# Spring loaded musket

howdybilly
disclaimer: bizarre question from physics novice

Using a smooth bore classic musket (approximately 1m length of barrel, musket ball (.0173m diameter)), what would the potential muzzle velocity of the musket ball be if the powder charge were replaced with a compressed spring?

PE=1/2kx^2

KE=1/2mv^2

Using a spring with k=128 Nm (used a calculator on engineer's edge where modulus of ridgity G was 25x10^6, wire diameter d was .2 inches, mean coil diameter D was .48 inches and number of active coils n was 40). Not sure if this is kosher or not.

Assuming friction and gravity are negligible - the musketeer keeps the weapon well lubed and is on the moon, if he pulls the trigger which releases the "charged" spring, what will the muzzle velocity be of the musket ball when it leaves the musket? Let's also assume that the spring when uncompressed, is the full length (1m) of the barrel.

PE=1/2*128*1^2=64 joules?
KE=64=1/2*.031v^2 so v=64.26m/s

64.26m/s, long way from a 1700's .69 caliber musket firing around 310m/s

Could that be right? What am I messing up?

## Answers and Replies

JaredJames
64.26m/s, long way from a 1700's .69 caliber musket firing around 310m/s

Could that be right? What am I messing up?

Given you know the exit velocity of the rifle, you may want to work it backwards - get the KE, stick it into the PE equation and see what spring you would need.

Then you can see if the required spring matches what you've got now.

howdybilly
Given you know the exit velocity of the rifle, you may want to work it backwards - get the KE, stick it into the PE equation and see what spring you would need.

Then you can see if the required spring matches what you've got now.

I did it backwards and entered the new spring constant k as 2979.1 Nm, plugged it into the spring calculator on the engineering website and keeping the diameters and modulus of ridgidity the same, I came up with a change in the number of active coils from 40 to 5.82.

I think there are probably some nature of elasticity issues that I'm neglecting but not sure.

so, rewritten - given a 1m long musket, standard .69 cal musket ball (approx .031kg) and a spring under compression with a constant of 1489 Nm (modulus of ridgitity 25x10^6psi, .25in wire diameter, .43 mean diameter and 5.82 active coils), a muzzle velocity of the musket ball could match the 310 m/s of a traditional powder musket.

PE=1/2*2979*1^2=1489.5 joules
KE=1489.5=1/2*.031v^2 so v=310 m/s

?? not sure if I'm using the right calculations. is a spring constant of 2979 Nm possible?

thanks

JaredJames
?? not sure if I'm using the right calculations. is a spring constant of 2979 Nm possible?

Not sure, but I'd say definitely not with the dimensions you're talking about.