Nuclear thermal rockets

[munlander1]

Just wanna leave this here to see what everyone thinks about it.

[monophonic]

Well, I for one think that that is one seriously broken link. 

[Vanamonde]

Not working for me, either. 

[Bill Phil]

Yeah, it's broken.

Actual link:

http://nasa%20reignites%20program%20for%20nuclear%20thermal%20rockets%20-%20universe%20today%20https//apple.news/Agf6Zk4hUMY-Rga8TjY-lmQ

There's an article name, but no website, I think.

Edited August 13, 2017 by Bill Phil

[StrandedonEarth]

He's probably trying to link to https://www.universetoday.com/136752/nasa-reignites-program-nuclear-thermal-rockets/

 

[Shpaget]

On 8/12/2017 at 6:55 AM, munlander1 said:

to see what everyone thinks about it.

That would be quite a bit of comments, so we better get started...

When it comes to space, I'm in favor of any technology study and experiment. I'd really like to see humanity in space in my remaining lifetime which, by the way, is getting shorter by a frightening rate of roughly 365 days per year.

Until we get a hang of fusion, nuclear fission is still the most energetic process we can control. We should use it. As for the actual proposal to restart it, I was not able to find anything about it on NASA site and the article linked by Stranded provides little detail.

[Scotius]

Same here. And we actually do have a good grasp on nuclear fission technologies. It's time to go beyond our tight horizon

[wumpus]

11 hours ago, Shpaget said:

Until we get a hang of fusion, nuclear fission is still the most energetic process we can control. We should use it. As for the actual proposal to restart it, I was not able to find anything about it on NASA site and the article linked by Stranded provides little detail.

It isn't even clear that fusion has any real advantages for spacecraft.  Both produce high energy and emit plenty of radiation that require shielding.  The biggest grip about fission is the waste products, and outer space is an ideal location for them (you can be reasonably sure they aren't going to bump into anyone else for the next few million years, at which point they will not longer radioactive).  The real advantage that fusion has is that you can use (presumably) the direct output of fusion (by opening the magnetic containment), possibly without subjecting any other materials to its heat.  This should give tremendous Isp, and doesn't even require breakeven fusion to do so (the energy could be produced by fission, for example).

One reason to get excited about using nuclear thermal rockets is that presumably the James Webb telescope research paid for enough research on closed-loop cyrogenics.  It needed helium, typically NTRs need hydrogen.  Note that returning from Mars might not need such extreme Isp, and we might get away with low efficiency O₂ or even CO₂.

Don't forget that if you want to travel to Mars, NASA already has cut most of the red tape for NTP by grandfathering research in the 1960s-1970s.  This means that it is "good to go" on a bit less stringent and allows easier funding from Congress (although very little can stop a congresscritter from throwing pork in a direction he wants).

I'd expect KSP players to be highly enthusiastic about NTRs, they are some of my favorite parts.  The biggest catch is likely the cryogenics: it might not be reasonable to expect to have hydrogen for the return trip (having hydrogen for the braking/insertion burn requires considerable engineering.  Nothing will stop the amount of leakage through all known mass over the stay on Mars, but it might be preferable to simply use what's left after the leakage to using CO₂ found on Mars).

[DDE]

I think non-H2 propellants for NTRs are undervalued. Also, you don't really need NTRs for the return vehicle; my current mission architecture of choice has a solar-electric single-use return craft.

[shynung]

10 hours ago, wumpus said:

The biggest catch is likely the cryogenics: it might not be reasonable to expect to have hydrogen for the return trip.

NTRs can run on almost any propellant. A solid-core NTR at 3200 K can attain a respectable 410 seconds of ISP on water, according to the Atomic Rockets site.

[DDE]

@shynung, I know there are limitation caused by interactions of neutrons with the propellant, meaning the engine must run with the propellant it was designed for... but I wonder if it's possible to optimize a motor to switch to a different remass closer to complete fuel burnout.

