I just saw
This and thought you guys would be interested. That, and i'm looking forward to seeing more.
There's a
thread started in February, originally based on the somewhat less recent announcement by Lockheed-Martin that they hope to have a useful fusion reactor based on old-fashioned magnetically contained plasma ready within the decade, where Slough et al's University of Washington proposal has also been discussed.
Needs more information. So this is a 150-ton system that can run off the solar cells aboard the ISS.
These media reports are of course soft and incomplete--mind, I have some reservations about Slough's own rigor, see below! But digging deeper a couple of layers, I found
a PDF of a paper by the team that gives us more useful information, though it still doesn't tell us just what sort of overall mass they expect a useful system to have, nor hard numbers as to the power input required (but he hopes to get it working to the point where it outputs 200 times the power needed to trigger the fusion pulse, so that's an estimate of sorts--if you knew the mass of the pulsed matter!
)
No, it doesn't have to run on solar power, it could feed back some of the power it generates to sustain the compression mechanism, but the team figured for the propulsive application, it would be simpler to rely on sun power for input and not try to capture any of the output. They do say though that regenerating the input energy from the output is entirely feasible. (Rightly or wrongly--it seems entirely plausible to me).
150 (actually, in the paper, up to 200 tonnes) would be the initial mass of the spacecraft being launched from LEO to Mars, on a 30 day trajectory--all up mass, craft including power plant plus propellant. Not the mass of the system itself.
So... what kind of thrust does it develop? I'm not sure this thing is any better than an ion engine from what the article tells us.
It's different from an ion drive. Different strengths, different weaknesses. What it is, is an intermittently pulsed fusion-powered thermal lithium (or aluminum, whatever--I think they only use Al now because it's more convenient and lithium is what they'd use on an operational spaceship) rocket. The power that drives the ship is from hydrogen fusion (deuterium-tritium) of a very small pellet that vaporizes a much larger mass of metal. I'll describe how I gather it's supposed to work below. But the energy comes from fusion; the choice to drive the compression stage with input solar power is one of alleged engineering convenience and amounts to choosing not to use any of the fusion-generated power to sustain the action, thus very slightly augmenting the thrust and one can think of the solar power as being indirectly input into the drive power.
Its supposed to be a fusion reactor. Given that we cant make one work at breakeven point ON EARTH with massive buidings and infrastructure, the article seems a tad, as in cockeyed, optimistic. For feasibility at all, let alone the engineering details needed to get a mass estimate. Good grief!
Indeed I'm considerably more excited by the prospect of the thing working as a power generator on Earth than as a rocket in space, especially if certain limitations the team seems to accept as inevitable cannot in fact be easily overcome. If it works just as they describe, it seems to me to make more sense to use it as a power generator on the spacecraft, recycling the metal that is most of the throughput mass, and use that power to drive some sort of ion or plasma or whatever sort of externally powered reaction drive one likes.
For one thing, even if the limitations I'm worried about can be addressed and the rate of fire and thus the thrust raised to something reasonable, the maximum ISP they envision is in the ballpark of 5000--and the press articles more typically talk about 3000. That's better than an LH2-LOX chemical rocket by far, better than a thermal fission rocket in fact by a factor of 3 to 5, but it's still not all that fast relative to the velocity changes one needs to go between planets in a matter of months. I can believe (with one big reservation!) the transit times their article talks about to Mars, but with the total mission delta-V, out to Mars, stopping there, then boosting away from there and braking to Earth orbit, of 60,000 m/sec, an ISP of 3000 means that something like 6/7 of the launch mass must still be propellant.
