# How high is the ISP of a fire extinguisher

After seeing this question Flying around on a fire extinguisher I wondered how high the ISP of a fire extinguisher is. What is the ISP of a fire extinguisher?

A conventional CO2 fire extinguisher could be used as a cold gas thruster. The maximum Isp would be determined by the temperature of the gas. CO2 is stored as supercritical CO2 inside the bottle. It cools as it expands and freezes into CO2 snow.

This question was addressed in Do cold gas thrusters fueled by supercritical CO2 have a higher Isp than those fueled by liquid CO2? along with entertaining video links.

This paper https://arc.aiaa.org/doi/abs/10.2514/6.2009-6553 attained an isp of 100sec using supercritical CO2 heated to 400K (260F). Isp is proportional to the sonic velocity so it would be expected to be about 87 sec at room temperature

However, CO2 is a poor choice for a fire extinguisher in an enclosed space such as a spacecraft and aircraft, except for localized fires. The space shuttle used Halon 1301 https://ntrs.nasa.gov/api/citations/19910011869/downloads/19910011869.pdf Isp is inversely proportional to molecular weight, Halon 1301 (MW=149) is much heavier than CO2 (MW= 44) so it would have a much lower Isp. Say, 27sec at room temperature with an optimized nozzle.

• CO2 might be a bad idea in any spacecraft or aircraft we have constructed to date, but the one depicted in Wall-E is the size of a city, it's probably fine there. Jan 9, 2023 at 21:02
• @Darrel Hoffman ... Actually, this paper on Freedom Space Station design ntrs.nasa.gov/api/citations/19910011869/downloads/… said "preferred extinguishing agent is carbon dioxide", No extinguisher they tested was perfect, and CO2 was the least of evils. Jan 9, 2023 at 22:56

Personal experience. Circa 2008, I replaced my home fire extinguisher. The old one was a type ABC, about 1 liter in volume and about 2 kg of propellant. Although it was still at full pressure and perfectly usable, I decided to have a little fun. I sat on a rolling chair and discharged the extinguisher.

The impulse was not even enough to disturb my hands, much less move the chair. Very disappointing.

I don't know what the exit velocity of the discharge was, but it landed about 10 feet away.

The ISP would probably be similar to compressed-gas systems used in spacewalks. On Gemini 4, Ed White used a can of compressed gaseous oxygen to briefly propel himself. Compressed gaseous nitrogen were later tested on Skylab and the Shuttle. The SAFER system is still in use, and has 1.4 kg of gaseous nitrogen, enough for a delta-V of about 10 ft/s.

• Fire extinguisher nozzles are not optimized for exit velocity. Cold gas propellant system nozzles are. Jan 8, 2023 at 23:24
• My very limited experience with powder-type extinguishers is that they make big mess and small propulsion. CO2 extinguishers are the other way around. Jan 9, 2023 at 4:26