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Water/steam as propellant?

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Water/steam as propellant?

Unread postAuthor: kjjohn » Sun Jul 18, 2010 12:44 am

Could you possibly increase the power of a combustion cannon by loading it with a cartridge containing regular water before the projectile? My theory is that you could possibly use a smaller chamber and still achieve the power of a larger cannon. Upon combustion, that water would (in theory) flash to steam, producing much higher pressures in the barrel.
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Unread postAuthor: Hotwired » Sun Jul 18, 2010 1:04 am

The energy in a combustion is not that great.

Consider how much fuel is actually in there. Few grams?

Then think about how much fuel you'd need to turn that water into steam if you were boiling it.

Steam cannons are entirely possible (has been done - Holman Projector). Flash boiling water cannons, not too sure anyone's done a working one. However boiling the crap out of some water trapped in a burst disk chamber will also definitely work.

But trying to vaporise any significant amount of water with a brief gas combustion isn't so good.
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Unread postAuthor: inonickname » Sun Jul 18, 2010 1:14 am

With a combustion, no. It's just an energy sink. You can have a metal chamber and burners outside, boiling the water to create pressure and steam, then using that as a pneumatic.

You'd be lucky to vaporize the water if you have any significant amount with a combustion setup. You don't have much energy to work with anyway, and water has a very high specific heat.
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Unread postAuthor: jimmy101 » Sun Jul 18, 2010 1:00 pm

Like inonickname said. Adding water would decrease (by a lot) the guns performance. A combustion is already partly steam operated (since one of the combustion products is water).

If you add water to the chamber then before you can convert it to steam, you have to heat the water from ambient to the BP. That heat will be completely wasted and is unrecoverable in something like a spudgun.

Once heated to the BP it takes additional heat to convert to steam (the temperature of the water doesn't change in this process). The steam will expand and can be used to push a projectile but to get the energy back the steam must cool enough in the pushing process to convert back to liquid water. If it doesn't then all the heat used to convert from liquid to steam is also wasted.

Look up the heat capacity of water and steam, and the heat of vaporisation, and you'll see it'll take a ton of energy to do what you want and most of the energy is wasted.
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Unread postAuthor: DYI » Sun Jul 25, 2010 10:26 pm

I'm going to go against the tide here and say that one could improve the performance of a combustion cannon with your idea. The type of combustion cannon that would benefit, however, is rather restricted and uncommon.

Consider the products of a typical hydrocarbon/oxygen combustion: CO<sub>2</sub> and H<sub>2</sub>O. It is a mixture of these gases which is providing the propulsion in any typical combustion launcher, along with nitrogen in the majority of cases. CO<sub>2</sub> is dragging down the SOS in the propellant gas mix, when compared with the other two components. Like all gas guns, regardless of propulsion method, muzzle velocity in a combustion is limited by the characteristics of the gas' expansion.

Increasing the pre-ignition chamber pressure in a combustion doesn't have a huge effect on the temperature produced, only the heat. As a result, with high enough pre-ignition pressures and light enough projectiles, one may well reach a point where it is beneficial to sacrifice a good deal of the energy released to the production of excess steam, which should have a somewhat higher SOS than the regular propellant gases.

Why's the class of launchers this would work for so restricted? They would have to run even higher pre-ignition pressures than HyGaC20 to see any really noticeable benefits. Perhaps the 320X butane/propane/air hybrid proposed by Ragnarok would be a candidate... :roll:

The original theory is partially correct; such a design could produce higher pressures on the base of the projectile, for certain lightweight projectiles (by increasing propellant gas SOS). It could not, however, increase the average pressure through the whole system, for the reasons mentioned by previous posters.
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Unread postAuthor: jimmy101 » Mon Jul 26, 2010 5:44 pm

I still don't think it'll work. It takes HUGE amounts of energy to convert liquid water at 70F to steam at 212F.

