Author Topic: Advancements in Electric Thrusters  (Read 104527 times)

Offline Genial Precis

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Re: Advancements in Electric Thrusters
« Reply #240 on: 03/28/2021 10:42 pm »
I have two related thoughts for this thread.

The first is that our understanding of the planet Mercury is very poor. With one or more of these very powerful electric thrusters with Isp 2000-5000 s, it ought to be possible to send rather fast solar-electric vehicles to Mercury to take better measurements from orbit and maybe even deploy a lander. There have been a total of three missions to Mercury launched so far, and the third, Bepi Colombo, doesn't arrive until 2025. It's very understudied compared to how weird it is as a planet.

The second is that large electric thrusters open up lots of destinations in the solar system for human spaceflight (including Mercury), but only if they're large enough. Is a 10 or 100 MW, or even GW class electric thruster on anyone's radar? Are there fundamental reasons why that would be much more difficult than 100 kW class thrusters?

Offline Vultur

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Re: Advancements in Electric Thrusters
« Reply #241 on: 03/29/2021 03:05 am »
Is a 10 or 100 MW, or even GW class electric thruster on anyone's radar? Are there fundamental reasons why that would be much more difficult than 100 kW class thrusters?

I'd like to know that too.

For the inner solar system (Mercury to Mars, maybe even the Asteroid Belt), orbit-to-orbit ships with massive solar wings could be the stage after something like Starship - going between say high-orbital stations (Gateway?), Phobos and Deimos, etc.

Offline tbellman

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Re: Advancements in Electric Thrusters
« Reply #242 on: 03/29/2021 06:06 pm »
Is a 10 or 100 MW, or even GW class electric thruster on anyone's radar? Are there fundamental reasons why that would be much more difficult than 100 kW class thrusters?

In principle, you don't need super-powerful thrusters; you can do it by having many thrusters instead.  So just mount a hundred, or a thousand, or ten thousand, 5 newton 100 kW electrical thrusters on your spacecraft, and add more solar panels to reach the wanted power levels.

In practice, it becomes difficult.  To reach 10 MW, you need solar panels that are roughly on the order of 200m×200m, massing a few tens of tonnes (I seem to remember current state of the art being about 4 kg/kW, including structures).  Solar panels capable of a gigawatt, would have sides more than a kilometer long, and mass a couple thousand tonnes.  With today's technology, it would be quite challenging to build.

Also, as the power and propulsion system becomes a larger portion of the spacecraft's mass, the advantage of increasing those in size gets lower, as they will spend most of their thrust just accelerating their own mass.  If the solar panels and engines are already 90% of the mass, doubling their size would only increase the acceleration by maybe 5%, and increasing their size by a factor ten would only give maybe 10% improvement.

Even if the Aricebo telescope hadn't broken down, I don't think anyone has a radar that can look far enough into the future to have such constructions within view. :D  Maybe we can check again in ten years time.

Offline Genial Precis

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Re: Advancements in Electric Thrusters
« Reply #243 on: 03/29/2021 06:23 pm »
Ahah, but you've failed to consider that I want to use them to go to Mercury, where solar panels are 5-10 times as powerful per area. That means  my mission planning can consider the state of the art to be less than 1 kg/kW, and that opens up solar-electric vehicles with substantial payload mass fraction.

Anyway, you're right that it would be large, though. If it's assembled in LEO, though, it only ever has to withstand less than 1E-2 m/s^2 acceleration, which ought to afford further mass reductions. I don't see a way any such thing gets underway sooner than 2030, so I think planning for assembly in LEO is not much of a compromise.

Offline tbellman

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Re: Advancements in Electric Thrusters
« Reply #244 on: 03/29/2021 07:53 pm »
Ahah, but you've failed to consider that I want to use them to go to Mercury, where solar panels are 5-10 times as powerful per area. That means  my mission planning can consider the state of the art to be less than 1 kg/kW, and that opens up solar-electric vehicles with substantial payload mass fraction.

But you will spend most of the transit time to Mercury further away, outside the orbit of Venus, so you only get a fraction of that increased sunshine density.

On the other hand, on the way home you will spend most of the time close to Mercury.

And then you have to take into account that the higher radiation levels close to the sun will cause the solar panels to detoriate quicker.

Quote from: Genial Precis
Anyway, you're right that it would be large, though. If it's assembled in LEO, though, it only ever has to withstand less than 1E-2 m/s^2 acceleration, which ought to afford further mass reductions. I don't see a way any such thing gets underway sooner than 2030, so I think planning for assembly in LEO is not much of a compromise.

