Nuclear Thermal Rocket (NTR) Propulsion

[Funkworks]

Looks like the idea is quite healthy:

"On Wednesday, NASA and DARPA announced they had selected Lockheed Martin to serve as the primary contractor to assemble the experimental nuclear thermal reactor vehicle (X-NTRV) and its engine. BWX Technologies will be one of Lockheed Martin’s partners, and it will develop the nuclear reactor and fabricate the high-assay low-enriched uranium fuel to power the reactor."

"The basic idea is straightforward: A nuclear reactor rapidly heats up a propellant, probably liquid hydrogen, and then this gas expands and is passed out a nozzle, creating thrust."

https://arstechnica.com/space/2023/...-nuclear-powered-rocket-engine-in-four-years/
https://www.engadget.com/nasa-picks...-rocket-thatll-take-us-to-mars-170035659.html
https://www.space.com/nasa-darpa-nuclear-thermal-rocket-draco-2026
https://www.nasa.gov/press-release/nasa-darpa-experts-to-discuss-nuclear-rocket-program-developments
It will be interesting to follow progress in the coming months and years.

 

[cls]

No matter what happens, it'll be fun!

Seriously, finally, way past time to try this.

 

[techrat]

Seriously, about time. I remember reading about this when I was a kid. Of course, in the books back then, I was told that by the year 2000, we'd have a moonbase and we'd already by trying to colonize Mars. I feel like we're 40 years behind.

 

[aerostadt]

Thanks for the update! I hope something good comes out of this. As one of the articles mentioned the NERVA program was cancelled in 1972.

 

[SDramstad]

Seriously, about time. I remember reading about this when I was a kid. Of course, in the books back then, I was told that by the year 2000, we'd have a moonbase and we'd already by trying to colonize Mars. I feel like we're 40 years behind.

We're not 40 years behind, we're 50 years behind.......

 

[sr205347d]

"The basic idea is straightforward: A nuclear reactor rapidly heats up a propellant, probably liquid hydrogen, and then this gas expands and is passed out a nozzle, creating thrust."

Why do they want to use liquid hydrogen? The stuff is a PITA to produce, store, and handle. Seems like they could use just about anything, since there is no combustion or other chemical reactions going on.

 

[aerostadt]

It is because from gas dynamics the equations show that specific impulse is proportional to the square root of the chamber temperature divided by the molecular weight. Let us say that we use water (steam) instead of hydrogen. That is a molecular weight of 18 versus 2 or a ratio of 9, which has a square root of 3. So, if the chamber temperature is the same for both propellants, the specific impulse for water will be 1/3 of that for hydrogen.

 

[OverTheTop]

Hydrogen gas has a very low density and requires large, heavy tanks to store. Even the LH2 tanks have to be large. There is a significant mass penalty, especially so for GH2.

 

[OverTheTop]

There is a great chapter in Rocket and Spacecraft Propulsion by Turner on nuclear rocket engines. I would actually highly recommend the book in general.

 

[Funkworks]

Why do they want to use liquid hydrogen? The stuff is a PITA to produce, store, and handle. Seems like they could use just about anything, since there is no combustion or other chemical reactions going on.

"Other propellants have also been proposed, such as ammonia, water, or LOX, but these propellants would provide reduced exhaust velocity and performance at a marginally reduced fuel cost. Yet another mark in favor of hydrogen is that at low pressures it begins to dissociate at about 1500 K, and at high pressures around 3000 K. This lowers the mass of the exhaust species, increasing Isp."

https://en.wikipedia.org/wiki/Nuclear_thermal_rocket
I think most designers would first select the ideal materials for the purpose (propulsion), and then see how practical or expensive it is to produce. Seems to me like any other gas would provide less thrust, and therefore result in a longer flight. So there's a trade-off between cost and how long a mission would last, which can be weeks, months or years, depending on how far into the solar system the rocket is meant to go.

 

[sr205347d]

I think most designers would first select the ideal materials for the purpose (propulsion), and then see how practical or expensive it is to produce.

Right. For example, SpaceX originally considered carbon fiber for the Starship because it has the best strength to weight ratio. But stainless steel is better because of ease of manufacturing, good strength at low as well as high temperatures, and of course, cost.

Perhaps methane might prove to be the more practical fuel for NTRs.

 

[aerostadt]

Molecular weight for methane is 18, which is about the same as water. Might as well use water.

 

[sr205347d]

Might as well use water.

There is that little problem with freezing.

I suppose the ultimate fuel choice may depend on where you are sourcing it.

You would rather not have to lift your interplanetary fuel out of the Earth's gravity well.

There is no methane on the Moon, but hydrogen could be produced there from the ice deposits in polar craters and solar energy.

Methane could be produced on Mars.

 

[techrat]

There is no methane on the Moon, but hydrogen could be produced there from the ice deposits in polar craters and solar energy.

Speaking of which, I wonder if using Helium-3 would be an interesting choice for a Nuke-Ular powered rocket.

 

[Antares JS]

Speaking of which, I wonder if using Helium-3 would be an interesting choice for a Nuke-Ular powered rocket.

Not really. It's a pain to get and we're not talking about fusion. The propellant of a nuclear thermal rocket is heated via nuclear energy and expelled from a nozzle, similar to how a conventional rocket works.

 

[sr205347d]

Speaking of which, I wonder if using Helium-3 would be an interesting choice for a Nuke-Ular powered rocket.

Sure! Fusion reactors are only 30 years away. Always have been.

 

[aerostadt]

We talked about lunar mining of Helium-3 in another TRF thread last year. The amount of lunar rock that needs to be mined and process is staggering. Robert Zubrin also talks about this issue in his book, "The Case for Space".
https://ui.adsabs.harvard.edu/abs/2014cosp...40E1515K/abstract

 

[Funkworks]

This is about a controlled nuclear reactor heating up a gas, much like where Homer Simpson works, except instead of being pressured against a turbine, the heated gas is expelled.

