Feasibility of fission fragment rockets

[Accelerando]

Fission fragment rockets are nuclear reactors that directly use the tiny particles undergoing fission as a source of thrust, taking advantage of nearly the full energy of each fission to achieve extremely efficient travel at the price of extremely low thrust. FFRs promise Isp in the millions; exhaust velocities exceeding 1% the speed of light. Winchell Chung has a nice long Atomic Rockets segment describing its potential capabilities and the basic principles of its operation, and relevant articles cap off the Wikipedia page.

credit Robert Werka via Project Rho

FFRs might be within our reach, but the earliest article was published in 1988, and the latest mentions in the last two decades don't indicate that any FFRs, rocket or pure reactor, have been built. The spinning fissile-disks system mentions the need to keep the fissile plates cool; the dusty plasma reactor needs magnetic confinement. What other problems are there? How might the hurdles in building an FFR be overcome?

Lastly, tangentially, would a fission sail, mentioned just below FFRs on the Atomic Rockets page, be a viable spacedrive? Essentially a sheet absorbing decay products layered on top of a radioactive sheet, with the emission of decay products propelling the ship along its axis.

[Red Iron Crown]

It's going to be extremely low thrust, but would probably be worth it for a high Isp "torch ship".

Unfortunately, development will be hampered since all things nuclear seem to be difficult to get funded because "OMGNUCULAR IS DANGEROUS".

[Nuke]

i figure nothing irradiating everyone and forcing them to watch doctor strangelove clockwork orange style wont fix.

this is one of those technologies you could probibly develop over the course of a couple decades provided the funding doesnt get cut and the protesters keep their nose out of it. in other words darpa probibly already has one sitting on a shelf somewhere.

[Accelerando]

Right in the secret warehouse along with shaped nuclear charges, yeah. Just hope they'll roll it out sooner or later... but what hurdles would they have faced in making the thing?

[MBobrik]

Well, as Red Iron Crown already said, the biggest insurmountable engineering problem is our civilizations radiophobia. the second is general disinterest in space travel. Solve those two and you will have all sorts of nuclear powered spacecraft flying around in the solar system within a decade ... But i think that this train already left the station. we are just too decadent to turn around. When the next civilization rises ( well, the one after the next, the next being a technophobic fundamentalist dystopia ), we can try again.

[Accelerando]

Then my question begs rephrasing: What technical hurdles must be overcome to build one of these things based on technology publicly known to exist?

[Nuke]

its essentially a fission reactor that you run in a constant meltdown state. fissile material vaporizes and is thrown out at very high velocities because of its insanely high temperature, giving you really high isp. cool stuff.

[sal_vager]

You are asking for technical hurdles in a technology that no one here has ever seen, probably no one ever has had the fortune to work on such a drive.

I'd guess though that the obvious hurdles would be the safety of the engineers while constructing it, as there will be radioactive components, and sufficient shielding of not only the crew (if any) but the electronics too would be a challenge.

The device looks very heavy if the Wikipedia images are anything to go by, so getting such a device into orbit would itself be a challenge.

[lajoswinkler]

You couldn't fire that thing in orbits around Earth. The contamination would be dreadful for the satellites. Clouds of horribly radioactive dust would do a great harm to the electronics.

Also, this is basically for unmanned missions only. The flux of gamma rays and neutrons emanating from an active fission core is... spectacular.

[Red Iron Crown]

You couldn't fire that thing in orbits around Earth. The contamination would be dreadful for the satellites. Clouds of horribly radioactive dust would do a great harm to the electronics.

Also, this is basically for unmanned missions only. The flux of gamma rays and neutrons emanating from an active fission core is... spectacular.

That's a solvable problem. We've been using active fission cores in naval vessels for half a century, I don't think getting one into a manned spacecraft with appropriate shielding is an insurmountable technical hurdle. Especially given that NERVA was being developed for manned flight.

[Rakaydos]

That's a solvable problem. We've been using active fission cores in naval vessels for half a century, I don't think getting one into a manned spacecraft with appropriate shielding is an insurmountable technical hurdle. Especially given that NERVA was being developed for manned flight.

The reactor isnt the issue, it's the propellant- Unlike NERVA, you're spitting pure radioactive waste out the back.

[Accelerando]

You are asking for technical hurdles in a technology that no one here has ever seen, probably no one ever has had the fortune to work on such a drive.

