Aeronautical engineering question

[Neubob]

Has anyone ever attempted to make a ramjet the size of a high-caliber rifle (.50 cal, 14.5mm, 20mm)?

I've been thinking about this problem for the last couple days, and am wondering if the idea would work.

Here's a very quick summary:

Mechanism:

The initial velocity imparted by the projectile's propellant should be enough to create the necessary compression to ignite the fuel once the projectile leaves the barrel. The spin of the projectile would serve not only to stabilize it as thrust continues to add velocity, but, if the fuel were stored in a recess at the tip of the connical centerbody , the centrifugal force would actually act as a pump, forcing the fuel from the narrow end of the cone, down to the wide end, towards injection holes close to the base of the inlet, and into the combustion chamber.

Benefits:

Even a brief burn period would markedly increase the projectile's velocity, or, at the very least, maintain close to muzzle velocity well downrange of the shooter. This would significantly increase the effective range of the rifle, as well as penetrative characteristics.

With no moving parts required, the projectile would still be cost effective when dealing with costly fragile targets.

Weapons such as  this 20mm rifle may be able to implement the ammo with little modification.

Any thoughts?

(Edit):   This apparently is not a new idea.

[Odee]

Sabot's come to mind. As well as the experimental "rocket pistol" which actually worked, but was deemed impractical

[Chairboy]

While not a ramjet, required reading for anyone interested in this subject is:

http://en.wikipedia.org/wiki/Gyrojet

The Gyrojet pistol (mentioned above) was pretty cool, and had many of the same limitations and advantages that the parent poster's idea would encounter.

Due to the properties of the rocket, with continued acceleration after it left the barrel, the Gyrojet had poor short-range power, but improved dramatically with distance. After about 75 yards (70 m) the motor would burn out, at which point the rocket had about 50% more power than the common .45 ACP round.

[Flatbar]

The prob as I see it is where the fuel would be stored and how would it be delivered in a high g environment.

Since the ram effect would suplement the velocity created by the powder charge, how would the projectile react once the initial velocity dropped off and the ram effect takes over. Some sort of guidance system would be needed IMO.

[Neubob]

Originally posted by Flatbar
The prob as I see it is where the fuel would be stored and how would it be delivered in a high g environment.

Since the ram effect would suplement the velocity created by the powder charge, how would the projectile react once the initial velocity dropped off and the ram effect takes over. Some sort of guidance system would be needed IMO.

Fuel storage and delivery are part of the novelty of this idea. The connical centerbody  would be the optimal place for a gastank. The spin of the round would force the fuel towards the back of the projectile (towards the wider end of the centerbody), and actually spray the fuel, at extremely high pressure into the cumbustion chamber. With a normal rifle round spinning at upwards of 180,000 rpm at the muzzle, this design actually takes advantage of that substantial energy. The spin is not interrupted after the ramjet kicks in, and so, its flight should remain stable. Also, the Ram effect would occur pretty much instantly, right out of the barrel, and last for only a second or so. If this proves to extend the range past the point where the spin can effecively stabilize the round, the pillars that hold the centerbody in place can be made to be curved. With the incoming air rushing over these pillars, the effect would be similar to having spin-inducing fins on the outside of the projectile. That being said, a guidance system is always a possibility, but it would also interfere with the inherent simplicity of the idea.

To fire this thing from a .50cal or 20mm rifle, all that would be required is a plastic wadding between the round and the powder charge, and a plastic or soft metal sheath to engage the rifling. At this point, the only issues I can come up with in terms of usability involve actually striking the target. With the projectile's design being dedicated to the ramjet, I don't know how destructive it would be on impact. Human targets don't need much, of course, and soft targets such as stationary aircraft and missiles would most likely be defeatable by the steel construction necessary to sustain the ramjet. However, the API(armor-piercing/incendiary) capabilities of modern large-caliber rifle ammunition would need to be addressed. Perhaps a larger centerbody to house an explosive/incendiary warhead would do the trick.

I think the key salable aspect of this idea is not so much that range is extended, rather, that sniping at distances that are currently on the high-end of the spectrum will become more routine. If the round retains most or all of its muzzle velocity out to, say, 1500 yards, issues such as winddrift and drop will be minimized. It will significantly decrease the guesswork in long-distance sniping, and increase the energy delivered to the target. Elite units implementing the .338 lapua and .408 cheytac will be still engage targets at 1500-2000 yards, but do so with more confidence. Engaging extremely distant targets with heavier calibers will become possible too, but then we're talking improved optics.