Water Rockets

[Guest doughbred]

From what I heard, Water rockets have higher Specific Impulse than Compressed Air. But, Compressed air has more fuel. So which one has a better overall performance?

Now what would happen if I replaced the compressed air with a rocket and bulkhead/ pusher plate in the water rocket? Will it perform better than a rocket with equal amounts of fuel?

Edited June 26, 2013 by doughbred

[astropapi1]

When in doubt, add more SRBs!

[K^2]

Depending on the pressure inside the bottle, there is an optimal water volume to get the most impulse out of the whole thing. I derived the formula for it once before. Let me think about it for a bit.

[Brotoro]

The compressed air is the propellant in both kinds of rockets. But if you have only compressed air in the rocket, you have very little reaction mass, so the result is poor. Add some water as reaction mass for the prepellant to push out, and you get better performance.

[K^2]

Propellant is what provides reaction mass. Water is the propellant in the water rocket. Air merely provides energy.

[Guest doughbred]

What if I replace the air with rapidly burning fuel, is what I am asking.

[K^2]

What you are suggesting with a chemical rocket is equivalent to adding some water to the fuel and just running a conventional rocket. This will increase specific impulse per weight of fuel, but it reduces specific impulse per total propellant weight, which reduces overall performance.

[Brotoro]

Propellant is what provides reaction mass. Water is the propellant in the water rocket. Air merely provides energy.

No. The propellant is what provides the energy. It propels the reaction mass. The reaction mass may also be the propellant (more typically the combustion product of a propellant in a chemical liquid fuel or solid fuel rocket), but the propellant is not necessarily the reaction mass. In a water rocket, the propellant is the compressed air in the bottle that pushes the water out. Similarly, in a can of spray paint, the propellant is the gas that expands to push the paint out of the nozzle. In a firearm the propellant is the burning powder that provides the expanding gas that propels the bullet out of the gun. The propellant propels that which is to be moved (in a rocket, that which is to be moved is the reaction mass).

[K^2]

What's the propellant of a NERVA rocket? Nope, it's hydrogen gas. Standard usage in any literature. Now look at ion drives. What's the propellant? Whatever inert gas that's being used. In context of a rocket, propellant is whatever you use for reaction mass. Source of energy is fuel. Again, if you look at NERVA, you'll notice that it uses nuclear fuel, not nuclear propellant.

[Brotoro]

What's the propellant of a NERVA rocket? Nope, it's hydrogen gas. Standard usage in any literature. Now look at ion drives. What's the propellant? Whatever inert gas that's being used. In context of a rocket, propellant is whatever you use for reaction mass. Source of energy is fuel. Again, if you look at NERVA, you'll notice that it uses nuclear fuel, not nuclear propellant.

What do you mean, "nope"? What is propelling the reaction mass (the hydrogen) out of the NERVA motor? The hydrogen itself, heated by the nuclear reactor, expands...and it is that gas pressure that propels the hydrogen out of the nozzle. Therefore it is acceptable to refer to the hydrogen reaction mass as propellant in the NERVA engine.

But in a water rocket, the water does not contribute to the propelling of the water out of the bottle. It is the compressed air that does this. It is sloppy to refer to the water as propellant.

In an ion engine, the reaction mass does not contribute the force that ejects it from the ion engine, so it is similarly sloppy to refer to the xenon reaction mass as "propellant"...but I'm sure people do. Just as there are people who would refer to the solid chunks of matter thrown out by a mass driver by electromagnetic acceleration as "propellant", but it would be sloppy to do so in that case as well, since the chunks of reaction mass contribute nothing to the the operation of being ejected. You might just as well call the air that gets pushed backwards by a rotating propeller as "propellant". I don't have to subscribe to sloppy thinking just because it has become popular usage.

[K^2]

It's not sloppy. It's proper usage of the word. The use of the word "propellant" to mean the expanding gas that drives something else is due to use of the word in things like aerosol sprays. If you want to spray-paint something, you need to accelerated the paint particles. Propellant is something that accelerates these particles. Id est, the compressed gas.

But in the rocket, your goal is not to propel the reaction mass. Your goal is to propel the rocket. Therefore, reaction mass is the propellant. And that usage is consistent across every type of the reaction engine.

[Brotoro]

The origin of the word propellant in rocketry is from the meaning of the word as used in gunnery.

[Guest doughbred]

So to go a given distance, would I need more or less kerosene and oxygen, or more.