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RandomFactor writes:

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

Original Submission