I've also noticed that LANTERN tankage is noticeably smaller for the same dV, and thus plan to use mixed LANTERN-pure H2 in future missions as well.

[magnemoe]

45 minutes ago, DDE said:

@shynung, I know there are limitation caused by interactions of neutrons with the propellant, meaning the engine must run with the propellant it was designed for... but I wonder if it's possible to optimize a motor to switch to a different remass closer to complete fuel burnout.

I've also noticed that LANTERN tankage is noticeably smaller for the same dV, and thus plan to use mixed LANTERN-pure H2 in future missions as well.

Changing the moderator staff positions could compensate for different fuel within some limits. 
An nuclear reactor don't burn out in reasonable time (years of use)
 

[KSK]

Well the less said about the article the better.

Nuclear thermal rockets - great idea in principle, especially if propellants other than hydrogen are considered. As @shynung pointed out, they'll run quite nicely on tap water, with an ISP comparable to hydrolox engines. Make your NTR a bimodal design, optimize it to use water as a propellant, apply liberal amounts of handwaving and you've got a nice interplanetary craft that's comparatively easy to refuel* elsewhere in the solar system and can use the reactor as a power source whilst in transit.

The big problem with NTR at the moment is political rather than technical. Launching chunks of enriched nukular fuel into orbit seems unlikely to be acceptable to the general public for the foreseeable future. Worse still, if you want a reactor that's light enough to consider launching into space in the first place, it's going to need fairly highly enriched fuel. See this Wikipedia table of critical masses for various pure isotopes - and that's assuming a spherical mass, which is the most efficient shape possible - and which NTRs are unlikely to be using.

 

*compared to a ship using hydrogen as a propellant.

[Kerbart]

9 hours ago, shynung said:

NTRs can run on almost any propellant. A solid-core NTR at 3200 K can attain a respectable 410 seconds of ISP on water, according to the Atomic Rockets site.

Unless you're able to mine it in space I really don't see the value of using water over a higher-I_SP propellant. I'm sure there are more compelling candidates than Hydrogen because of storage & density properties, making up for a lesser I_SP. And if we're mining fuel "for free" in the asteroid belt I think there will be value in an engine that can run practically anything as long as it's fluid. 

[Tullius]

7 hours ago, KSK said:

The big problem with NTR at the moment is political rather than technical. Launching chunks of enriched nukular fuel into orbit seems unlikely to be acceptable to the general public for the foreseeable future. Worse still, if you want a reactor that's light enough to consider launching into space in the first place, it's going to need fairly highly enriched fuel. See this Wikipedia table of critical masses for various pure isotopes - and that's assuming a spherical mass, which is the most efficient shape possible - and which NTRs are unlikely to be using.

You can probably design around the political problems, after all we have launched nearly 10kg of plutonium 238 on New Horizons, and even more on Cassini. You will only need to fulfill 2 conditions:

• You can prove that it is the only sensible propulsion system for this type of mission.
• Make it as safe as possible: On the Apollo missions the RTG for the science package was transported in a casing capable of surviving reentry, preventing the spreading of the plutonium in the atmosphere should the mission result in a failure and the lunar module reenter (as has happened on Apollo 13).

If you observe these restrictions, it is probably doable to launch a NTR into space.

On the other hand, you probably want to do tests on Earth, which could produce a quite significant amount of radioactive waste, which in turn could increase program costs and political opposition to it significantly.

Edited August 14, 2017 by Tullius

[magnemoe]

26 minutes ago, KSK said:

Well the less said about the article the better.

Nuclear thermal rockets - great idea in principle, especially if propellants other than hydrogen are considered. As @shynung pointed out, they'll run quite nicely on tap water, with an ISP comparable to hydrolox engines. Make your NTR a bimodal design, optimize it to use water as a propellant, apply liberal amounts of handwaving and you've got a nice interplanetary craft that's comparatively easy to refuel* elsewhere in the solar system and can use the reactor as a power source whilst in transit.

