If you reference some of the links off this page, a nuclear powered engine of this type does give a higher ISP 875 vs 350 for the very efficient SSMEs.

Of course, other technologies, Ion and plasma are much higher (a quick google gives an ISP of 3300 for early ion drives).

Of course, Ion and plasma are low thrust, and cannot be used for ascent or descent, but neither can NERVA class motors, as their exhaust is radioactive.

Space nuke boffin: NASA Moonbase needs nuclear rockets
By Lewis Page
Published Saturday 30th June 2007 07:02 GMT

One of America’a top nukes-in-space boffins says it’s time to consider nuclear-powered rockets again. He reckons atomic boosters could cut the cost of NASA’s upcoming Moonbase plan.

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This guy runs the premier RTG battery (nuclear batteries) manufacturer in the US, The Center for Space Nuclear Research, and he is someone who has a hammer, and sees everything as a nail.

But Howe reckons that there’s more to nukes in space than just providing electricity. He says that nuclear power should be used for propulsion, too. According to an article in New Scientist, his plan is to update a 1960s design called Nuclear Engine for Rocket Vehicle Application (NERVA) to carry payloads from Earth orbit to the Moon.

NERVA-type rockets use a fission reactor to heat up hydrogen and blast it out of the thrust nozzle at extremely high speed, faster than can be achieved by normal chemical-powered boosters. This allows a nuclear-driven spacecraft to achieve more with a given amount of liquid fuel, or “reaction mass.” The NERVA test programme had its problems – not least the fact that the reactor tended to come apart and fire itself out of the exhaust – and was terminated in 1972 during NASA budget cuts.

Howe and his team reckon that the greater efficiency of nuclear drive would allow each Moon shot to carry an additional eight tons of payload, which would mean fewer launches being needed. He thinks the savings from a lower number of launches would more than offset the cost of updating the original NERVA design, perhaps yielding overall savings of as much as $2bn.

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Here is an interesting rundown on the technologies that NASA is looking at:

Rovers, reactors and ‘green’ rockets all on NASA’s future-technology list

Aviation Week & Space Technology
06/04/2007, page 50
Frank Morring, Jr.
Washington

Some advanced technology for long-term human space exploration is already getting on-orbit checkout as NASA targets about $350 million from its tight exploration budget this year on the long-lead items that may enable a return to the Moon en route to Mars.

There is some neat stuff here, but also some scary stuff.

But the program also is funding advanced-technology research ranging from composite space radiators that might save weight on Orion to a 5-ton nuclear reactor that would generate power for the outpost NASA plans to build on the Moon as a proving ground for expeditions to Mars.

The reactor is one of the scary things. The tech is described below, and I’ll explain why it’s scary (it’s notbecause it’s a nuclear reactor) after that.

One such system is getting a workout on the Orbital Express mission being conducted by the Defense Advanced Research Projects Agency (Darpa). Although the satellite-servicing testbed has had problems in orbit (AW&ST May 28, p. 22), they were unrelated to the Advanced Video Guidance Sensor provided by NASA.

Evolved over the past decade at Marshall Space Flight Center in Huntsville, Ala., and previously space-tested on two space shuttle flights and the failed Demonstration of Autonomous Rendezvous Technology mission in 2004, the sensor is designed for short-range rendezvous guidance between cooperating spacecraft.

I worked at the battery manufacturer for DART and did some (very) minor work on the batteries. It should be noted that the batteries worked just fine.

If it turns out that such a large piece of PICA can’t handle the lunar-reentry heat flux, which is four to five times greater than that experienced by a reentering space shuttle, NASA will turn to two more contracts for backup heat-shield material. Boeing’s Huntington Beach, Calif., facility will receive as much as $10 million to conduct early studies of a proprietary material known as the Boeing Phenolic Ablator (BPA). Textron Systems of Wilmington, Mass., won a contract worth as much as $24 million for preliminary work on two proprietary materials–Dual Layer, and a material called Avcoat that was used on the Apollo capsule.

