Author Topic: electric pumped rocket cycle  (Read 14147 times)

Offline msat

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Re: electric pumped rocket cycle
« Reply #20 on: 02/12/2018 04:16 PM »


I was under the impression that scaling down axial/centrifugal pumps was the source of the 5000 line, as the machining difficulty increases non-linearly, adding cost. Nobody is 3D printing unfinished pump blades at super small sizes (yet...).

This is more true for axial types than centrifugal compressors. That is in large part why gas turbines below a certain power level nearly all use centrifugal compressors exclusively, and the next step up use a combination of axial low pressure stages followed by a centrifugal high pressure stage.

I have to confess that I'm more familiar with their use in turbocompressors than turbopumps, so the scaling would be different, but we're not talking about something all that small here, or at least I suspect not. If we were talking about centrifugal units equivalent small RC aircraft glow engines, then the degree of difficulty in attaining a certain performance would be more unfavorable for the former. That said, I had come across a research paper a while back about the development of an incredibly small gas turbine engine manufactured using MEMS technology. That engine used a centrifugal compressor.

Offline ArbitraryConstant

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Re: electric pumped rocket cycle
« Reply #21 on: 02/12/2018 08:02 PM »
Given RL's figure of 95% efficiency, even if there was any room for improvement by using piston pumps, you end up with a more complex (probably less reliable and heavier - look at how tiny those volute casings are!) system that also has a less desirable pulsed output. Axial and centrifugal turbomachinery is perfectly suited for the role of rocket pumps, and are completely capable of producing the necessary fluid pressures common in rocket motors.
I wonder if there aren't different ways to structure this.

For example, a small reciprocating pump may not be ideal for feeding the combustion chamber directly, but it may be able to feed a high pressure reservoir from a low pressure reservoir, such that a pressure fed system from the high pressure reservoir would have higher combustion chamber pressure than would otherwise be optimal, but the system mass is reduced since the low pressure reservoir doesn't have to handle >combustion chamber pressure. This might even be useful for RCS. A reciprocating pump is useful here because it would be able to handle a larger pressure gradient.

edit: Should also be possible (but not necessary) to recharge batteries for a system like this. So really it's reducing tankage mass by using a lower pressure for the low pressure reservoir (which should contain most of the total mission propellant), reducing pressurant mass by reducing the gasses needed to pressurize the system and partially replacing them with batteries, and potentially (though not necessarily) reusing the batteries by recharging them from other energy sources like solar or RTG. Recharging a battery is no great engineering feat but in this case the total mission specific energy of a battery could easily be several times that of the battery over a single charge, which would be significant if it replaces helium bottles that are only used once.

This seems like it could be useful for RCS on a crewed vehicle but also propulsion in a smaller uncrewed probe.
« Last Edit: 02/12/2018 10:37 PM by ArbitraryConstant »

Offline Asteroza

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Re: electric pumped rocket cycle
« Reply #22 on: 02/13/2018 03:43 AM »
For an RCS methodology, could one potentially run overspecced reaction wheels double duty as energy storage flywheels? Trickle charge the wheels by whatever means is acceptable...

Offline sanman

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Re: electric pumped rocket cycle
« Reply #23 on: 02/13/2018 11:24 AM »
If cryo-propellant is being used, then could its cryogenic temperatures be exploited to enhance the electrical properties and performance of the electric turbo-pump? The low temperatures associated with Liquid Oxygen can easily reach the critical temperatures for various warm superconductor materials. And if you include Liquid Hydrogen, then practically everything can be made superconductive.

How can superconductivity be exploited to benefit an electric turbo-pump?

(Thanks for this great thread, btw.)
« Last Edit: 02/13/2018 01:23 PM by sanman »

Offline Klebiano

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Re: electric pumped rocket cycle
« Reply #24 on: 02/13/2018 02:53 PM »
If cryo-propellant is being used, then could its cryogenic temperatures be exploited to enhance the electrical properties and performance of the electric turbo-pump? The low temperatures associated with Liquid Oxygen can easily reach the critical temperatures for various warm superconductor materials. And if you include Liquid Hydrogen, then practically everything can be made superconductive.

