Selected from original post, by Tkendrick
They are currently working on a new projectile concept which may or may not be applicable to rifles. (I have my own opinion, but I'm not anywhere near smart enough or educated enough to make a guess)
Someone has come up with a concept that goes like this:
The projectile (bullet) is hollow, made from a hard material, my guess is they're working with depleted uranium or solid copper or something in between, don't really know.
The bullet is milled, not cast, and looks like a standard bullet, except that it is hollow. The opening narrows down the farther back you go, until just before the base, where it opens up, constricts again, and finally forms a bell shape like a rocket nozzle.
The theory that they are working on, and are apparently having enough success that someone is willing to spend the money to test it, is that once the bullet hits a certain speed, the air entering the front of the bullet is compression heated to the point that it creates plasma and actually provides thrust which causes the bullet's velocity to increase as it moves down range.
This sounds like a scram jet to me, except there is no fuel.
Selected from other posts:
1.
A scramjet type engine needs to be at hypersonic speed in order to obtain the amount of air for combustion...that means at least Mach 5 or 6. That means as a minimum the projectile must be attaining an initial speed of 6,000 fps. I'd still like to know what gun shoots a projectile at these speeds. Kinetic tank rounds are close in speed, but tank rounds are not hollow and loose speed quickly, as does any round.
The missile described in
http://www.globalsecurity.org/wmd/systems/slam.htm
was intended for Mach 3. Many modern rifle bullets approach this velocity out of the muzzle.
In addition, this poster is still talking about
combustion as the source of the heat. I pointed out several times that combustion isnot needed to heat the air.
2.
230RN, you mean the bullet is going to be hot enough from powder flame and barrel friction to transfer energy to the air passing through the core?[
No. I did not specify a source to heat the projectile. I
suggested a possible source of heating it was to fill the hollow cavity with thermite. Which would bring the projectile to probably a white heat
after which the projectile would turn into a ramjet. Read my posts.
Missile packed with thermite gets started, thermite is ignited, perhaps by powder gases from the discharge, perhaps by a little demon following along with a Bic lighter. The thermits heats up the bullet to,
let's say, a white heat, and in the meantime perhaps provides a bit of a rocket thrust.
When the thermite burns through from the base of the projectile to the front, it thereby now leaves the front of the ramjet cavity open. It is assumed that the bullet is now at a speed where the ramjet effect can take place.
The air enters the white-hot bullet, is heated by the white-hot bullet itself, and is accelerated out the rear, thereby providing thrust, which if it does not actually accelerate the bullet, will at least help it along in its flight.
Now: Important: I used the term "white heat," just as an example, but the actual temperature required might be only a dull red. The point is that at whatever temperature that is required to achieve the ramjet effect, that's what the bullet is heated to.
Maybe white heat. Maybe only 1000 dF. We don't know. I am just providing a conceptual basis for an explanation of what they were trying to do.
I don't think the remark in the OP, "the air entering the front of the bullet is compression heated to the point that it creates plasma and actually provides thrust which causes the bullet's velocity to increase as it moves down range." is what's happening. The only part of it I see as possibly true is the "heating" part.
3.
Chamber shape may be right, but velocities are too low for scramjet reaction.
I'm talking about a simple ramjet. I don't know who brought up scramjets. I believe, but cannot document, that simple ramjets do not have to run in the supersonic range. However, see above about Mach 3.
4.
After combustion, if the exhaust is supersonic, the nozzle must be shaped in a diverging cone to continue the acceleration of the gasses.
As was the decribed internal shape of the object described in the OP. However, again, don't get stuck on combustion as the source of the heat required.
I'm probably running over the limit on a single post here, but remember the old quote, which is probably apocryphal, nevertheless pointed:
"What? Put an electric motor on a razor? That's crazy! You'd slash your face to ribbons, Dr. Braun!"
The point here is that while one person is thinking of a razor blade being spun around by an electric motor, Dr. braun was thinking of the electric razor which we all use, invented by Braun, and which is still around.