NASA's Nuclear Thermal Engine

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Winston

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NASA's Nuclear Thermal Engine Is a Blast From the Cold War Past
Nuclear thermal propulsion, which was studied in the Cold War for space travel, could make a comeback to fly humans to Mars

https://www.popularmechanics.com/space/moon-mars/a18345717/nasa-ntp-nuclear-engines-mars/

nuclearthermalpropulsionengine-8-1519159474.jpg
 
The article had an ad-blocker-blocker, so I wasn't able to read it - and therefore don't know if the article spoke to a lingering question I've had about these systems, going all the way back to Heinlein:

The exhaust temperature can't exceed the melting point of the reactor core, correct? Doesn't that limit it to very slow velocities?
 
The exhaust temperature can't exceed the melting point of the reactor core, correct? Doesn't that limit it to very slow velocities?

That is correct, but the performance is still higher than chemical rockets, because the hydrogen molecular weight is so low. In general the exhaust velocity and/or the specific impulse is proportional to the square root of the chamber temperature divided by the molecular weight. The higher this ratio can be made (lowering the molecular weight helps a lot) the better the performance. The SSME pushed the chemical rocket performance up to about 430 sec. A nuclear thermal rocket can about double this. The classical downside of a thermal rocket is that a way is needed to remove the isotope decay heat of the core after the core has been shut down. I would think one advantage might be that substantial fissionable material might be left after shutdown that the rocket could be refueled with hydrogen to start another run.
 
Let's see... H2/O2 flame temp ~3000K. Uranium melting point ~1400K. H2 MW = 2. What MW to use for comparison - the combustion product? That's what carrying momentum away. H2O = 18.

37.4/2 = 18.7
54.8/18 = 3.0

Hmmm... maybe.

If one wasn't going to use the exhaust nozzle again, I'd be tempted to jettison it and attach a unit that turned it into a stirling engine. Cycle H2 through the remaining core and generate power for an ion drive. Get a high thrust system and a low thrust system in one.
 
Actually, optimized liquid hydrogen chemical rocket engines do not operate at the stoichiometric ratio. They are hydrogen-rich in order to get the the molecular weight of the exhaust products down. Commercial reactors never use pure uranium metal. They use uranium dioxide powder pressed into pellets and sintered to form ceramic-like pellets. The center-line temperature of such pellets can be on the order of 4000 Deg.F (or about 2500 Deg.K.). George Sutton in "Rocket Propulsion Elements" lists operating nuclear thermal rockets as having gas temperatures on the order of 5400 Deg.R. or 3000 Deg.K. Nuclear thermal rockets are not new. The US did extensive testing (many hours of operating time) from 1959 to 1972 with projects such as Kiwi, NERVA, etc. The Russians did testing, too.
 
These things do have some advantages like high Isp and some others. They are quite safe and not radioactive until they are activated for the first time, then the radioactive decay products with long half-lives get produced. This makes them safe to launch to orbit before firing them up, but any failures have to be calculated to produce an orbit that will never impact Earth if they occur.

They do have a disadvantage in that extra gas needs to be carried to cool the reactor after the notional "shutdown". This cooling process also produces thrust and can take a LONG time, so precise burn durations are not an option with this sort of motor.

Core temperatures are really pushed to get the Isp up, generally quite close to the material failure points. Cores I think were typically ceramics with the fissile material embedded in them. The early ones had problems with bits of the cores being ejected in the exhaust :eek:

See if you can get your hands on the eBook (or book) "Rocket and Spacecraft Propulsion" by Turner. There is a really good chapter or two on this stuff. Quite enlightening!
 
For more than you ever wanted to know I suggest report LA-2091 on Dumbo. (Can be found on the NASA technical reports server.)

For a shorter version I recommend an article from the December 1975 issue of Analog magazine by Donald Kingsbury. (For those without that issue in their library, a PM could get you a copy of the article.) This includes a design for an F1 class nuclear rocket motor.
 
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