How can superconductivity be exploited to benefit an electric turbo-pump?

(Thanks for this great thread, btw.)

There is a lot of work in superconducting wind turbine generators that problably can be implemented in electric turbo-pump.
Example: http://iopscience.iop.org/article/10.1088/0953-2048/23/3/034019/meta

« Last Edit: 02/13/2018 03:01 PM by Klebiano »

Offline ArbitraryConstant

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Re: electric pumped rocket cycle
« Reply #25 on: 02/13/2018 04:48 PM »
How can superconductivity be exploited to benefit an electric turbo-pump?
There are HTS motors/generators, they offer higher power density and IIRC can do away with gearing because they can operate at very low RPM. I don't think low RPM is an issue for electric pumped rocket engines though. Also seems like you may need a relatively large engine requiring many megawatts of pump power before it was a mass savings. I also tend to agree with the comment earlier that by the time you had an engine that could benefit you'd likely have an engine where it was worth it to go to a gas generator cycle. That said, technology is always changing and if one day there's an HTS motor off the shelf that's 20 MW and fits in your pocket, might be worth it.

Offline sanman

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Re: electric pumped rocket cycle
« Reply #26 on: 02/13/2018 06:35 PM »
Would superconductivity significantly lower the electric power input requirements to run an electric turbo-pump? Using Rocket Lab's Electron rocket as an example, if its electric turbo-pump was based on superconductors, then would its required battery mass be appreciably lower?

Offline ArbitraryConstant

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Re: electric pumped rocket cycle
« Reply #27 on: 02/13/2018 07:52 PM »
Would superconductivity significantly lower the electric power input requirements to run an electric turbo-pump? Using Rocket Lab's Electron rocket as an example, if its electric turbo-pump was based on superconductors, then would its required battery mass be appreciably lower?
I think someone mentioned on the previous page that it's already 95% efficient so I don't think there's much to be gained there. Even a superconducting motor is not 100% efficient because energy is lost to eddy currents. Mostly it's just smaller.

Offline Lar

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Re: electric pumped rocket cycle
« Reply #28 on: 02/13/2018 10:45 PM »
I was thinking about how you might include a jettisoned battery pod in ISP but the rocket equation really wants things to be continuous. :P
I think the ejection velocity of a battery pod is low enough that you can ignore any thrust contributions :)

But it's a step change in mass... so wouldn't you solve to the timepoint of ejection, then decrease the mass and solve the rest of the way?

Would superconductivity significantly lower the electric power input requirements to run an electric turbo-pump? Using Rocket Lab's Electron rocket as an example, if its electric turbo-pump was based on superconductors, then would its required battery mass be appreciably lower?
I think someone mentioned on the previous page that it's already 95% efficient so I don't think there's much to be gained there. Even a superconducting motor is not 100% efficient because energy is lost to eddy currents. Mostly it's just smaller.
Reduced mass of the motor or just more dense?
« Last Edit: 02/13/2018 10:47 PM by Lar »
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Offline ArbitraryConstant

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Re: electric pumped rocket cycle
« Reply #29 on: 02/13/2018 11:53 PM »
I was thinking about how you might include a jettisoned battery pod in ISP but the rocket equation really wants things to be continuous. :P
I think the ejection velocity of a battery pod is low enough that you can ignore any thrust contributions :)

But it's a step change in mass... so wouldn't you solve to the timepoint of ejection, then decrease the mass and solve the rest of the way?
Yup I think so, they would model it like the fairing jettison.

In the limit if you had arbitrarily small batteries being jettisoned continuously it would converge on behaving like a gas generator and you would include it in the ISP, but with 1 jettison in the only known implementation it doesn't really make sense to do it that way. Just messes with my expectations from thinking about other rocket cycles.

Reduced mass of the motor or just more dense?
Reduced size and mass, dunno about density.