Besides, the change in the SOS for dry versus moist air is darn near zero. Or at least, so close to zero that it will be impossible to measure a difference in performance. IIRC, for air at 1 ATM the difference in the SOS for 50% versus 100% humidity is a couple tenths of one percent. So the SOS would go from 1100 FPS to 1103 (ish) FPS. You could accomplish the same increase in performance by just firing the gun while you were walking forwards. :D
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Unread postAuthor: DYI » Mon Jul 26, 2010 10:44 pm

Let's take our propellant mix as methane/air, for ease of calculation.
I don't think we'll have much trouble agreeing that the propellant gases produced by a 200X or a 500X methane/air mix will have very similar speeds of sound. Using a 1L chamber:
200X
Approximate combustion energy: 0.74MJ
Total mass of propellant gases: 256g

500X
Approximate combustion energy: 1.86MJ
Total mass of propellant gases: 640g

Assuming that we've already reached roughly the speed of sound in the propellant gas with our projectile of choice at 200X, 500X gives us an extra 1.1MJ to play around with. In theory, the excess alone could boil almost a pound of water (420g). Obviously, we'd use a good deal less than that to allow it to reach a high enough temperature to be useful. Considering the SOS in steam as opposed to nitrogen and carbon dioxide, and the amount we might use here in comparison to the total gas present (and that much of its mass will loiter well behind the projectile), I contend that a noticeable, positive increase in velocity for certain lightweight projectiles in very high pre-ignition pressure launchers would be possible.

Worthwhile? Not in many cases, especially when one considers the possibilities presented by using helium or hydrogen as buffer gases. Noticeable though? The gases would be very "moist", as you would put it, Jimmy :wink:
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Unread postAuthor: Technician1002 » Tue Jul 27, 2010 1:12 am

Remember the boiling pressure is higher in a high mix. Boiling is nothing more than the formation of bubbles anywhere in the liquid when the vapor pressure is trying to exceed the pressure in the liquid. The phase change takes away heat and maintains the temperature. This phase change temperature is pressure dependant.
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Unread postAuthor: Ragnarok » Tue Jul 27, 2010 5:19 am

Technician1002 wrote:Remember the boiling pressure is higher in a high mix.

Higher, but actually, the enthalpy change of vaporisation of water is lower as pressure (and boiling temperature) go up..

Indeed, at more than water's ~650K critical temperature, it is zero.
You still have to heat said water, but you do not need to boil it.

~~~~~

DYI is potentially right here. You'd ultimately need to work out whether the increase in SOS would be worth the lost pressure, and, as has been suggested, it would be inferior to say a fuel/oxygen/helium mix.

To use the 320x mix as an example, if the nitrogen in the air were replaced by a helium buffer gas, the speed of sound in the mixture goes from ~1000 m/s up to ~1930 m/s, peak pressure goes up from 3200 atm to 4060 atm - both of which owing some of that to lower heat capacity of the gasses increasing temperature from 2830K to 3590K.
Fiddle with the proportions a little (as, ultimately, the buffer gas does not affect the partial pressures of the fuel or oxygen), and >2000 m/s SOS values are possible, if at the expense of temperature (which is probably a good thing, as it will reduce heat loss).

DYI wrote:Perhaps the 320X butane/propane/air hybrid proposed by Ragnarok would be a candidate... :roll:

Actually, on that note, I've been thinking about whether it would be possible to achieve a hybrid where part of the fuel (be it propane, butane or whatever) is still liquid on the point of firing, using the heat of the combustion to vaporise the fuel.

Given that the enthalpy change of vaporisation for propane is 15.7 kJ per mole, but the enthalpy change of combustion is 2044 kJ per mole, provided you could still achieve ignition with the lean fuel/air mix (which would be the limiting factor, although spectacular ignition could help get you closer to the lower explosive limit), and solve the metering issues, then there would seem to be a considerable net gain.

Like that, I'd say 500x mixes of propane/butane/air could be possible.
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