The big problem with NTR at the moment is political rather than technical. Launching chunks of enriched nukular fuel into orbit seems unlikely to be acceptable to the general public for the foreseeable future. Worse still, if you want a reactor that's light enough to consider launching into space in the first place, it's going to need fairly highly enriched fuel. See this Wikipedia table of critical masses for various pure isotopes - and that's assuming a spherical mass, which is the most efficient shape possible - and which NTRs are unlikely to be using.

*compared to a ship using hydrogen as a propellant.

NTR has two major issues, its the cost of develop it and the lack of missions, like in KSP you need an manned mission or an huge probe mission (europa sample return) for it to make sense. For small probes the engine is to heavy. 

Political its probably more of an congress issue, you have an expensive program with limited use and some politicians would fear the critic from pressure groups, NTR are common and far more of an radiation issue on fail than an non started reactor. 
People protests against nuclear plants in their backyard not nuclear submarines. Am talking huge protest not some loons who also protest because the sun set. 

Neither China nor Russia has shown much interest and they don't have political problems with it. 
makes me wonder if an NTR upper stage would be practical for an manned Moon mission? Know the US looked into it but could not get NTR ready in time. 
We know China aim for this. 

Benefit would be single use short burn time with return to earth in a week if fail, the high ISP should reduce the fuel requirements for the upper stage who also do moon injection. 
The short burn time and then eject would reduce the need for shielding.
Shield and engine weight would be an downside here. You can not use an long boom to get distance from reactor either. 

 

[DDE]

1 hour ago, magnemoe said:

An nuclear reactor don't burn out in reasonable time (years of use)

Many sources claim that NERVA would have burnt out its fuel (yes, fuel) in one TMI, hence many 1960s designs that used multiple nuclear stages.

[wumpus]

12 minutes ago, DDE said:

Many sources claim that NERVA would have burnt out its fuel (yes, fuel) in one TMI, hence many 1960s designs that used multiple nuclear stages.

I suspect it is more a matter of ejecting the core to avoid cooling issues.  Running "exhaust mass" to cool the core is pretty disastrous for Isp, and I'm guessing it needs more cooling gas than the mass of the core.  My guess is that any modern design that cools down the core for re-use is doing so to avoid the political issues of lifting up more "fuel rods".  Turning a NERVA off is a pretty hard problem: ejecting the core is an easy solution.

[Racescort666]

Ever since the proposal for Neptune/Uranus missions was shared, I have been thinking about hypothetical ways to improve the mission, specifically: time of flight. One of the things that irked me about the proposal was the injection burn dV limit: 4.5 km/s. We've all played KSP enough to know that this is a sizable feat to accomplish when your ISP is limited to <=325s so the report makes a good case for the 4.5 km/s limit for good reason. But it also assumes storable bi-propellant propulsion, read: A50 and NTO.

If NTR using water as a propellant were on the table and the ISP of NTR+H2O is 410, the injection could be increased by over 1 km/s. I'm not really sure what this means in regard to time of flight and their flight was pretty complex for getting there with multiple Earth flybys. 

You could even get really creative with an Earth orbit rendezvous to load the water in the upper stage (which is hypothetically much easier to do than with transferring cryogenic fuel) to reduce launch mass. 

[KSK]

5 hours ago, DDE said:

Many sources claim that NERVA would have burnt out its fuel (yes, fuel) in one TMI, hence many 1960s designs that used multiple nuclear stages.

I'd like to see those sources. Not in a snarky way but just out of personal curiosity. I did a fair bit of reading up on NTRs as research for my First Flight novel and found some pretty interesting stuff, including this NASA overview report of Project Rover. Section II is about the basics of nuclear thermal propulsion and the very first paragraph states that:

"The advantage of a nuclear rocket is that it can achieve more than twice the specific impulse of the best chemical rockets. lor a Mars mission, a 5000 MW engine would burn less than an hour to provide the necessary velocity for the mission. The major disadvantage of a nuclear engine is that its exhaust is radioactive, and hence it probably is useful only as an upper-stage engine, operating outside the earth's atmosphere. But the simplicity of design, and the fact that it can start, stop, and restart make it an attractive alternative to conventional chemical rocket engines. In addition, the nuclear engine can be started by using only energy generated by the system itself."

