How does an Electric Solid Propellant rocket work?

I saw the section in Wikipedia about Electric Solid propellants but I can't figure out what it really is, and how it works. It sounds like a solid propellant rocket that you can start and stop multiple times by turning it ON and OFF - a restartable solid fuel rocket.

How!?

Are there any examples of this type of rocket being put to use? (beyond self-validation)

– SF.
Jul 1 '16 at 10:20
• @SF. $\text{Wow!}^N$ This would be phenomenal if it ever turns out to be save enough for a shared payload space full of nanosatellites. The article is a useful summary/survey, but I still don't know how it actually works. Is it a high voltage radial discharge localized across the exposed surface? It works in vacuum? For a small engine, would the electrical power be Watts or kilo Watts?
– uhoh
Jul 1 '16 at 11:16
• The charging circuit consists of a variable high - voltage power supply, a capacitor - charging circuit and an electrostatic voltmeter. Testing is begun at 0.25 joule. A 0.02 microfarad capacitor is connected to the discharge circuit and charged to 5 kV. - so, an equivalent of a static charge; very low power but high voltage. Possibly a piezzo crystal could be helpful. I'll try to compile an answer when I have the time. There's another paper: campus.mst.edu/aplab/index_files/AIAA-2015-4185.pdf
– SF.
Jul 1 '16 at 11:39
• @SF. OK that's great thanks! The trail leads to both the term, and the company name Digital Solid State Propulsion. Any web site that can combine Frank Zappa and Plasma Cannons is worth a read in my book. fyi: There's a section on the use of arrays of small thrusters in the SpinSsat Mission Overview.
– uhoh
Jul 1 '16 at 12:21
• One beautiful thing there seems it's not just restartable. It can be "restarted" and "extinguished" up to 60 times per second. Which in practice means full throttlability,
– SF.
Jul 1 '16 at 16:57

Basing on two papers I had managed to find: Electrical Solid Propellants: A Safe, Micro to Macro Propulsion Technology and Plasma Plume Characterization of Electric Solid Propellant Pulsed Microthrusters.

First, the concept isn't new; it's an incremental research on Pulsed Plasma Thrusters which were used first on soviet Zond 2 and Zond 3, starting in 1964. In Pulsed Plasma Thrusters, all the combustion energy comes from electric discharge, which evaporates a small portion of inert (non-combustible) solid propellant (usually teflon), the process producing a "puff" of charged plasma (moving at relatively low speed), which is subsequently propelled through ion engine principles.

The Electric Solid Propellant (ESP) does away with the ion engine part, but instead uses an almost combustible (just barely self-extinguishing) propellant which evaporates rapidly enough to produce a reasonable exhaust velocity. It also replaces moving propellant stick and the fixed "spark plug" electrode with a non-movable structure of electrodes, conductive (but resistive enough to create plasma) propellant and ablating insulator, ablation of which (during combustion of the propellant) exposes more of the electrode, allowing the electric pulse to travel with the front of the propellant.

There is a number of different ESPs, varying in properties

• ASPEN - an early formulation; response time of order of 0.1s, may be difficult to extinguish after achieving higher pressures
• HIPEP - minimum smoke, non-metalized, good burning rate and ignitability properties, wide temperature range
• ESP ANAV - high aluminum content, high insensitivity to hazards (flame, spark, impact, friction), can self-sustain after ignition (though electricity still regulates burn rate).
• BADB - similarly, can self-sustain and offer better performance, but require additional processing to passivate surface active characteristics to retain acceptable storage and service life.

Specific impulse in all these is on par, or somewhat below that found in classic SRBs - of order of 200-230s. The primary differences are in smoke signature, environmental vulnerability and ignitability characteristics.

Thanks to the plastisol process, they can be prepared with standard lab equipment and just "cast" like resins and the likes - simply poured into the prepared thruster, in room temperature. "Curing" occurs in temperatures of order of 35C over 7 days. This way, for example, microthruster modules are prepared. Such modules can be installed directly in nanosatellites.

The microthrusters like pictured above can fire for 1.5s, providing 5-10 millinewtons of thrust (x6 in the sextuple modules). The microthrusters can be activated in very short pulses - of order of 0.02s or less, to provide a fine regulation of impulse.

Of course the propellant can also be used in larger assemblies.

Currently, a test article microsatellite "SpinSat" utilizing the microthrusters was launched from ISS on 28th November 2014. The results have not been published yet.

• This sounds like something that might be produced on a 3D printer Jul 2 '16 at 21:24
• @HowardMiller: Not simply; all that copper for electrodes etc. But I could picture a giant electronics manufacturer making such microthrusters next to capacitors, coils and fuses; they really look like parts to be soldered into a PCB.
– SF.
Jul 2 '16 at 23:09