Author Topic: Plasma Jet Electric Thruster Kickstarter  (Read 10563 times)

Offline Robotbeat

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Re: Plasma Jet Electric Thruster Kickstarter
« Reply #20 on: 02/09/2013 05:03 pm »
So for a Mars mission high thrust is important! Let's say that we are using a methane engine with Isp of 380s. A low thrust electrical engine would require an Isp of at least 1520s to be as effective. Of course most electrical engines have an optimum Isp over 2000s, but that does not include the extra weight of the solar panels...
Mars missions have long cruising periods, and so the optimum ISP is quite large for state of the art solar tech.
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Offline Robotbeat

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Re: Plasma Jet Electric Thruster Kickstarter
« Reply #21 on: 02/09/2013 05:38 pm »

I was under the impression that some orbital capture maneuvers required fast delta V change...
No, nothing in-orbit actually /requires/ high thrust. There is a performance benefit (from the Oberth effect), but it's still dwarfed by the Isp advantage of electric propulsion.

see emphasis
I didn't miss it the first time. You just have to start thrusting earlier. You miss out on most of the Oberth effect (and trip time is slightly longer), but it's not a huge deal because of the huge advantage in Isp, which can also counter-act the delta-t penalty.

For trans mars injection and mars capture, the oberth effect can make a very big difference. Electrical delta-v from earth C3=0 to mars C3=0 is roughly 6km/s, while chemical delta-v is roughly 1.5km/s. So a factor of 4.

So I think discounting the oberth effect is not warranted unless you have a very good power source, or your destination does not have significant oberth effect (e.g. an asteroid).
Infinitely low-thrust assumption, yes, it would be almost 6km/s. that would assume spiraling around the sun for a very long time, years and years. But that assumption is clearly invalid here. The actual result would be closer to a Hohmann transfer, which still isn't great but is better than your factor of 4. And, of course, even the assumption of infinite thrust breaks down for chemical rockets as well.

This paper shows the transfer between Earth and mars for far less than your 6km/s: http://goo.gl/2USvJ

And again, solar electric keeps improving.
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Offline manboy

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Re: Plasma Jet Electric Thruster Kickstarter
« Reply #22 on: 02/09/2013 08:58 pm »
Wasn't commenting the machine in original post, pointing out that high thrust/low Isp electric propulsion is not a break through, solutions been available. Then again, why use them because chemical solutions make more sense.
Depends on where you want to go.
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Offline rklaehn

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Re: Plasma Jet Electric Thruster Kickstarter
« Reply #23 on: 02/09/2013 09:09 pm »
For trans mars injection and mars capture, the oberth effect can make a very big difference. Electrical delta-v from earth C3=0 to mars C3=0 is roughly 6km/s, while chemical delta-v is roughly 1.5km/s. So a factor of 4.

So I think discounting the oberth effect is not warranted unless you have a very good power source, or your destination does not have significant oberth effect (e.g. an asteroid).
Infinitely low-thrust assumption, yes, it would be almost 6km/s. that would assume spiraling around the sun for a very long time, years and years. But that assumption is clearly invalid here. The actual result would be closer to a Hohmann transfer, which still isn't great but is better than your factor of 4. And, of course, even the assumption of infinite thrust breaks down for chemical rockets as well.

This paper shows the transfer between Earth and mars for far less than your 6km/s: http://goo.gl/2USvJ

Interesting.


The paper gives the delta-v for Earth Mars transfer and capture to a 5-day Mars elliptical orbit as 2600m/s. That is for a hyperbolic excess of 0.6km2/s2, which is a velocity at infinity of just sqrt(11.22+0.6)-11.2=0.027km/s. So 2600m/s for almost the entire interplanetary part earth c3=0 to mars c3=0.

This indeed an impressively low delta-v. But they get this very good delta-v by having a very long trip time of two and a half years, from 2007-01-08 to 2009-07-26. Such a long delta-v would not be acceptable for a manned mission.


For the way back they have a delta-v budget of 3150m/s for Mars-Earth transfer. But that is not mars c3=0 to earth c3=0, but just mars c3=0 to an earth intercept trajectory. That is pretty much what you would expect for the infinitely low thrust assumption. The reason for that is that they are in a hurry on the way back because they have valuable cargo on board, so they can't optimize the trajectory for low delta-v. But even so the trip time just from mars c3=0 is 2011-06-09 to 2012-05-29, almost a year. The spiral out time would have to be added. This is much longer than what you would have with a high thrust propulsion system.

So 6km/s is pretty realistic unless you have the luxury of optimizing your trajectory for delta-v and not trip time.


Note that I do see the attraction of electric propulsion. Instead of a hypothetical mars mission leaving the staging point at EML2 to perform a "death swoop" close flyby to get maximum benefit of the oberth effect, you just slowly accelerate away without ever having to get close to earth and to cross the van allen belts. Also, the low thrust lets you design your spacecraft in a more modular way, with light structure.