I would expect more like 2040 for a mission to Mercury with humans on board.  (I'm hoping 2029 for first humans on Mars.)  So yes, by that time, I also hope assembling large structures in space will be viable.  Although building kilometer-sized solar panels in low Earth orbit might not be good, due to atmospheric drag and collision risks.

Offline Genial Precis

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Re: Advancements in Electric Thrusters
« Reply #245 on: 03/29/2021 08:49 pm »
If you stage your mission to Mercury out of Earth Moon lagrange point you only need 2.5 km/s to enter Mercury transfer orbit. Therefore an acceleration of 1e-3 m/s^2 at Earth is adequate to complete the initial boost in a month. For this, if the specific impulse is 5000 s the minimum power to weight ratio must be 50 W/kg, so I say the vehicle will require 0.2 m^2 of solar panels per ton of overall mass. If I can get solar panels for 1 kg/m^2, this is fine and means 20% of the vehicle mass is in solar panels.

At 0.47 AU, the vehicle's maximum acceleration would increase to 4.5e-3 m/s^2 (neglecting propellant consumption), allowing it to complete the 7.5 km/s burn to go from Mercury transfer orbit to low Mercury orbit in 20 days. The burns for the reverse maneuvers to return to Earth would be slightly faster due to propellant consumption, and require ~20 km/s overall

Overall, the vehicle would devote 33% of mass to propellant, 20% to solar panels as previously mentioned. The best electric thrusters deliver about 1 kW/kg, so the engine mass would be 5% and the payload and propellant-tank fraction would be 42%. The above is in no way optimized, but it indicates that a direct mission to Mercury orbit and back is more or less within the scope of near-term technological possibilities. A large vehicle on that mission could even deploy a lander and ascent vehicle in order to return samples to Earth.

Offline Vultur

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Re: Advancements in Electric Thrusters
« Reply #246 on: 04/01/2021 05:37 pm »
What's the largest object that we could land on/take off from (or maybe "rendezvous" would be more accurate for very small asteroids etc.) with current-technology electric propulsion?

I think the NSTAR engine used by Dawn would be (barely) good enough for tiny objects like Bennu (Wikipedia claims Bennu's gravity is 6.7 micro-g, so 0.09 N should be able to accelerate 1200 kg at 0.000075 m/s = about 7.6 micro-g, unless my math is off).

But how much better could we do? Can we trade Isp for thrust to land on, say, Deimos?

Offline KelvinZero

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Re: Advancements in Electric Thrusters
« Reply #247 on: 04/01/2021 10:54 pm »
What's the largest object that we could land on/take off from (or maybe "rendezvous" would be more accurate for very small asteroids etc.) with current-technology electric propulsion?

I think the NSTAR engine used by Dawn would be (barely) good enough for tiny objects like Bennu (Wikipedia claims Bennu's gravity is 6.7 micro-g, so 0.09 N should be able to accelerate 1200 kg at 0.000075 m/s = about 7.6 micro-g, unless my math is off).

But how much better could we do? Can we trade Isp for thrust to land on, say, Deimos?
You could add some kind of spring.. that could be wound up with a tiny electric motor on very low gear, or just bounces off asteroid and takes a sample as it does like Hayabusa2.

https://en.wikipedia.org/wiki/Deimos_(moon)#Physical_characteristics
"Escape velocity from Deimos is 5.6 m/s. This velocity could theoretically be achieved by a human performing a vertical jump."

It would also be useful to be able to change location on the asteroid with minimal energy and no propellant cost.

Offline magicsound

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Re: Advancements in Electric Thrusters
« Reply #248 on: 12/03/2021 02:30 am »
Ion thrusters have very high specific impulse, but depend on heavy ions. Once away from Earth, refueling becomes an issue. There's ample Argon in Mars' atmosphere, but certainly not on the moon or in the asteroid belt.

Consider another possibility - high-pressure electrolysis to generate H & O from water. Such systems have reached hundred bar output pressure at small commercial scale[1] and much higher in DOE-funded experiments[2]. That's enough to feed a small rocket engine directly (without pumps). In my conception a tiny combustion chamber and large nozzle would give a pretty high specific impulse, needing only water for fuel (available from comets etc) and direct electricity from PV. With no moving parts such an engine would give the extended service life needed for low thrust in-space propulsion.

