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Solar electric hydrolox rockets


Streetwind
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*points to the title* Now there's a combination of words I never expected to see... or at least not in a serious context! :P

But a company called Tethers Unlimited is quite serious about it. They've built a hydrolox thruster for microsatellites - main propulsion for large cubesats, or attitude thrusters for slightly larger craft. However, it doesn't carry cryogenic liquid oxygen and liquid hydrogen. No, it carries plain old de-ionized water. And it will use electricity from the satellite's solar panels to electrolyse that water, creating oxygen and hydrogen which is then injected into the combustion chamber.

The main idea behind it is that small satellites that ride as secondary payloads often struggle to find rideshares if they carry propulsion, because those propulsion systems and the fuel they contain pose risks to the primary payload (or to other secondaries). These risks need to be insured, which costs insurance money, and mitigated, which costs development money. But a spacecraft carrying water needs neither of those two, because the propellant is "non-toxic, non-explosive, and unpressurized". It's also dirt cheap, and (in a future scenario) can be easily replenished by ISRU in space.

So much for the theory, but in practice? The system may launch into space for the first time as early as 12 months from now. It's been test fired plenty of times on the ground. (Source)

 

What do you guys think? Hydrolox has the potential for the best specific impulse by a wide margin among the chemical propellants typically used today, and removing the need to carry the low density, hard cryogenic LH2/LO2 bipropellant instantly removes its biggest drawback. But how much extra dry mass and volume does the electrolysis equipment require? How much effort does it take to consistently draw the right amount of liquid from an unpressurized tank in zero gravity? How much electrolysis power can a cubesat's solar arrays provide - will that be enough gas to build sufficient combustion chamber pressure and temperature to actually get the nice, high Isp? Or is the system ultimately no better than a common hydrazine thruster, and its only advantage is the propellant safety? And what about comparing other options for solar electric supported propulsion, such as electrothermal rockets (resistojets)?

Edited by Streetwind
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I assume the H2 and O2 is stored pressurized under the pretty low pressure created by the electrolyse and then burned in pulses. This eliminate the need for an minimum power 
Benefit is that the system is pretty simple and the pulse trust is decent, downside is that an ion engine would have higher ISP and is just as simpler if not more. 

Now if you could resupply water this might be an nice idea for larger satellites too, again main competitor is ion engines. 
 

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You can't really build a true hall effect or gridded ion plasma thruster in a cubesat format though. Even for the tiniest of units, the power output is simply not enough. True electric plasma rockets need a huge amount of power, and you cannot really downsize them as well as chemical units. Too little power throughput and voltage simply results in the Isp crumbling to dust. So electric engines used on microsatellites tend to be arcjets, pulsed plasma thrusters and other such things that get Isp's around the 1,000s mark. Still a lot, true, but having three times the specific impulse of a chemical thruster but only one thousandth of the thrust is a tradeoff that isn't all that favorable. The all-electric system has a significantly higher dry mass and needs to fly dV-wasting spiral trajectories, after all.

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Interesting. I've been pondering this sort of system on a much larger scale as a plot point in a story I'm writing.

So I'll see this crazy and raise you another one - how far could you reasonably scale this up?  My guess is - not very -  but I'd be curious to see if anyone can throw some numbers at me. :)

 

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That's what I'm wondering. How do they compare? :P

Water is probably not a good working fluid for a resistojet though, since it vaporizes at such a low temperature, and the act of vaporization keeps more water away from the heating element. This will limit how much heat you can put into water in the short time it flows through the engine. Even good resistojet working fluids have Isp's in the 500s-600s range, so if you're using a bad fluid and need to compete with hydrolox combustion around maybe 400s, it may not work out so well.

Edited by Streetwind
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"De-ionized water".  How much anti-freeze do you think it needs mixed in?  I'd expect all kinds of issues in full sunlight as well as night.  Or do they just let it start to freeze and time the burns well after "dawn" (and assume the thing can handle any boiling it might have)?

I've heard that PVC ablation is a cheap way to make an ion engine, but never found out how cheap.  I'd still prefer something like a boring old hybrid rocket, and note that the Isp shouldn't be much of an issue assuming the weight of the water isn't a large portion of the satellite (i.e. not going to the Moon of something).  Note that while the hybrid rocket needs to be ignited, so does our hydrolox engine as well.  I'd just assume that microsat builders aren't at all interested in dealing with hypergolics nor the paperwork, permissions, and procedures.

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PVC ablation... that reminds me... @Streetwind, using a resistojet doesn't even restrict us to water. Some of them are solid-fuelled, e.g. Teflon; while an electrolytic hydrolox system has capacity for short, high-g pulses, a resistojet could be capacitor-powered. Both would put a constant drain on the solar panels.

We really need to see those numbers.

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58 minutes ago, DDE said:

PVC ablation... that reminds me... @Streetwind, using a resistojet doesn't even restrict us to water. Some of them are solid-fuelled, e.g. Teflon; while an electrolytic hydrolox system has capacity for short, high-g pulses, a resistojet could be capacitor-powered. Both would put a constant drain on the solar panels.

We really need to see those numbers.

Teflon? Aren't you thinking of pulsed plasma thrusters here?

A resistojet is an eletrothermal rocket - in other words, it works like a NERVA, except that the heat is created by electrical current in an incandescent filament and not a nuclear reactor. You need liquid or gaseous propellant for it to work, ideally something that that doesn't change phases while passing over the filament. (In a NERVA, the liquid hydrogen is turned gaseous before entering the main heat exchanger, thus fulfilling that requirement.)

Pulsed plasma thrusters often use sticks of solid teflon, which they ablate and ionize with a brief electric pulse discharge.

1 hour ago, wumpus said:

Note that while the hybrid rocket needs to be ignited, so does our hydrolox engine as well.  I'd just assume that microsat builders aren't at all interested in dealing with hypergolics nor the paperwork, permissions, and procedures.

There are tried and true methods for igniting non-hypergolic bipropellant motors in space, such as electric spark plugs and heating lasers. :)

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19 minutes ago, Streetwind said:

Teflon? Aren't you thinking of pulsed plasma thrusters here?

There are tried and true methods for igniting non-hypergolic bipropellant motors in space, such as electric spark plugs and heating lasers. :)

1.  That sounds right.

2. I can't remember a laser being used, but lighting hydrolox isn't hard (I think it is the easiest of non-hypergolic fuels).  Hypergolics remain in use, and I'm guessing because igniters keep failing.  From memory, I think The Register's Lohan project had a failed electric igniter at altitude.  Things like that are hard to test, and can't be assumed working until tested.  Testing (in the correct environment) is going to be hard for smallsat builders (especially the amatures).

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53 minutes ago, wumpus said:

Testing (in the correct environment) is going to be hard for smallsat builders (especially the amatures).

Well, this unit is sold as a preassembled all-in one system. The water tank, electrolyzer, plumbing, thruster and ignition, and electronics are all included. The smallsat builders only need to bolt it into the frame and hook it up to their flight control computer. As such, I imagine the thruster comes with a pre-tested warranty, so the sat builders don't need to do more than a functional test.

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