This Kickstarter project is less than halfway to its goal, with only four days to go.
13 thoughts on “Electric Plasma Thrusters”
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This Kickstarter project is less than halfway to its goal, with only four days to go.
Comments are closed.
Plasma thrusters are already in use on satellites now, so what’s the point?
Ummmm…no.
Ion thrusters are not plasma-jet thrusters. The latter are lower Isp, but much higher thrust.
That’s only half of it, though it’s a big half. Ion engines and hall thrusters are extremely low thrust. With the electric thrusters we have today it takes months to generate substantial delta V (for example, it takes a year for the Dawn spacecraft to generate even just 2.3 km/s delta V). This makes such propulsion unsuitable for manned spacecraft and also only a little helpful in unmanned spaceflight. Pulsed plasma thrusters can generate one to two orders of magnitude more thrust from the same sized engine, which makes high delta V missions vastly more possible (being able to generate > 10 km/s of delta V in a year instead of in a decade) and makes their use with manned missions possible. For example, on a large manned Mars mission it would take such thrusters only a matter of weeks to generate enough delta V to send a ship from Earth orbit to Mars. Even if it didn’t reduce the travel time of such missions it would still be able to vastly increase the amount of payload capable of being delivered to Mars.
But wait, there’s more. Ion engines struggle with the problem that they are shooting high energy plasma right through the most important part of the whole engine, the acceleration grid. This limits the lifetime of an ion engine, and also limits the propellant choices. Pulsed plasma thrusters also have components which can potentially erode over time but not to the same degree and they are almost certainly capable of longer service lives than ion engines in terms of total delta V delivered. Which means, that using such thrusters on reusable interplanetary cargo and crew shuttles is a possibility. As is potentially using propellant from off-Earth sources.
In total these types of engines have an impressive potential to open up the solar system to exploration and colonization, this research is easily worth setting aside a few dollars to support.
Plus they don’t need orders of magnitude nuclear power like VASIMIR to operate. They work fine on solar power. I hope some of our billionaire visionaries see the potential and look into hiring these guys.
I can see Musk saying, let’s skip these test and just build a real engine. We’ll test that.
The short version, and apologies for not being able to make the short version more informative and persuasive: I do this for a living, Ph.D. in astronautical engineering, twenty years in advanced spacecraft propulsion, one new type of pulsed plasma thruster developed and flown successfully. And one resounding “No!” vote on the Hyper-V proposal. I won’t be contributing, and I recommend you all don’t either. There are things worth doing in the field of electric/plasma propulsion, but there are probably no revolutions left to be had. And if there are, this isn’t it.
The long version:
We already have Hall thrusters that work quite nicely over a broad range of performance. They are relaible, relatively inexpensive, the large ones are fairly durable, and there are good preliminary results on making them with propellants we can find on the Moon and probably on Mars. There are other sorts of plasma thruster under advanced development, well ahead of anything Hyper-V can show, that promise modest improvements over Hall thrusters, but what we’ve already got is pretty good, and pretty close to the best we are ever going to get in this area.
If Hall-effect thrusters don’t generate enough thrust for your tastes, then probably no electric or plasma thruster ever can generate enough thrust for you and there is not much point in looking further. Present Hall-effect thrusters convert electric power to thrust with 60+% efficiency at a specific impulse of 1500+ seconds. The only ways to increase thrust without violating well-established laws of physics are to increase power, increase efficiency, or decrease specific impulse.
As the electric power supply (whether solar or nuclear) already dominates the mass and cost of the system, “add more power” just begs the question – and doesn’t accomplish anything that you couldn’t have done just as well with a more powerful Hall thruster (or a cluster of smaller ones). Whatever level of power you can deliver, we cah channel efficiently through thrusters we have already got. Efficiency can’t go above 100%, and almost certainly won’t go above 90%. Reducing specific impulse negates the whole point of electric or plasma thrusters in the first place, and reducing it by a factor of three would put you squarely in competition with high-thrust chemical rockets.
There are marginal gains to be had in better EP systems, yes, and in an enterprise as marginal as deep space travel even those marginal gains are worth pursuing. But anyone promising order-of-magnitude gains, anyone saying that if we invest in their plasma drive we get order-of-magnitude improvements in mission performance, that we get to Mars in weeks while those losers with their Hall thrusters take years, those people haven’t done the math. Or they have, and they are knowingly selling snake oil.
And if you genuinely are looking for the marginal improvements that are to be had, you probably don’t want to be looking at pulsed systems and you almost certainly don’t want to be looking at the sort of gas-fed pulsed plasma thrusters that have been exhibiting underwhelming performance since the 1960s in spite of a whole lot of really smart people trying to make them work. Pulsed operation means blowing energy on start-up and shut-down transients over and over and over; you won’t get it all back, it isn’t a trivial part of your power budget, and it pretty much guarantees you won’t beat a decent Hall thruster’s 60% efficiency. Pulsed operation of high-current systems also means burning out lots and lots of semiconductors, or eroding cathodes or whatever, all of which is annoying and expensive in the laboratory and intolerable halfway to Mars.
You probably also want to have your advanced spacecraft propulsion systems developed by people with some actual expertise in spacecraft propulsion. Their website and prior publications make it clear that Hyper-V is a bunch of fusion researchers with a failed fusion reactor scheme looking for a new home. There’s actually precedent for failed fusion reactor schemes being legitimately interesting in propulsion applications, but you need people who are familiar with all the work that has already been done in the field so they don’t repeat all the same mistakes of the past fifty years. There’s no evidence of that knowledge or experience in anything I see from Hyper-V.