But with a high thrust architecture you can do in situ resource usage more easily, since LOX/LH2 or LOX/Methane are much easier to manufacture on mars or phobos than good ion engine fuel like xenon. And a spacecraft that is capable of high thrust maneuvering could also more easily made to survive aerocapture and aerobraking.

Quote
And again, solar electric keeps improving.

Indeed. So maybe in a few years you will be able to get the 2600m/s delta-v to mars without having an unacceptably long trip time. But then there are possible improvements on the high-thrust side as well, such as a demonstration of inflatable reentry technology.

One thing I would love to see would be electric propulsion capable of using mixed mass molecules with high specific ionization energies such as water or ammonia without losing too much efficiency. In fact I asked the initiators of the kickstarter this question:

Me:
Will the thruster be capable of working with mixed molecular mass molecules such as water without losing too much efficiency?

Doug Witherspoon Ph. D. :
In principle, the thruster can work with almost any gas or liquid, and even some forms of solids. But there are trade-offs in performance and  practicality with the different materials. Efficiency in the mixed  molecular cases can, in principle at least, be at least partly maintained, but only if the so-called frozen-flow losses (such as those associated with molecular bonds and ionization) can be recovered in the exhaust flow. This typically can only be achieved if the density is high enough to allow three-body recombination collisions. Our density is indeed high, so there is hope that we could achieve high efficiency in those cases too, but we won't know until we test it. Liquids present a special problem in that mass flow rates require very small orifices which can become clogged from impurities in the liquid. We once experienced intermittent problems with exactly that problem 20 years ago doing research on a pulsed capillary discharge thruster using water as the propellant. It works, but extreme care is required, and we would prefer to design something more inherently robust. Monatomic gases are definitely best and easy to work with. Xenon is the propellant of choice for ion thrusters and probably also for ours. On the other hand, argon is much cheaper and works pretty well too.

Offline Robotbeat

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Re: Plasma Jet Electric Thruster Kickstarter
« Reply #24 on: 02/09/2013 09:37 pm »
Yeah, I think Xenon is a great propellant for a lot of reasons. Agreed there.


But optimum exhaust velocity is related to how long you thrust and the specific power.

characteristic velocity: sqrt(2*totalelectricalefficiency*totaltimethrusting*totalspecificpowerofpropulsionsystem)

(characteristic velocity is pretty close to optimal, but you'd want a little less than the characteristic velocity for your exhaust velocity because you can take advantage of the Oberth effect better)

But....


There's an interesting fact about the Oberth Effect... The effect is greater the less delta-v you supply! So even a small chemical burn on an otherwise solar-electric propulsion system will be worth it. And in fact, the smaller the burn, the greater the multiplier. http://en.wikipedia.org/wiki/Oberth_effect#Detailed_proof

Calculate the multiplier for a 10m/s burn at perigee, and you see the multiplier is almost 50, while if you do a 1km/s burn, the multiplier is just 5. So even using RCS jets at perigee would give you a quite significant boost, well worth the IMLEO.

I guess this is why Boeing's Mars SEP architecture uses a chemical kick-stage. It really is an optimal arrangement to have a small chemical stage, more optimal than either a fully SEP or fully chemical architecture. (This also means it's really hard to decrease mission time by using a very large perigee burn in an all-chemical--or even all-NTR--architecture since the larger the burn, the less the Oberth multiplier is...).
« Last Edit: 02/09/2013 09:42 pm by Robotbeat »
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Offline Robotbeat

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Re: Plasma Jet Electric Thruster Kickstarter
« Reply #25 on: 02/09/2013 10:11 pm »
Also, electric propulsion, especially solar-electric propulsion, benefits disproportionally when talking about short-stay Mars missions. This is for three reasons:

Because of the higher Isp (and smaller Oberth effect for chemical compared to long-stay) and the exponential nature of the rocket equation, the reduction in IMLEO is yet more obvious.

Also, each trip leg may be just as long or even longer than the long-stay missions, so there's more time to build up thrust, thus the optimal Isp is even higher than for long-stay (given same specific power).

And finally, often you have to swing closer to the Sun, even doing a powered Venus flyby (using chemical thrusters), which means you partially get an Oberth effect from being further in the Sun's gravity well (even for the low-thrust propulsion part) AND your solar arrays can produce twice as much power at that point than they can at Earth (this is the corollary to the reduction in power at Mars... you get a really big boost if you go near Venus's orbit).

So those in favor of the shortest round-trip Mars missions feasible would do well to support a solar-electric architecture, with the addition of a small--perhaps even hypergolic--chemical stage or two for being able to get the Oberth bonus at departure from Earth, powered swing-by of Venus, and departure from Mars... with multipass aerobraking (not aerocapture) of the transfer vehicle when appropriate and hyperbolic reentry of a capsule back to Earth.

