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Comment by rbanffy | Hacker News Reader

> Obviously anything less than break-even is not worth having

No. Not at all. We are talking propulsion, using fusion reactions to pass on energy to the propellant and convert that into thrust. Although it would be desirable, it's not required to make it a net-positive reaction - just giving the propellant more energy than a chemical reaction is enough to be more efficient than a chemical rocket.

As the article mentioned, you can power this rocket with an ISS worth of solar panels (which is quite a lot of mass). Or, as it didn't mention, a very small fission reactor (provided you could negotiate putting a 200 KW reactor in space).

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>> Obviously anything less than break-even is not worth having

> No. Not at all.

Yes -- if the fusion reactor didn't achieve break-even, the designers would be better off using an ion thruster. Also, the NASA documents that describe the project assume that break-even must be achieved:

http://www.nasa.gov/directorates/spacetech/niac/2012_phaseII...

Quote: "an in-depth analysis of the rocket design and spacecraft integration as well as mission architectures enabled by the FDR need to be performed. Fulfilling these three elements form the major tasks to be completed in the proposed Phase II study. A subscale, laboratory liner compression test facility will be assembled with sufficient liner kinetic energy (~ 0.5 MJ) to reach fusion breakeven conditions."

They are talking about doing away with the present lines of fusion research, which haven't achieved break-even, and using a different method. But they don't suggest that this, or some variation, might be used for conventional power generation.

> Although it would be desirable, it's not required to make it a net-positive reaction

Yes, it is -- that is required. Were this not true, the designers would be better off using an ion thruster, which already exists and is quite efficient.

> As the article mentioned, you can power this rocket with an ISS worth of solar panels ...

Yes, that's in the description, but the power available (200KW near earth, 100KW near Mars) is not enough to propel the relatively heavy craft to the mission profile (i.e. 30 days to Mars) without some other source of energy, like from a net fusion power gain > 1.

A rocket only needs to achieve thermal break-even. A power plant needs to achieve electrical break-even, which includes the unavoidable loss from converting the heat to electricity in a heat engine.

In this way a system can be simultaneously useful for space travel but useless for power generation.

> A rocket only needs to achieve thermal break-even. A power plant needs to achieve electrical break-even ...

Yes, all true. But if a fusion reactor ever achieved break-even, that would be such a breakthrough that the specifics would be reduced to footnotes, and both thermal and electrical applications would soon follow.

> In this way a system can be simultaneously useful for space travel but useless for power generation.

My point is that if break-even were to be achieved, it would be break-even for both applications. The reason is that the plasma conditions for fusion break-even would have much more in common in the two cases than the differences.

If fusion doesn't provide a substantial energy gain, it's not worth the effort. Existing ion thrusters are quite efficient (60+%) at translating electrical power to exhaust kinetic energy.

Indeed, but they offer very low thrust. For manned space travel, you may need something that can give you a higher acceleration, even if at the expense of efficiency.

Also, fusion reactors designed for power generation have very different goals than fusion rockets. With power generation, particles leaving the reactor may be considered wasted energy. With rockets, the whole idea is to have particles leaving the reactor in a certain direction taking as much energy as possible with them. You just point the jet at the direction opposite to the one you want to go.

Well, controlled fusion hasn't reached energy breakeven at all, even taking into account the kinetic energy of the reaction products. But the low thrust of current ion engines is mostly due to power restrictions (https://groups.google.com/group/sci.space.science/msg/0cb332...). If your spaceship has a mass of one ton and you want an acceleration of 1 centigee with an exhaust velocity of 30 km/s, you will need at least:

0.5 * thrust * velocity = 0.5 * (0.1 m/s^2 * 1000 kg) * 30 km/s = 0.5 * 100 N * 3E4 m/s = 150E4 W = 1.5 MW

of power.

If you don't mind waiting a year.

Existing ion thrusters don't give enough thrust to get to Mars in 30 days.

"kinetic energy" means nothing alone, thrust and specific impulse are better metrics to look at. Doing a lot better than ion thrusters at either without being too much worse in the other and energy efficiency would be interesting.

However, the article quotes an exhaust velocity of 30km/s (or ISP of 3000s), and using 200kW doesn't leave much room to beat http://en.wikipedia.org/wiki/HiPEP on thrust without counting on net energy gain. The linked slides also claim net energy gain.

Yes, I agree. My point was that you cannot do substantially better than ion engines without having energy gain and, in fact, you are probably going to do much worse due to the weight of the "fusion hardware".

I'm quite optimistic about magnetized inertial fusion. But the idea of doing the job much better than the Z-machine, with something lightweight enough to carry into space and in less than 10 years seems to me... unlikely, to put it mildly.

>in fact, you are probably going to do much worse due to the weight of the "fusion hardware".

Ion engines have weight, too. What leads you to believe that the fusion hardware will be heavier?

In the short run:

- Ion thrusters are way simpler and they are trying to do something that is much easier.

- Ion thrusters are relatively mature, working "fusors" don't exist.

- Existing machines that are trying to get fusion breakeven are building-sized.

- Trying to start a fusion reaction is hard, it only makes sense if you get a commensurate result (energy gain).

In the long run, I'm very optimistic about fusion.

Ion drives are highly efficient, but have ridiculously low thrust. Which doesn't help at all when you are trying to reach some point quickly.

If you can construct an ion drive with enough thrust to match this proposed fusion drive (or even a NERVA: http://en.wikipedia.org/wiki/Nuclear_thermal_rocket), talk to NASA, I am sure they will be interested in buying several from you.

> Which doesn't help at all when you are trying to reach some point quickly.

Sure it does. You just leave it turned on. This thruster assumes 6 days of thrust then 24 days of coasting. With an ion thruster you leave it on for all 30 days.

There are no high thrust systems that can just be left on - even nuclear ones are used for a short period then turned off. The idea of an ion thruster is that you leave them on, and achieve the same total thrust, over the same time.

> With an ion thruster you leave it on for all 30 days.

With a thrust of - at best - 5 newtons, you won't achieve the goal of getting to Mars faster. You may get there cheaper and using less fuel, but for pure speed you lose. Ion thrusters are good for very long trips when you're going to leave the engine on for months, or for trips where the total time doesn't matter much, only fuel economy (e.g. for cargo shipments or probes).

5 newtons won't get you there in 1 month, but according to http://en.wikipedia.org/wiki/VASIMR it will in 5 months which is still pretty good, and much better than a standard rocket.

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