How to generate energy from recoil?
Yesterday I was reading through this question and it got me thinking: would it be possible on a ship or building mounted railgun to convert the energy from the recoil of the shot in order to partially recharge its capacitors for the next shot?
This post was sourced from https://worldbuilding.stackexchange.com/q/49429. It is licensed under CC BY-SA 3.0.
1 answer
Using Michael Karnerfors' answer as a basis for the numbers, only instead of directly attaching the rail-gun to the ship so that everything moves as a whole, attaching the gun to a flywheel (or something similar), which is then connected to the ship so that only the gun moves (and the flywheel rotates) so that KERS can be used as in AmiralPatate's answer allows for the kinetic energy of the gun to be recovered:
Note: This answer takes the rail-gun to be intrinsically 'not the ship', so that the gun will move regardless of how it is attached. While it could also be completely built in to the ship (or built into the ground) so that it cannot turn/move independently of whatever it's on, in which case Michael Karnerfors' answer applies completely, we like our guns to be able to do things like turn and maybe even move independently of whatever it's placed on.
Taking the mass of the rail-gun to be $~1.5\times 10^5kg$ as in here, gives the speed of the railgun immediately after firing as $v = \frac{2.5\times 10^4}{1.5\times 10^5} = \frac{1}{6}m s^{-1}$, which gives a kinetic energy of $$KE = \frac{1}{2}\times 1.5\times 10^5\times \frac{1}{36} = \frac{6250}{3}J$$
At this point, there are two options: either the gun can be directly attached to the ship without any recoil and whichever method used of attaching it needs to withstand the large stresses that this creates or some recoil system can be used to help prevent these stresses. Using the recoil option allows for KERS as well as decreasing these stresses, which decreases damage done to the ship/the mechanism that attaches the gun to the ship. Otherwise, the resulting reaction force that exists as a result of the projectile leaving the gun will cause a torque that results in a stress on the mechanism that attaches the gun to the ship. Using something that decreases this force (something that dissipates recoil) decreases this stress and so decreases the damage done to this mechanism.
To forestall the argument about needing to put as much energy into the projectile as possible, I'll point out now that the momentum and energy of the projectile are already well defined and the mass of the overall system or how the gun recoils doesn't change this. It does increase the amount of energy needed to be put in the system in the first place, but we want the gun to move around a bit, so it'll have to have some recoil in this case anyway (as well as for the above reasons).
In any case, the energy required to fire said projectile is $3.125\times 10^7J$, about 4 orders of magnitude greater than the recoil of the gun, so, while KERS can be used in principle, in practice, there is no real point. Decreasing the mass of the gun is pointless as more energy is needed overall.
However, there are large amounts of excess heat generated and large amounts of electricity used in a rail-gun, so there may be other ways of regenerating some of the used energy. Having said that, rail-guns that have been created so far have a tendency to get heavily damaged, so allowing the guns to move might decrease the amount of damage caused to the guns.
In summary:
It is possible to use energy from the recoil to partially charge the capacitors, but in practice, you're recovering less than 0.1% of the energy required to fire the next shot, so there's no real point unless it decreases damage done to the gun
0 comment threads
0 comment threads