6 ms·
In fission, it seems that most of the energy release is in the form of the kinetic energy of the daughter nuclei rather than gamma radiation or the kinetic ener
by mannykannot 2y ago
In fission, it seems that most of the energy release is in the form of the kinetic energy of the daughter nuclei rather than gamma radiation or the kinetic energy of neutrons (from Wikipedia: For uranium-235 (total mean fission energy 202.79 MeV), typically ~169 MeV appears as the kinetic energy of the daughter nuclei, which fly apart at about 3% of the speed of light, due to Coulomb repulsion. Also, an average of 2.5 neutrons are emitted, with a mean kinetic energy per neutron of ~2 MeV (total of 4.8 MeV.) The fission reaction also releases ~7 MeV in prompt gamma ray photons.)[1]
Given this, I'm guessing that, for direct conversion to be at all efficient here, a significant fraction of this energy would have to be converted into electrical potential energy rather than be dissipated as heat in collisions between these nuclei and any part of the apparatus. Are there any nascent technologies of this sort?
[1] https://en.wikipedia.org/wiki/Nuclear_fission https://en.wikipedia.org/wiki/Nuclear_fission
- db48x 2y agoIt’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.
- drivebyhooting 2y agoThat sounds very cool. Like a nuclear ion drive. Is there a technical paper about it?
- Intralexical 2y agoSounds like what Wikipedia calls a "Fission-fragment rocket", I think? So check the citations for papers: https://en.wikipedia.org/wiki/Fission-fragment_rocket https://en.wikipedia.org/wiki/Fission-fragment_rocket
- mannykannot 2y agoVery 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.
- db48x 2y agoWikipedia has a short list: https://en.wikipedia.org/wiki/Direct_energy_conversion https://en.wikipedia.org/wiki/Direct_energy_conversion
- jpk 2y agoNow 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?
- db48x 2y agoOrbit is sufficiently safe. The exhaust products are individual atoms rather than grains of rock that can fall on a roof.
- jpk 2y agoYeah, but those atoms are fission products!
- Intralexical 2y agoEh, they're moving at 3-5% the speed of light. Just don't point it towards the planet.
- lazide 2y agoOne 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.
- db48x 2y agoYes, 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.
- epistasis 2y ago> which fly apart at about 3% of the speed of light, due to Coulomb repulsion So is that at first an electrical force that results in kinetic acceleration?
- tmiku 2y agoWhat do you mean by "at first"? The protons are pushing each other away via that Coulomb repulsion while they're all in the pre-fission nucleus, but at that time it's not enough to counteract the strong force (https://en.wikipedia.org/wiki/Strong_interaction https://en.wikipedia.org/wiki/Strong_interaction) holding the nucleus together. Once that once nucleus has been split into parts that can move freely, the daughter nuclei are both accelerated by an electromagnetic force.
- epistasis 2y agoWell, yeah, once the distance gets out of the well of the strong force, the nuclei are at the top of a big electrical force gradient that gets converted to kinetic energy. With proper ionization of the daughter atoms that kinetic energy could potentially be converted back to electric, well, potential.