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Comment by PaulHoule | Hacker News Reader

Our energy scale is set by the energy scale of chemistry. When you run it is powered by chemistry. Chemical rockets are powered by chemistry.

Hypothetically you could go faster if you used fission or fusion energy but practically the chemical bonds get in the way. Even

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

is limited by the strength of chemical bonds of the reactor so it gets a factor of 2 or 3 or so on exhaust velocity compared to a chemical rocket.

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This is why somewhat realistic high-power fusion rockets like those portrayed in The Expanse require magnetic nozzles. The fusion plasma would never come into contact with the material the engine is made of or it would melt.

We could build a decent fusion pulse drive today if we had higher temperature more compact superconductors and super-efficient compact lasers that could fit in a spacecraft and ignite a strongly net-positive inertial confinement fusion pulse.

Our superconductors are almost good enough, but our high-power lasers are way too inefficient and bulky. We can't even make economically viable ICF on Earth with current lasers.

D + He3 and H + B are attractive precisely because the product is entirely charged particles that you can catch with such a magnetic nozzle. D + T is easier to ignite, but releases a lot of energy in neutron kinetic energy which you need to absorb in some material which lowers the temperature. D + D is the most common fuel on outer solar system and interstellar bodies but it also releases neutrons and might be most valuable in an energy system in that it products He3 and T some of which could be separated from the plasma and used to fuel reactors that are either aneutronic or low ignition energy.

Yep, there was a physicist who did the math on the Epstein Drive (unfortunate name) in The Expanse and found that it was at the edge of possibility if you use very efficient lasers, ICF, and D+He3 fuel. There is mention of helium-3 and fuel pellets in the books, so it sounds like that was the idea.

The biggest un-realism in The Expanse is the lack of huge heat-sinks, at least in the show. (They aren't mentioned in the books but I assumed they'd be there.) Without heat-sinks even if the drives were >95% efficient the ship would melt. Also in the show the thrust plumes from the engines are portrayed as looking like grill flames. In reality they'd look more like beams of light fading off into space.

The path not taken in fusion research is

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

the argument is that "highly efficient laser" might be an oxymoron and if you were serious about commercial fusion you might trade lasers for much more efficient particle accelerators that run at a viable shot rate. Trouble is that you need heavy ions (lead) at 8GeV and it takes multiple barrels that are a km long or so... A huge machine that might be competitive with lasers for a commercial power plant but that can't be built in a subscale prototype. It might not be compatible with the magnetic nozzle though as 8GeV is not relativistic for lead ions.

I'd never even heard of HIF. I wonder how it could scale with today's superconductors, which are far better than what we had in the 70s.

See https://cds.cern.ch/record/2743157/files/Seeman2020_Chapter_... you can get better Q for superconducting linacs and also operate them in CW mode without fear of burning them up. Off the cuff I'd speculate that superconducting magnets that can support 2x the magnetic field might support 2x the accelerating gradient but we are talking AC operation here, not DC.

I thought laser fusion only exists for nuclear weapons research, and there isn’t any path from the current experiments to a power plant.

That’s what I think. The efficiency of lasers is awful and they take hours to cool off after a shot whereas a commercial fusion power plant needs a shot rate between one every few seconds to several per second.

A heavy ion power plant is possible in terms of the physics but needs to be the scale of a fission power plant to work at all and is projected to cost maybe 2x what an AP1000 costs assuming everything goes well and we know things usually work worse than you expect. So nobody is interested in funding a full-scale prototype, a reasonable development plan is you build several linac barrels and a test fusion facility and expect to rebuild that and add more barrels. It probably costs about what Musk spent on Twitter in the end.

That’s the funding pitch but I’ve heard the scientists claim they want to study it for possible power use.

Maybe they have some sort of thermally excited laser material and use hybrid electro/thermal lasers to "heat pump" waste heat back into the laser system.

The entropy has to go somewhere. Laser beams carry no entropy.

10.1103/PhysRevLett.109.193601

K. Sandner and H. Ritsch, Physical Review Letters (2012); Temperature Gradient Driven Lasing and Stimulated Cooling

Now tell me about all the waste heat such a scheme would generate. Yes, you can generate work from a temperature difference. You cannot convert that thermal energy to work with 100% efficiency.

In Dragons’s Egg[0] by Robert L. Forward the neutron star alien race are nucleonics life forms which experience timescales a million times faster than humans.

[0] http://mdpub.github.io/cheela

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