CubeSat Rocket Thruster Is So Small It Has To Be Made Like Microchips (newatlas.com) 24
An anonymous reader quotes a report from New Atlas: Imperial College is developing a rocket thruster called the Iridium Catalysed Electrolysis CubeSat Thruster (ICE-Cube Thruster) that is so small that it can only be fabricated using techniques originally designed for making silicon chips. The entire thruster chip is about the length of a fingernail, with the combustion chamber and nozzle only measuring 1 mm long. It also requires only 20 watts of electric current to operate and in a test campaign generated 1.25 millinewtons of thrust at a specific impulse of 185 seconds on a sustained basis. To put that into perspective, that's half a billion times less thrust than the engines used on the Space Shuttle.
However, the party trick of the ICE-Cube Thruster is that it uses ordinary water as its propellant, which is about as non-explosive and non-flammable as you can get. Onboard electric current creates electrolysis to break down the water into hydrogen and oxygen, which is fed into the combustion chamber to ignite, generating thrust to maneuver the CubeSat. Using water is not only very green, it also reduces payload because no pressurization is needed to store it, so storage and handling systems can be lighter and simpler. However, fabricating the combustion chamber and nozzle for the thruster in what is essentially two dimensions required taking a page from microelectronics by using the Micro-Electrical Mechanical Systems (MEMS) technique normally employed for machining silicon wafers for processors to sub-micrometer tolerances.
However, the party trick of the ICE-Cube Thruster is that it uses ordinary water as its propellant, which is about as non-explosive and non-flammable as you can get. Onboard electric current creates electrolysis to break down the water into hydrogen and oxygen, which is fed into the combustion chamber to ignite, generating thrust to maneuver the CubeSat. Using water is not only very green, it also reduces payload because no pressurization is needed to store it, so storage and handling systems can be lighter and simpler. However, fabricating the combustion chamber and nozzle for the thruster in what is essentially two dimensions required taking a page from microelectronics by using the Micro-Electrical Mechanical Systems (MEMS) technique normally employed for machining silicon wafers for processors to sub-micrometer tolerances.
But water is heavy (Score:2)
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So how long will the propellant they can afford to send up really last?
The weight doesn't matter in this context. I believe the entirety of force generated comes from throwing mass in the opposite direction the rocket wants to go, so more mass is synonymous with more propellant. The difference between chemical propellants and water is that chemical propellants provide their own energy to launch the mass in some direction. Using water assumes electricity will be available, but it doesn't overcome the fundamental problem of carrying more fuel weighs more and requires more fuel.
Re:But water is heavy (Score:5, Informative)
The problem of the amount of fuel you can carry is that this new thruster is *extremely* inefficient. Specific impulse is a measure of fuel efficiency, basically how much thrust you get relative to propellant mass. The advantage of electric propulsion is normally their extreme efficiency. Hall effect thrusters are among the least efficient electric thrusters, and the hall effect thrusters in SpaceX's current Starlink satellites has a specific impulse of 2,500 seconds. SpaceX's best chemical rocket, the Raptor, has a specific impulse of 363 seconds. This new MEMS thruster has a specific impulse of 185 seconds. Any savings in thruster mass are meaningless in the face of the terrible efficiency because you'll need to carry so much more fuel as to erase the mass savings from the tiny thruster.
Hall effect thrusters have been used on cubesats before too. You lose some efficiency, but the Exotrail ExoMG is designed for cubesats as small as 10kg, produces 1.5 to 2 millinewtons of thrust, and does it with a specific impulse of 800 seconds.
Re:But water is heavy (Score:4, Interesting)
One of the problems I see with using the Exotrail ExoMG thrusters is the comparatively large size of the magnets needed to contain the ion flow, plus the pressurized propellant. The ICE-Cube thruster was specifically designed for extremely small cube sats, some not much larger than cell phones.Exotrail ExoMG [exotrail.com]
A pico cube sat about the size of a soda can could have six thrusters arranged on all three axes sharing a common water source. The size and mass of six hall effect thrusters alone would probably be the equivalent size of the soda can, plus pressurized propellant gas, plus a much larger power source. In some cases the terrible efficiency would be offset by the huge size reduction for extremely small cube sats.
Re:But water is heavy (Score:4, Interesting)
This new MEMS thruster has a specific impulse of 185 seconds.
This is withing the range of traditional monopropellant thrusters, but presumably without their chemical hazards. So I wouldn't be worried about a lack of use cases.
Re:But water is heavy (Score:4, Informative)
There is an advantage to water as a propellant that's easily overlooked. It's much less explodey, to the point of being practically inert, compared to Hydrazine or a high pressure compressed gas like Xeon.
