24 ms·

DARPA moving forward with nuclear thermal engine design

215 points by tectonic 4y ago

_Microft 4y ago
If I remember correctly, SpaceX would be happy to experiment with nuclear-thermal propulsion but cited the lack of a engine test stand as reason why they aren't actively working on it. I'll see if I can find a quote for that. I am rather sure that it was by Gwynne Shotwell, COO of SpaceX. (Edit: progress! I think it's in a talk by her at MIT Road to Mars 2017. Too bad I cannot find a recording of that). NERVA is another term to search for if you are interested in nuclear-thermal propulsion. https://en.wikipedia.org/wiki/NERVA https://en.wikipedia.org/wiki/NERVA
- oldstrangers 4y ago
  Whats the relevance of SpaceX here?
  - robonerd 4y ago
    It's tangentially related insofar as SpaceX says they're planning to go to Mars, and this NTR engine is also for going to Mars. But according to the DARPA announcement, DARPA determined that Falcon 9 doesn't presently support this sort of liquid hydrogen payload. They suggest Vulcan Centaur could do it with fairing modifications. (Vulcan Centaur hasn't flown yet. Where are the engines, Jeff??)
    - mlindner 4y ago
      It's not really related as SpaceX has no plans to use them and isn't exactly interested in doing so as they don't see them as needed. Also NTR are kind of a tossup on efficiency as while you get somewhat better fuel efficiency, their mass is huge because you're lugging an entire nuclear reactor core along with with you. The thrust to weight ratio isn't great.
      - leereeves 4y ago
        Can you mount the nuclear reactor far away from the crew module and reduce the mass of shielding, as in old sci-fi?
        mlindner 4y ago
        It's not just the shielding. An NTR engine needs a lot of the same plumbing that other engines need, including turbopumps, and they have additional cooling requirements because of the much hotter fuel. Add on to that the already very heavy Uranium and control rods. Add on to that I'm not quite sure how you prevent the engine's nuclear reactor from going into meltdown once it shuts off. The residual heat from the decay products in the seconds to minutes after shutdown will be substantial and that heat needs to go somewhere or it'll cause a reactor meltdown the instant you shut off the engine. So you need all the hardware to dump heat somewhere (presumably radiators and a cooling system that pumps hydrogen through the reactor while it's shut off) so that's even more mass. The only way NTR really makes sense to me is if your spacecraft is truly massive, but literally no one has anything like that even in planning stages.
        Robotbeat 4y ago
        The fuel is actually cooler for NTR than chemical. With chemical, the peak heat can occur in the gaseous state away from anything solid, but for conventional nuclear thermal, the peak heat is generated in solid material and needs to conduct through to fluids, which are therefore at lower temperatures. And the way they handle shut down is they continue a small flow of propellant through the engine until the core cools off and the hottest, shortest lived stuff decays away. NTRs usually run for a few hours at most, not years, so the decay heat a few minutes after shutdown isn’t that bad.
        mlindner 4y ago
        > The fuel is actually cooler for NTR than chemical. With chemical, the peak heat can occur in the gaseous state away from anything solid, but for conventional nuclear thermal, the peak heat is generated in solid material and needs to conduct through to fluids, which are therefore at lower temperatures. Pretty sure this can't be true. In order to have a higher exhaust velocity the fuel temperature needs to be higher than chemical propulsion.
  - mlindner 4y ago
    There isn't one really other than SpaceX COO (or was it Musk?) making a single passing reference to them in response to a question at a conference keynote a few years ago.
  - ClumsyPilot 4y ago
    As much as i like them, SpaceX approach of move fast with explosion is best kept away from nuclear :) Also they don't seem to have relevant experience
- dotnet00 4y ago
  Given the regulatory delays and uncertainty of somewhat safer things like approval for orbital launches from Starbase, I imagine that SpaceX would not be all too eager to experiment with NTR given the regulatory environment for anything nuclear and that they want to get Starship flying humans within this decade. The regulatory environment is bad enough that I still expect this to eventually get cancelled again, only to be taken seriously when eventually another country is close to catching up technologically.
  - throwaway0a5e 4y ago
    > imagine that SpaceX would not be all too eager to experiment with NTR given the regulatory environment for anything nuclear and that they want to get Starship flying humans within this decade. Or they might want to do it anyway knowing it would never be allowed to launch in order to drag the overton window in a more permissive direction.
