6 ms·

Can someone explain how uranium turns into thrust?

the8472 6y ago
https://en.wikipedia.org/wiki/Nuclear_thermal_rocket https://en.wikipedia.org/wiki/Nuclear_thermal_rocket
tedk-42 6y ago
When fuel burns, it's the velocity of the exhaust gas that propels the rocket (F=ma). Imagine the same with a nuclear rocket (a reactor getting hot) and then firing out something very quickly through the exhaust. Same F=ma law applies.
- anamexis 6y ago
  I think they were specifically asking about the "firing out something very quickly" part.
- mac01021 6y ago
  But concretely, what is the reaction mass made of and what is the most reasonable mechanism for propelling that mass with the energy of the nuclear reaction?
  - xupybd 6y ago
    Normally liquid hydrogen. Just pass the hydrogen past the reactor. Cooling the reactor and heating the hydrogen. The expansion due to heat send the hydrogen out of the nozzle at high speed, creating thrust. More efficient than a chemical hydrogen and oxygen rocket due to the high energy to weight ratio you get from nuclear fuel.
    - digdigdag 6y ago
      What happens when the rector runs out of fuel? Control rods permanently insert or it just, you know, proceeds to melt down once arrived at Mars?
      - lumost 6y ago
        Good design would either reuse the reactor for other purposes, or ensure it runs out of fuel at roughly the same time as the rocket runs out of reaction mass.
      - db48x 6y ago
        If it's out of fuel then it can no longer generate any heat. The position of the control rods wouldn't matter, and it couldn't melt down. If you're referring to reaction mass (the hydrogen gas), then yes, you would need to insert the control rods to stop the reaction. Or land the reactor and use it to power your Mars colony. If you do the latter, then you will have had to build it with another cooling mechanism in mind. That could increase the mass of the reactor and reduce your payload capacity, so you might not do it. Instead, you might include just enough uranium to make the trip, and no more.
      - Dylan16807 6y ago
        Nobody's going to make a reactor that can't turn off.
      - dajomu 6y ago
        Why would it melt down if the fuel has run out?
    - dmurray 6y ago
      How much more efficient is it? The fact that you have to carry propellant with you anyway seems like a big factor in favour of traditional rockets, where the fuel becomes the propellant. And burning the hydrogen way hotter seems like a big engineering problem - rocket motors already run at temperatures which pose materials science problems, increase that a few thousand degrees and you might as well say we should run the spaceship on fusion power. I don't doubt that the physics works, and it's important to note that say a 2x gain in thrust-to-mass ratio would lead to enormous advances in trips out of Earth's gravitational well (10x? 100x?) due to the tyranny of the rocket equation. But I'm curious whether this is the 2x gain of replacing oxygen+hydrogen with hydrogen, plus a big complex reactor, or the millionfold gain of replacing hydrogen fuels with uranium. Edit: TFA says twice the specific impulse, and presumably that's the optimistic estimate. But still very good!
      - giantrobot 6y ago
        The gains from an NTR over chemical rocket has to do with the temperature, thus velocity, of the exhaust.
      - valuearb 6y ago
        NTR is not anywhere near 2x chemical because of parasitic losses, in fact it may only barely offer higher net DeltaV. NTR requires heavy engines, heavy shielding and heavy radiators to keep cool. The final NERVA prototype was as close to a functional NTR as ever built, and it massed 40,000 lbs while only generated 55,000 lbs of thrust, with a maximum ISP of 710 seconds. A SpaceX Raptor only has a Mac ISP of 380 seconds, but masses only 3,000 lbs, and produces 500,000 lbs of thrust. Add another 50,000 lbs dead mass to the NTR for shielding and cooling, and you see why Raptor will get humans to Mars well before any NTR and just as quickly.
        dr_zoidberg 6y ago