(added 4Dec21) A water-based ion drive has been developed, funded by the UK Space Agency, and is currently in production, with both electrolysis (isp 300s) and microwave-based (isp 1000s) systems:
https://www.urathrusters.com/#products

[1]https://thsenergy.com/products/hydrogen-gas-generators/h-genie-high-pressure-hydrogen-generator/
[2]https://www.hydrogen.energy.gov/pdfs/review15/pd117_mittelsteadt_2015_o.pdf
« Last Edit: 12/04/2021 06:16 pm by magicsound »

Offline Tywin

Re: Advancements in Electric Thrusters
« Reply #249 on: 10/12/2022 08:48 am »
Looks like China responds to the X3 of Aerojet...

https://twitter.com/CNSpaceflight/status/1486798962963070979

Amazing this HET-450...
The knowledge is power...Everything is connected...
The Turtle continues at a steady pace ...

Offline InterestedEngineer

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Re: Advancements in Electric Thrusters
« Reply #250 on: 03/05/2023 11:20 pm »
When an electric thruster (say a Hall Thruster) states an efficiency of 60%, does that mean 40% of the energy input is waste and 60% is kinetic energy, the latter which applies directly to kinematic equations?

If so, of that 40% waste, how much of the waste heat goes out with the reaction mass (the ions) and thus doesn't have to be mitigated, and how much of that 40% has to be radiated away with special radiators?


Offline Robotbeat

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Re: Advancements in Electric Thrusters
« Reply #251 on: 03/06/2023 03:02 pm »
Yeah, 60% of the input energy is kinetic energy. You can verify this by multiplying the thrust times Isp times g.

And I’d say most of the losses don’t need to be radiated away via low temperature radiators.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline InterestedEngineer

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Re: Advancements in Electric Thrusters
« Reply #252 on: 03/06/2023 03:22 pm »
I guess one could take the temperature of the exhaust, multiply it by the specific heat, and that gives the heat content of the exhaust.

Except the exhaust is ions, so there's the energy of ionization in them. I think.

Offline Robotbeat

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Re: Advancements in Electric Thrusters
« Reply #253 on: 03/06/2023 03:43 pm »
Yeah, for plasma, the question of specific heat becomes complicated, and this overall isn’t that fruitful of an approach to the problem.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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Offline Paul451

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Re: Advancements in Electric Thrusters
« Reply #254 on: 03/06/2023 03:52 pm »
If so, of that 40% waste, how much of the waste heat goes out with the reaction mass (the ions) and thus doesn't have to be mitigated, and how much of that 40% has to be radiated away with special radiators?

AIUI, most of the energy not producing thrust is spent ionising the propellant, so it's not really "waste" as such, and it definitely goes out with the exhaust.

I believe most of the remaining loss is called "dissipated power" (or equiv.) and is the true waste heat that is radiated from the thruster, but I can't find any clear figures for actual thrusters. But using that phrase might help you/others doing searches.

[Additionally, the overall electrical system will also produce waste heat (solar, batteries, voltage conversions, etc), but that isn't counted against the thruster. Just be careful when you are using thruster input energy to work out solar array area that you are allowing for systems efficiencies, not just thruster efficiencies.]



490+ page text book on ion drives that is considered the definitive source:

Fundamentals of Electric Propulsion: Ion and Hall Thrusters
. Dan M. Goebel and Ira Katz. JPL, 2008.

Quote
7.3.3 Dominant Power Loss Mechanisms

In preparation for examining the terms that drive the efficiency of Hall thrusters, it is useful to examine the dominant power-loss mechanisms in the thruster. Globally, the power into the thruster comes from the discharge power supply. The power out of the thruster, which is equal to the input power, is
given to first order by

Pd = Pb + Pw + Pa + PR + Pion ,  (7.3-27)

where Pb is the beam power given by IbVb , Pw is the power to the channel walls due to ion and electron loss, Pa is the power to the anode due to electron collection, PR is the radiative power loss from the plasma, and Pion is the power to produce the ions that hit the walls and become the beam. Additional loss terms, such as the power that electrons take into the beam, the ion power to the anode, etc., are relatively small and can usually be neglected.

In Hall thrusters with dielectric walls, the power loss due to electron and ion currents flowing along the radial magnetic field through the sheath to the channel walls ( Pw ) represents the most significant power loss.

Offline InterestedEngineer

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Re: Advancements in Electric Thrusters
« Reply #255 on: 03/06/2023 07:42 pm »
If so, of that 40% waste, how much of the waste heat goes out with the reaction mass (the ions) and thus doesn't have to be mitigated, and how much of that 40% has to be radiated away with special radiators?

AIUI, most of the energy not producing thrust is spent ionising the propellant, so it's not really "waste" as such, and it definitely goes out with the exhaust.


If that's true, then a lot of our models are wrong, because we are thinking we need passive radiators proportional to the wasted energy of the drive.

Which isn't true.  Maybe no radiators needed at all for SEP.

 

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