hendler 4y ago
Link with anchor https://orbitalindex.com/archive/2022-05-25-Issue-170/#darpa-moving-forward-with-draco-nuclear-thermal-engine https://orbitalindex.com/archive/2022-05-25-Issue-170/#darpa...
XorNot 4y ago
I really hope this gets off the ground - literally and figuratively. NTRs would be a game changer for Sol exploration and open up some real serious options for things like intercepting interstellar objects.
robonerd 4y ago
The PDFs here have a lot more information: https://sam.gov/opp/af490b568d2a438498afa1e80bce63e5/view https://sam.gov/opp/af490b568d2a438498afa1e80bce63e5/view A few takeaways; they intend for such an engine to eventually support long duration human spaceflight (going to Mars.) The propellant for the NTR engine to be liquid hydrogen. One of the problems DARPA anticipates with using such an engine for such a mission is needing to store liquid hydrogen longer than the present state of the art. The PDF doesn't seem to mention it, but I think the Advanced Cryogenic Evolved Stage (ACES) is probably relevant to this project. Does anybody know what kind of duration they expect to get from ACES? I'm not sure but I think it's weeks, not months.
- ceejayoz 4y ago
  Could you make the hydrogen en-route? Solar power to crack water, use the oxygen for breathing? https://www.nasa.gov/content/space-applications-of-hydrogen-and-fuel-cells https://www.nasa.gov/content/space-applications-of-hydrogen-... says this is done on the ISS currently.
  - nicoburns 4y ago
    Where are they going to get the water from?
    - ceejayoz 4y ago
      You'd be bringing oxygen and hydrogen along anyways. Why not bring it in water form?
    - evgen 4y ago
      Much easier to store water for long duration than to store hydrogen. It even serves a useful purpose as radiation shielding for some solar events.
  - robonerd 4y ago
    Maybe? I believe LH2 has about 35 mols of hydrogen per liter, while water is 55 mols per liter. Storing hydrogen as water seems practical from that perspective, but what of the power needed to split that water? I think you'd need quite a lot of power to split that much water fast (starting a few days before running the engine.) Splitting it slowly over time using solar energy would seem to still leave you with a storage problem, but perhaps a more tractable one. Maybe instead of electrolysis, they could use heat from the reactor? Thermolysis needs 2500 C though.
jahabrewer 4y ago
This would be for a ship that stays in space, right? (as in, not using an NTR from ground to orbit)
- Robotbeat 4y ago
  That’s right. NTR not that useful for launch to orbit anyway due to the really terrible thrust to weight ratio (compared to chemical) and the poor density.
danans 4y ago
Darn, I clicked on it hoping for a new terrestrial energy production technology.
- ncmncm 4y ago
  True, what we have certainly won't do. Fortunately, it doesn't need to.
Stevvo 4y ago
We have known how to build working NTRs since before the moon landings. They are a proven technology but we decided is was not worth the risk to fly them. What changed? Or will this rocket stay firmly on the ground?
- Robotbeat 4y ago
  It’s not risk but cost and also difficulty in ground testing safely. What changed is they may not test them on the ground but in orbit. Just design it very conservatively and launch to a safe orbit and test there. Technology can just progress, nothing massive needs to change. DARPA sees that the time is ready to advance this technology once again. They will test it first at very small scale. The purpose is deep space space force robotic vehicles being able to make lots of maneuvers (to avoid ASAT? To do multiple missions? Changing orbit to avoid detection?) with high thrust, ie quickly.
- maccam94 4y ago
  From what I've read NERVA was actually killed by Nixon for political (cost) reasons. https://en.wikipedia.org/wiki/NERVA#Cancellation https://en.wikipedia.org/wiki/NERVA#Cancellation
timcavel 4y ago
marktangotango 4y ago
Presumably this is a solid core design and since these would never fly in the atmosphere anyway, I've always thought that going all in on nuclear salt water engines would be the way to go [1]. These things are so high performance, I bet even a small/micro one could enable tic tac levels of performance, buts that just a guess. [1] https://en.wikipedia.org/wiki/Nuclear_salt-water_rocket https://en.wikipedia.org/wiki/Nuclear_salt-water_rocket
deleted 4y ago
[deleted]
TedDoesntTalk 4y ago
> One key challenge with nuclear reactors in space is the risk of contaminating Earth What?
- moffkalast 4y ago
  ONE KEY CHALLENGE WITH NUCLEAR REACTORS IN SPACE IS THE RISK OF CONTAMINATING EARTH
  - doctor_eval 4y ago
    Well, I laughed, anyway.
- bobsmooth 4y ago
  Should the rocket explode during launch.
  - DennisP 4y ago
    That's not much of a risk, if you haven't ground-tested the engine. Nuclear fuel is barely radioactive before fission. It's the waste products of fission that you have to watch out for.

