo engines. Meanwhile, SpaceDev wants to extend its hybrid rocket technology to a spaceship project it calls the “Dream Chaser.” So you can expect to see a lot more not-so-alien hybrids on the road to outer space.

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