Their aim is to move a heavy object with as little fuel as possible not necessarily produce perfect fusion. (although obviously that would be nice for everyone!)
But "break-even" means yielding more energy than is required to start the reaction. Obviously anything less than break-even is not worth having -- a chemical rocket would be more efficient.
> Their aim is to move a heavy object with as little fuel as possible not necessarily produce perfect fusion.
This misses the point that present laboratory fusion reactions require more power than they release. It's not a question of "perfect fusion" but any fusion that has a net positive energy yield.
Why not? The energy to start the reactor comes from solar power, so they didn't have to carry it up with them.
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).
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.
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.
Existing ion thrusters don't give enough thrust to get to Mars in 30 days.
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.
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.
Ion engines have weight, too. What leads you to believe that the fusion hardware will be heavier?
- 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.
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.
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 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).
> 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.
In this way a system can be simultaneously useful for space travel but useless for power generation.
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.
To represent an advantage over an ion thruster, it must be both. And the NASA documents that describe the project specify a fusion power gain > 1:
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."
The phrase "fusion breakeven conditions" means the reaction must produce more power than is requires to start it. This is a requirement to justify the project over other approaches.
No radiators implicitly mean no electricity generation, because it creates waste heat that has to be utilised.
http://en.wikipedia.org/wiki/Fusion_rocket
Proposed concept is a direct drive, and it actually claims they fixed the problem with neutrons (they are caught by the compressing metal)
As for breakeven conditions they merely state that the power that the fusion generates must be at least equal to power used to start/sustain it. http://en.wikipedia.org/wiki/Fusion_energy_gain_factor Theoretical limits of efficiency of heat engine is around 70-80% (roughly) http://en.wikipedia.org/wiki/Thermal_efficiency and you have to use those to convert heat generated in controlled fusion into electricity. To actually achieve net power generation in a power plant, you have to generate a lot more heat that electricity, while for propulsion you merely care about actual raw (so to speak) power.
Ion thrusters have their own issues, while their ISP is considerably better than chemical rockets, maximum thrust is abysmal, but it is hard to argue about that due to the lack of data on FDR. However, with ion thrusters there are other issues (for example:wear, charge imbalance)
Yes, but (apart from stars and weapons) this has never been achieved anywhere, ever. This is a big issue, bigger than most people realize.
https://en.wikipedia.org/wiki/Fusion_power
Quote: "As of July 2010, the largest experiment by means of magnetic confinement has been the Joint European Torus (JET). In 1997, JET produced a peak of 16.1 megawatts (21,600 hp) of fusion power (65% of input power) ... Fusion powered electricity generation was initially believed to be readily achievable, as fission power had been. However, the extreme requirements for continuous reactions and plasma containment led to projections being extended by several decades. In 2010, more than 60 years after the first attempts, commercial power production was still believed to be unlikely before 2050."
My point is if this project creates a net gain > 1, it will address a lot more than the problem of getting to Mars.