Author Topic: Advancements in Electric Thrusters (Read 130704 times)

Paul451 · « **Reply #60 on:** 02/21/2015 04:25 am »

Quote from: Asteroza on 02/20/2015 10:47 pm

The "spiral is bad" argument gets a lot weaker if the system proposed by Tethers Unlimited to drain the inner (and also outer?) Van Allen belt is practical.

Heh. Do an NSF search on the topic. It's one of those threads. A bunch of very angry people do not want it discussed. It's immoral and dangerous and shouldn't even be discussed on this, a respectable site. And simultaneously it's impossible and stupid and therefore doesn't need to be discussed.

Quote from: Impaler on 02/21/2015 12:08 am

as far as I can tell their is nothing preventing any Electric propulsion operating in ambient air-pressure

I'd be surprised if at least some of them didn't get completely borked.

Quote from: Impaler

they just don't operate the SAME as in a vacuum.

That was kind of my question, how sensitive are they, how close to your operating environment do you have to go to get solid data. Do the different technologies vary in their sensitivity. Do some allow clever workarounds for testing, like using a low molar-mass gas at ambient pressure. I can't find any quick references on it, thought you might know something more specific.

tea monster · « **Reply #61 on:** 02/21/2015 08:04 am »

Quote from: Paul451 on 02/21/2015 04:25 am

Heh. Do an NSF search on the topic. It's one of those threads. A bunch of very angry people do not want it discussed. It's immoral and dangerous and shouldn't even be discussed on this, a respectable site. And simultaneously it's impossible and stupid and therefore doesn't need to be discussed.

You mean, one of those "If God had meant Man to fly in space, he would have given him gimbled RL10s" thread?

If some of these thruster designs have a VASIMIR-style high-thrust mode, can't we just drop a gear to get out of the gravity well then shift back to high gear to do the transit? Is that going to use an unfeasable amount of power or propellant?

Secondly, how many of these Elf thrusters or nested Halls could you conceivably fit onto the back of one of these vehicles? Every design I've seen is a JIMO-style long thin flying girder with an inflatable hab on one end and a flimsly looking panel with some engines tagged on the other. There are going to be big issues with delivering enough power to a silly amount of thrusters, then you will have to have a lot more fuel to feed them. I've throught of a more rounded girder 'pusher plate' packed with thrusters. You could ring the plate with solar arrays spanned out in a circle. the instruments, fuel and hab rest in the middle.

I don't know at what point you ballance the amount of power and fuel you can realisticly deliver to the engines with the amount of thrust you can get.

MP99 · « **Reply #62 on:** 02/21/2015 03:38 pm »

Quote from: tea monster on 02/21/2015 08:04 am

If some of these thruster designs have a VASIMIR-style high-thrust mode, can't we just drop a gear to get out of the gravity well then shift back to high gear to do the transit? Is that going to use an unfeasable amount of power or propellant?

Not a proper answer to your question, but if you tuned VASIMR down to Isp of 1,000, it will probably need more prop than chemical at 450s, due to impulsive burn and Oberth effect.

Cheers, Martin

Burninate · « **Reply #63 on:** 02/21/2015 05:34 pm »

There's no issue with spending most of your time in the gravity well; Spending a lot of time in the gravity well doing SEP spiral-out *is* the shortcut you are adopting, in order to make it so that you only need 400-1000m/s worth of chemical propellant once you're ready to leave, to get onto a low energy Mars transfer trajectory. The proximity to Earth until the very last minute, when you're ready to put humans onboard, is a feature rather than a bug.

Impaler · « **Reply #64 on:** 02/21/2015 05:34 pm »

Quote from: Paul451 on 02/21/2015 04:25 am

That was kind of my question, how sensitive are they, how close to your operating environment do you have to go to get solid data. Do the different technologies vary in their sensitivity. Do some allow clever workarounds for testing, like using a low molar-mass gas at ambient pressure. I can't find any quick references on it, thought you might know something more specific.