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OK, here's what it's supposed to be. One takes a strip of metal foil--ideally lithium at least for the propulsion application--that, in the paper, masses 370 grams, and presumably can be scaled up or down within certain ranges for different applications--and arranges it as a ring, a good fraction of a meter in diameter. Then there's a pellet of fusible material, in this case a mix of deuterium and tritium, being shot down the axis of the ring; as it approaches the ring center magnetic fields rapidly crush the ring (I think I saw the speed of the metal inward estimated as reaching 4000 m/sec) so that crumples around the pellet, dead center when the metal gets there. The momentum of the metal compresses and heats the pellet to temperatures and pressures where fusion occurs--I gather the reaction won't fuse all the potential fuel except at the very highest possible gains they might hope to achieve--and the released heat vaporizes the metal. The paper mentions the metal layer being something like 5 centimeters thick as fusion is happening, and that this would be enough to absorb most of the neutrons the reaction puts out, so essentially all the released energy goes into heating the metal (and presumably the leaking neutron flux is low).
Now we have essentially 370 grams of metal that will of course explode thermally; the magnetic fields form a sort of nozzle that guides the thermalized plasma back to form a rocket pulse, and from the estimates given of the metal moving at 30 to 50 km/sec exiting the nozzle, we can see that the lithium outmasses the fused fuel by a factor of a million or so.
First reservation--in the first article I saw about it, Slough is
hoping to get the rate of pulsing
up to once a minute!
That's right, not 60 Hertz; 1/60th of a Hertz!
When we look at the mechanism we can see why; it is necessary to arrange another ring of metal foil around the focal point of the reaction; obviously it would take time to do that.
But on the other hand--suppose he does get the metal to come flying out at 50,000 meters/sec. A kilogram would have kinetic energy of one and a quarter Gigajoules, since we're dealing with a bit more than a third of that mass, it's still 500 megajoules or so; doled out over 60 seconds until the next pulse, that's 8 megawatts.
As a power generator, one would simply have another magnetic field downstream, to brake the plasma pulse to nearly a stop, using its kinetic energy to pump up the field. Obviously the slowed, cooled and spent lithium needs to be allowed to vent out of the chamber anyway or it will accumulate there and clog things up--so there the lithium is, needing to be collected and removed anyway but available to be re-fashioned into more foil for later pulses.
As a rocket--obviously the thrust depends on how big a mass of metal the mechanism can squeeze to initiate how large a fusion reaction. If we were talking a whole kilogram, or 10 tonnes, and yet we can fuse enough fuel to vaporize it all to the same 30-50 km/sec exhaust speed, we'll raise the impulse by 3 to 30,000.
But of course that's "raising" it from around 20,000 kg-meter/sec, which is macroscopic enough, but with 60 second intervals between pulses, that could only accelerate a third of a tonne at a rate of 1 m/sec^2, or just 30 kg at one gravity--average. Obviously it would give that 30 kg mass a big 600 meter/sec jolt, then it would coast (assuming a 60 G shock didn't smash it!) until the next pulse. But of course while I can well believe the magnets and stuff involved can weigh in at a lot less than 10 tonnes or even 1 tonne, I doubt that it can mass just 30 kg!
The accelerations he hopes to reach are in fact more like an average of 1/200 of a G, or 5 centimeters/sec^2--again in the form of a jolt of 3 meters/sec with a 60 second wait for the next one. Imagine being in a car going 6 mph that slams into a wall, once a minute.
A single pulse drive with its metal charge in the range of 1/3 a kg as his article discusses could only drive 7 tonnes at this rate; obviously we'd need a battery of 30 or so to drive 200 tonnes! That of course is an opportunity as well as cost--it means the pulsing would be smoothed out, each jolt only being 10 cm/sec on the whole mass, and one coming every 2 seconds--one might better imagine effective shock absorbers that smooth that out into a steady, slow push.
And if the assembly to pulse a 370 gram mass once a minute masses a tonne all up, then I've proposed a drive that masses 30 tonnes, for a 200 tonne spaceship, that drives it at 1/20 meter/sec^2 acceleration.
Obviously I'd hope that the rate of fire could be considerably improved! If it could be got up to once every 10 seconds we'd increase the thrust by a factor of 5, or alternatively could get rid of 4/5 of the pulse driver units. And we could fiddle with the mass of the package that gets pulsed; I don't think 370 grams is written in stone on the foundations of the Universe; why not a kilogram, or more? (Or less--10 grams, a single gram, assuming the mechanism that drives the masses to compression can also scale down--since it's momentum driven, I doubt that though). I'd think there would be tradeoffs and practical limits on the scale of each pulse; I've thought of some approaches to get the rate of fire up.