You're right. I guess I'm just hoping some nuclear engineers or physicists will be able to shed some light on how difficult this sounds.

Indeed, Red, as for instance we keep distance by mounting the engines on the front of the vessel and hauling the crew compartment behind on a tether, protected by a tungsten shadow shield somewhere in the middle, as with the Valkyrie antimatter-ship concept. The creator of the concept writes that we can make the shadow shield mass significantly less than on a push-engine configuration.

Here's how we can shave off many tons of shielding.

Put the engine up front and carry the crew compartment ten kilometers behind the engine, on the end of a tether. Let the engine pull the ship along, much like a motorboat pulling a water skier, and let the distance between the gamma ray source and the crew compartment, as the rays stream out in every direction, provide part of the gamma ray protection - with almost no weight penalty at all. (ed. note: this should remind you of "Helios") We can easily direct the pion/muon thrust around the tether and its supporting structures, and we can strap a tiny block of (let's say) tungsten to the tether, about one hundred meters behind the engine. Gamma rays are attenuated by a factor of ten for every two centimeters of tungsten they pass through. Therefore, a block of tungsten twenty centimeters deep will reduce the gamma dose to anything behind it by a factor of ten to the tenth power (1010). An important shielding advantage provided by a ten-kilometer-long tether is that, by locating the tungsten shield one hundred times closer to the engine than the crew, the diameter of the shield need be only one-hundredth the diameter of the gamma ray shadow you want to cast over and around the crew compartment. The weight of the shielding system then becomes trivial.

[ScallopPotato]

10km tether? Well sure as hell won't be testing that in KSP.

[Red Iron Crown]

The reactor isnt the issue, it's the propellant- Unlike NERVA, you're spitting pure radioactive waste out the back.

That's true, we definitely need to be careful about where the engine gets used, but I don't think it precludes a crewed vessel.

10km tether? Well sure as hell won't be testing that in KSP.

Don't worry, in Kerbal scale that's only 900m.

[lajoswinkler]

That's a solvable problem. We've been using active fission cores in naval vessels for half a century, I don't think getting one into a manned spacecraft with appropriate shielding is an insurmountable technical hurdle. Especially given that NERVA was being developed for manned flight.

With submarines, you can have lots of mass onboard. In space, all that mass needs to be hauled up and then pushed. All our stuff in space is so lightweight so that chemical engines can push it fast enough. This engines does have a high Isp, but the mass of the shield and the reactor would be really something. The effectiveness of this propulsion doesn't take into account that you need to protect the crew, and you need large mass for that.

[Anton P. Nym]

That's a solvable problem. We've been using active fission cores in naval vessels for half a century, I don't think getting one into a manned spacecraft with appropriate shielding is an insurmountable technical hurdle. Especially given that NERVA was being developed for manned flight.

Aboard a 6000 ton submarine, a 100 ton* shielded reactor isn't much of a problem. In a spacecraft, however, it's a HUGE problem.

NERVA got around the shielding problem by ducking most of it. It used a small shadow shield that just protected the crew compartment, and relied on fuel mass for the rest. Anything not in a fairly narrow cone around the bow was exposed to the unshielded core. (Rendezvous and docking was... fraught, to put it mildly.)

-- Steve

* wild-assed guess; I have no idea how heavy the reactor plant is on a sub or carrier, and a hasty search didn't turn up any numbers.

[andrewas]

Does anyone know how the weight of the needed shielding varies with the power of the reactor? I believe it works in our favor as the engine scales up, doubling the reactor power means you need to add another halving thickness to the shield, so the TWR goes up.

[ZetaX]

An r-fold radius means r³ times radiation on a r² surface, thus to get the same radiation dampening you will need shielding proportional to log® in thickness, or proportional to r²·log® in mass. This ignores radiation absorbed within the reactor, which would only give some lower constant factor in the result anyway. Thus a bigger one is more efficient as the fraction of mass by the shielding is proportional to log®/r, tending to 0.

[Red Iron Crown]

I have a feeling that the high Isp of the engine will make the weight of the shielding worthwhile, especially if the shielding is minimized by using a shadow shield, either NERVA style or the more exotic tether arrangement.

[TechnicalK3rbal]

-Snip-

In other words darpa probibly already has one sitting on a shelf somewhere.

Probably true