        TWR is only important when you have to climb up the gravity well. Once you're up in orbit, high ISP is king to get you anywhere. Other comments already highlighted that NTRs are a middle ground between superefficient, ultra-high ISP electric drives, and very high TWR chemical rockets. So that's where this sits: high enough TWR to be practical, but way better ISP than chemical (though still far from electrical).
  - deleted 6y ago
    [deleted]
  - credit_guy 6y ago
    Most likely the reaction mass is hydrogen. The NERVA project [1] used it, and the mechanism was quite straightforward: heat the hydrogen to 2250 deg Celsius and let it go. The vacuum specific impulse obtained by NERVA was 841 s (the Space Shuttle main engine had a specific impulse of 453 s) [1] https://en.wikipedia.org/wiki/NERVA https://en.wikipedia.org/wiki/NERVA
    - valuearb 6y ago
      This is another shortcoming of NTRs, as Hydrogen is tiny and slippery, leading to significant amounts of leakage over long trips.
godelski 6y ago
- Pressurize gas (typically hydrogen) - Send gas through nuclear reactor to make it hot - Send hot gas through nozzle This is essentially the what you are doing. Like if you sat on a chair and aimed a fire extinguisher you'd go flying (cold gas thruster, typically used in RCS). You'd go faster if you super heated the gas before it exited the nozzle.
cwojno 6y ago
This article didn't explain it at all, so you're right to ask. tl;dr: Gaseous propellant (I'm guessing hydrogen) is heated with fission then pointed in the opposite direction of intended travel. The uranium in this design is not a propellant, but a heat source. Aside: you can use photons/heat as a propellant, but it's thrust is very low https://en.wikipedia.org/wiki/Pioneer_anomaly https://en.wikipedia.org/wiki/Pioneer_anomaly. Ideal propellants typically have a high exit velocity and low mass. That gives you the longest amount of "burn" time, and the greatest amount of control for the weight. https://en.wikipedia.org/wiki/Specific_impulse https://en.wikipedia.org/wiki/Specific_impulse Back in the day when the US was building more of these nuclear rockets, the propellant of choice was typically hydrogen https://en.wikipedia.org/wiki/NERVA https://en.wikipedia.org/wiki/NERVA. Old timey video explaining it https://youtu.be/eDNX65d-FBY?t=238 https://youtu.be/eDNX65d-FBY?t=238. I'm assuming this proposed design would also use hydrogen, but I couldn't find any sources on the propellant for their design. Liquid hydrogen served to keep the reactor cool as it transitioned from liquid to gas as that phase change absorbs energy. The gas is the directed through the reactor core where the gas heats up. As gases heat up, they absorb energy, their average particle velocities increase. Eventually, the hydrogen molecules (mostly H2 or H-H gaseous hydrogen), makes it to the nozzle and is ejected. The high-velocity hydrogen is what actually provides the bulk of the thrust to the spacecraft. Compare this to Project Orion (https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion) https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...) which intended to detonate nuclear warheads and the craft essentially rode the shock wave into the stars. I would classify this method of propulsion, not safe.
- mrfusion 6y ago
  What happens when the hydrogen runs out? Control rods to slow the reactor? Or eject it?
  - giantrobot 6y ago
    The primary failsafe mode for an NTR would be to insert control rods to stop fission. Without the hydrogen the core wouldn't be able to cool itself and would melt down. There are however NTR core designs with closed circuit cooling. The core would be kept at a low critical state (hot but not melting) and circulate a coolant through the core and into a generator and from there to radiator panels. When the NTR wasn't providing thrust it would provide electrical power. When thrust is needed the coolant loop would cut off and hydrogen would be pumped through the core. Provided no mechanical breakdown in the coolant/generator loop an NTR could provide power for years.

