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31

At their core nuclear rockets working by heating a working fluid and running it out a nozzle are still constrained by the same physics as a chemical rocket where exhaust temperature cannot be much higher than the melting point of nozzle (cooling the nozzle lets you cheat a bit), putting limits on how much energy can go into the fuel. Nuclear rockets get ...


22

At least for the NERVA, after shutdown, a residual, thrust-generating cooldown flow of propellant was run through the reactor. From the engine specs 3.1.1.1.5.1.4 Shutdown and Cooldown Shutdown consists of throttling, throttle hold, temperature retreat and pump tailoff, and is initiated by a command signal to depart from rated conditions and is completed ...


21

The one exception to this fact is Project Orion Not quite. Project Timberwind was a solid-core NTR using a pebble-bed reactor design that combined high Isp with a moderate T/W of 30. The DUMBO NTR used a quite different core design, and had predicted T/W ratio of 70. Still somewhat shy of a good modern chemical rocket, but with a much better specific ...


16

You have the same problem in any rocket. The pressure at the pump outlet must be higher than the chamber pressure. What you are missing, is that the density of your propellant drops by heating, both in chemical reactions, as well as by being fed through a nuclear core. For a nuclear thermal rocket some of the heated (and thus expanded) hydrogen is tapped ...


15

Yes they considered the disassosciation of hydrogen According to this source : Previous testing used a maximum temperature of 2,750° K, short of the 3000+° K design temperature for the NCPS. The NTREES facility is designed to test fuel elements and materials in hot flowing hydrogen, reaching pressures up to 1,000 pounds per square inch and ...


14

It's not dangerous. The core would never be operated on Earth, and so would not become radioactive like you're thinking. A nuclear reactor on Earth that has been in operation is extremely radioactive due to the fission products, but the original fuel was not. The unburned U-235 fuel has a half-life of 700 million years, which means that its ...


13

Some of the hydrogen will be disassociated. For the reaction mass that is not dissociated, and passes through the engine in the form of diatomic hydrogen, in addition to the three translational degrees of freedom, heat energy is also put into the vibrational and rotational energy of the hydrogen molecule. So the energy stored is 6/2kT, not 3/2 kT as in the ...


12

Whether the reported article is accurate? It's rather bad in my opinion. The news is outdated and incorrect. This part was completely incorrect: After a multi-decade hiatus, both NASA and the Russian Federal Space Agency (which developed many of its own NTRs during the Cold War but never physically tested their designs) announced in April 2012 that they ...


12

Any sensible answer to your question should make an implicit assumption on the number of engines to be ordered overall. That is, we need to know the number of planned manned flights to Mars and other important destinations, since fixed (sunk) costs of an NTR-based program are significant. The following ideas are based on a few NASA documents discussing NTRs. ...


10

Is it currently possible to shut down an actual rocket (not test reactor with external cooling) while in space and then restart it, without melting significant parts of the engine? If so, how? It was certainly intended that NTRs under development were restartable, but more importantly a clean shutdown of the engine is clearly desirable so you get good ...


10

In the past 6 years NASA and the DOE have put a fair amount of effort into estimating the cost to recapture NTP technology assuming either graphite based or tungsten CERMET fuel. The estimates to include fuel development, reactor development, reactor testing and infrastructure ranged from $4 to 6 billion over 10 to 15 years. However, where there has been ...


9

Reactors are, in general, tuned so that the proportion of delayed neutrons (on the order of seconds to minutes after each fissioning) out of all neutrons is enough to make the reactor more or less dynamically stable. (Specifically, the prompt neutrons released within nanoseconds, which are upwards of 95% of all neutrons, aren't enough to match the neutrons ...


8

The American NERVA program developed a design expected to produce up to 333kN (75klbf) of thrust in a 6.8 ton package, for a thrust to weight ratio of about 5:1. It was never completed or flown. NERVA 2 was supposed to produce 867kN (195klbf) from 11.9 tons, a thrust to weight ratio of 7.5:1. Those are figures for thrust in vacuum. The Russian RD-0410 was ...


8

As of 2012 (Nuclear and Emerging Technologies in Space meeting at LPI) the outline of development program is as follows: See this pdf file for links to papers. (note: only tangible test & evaluation items listed) Manufacture and test fuel elements in two compositions: NERVA composite, and Uranium dioxide UO2 in tungsten ceramic-metallic "cermet". ...


7

It was DoD, not NASA, but there was a post-NERVA nuclear rocket research program named Project TIMBERWIND. The summary of the project at its conclusion can be gleaned from this report.


7

I can't find any sources that corroborate, confirm, or falsify the article, so I'm going to treat it as if it was accurate, in order to answer the question. That said, given some of the articles (complete with an obscenity or two) that popped up when I first saw the site, I wouldn't exactly cite this thing in a publication, report, essay, paper, . . . You ...


5

A couple of things: The reference OP provided for cosmic rays states: Cosmic Ray Composition: Cosmic rays include essentially all of the elements in the periodic table; about 89% of the nuclei are hydrogen (protons), 10% helium, and about 1% heavier elements. This implies that neutrons are not a significant constituent of cosmic rays. Since fission is ...


4

The reason is that you have to carry a fission reactor, which is large & heavy, and a heat exchanger which is also probably fairly heavy. This is all especially heavy if you want to avoid the working fluid from ending up radioactive, and also if you want to avoid the payload from getting irradiated. If you're willing to forego some or all of this ...


4

The relation is quite straightforward. The energy injected into a unit of propellant before it evaporates and spent on evaporating it is minuscule comparing to energy applied to the propellant as gas; and for gas of similar pressure and temperature the number of particles per unit of volume varies very little. As result, approximation that assumes each ...


4

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 ...


4

50 years old abort routines are hard to come by, but the NERVA-specs (unclassified 9/8/1970) clearly specify a malfunction mode in which the engine should be able to provide a minimum of thrust and impulse. The values stated (thrust of 30000 pounds, impulse of 10⁸ lb-seconds) are enough to direct the upper stage away from densely populated areas, towards a ...


3

Neutron flux created by cosmic rays would be small and would actually make control of the reactor slightly easier in terms of neutron economy. The more non-prompt neutrons are involved in a reactor, the farther you can get from prompt criticality while maintaining the reaction, which makes safety easier. This slight benefit would be outweighed by the ...


3

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 ...


1

To copy the key part of this answer (and this related earlier answer)... an important figure of merit in a rocket engine is the "characteristic velocity": $$c_* \propto \sqrt{\frac{T_t}{M_w}}$$ where $T_t$ can be taken to be the exhaust temperature, and $M_w$ is the molecular weight of the gas species in the exhaust. (The "proportional" ...


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