I honestly don't know, I've read reports on the testing of these thrusters and they seem be be very persnickety about the conditions of the vacuum chamber so I'm concluding that it is important only by inference. I do recall that some lighter propellent gasses (Krypton) are used in testing because they are easier to pump out or are cheaper and extrapolations are made to how the thrusters would operate with a different gases like Xenon.

Impaler · « **Reply #65 on:** 02/21/2015 05:42 pm »

Quote from: tea monster on 02/21/2015 08:04 am

Secondly, how many of these Elf thrusters or nested Halls could you conceivably fit onto the back of one of these vehicles? Every design I've seen is a JIMO-style long thin flying girder with an inflatable hab on one end and a flimsly looking panel with some engines tagged on the other. There are going to be big issues with delivering enough power to a silly amount of thrusters, then you will have to have a lot more fuel to feed them. I've throught of a more rounded girder 'pusher plate' packed with thrusters. You could ring the plate with solar arrays spanned out in a circle. the instruments, fuel and hab rest in the middle.

I don't know at what point you ballance the amount of power and fuel you can realisticly deliver to the engines with the amount of thrust you can get.

ELF is a tube only 0.5 m in diameter, we could easily put enough of them on the vehicle to get the necessary thrust without having to make some kind of huge pusher-plate. The X3 is about a meter in diameter so it is lower thrust density but even it is fine if your vehicle is in the 1-2 MW range which is the reasonable near-term power availability. At 250 kw each you need 8 thrusters to use 2 MW and that comes out to 2 m^2 for the ELF and 8 m^2 for the X3.

Concerns over thrust density were legit when we only had tiny HALL thrusters and Ion-Engines (Ion type drives having fundamental thrust-density limits), but it is no longer an issue.

Paul451 · « **Reply #66 on:** 02/22/2015 01:28 am »

Impaler,
Thanks. Even your "I don't knows" are informative.

dkovacic · « **Reply #67 on:** 02/22/2015 12:14 pm »

Impaler,

I noticed in your opening post on this thread you completely skipped electrothermal thrusters. Are you aware of their status of development? Last effort I heard of was ORBITEC Advanced MET.

Impaler · « **Reply #68 on:** 02/22/2015 05:07 pm »

Is that the system they are going to use on that CubeSat on Kickstarter? Sounds interesting but do you have the numbers for ISP and alpha? We will need to know that to figure out if it is a viable propulsion system for interplanetary transport.

Also there are a number of other EP types I did not go over, the reason is I'm simply not familiar enough with all of them to give a good description. If anyone can provide the data I'd be glad to update the First post.

Nilof · « **Reply #69 on:** 02/22/2015 07:06 pm »

Electric propulsion for cubesats is an interesting topic. On the power side of things, I'd expect that good alphas should be relatively easy as long as the thruster shows promise, since at these sizes the solar sail + thin film approach should work well.

The CAT thruster that was on Kickstarter was supposed to fly for the first time this year, though I don't know if the whole Dnepr debacle between Russia and Ukraine has delayed the launch.

It is apparently supposed to use Iodine as a propellant, which is an interesting choice for a volume-limited spacecraft. It is almost twice as dense as liquid Xenon(which is already insanely dense), has similar ionization energies, and since it is not pressurized it does not have stored energy issues that could annoy the primary customer in a cubesat context.

dkovacic · « **Reply #70 on:** 02/22/2015 08:34 pm »

Quote from: Impaler on 02/22/2015 05:07 pm

Is that the system they are going to use on that CubeSat on Kickstarter? Sounds interesting but do you have the numbers for ISP and alpha? We will need to know that to figure out if it is a viable propulsion system for interplanetary transport.

Also there are a number of other EP types I did not go over, the reason is I'm simply not familiar enough with all of them to give a good description. If anyone can provide the data I'd be glad to update the First post.