https://en.wikipedia.org/wiki/Nuclear_thermal_rocket https://en.wikipedia.org/wiki/Nuclear_thermal_rocket

When fuel burns, it's the velocity of the exhaust gas that propels the rocket (F=ma). Imagine the same with a nuclear rocket (a reactor getting hot) and then firing out something very quickly through the exhaust. Same F=ma law applies.

anamexis 6y ago
I think they were specifically asking about the "firing out something very quickly" part.
mac01021 6y ago
But concretely, what is the reaction mass made of and what is the most reasonable mechanism for propelling that mass with the energy of the nuclear reaction?
- xupybd 6y ago
  Normally liquid hydrogen. Just pass the hydrogen past the reactor. Cooling the reactor and heating the hydrogen. The expansion due to heat send the hydrogen out of the nozzle at high speed, creating thrust. More efficient than a chemical hydrogen and oxygen rocket due to the high energy to weight ratio you get from nuclear fuel.
  - digdigdag 6y ago
    What happens when the rector runs out of fuel? Control rods permanently insert or it just, you know, proceeds to melt down once arrived at Mars?
    - lumost 6y ago
      Good design would either reuse the reactor for other purposes, or ensure it runs out of fuel at roughly the same time as the rocket runs out of reaction mass.
    - db48x 6y ago
      If it's out of fuel then it can no longer generate any heat. The position of the control rods wouldn't matter, and it couldn't melt down. If you're referring to reaction mass (the hydrogen gas), then yes, you would need to insert the control rods to stop the reaction. Or land the reactor and use it to power your Mars colony. If you do the latter, then you will have had to build it with another cooling mechanism in mind. That could increase the mass of the reactor and reduce your payload capacity, so you might not do it. Instead, you might include just enough uranium to make the trip, and no more.
    - Dylan16807 6y ago
      Nobody's going to make a reactor that can't turn off.
    - dajomu 6y ago
      Why would it melt down if the fuel has run out?
  - dmurray 6y ago
    How much more efficient is it? The fact that you have to carry propellant with you anyway seems like a big factor in favour of traditional rockets, where the fuel becomes the propellant. And burning the hydrogen way hotter seems like a big engineering problem - rocket motors already run at temperatures which pose materials science problems, increase that a few thousand degrees and you might as well say we should run the spaceship on fusion power. I don't doubt that the physics works, and it's important to note that say a 2x gain in thrust-to-mass ratio would lead to enormous advances in trips out of Earth's gravitational well (10x? 100x?) due to the tyranny of the rocket equation. But I'm curious whether this is the 2x gain of replacing oxygen+hydrogen with hydrogen, plus a big complex reactor, or the millionfold gain of replacing hydrogen fuels with uranium. Edit: TFA says twice the specific impulse, and presumably that's the optimistic estimate. But still very good!
    - giantrobot 6y ago
      The gains from an NTR over chemical rocket has to do with the temperature, thus velocity, of the exhaust.
    - valuearb 6y ago
      NTR is not anywhere near 2x chemical because of parasitic losses, in fact it may only barely offer higher net DeltaV. NTR requires heavy engines, heavy shielding and heavy radiators to keep cool. The final NERVA prototype was as close to a functional NTR as ever built, and it massed 40,000 lbs while only generated 55,000 lbs of thrust, with a maximum ISP of 710 seconds. A SpaceX Raptor only has a Mac ISP of 380 seconds, but masses only 3,000 lbs, and produces 500,000 lbs of thrust. Add another 50,000 lbs dead mass to the NTR for shielding and cooling, and you see why Raptor will get humans to Mars well before any NTR and just as quickly.
      - dr_zoidberg 6y ago
        TWR is only important when you have to climb up the gravity well. Once you're up in orbit, high ISP is king to get you anywhere. Other comments already highlighted that NTRs are a middle ground between superefficient, ultra-high ISP electric drives, and very high TWR chemical rockets. So that's where this sits: high enough TWR to be practical, but way better ISP than chemical (though still far from electrical).
- deleted 6y ago
  [deleted]
- credit_guy 6y ago
  Most likely the reaction mass is hydrogen. The NERVA project [1] used it, and the mechanism was quite straightforward: heat the hydrogen to 2250 deg Celsius and let it go. The vacuum specific impulse obtained by NERVA was 841 s (the Space Shuttle main engine had a specific impulse of 453 s) [1] https://en.wikipedia.org/wiki/NERVA https://en.wikipedia.org/wiki/NERVA
  - valuearb 6y ago
    This is another shortcoming of NTRs, as Hydrogen is tiny and slippery, leading to significant amounts of leakage over long trips.

godelski 6y ago

- Pressurize gas (typically hydrogen) - Send gas through nuclear reactor to make it hot - Send hot gas through nozzle This is essentially the what you are doing. Like if you sat on a chair and aimed a fire extinguisher you'd go flying (cold gas thruster, typically used in RCS). You'd go faster if you super heated the gas before it exited the nozzle.

cwojno 6y ago

This article didn't explain it at all, so you're right to ask. tl;dr: Gaseous propellant (I'm guessing hydrogen) is heated with fission then pointed in the opposite direction of intended travel. The uranium in this design is not a propellant, but a heat source. Aside: you can use photons/heat as a propellant, but it's thrust is very low https://en.wikipedia.org/wiki/Pioneer_anomaly https://en.wikipedia.org/wiki/Pioneer_anomaly. Ideal propellants typically have a high exit velocity and low mass. That gives you the longest amount of "burn" time, and the greatest amount of control for the weight. https://en.wikipedia.org/wiki/Specific_impulse https://en.wikipedia.org/wiki/Specific_impulse Back in the day when the US was building more of these nuclear rockets, the propellant of choice was typically hydrogen https://en.wikipedia.org/wiki/NERVA https://en.wikipedia.org/wiki/NERVA. Old timey video explaining it https://youtu.be/eDNX65d-FBY?t=238 https://youtu.be/eDNX65d-FBY?t=238. I'm assuming this proposed design would also use hydrogen, but I couldn't find any sources on the propellant for their design. Liquid hydrogen served to keep the reactor cool as it transitioned from liquid to gas as that phase change absorbs energy. The gas is the directed through the reactor core where the gas heats up. As gases heat up, they absorb energy, their average particle velocities increase. Eventually, the hydrogen molecules (mostly H2 or H-H gaseous hydrogen), makes it to the nozzle and is ejected. The high-velocity hydrogen is what actually provides the bulk of the thrust to the spacecraft. Compare this to Project Orion (https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion) https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...) which intended to detonate nuclear warheads and the craft essentially rode the shock wave into the stars. I would classify this method of propulsion, not safe.

mrfusion 6y ago
What happens when the hydrogen runs out? Control rods to slow the reactor? Or eject it?
- giantrobot 6y ago
  The primary failsafe mode for an NTR would be to insert control rods to stop fission. Without the hydrogen the core wouldn't be able to cool itself and would melt down. There are however NTR core designs with closed circuit cooling. The core would be kept at a low critical state (hot but not melting) and circulate a coolant through the core and into a generator and from there to radiator panels. When the NTR wasn't providing thrust it would provide electrical power. When thrust is needed the coolant loop would cut off and hydrogen would be pumped through the core. Provided no mechanical breakdown in the coolant/generator loop an NTR could provide power for years.