For even faster transits, you could even have hyperbolic entry at Mars... but you'd have to pre-land a seperate ascent vehicle, though that's probably a good idea anyway.
« Last Edit: 02/09/2013 10:12 pm by Robotbeat »
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Offline rklaehn

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Re: Plasma Jet Electric Thruster Kickstarter
« Reply #26 on: 02/10/2013 09:49 am »
Also, electric propulsion, especially solar-electric propulsion, benefits disproportionally when talking about short-stay Mars missions.

I am not a big fan of short-stay missions, to put it mildly.

- The mars radiation environment is relatively benign (comparable to ISS without any shielding, and can be made comparable to high altitudes on earth without unreasonable effort).

- With 0.38g and and a lot of activity in heavy space suits you would not have to worry about zero-gravity effects.

So there is no downside whatsoever in waiting for a good return launch window on the surface of mars.

Not to mention that the whole point of the expedition is to stay on the surface of mars, and the most dangerous part is the transit. So the risk/reward factor is definitely much better for a long-stay mission or a one-way colonization mission...


Electric propulsion has a very large advantage for missions to asteroids because in that case the destination does not allow for any oberth effect tricks. So if NASA or somebody else does a mission to near earth asteroid first, they will most likely use electric propulsion for that and develop the technology so much in the process that it will make sense to use it for mars as well.

And the new trend for "all electric" communication satellites that do the GTO-GEO trip using electric propulsion will be a huge boost for the proponents of electric propulsion. High-thrust electric engines will become truly commercial off the shelf, and repeated crossings of the van allen belts will become routine.


Yeah, I think Xenon is a great propellant for a lot of reasons. Agreed there.

Too bad that you won't find it on mars or asteroids. Do you know of any electric propulsion concepts that can use water without too much efficiency loss? That would be a major breakthrough for manned missions.

- Imagine instead of having a separate propellant tank, have all your consumables and shielding double as propellant. You would not have to take dried food and try to get a 99% closed life support system. Instead you would just take frozen food, use a less ambitious life support system (90%?), and extract the excess water from the atmosphere for use as propellant. You could arrange all the frozen consumables as a solar particle event storm shelter as well.

- You could also refuel everywhere where there are volatiles available. Mars, lunar north pole, phobos and many near earth asteroids would immediately become fuel stations. Just let the volatiles form volatile-rich regolith evaporate into a pressurized volume and claim the volatiles using the life support system, then remove the dry dirt.

The only company I know of that is working on something like this is orbitec. They have an engine that can work in high-thrust, chemical mode by burning gaseous hydrogen and oxygen, and in low-thrust, high Isp mode by using microwave electrothermal mode.

But I don't have the impression that they are actively working on this. I could not find any specs (efficiency, Isp) for the high-Isp mode of the engine anywhere.

That is the reason I asked Dr. Witherspoon about water as propellant. If I got his answer correctly, you should be able to use water in a pulsed plasma thruster. He says that you might even get some of the recombination energy back as thrust.

There will be some loss of efficiency compared to xenon, but as long as T/W does not suffer too much that might be acceptable given all the benefits mentioned above.

(With solar cell specific power improving, engine T/W becomes more important. There is no point of having a solar cell with 1000W/kg if your engine is 100W/kg).

Quote
There's an interesting fact about the Oberth Effect... The effect is greater the less delta-v you supply! So even a small chemical burn on an otherwise solar-electric propulsion system will be worth it. And in fact, the smaller the burn, the greater the multiplier. http://en.wikipedia.org/wiki/Oberth_effect#Detailed_proof

Calculate the multiplier for a 10m/s burn at perigee, and you see the multiplier is almost 50, while if you do a 1km/s burn, the multiplier is just 5. So even using RCS jets at perigee would give you a quite significant boost, well worth the IMLEO.

I guess this is why Boeing's Mars SEP architecture uses a chemical kick-stage. It really is an optimal arrangement to have a small chemical stage, more optimal than either a fully SEP or fully chemical architecture. (This also means it's really hard to decrease mission time by using a very large perigee burn in an all-chemical--or even all-NTR--architecture since the larger the burn, the less the Oberth multiplier is...).

For a manned mission you will have a powerful electrolysis unit anyway as part of the life support system. You could store excess hydrogen and oxygen using high pressure compressed storage (using as low a temperature as you can get with passive cooling, but not cryogenic). Then you could burn this in a chemical rocket engine for the occasional oberth-multiplied chemical boost.

Compressed storage of hydrogen in carbon fibre bottles has become surprisingly effective for fuel-cell electric cars. If you could reduce the safety factor a bit and use lower temperatures (using passive cooling), you could get some decent delta-v of a few 100m/s for one burn without having to liquefy the propellants at all.

Offline Robotbeat

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Re: Plasma Jet Electric Thruster Kickstarter
« Reply #27 on: 02/10/2013 03:04 pm »
Argon is pretty available at Mars, a little easier than water, actually. The atmosphere is 2% argon. Argon works just like Xenon with no residue but isn't as dense.

You can possibly use also anything in an electric thruster.

And I agree with you WRT a short-stay vs long-stay mission.
« Last Edit: 02/10/2013 03:05 pm by Robotbeat »
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