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antoniseb
2005-Aug-31, 07:54 PM
Here's a New Scientist article
http://www.newscientistspace.com/article.ns?id=dn7928
about some recent criticism for nuclear-electric fission projects, including most specifically the Prometheus project.

Generally, the complaint is that for many projects, having these hot sources of radiation will blind or impare sensative observing missions. The article seems to imply that such reactors on the moon or other large bodies would be much less of an issue.

There was also a part of the article which decries using nuclear electric (ion power) for manned space flight, saying that using the reactions to heat Hydrogen directly is more efficient.

ASEI
2005-Aug-31, 08:59 PM
A few words on the article:
JIMO isn't going to neptune. It's going to jupiter. Second of all, you can't do nuclear electric propulsion with an RTG - there's not enough power to put out serious amounts of thrust or Isp.

Nuclear rockets of either the electric or the thermal variety are practically necessary for human interplanetary missions. Even if you dropped nuclear propulsion from the picture, what would power your life support? But then again, without the high Isp of nuclear propulsion, you would need tens to hundreds of tons of fuel in orbit for every ton of spacecraft. That isn't going to happen at today's launch rates. We couldn't even manage to finish a 500 ton (originally) space station.

If your instruments are trying to look at gamma rays, you could always store power in a battery, shut down the reactors, and take some measurements. Or reel it out on a long cable away from the vehicle (reactors do operate by the inverse square law, you know). Or sheild the instrument better. Or . . . I think figuring out how to deal with sheilding an instrument (BTW, why don't cosmic rays ever cause these sorts of problems?) is well worth having more than 100W of mission power.


Direct nuclear thermal rockets are more efficient in terms of putting power into the jet. They have more thrust, but they are less fuel efficient. 150000 N at 1000 sec isp vs 1500 N at 4000+ sec isp.

For planetary maneuvers, nuclear thermal rockets can get you on much more reasonable transfer paths (hohman, or close to it), where electric propulsion would have to follow a long slow spiral. But in interplanetary flight, when your path is a long arc between planets, all the way across the solar system, slow accelerations can build up to huge dvs over the course of a month or so. Electric propulsion could take you on a faster course with the same amount of fuel, widening your launch windows and shortening the interplanetary transit time.

If you're going from Earth to a planet further than mars, (say, to jupiter or saturn) nuclear electric propulsion wins, hands down, in terms of transit time. Even on massive missions.

Guest
2005-Sep-01, 07:38 PM
A quick opinion

You need nuclear energy only if you go to Jupiter and farther. And nuclear rocket only if you want to send people that far away.

Nuclear rockets will be extremely expensive to develop , they are heavy ,dirty and dangerous and quite frail in fact , because you want them to be efficient and then the lightest possible . Cooling panels can be punctured by meteors by example. pumping liquid sodium or melted salt for years ... !!
I believe solar energy is quite enough for the inner solar system and the years to come. It is a proven working system and it will be improved furthermore.

Guest_galacsi
2005-Sep-01, 07:41 PM
Originally posted by Guest@Sep 1 2005, 07:38 PM
A quick opinion

You need nuclear energy only if you go to Jupiter and farther. And nuclear rocket only if you want to send people that far away.

Nuclear rockets will be extremely expensive to develop , they are heavy ,dirty and dangerous and quite frail in fact , because you want them to be efficient and then the lightest possible . Cooling panels can be punctured by meteors by example. pumping liquid sodium or melted salt for years ... !!
I believe solar energy is quite enough for the inner solar system and the years to come. It is a proven working system and it will be improved furthermore.
hello

guest is Galacsi

i registered before entering the forum i swear it !

Joff
2005-Sep-02, 10:57 PM
Originally posted by Guest_galacsi@Sep 1 2005, 07:38 PM
Nuclear rockets will be extremely expensive to develop , they are heavy ,dirty and dangerous and quite frail in fact , because you want them to be efficient and then the lightest possible . Cooling panels can be punctured by meteors by example. pumping liquid sodium or melted salt for years ... !!
It sounds like you already have the "scary" nuclear option tagged for rejection. :rolleyes:

"extremely expensive... heavy, dirty and dangerous and quite frail" - no room for innovation there. However I don't see how you get from (*efficient and as light as possible*) to (*quite frail*). In all likelihood, a nuclear thermal rocket could be sufficiently powerful that "heavy" wouldn't matter so much and extra weight could be devoted to countering "dangerous". If I understood "dirty" I'm sure that could be dealt with too.