That being said, it's also possible to miniaturize an electric motor simply by giving it better cooling, liquid cooled copper wires can carry extremely high current and I suspect the mass flow rate of RP-1 is more than enough if they decide to use it for that.

It's also possible they do something else, like have some material embedded that can take up the excess heat by phase change. I recall the lunar rovers did this with a wax material. The batteries may also require cooling, and especially in the case of the upper stage batteries where they jettison I doubt they were trying to flow propellant into a detachable battery pod.

Offline sanman

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Re: electric pumped rocket cycle
« Reply #30 on: 02/14/2018 02:25 AM »
That being said, it's also possible to miniaturize an electric motor simply by giving it better cooling, liquid cooled copper wires can carry extremely high current and I suspect the mass flow rate of RP-1 is more than enough if they decide to use it for that.

It's also possible they do something else, like have some material embedded that can take up the excess heat by phase change. I recall the lunar rovers did this with a wax material. The batteries may also require cooling, and especially in the case of the upper stage batteries where they jettison I doubt they were trying to flow propellant into a detachable battery pod.

Phase change shouldn't necessarily be the only way - any endothermic reaction should do - look at Zubrin's pioneering work on the Acetylene-Methane reaction for the National Aerospace Plane:

http://adsabs.harvard.edu/abs/1994JBIS...47..241Z

Couldn't that kind of reaction be used to cool an electric motor/turbo-pump/battery?

« Last Edit: 02/14/2018 02:27 AM by sanman »

Offline msat

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Re: electric pumped rocket cycle
« Reply #31 on: 02/14/2018 01:54 PM »
While this reply is going to be on electric pumps in general, it'll largely be based around how it applies to something like the Electron LV.

I'm confident that power density of an electric motor (at least in terms of mass) could be increased by taking advantage of the cooling properties of pumped propellants, especially if they're cryogenic. What isn't so clear is what is the lower limit in power output where the complexity outweighs the benefits, or where the benefits even become impossible to realize. Taking the Rutherford for example, we have an already impressive level of performance from its pumps, at least relative to their volume (we don't know the mass, but it probably could be inferred). What would happen to its volume if Rocket Lab tried to implement a cooling system? Some parts might shrink but others would increase due to added coolant paths, and maybe some additional external plumbing. I can't say which way it would end up other than: more complex. And even if there was a benefit in terms of performance, would it be worth it? Not to mention the different cooling properties of LOX and RP1 which would necessitate very different designs for each motor (unless the same fluid was used for both.. that brings its own dangers).


I suspect that it would be easiest to achieve the potential benefits of cooling on a scale larger than the individual pump motors on the Rutherford, i.e. one pump feeding multiple combustion chambers. This is perfectly acceptable, as we know it has served the Russians very well. Of course, everything has drawbacks. For one, a pump failure (even if there are multiples) is likely to result in a LOM. It also makes it harder to scale the LV since it more closely couples propulsion to a given airframe. For example, if Rocket Lab wanted to build a larger first stage, it would be a lot easier to do it with the rocket motors they currently produce "simply" by using more of them. They have a built-in flexibility due to their use in an independent but clustered configuration. If several thrust chambers shared a pump then any scaling would require some multiple of thrust chambers.

I also mentioned metal-air batteries (using LOX, or heated GOX) earlier in this thread. I think there's quite a bit of potential there in terms of battery performance being significantly higher than current common chemistries. Definitely worth investigating in this application. But once again, you have to decide when to stop as it doesn't make sense to spend more effort than you would on a turbine-based system that its unlikely to ever outperform.

One of the banes of the orbital rocket industry, especially in the US, IMO is the desire for maximum performance. I commend Rocket Lab for their approach. They certainly didn't build a neanderthal rocket by any means, but they achieved their performance target in a design that's not overly complex by being open minded and using modern manufacturing techniques and technologies. This rocket may have not been possible not all that long ago, but it is now and they seized every opportunity. Good for them!

