I’d like to see them explain the benefits of this over solar power. They write in the article that it’s much faster, but that’s true for all chemical propulsion.
The benefit of solar power is that it’s almost limitless.
The benefit of solar power is that it’s almost limitless.
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.
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.
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.
Edit: reaction mass becomes the problem, say to move a ship 10^4kg, 2x10^9 m (the distance to mars) at 1g, I adapted an answer from [1]
So say the craft has a mass of 10^4 kg. To accelerate at 1 G you need F = ma = (10^4 kg)(10 m/s^2) = 10^5 N. To get a force of 10^5 N over 2x10^9 m (distance to mars), you would need (10^5 N)(2x10^9 m) = 2x10^14 J, lets say 10^14J. Antimatter is the most energy dense material we know. To get that from antimatter you would need m = E/c^2 = 10^14 J/10^17 m^2/s^2 = 1gram. So you'd only need 1gram of antimatter - is that right?
CERN has made around 1 nanogram so far, 1gram would cost around $25 billion in a 2006 estimate, its down from $62 trillion in a 1999 estimate [2], so it may be feasible one day...
Edit2: The moon is 4x10^8m, so you'd need 0.1gram of antimatter, you'd be there in an hour or so.
[1] https://forum.nasaspaceflight.com/index.php?topic=34996.0
The advantages over solar power is thrust (which, within the solar system at least, is important if you want to get somewhere fast), and the fact that it works further out from the sun, which means you could use it to get mass to the outer planets (or mars) relatively quickly. Solar propulsion, yes, is limitless, but and can reach much faster speeds, but it can take so long to reach them, that by the time you're going faster than the NTR, the NTR has already arrived at its destination.
The advantage over chemical propulsion is efficiency.
One big problem with solar energy is that it is a diffuse power source (i.e. energy per unit area is low), and gets exponentially more diffuse the further from the sun you go.
The second is that though solar energy is limitless, the components that need to use chemistry to turn solar energy into electricity, are not.
The big difference there is thrust. Ion engines are efficient but have very low thrust, so it can take months to change your orbit. Chemical and nuclear propulsion is high thrust, so we're talking minutes instead. Nuclear has higher propellant (i.e. mass) efficiency than chemical as well.
Which means the solar array must be twice as large for same output on Mars alone, and solar propulsion is probably completely infeasible due to weight and volume constraints, if not structural as well, on Saturn orbit and beyond.
For missions further out or for Mars missions that cannot accommodate twice larger wings, we must “carry our own Sun” for power.
Which isn’t wrong at all, after all the Sun is a natural fusion reactor so solar panels aren’t like it’s free of stigmatic nuclear technology.
Oh and also nuclear propulsion aren’t considered chemical by the way...
Solar by itself doesn't provide reaction mass (outside of solar sails, which are very low thrust). You would probably be combining it with ion thursters, which have really high exhaust velocities, but low thrust to weight ratios.
The questions here include:
- whether you can get more efficient propulsion overall when the heat comes from a reactor instead of from a reaction in the fuel
- whether switching from a fuel/oxidizer energy-source/mass combo to another working-mass-only substance might net you other engineering advantages (storage concerns, perhaps? though if you're switching to hydrogen like one often considers for nuclear rocket working mass, you will have plenty of storage problems)
- what sort of engineering disadvantages the nuclear reactor component brings (added mass, heat dispersal issues, etc)