“We’re trying to relearn how to make that material,” Moore says. “It’s been a generation, and a lot of the people have retired who have done the Apollo heat shield, so we have lost a lot of that expertise.”

They could also talk to the Russians.

Also under study are new fluids to carry the heat away from Orion electronics into the radiators–a mixture of propylene glycol and water is the baseline.

Don’t confuse Ethylene Glycol and Propylene Glycol. The former is normal poisonous anti-freeze, the the latter is an alternative anti-freeze that is in the “pet safe” stuff, which is so non-toxic that you see it added to salad dressing.

Other advanced technologies under study for Orion include lightweight parachute material that can also fit into a smaller volume, and an amine swing-bed for carbon dioxide and moisture removal from the crew compartment. A bed of very small plastic beads is coated with chemicals that absorb CO2 and water vapor, and then release it when vented to the space vacuum.

NASA is developing a prototype amine swing-bed system to remove carbon dioxide and moisture from the Orion crew compartment.Credit: NASA/JSC

I think that this is of more concern for the Mars missions than the moon missions, where the use of conventional CO₂ sorbents might require too much weight and volume.

For crew safety, a team at NASA’s Glenn Research Center in Cleveland is working on a “green” Orion attitude-control thruster system. Instead of requiring storage of highly toxic hydrazine fuel near the crew compartment, the system would burn gaseous oxygen and methane.

I think that this might be more of an issue of ISP (fuel economy) than environmental friendliness. The room temp propellants typically have a lower ISP, but Methane is much easier to store than liquid hydrogen, having a higher boiling point.

The dual propellant thrusters are more difficult to control precisely, but much more efficient.

Advanced preparations for the shuttle-derived Ares I crew launch vehicle that will carry Orion to orbit are also underway, piggybacking at times on routine shuttle-program tests to gather data. A case in point was the May 24 static test of a four-segment shuttle Reusable Solid Rocket Motor (RSRM) in Utah, which provided motor-signature data for a study at Ames Research Center on ways to detect impending failure in the five-segment RSRM that will serve as the Ares I first-stage.

Good move, but a better move is not to use solid rockets on man-rated systems. I continue to be dubious of solid rocket boosters on manned systems.

To help astronauts establish a lunar outpost and explore the surrounding terrain, the advanced-technology program is studying rovers, a nuclear power source and in situ resource utilization (ISRU) techniques. One promising activity at the Pasadena, Calif.-based Jet Propulsion Laboratory has field tested the All-Terrain Hex-Legged Extra-Terrestrial Explorer (Athlete), a six-legged vehicle as tall as a man that can move heavy payloads such as habitats around the surface either on wheels or by “walking.” Another project at Houston-based Johnson Space Center, dubbed Scout, is an updated version of the unpressurized moon buggy that hauled Apollo astronauts around the lunar surface.

Two Athlete rovers undergo testing in California’s Dumont Dunes. Athlete can roll over rough terrain using its wheels or its six legs, transporting heavy payloads during construction of the planned lunar outpost.Credit: JET PROPULSION LABORATORY

I think that this just looks cool, so here is the picture.

A big objective of the lunar outpost will be to test exploration technologies and techniques that can enable subsequent human exploration on Mars. Engineers at JSC are working on a variety of ISRU techniques to extract oxygen from the lunar regolith, as much for the experience as the resource.

Along the same lines, NASA and the Energy Dept. are working under a scaled-back version of the old Project Prometheus to advance some of the technologies that would be needed for a five-metric-ton, 40-kw. fission reactor that would take over from the solar power plant scheduled to provide initial power for the outpost. It would be fueled with uranium dioxide and cooled with a sodium/potassium liquid alloy.

This scares the hell out of me. Based on discussions with a Naval Nuke over a decade ago, metal cooled reactors are VERY twitchy things, with reaction times so fast that you can’t operate them manually.

They are much more compact though, hence their use in the Soviet’s Alpha class boats, where they worked (though their nuclear personnel glowed in the dark), and the unsuccessful use in the USN’s original Seawolf, which was our 2nd nuclear boat.

They replaced it with a pressurized water reactor a few years after it was installed.