Tiny Ion Engine Runs On Water 103
symbolset writes "Discovery News is covering a project by two engineers from the University of Michigan to pair cubesats with tiny ion engines for inexpensive interplanetary exploration. The tiny plasma drive called the CubeSat Ambipolar Thruster (CAT) will ionize water and use it as propellant with power provided by solar cells. In addition to scaling down the size of ion engines they hope to bring down the whole cost of development and launch to under $200,000."
I'm sure there is a drought in space joke somewher (Score:5, Interesting)
.....but more practically: how much thrust/impulse/whatever would you be able squeeze out of an amount of water that can be carried by a tiny cubesat? The article implicitly compares it favorably to current Xenon/Krypton based systems, but made no effort to explain why. Any slashdoter willing to work out the math?
Re:I'm sure there is a drought in space joke somew (Score:5, Interesting)
Re:No, it runs on sunlight. (Score:1, Interesting)
Re:I'm sure there is a drought in space joke somew (Score:5, Interesting)
Thats true, but the issue in a cubesat is going to be all about total propellant mass fraction (The fraction of the vehicle mass at launch made of of stuff you can sling out the back at high speed), so while Xe is a better reaction mass if you have the space for the tank, it may well be that in this particular use case the higher storage density (and thus the ability to fit more of it into a tiny tank) actually trumps the heavier ion.
Space propulsion is all about propellant mass fraction and exhaust velocity, as those two numbers define how much delta V you can get out of your available fuel.
The problem with light ions in this situation is that the momentum transferred is simply the product of exhaust mass and exhaust velocity, the energy required to produce that exhaust velocity is 1/2 mv^2, thus a heavier ion travelling more slowly requires less energy input to the accelerator for a given amount of momentum transfer then a light ion moving fast.
However if you have surplus electrical power, and are not too concerned about producing large accelerations (even by ion drive standards), and can solve the corrosion and thermal management problems, it might actually be a reasonable tradeoff.
All space propulsion is tradeoffs between energy/reaction mass/specific impulse/acceleration, there are no really right answers here, and having another validated tool in the box is always going to be useful.
Why is water better than Xe? (Score:5, Interesting)
As has been previously mentioned, the key question of space propulsion is how much thrust can you get for a given mass of propellant? The usual measure of this is Isp, which is thrust per weight flow rate of propellant. While it seems unlikely that water will beat Xe due to having lower mass per ion, it does have several key advantages, which are not really in the article except the first one:
1. Smaller storage tank can be used for liquid water as opposed to a gas. This is especially important if you're trying to piggyback with another satellite.
2. Gas will leak out over time, requiring more expensive hardware to contain it. You need something able to handle the expansion and contraction associated with sunlight, plus the very high pressure. That's a lot of seals, and getting seals that won't degrade in space is not that trivial- it's a harsh environment, especially from a radiation standpoint.
3. This is just something that occurred to me, but a large fraction of the weight on a spacecraft is a radiator, because the only way to get rid of heat in space is radiative heat transfer, which is much less efficient than convection. (and if you are generating power and thrusting, you are making heat) If you utilized the water as the working fluid in the radiator, you might be able to simplify another subsystem. I don't know if they actually did this.
So in summary:
It is unlikely that water produces a more efficient propulsion system, but it may well produce a simpler, cheaper, and easier to transport one.
Disclaimer: No actual math was done for the writing of this post. If you have math to prove me wrong, please do so.
Re:I'm sure there is a drought in space joke somew (Score:4, Interesting)
Re:No, it runs on sunlight. (Score:5, Interesting)
Until, that is, you get out to the Oort cloud where all the water is.
Ceres [wikipedia.org] was the first asteroid discovered and is now classed as a "minor planet". It is a main belt asteroid between Mars and Jupiter. It has a diameter of about 1000 km, and is believed to have a mantle composed almost entirely of water ice that makes up one third of the body's mass. 200 quadrillion metric tons of water ice should be more than sufficient, as it's more than all the fresh water on Earth and ionizing that much water to plasma would take more energy than we have until fusion is worked out. For scale, these interplanetary cubesats would launch with 1.5 kg of water for propellant, or 1/1*10^20 of that. The water ice should be relatively pure as it was gravitically distilled during the formation of the body. Ceres may even have liquid water still. It is a main asteroid belt object near enough to the sun that solar energy is effective for solar water distillation and solar cells. It has an equatorial surface gravity of 0.03g, and an escape velocity of 500 m/s. On the surface it has a thin coat of iron and silicon rich minerals that would also be useful. It is believed to own 1/3rd the mass of the entire main asteroid belt, or just over 1% of the mass of the Moon, 0.01% the mass of the entire Oort cloud all in one convenient low gravity place close enough to the sun for solar cells to work. How much more water the Oort cloud has than Ceres is irrelevant if Ceres has more than we can use in a more convenient place.
In just over 18 months, February 2015, the NASA Dawn [wikipedia.org] mission will arrive there and survey Ceres. NASA Dawn uses an ion engine also, and its delta-V is not very different from that proposed for these cubesats.
On the energy thing: Solar cells are perhaps misnamed because of their historical use. It turns out they convert energy quite well if you point a laser at them. Even better as the laser can be tuned to the maximum conversion frequency of the cell. As the energy of lasers don't diminish as much over distance as other methods it is quite possible to power the "solar cells" of distant craft by pointing lasers at them from closer to the sun where insolation is higher using solar cells as energy input. We think of lasers as a thing that has perfect collimation that makes them useless for powering solar cells but in fact at interplanetary distances past the utility of the sun's energy the beam will be larger than the craft. Our deep space craft need not run out of "solar" energy ever while we care to illuminate them with space-based lasers to the limit of their capacity, even to interstellar space. This is much preferable to carrying the energy with the craft even with nuclear fission fuels. Maybe one day we'll make a craft that goes out to the Oort cloud, eats a comet to refuel on its way to the stars. But that's in the distant future. For now the Oort cloud is off the table.
Because filters exist you can even still use communication lasers on a different frequency from your power lasers. Communication lasers don't suffer from physics as much as radio transmissions do, as long as you use space based relays in Earth orbit with radio ground comms. The Earth's atmosphere plays hob with communication lasers. You still have periods twice a year when you can't talk to the thing, and can't send it power. If you have power and comm relays on Ceres this drops to once in 800 years when both Earth and Ceres are conjoined.
Ionizing the water is part of the process of the engine converting it to plasma, so ordinary water is the input - not some special "ionized" water.
In short, if water works as an ion engine propellant then we've cracked the nut for Man's exploration of the solar system and the stars. Your issue about having to go out to the Oort cloud for water is, of course, ridiculous.
Re:I'm sure there is a drought in space joke somew (Score:5, Interesting)