I dont think do. CAT is somewhat similar but they obviously target higher isp than electrothermal. Arcjet and microwave electrothermal thruster (MET) can target isp between 800 and 1000.

The benefit of MET is that they can consume for example water. Their isp helps because fot the same power and dV they require shorter burn time (equal to isp ratio).

Robotbeat · « **Reply #71 on:** 02/22/2015 10:48 pm »

Yeah, if you're stuck with low Isp, you can still make the most of it by doing targeted burns when near perigee. This gives you some of an Oberth effect just over many orbits.

Even though no doubt the performance will be inferior at first, electric propulsion on cubesats (especially any type with a decent mass fraction... as in 25-50% of the spacecraft mass available as propellant, not just a few percent) will be hugely enabling. Interplanetary cubesats have such great potential for low-cost exploration that we really should be putting a little more resources into it. We just need a team with proven execution on ambitious low-cost projects and, say, $10-20 million.

Interplanetary microsats/cubesats (especially with advanced unfolding/unrolling thin film solar arrays and such) will enable persistent presence around every object of significance from Venus to Saturn inclusive with a relatively small budget, allowing us to use larger spacecraft in a more targeted, enlightened manner (and also freeing up resources for a couple ice giant missions...).

Asteroza · « **Reply #72 on:** 02/23/2015 12:15 am »

Quote from: dkovacic on 02/22/2015 08:34 pm

Quote from: Impaler on 02/22/2015 05:07 pm
Is that the system they are going to use on that CubeSat on Kickstarter? Sounds interesting but do you have the numbers for ISP and alpha? We will need to know that to figure out if it is a viable propulsion system for interplanetary transport.

Also there are a number of other EP types I did not go over, the reason is I'm simply not familiar enough with all of them to give a good description. If anyone can provide the data I'd be glad to update the First post.
I dont think do. CAT is somewhat similar but they obviously target higher isp than electrothermal. Arcjet and microwave electrothermal thruster (MET) can target isp between 800 and 1000.

The benefit of MET is that they can consume for example water. Their isp helps because fot the same power and dV they require shorter burn time (equal to isp ratio).

Wasn't one of the stretch goals for CAT (unfortunately not reached) was the water propellant version dev/flight test?

Paul451 · « **Reply #73 on:** 02/23/2015 07:23 am »

Quote from: Robotbeat on 02/22/2015 10:48 pm

Interplanetary microsats/cubesats

Can you broadcast powerfully enough with a cubesat/nanosat to do useful work at that distance, or do you need a larger, more traditional orbiter at each target-world to act as a relay for the cube-flock?

(I figure you're not saving much if you have to hog the whole DSN for each cubesat.)

Impaler,
The CAT guys claim 20km/s V_e, so ~2000s Isp. (Bit high for Earth orbit, but fine for interplanetary.) With 20mN from 10 watts (or 200mN from 100W "pulsed"), so 0.2N/kW, 5kW/N. And 90% efficiency from source to RF... but I can't find anything for RF to fuel, which is presumably the figure you need.

(AMET's numbers vary from pdf to pdf.)

dkovacic · « **Reply #74 on:** 02/23/2015 08:02 am »

Quote from: Paul451 on 02/23/2015 07:23 am

Quote from: Robotbeat on 02/22/2015 10:48 pm
Interplanetary microsats/cubesats

Impaler,
The CAT guys claim 20km/s V_e, so ~2000s Isp. (Bit high for Earth orbit, but fine for interplanetary.) With 20mN from 10 watts (or 200mN from 100W "pulsed"), so 0.2N/kW, 5kW/N. And 90% efficiency from source to RF... but I can't find anything for RF to fuel, which is presumably the figure you need.

(AMET's number vary from pdf to pdf.)

CAT seems to use electrothermal approach to heating gas (just like MET), but it goes to higher temperatures(and different frequency) which give higher ISP and ionizes the propellant. The problem I see is that the energy used for ionization is essentially lost, so the efficiency goes way down. That is why ion drives need really high ISP to have high efficiency. So this source to RF efficiency is not end-to-end efficiency. CAT was probably proposed becuase it seemed easy to scale down for cubesat applications.