Offline ArbitraryConstant

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Re: electric pumped rocket cycle
« Reply #32 on: 02/14/2018 06:29 PM »
And even if there was a benefit in terms of performance, would it be worth it? Not to mention the different cooling properties of LOX and RP1 which would necessitate very different designs for each motor (unless the same fluid was used for both.. that brings its own dangers).
For the purpose of cooling a motor I don't think it's necessarily the case that the shaft would have to be sealed against the coolant as coolant channels could be run around the coils without having to touch the shaft. Either propellant could be used as coolant for both motors.

For LOX... I think it is highly non-obvious how that ends up. Reduced resistivity is a benefit which would improve the power density of the motor, on the other hand materials compatibility is an issue, it's paramagnetic and the motor would be subject to likely multi-tesla magnetic fields, and there may be issues with condensation on the launch pad, etc. Do they do a heat exchanger with some other material as coolant like LN2 or gaseous helium/neon/etc? It ends up being pretty complicated compared to RP-1.

Conversely a phase change cooling solution as was used for the lunar rovers, or in other areas such as aircraft black boxes designed to withstand fires, may well be adequate for the single digit minute burn times typical of a chemical rocket engine. It would be completely sealed with no mechanical components to fail, and would be adequate for a test fire followed by a launch a day or two later.

Where these tradeoffs end up is highly non-obvious to me with an amateur analysis. I also think it's non-obvious when an engine would be worth switching to pumped coolant or cryo coolant or whether that line is after it would already be worth it to switch to another cycle. I don't think Rutherford uses propellant as coolant as the plumbing to do this is not visible, but I'm guessing they do need something. They didn't build a custom motor on a whim, and others building motors definitely know what they're doing. If they got such a big improvement it's likely because nobody was building motors for that specific duty cycle hence RL was able to make optimizations that wouldn't make sense for anyone else.

I suspect that it would be easiest to achieve the potential benefits of cooling on a scale larger than the individual pump motors on the Rutherford, i.e. one pump feeding multiple combustion chambers.
Yup, agreed.

I also mentioned metal-air batteries (using LOX, or heated GOX) earlier in this thread. I think there's quite a bit of potential there in terms of battery performance being significantly higher than current common chemistries. Definitely worth investigating in this application. But once again, you have to decide when to stop as it doesn't make sense to spend more effort than you would on a turbine-based system that its unlikely to ever outperform.
Indeed. I think this is one area where there is enormous potential for different concepts because electricity is so easily fungible.

Referring to the "Electric feed systems for liquid propellant rocket engines" paper lined in my OP they mention Lithium-Sulfur batteries as being one possibility and they do some analysis of when these would be useful. Depending on criteria like burn length a battery can be either power limited or energy limited. Li-Po tends to be energy limited, Li-S tends to be power limited. My suspicion is that a metal-oxygen cell would be even more power limited. This doesn't necessarily prevent its use but we would need to be mindful of its limitations, it would tend to be useful for missions with low thrust to weight and long burn times. For example the Briz-M stage used in Russia is used as a third/fourth stage for orbital insertion, hence does not require high thrust and has a burn time of thousands of seconds, something this would likely be where you'd use a metal-air cell.

We can also consider a hybrid battery architecture. It would be quite different but have many of the same characteristics and implications of IVF from ULA. We can also draw comparisons to electric vehicles, fuel cell vehicles tend to have a battery to handle things like bursts of acceleration, and I believe Tesla also filed for a patent where they had a hybrid lithium-ion and metal-air battery pack. In this case the primary pumping power would be provided by Li-Po cells as is currently the case on Electron, but LOX boiloff would be used to drive a metal-air cell that could be used to recharge the Li-Po cells as well as provide power to the stage and payload(s). We can also consider a fuel cell in this application if the fuel is something like methane. For both fuel cell and metal-air I doubt the power to weight ratio is sufficient for a high thrust rocket stage, but these approaches would both be very friendly to miniaturization compared to the ICE engine used by IVF. In this way an electric pumped stage could seek to provide many of the advantages of IVF (multi-day on-orbit endurance, unlimited restarts, power and heat) at much lower cost.

