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« A History Of Two Wars | Main | Speak For Yourself »

Another Casualty Of Shuttle-Derived Vehicles?

If this story is true, it looks like Prometheus is dead. No nuclear propulsion for the foreseeable future.

This part, though, is a little puzzling:

NASA had hoped to develop the nuclear propulsion system to carry spacecraft beyond the solar system, according to a June 19 story in the Times Union.

That's news to me. I thought the program's purpose was to enable easier exploration of the solar system, not to go beyond it.

Posted by Rand Simberg at September 11, 2005 11:03 AM
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Prometheus Cancelled?
Excerpt: I can't find this anywhere else, and the Prometheus website gives no indication of it. It may just be that NASA no longer sees the need to work with the NAVY in developing the reactor. I have a lukewarm attitude towards Prometheus because it would ...
Weblog: The Star Spangled Cosmos
Tracked: September 12, 2005 03:57 PM

Prometheus was a joke. The thrust/mass of the system was far too low to be interesting. Freeman Dyson's comment was that it would set nuclear propulsion back a generation.

Good riddance!

Posted by Paul Dietz at September 11, 2005 11:19 AM

The whole thing doesn't make much sense to me. Nuclear propulsion is not going to be used in the atmosphere anytime soon, even if it is done in such a way as to release no radiation. Therefore, I don't see how this conflicts with shuttle-derived launchers--in fact, it enhances their ability to deliver missions to the Moon or Mars.

I'm guessing it was a near-term budget sacrifice--and just as short-sighted as most.

I think the "beyond" thing is just the usual media cluelessness.

Posted by Big D at September 11, 2005 11:21 AM

Paul: Did the current effort have anything in common with the original "Prometheus" design other than the name and the use of nuclear propulsion?

Posted by Big D at September 11, 2005 11:23 AM

There was no programmatic conflict. The conflict was a budgetary one. Griffin doesn't have enough money to build all the new launch systems that he wants, so in addition to asking OMB for more money, he's raiding other programs.

Posted by Rand Simberg at September 11, 2005 11:33 AM

Big D: I was refering to the nuclear-electric propulsion system that was to be used in JIMO, and which they tried to repurpose for closer to home activities (where chemical propulsion is even more competitive, not that JIMO could have gotten to Jupiter faster than chemically-propelled spacecraft.)

As for nuclear power for lunar bases, and nuclear-thermal propulsion: if the plans for VSE have regressed to a couple of Apollo-redux missions per year, with stay times measured in days, then they don't really need that, do they?

Posted by Paul Dietz at September 11, 2005 12:00 PM

"No nuclear propulsion for the foreseeable future."

Not neccesarily true. Mike Griffin is a big fan of nuclear thermal propulsion. Prometheus was nuclear electric. This may not be the end of nuclear propulsion as much as the refocusing of it to a higher thrust/lower ISP design better suited for lunar and Mars missions.

"'Nuclear energy is a core requirement in exploration,' Griffin added. 'I believe nuclear thermal propulsion is the most intelligent way to go to Mars. And development of these systems has been a historical core competency at Glenn.'"


J. Random American

Posted by at September 11, 2005 01:34 PM

The Garriot/Griffin report here might also give you a glimpse of what to expect:

Compare the lukewarm treatment of nuclear electric on page 24 with its endorsement of nuclear thermal: "Of the technologies so far proposed for radically transforming the architecture of Moon and Mars exploration, nuclear thermal propulsion (NTP) is among the most credible in terms of both fundamental
physics and engineering development

Given Mike's past work on nuclear thermal, the move to cut back on Prometheus is not surprising at all.


Posted by J Random American at September 11, 2005 01:42 PM

Certainly we are going to someday need nuclear electric and nuclear thermal propulsion, but do we really need it right now? It seem to me that the money would be better spent on chemical rocket development, particularly high thrust liquid hydrocarbon engines.

What nuclear buys you is propulsive efficiency, saving reaction mass (fuel). Another alternative investment would be a high flight rate, reusable launch system to carry large amounts of fuel into LEO to be used for chemically powered Moon and Mars missions. I think the economic benefits of developing such a launch system would be greater than the benefits of a nuclear propulsion system.

What is needed for deep space missions is more spacecraft electrical power. A small reactor, on the 10 kW power scale, coupled with better energy conversion technology (Stirling engines) would be a much less ambitious, more achievable goal than a full propulsion system. Higher power would allow more instruments and enable a faster data transmission rate back home. One could probably make a good argument by comparing bytes of data returned vs dollars spent in developing conventional missions like Cassini and a nuclear powered mission.

Posted by Frank Johnson at September 11, 2005 02:36 PM

An advanced PV system might have a mass of 10 to 15 kg/kW (at 1 AU), so your 10 kW system would have to be rather light to compete. Can you really make a reactor that small? Shielding doesn't scale down well.