The base cubesat specification specifically forbids chemical or pressurized gas propellants so you have to get an engineering review and waiver if you want it on your kit.
There is another hypothetical advantage, too. If you skedaddle out to the belt and find a hydrate-rich rock out there you can back out your own fuel. That's extremely unlikely to happen in the near future though, thus the hypothetical.
Back to the technology. This isn't the first MEMS scale ion thruster. There was a paper about 6 years ago with one that used liquid mercury as the propellant. I'm glad to see the technology evolve.
Its about gravimetric energy density not just mass (Score:2)
So how long will the propellant they can afford to send up really last?
What fuel has a better gravimetric energy density than 2 Hydrogens and an Oxygen?
Re: (Score:3)
Water isn't that much heavier than hydrazine, and anyway thrusters are a last resort. Magnetorquers and reaction wheels come first.
Roundabout Resistojet (Score:2)
https://ntrs.nasa.gov/citation... [nasa.gov]
I'm guessing the secret sauce here is that electrolysis lets you use low power slowly to get high temperature combustion and high Isp instead of having to dump all that power into your propellant directly.
Neat stuff, but I'd bet that another caveat here is that that 1mN figure is a peak power off a very low duty cycle. Suitable perhaps for momentum dumps but maybe not stationkeeping.
Re: (Score:2)
https://ntrs.nasa.gov/citation... [nasa.gov]
I'm guessing the secret sauce here is that electrolysis lets you use low power slowly to get high temperature combustion and high Isp instead of having to dump all that power into your propellant directly.
Neat stuff, but I'd bet that another caveat here is that that 1mN figure is a peak power off a very low duty cycle. Suitable perhaps for momentum dumps but maybe not stationkeeping.
I'm guess that electrolysis is about storing fuel (water) that is incredibly safe and common. That way these cubsats can be payload, hitch hikers, on other missions without raising safety concerns.
Re: (Score:2)
Safety has nothing to do with it. These things would basically compete with the smallest hall effect thrusters, which use xenon propellant, and xenon is a noble gas, pretty safe and inert. Arguably safer and easier to store than water, which is pretty unstable in space (exposed to vacuum it will boil, kept under pressure it will freeze, depending on sunlight exposure).
Re: (Score:3)
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And somebody else posted that some of these cube sats are banned from having pressurized gas in the first place. In which case, water might be their only option.
Re: Roundabout Resistojet (Score:2)
Some are banned from having any sealed cavities, since they might go boom in vacuum. You have to drill little holes in any components that might not equalize by themselves.
I'm sick of "furlongs per fortnight" comparisons (Score:2)
"To put that into perspective, that's half a billion times less thrust than the engines used on the Space Shuttle."
How about "putting it into perspective" by describing something people can relate to, like maybe the amount of force you use when scratching an itch giving your lady a butterfly kiss, but applied for many days.
Re: (Score:2)
Yeah that didn't really put it into perspective at all, but I can help: The engines on the Space Shuttle have roughly half a billion times more thrust than those on a little cubesat!
Re: (Score:2)
giving your lady a butterfly kiss
This is Slashdot. Bringing members of the opposite sex into a discussion is a purely theoretical domain for which we do not have experience.
Water freezes into a solid (Score:2)
Re:Water freezes into a solid (Score:5, Informative)
Water does freeze, if it gets cold enough. But the satellites will be in vacuum, which is a near-perfect insulator (think thermos flasks), so they will cool very slowly as heat can only radiate not convect. They're also in sunlight at least half their orbit, and in view of the warm Earth constantly, which will heat them. Plus of course the payload will be producing heat. The usual problem with heat on spacecraft is keeping them cool, not keeping them warm.
Way too in-effective (Score:2)
Re:Way too in-effective (Score:5, Informative)
The Space Shuttle engines had an Isp of 450, not 4500.
But this engine does not compete with other rocket engines. At the moment, there are no rocket engine options for cubesats because even the smallest available engine takes up more than 1 dm^3, i.e. is larger than the cubesat it's supposed to go into.
For a cubesat, the constraints are available space, and safety regs imposed by the companies that launch cubesats (no gases stored at high pressure, and no flammable liquids either).
Re: (Score:2)
Here's a comparable engine used for similar tasks for larger satellites:
https://www.space-propulsion.c... [space-propulsion.com]
ISP of 220. More than 185 for sure, but it's not wildly out of the range. And of course, the tiny one does have a number of advantages, such as inert, nontoxic fuel and being able to scale down to a very small size.
Water non-explosive (Score:2)
These people have never seen a steam locomotive boiler explosion. Low water, crown sheet softens, contents of boiler (superheated water) exit via the firebox, boiler end up in near by corn field. Engine crew suffers.
Inkjet for space? (Score:2)