4 ms·

Or just use a bunch of Hall thrusters, and not lob fissile material into Earth's atmosphere.

ashtonkem 6y ago
The problem is that Hall-effect thrusters provide basically no thrust. Most Hall-effect thrusters provide millinewtons of thrust, with the best lab model producing 5.4N and the planned AEPS motor producing a theoretical max of 2.356N. A single Merlin 1D motor produces 690kN of thrust at sea level. You would need over 290,000 AEPS engines to match one Merlin 1D engine. At 40kW per engine, this would use 11.7 gigawatts to run, so you’d also have to add in the weight for a dozen or so nuclear power plants. Keep in mind that the falcon 9 uses nine Merlin 1D motors for the first stage.
- CydeWeys 6y ago
  > You would need over 290,000 AEPS engines to match one Merlin 1D engine. Even worse, even an infinite number of AEPS engines would not match a single chemical rocket engine of any type, because Hall effect thrusters have a thrust-to-weight ratio of significantly less than 1. So it can't even lift itself off the pad, let alone anything else.
  - ashtonkem 6y ago
    If I’m doing my math right, the 25kg AEPS engine would need to produce 245N to even hover without any payload. So it’s theoretical max is about 0.9% of what’s needed to even lift itself.
    - CydeWeys 6y ago
      And of course these figures get much worse when you consider the other components that are a necessary part of the engine working, namely, the xenon fuel tank and massive solar panel array. For a relevant comparison, the thrust-to-weight ratio of the Merlin 1D engine by itself is something like 180:1, but the overall thrust-to-weight ratio of the entire rocket at launch is only 1.4:1. So just in order to get a rocket that can lift off at all and deliver useful payload to orbit you need at least a 3-digit thrust-to-weight ratio on the engine itself. Ion thrusters don't remotely come close.
  - rbanffy 6y ago
    > So it can't even lift itself off the pad, let alone anything else. This is only a problem for take off from large bodies. Mass per mass, you'll get a lot more delta-v from a Hall engine than from a Merlin. It'll just take a lot longer to get to that point. Which brings up an interesting option for NTRs: bimodal propulsion. From a single propellant tank you could drive both an NTR and a Hall thruster, depending on your acceleration needs. Hall thrusters aren't heavy (certainly not compared to fission reactors) and you could use your NTRs when you needed high acceleration (such as to reduce transit times or to take off and land from a big rock) and use the low-power extreme-efficiency Halls for most of the trip when you'd be coasting.
    - CydeWeys 6y ago
      > This is only a problem for take off from large bodies. Yes, that's what we're talking about here. > Mass per mass, you'll get a lot more delta-v from a Hall engine than from a Merlin. Yes, that's what specific impulse measures (which is what most of the larger conversation has been about). As for bimodal propulsion, it makes you wonder if that would be worth doing at all, or if it wouldn't be better to just have separate engines and fuel tanks so each can do what it's best at. I suspect it might end up being less weight and complexity just to use an ion thruster as designed using xenon propellant. Xenon fuel tanks aren't typically all that big anyway.
      - rbanffy 6y ago
        Separate fuel tanks would demand the design to be more mission-specific. Ideally we want reusable spacecraft that can do multiple roles.
- greesil 6y ago
  We're talking about orbit raising, not launch, which the nuclear thermal can't do. I can't believe I've been downvoted this much when I probably know more about electric propulsion and orbital dynamics than anyone in this thread.
greesil 6y ago
Also, downvoted for not wanting to accidentally create radioactive fallout? Do the cost benefit analysis when there's plenty of other technologies for moving mass around off planet and to other planets.

The problem is that Hall-effect thrusters provide basically no thrust. Most Hall-effect thrusters provide millinewtons of thrust, with the best lab model producing 5.4N and the planned AEPS motor producing a theoretical max of 2.356N. A single Merlin 1D motor produces 690kN of thrust at sea level. You would need over 290,000 AEPS engines to match one Merlin 1D engine. At 40kW per engine, this would use 11.7 gigawatts to run, so you’d also have to add in the weight for a dozen or so nuclear power plants. Keep in mind that the falcon 9 uses nine Merlin 1D motors for the first stage.

CydeWeys 6y ago
> You would need over 290,000 AEPS engines to match one Merlin 1D engine. Even worse, even an infinite number of AEPS engines would not match a single chemical rocket engine of any type, because Hall effect thrusters have a thrust-to-weight ratio of significantly less than 1. So it can't even lift itself off the pad, let alone anything else.
- ashtonkem 6y ago
  If I’m doing my math right, the 25kg AEPS engine would need to produce 245N to even hover without any payload. So it’s theoretical max is about 0.9% of what’s needed to even lift itself.
  - CydeWeys 6y ago
    And of course these figures get much worse when you consider the other components that are a necessary part of the engine working, namely, the xenon fuel tank and massive solar panel array. For a relevant comparison, the thrust-to-weight ratio of the Merlin 1D engine by itself is something like 180:1, but the overall thrust-to-weight ratio of the entire rocket at launch is only 1.4:1. So just in order to get a rocket that can lift off at all and deliver useful payload to orbit you need at least a 3-digit thrust-to-weight ratio on the engine itself. Ion thrusters don't remotely come close.
- rbanffy 6y ago
  > So it can't even lift itself off the pad, let alone anything else. This is only a problem for take off from large bodies. Mass per mass, you'll get a lot more delta-v from a Hall engine than from a Merlin. It'll just take a lot longer to get to that point. Which brings up an interesting option for NTRs: bimodal propulsion. From a single propellant tank you could drive both an NTR and a Hall thruster, depending on your acceleration needs. Hall thrusters aren't heavy (certainly not compared to fission reactors) and you could use your NTRs when you needed high acceleration (such as to reduce transit times or to take off and land from a big rock) and use the low-power extreme-efficiency Halls for most of the trip when you'd be coasting.
  - CydeWeys 6y ago
    > This is only a problem for take off from large bodies. Yes, that's what we're talking about here. > Mass per mass, you'll get a lot more delta-v from a Hall engine than from a Merlin. Yes, that's what specific impulse measures (which is what most of the larger conversation has been about). As for bimodal propulsion, it makes you wonder if that would be worth doing at all, or if it wouldn't be better to just have separate engines and fuel tanks so each can do what it's best at. I suspect it might end up being less weight and complexity just to use an ion thruster as designed using xenon propellant. Xenon fuel tanks aren't typically all that big anyway.
    - rbanffy 6y ago
      Separate fuel tanks would demand the design to be more mission-specific. Ideally we want reusable spacecraft that can do multiple roles.
greesil 6y ago
We're talking about orbit raising, not launch, which the nuclear thermal can't do. I can't believe I've been downvoted this much when I probably know more about electric propulsion and orbital dynamics than anyone in this thread.

greesil 6y ago

Also, downvoted for not wanting to accidentally create radioactive fallout? Do the cost benefit analysis when there's plenty of other technologies for moving mass around off planet and to other planets.