Posted by Paul Dietz at September 11, 2005 04:19 PM

"An advanced PV system might have a mass of 10 to 15 kg/kW"

Is that correct? Why then is the ISS solar power system so massive? Is the correct ratio really per watt instead of per kilowatt?

Posted by Brad at September 11, 2005 07:06 PM

Prometheus is Dead?

I think too much is being read into the Times Union story.

"The National Aeronautics and Space Administration has pulled the plug on a $65 million nuclear propulsion research program at Knolls Atomic Power Laboratory,..."

Project Prometheus has a budget of several hundred million dollars. The Times Union Story only talks about the cuts to the Knolls Atomic Power Laboratory.

Posted by Brad at September 11, 2005 07:16 PM

Brad: I don't know why ISS's PV arrays are so massive (about 93 kg/kW, I think). Perhaps they're optimized to minimize drag instead of mass, or maybe they have to be strong enough to withstand errant RCS jets from shuttles?

Posted by Paul Dietz at September 11, 2005 07:37 PM

Quick google produced this on photovoltaics.

"A flexible, lightweight fold-up solar array with a mass of 1 kilogram per square meter of collecting area was developed by JPL antler the APSA program, in conjunction with TRW's Space and Electronics Group. The array design incorporates both thin silicon cells and thin gallium arsenide on germanium substrate cells. The cells are attached to a flexible Kapton polyamide blanket. A fiberglass deployment mast is used. The complete array system produces 135 watts per kilogram"

Any yet the ISS will generate only about 110 kilowatts using solar power structures massing over 68 metric tons.

Posted by Brad at September 11, 2005 08:00 PM

I believe (as in, I'm not sure) that the ISS had to meet stringent flexibility requirements, which are not needed in other missions.

Posted by David Summers at September 12, 2005 08:40 AM

If my memory and math isn't off, the Prometheus reactor would have had an output equivalent to a 150 horsepower engine. Seems hardly worth the trouble. For comparison, a diesel locomotive generates 7800hp (5 megawatts), and a nuclear submarine quite a bit more I believe. Apparently the thrust of JIMO was to be so low that it would require 3 years (!) just to leave earth orbit on its own power.

Have there been any experiments to develop a nuclear engine combining both thermal and electric propulsion? I'm imagining a system that heats the reaction mass (say hydrogen) by passing it through the reactor, followed by an electric/magnetic system that strips off the electrons, accelerates the + and - charged components separately out the back, where they recombine outside the nozzle.

Posted by lmg at September 12, 2005 10:09 AM

Img: I think P&W's TRITON concept comes somewhat close - it's ordinary NTR with capability for electricity generation which can be used for NEP.

Read interview with it's designer at

Posted by jv at September 12, 2005 02:21 PM

Given that I spent this summer working on programs related to Prometheus and space reactors, I feel moderately qualified to comment on this.

First, we all knew Prometheus was dying, largely because of mission creep. Not only did Prometheus have to provide X amount of power for Y years, the program was also designed to build up the space nuclear infrastructure of the US (test facilities, fabrication, etc.) which meant it was becoming increasingly cost ineffective.

As far as no new nuclear propulsion, that is probably correct as Griffin is focused on reactors for surface power right now. He knows we don't need nukes to get to the moon, chem works fine for that. But we do need nukes to stay there for more than 2 weeks (because PV's don't work for 2 weeks out of every month due to darkness).

And Paul, I designed several reactors that had higher power densities than what you specifid for PV's, including shielding. (If I'm allowed to neglect the probability of a water immersion after launch, I can get much higher than even that).

Also, as I understand it, the biggest problem for using the chemicals for a mission to the icy moons was having enough fuel to change orbits once you are there. The beauty of NEP is that, even if it is slow, you are much more efficient in terms of fuel usage, which expands your mission capability.

Posted by Matthew at September 12, 2005 03:42 PM

Oops, just reread you Paul. I thought you said W/kg. A lot of the reactors we were looking at were on the order of 100 W/kg when you include all shielding control and power conversion systems. We can probably go significantly higher than that though as Stirling tech improves.

Posted by Matthew at September 12, 2005 03:50 PM

Matthew: is that for systems with a power output of 10 kW(e)?

Posted by Paul Dietz at September 12, 2005 04:56 PM

We were looking at systems of about 25-30 kWe.

Posted by Matthew at September 13, 2005 09:05 PM

The 'beyond the solar system' bit may have been a reference to TAU, the Thousand Astronomical Unit mission. This mission concept would involve a nuclear-electric vehicle designed to reach 1000 AU in 50 years.

Posted by Paul Dietz at September 19, 2005 01:54 PM

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