Aerospiked recoiless rifle

[farmerben]

I observe that military reccoiless rifles have either cone exhaust or a straight tube, often with multiple venturis to spin the gas.  If you watch them fire, the exhaust usually kicks up a cone spreading to mushroom shaped cloud of dust.  

Based on what I know about rocket nozzles this does not seem optimal  Directing the escape gases straight back provides the greatest impulse to the projectile.  So a bell nozzle with a straight throat would be fine.  And also an aerospike might be awesome for the exhaust of a recoiless rifle.   Imagine a sharp tongue of flame pointing straight back behind the shooter. 

Based on what I've read, US citizens have a compelling argument to legally own a muzzle loading, blackpowder, recoiless rifle, provided the ignition cap is not fixed the to ammunition.  I'm not the first person to have that idea, but there is no good info about anybody actually building it.  I won't be building one either, it just sounds cool.

 

[sevenperforce]

6 minutes ago, farmerben said:

I observe that military reccoiless rifles have either cone exhaust or a straight tube, often with multiple venturis to spin the gas.  If you watch them fire, the exhaust usually kicks up a cone spreading to mushroom shaped cloud of dust.  

Based on what I know about rocket nozzles this does not seem optimal  Directing the escape gases straight back provides the greatest impulse to the projectile.  So a bell nozzle with a straight throat would be fine.  And also an aerospike might be awesome for the exhaust of a recoiless rifle.   Imagine a sharp tongue of flame pointing straight back behind the shooter. 

A recoilless rifle does not use rocket exhaust to impart impulse to the projectile. Rather, the projectile is fired using explosive pressure out of a rifled barrel in the same way as a conventional gun. The difference is that there are openings in the back of the barrel which allow a portion of the exhaust gases to escape out the rear of the gun, producing forward thrust to the gun (not to the projectile) to counteract the recoil "kick" from the shot.

The primary job of the exhaust gases is too push the projectile down the barrel so that's what the system is optimized for. The holes which allow spent exhaust to escape rearward and counterbalance recoil cannot be optimized for thrust, so the escaping exhaust is never "choked" as in a conventional rocket engine. It has more in common with air escaping from a balloon.

Note that despite the appearance of an aerospike producing a straight column of fire is deceptive; it is less efficient at any altitude than a conventional nozzle sized for that altitude.

[farmerben]

Good reply, severperforce!  

So the more choked the exhaust the higher the pressure driving the projectile.  

The shape of the exhaust nozzle determines how much forward the thrust the gun will get, to counterbalance the rearward force on the interior face of the choke.  The difference between a good and bad nozzle is mainly how much recoil is felt, less so the projectile.  

A reccoiless also had to deal with sudden changes in pressure and flow rate.  In muzzle loading arms it is not uncommon for the breach face to be round or conical, allowing more propellant to burn late.