It’s been proposed as an extremely efficient spaceship drive. Make 1µm beads of fissile fuel embedded at below critical density in aerogel. A thin sheet of aerogel with these fuel pellets in it is then inserted into the engine right inside a very strong magnetic field. Bombard the fuel with neutrons to ramp up the fission rate and those same daughter nuclei come streaming out of the pellets. They are still charged, so in the magnetic field they take a curved path. They push the magnet forward as the field pushes them out the back of the engine. With an exhaust velocity of 3% of the speed of light, they can get enough ΔV out of a few kilograms of fuel to zip out to Pluto and back in a few years.
Now knowing that small quantities of space dust settle on our roofs, I presume you'd have to reach some safe distance from populated planets before using such an engine?
But they’re also just individual atoms. They’re not a chunk of radioactive material that can sit on your roof for years.
Besides, the whole ship would have maybe 15kg of fuel. That means that there can only be a maximum 15kg of fission products. It would burn fuel that for several years, spreading the exhaust out over the distance from here to Pluto and back.
Irrelevant. They are moving fast but they are tiny individual atoms. There would be no harm in having them impact the atmosphere. They would bump into a few thousand or perhaps a million gas molecules on their way down to the surface, turning their tiny kinetic energy into a tiny amount of heat. And since there is only a few kilograms of fuel, there can only be a few kilograms of exhaust. With a low thrust and high specific impulse, the engine would burn that fuel continuously for most of the trip, spreading the exhaust from here to Pluto and back. It’s not going to be sufficiently concentrated to bother anybody.
In fact, even though the fuel pellets are only about 1µm across, they still bump into enough atoms on their way out that the pellets heat up significantly. A major engineering consideration of the engine would be to absorb or deflect the heat radiated by the >1000°C fuel pellets without letting that heat quench your superconducting magnets.
On the one hand, I also think it's likely safe: we've had a huge nuclear reactor in the sky since before life happened, the atmosphere and the magnetosphere are pretty effective barriers. (I think they're more worried about actinides than impactors).
On the other, I'm not sure where your 15 kg came from.
This matters, because fancy fuels matter a lot more for higher-mass or high-Δv payloads than smaller ones.
A fission fragment rocket can be Isp of 1,000,000[1] depending on the exact details — thrust is proportional to momentum (mv), not energy (0.5(mv^2)), and that means four million times the energy density is two thousand times the momentum and thrust, so that 15 kg is like 30 tons of conventional propellant: a nice saving, but you'd use a lot more than that for e.g. a manned mission to Mars.
For missions where the payload rather than the speed is critical, fuel is also a small fraction of total mass, so you also get a performance boost from being able to approximate the Tsiolkovsky rocket equation as linear.
But that's perhaps another factor of 10, which is still roughly 25% of a Starship upper stage, so even then I'd expect at least 60 kg even if the engine itself can be considered negligible in both cases.
And that's likely to be burned through much sooner than Pluto, though it depends on the details of the design. The ship would likely melt if you tried to thrust at 1 gee, but I think it would still be comparable to the Earth-Moon distance, give or take a factor of 3.
If you want something that burns from here to Pluto, then… huh, I was going to say you're likely back in Tsiolkovsky's realm, but apparently still not, and also still sub-relativistic (~ 1 milli-c for the specific values I was using).
Which is still safe, I just don't think it's quite as trivial as you say.
Honestly, I might have misremembered the fuel mass. I tried looking for the paper I read about this design, but I couldn’t find it. It’s been a whole year since I read it, and I guess they don’t think hosting it is useful anymore.
One of my fav. points in one of the classic sci-fi books (a deepness in the sky maybe?) is how the main engines of their ships (bussard ramjet fusion drives) were also such incredibly powerful weapons, that merely pointing one (even when off) at anyone was seen as a clear declaration of war/hostile intent.
Which, if you do the math, is definitely the case. Ain’t nobody walking away from getting one of those to the face.
Yes, Vernor Vinge’s A Deepness in the Sky had Bussard ramjets, and using one inside a civilized solar system was usually a great crime since the magnetic fields may be extremely large. But you can’t point them; they’re more of an area–effect weapon.
Larry Niven’s Known Space stories coined the phrase Kzinti Lesson when a peaceful Human crew turned their photon drive on an attacking Kzinti ship, slicing it in half. The Kzinti had acquired an anti–gravity drive from aliens (who they then enslaved and ate) so they didn’t have a good visceral sense of the energy required to visit the stars the hard way. Their telepaths kept reporting that the Humans were peaceful and didn’t have any weapons on their ship right up until the ship flipped over and sliced them in half. That kind you can point.
I thought the Fusion Drive plume also came up - but perhaps I’m thinking of a different book?
At the accelerations most sci-fi happens at (even fractions of 1G), the energy coming out the business end of any sort of drive based on extrapolations of currently known physics are going to be in the TJ+ range, and often with relativistic particle velocities.
Probably not as focused as a laser type drive, but in a vacuum such a fusion type drive would melt any known materials at multi-km distances and likely cause extreme x-ray emissions from whatever it was vaporizing when it came in contact with it. In atmosphere, it would likely just vaporize the space craft along with anything in a several mile radius.
It’s basically carrying around a directable, continuously exploding hydrogen bomb.
Very interesting - I had not heard of this before. I see that the entry on the “dusty plasma” design says that the exhaust flow can be decelerated for power (presumably that applies to other designs as well.) It does not mention how the deceleration is performed (electrostatically, a bit like a reverse Van der Graaff particle accelerator?) or how efficient and compact this could be.
