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It's difficult to see how DT fusion could be better in space than alternatives. DT fusion reactors will be extremely large and complex. They will be much heavier than and much less reliable than alternatives. That's not good for space applications.

The high energy density of the fuel is irrelevant if the reactor is very massive and if only a tiny fraction of the reactor's mass in fuel can be fused over the lifespan of the reactor. IIRC, it would take something like 700,000 years for ITER to fuse its own mass in fuel, if it could be operated continuously forever (which it can't, of course.)

You are correct, but I wonder if space travel applications allow us to take certain liberties with the reactor design that are not feasible down on Earth. E.g. a simple(r) confinement design that expels the fusion products could work well in space, but is a non starter on Earth for obvious reasons.

I'm not sure how much of the design on Earth is bulky and cumbersome because it needs to be vastly safer and more complicated than what a rocket engine needs to be. Taken to the extreme, nuclear propulsion in space can be very dumb: just detonate nukes some distance behind the rocket and ride the shockwave with a pusher plate. It's a radically different design than anything we'd attempt for terrestrial applications.

Sure, but five orders of magnitude worth of liberties?

The Orion spacecraft concept you mention will use fission bombs on any vehicle we might build soon. Thermonuclear driven interstellar Orion has been proposed, but the vehicle would be enormous (and take 400 years to reach the nearest star.)

You are implying ion drives I assume. Ion Drives work by accelerating particles to _really_ fast speeds (like fractions of light-speed) and use that to generate thrust. Meaning it requires almost no "fuel" but requires A LOT of power for the amount of thrust generated.

A fusion power generator coupled with ion drives could potentially generate a lot of thrust for little mass since the fusion fuel also has very little mass.

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

He might be talking about a "direct fusion drive," which is not an ion drive. It's basically a magnetic-confinement device but you leave part open in such a way that fusion products exit to produce thrust.

In fact I meant it in a general sense, and both ion drive and a more classic confinement engine like design that expels the fusion products. I don't know enough about the intricacies to argue for one vs the other, but fusion fuel is the most energy dense fuel we have within reach, by a huge margin. The variable then becomes the mass of the engine (and supporting plumbing), but we have control over this.

Can we shrink a fusion reactor small enough to make the rocket as a whole feasible? I don't know. My hope is that for space travel, we can make certain tradeoffs that are not feasible on Earth - e.g. expelling radioactive waste products into the environment. A rocket engine doesn't need to be all that efficient at generating electricity either, it mostly needs to be efficient for propulsion, which might make it's design much simpler.

For example, here's a fission-fusion one that looks like it only needs engineering to be workable:

https://www.nasa.gov/general/pulsed-fission-fusion-puff-prop...

> Ion Drives work by accelerating particles to _really_ fast speeds (like fractions of light-speed)

Sure, if the fraction is 1/10000. Very high exhaust velocity is not optimal since it increases the power needed per unit of thrust.

> including a massive fusion reactor up in the sky which will continue operating for the foreseeable future :)

I assume this refers to the sun. Other responses don't seem to have made the same conclusion.

Even if you had a fully functional reactor in space you now have a massive thermodynamics problem on your hands. How do you get rid of all that heat?

Near-earth space has an abundance of solar energy convertible to electrical energy. Deep space is an interesting potential use case for fusion, but fission-powered electric space propulsion via RTGs has the virtue of having demonstrated nearly half a century of continuous operation

(and after it leaves the earth's atmosphere, radioactivity ceases to be a concern)

As far as I know there has never been an RTG powered electric rocket engine. RTGs are extremely expensive per unit of power output. Spacecraft like that ion engine propelled one that was sent to Ceres use solar power.

Voyager and Pioneer used RTGs, as well as some more recent longer range missions and the planned Dragonfly to Titan. Agree that solar has much better unit economics, but I don't see any reason why the same wouldn't be true of fusion too.

And none of those use RTGs to power electric rocket engines.

(EDIT: thrusters are rocket engines, just very small ones. All those missions used small chemical rockets for maneuvering, using storable propellants like hydrazine.)

Edit: I stand corrected on Voyager/Pioneer exclusively using hydrazine, though you could get similar thrust from a modern HET without needing insane amounts of RTG power, and run other electrical equipment off that extra power when you've reached target velocity and aren't firing the thrusters for attitude control. The higher Isp of the EP system is an advantage too...

I dont want to undermine your enthusiasm.. But unfortunately, even fussion based energy generation looks like sci-fi. There is reason why certain things happen in objects of certain mass. You cant cheat on physics.

The neutron corrosion alone is very serious problem that I doubt can be fixed. There is only one material that can withstand it, its called neutronium and we all know why we cant even use it on earth, not to mention about producing one. The Li-7 trick they are currently trying to use is not convicing me. Whats the point of it if you need to stop reactor every few years to replace entire Li-7 shielding. Lithium itself aint cheap and we basically will destroy it in such reactors (non-recycleable).

The hope lies in Fission Reactors, breeder one. Unfortunately, there is big tabu here because those can be used for enrichment, and so, for nuclear weapons.

The materials question is interesting to me because my Masters is mostly related to Material Science. My thesis advisor did some research in this area--mostly with ZR3AL as I recall. My thesis was with a related material (NiAl) for potential uses related to turbines. The aluminide alloys never really worked out for a variety of reasons. My advisor ended up pretty much doing water ice-related research full-time (which he had just started doing when I was a grad student) and that's pretty much what he spent his career doing. The US Army has? a lab in Hanover NH and, at the time, they were very interested in better understanding the properties of ice, in part, presumably because of the possibility of a land war with the Soviet Union in Alaska.

> The neutron corrosion alone is very serious problem that I doubt can be fixed.

It can be fixed, because some fuels don't produce neutrons. But aneutronic fusion is like two orders of magnitude more difficult than Deuterium-Tritium. Still, it might be necessary for any reasonable application. One of my professors was a big proponent in aneutronic fusion even for Tokamaks.