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At the very end, NERVA (Nuclear Engine for Rocket Vehicle Application) had developed nearly flight-ready engines able to:

  • operate for periods in excess of an hour while providing maximum thrusts up to 220,000 lb (1000 kN),
  • reactor core temperatures of 2500° K,
  • give a specific impulse over 850 seconds (roughly double than chemical rockets),
  • have a thrust-to-weight ratio of better than 3:1.

Despite these results, what made NASA-(AEC/DoE) drop the NERVA program ?

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  • $\begingroup$ Lack of funding, lack of mission. Please also remember that NERVA was a joint NASA-(AEC/DoE) program. $\endgroup$ – Deer Hunter Aug 10 '13 at 9:33
  • $\begingroup$ @DeerHunter: can be security problems considered as well? $\endgroup$ – user55 Aug 10 '13 at 10:37
  • $\begingroup$ @trapo - what do you mean by security? Jackass Flats was pretty secure. $\endgroup$ – Deer Hunter Aug 10 '13 at 10:54
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    $\begingroup$ Wikipedia's NERVA article includes a section titled Loss of political support and cancellation. $\endgroup$ – coleopterist Aug 10 '13 at 17:45
  • $\begingroup$ Nukes are scary. $\endgroup$ – Organic Marble Jan 11 '15 at 15:56
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Once its initial incarnation and the Mars program was cancelled for cost reasons there was no longer a need for anything like it. As the upper stage of a Saturn 5 it would create a much more powerful rocket than what we used to go to the moon; but with Apollo being wound down and no successor in store there wasn't anything it would be beneficial for to resurrect it.

I'm not sure where your 3:1 thrust to weight ratio came from; while I've seen multiple claimed performance numbers for what Nasa achieved before the program was cancelled; but none of them had a rocket stage with a fueled thrust to weight ratio greater than one, leaving them only usable as upper stages. The highest I found was in Encyclopedia Astronautica (bottom of the page) which listed a fueled mass of ~178 tons and an 867kN thrust for a thrust to weight ratio of 0.5. Wikipedia's description of what otherwise appears to be the same system lists a much lower thrust of 330kN for a 0.2 ratio. This leaves it unusable for anything but an upper stage.

Even if modern designs could get a thrust to weight ratio slightly above 1. the political impossibility of dropping reactors from spent stages would mean that to use it as first stage you'd need performance high enough to also carry hardware to make a safe landing afterward or to serve as an SSTO. These days I suspect nuclear distrust would make even using it as an upper stage impossible under any but extraordinary circumstances.

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    $\begingroup$ A cold, never operated, fission reactor is safer to launch than other nuclear stuff we launch with some regularity, i.e. large amounts of actively decaying radioisotopes. If we can get approval to launch those, I don't see why we couldn't get approval to launch a cold fission core. You would just have to show that there is a vanishingly small probability that it could inadvertently reenter Earth's atmosphere after being fired up. The hard part is how and where you would test the design before you launch it. $\endgroup$ – Mark Adler Aug 11 '13 at 21:29
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    $\begingroup$ @MarkAdler when has rationality ever entered the nuclear debate? $\endgroup$ – Dan Neely Aug 11 '13 at 22:16
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    $\begingroup$ In my experience limited to the world of launch approval, it has been rational. I have no experience with, say, the approval of nuclear power plants. I suspect that that is a different world. $\endgroup$ – Mark Adler Aug 12 '13 at 5:27
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I know a man who worked on it. All my knowledge of the nuclear program comes from him.

The design objective was a single engine to be reused (fancy that--they're talking about reusable rockets in 1970) for a large number of TLI burns. Specifically, they ignited the test engine on the ground 10 times.

He says it was cancelled due to a lack of a mission when Apollo shut down and there wasn't any moon program anymore. It was intended for Apollo's successor, which never happened.

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