One of the banes of the orbital rocket industry, especially in the US, IMO is the desire for maximum performance. I commend Rocket Lab for their approach. They certainly didn't build a neanderthal rocket by any means, but they achieved their performance target in a design that's not overly complex by being open minded and using modern manufacturing techniques and technologies.
Indeed!

Offline john smith 19

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Re: electric pumped rocket cycle
« Reply #33 on: 02/14/2018 08:14 PM »
One of the banes of the orbital rocket industry, especially in the US, IMO is the desire for maximum performance. I commend Rocket Lab for their approach. They certainly didn't build a neanderthal rocket by any means, but they achieved their performance target in a design that's not overly complex by being open minded and using modern manufacturing techniques and technologies. This rocket may have not been possible not all that long ago, but it is now and they seized every opportunity. Good for them!
Any comments on battery technology needs to consider what's Commercial Off The Shelf technology today.

Is there some super duper battery tech that can do better? Probably.

But can you phone multiple suppliers and get an actual delivery date for it in the repeat quantities you need? If you can't then your LV's success depends on nothing happening to that supplier.

That's the situation Orbital are in with their SRB supplier for Pegasus. Effectively the cost of those stages sets the price for Pegasus.

The next question is wheather RL build their own electric motors. People have built effectively open frame motors that run immersed in non conducting fluids. Others have run motors "canned," driving the pump impeller through some kind of magnetic coupling, leaving just the power cables to go through the wall.

However if better thermal management is needed probably the simplest answer is to use heat pipes to extract the heat. They are in widespread use within modern laptops and, per unit of cross sectional area, can be 6x more efficient than an equal cross section of pure Copper.

This is now pretty well understood technology, with substantial history in satellite systems, like cooling the transponders in comm sats.

For expendable rockets KISS is always a good idea.
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Offline ArbitraryConstant

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Re: electric pumped rocket cycle
« Reply #34 on: 02/15/2018 02:08 AM »
However if better thermal management is needed probably the simplest answer is to use heat pipes to extract the heat. They are in widespread use within modern laptops and, per unit of cross sectional area, can be 6x more efficient than an equal cross section of pure Copper.
Heat pipes don't eliminate the heat, they're a simple way to move heat around. Something to reject the heat on the cold side of the heat pipe is required. In a laptop that's ambient air. Where does the heat pipe in a rocket reject the heat? Radiators? Too big and heavy. Propellant? Sure, but why have a heat pipe when the propellant could be run through the motor like it already is for combustion chamber regenerative cooling? Heat pipes are great when they eliminate a pump that can break but a Rutherford engine already has pumps.

Offline john smith 19

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Re: electric pumped rocket cycle
« Reply #35 on: 02/15/2018 06:19 PM »
Sure, but why have a heat pipe when the propellant could be run through the motor like it already is for combustion chamber regenerative cooling? Heat pipes are great when they eliminate a pump that can break but a Rutherford engine already has pumps.
Because high current and LOX don't mix very well. A heat pipe puts 2 barriers between the motors and fluid (one at each end) which can be an important safety feature.
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Offline ArbitraryConstant

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Re: electric pumped rocket cycle
« Reply #36 on: 02/16/2018 12:54 AM »
Because high current and LOX don't mix very well. A heat pipe puts 2 barriers between the motors and fluid (one at each end) which can be an important safety feature.
Ok but is cryogenic cooling worth it in the first place? This is unnecessary if the coolant is room temperature kero.

Also for some applications no cryo propellant is available. For example an article was posted today about a replacement engine for the Orion service module. I suspect battery pumped could improve performance over the AJ10.

Offline john smith 19

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Re: electric pumped rocket cycle
« Reply #37 on: 02/16/2018 12:41 PM »
Because high current and LOX don't mix very well. A heat pipe puts 2 barriers between the motors and fluid (one at each end) which can be an important safety feature.
Ok but is cryogenic cooling worth it in the first place? This is unnecessary if the coolant is room temperature kero.

Also for some applications no cryo propellant is available. For example an article was posted today about a replacement engine for the Orion service module. I suspect battery pumped could improve performance over the AJ10.
Either is only a problem if Battery performance is dropping too much as they get hotter on discharge or motor performance is falling off too far with temperature rise during operation.

