At the 36th annual Joint Propulsion Conference held here this week, two sessions and at least a dozen papers were devoted to space propulsion using nuclear thermal rockets (NTR). This type of propulsion is rooted in the Nuclear Engine for Rocket Vehicle Applications (NERVA) program that was terminated in 1973, seemingly with its coffin nailed shut.

"Maybe those nails haven't been completely pulled out," admitted Michael Stancati of SAIC in Schaumberg, Illinois. SAIC has supported NASA on a number of planetary mission studies. "But the thinking in the engineering community is that despite sporadic low levels of funding, NTR is the only credible option that makes sense for what we want to do."

It is a significant step forward since, for more than two decades after the demise of NERVA, NASA put very little money into space nuclear propulsion. Growing anti-nuclear feelings in the U.S. in the 1970s and 1980s did away with support for nuclear technology.

In recent years, the agency has also sought to avoid drawing attention to nuclear options; rather, it put a great deal of study into concepts for all-chemical and other propulsion schemes for sending humans to Mars. But several engineers have kept returning to the fact that the United States had once tested, on the ground, virtually all of the elements for a nuclear-propelled spacecraft.

NERVA was basically a graphite-core nuclear reactor with a liquid-hydrogen tank on one end and a rocket nozzle on the other. It also seemed to have a single purpose.

"Pigeonholing it for Mars led to its demise," said Stanley Borowski, a nuclear engineer managing NTR studies of NASA's Glenn Research Center since 1990 and co-chair of the sessions at the conference this week.

Borowski sees NTR as being a "three-fer," offering three mission types for the price of a single engine.

"We can use it to fly missions to the moon, to Mars and to near-Earth asteroids -- all with the same vehicle," he said. The asteroid mission is touted as a parallel to Apollo 8's mission to orbit the moon -- it would demonstrate everything needed for a landing mission other than the lander itself. A one-year asteroid mission would involve a one-month stay and provide valuable samples and data that could be used in determining how to tackle an asteroid headed for Earth.

This won't be your father's NERVA though. Robert Sackheim, assistant director and chief engineer for space propulsion systems at NASA's Marshall Space Flight Center, said that NASA and its contractor teams are working to combine lessons from NERVA with a number of new technologies to fashion "a non-controversial program."

For starters, the engine won't be test-fired in the open as NERVA was at Jackass Flats, Nevada. The new NTR engine could fire into a hole in the ground and the exhaust products would be caught by diatomaceous earth. Or, at the Idaho National Engineering Laboratory -- where the U.S. Navy tests new submarine reactors -- it could fire through a special filtration system that would trap fission products like xenon before the non-radioactive hydrogen exhaust is chemically burned.

Even before then, explained Sackheim, electrical heaters will stand-in for reactor fuel rods to test the "safe, affordable fission-engine" concept.

It also seems likely that Russia will participate in the program since they have experience with more than 30 space-reactor launches to the United States' one (other types of nuclear-power sources use the heat of radioactive decay to generate electricity). Russia also appears to have developed robust fuel rods that would be needed in the nuclear engine. Building on this, the University of Florida has been developing new fuel elements for NASA.

Sackheim emphasized that the flight engines will be non-radioactive at launch, and that "Safety features must be obvious to regulators and the public."

Another key feature of the new NTR engine is that its thrust is down to 15,000 pounds, as little as one-sixteenth that of the larger NERVA concepts, making it more affordable to build. What it lacks in brute power it makes up in duration. Several components such as the hydrogen pump and large nozzle can be adapted from Pratt & Whitney's new RL 50 engine, a descendant of the RL 10 that dispatched robotic Surveyor landers to the moon in the 1960s, further reducing development costs and lead time, the engineers claim.

"This engine could be ready to fly in 10 years," Borowski claims. It has enough design margin that it could also run in idle mode to power refrigeration systems to keep liquid hydrogen liquid, and provide all the electricity that the crew needs. This is known as the bimodal approach.

This time around, the design process involves a number of critics as monitors and keeps an eye on public reception. Anticipating a range of questions about nuclear power in space, Borowski said that the project office is preparing a "frequently asked questions" brochure to be distributed later this year.

As one of NASA/Glenn's public speakers, Borowski has spoken to groups ranging from kindergartners to retirees and gotten a nearly uniform response: "Why hasn't this country developed this option yet?" he said.

Sackheim reported a similar experience in speaking, even following the losses of two Mars missions.

"I was amazed at how forgiving the public was," he said. "They want a focus like Apollo."

"Does that mean it's okay to say the N-word now?" asked a voice from the back of the packed room.

"Yes," replied Sackheim, directly and unapologetically.

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