Thanx for the input.
I thought the rail-gun like plasma pulse propulsion looked intriguing, but I was not aware of the drawbacks of pulsed propulsion.
I did already know of Hall effect plasma thrusters, and how mature the technology is. It surprises me how many space enthusiasts think VASIMR is some kind of magic technology, while Hall thrusters are already here and for most practical applications just as good.
For manned missions to Mars I’m fond of a mixed mode propulsion, using a combination of nuclear thermal and a 2 megawatt power class electric propulsion. That way a manned spacecraft needn’t spend a year spiraling out from orbit, or require an enormous power source (and radiator!) for the electric engine.
1 meganewton = 1 billion millinewtons
Hall thrusters produce milli-newtons of thrust. A chemical engine like the Merlin produces mega-newtons of thrust. That would be some cluster of hall thrusters to get somewhere in the middle of that gap.
You’re saying these plasma thrusters can’t better than 300s and do it with thrust somewhere in that gap?
Make that 450s
Short answer: Yes, I’m saying they can’t do that. And the math says I’m right, however much your intuition says otherwise.
For a plasma thruster produce even one kilonewton of thrust at a specific impulse of 500 seconds, would require about 2.5 megawatts of input power used at 100% efficiency. The largest nuclear reactor ever seriously proposed (but not built) for space use, had a projected electric power output of 100 kilowatts, so if you’ve got a couple dozen of them…
If you’ll settle for mere newtons of thrust, we’ve already got Hall thrusters that can do that – they aren’t all down in the millinewtons any more. If yo’ve got the power for tens, hundreds, or even thousands of newtons of thrust, we can build Hall thrusters to handle that as well. Or ion thrusters, MPD thrusters, probably even a VASIMR or an FRC. We don’t need new physics or even much new technology for any plasma-drive application we can realistically envision in this generation or the next.
But the gap between chemical and electric propulsion is real, it is substantial, it is deeply rooted in fundamental physics, and only thing even remotely on the horizon that can fill that gap would be nuclear-thermal propulsion. Or possibly laser-thermal if you don’t mind being closely tied to a massive and expensive piece of infrastructure.
The kickstarter project to build a billion-watt laser array or open-cycle nuclear reactor, I’m not terribly optimistic about that one.
Makes sense. So Dragon has two panels with1,500 W average, 4,000 W peak. So a general purpose ship with more panels might have 10kW available for a drive. From Wikipedia…
…the Hall thruster on SMART-1 could be throttled over a range of power, specific impulse, and thrust.
Discharge power: 0.46–1.19 kW
Specific impulse: 1,100–1,600 s
Thrust: 30–70 mN
So 10 kW could power ten to twenty of these giving us up to 1.4 Newtons.
That doesn’t sound so bad. I haven’t figured out the fuel mass for a 50 mt ship the Falcon Heavy could launch yet. Nor the trip time to mars. Hey, any real rocket scientists want to step up?
The operating envelope for the SMART-1 thruster isn’t a nice rectangle; you can’t simultaneously have 0.46 kW and 70 mN. Also, subtlety, “discharge power” is not total power. So more like 0.8 Newtons than 1.4.
But even with 1.4 Newtons of thrust, a Dragon would take roughly two years to travel from LEO to Mars – about half of that time just spiraling out to Earth escape. The required propellant would be about six tonnes, which is coincidentally just about the total payload capacity of a Dragon – so I hope you weren’t planning to carry anything useful.
A 50-tonne Falcon Heavy with twenty percent of its mass devoted to the most potent space power supply presently on the drawing board (the Boing SLASR solar array architecture), would have ~3 Megawatts of power to play with. Run that through a plasma thruster with ~3000 seconds of Isp, and you get 150 Newtons of thrust. That’s enough for LEO to Mars in a bit under six months, consuming 22.5 tonnes of fuel in the process.
Assuming a 0.15 stage mass fraction, that leaves you with ten tonnes of useful payload in low Mars orbit, which isn’t too bad. That’s with a cluster of standard Hall-effect thrusters; if you use Hyper-V’s wonder-thruster it will probably come in a bit worse because I very much doubt any pulsed system can beat a good Hall thruster at high power.
You can actually do a bit better still if you keep one of those antique chemical rockets around as well – there’s a brief period just around Earth escape where, if you do it right, the Oberth effect gives you enormous leverage on the first km/s or so of post-escape delta-V. But it is a brief period, minutes, so you need a true high-thrust system to take advantage. Same applies for Mars arrival if you aren’t doing aerocapture.
As Von Braun et al understood full well, going to Mars and doing it in style does not require hyperadvanced propulsion technology. It does require that you be smart about how you use your various bits of fifty-year-old technology.
Thank you.
3 Megawatts … 3000s … 150 Newtons of thrust. That’s enough for LEO to Mars in a bit under six months, consuming 22.5 tonnes of fuel in the process.
So it takes about the same time but xenon at $1200 per kg. would be $27m plus another $100m to orbit vs. about a billion dollars for conventional chemical fuel. That eventually changes once we have lunar fuel.
Hall thrusters are off the shelf parts. I assume the crew could meet up with the ship separately after it has spun up around the earth? Artificial gravity would be a lot trickier under continuous thrust so the little bit provided by the thrusters is probably all you’d have for the journey.
I was assuming 50 mt dry wgt. for the ship to which you add crew and provisions but that doesn’t matter for sake of this argument. Or does it? That 3 MW only applies to this smaller ship. Once in mars orbit you could beam some of that power to the martians but I’m thinking a much larger mission than 10 ton is required.
We’ve got options.