I'd suggest that right now either is a theoretical problem, rather than an actual inhibitor of mission success.

If you're talking about the Shuttle OMS engine I thought it was called the RS72 and I recall a joint project with one of the Arianespace contractors to do a pumped version, which the contractor (Safran?) called the Astus II.

My instinct is this is just the usual NASA contractor dance before the original contractor is awarded the money to restart production.
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Offline msat

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Re: electric pumped rocket cycle
« Reply #38 on: 02/16/2018 01:54 PM »

For the purpose of cooling a motor I don't think it's necessarily the case that the shaft would have to be sealed against the coolant as coolant channels could be run around the coils without having to touch the shaft. Either propellant could be used as coolant for both motors.

I think this depends on motor type and power density. In an interview with Elon Musk, someone stated they considered the Tesla Model S motor to have an impressive power to weight ratio. After informing the person that turbopumps are much more impressive in that regard, he started talking about how one of the bigger challenges in that motor's design was cooling. Here's the relevant bit: "In the Model S, we coaxially cool the rotor in order to have high steady state." This comment might reaffirm what you said, as in the case of the Tesla motor, it is the outside perimeter of the stator that's cooled, and so presumably it's able to absorb heat radiated from the rotor, rather than being directly cooled. But a much higher power density than the Model S motor would be desired for rocket pump use. There are motor types that don't induce currents in the rotor to generate magnetic fields and thus greatly minimize heating in that component, such as switched reluctance motors - which is the type I really suspect is used in the Rutherfords. It's somewhat tough to gauge how impressive their power density is compared to other electric motors since ~50hp @ 40k RPM (which equates to ~6.5ft-lbs of torque) isn't at all a common combination. Their stated efficiency, however, is impressive.


Quote
For LOX... I think it is highly non-obvious how that ends up. Reduced resistivity is a benefit which would improve the power density of the motor, on the other hand materials compatibility is an issue, it's paramagnetic and the motor would be subject to likely multi-tesla magnetic fields, and there may be issues with condensation on the launch pad, etc. Do they do a heat exchanger with some other material as coolant like LN2 or gaseous helium/neon/etc? It ends up being pretty complicated compared to RP-1.

It would actually be pretty interesting to know what would happen to LOX in/around a really strong magnetic field. One way I envisioned a potential motor design was where the coil windings were made of copper tubing with the LOX flowing through them. The stator lamination stacks could obviously also be cooled.

Quote
They didn't build a custom motor on a whim, and others building motors definitely know what they're doing. If they got such a big improvement it's likely because nobody was building motors for that specific duty cycle hence RL was able to make optimizations that wouldn't make sense for anyone else.

I think this is really it. It's a lot easier to highly optimize a design that has a narrow operating range. And as I stated above, since it's in an uncommon range and power output combination, it might appear a lot more impressive than it actually is from a technical standpoint - this isn't meant to disparage RL in any way, but rather point out the lay person and amateur's lack of understanding.

Quote
My suspicion is that a metal-oxygen cell would be even more power limited.

..

We can also consider a hybrid battery architecture. It would be quite different but have many of the same characteristics and implications of IVF from ULA.

...

In this way an electric pumped stage could seek to provide many of the advantages of IVF (multi-day on-orbit endurance, unlimited restarts, power and heat) at much lower cost.

While the discharge rate for common metal-air battery configurations is unfavorable compared to other chemistries, I'm not sure that it has to be this way, particularly for a narrowly defined and specialized use case. Neither should it preclude a mixture of technologies such as a variation of flow/pumped batteries to enhance ion transport between the electrodes and electrolyte.

I'd like to know how such a system would compare to the IVF concept. I suspect the electric route would be more robust.



Offline msat

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Re: electric pumped rocket cycle
« Reply #39 on: 02/16/2018 01:57 PM »

Because high current and LOX don't mix very well.

Why is this? My understanding is that oxygen is an insulator except at extreme pressures.

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