Fuel element erosion was certainly a problem to be solved (and if I recall correctly, was solved, to an acceptable extent) which would ultimately limit the endurance of an NTR but a number of Rover test reactors were fired on numerous occasions, stopped, and restarted. Throttling was also demonstrated - reactors would be run at low power for a certain amount of time, then ramped up to full power. So turning a NERVA off was a solved problem (I think) at the end of Project Rover, so not an insurmountable problem for modern NTRs one would hope.

Basically they used control drums - think a cylinder of neutron absorbing material which is partially coated with a neutron reflecting material. The more of that coating is facing towards the fuel elements, the faster your nuclear reaction goes because you're reflecting all the stray neutrons from the fission reaction right back into the reactor. Conversely, turning the drum so that no reflector is facing the fuel elements, will damp the reaction down. Not to mention that the propellant (for hydrogen containing propellants anyway) is a moderator and has a significant effect on reactor power. In one of the later test designs they could basically stably control the reactor power by varying propellant flow and only had to use the control drums for fine tuning.

6 hours ago, Kerbart said:

Unless you're able to mine it in space I really don't see the value of using water over a higher-I_SP propellant. I'm sure there are more compelling candidates than Hydrogen because of storage & density properties, making up for a lesser I_SP. And if we're mining fuel "for free" in the asteroid belt I think there will be value in an engine that can run practically anything as long as it's fluid. 

Building an NTR that can run on anything is a hard problem. Designing a fuel element coating that can protect against hot reducing propellants (such as hydrogen) or hot oxidising propellants (such as water) isn't too bad but (or so I remember reading) designing a coating that can deal with both is tough. I suppose you could have two sets of channels through the reactor, each with an appropriate coating for the different propellant types but then you'd likely run into all sorts of thermal problems, not to mention those other interactions that @DDE mentioned. I doubt it's impossible but I can certainly imagine it being challenging, to put it mildly.

 

[DDE]

@KSK, referencing @nyrath:

Quote

Dr. John Schilling figures that as an order of magnitude guess, about one day of full power operation would result in enough fuel burnup to require reprocessing of the fissionable fuel elements. (meaning that while there is still plenty of fissionables in the fuel rod, enough by-products have accumulated that the clogged rod produces less and less energy) A reprocessing plant could recover 55-95% of the fuel. With reprocessing, in the long term each totally consumed kilogram of plutonium or highly enriched uranium (HEU) will yield ~1E10 newton-seconds of impulse at a specific impulse of ~1000 seconds.

 

[MaverickSawyer]

I'm curious to see if anyone's considered making an NTR run on methane, and what issues that may pose.

[Racescort666]

4 minutes ago, MaverickSawyer said:

I'm curious to see if anyone's considered making an NTR run on methane, and what issues that may pose.

Also in the link that @shynung shared: http://www.projectrho.com/public_html/rocket/enginelist.php#ntrsolidcore

Quote

Methane is nasty because it breaks down into Hydrogen and Carbon, the latter tends to clog the reactor with soot deposits.

 

Edited August 14, 2017 by Racescort666
added link

[sevenperforce]

18 minutes ago, MaverickSawyer said:

I'm curious to see if anyone's considered making an NTR run on methane, and what issues that may pose.

I've considered it but my supply of plutonium is a bit low.

EDIT: In all seriousness, one of the big things with NTRs is the design. You can get up into some really wicked exhaust velocities if you're willing to bump up to a pebble-bed reactor.

Edited August 14, 2017 by sevenperforce

[tater]

A friend of mine in contact with someone at Marshall who is intimately involved said that they could fly a NTP testbed in a relatively short time period if they could secure resources for it. They are pretty confident.