Rail-Guns: Recoil Comparisons and Momentum Questions

[Jasper]

I have a question about rail-guns. Let’s say that you have a rail-gun and a normal gun both have the same muzzle velocity and fire a projectile of the same mass. Which of these two weapons would have the most recoil?

It seems to me like this problem involves momentum right? Is there such a thing as momentum absorption? Something like force*time?

If so then the rail-gun would seem to have the higher recoil. I am probably not even on the right track… Could someone explain this better and which one would recoil more?

Thank

 

[Astronuc]

The recoil is the result of the force require to accelerate a projectile - equal and opposite (reaction) forces.

The guns are much heavier than the projectiles, but the projectiles can be accelerated to very high velocities in short distances.

With the same projectile mass and muzzle velocity, the forces on the rail-gun and a normal gun would be the same. The response of the rail-gun/normal gun depends on their masses. The heavier the gun the less recoil acceleration (and lower momentum) if would experience.

If the normal gun and rail-gun had the same mass (and length over which the projectile is accelerated), they would experience the same recoil for the same projectile mass and acceleration.

 

[Jasper]

This would be true even though most of the normal guns acceleration occurs earlier but the rail-gun accelerates its projectile at a constant rate? The recoil force would not feel more spread out?

 

[NateTG]

I have a question about rail-guns. Let’s say that you have a rail-gun and a normal gun both have the same muzzle velocity and fire a projectile of the same mass. Which of these two weapons would have the most recoil?

It seems to me like this problem involves momentum right? Is there such a thing as momentum absorption? Something like force*time?

If so then the rail-gun would seem to have the higher recoil. I am probably not even on the right track… Could someone explain this better and which one would recoil more?

It's not necessarily something that gets gone over explictly in class, but:
Force * time = mass * velocity = momentum

The recoil is essentially the same.

In practice conventional (i.e. chemical propelled) artillery launches things other than the projectile (like some of the explosive gasses), and because conventional artillery does not have constant acceleration capability (conventional guns usually have most of the acceleration take place at the beginning of the shot), odds are that the conventional artillery piece would experience more recoil, both in the sense that the maximal force on the gun is higher, and in the sense that the total momentum change of the gun is higher. However, this is an practical efficiency issue, rather than some fundamental difference between the guns, and it's readily concievable that a rail gun that uses some kind of sled to accelerate a projectile could have more recoil per projectile mass than a conventional gun.

 

[russ_watters]

This would be true even though most of the normal guns acceleration occurs earlier but the rail-gun accelerates its projectile at a constant rate? The recoil force would not feel more spread out?

In both cases the acceleration is so high it may as well be instantaneous: the bullet is out the muzzle before you notice that the gun is recoiling.

 

[Astronuc]

If the normal gun and rail-gun had the same mass (and length over which the projectile is accelerated), they would experience the same recoil for the same projectile mass and acceleration.

The original statement did not specify how the charge-propelled projectile would accelerate. Clearly, if the acceleration of the chemically propelled charge is higher than the acceleration of the mass in the rail-gun, then the reaction force on the normal gun is higher. On the other hand, the force (pressure * area) of the normal-gun chamber falls off as the projectile moves forward in the chamber, so the acceleration, which is initially higher than that of the rail-gun, decreases to a level below - given the constraint on the problem that the muzzle velocity is the same for both cases.

 

[Hdeasy]

The recoil in the rail gun is unusual as the force is from the cross product Lorentz force, so the immediate reaction at the projectile on the rail is sideways, i.e. not linear. The back reaction is curiously due to the force on the battery caused by the magnetic field of the side bars on the current flowing between the electrodes.