215 points by tectonic 4y ago

If I remember correctly, SpaceX would be happy to experiment with nuclear-thermal propulsion but cited the lack of a engine test stand as reason why they aren't actively working on it. I'll see if I can find a quote for that. I am rather sure that it was by Gwynne Shotwell, COO of SpaceX. (Edit: progress! I think it's in a talk by her at MIT Road to Mars 2017. Too bad I cannot find a recording of that). NERVA is another term to search for if you are interested in nuclear-thermal propulsion. https://en.wikipedia.org/wiki/NERVA https://en.wikipedia.org/wiki/NERVA

oldstrangers 4y ago
Whats the relevance of SpaceX here?
- robonerd 4y ago
  It's tangentially related insofar as SpaceX says they're planning to go to Mars, and this NTR engine is also for going to Mars. But according to the DARPA announcement, DARPA determined that Falcon 9 doesn't presently support this sort of liquid hydrogen payload. They suggest Vulcan Centaur could do it with fairing modifications. (Vulcan Centaur hasn't flown yet. Where are the engines, Jeff??)
  - mlindner 4y ago
    It's not really related as SpaceX has no plans to use them and isn't exactly interested in doing so as they don't see them as needed. Also NTR are kind of a tossup on efficiency as while you get somewhat better fuel efficiency, their mass is huge because you're lugging an entire nuclear reactor core along with with you. The thrust to weight ratio isn't great.
    - leereeves 4y ago
      Can you mount the nuclear reactor far away from the crew module and reduce the mass of shielding, as in old sci-fi?
      - mlindner 4y ago
        It's not just the shielding. An NTR engine needs a lot of the same plumbing that other engines need, including turbopumps, and they have additional cooling requirements because of the much hotter fuel. Add on to that the already very heavy Uranium and control rods. Add on to that I'm not quite sure how you prevent the engine's nuclear reactor from going into meltdown once it shuts off. The residual heat from the decay products in the seconds to minutes after shutdown will be substantial and that heat needs to go somewhere or it'll cause a reactor meltdown the instant you shut off the engine. So you need all the hardware to dump heat somewhere (presumably radiators and a cooling system that pumps hydrogen through the reactor while it's shut off) so that's even more mass. The only way NTR really makes sense to me is if your spacecraft is truly massive, but literally no one has anything like that even in planning stages.
        Robotbeat 4y ago
        The fuel is actually cooler for NTR than chemical. With chemical, the peak heat can occur in the gaseous state away from anything solid, but for conventional nuclear thermal, the peak heat is generated in solid material and needs to conduct through to fluids, which are therefore at lower temperatures. And the way they handle shut down is they continue a small flow of propellant through the engine until the core cools off and the hottest, shortest lived stuff decays away. NTRs usually run for a few hours at most, not years, so the decay heat a few minutes after shutdown isn’t that bad.
        mlindner 4y ago
        > The fuel is actually cooler for NTR than chemical. With chemical, the peak heat can occur in the gaseous state away from anything solid, but for conventional nuclear thermal, the peak heat is generated in solid material and needs to conduct through to fluids, which are therefore at lower temperatures. Pretty sure this can't be true. In order to have a higher exhaust velocity the fuel temperature needs to be higher than chemical propulsion.
- mlindner 4y ago
  There isn't one really other than SpaceX COO (or was it Musk?) making a single passing reference to them in response to a question at a conference keynote a few years ago.
- ClumsyPilot 4y ago
  As much as i like them, SpaceX approach of move fast with explosion is best kept away from nuclear :) Also they don't seem to have relevant experience
dotnet00 4y ago
Given the regulatory delays and uncertainty of somewhat safer things like approval for orbital launches from Starbase, I imagine that SpaceX would not be all too eager to experiment with NTR given the regulatory environment for anything nuclear and that they want to get Starship flying humans within this decade. The regulatory environment is bad enough that I still expect this to eventually get cancelled again, only to be taken seriously when eventually another country is close to catching up technologically.
- throwaway0a5e 4y ago
  > imagine that SpaceX would not be all too eager to experiment with NTR given the regulatory environment for anything nuclear and that they want to get Starship flying humans within this decade. Or they might want to do it anyway knowing it would never be allowed to launch in order to drag the overton window in a more permissive direction.

hendler 4y ago

Link with anchor https://orbitalindex.com/archive/2022-05-25-Issue-170/#darpa-moving-forward-with-draco-nuclear-thermal-engine https://orbitalindex.com/archive/2022-05-25-Issue-170/#darpa...