Incidentally I believe the next Mars Rover will have a RTG, which is clearly not as far as Jupiter.

projectorion
2005-Sep-03, 05:45 AM
New Scientist
31 Aug

The title is a little misleading. It starts with Astronomy concerns over radiation from near earth reactors (sounds a little absurd to me).


NRC report finds that the reactors would be virtually useless for - and could even hamper - observations of astrophysical phenomena beyond our solar system.



Sounds like a proximity and shielding problem.



could effectively "blind" space telescopes such as Hubble, Spitzer, and Swift

Only if we operate the reactor next to the things. I thought Prometheus was about trips to the gas giants. Well, Jimo is dead so anything might be on the cards now.

Then it goes on to criticise fission reactors in general. The scientists in this article don't like nuclear electric rockets.


Nor did the panel find that NASA's nuclear programme would support its planned human missions. The NRC acknowledged that fission reactors would be useful for both space travel and long-term human bases on the Moon or Mars. But it said it is not clear whether the nuclear electric propulsion NASA is pursuing is "adequate for either application".

It isn't. We need an Orion programme.
:)



Another reactor technology that uses fission to heat hydrogen so it can be forcefully expelled to provide rocket thrust might get astronauts to Mars more quickly, the panel writes.

NERVA making a comeback? Goody!


But the NRC cautioned that significant hurdles remain for the technology to actually be practical.

That's talking about nuclear-electric. If they think that's hard then NERVA will give them heart palpitations to go with their kidney stones. Continuous burn systems are just too hot.



Anthony Hyder, a physicist at the University of Notre Dame University in Indiana, US, points out that RTGs can only generate a few hundred watts of electricity. He says any missions requiring more than that would have to use many more RTGs - the equivalent of using a "trunk full" of flashlight batteries to start a car. "It's much easier at that point to graduate from radioisotopes to fission reactors," he says.

A few hundred watts! Is that all we can scrounge out of hot rocks? Not really enough to operate all the electronics we'd like to send is it. I mean if you really want to map new worlds with high tech camera's, send robotic explorers down(I'd prefer humans), run all sorts of tests and transmit messages millions of miles across space then you really want more power than a standard refrigerator consumes. Don't you?



Death knell
However one scientist, who wished to remain anonymous, told New Scientist that public concern over the safety of nuclear reactors resulted in a Catch-22 situation: "You're not going to develop it until someone says they need it and no one is going to say they need it because they know it's a death knell for their programme."

Space researchers generally believe spacecraft reactors can be used safely, for example by launching the reactor in pieces before assembling and starting it in space.

While reactors would definitely boost a mission's power level, the technology does come at a heavy financial cost. NASA projects Prometheus will cost $3 billion between now and 2010. In the agency's 2006 budget request, the money was scheduled to come from "exploration systems" - and not the science budget.

But Bernstein says he is worried about the effect of the cost on NASA's other missions. "If you're going to make this a priority, then what gets deprioritised?" he asked New Scientist. "It's not free."



I don't like the sound of that. Not one bit. This could be the shortest nuclear initiative in history if President Bush doesn't move his attention away from secular warfare to outward expansion and colonisation instead. Prometheus has never had much attention in the news and most people don't even seem to know about it. Hardly surprising after the last Shuttle disaster stole all the publicity away from the planned announcement of Prometheus.

I find it so ironic that even in death those stupid useless shuttles may have stuffed up our chances of a real space program. With nobody aware of or caring about it's demise, the entire nuclear initiative could follow in JIMO's wake, vanishing without a murmur of protest from anyone people would want to hear from.

All the other programs got shelved because nobody cared. Maybe it's time we made people care.