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Solar power is not a method of propulsion, it's a method of collecting energy. Perhaps you mean ion engines paired with solar panels? If so, the difference in

Solar power is not a method of propulsion, it's a method of collecting energy. Perhaps you mean ion engines paired with solar panels? If so, the difference in thrust is massive, as in several orders of magnitude - to the point where getting anything larger than a satellite moving with electric-based propulsion would take many orbits of acceleration. You also run into issues of not being able to collect power when in shadow, needing power storage banks (batteries are very heavy), and diminishing power output as you move away from the sun (ex. at Mars, solar energy density is 40% less than at Earth).

bryanlarsen 6y ago
You have to have some pretty big engines before the mass of the reactor is a small enough portion of the mass that the Thrust to Weight ratio of a nuclear thermal engine + tank + reaction mass is better than that of a chemical engine + tank + fuel. The only place that kind of thrust is needed is getting off of Earth, and a nuclear engine doing that is a non-starter. And inside the orbit of Jupiter, solar + ion engines are way more efficient. The Ars commenters are very well informed, they go back and forth on this a lot. I highly recommend reading the comments there.
- numpad0 6y ago
  I think you’re overestimating the minimum size of a nuclear device, they can be small as a larger watermelon.
  - bryanlarsen 6y ago
    That's still several orders of magnitude more massive than the chemical and ion thrusters used on small satellites. But don't take my word for it, there are 12 pages of intelligent comments on the Ars article, comments mostly of much higher quality than any comment here, including and especially mine. And those comments are fairly disparaging. Unless you're going past Jupiter, in which case Nuclear-Electric-Propulstion is your best bet. And nuclear-electric is very different than nuclear-thermal.
- marcinzm 6y ago
  The implied goal is to send humans to places in the solar system. That means life support, food, water, etc. So a lot of mass anyway. Worse, the longer the trip the more mass you need to bring onboard. Nuclear would allow for the shortest trip durations while ion engines would be the longest. With ion engines you have the issues of micro-gravity health risks on the crew, psychological risks and the need for a lot more supplies.
  - bryanlarsen 6y ago
    For super long trips, efficiency is king. Coupled with the ability to ability to burn continuously, solar-electric and nuclear-electric are way more efficient than nuclear-thermal. Ion engines may have low thrust -- just use a lot of them. Nuclear-thermal's Achilles heel is that you can't use it to boost off Earth. So you need a high thrust chemical engine to get it into space. It's hard to make the numbers work -- nuclear-thermal may be more efficient than that chemical engine, but it's dead mass while you're boosting it out of the Earth's gravity well. If instead you can use the same chemical engine to get to Mars that you used to boost yourself out of Earth, you don't have that dead mass. If you want to get to Mars fast, just boost some extra fuel on an extra ship. Nuclear-thermal looks nice on paper; it's probably the highest efficiency high thrust engine that's achievable with today's materials. But it's hard to find a good use case for it. A manned Earth->Mars run used to be it; but SpaceX's plan for in-space-refueling shortens the trip time just as much, if not more.

bryanlarsen 6y ago

You have to have some pretty big engines before the mass of the reactor is a small enough portion of the mass that the Thrust to Weight ratio of a nuclear thermal engine + tank + reaction mass is better than that of a chemical engine + tank + fuel. The only place that kind of thrust is needed is getting off of Earth, and a nuclear engine doing that is a non-starter. And inside the orbit of Jupiter, solar + ion engines are way more efficient. The Ars commenters are very well informed, they go back and forth on this a lot. I highly recommend reading the comments there.

numpad0 6y ago
I think you’re overestimating the minimum size of a nuclear device, they can be small as a larger watermelon.
- bryanlarsen 6y ago
  That's still several orders of magnitude more massive than the chemical and ion thrusters used on small satellites. But don't take my word for it, there are 12 pages of intelligent comments on the Ars article, comments mostly of much higher quality than any comment here, including and especially mine. And those comments are fairly disparaging. Unless you're going past Jupiter, in which case Nuclear-Electric-Propulstion is your best bet. And nuclear-electric is very different than nuclear-thermal.
marcinzm 6y ago
The implied goal is to send humans to places in the solar system. That means life support, food, water, etc. So a lot of mass anyway. Worse, the longer the trip the more mass you need to bring onboard. Nuclear would allow for the shortest trip durations while ion engines would be the longest. With ion engines you have the issues of micro-gravity health risks on the crew, psychological risks and the need for a lot more supplies.
- bryanlarsen 6y ago
  For super long trips, efficiency is king. Coupled with the ability to ability to burn continuously, solar-electric and nuclear-electric are way more efficient than nuclear-thermal. Ion engines may have low thrust -- just use a lot of them. Nuclear-thermal's Achilles heel is that you can't use it to boost off Earth. So you need a high thrust chemical engine to get it into space. It's hard to make the numbers work -- nuclear-thermal may be more efficient than that chemical engine, but it's dead mass while you're boosting it out of the Earth's gravity well. If instead you can use the same chemical engine to get to Mars that you used to boost yourself out of Earth, you don't have that dead mass. If you want to get to Mars fast, just boost some extra fuel on an extra ship. Nuclear-thermal looks nice on paper; it's probably the highest efficiency high thrust engine that's achievable with today's materials. But it's hard to find a good use case for it. A manned Earth->Mars run used to be it; but SpaceX's plan for in-space-refueling shortens the trip time just as much, if not more.