4 ms·

Darn, it seems nuclear thermal propulsion is not the same as the Project Orion "spacecraft intended to be directly propelled by a series of explosions of atomic

Darn, it seems nuclear thermal propulsion is not the same as the Project Orion "spacecraft intended to be directly propelled by a series of explosions of atomic bombs behind the craft (nuclear pulse propulsion)". https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion) https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...

Symmetry 6y ago
Yeah, this is nuclear thermal where uranium is burned as fuel to heat a hydrogen propellant. That limits the heat and energy the propellant can absorb so while the efficiency end up being high due to using all-hydrogen fuel with a very low molecular weight the thrust isn't that great. These are good for second stages or transfer once in orbit but they don't make very good booster engines. Well, not unless you're going to do something really exotic like a gas phase nuclear lightbulb design[1]. But thankfully it won't be releasing any radiation into the atmosphere if it's working properly, the hydrogen fuel has to absorb two neutrons on its pass through to turn radioactive and it really doesn't want to absorb that second neutron. And if something goes wrong before its turned on uranium, even enriched uranium, isn't that radioactive and honestly there are scarier heavy metals that aren't radioactive at all. Orion, by contrast, uses its bombs effectively as both fuel and propellant and achieves far greater temperatures/efficiencies and power. It's a stupendously efficient and effective engine. However, that does mean that you've got nuclear reaction byproducts coming out of the engine which are pretty radioactive and also will tend to fall out of the atmosphere onto us down below in a reasonable time. Each one only produces a little but you'll be setting off many of these and overall you're looking at the same level of fallout as a multi-megaton thermonuclear bomb airburst. And we banned open air test of those for a reason. Still, if you're launching from some place that's already exposed to cosmic radiation and the solar wind, like the surface of the Moon, it might be a useful means of transport there. [1]https://en.wikipedia.org/wiki/Nuclear_lightbulb https://en.wikipedia.org/wiki/Nuclear_lightbulb
- Sharlin 6y ago
  Just to be clear, uranium is not literally burned in a nuclear thermal rocket. Unless things are very very wrong and you’re not going to space today. Nuclear thermal propulsion is simply using a fission (or more speculatively fusion) reactor to heat hydrogen gas which is then allowed to escape at high velocities from the business end of the rocket. In theory the heat source could be anything, but very few things offer remotely as much W/kg as nuclear.
  - Symmetry 6y ago
    Saying that nuclear fuels "burn" when you cause them to undergo fission in a reactor is a common colloquialism. Hence standard terms like "burnup rates" when describing the efficiency of reactors. https://en.wikipedia.org/wiki/Burnup https://en.wikipedia.org/wiki/Burnup
    - catalogia 6y ago
      Nuclear fusion is also sometimes described as "burning plasma."
  - regularfry 6y ago
    > Unless things are very very wrong and you’re not going to space today. More accurate, possibly, to say you're not going to space in a controlled fashion today.
- baybal2 6y ago
  > It's a stupendously efficient and effective engine. You have to add the weight of a pusher plate system, and account for blasts still happening quite a distance from it. Your ISP will vary wildly depending how much "fuel" you will carry, and what yields. To beat a nerva, it will definitely need to be patently gigantic
tomatotomato37 6y ago
Project Orion is outdated anyway, we have technology like laser detonation and computer coordination such that you can detonate tiny fuel pellets continuously rather than bouncing off of entire bombs with a looney tune pogo stick arrangement. https://en.m.wikipedia.org/wiki/Project_Daedalus https://en.m.wikipedia.org/wiki/Project_Daedalus
- ecpottinger 6y ago
  Sorry, no we do not yet, otherwise the first fusion reactors would bo online maling power right now.
  - tomatotomato37 6y ago
    The standards for propulsion are lower than that for electric power; you don't really need a continuous nuclear explosion flung out the back to be net-positive electrically, you just need it to happen.

Symmetry 6y ago

Yeah, this is nuclear thermal where uranium is burned as fuel to heat a hydrogen propellant. That limits the heat and energy the propellant can absorb so while the efficiency end up being high due to using all-hydrogen fuel with a very low molecular weight the thrust isn't that great. These are good for second stages or transfer once in orbit but they don't make very good booster engines. Well, not unless you're going to do something really exotic like a gas phase nuclear lightbulb design[1]. But thankfully it won't be releasing any radiation into the atmosphere if it's working properly, the hydrogen fuel has to absorb two neutrons on its pass through to turn radioactive and it really doesn't want to absorb that second neutron. And if something goes wrong before its turned on uranium, even enriched uranium, isn't that radioactive and honestly there are scarier heavy metals that aren't radioactive at all. Orion, by contrast, uses its bombs effectively as both fuel and propellant and achieves far greater temperatures/efficiencies and power. It's a stupendously efficient and effective engine. However, that does mean that you've got nuclear reaction byproducts coming out of the engine which are pretty radioactive and also will tend to fall out of the atmosphere onto us down below in a reasonable time. Each one only produces a little but you'll be setting off many of these and overall you're looking at the same level of fallout as a multi-megaton thermonuclear bomb airburst. And we banned open air test of those for a reason. Still, if you're launching from some place that's already exposed to cosmic radiation and the solar wind, like the surface of the Moon, it might be a useful means of transport there. [1]https://en.wikipedia.org/wiki/Nuclear_lightbulb https://en.wikipedia.org/wiki/Nuclear_lightbulb

Sharlin 6y ago
Just to be clear, uranium is not literally burned in a nuclear thermal rocket. Unless things are very very wrong and you’re not going to space today. Nuclear thermal propulsion is simply using a fission (or more speculatively fusion) reactor to heat hydrogen gas which is then allowed to escape at high velocities from the business end of the rocket. In theory the heat source could be anything, but very few things offer remotely as much W/kg as nuclear.
- Symmetry 6y ago
  Saying that nuclear fuels "burn" when you cause them to undergo fission in a reactor is a common colloquialism. Hence standard terms like "burnup rates" when describing the efficiency of reactors. https://en.wikipedia.org/wiki/Burnup https://en.wikipedia.org/wiki/Burnup
  - catalogia 6y ago
    Nuclear fusion is also sometimes described as "burning plasma."
- regularfry 6y ago
  > Unless things are very very wrong and you’re not going to space today. More accurate, possibly, to say you're not going to space in a controlled fashion today.
baybal2 6y ago
> It's a stupendously efficient and effective engine. You have to add the weight of a pusher plate system, and account for blasts still happening quite a distance from it. Your ISP will vary wildly depending how much "fuel" you will carry, and what yields. To beat a nerva, it will definitely need to be patently gigantic

tomatotomato37 6y ago

Project Orion is outdated anyway, we have technology like laser detonation and computer coordination such that you can detonate tiny fuel pellets continuously rather than bouncing off of entire bombs with a looney tune pogo stick arrangement. https://en.m.wikipedia.org/wiki/Project_Daedalus https://en.m.wikipedia.org/wiki/Project_Daedalus

ecpottinger 6y ago
Sorry, no we do not yet, otherwise the first fusion reactors would bo online maling power right now.
- tomatotomato37 6y ago
  The standards for propulsion are lower than that for electric power; you don't really need a continuous nuclear explosion flung out the back to be net-positive electrically, you just need it to happen.