Rangefinder-Adjusted Electric Crossbow

The crossbow is a more complex derivative of the ancient recurve longbow. To be an effective weapon, a longbow requires an operator with decades of physical adaptation, experience and constant practice. As such, the trained archer gained social status and value befitting their worth on the battlefield. Conversely, the crossbow can be wielded by almost any disposable shambolic clodhopper. This means a reasonably effective force can be made out of the nearest group of low-grade troglodytes. This explains the popularity of crossbows among the French.

To draw a light crossbow, the operator can simply pull back the bowstring. Heavier, more powerful bows for warfare require a separate mechanism. Stronger peoplw would be one way, another promising direction might be to use trained primates. I reckon a gorilla would handle 500lb with minimal fuss and would be handy when it came to the melee, but "Mixed Primate Archery Corp" is an idea for a different day.

Real world mechanisms included one system that used a loop on the front of the bow that was held down with the foot while the string was lifted via a hook attached to the belt. This allowed use of the stronger back/leg muscles and therefore a more powerful bow. A second system used a hand crank or windlass arrangement. The string could be wound back, spreading the work load over several seconds.

The windlass arrangement could easily be modernized. The power demands for a modern hunting bow are large but manageable. Brief research suggests 350lb draw weight over an average of 13" is typical for the larger end, which is where we want to be. Converting that to sensible units gives a draw energy of ~4800J, which if you wanted to load in say 3 seconds requires 1600W. That is at the larger en of "hobby grade" electric motors, so we're in business.

Those motors are big, and an electric crossbow already exists <link> where they've clearly had to dial down the draw weight (230lb) to compensate for the bad engineering.

So, innovation #1. Have the cocking mechanism mounted with a spring opposing the force of the main spring system. This will work a bit like the counter weight in a lift/elevator system. Rather than have a brutish motor that can lift the fullest car, have a counter weight of half the maximum so you can use a smaller motor. Here, the electric motor will move the cocking mechanism forward, compressing a spring of say half the main spring value. 350/2 = 175lb in my example. It hooks the bowstring and moves backwards with half the force coming from the motor, half from the spring. Now you could use say a 1000W motor, get a faster draw and a much more managable motor size.

Innovation #2. When you want to shoot stuff, particularly stuff that dislikes being shot, the stuff seldom hangs around at exactly the range you have your scope dialed in for. With a rifle, and to some extent bows, you have a projectile designed to give as consistent a flight path as possible. To do this, they are often fired with the exact same energy, as far as can be controlled. Then, with calibration, you can alter the angle of the scope to the barrel and have the object you're looking at be on the exact flight path of the bullet.

More sophisticated guns, such as those aboard HMS Warspite used to open the D-Day bombardments, can vary the amount of energy given to the projectile, the flightpath can be modified by angle and power independently*.

We can do the same with our electric crossbow. Imagine looking down your scope and panning across a landscape looking for stuff to shoot, as you pan, the objects in view will be at different ranges, there might be a bush 50 yards ahead or a collection of trees 125 yards away. The laser rangefinder built into the scope relays the distance to the objects you are surveying and adjusts the power of the bow proportionally by moving the string forward/backwards through the travel.

I imagine a cylinder/piston arrangement with appropriate pressures on either side of the piston would make a cleverer spring design.

*You can use this to pull some clever tricks, such as shooting a great high arc and a shallow lob from the same gun so that 2 shells arrive at the same time.