Rocket scientists at Purdue University in west Lafayette, Indiana have come up with a new approach to plasma thrusters which will potentially increase their reliability and efficiency making them more suitable for softball sized nanosatellites, which are becoming more and more common.
Plasma thrusters have traditionally used one of two approaches to fuel. A solid propellant, usually Teflon (polytetrafluoroethylene, that is ablated and vaporized and then passed through a field that accelerates it.
The problem is that this ablation is a hit-and-miss process. The rate is difficult to control, and this can make the thrust non-uniform. Also, the Teflon surface sometimes breaks down and ejects debris in the form of macroparticles that interfere with the engine operation.
What's more, the igniter that triggers the flashover process can become damaged over time. All these problems ultimately limit the efficiency of the solid-fuel plasma thrusters to less than 15%.
The other common way is to store the propellant as a gas. This increases the efficiency of a plasma thruster by up to 70%.
But these systems are bulky and complex, and the gas itself has a significantly larger volume than an equivalent solid mass. That makes it hard to build into a nanosat.
According to lead author Adam Patel, these issues can be addressed by storing the propellant as a liquid, which "could potentially overcome several disadvantages associated with traditional pulsed plasma thruster devices"
The team has built and, using a vacuum chamber, tested a proof-of-principle micro-propulsion system fed by liquid propellant. The liquid they used was pentaphenyl trimethyl trisiloxane (C33H34O2Si3), a viscous liquid with low vapor pressure that is also an excellent dielectric.
The advantage of this kind of igniter is that the threshold voltage is always the same, and so the amount of energy required for flashover is always limited. This limits the potential damage to the flashover assembly over time.
In tests, Patel and co used the igniter for upwards of 1.5 million flashover events without observing any significant damage to the device. Other designs can sometimes fail after only 400 firing cycles.
The test device was able to generate an exhaust velocity of 32km/sec and 5.8 Newtons of thrust making it a potentially (not)solid option for future nanosats.
Reference
arxiv.org/abs/1907.00169 : Liquid-Fed Pulsed Plasma Thruster for Propelling Nanosatellites
(Score: 0) by Anonymous Coward on Monday July 22 2019, @08:44AM (8 children)
N/T
(Score: 3, Funny) by Runaway1956 on Monday July 22 2019, @11:25AM
It poisons the space guppies, which in turn, poison all the space fish that feed on space guppies.
(Score: 2) by Immerman on Monday July 22 2019, @01:53PM (6 children)
Well, mercury was a propellant for ion thrusters, but I see no mention of usage in plasma thrusters. And it was largely replaced by Xenon over concern of the toxicity of ground testing and concerns about condensation on the spacecraft during long operations - mercury doesn't play that nicely with... lots of other stuff.
There's also the challenge of keeping it frozen - space is cold, but things *in* space tend to be quite warm, at least this close to the sun. Earth's effective temperature (The expected temperature of something the same color, with no atmosphere, etc. complicating things) is -18C. Cold, but still well above mercury's -39C melting point.
(Score: 2) by ElizabethGreene on Monday July 22 2019, @03:21PM (5 children)
Mercury isn't great for another reason. It's heavy. Exhaust velocity factors heavily into Specific Impulse and it takes more oomf to accelerate Mercury (atomic weight 200) vs. PTFE (atomic weight 100). Oomf is expensive in all the metrics that matter; dollars, mass, and volume.
(Score: 2) by deimtee on Monday July 22 2019, @04:03PM (3 children)
While particle weight is important for thermal exhausts it doesn't really factor into ion thrusters apart from tuning the electric field to match it. Hg takes twice as much energy to accelerate it, but it provides twice the thrust also.
If you cough while drinking cheap red wine it really cleans out your sinuses.
(Score: 1) by khallow on Monday July 22 2019, @09:51PM (2 children)
(Score: 2) by deimtee on Monday July 22 2019, @11:06PM (1 child)
That really only matters for thermal exhausts. Isp inversely correlates with molecular weight for a given gas temperature.
If you are accelerating it electrically to a velocity of 100km/s, ion size doesn't change your Isp, you just need to design the thruster to match the ion size. At those exhaust speeds you are so far above any solid material temperature it doesn't matter whether the effective exhaust temperature is 100,000K for a light ion or 200,000K for a heavy one.
If you cough while drinking cheap red wine it really cleans out your sinuses.
(Score: 1) by khallow on Monday July 22 2019, @11:58PM
Lighter molecular weight is inversely proportional to higher exhaust velocity at fixed energy of a molecule of the propellant.
Except that as part of those adjustments the dimensions of the engine can become prohibitive. For example, the normal isotope distribution of mercury has an molar mass of 230, while helium has a molar mass of 4. Ionized by removing one electron they would have the same charge. If one is attempting to accelerate at the same rate, one needs roughly 50 times the force on the mercury atom as an ionized helium atom. An electrostatic accelerator which accelerates the ion across a voltage drop is going to need 50 times the voltage. To prevent arcing, that means the structure will need roughly 50 times the distance between electrified components.
(Score: 2) by Immerman on Tuesday July 23 2019, @12:31AM
My impression is that larger atoms tend to be preferred for ion drives, though I don't recall the exact rationale - hence Xenon being the preferred nontoxic alternative, with its mass of 131 AMU. I mean, otherwise they could just use hydrogen, right?
I think there was also a second important factor: how easy it is to ionize, with the ratio of reasonably achievable ionization level per AMU being an important factor.
(Score: 2) by richtopia on Monday July 22 2019, @10:46AM
Just another application of pentaphenyl trimethyl trisiloxane, the miracle chemical.
Sarcasm aside, this was probably a difficult find. I remember reading about the propulsion choice a few years ago, and there was no obvious choice. Noble gasses like Xe are often used, but difficult to store. At the time buckey balls were being investigated for their high weight and stability.
