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://www.nasa.gov/general/pulsed-fission-fusion-puff-prop...
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.
Sure, if the fraction is 1/10000. Very high exhaust velocity is not optimal since it increases the power needed per unit of thrust.
(and after it leaves the earth's atmosphere, radioactivity ceases to be a concern)
(EDIT: thrusters are rocket engines, just very small ones. All those missions used small chemical rockets for maneuvering, using storable propellants like hydrazine.)
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.
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.
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.)
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.
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.
I assume this refers to the sun. Other responses don't seem to have made the same conclusion.
Instead, I think need to find some way of generating a lot of power over extended periods of time if we want to crack space travel beyond lobbing items into Earth orbit. Particularly for human space travel.
We need significantly more energy density than what chemical motors can provide, and nuclear is the easiest and most "within-reach" that we know of. Things like anti-matter and even more exotic forms of propulsion are pure sci-fi for the time being (in terms of practicality). I suppose fission could work in a very primitive way of basically detonating small nukes behind a pusher plate, but fusion feels so much more elegant (at least to a layman). Mostly in the sense of not having to use effectively the same nukes we stick in weapons. Something tells me that this will never fly (excuse the pun), for obvious reasons.