Dead Space Question

[Hubb]

Nothing is in the wiki on this topic and searching didn't reveal too much.

My current design is going to call for dead space. My question is, how much is too much dead space? I know GGDT will show the effects of dead space but every simulation I've ran, it doesn't really effect the shot too much (maybe 1 or 2 fps).

Thanks

[MaxuS the 2nd]

Why would you want dead space?

[bigbob12345]

dead space wont really affect performance that much at all and is essential if you are looking to break the sound barrier with air(the air needs to heat up before it can get to the sound barrier)
It will even increase performance in some circumstances

[Hubb]

The reason I "want" dead space is because I'm building a magazine fed launcher. The bolt and receiver portions are going to create dead space. It's going to be about 7 or 8 inches of dead space, but the barrel is 1/2".

bigbob: I don't really want to break the sound barrier (yet) and am not too sure of this dead space helping the launcher. But I appreciate the post.

Any other answers?

[MaxuS the 2nd]

I think you should still try to have as little dead space as is possible.

[Hubb]

I have, however,

The reason I "want" dead space is because I'm building a magazine fed launcher. The bolt and receiver portions are going to create dead space.

[clide]

Unless you have a stupid amount of dead space or a tiny chamber it won't really matter much. Like bigbob said, more dead space can even sometimes improve performance slightly.

[Gippeto]

Dead space could be thought of as "ineffective" barrel length. It has to be filled, but you get no benefit from it.(the projectile is not accelerating)

It also contributes to total pressure drop in the system.(for intents and purposes, you have a larger barrel volume, P1*V1=P2*V2). Higher average pressure = higher average acceleration.

While it is an admirable goal to keep this waste in check, a compromise is necessary if you wish to make a breech loading mechanism.

And finally, as you are not building a space shuttle;

Keep it as small as you can, and don't worry about it.

[Hubb]

Thanks, guys. I really am trying to keep it as small as I can.

The launcher will shoot paintballs (5) from 1/2" sdr. The C:B ratio will be (hopefully) 1:1. And, in that case, the small amount of dead space (speaking of cubic inches) should not matter much.

[clide]

Dead space could be thought of as "ineffective" barrel length. It has to be filled, but you get no benefit from it.(the projectile is not accelerating)

It also contributes to total pressure drop in the system.(for intents and purposes, you have a larger barrel volume, P1*V1=P2*V2). Higher average pressure = higher average acceleration.

When you consider flow restrictions it is not quite as simple. The case where dead space can actually increase performance is caused from a valve being able to fill the dead space to a certain pressure when the projectile is hardly moving, but when the projectile really gets moving the valve isn't able to keep up. Once you get to the point where the valve can't keep up, you end up with pressure in the barrel dropping faster than in the chamber. The dead space can act as a secondary chamber that maintains pressure in the barrel better than a restrictive valve when the projectile is moving quickly.

[Gippeto]

Thanks Clide, I never thought of it quite like that before.

Always learning something new.