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The private space company Pixar has developed an autonomous space vehicle (Wall-e) propelled by a cold gas thruster Hand-Held Maneuvering Unit as demonstrated in this press release:

enter image description here

From https://scienceonblog.wordpress.com/2019/03/14/propulsion-in-wall-e/

And in this video

The company is cagey about the proprietary propellant, but informed sources report it is supercritical CO2, stored above the critical point, at 40*C and 75atm.

enter image description here

The theoretical Isp for CO2 cold gas thrusters is reported at 67sec https://en.wikipedia.org/wiki/Cold_gas_thruster

Question: Is this Isp figure improved by storage of CO2 as a supercritical fluid?

I ask since (I believe) there is no heat of vaporization penalty to be paid converting supercritical gas to gaseous phase ("A" in the above diagram), unlike converting liquid to gaseous phase ("B" in the diagram). Cold gas thrusters have advantages of cost and simplicity at the expense of low Isp. A boost in Isp by altering storage conditions would be welcome.

Addendum: This paper https://arc.aiaa.org/doi/abs/10.2514/6.2009-6553 describes a rocket design which achieves a Isp greater than 100 using supercritical CO2

Woody
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  • I love this cute little question, but it's unanswerable because it's asking about something that just is completely made up. Maybe ask "what would the isp of a fire extinguisher be," (as inspired by wall-e) – Anton Hengst Aug 23 '22 at 03:42
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    @AntonHengst ... the question is really "Do cold gas rockets fueled by supercritical CO2 have a higher Isp than those fueled by liquid CO2?" This question is answerable. – Woody Aug 23 '22 at 04:36
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    @Woody if that's your actual question, then the title is misleading. Cute and all, but you're making it hard for other people to benefit from the answer to the question (if anyone does) – Erin Anne Aug 23 '22 at 04:43
  • Please reask/retitle the question! I'd like to see the answer. – Anton Hengst Aug 24 '22 at 05:57
  • If you actually read the paper linked in the question, the sustained Isp is 40. There's a instantaneous spike up to ~100 at the start of the blowdown. – Organic Marble Aug 24 '22 at 17:01
  • @OrganicMarble ... yes. That spike is of interest. I read a description of a proposed solar thermal thruster which uses a "thermal capacitor" approach to produce intermittent thrust pulses. This would pair nicely with with a high Isp spike. But is this spike a feature of supercritical CO2? And if "yes", is it due to the absence of a phase change energy barrier? – Woody Aug 24 '22 at 17:20
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    Note that cold gas thrusters wouldn't use isothermal expansion (vertical line as drawn on your p-T diagram), as that would require supplying heat to keep the temperature constant as the propellant expands. A thruster that does that is called a resistojet. You can assume that the expansion in a CGT is approximately adiabatic (temperature drops upon expansion). – TooTea Aug 26 '22 at 13:11

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