But even if a reasonably sized set of driver units could between them generate not 1/200 of a G but say a half G, suitable for TLI and Solar System transit injection in one quick thrust--still, we have the limited ISP to think about. It's large, but not huge compared to Solar system travel delta-V's we'd want.
Meanwhile--Slough and co-writers are very pleased that they can think in terms of a 200 tonne craft all up, that can be launched to LEO by a very big rocket. They don't seem to realize that if their acceleration is far below the level of the gravitational acceleration where they are, that they won't arc out on an escape trajectory but on a slow spiral tour of LEO and MEO (including those scenic Van Allen radiation belts!
) that will waste a huge boatload (literally!) of reaction mass. They need something to boost them up high enough that 1/200 of a G is comparable to the pull Earth is putting on them. Well, that's just 14 Earth radii out--but the rocket that can send them up that high first is going to account for much of the delta-V of the mission. The same thing needs to be done approaching Mars, and leaving Mars, and braking to parking orbit at Earth.
This is why I'd want the rate of fire way up there, and hope that the mass of a single pulse unit scales down with the pulse mass, and that small pulse masses will work, so that there can be lots of drivers each generating small pulses out of synch with each other for a net thrust in the half-g range or so, to eliminate that intermediate rocket.
And why it might be smarter to go with the device as a power generator, converting a store of fusible fuel to power via metal that gets recycled, and the power used to drive an ion drive or some such, provided the ion drive has a higher ISP than 5000.
Meanwhile, if this contraption can work at all--and I'd never have thought such a simple contrivance could reasonably be expected to produce fusion-inducing conditions, but on that point I'll take Dr. Slough et al's word for it, and trust they will be kicked out of their positions at U-Wash if the basic physics claims they are making are all mendacious hooey, tenured or not--even if the limits that make it unattractive as a space drive cannot be overcome, I think it can probably transform the world situation by providing a long term solution to power generation that would be feasible as such immediately. In other words, we would never learn whether we might have gotten by on renewable energy, or vice versa done perfectly fine with suitable fission reactors. If the U-Wash team can get better than breakeven on the reaction--and the authors hope to go as high as 200 times input energy, and would settle for a mere 100--then I foresee no problems at all using the system for ground-based power generation, even if the installations turn out to mass 20 tonnes for a paltry 8 megawatts! The point is, their "fuel," when we bear in mind we have to recover the metal just to keep it from accumulating and clogging up the works, and so can easily recycle it, is abundant in sea water. There's plenty of it, available to everyone in the world, and so a huge constraint on the ongoing economic development of the world to First-World standards has been removed.
So why in God's name is the U-Wash team getting funding from NASA for a dubious extraterrestrial application, when first developing it as a power-generation technology seems so obvious?
Maybe because they and I are dunces, me for believing them, them for seriously believing that they can get fusion this way. Well, I don't know enough to judge, but if these people are willing to stake their reputations like this, I can believe, in an impressionistic way, that fusion can happen this way, and have to believe for now that the consensus of physicists can't readily prove it can't.
And maybe because businessmen are dunces; having been fleeced by fraudulent claims of cold fusion, "free energy," perpetual motion, ad nauseum, they don't know good science from bad and so no reputable capitalists who would back a straight energy generation plan are coming forward. Nor is the US government generous with money for pure scientific research. But there happens to be a pot of money currently allocated to developing advanced space drives for NASA. As advertised, it is dubious in that application, though as I said above I see room for improvement, if the basic technique works at all. Getting NASA to fund the scheme in the hopes not only of answering that question first and then having to overcome a half dozen other hurdles before it becomes useful to them is a roundabout way of getting that first question answered, but if the answer is yes then the other issues are probably solvable, and if they are not--the world will be entering an era of general prosperity based on more abundant power, and presumably the rising tide will lift even NASA's boat, and some of the other schemes they are funding right now will be affordable.
If it can't work--this is how the government is currently willing to risk taxpayer dollars to find that sort of thing out.
