evacuated barrel

[Sticky_Tape]

Hi I am making a triggered burst disk rifle and I am wondering if evacuating the barrel will make the rifle shoot faster that is what I am going for will this do much of anything? I am aware of the vaccum olny being -14.7psi so there wouldn't be much differece but would it help?

[Ragnarok]

Yes, although, consider that burst disks work on pressure differential, so there would be no increase in overall pressure... with the same disks. Thicker disks could however be used to gain the benefit.

A part of the change would arise from the suppression of funny pressure effects ahead of the projectile, which work against approaching the sound barrier. No air - no pressure effects, simple.

[psycix]

I had this idea for a long time:
When approaching the sound barrier, and you are using helium (or heated combustion gases) the largest thing holding you back is the air in front of the projectile.
If the projectile goes over the speed of sound, the air in the barrel also has to move at the speed of sound, and air doesnt like to flow that fast, thus generating alot of back pressure.

Just have a second burst burst disk at the muzzle end (a thin one) made to hold no more then 14.7 psi.

[Sticky_Tape]

Yes, although, consider that burst disks work on pressure differential, so there would be no increase in overall pressure... with the same disks. Thicker disks could however be used to gain the benefit.

A part of the change would arise from the suppression of funny pressure effects ahead of the projectile, which work against approaching the sound barrier. No air - no pressure effects, simple.

I don't think you get it I want to increase volocity by creating a vacuum in the barrel doesn't have anything to do with the burst disks I know you would have to make them to handle 14.7psi more and all. And creating a vaccuum in the barrel wouldn't work against the reaching of the soundbarrier it would probably help it.

[jackssmirkingrevenge]

Evacuating the barrel means that your projectile is accelerating in zero air resistance, it's not just about the pressure difference. If NASA does it with their test guns it must be a good thing

[psycix]

Well, if you create a vacuum in the barrel, you can consider your effective chamber pressure as: chamber pressure + 14,7psi.
Also, the muzzle velocity will increase further because of having no air resistance. Though we do not know how much that makes a difference yet, but I suppose the effects get more intense as you approach the sound barrier.

[Sticky_Tape]

Soooooooo why don't people use this in longbarreled hybrids?

[Hotwired]

It's more hassle than it's worth thats why

Another pump and another burst disk for very little benefit is in the same category as having an uber-long barrel to get the last drop of energy out of a pneumatic.

[al-xg]

Wouldn't it be sad if the projectile slowed back down to subsonic speed as it ruptures the burst disk at the end of the barrel

[Sticky_Tape]

olny having to burst through 1 burst disk of aluminum foil 1/2 holds about 20psi at supersonic speeds wouldn't slow it down to subsonic very easily maybe if you had about 15 disks which holds about 300psi.

[psycix]

Indeed, the second burst disk is very thin (as thin as possible) because it doesnt need to hold back much more then 14,7 psi.
An APFSDS round going way over mach1 doesnt seem to care about that I think.

[al-xg]

APFSDS use burst disks ? or you just implying he'd be using saboted projeciles ?

[jackssmirkingrevenge]

APFSDS use burst disks?

Definitely not, it's a conventional cartridge weapon (though modern rounds have exotic propellants and combustible cartridge cases)

[Ragnarok]

I don't think you get it I want to increase volocity by creating a vacuum in the barrel doesn't have anything to do with the burst disks I know you would have to make them to handle 14.7psi more and all. And creating a vaccuum in the barrel wouldn't work against the reaching of the soundbarrier it would probably help it.

I'm not sure you read what I said:

A part of the change would arise from the suppression of funny pressure effects ahead of the projectile, which work against approaching the sound barrier. No air - no pressure effects, simple.

I said exactly the same thing as you did - I know I used some funny negatives, but nonetheless...

To use different words:
A part of the change from the use of an evacuated barrel is the reduction of pressure effects. These pressure effects will make it harder to breach the sound barrier, so reducing them (by creating a vacuum in the barrel) will make it easier.

Trust me, I've considered the idea many a time, but it works against my rule of keeping launchers practical - the reason I won't build a burst disc launcher.
Efficency is important to me, but not at the cost of practicality and reliability.

[Sticky_Tape]

But why don't you go against your rule and make a super powered launcher? As to that I mean Maybe high mix hybrid with a evacuated long barrel small bore. Imagine being the first? spudder to get a 1/2'' projectile past mach4.