(Score: 2) by Muad'Dave on Monday July 22 2019, @11:14AM (4 children)
With a melting point of -25C, this stuff will have to be kept warm to stay liquid in space.
(Score: 2) by Runaway1956 on Monday July 22 2019, @11:30AM (2 children)
Isn't that a fairly easy task? Everything is in a vacuum, so no conduction, or convection. Radiation isn't a huge problem. Everything I've read says that overheating is a bigger problem in space, than getting too cold. Getting too much sunlight is the greater danger, if I've got it right.
Of course, I've never seen any charts or anything, showing how quickly, or how slowly, heat is lost into space. If the typical comm satellite loses 1 degree per year, it seems you have a lot of time in which to correct cold fuel tanks. A single small solar power panel ought to take care of that.
(Score: 2) by VLM on Monday July 22 2019, @01:21PM
Passive thermal control is pretty old stuff now. In the 80s it was easy for any ham radio guy with standard HF gear to listen to the Russian RS-10 and RS-12 satellites which transmitted on HF bands in morse code, the transmissions were like "register name space value" format continuously (well, when the sat was overhead LOL) and you'd run the register number thru a formula and get the internal temperature of the satellite (or a zillion other sensors) and then you could graph it.
Modern sats are not so user friendly but do the same thing digitally using somewhat more obscure frequencies and modulation methods.
Anyway, yeah, its pretty much a solved problem for a long time and its within the range of hobby activity to receive, decode, and analyze satellite internal temps.
It kinda sucks that AMSAT and similar groups moved away from EZ-Sats like that and toward drastically higher performance but drastically harder to use sats. They really need an outreach program for, like, a HF PSK-31 telemetry transmitter.
(Score: 5, Informative) by ElizabethGreene on Monday July 22 2019, @03:30PM
Spacecraft thermal control volumes 1 and 2 are a fascinating read if you really want to know. The short version is that you need a sun shade, attitude control to keep the sunshade between you and the sun, thermal isolation between hot and cold components, and a way to generate heat to keep it from getting too cold.
(Score: 3, Interesting) by Immerman on Monday July 22 2019, @02:00PM
Depends where you use it. Earth's effective temperature (i.e. the excepted temperature based solely on its albedo and sun exposure, before atmospheric effects, etc are taken into account) is 255K (-18C). A true blackbody in our orbit would be 279K (6C). And that's before you consider the additional heat radiating from Earth. So really, keeping satellite propellant above -25C is unlikely to be a serious problem. Might not be the best propellant choice for a mission to Neptune though.
(Score: 3, Interesting) by VLM on Monday July 22 2019, @01:26PM (2 children)
Wonder how well the pressurized liquid valve handled that. Air/Liq tight and millions of operations over decades of use in a comsat is going to weight the same no matter if its boring xenon or this cool new liquid.
Historically the concern of fooling around with cesium and liq mercury was conductive ionized clouds that either impaired remote control and access or worried the engineers that stuff would plate back on parts of the sat and short stuff out. Yeah some freaky organic chem is a great insulator as the article says, but charred carbon debris conducts as well as any other graphite.
(Score: 5, Informative) by Immerman on Monday July 22 2019, @02:08PM (1 child)
> Air/Liq tight
That's just it though - liquid tight is a LOT easier to achieve than air tight. Even more so when the liquid is a molecule composed of 73 atoms - that stuff is a LOT bigger than water, and probably has fairly high surface tension as well what with all the hydrogen polar bonds between molecules. And especially when a similar mass of gas would need t be kept at much higher pressures to fit within the available volume constraints. And this stuff has a low vapor pressure, meaning that the valve won't have to contain it against significant pressure probably only some small fraction of atmospheric pressure.
(Score: 2) by VLM on Tuesday July 23 2019, @01:26PM
Those are valid points; I was right that they can't get out of dealing with valves that easily, you're also right that valve will be much less of a heavy PITA than some alternatives.
(Score: 5, Interesting) by ElizabethGreene on Monday July 22 2019, @02:29PM
The summary misses a key disadvantage of gas thrusters. Cubesat launchers don't like compressed gasses (or other energetics) on their payloads. It can be done, but it is much easier to get somebody to tote your secondary payload if it doesn't have compressed gasses or volatile materials (e.g. fuel/oxidizer) on it.
(It's a lot more paperwork and review to make sure you don't endanger the primary payload.)
(Score: 2) by Rupert Pupnick on Monday July 22 2019, @04:07PM (4 children)
Do we really have to name things that are softball sized with a nano- prefix?
If real satellites were roughly 10^9 times bigger, I’d be OK with it, though.
Who the hell comes up with these names?
(Score: 2) by istartedi on Monday July 22 2019, @04:33PM (3 children)
Who? Microcephalics maybe, even though their heads aren't 1/1000 the size of regular heads. Oh, and don't get me started on Micronesia or Mega Millions.
Appended to the end of comments you post. Max: 120 chars.
(Score: 2) by Rupert Pupnick on Monday July 22 2019, @05:24PM (2 children)
Exactly. Micro- is entirely too colloquial and lacks the high tech sheen of nano-.
(Score: 3, Informative) by deimtee on Monday July 22 2019, @11:16PM (1 child)
Well the Moon masses 7.4x1022 Kg so these are really somewhere between a zepto-satellite and a yocto-satellite.
If you cough while drinking cheap red wine it really cleans out your sinuses.
(Score: 2) by Rupert Pupnick on Tuesday July 23 2019, @12:16AM
If I were CEO of RupeTech, I’d hire you as Chief Marketing Officer. You’ve got a keen sense of both the factual and the stylistic!