XorNot 4y ago

I really hope this gets off the ground - literally and figuratively. NTRs would be a game changer for Sol exploration and open up some real serious options for things like intercepting interstellar objects.

robonerd 4y ago

The PDFs here have a lot more information: https://sam.gov/opp/af490b568d2a438498afa1e80bce63e5/view https://sam.gov/opp/af490b568d2a438498afa1e80bce63e5/view A few takeaways; they intend for such an engine to eventually support long duration human spaceflight (going to Mars.) The propellant for the NTR engine to be liquid hydrogen. One of the problems DARPA anticipates with using such an engine for such a mission is needing to store liquid hydrogen longer than the present state of the art. The PDF doesn't seem to mention it, but I think the Advanced Cryogenic Evolved Stage (ACES) is probably relevant to this project. Does anybody know what kind of duration they expect to get from ACES? I'm not sure but I think it's weeks, not months.

ceejayoz 4y ago
Could you make the hydrogen en-route? Solar power to crack water, use the oxygen for breathing? https://www.nasa.gov/content/space-applications-of-hydrogen-and-fuel-cells https://www.nasa.gov/content/space-applications-of-hydrogen-... says this is done on the ISS currently.
- nicoburns 4y ago
  Where are they going to get the water from?
  - ceejayoz 4y ago
    You'd be bringing oxygen and hydrogen along anyways. Why not bring it in water form?
  - evgen 4y ago
    Much easier to store water for long duration than to store hydrogen. It even serves a useful purpose as radiation shielding for some solar events.
- robonerd 4y ago
  Maybe? I believe LH2 has about 35 mols of hydrogen per liter, while water is 55 mols per liter. Storing hydrogen as water seems practical from that perspective, but what of the power needed to split that water? I think you'd need quite a lot of power to split that much water fast (starting a few days before running the engine.) Splitting it slowly over time using solar energy would seem to still leave you with a storage problem, but perhaps a more tractable one. Maybe instead of electrolysis, they could use heat from the reactor? Thermolysis needs 2500 C though.

jahabrewer 4y ago

This would be for a ship that stays in space, right? (as in, not using an NTR from ground to orbit)

Robotbeat 4y ago
That’s right. NTR not that useful for launch to orbit anyway due to the really terrible thrust to weight ratio (compared to chemical) and the poor density.

danans 4y ago

Darn, I clicked on it hoping for a new terrestrial energy production technology.

ncmncm 4y ago
True, what we have certainly won't do. Fortunately, it doesn't need to.

Stevvo 4y ago

We have known how to build working NTRs since before the moon landings. They are a proven technology but we decided is was not worth the risk to fly them. What changed? Or will this rocket stay firmly on the ground?

Robotbeat 4y ago
It’s not risk but cost and also difficulty in ground testing safely. What changed is they may not test them on the ground but in orbit. Just design it very conservatively and launch to a safe orbit and test there. Technology can just progress, nothing massive needs to change. DARPA sees that the time is ready to advance this technology once again. They will test it first at very small scale. The purpose is deep space space force robotic vehicles being able to make lots of maneuvers (to avoid ASAT? To do multiple missions? Changing orbit to avoid detection?) with high thrust, ie quickly.
maccam94 4y ago
From what I've read NERVA was actually killed by Nixon for political (cost) reasons. https://en.wikipedia.org/wiki/NERVA#Cancellation https://en.wikipedia.org/wiki/NERVA#Cancellation

timcavel 4y ago

marktangotango 4y ago

Presumably this is a solid core design and since these would never fly in the atmosphere anyway, I've always thought that going all in on nuclear salt water engines would be the way to go [1]. These things are so high performance, I bet even a small/micro one could enable tic tac levels of performance, buts that just a guess. [1] https://en.wikipedia.org/wiki/Nuclear_salt-water_rocket https://en.wikipedia.org/wiki/Nuclear_salt-water_rocket

deleted 4y ago

[deleted]

TedDoesntTalk 4y ago

> One key challenge with nuclear reactors in space is the risk of contaminating Earth What?

moffkalast 4y ago
ONE KEY CHALLENGE WITH NUCLEAR REACTORS IN SPACE IS THE RISK OF CONTAMINATING EARTH
- doctor_eval 4y ago
  Well, I laughed, anyway.
bobsmooth 4y ago
Should the rocket explode during launch.
- DennisP 4y ago
  That's not much of a risk, if you haven't ground-tested the engine. Nuclear fuel is barely radioactive before fission. It's the waste products of fission that you have to watch out for.