The numbers for CAT are a bit misleading to me. For isp of 2000s and 1mg/s propellant flow, I get 20mN force and 200W power (assuming 100% efficiency).

The advantage of MET is that is stops the heating of the gas before it turns to plasma. So its ISP is limited to less than 1000s. By replacing 3000s Hall thruster with 1000s electrothermal thruster transfer time from GTO to GEO would be reduced from six to two months (and it would use three times as much fuel, of course).

MET would really shine in space economy where you have multiple sources of propellent distributed around the solar system. And its propellant can double act as a radiation shield, source of oxygen and drinking water.

Nilof · « **Reply #75 on:** 02/23/2015 08:47 am »

The numbers for CAT on their page are actually 2mN/10 watts, 20mN/100 watts. Paul got the power/thrust ratio right, so clearly it was a typo. Also, while it mostly empatized water in the kickstarter, now it is mostly about Iodine since the latter is five times denser which means five times the reaction mass. That and Iodine is easier to ionize.

Regarding cubesat thrusters with ISP's below 1000s, I like the Tethers unlimited hydros thruster. Effectively, it electrolyses water and then acts as a Hydrolox engine, or it can act as a cold gas thuster if it is used as a maneuvering thruster. The electrolyzing allows it to save up energy for most of it's orbit and then perform an impulsive burn at one point, which means it can use the Oberth effect properly.

Paul451 · « **Reply #76 on:** 02/23/2015 10:24 am »

Quote from: Nilof on 02/23/2015 08:47 am

The numbers for CAT on their page are actually 2mN/10 watts, 20mN/100 watts. Paul got the power/thrust ratio right, so clearly it was a typo.

I probably just worked out the power/thrust wrong as well and the two happened to cancel out.

tea monster · « **Reply #77 on:** 03/10/2015 01:59 pm »

I found this PDF on the web about nested Hall thrusters. Some interesting info.

I think for my next project I'm going to model one of these things.

Star One · « **Reply #78 on:** 03/10/2015 03:08 pm »

This news seems to fit in here.

CNES Gives All-Electric Satellite Research a $30 Million Jolt with More To Come.

Quote

The French space agency, CNES, on March 9 said a government research program stimulating French industry development of all-electric-propulsion satellites has disbursed 25 million euros ($30 million) for its first phase, with more to come to help with in-flight technology validation.

CNES is motivated by industry forecasts saying that, by 2020, 50 percent of all commercial telecommunications satellites will be all-electric, a design that affords substantial weight savings compared to satellites using chemical propellant.

http://spacenews.com/cnes-gives-all-electric-satellite-research-a-30-million-jolt-with-more-to-come/#sthash.sXpFIMs3.dpuf

dkovacic · « **Reply #79 on:** 03/12/2015 08:41 am »

I found really interesting lecture by Ben Longmier, which discusses status of CAT a year ago:

Highlights:
- use of permanent magnets for forming the converging/diverging nozzle (no physical nozzle)
- optimized for 5W-200W power range
- pretty strong local magnetic field (dies quickly
- tested with argon, moving to Xenon
- for space applications, they plan to use iodine, because it solid metal, high density, has high vapor pressure (they just need a heater with moderate temperature - 60C to produce gas directly into the RF chamber
- H20 good for high ISP, not really efficient at lower ISP
- optimized for 1000s Isp, highest thrust to power (100mN/kW), that is optimal system when including tanks and solar power sources

But the coolest part is concept interplanetary Jupiter mission discussed at the end of the lecture. It would be 6U cubesat, dual CAT, 15kg iodine fuel (that would take around 3U of space), 1000s isp, 130W solar power at 1AU (earth orbit). With 7kg payload, it would be capable of reaching Jupiter in 3 years and possibly entering orbit.