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Comment by Symmetry | Hacker News Reader

In terms of fusion drivers I've always liked NASA's PuFF[1] or Pulsed Fission Fusion. The idea is to use a normal z-pinch pulsed fusion setting, which won't normally be self sustaining. But if you surround the D-T mix with depleted uranium it'll adsorb a lot of the fast neutrons from the fusion and trigger a round of fission generating more energy. The recoil against the magnetic compressor/nozzle should be enough to generate enough energy for the next pulse and the reaction byproduct are emitted to become your propellant.

[1]https://www.nasa.gov/puff

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we don't have fusion power on earth (except for bombs) so why could we magically create it (and launch it) in space?

Fusion on earth (that people talk about) is about generating electricity. Fusion on spacecraft is about shooting out exhaust gas as quickly as possible and storing energy at high density. The rocket equation relates the exhaust rate to the propelent mass requirements, which determines the feasibility of the spacecraft. It's a completely different optimization problem, and one that has historically favored nuclear energy sources in general.

To clarify further, from TFA: "While early versions of the Helicity Drive wouldn’t exceed the Lawson criterion (more energy out than in) and would require external (presumably solar) power, the energy added to the exhaust from fusion would increase the system’s exhaust velocity and ISP (giving somewhere in the neighborhood of 10,000 s isp, vs a maximum of ~400 for chemical rockets)"

So it is not able to generate more energy than it takes to operate, just like the fusion devices we can already create on earth, but can create really fast exhaust which makes the rocket more efficient.

In 1998, Robert A. Freitas published a detailed design of an artificial red blood cell with several useful properties, (stores 236 times as much oxygen as a natural red blood cell, indefinite shelf life) but with the minor downside that it would require atomically precise diamond fabrication to actually construct. https://www.tandfonline.com/doi/pdf/10.3109/1073119980911768...

Was that paper a waste of time?

It's always shocking to me how inefficient hemoglobin is. Less than 1/500th the mass of oxygenated hemoglobin is oxygen.

I'm similarly always a little surprised at just how inefficient photosynthesis is compared to a solar cell. Once I realized it was more than an order of magnitude I lost a lot of interest in biomass (much less ethanol) as a fuel source for sure...

Solar cells do one thing. Plants do many things.

There is value in creating designs that are feasible if some technological breakthroughs is made. If we manage to make fusion work, then someone can then capitalise on the existing designs. It's also a great motivator for people to work on those hard problems that could enable many fantastic possibilities. It's also just fun to let your imagination not be constrained by what is possible today, but what might be possible in the future.

For a power plant, you need to get more energy out than you put in. The best we've done is output that's about 70% of the input energy.

But a plasma rocket works without any fusion at all. If you get that same 70%, then you're doing 70% better than a plasma rocket without fusion.

On top of that, a power plant needs to run reliably for thirty years or so, withstanding intense neutron radiation, and breeding new tritium fuel from lithium. A fusion rocket might only need to fire for several days for a trip to another planet, and won't breed its own tritium.

The NASA Puff proposal would be more aptly described as a boosted fission engine.

From their intro [1]

  This synergy has been observed in the development of other fission-fusion devices.

You may be wondering what "other fission-fusion devices" were ever developed. The answer is of course, nuclear bombs.

The first fission-fusion device was not a thermonuclear bomb, but a boosted fission one [2]. In such a device, there's a little bit of Deuterium and Tritium in the center of the Plutonium pit. When the Plutonium starts undergoing fission, the temperature increases to tens of millions of degrees, and the D-T fusion reaction happens; the energy produced by that reaction is negligible. What the reaction does is release a huge amount of neutrons that go and split nuclei of Plutonium, therefore boosting the yield of the fission in the bomb.

The idea with Nasa's Puff proposal is the fusion is triggered first, using a plasma confinement device, similar to a Tokamak, but linear, not toroidal. The device is well below breakeven. But you don't care about the energy produced by the fusion, only that enough neutrons are released to go on an split a bunch of fissile nuclei. Few people are aware, but not only Uranium and Plutonium (and other heavy nuclei) can undergo fission, but also Lithium. Lithium can't sustain a chain reaction, but if external neutrons (or protons) are provided, it is happy to split and release a respectable amount of energy in the process. Pound per pound about as much energy as Uranium and Plutonium.

The end result is a very hot plasma of millions of degrees that is expelled and provides huge thrust and specific impulse. Of course the ejected plasma will be radioactive, and that's why this design can't be used on Earth to produce energy, or other useful things.

But in space, blowing some radioactive stuff is not a problem: it will simply spread out and go on and on forever and ever. 99.9999999% of it will not encounter anything for billions of years. The few particles that will, by luck of the draw, encounter something will be no different than other cosmic particles coming from supernovas, or other sources.

[1] https://www.nasa.gov/puff/intro

[2] https://en.wikipedia.org/wiki/Boosted_fission_weapon

> we don't have fusion power on earth

We do though, you can build a fusion reactor in your garage

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

What’s hard is making a fusion reactor that produces more energy than it consumes, and does so economically.

It's a fusor, a glorified toy for making neutrons, not a fusion reactor.

This is the critical leap into fantasy.

We have to get some form of self-sustaining fusion, i.e., producing energy, before we have a shot at making an engine out of it.

We don't. An engine doesn't need to be energy-positive, it just needs to produce thrust.

Well, you don't need to care about safety nearly as much.

safety is not what is limiting fusion development on earth - we simply can't do it effectively

Ofcourse we can - that's what a hydrogen bomb does!

The Teller–Ulam design and the NIF do the exact same job - they heat up and compress hydrogen to trigger fusion. Its just that NIF and has to fuck around with lazers that are perfectly focused on tiny gold pellets with micrograms of hydrogen. The hydrogen bomb deals with literal tons of material at once.

If you dont care about safety and environment, you concrete over one of the great lakes, heat them up by exploding thermonuclear bombs and use the steam to generate elctricity.

And even if we could, safety isn't a concern for fusion power plants.

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