Daily Archives: August 15, 2014

Railgun water pistols

August is meant to be the silly season so here’s my contribution.

There is something about railguns that generates ‘I want one of those’ thoughts.

bae railgun

BAE Systems Railgun


See: http://www.popsci.com/article/technology/navy-wants-fire-its-ridiculously-strong-railgun-ocean

about the US Navy 5000mph projectile. Or:


Most people have no idea how fast normal missiles go so it’s hard to visualise it, but many missiles travel below the speed of sound and only a few go faster than Mach 2. So 5000mph is pretty fast. Few bullets go faster than 1000mph.

Rail guns are not the old-fashioned big guns that used to be transported by rail. They are electromagnetic guns that send enormous currents down two rails and use a projectile or sled to electrically join the two rails. Magnetic forces propel the sled at high speed.

Anyway, I wondered whether you could make a better water pistol for domestic use using rail gun technology. If you attempt to get rid of a cat from your garden by squirting it with water from a water pistol such as the wonderful Super Soaker, it runs away as soon as it hears the hiss from the escaping water, or so I’ve heard. Please don’t do this – cats are probably nice gentle creatures at heart that have the utmost respect for bird life, always poo in the bushes and away from the flower beds and are probably innocent victims of specist slurs.

Rail guns use either the projectile itself or a sled as part of the conducting circuit. Salty water conducts, so a water jet might work a bit, but probably not enough. Nor milk, for much the same reason. I suspect that far too much of the electricity would be converted to heat and you’d risk scalding the cat, which would be cruel. However, using a sled within the water pistol to push the water out would probably work a lot better.

The Navy railgun uses 25MW of electricity to fire its projectile at 5000mph, with 32MJ of energy. That is  probably a bit over the top for a water pistol and it is also illegal in some countries to make one that could be used as a serious weapon unless you have a license. It would also make rather a mess of the cat. 55mph or 25m/s is probably adequate (according to http://www.tomsguide.com/us/water-gun-tests,review-1313-9.html, a Super Soaker only has a muzzle velocity of around 50 ft per second or 15m/s, so 25m/s is a LOT better, but still less than half the speed of a BB gun), and by the time the cat takes a soaking it would be much slower so wouldn’t hurt it. Even for the same mass, E=0.5MV^2 rule says that 55mph needs about 10,000 times less energy, but we also don’t need as much mass. Even 10ml (two teaspoonfuls) of water is enough to wet a cat enough that it won’t come back for a while and small enough volume to fit in compartment to fire as a package rather than as long feeble squirt. 0.5 x 0.01kg  x 25^2 = 3.125J, roughly one ten millionth as much as the navy gun. (Or you could discourage 10 million cats using the navy version).

The water has to reach that 25m/s in the 75cm that a reasonable water pistol sniper rifle could be so it has to accelerate at 33m/s^2, only about 3.5g. That all sounds very feasible to me. 0.75=-.5 x 33 t^2 gives the time t as 0.2s, which gives a power requirement of 15W. Easy.

A quick googling shows that you can easily get 18v, 3.0 amp hr lithium ion batteries for reasonable price (below $75) that produce over 50W for up to an hour. A superior water pistol could use rapid fire to issue three such water packets per second, 60% faster than a Super Soaker, and emptying a 300ml water tank in 10 seconds. So our superior rapid-fire water rail gun sniper rifle sounds entirely feasible. Far better than a Super Soaker, and still far safer than a BB gun.

I want one of those.

I guess in principle, it doesn’t really have to be a rail gun. It could just use some sort of chain to push out compartments of water at the same speed, with the same power limits. But that would just be boring. Sometimes style is as important as substance.