CO2 Cartridge discharge thrust?

I am trying to figure out how much thrust would be created with a full discharge of a typical small CO2 cartridge.  Way back when in High School we built CO2 powered drag racing cars in shop.  (I can't remember how we dischardged them?).  But I want to rig a similar device to discharge underwater.  I want it to give a "turbo boost" to a fairly heavy model boat after the boat gets moving a bit.   So it doesn't have to get the boat moving or up to a speed. It's more to give the boat a good push after it gets going.

How much thrust would that generate? In water?  I could probably rig 2 cartridges to fire simultaneously on each side of the boat.

Denger4 years ago
Hey, I built one of those back in junior high shop class, too.  Ah! those were the days, when public school students could still learn how to fabricate things out of bits of wood and metal rather than just out of bits, bytes and pixels... but I digress.

In the United States, pre-charged CO2 propellant cartridges come in various sizes: 8, 12 and 16 gram, for instance, with 12 gram typically being most common (and least expensive). In our school's CO2 cars, we had used the 8 gram "food grade" CO2 cartridges, typically available in supermarkets and used in seltzer bottles and whipped cream canisters. They are about the same diameter and about 25% shorter than the more common 12 gram size, but more expensive.

These 8 gram cartridges are also commercially available online in 10-packs from Boy Scouts of America's ScoutStuff web catalog as well as a dual CO2 piercer/launcher made for model jet car races such as you mentioned. 

Another similar means of piercing a single CO2 cartridge was the Crosman Jet-Line Model 101 conduit gun, which (unlike other Crosman pistols) fires not a BB, pellet or paintball, but rather the CO2 propellant bottle itself!  It's basically a handheld spring-loaded piercing needle tool.  More information on the Model 101 can be found here (in German), but if you use Bing Translator, you'll get the idea.  I have seen conduit guns like this available recently in the electrical department of a local big box home improvement retailer.

The actual thrust produced by 8 and 12 gram cartridges seems to be a closely guarded secret (at least on the internet at this time).  If you know the CO2 mass, the temperature and the pressure, you can probably calculate the figures you desire using Boyle's Law, Newton's Third Law of Motion and the coefficient of friction between your craft's hull and the water.  It's been a long, long time since high school physics class, and I don't know that I could have provided you an accurate answer even back then.  You'll have to do your homework.  You may find PITSCO's website useful for reference.

The bit which seems tricky to me is the part of your proposed design which seems to call for a piercing apparatus which will function while your craft is already underway, posing a potential difficulty for the stationary piercing device technologies listed above. 

This sounds like a fun and exciting project.  Hopefully, you can publish an Instructable when you've done it; I'd be interested to read it.  Good luck!