For space flight, I would summarize the choice of gas as a trade-off between cost and mission duration. If you want to fly far in the least amount of time, you use hydrogen. If you're ok with a shorter trip that lasts longer, you pick another gas. So it depends on what the clients/investors want exactly.

I don't think digging up resources from a another planet or moon is part of this particular plan. Demonstrating nuclear-heated gas propulsion, in space, would in itself be a first. Once that is successful, you attract investments to move ahead with other ideas that rely on the working demonstration.

 

[aerostadt]

I don't think digging up resources from a another planet or moon is part of this particular plan. Demonstrating nuclear-heated gas propulsion, in space, would in itself be a first. Once that is successful, you attract investments to move ahead with other ideas that rely on the working demonstration.

I tend to think that goes back to the first post story. It is necessary to have a basic demonstration, which has not been done, yet. There are several basic issues that will need to be addressed later; like how is the radiation handled, especially for human flight, how is decay heat handled after the core is shut-down, can the core be re-started again and how many times before the core must be re-fueled. These are questions that should be addressed in order to fully take advantage of a nuclear thermal rocket, but a starting point is needed first.

 

[Wayco]

It is because from gas dynamics the equations show that specific impulse is proportional to the square root of the chamber temperature divided by the molecular weight. Let us say that we use water (steam) instead of hydrogen. That is a molecular weight of 18 versus 2 or a ratio of 9, which has a square root of 3. So, if the chamber temperature is the same for both propellants, the specific impulse for water will be 1/3 of that for hydrogen.

HUH? You can really tell who the real rocket scientists are on this thread, can't you...

 

[techrat]

HUH? You can really tell who the real rocket scientists are on this thread, can't you...

What.. wait... there's SCIENCE involved? Shoot, I just build rockets.

 

[Grog6]

The big problem they ran into in the 50s was that when you run cryo h2 across/thru a red hot ceramic reactor it shatters, and gets spit out the nozzle. There's video, online, and a description in one of the books I read.

 

[Ronz Rocketz]

"Other propellants have also been proposed, such as ammonia, water, or LOX, but these propellants would provide reduced exhaust velocity and performance at a marginally reduced fuel cost. Yet another mark in favor of hydrogen is that at low pressures it begins to dissociate at about 1500 K, and at high pressures around 3000 K. This lowers the mass of the exhaust species, increasing Isp."

https://en.wikipedia.org/wiki/Nuclear_thermal_rocket
I think most designers would first select the ideal materials for the purpose (propulsion), and then see how practical or expensive it is to produce. Seems to me like any other gas would provide less thrust, and therefore result in a longer flight. So there's a trade-off between cost and how long a mission would last, which can be weeks, months or years, depending on how far into the solar system the rocket is meant to go.

I used to think about it every time I drove by on I-95.

https://goo.gl/maps/QucKyDgKANL1H4hi7
What's cool is when I scan around the map and see buildings with cars parked in front. I wonder what they're doing in there...

As far as I know, there's no nuclear tests so what devices are they assembling here?



Uh oh, I hear black helicopters in the distance. I gotta go...

 

[Wayco]

What.. wait... there's SCIENCE involved? Shoot, I just build rockets.

Yeah, I'm with you on that. The only thing that works for me is I figure things out. I Don't use formula's to determine how big my ejection charge needs to be. Just add more black powder until it works.

 

[aerostadt]

There is a lot of good information in this thread about NTR. Aside from all the technical aspects discussed, just the proof of concept needs to be shown. That is where the very first post is important. A large NTR put into space has not been demonstrated. There are all kinds of safety issues involved such as what happens if there is a launch abort and/or unplanned re-entry. I am wondering how control rods can be moved in out of the core. Obviously, electric motors can do this, no problem. However, the option of having the control rods drop by gravity as in many commercial light water reactors is not an option. The basic NTR concept has yet to be demonstrated. (I suspect that he plutonium thermal electric generators used in interplanetary space probes were smaller in total energy. Even those devices had their detractors.)

 

[OverTheTop]

There are all kinds of safety issues involved such as what happens if there is a launch abort and/or unplanned re-entry. I am wondering how control rods can be moved in out of the core.

If you have a read of the book chapter I linked earlier your questions should be answered. Post #9.

TLDR: The reactor has no significant radioactivity or radiation until the reactor is actually started up and other short-lived decay products are created. If this is done after launch then anything falling back to earth in the event of an anomaly will be essentially a big inert mass. Regarding control rods, one strategy is to use a neutron reflector on the outside of the core instead. It would be driven by electric motor(s).

 

[OverTheTop]

Here is a good article by the Royal Aeronautical Society
https://www.aerosociety.com/news/atomic-rockets-are-back

 

[aerostadt]

TLDR: The reactor has no significant radioactivity or radiation until the reactor is actually started up and other short-lived decay products are created. If this is done after launch then anything falling back to earth in the event of an anomaly will be essentially a big inert mass. Regarding control rods, one strategy is to use a neutron reflector on the outside of the core instead. It would be driven by electric motor(s).

That is a good point. In my early engineering days (about 1972) I visited the Westinghouse facility in Rayleigh, N. Carolina for making Uranium dioxide fuel pellets for Pressurized Water Reactors (PWR). Enriched Uranium Hexaflouride (UF6) was brought in from the Atomic Energy Commission (AEC, which was later renamed the Nuclear Regulatory Commission (NRC) ). There are a number of chemical reactions to convert UF6 into UO2. The UO2 powder was pressed into pellets and then put through a sintering oven. We were probably only 20 or 30 feet away from the pellets and then was no concern about radiation.