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posted by martyb on Monday July 22 2019, @08:32AM   Printer-friendly
from the not-a-solid-step dept.

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


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  • (Score: 2) by Immerman on Monday July 22 2019, @01:53PM (6 children)

    by Immerman (3985) on Monday July 22 2019, @01:53PM (#869941)

    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.

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  • (Score: 2) by ElizabethGreene on Monday July 22 2019, @03:21PM (5 children)

    by ElizabethGreene (6748) Subscriber Badge on Monday July 22 2019, @03:21PM (#869979) Journal

    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)

      by deimtee (3272) on Monday July 22 2019, @04:03PM (#869990) Journal

      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)

        by khallow (3766) Subscriber Badge on Monday July 22 2019, @09:51PM (#870097) Journal
        The previous poster mentioned specific impulse. The faster the propellant exits, the less propellant mass is needed to get the same delta-v though the trade off is weaker thrust-weight ratio. High specific impulse is useful for satellite station keeping since one can usually do without high thrust for the entire lifespan of the satellite once it has reached orbit.
        • (Score: 2) by deimtee on Monday July 22 2019, @11:06PM (1 child)

          by deimtee (3272) on Monday July 22 2019, @11:06PM (#870118) Journal

          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

            by khallow (3766) Subscriber Badge on Monday July 22 2019, @11:58PM (#870137) Journal

            That really only matters for thermal exhausts. Isp inversely correlates with molecular weight for a given gas temperature.

            Lighter molecular weight is inversely proportional to higher exhaust velocity at fixed energy of a molecule of the propellant.

            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.

            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

      by Immerman (3985) on Tuesday July 23 2019, @12:31AM (#870147)

      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.