Author Options:

Spring-loaded cannon Answered

My friend and I are considering building a spring loaded cannon. The idea right now is to have a pvc pipe for the cannon chamber, and pvc pipes flanking it to hold springs in parallel. A cross-bar will go through all 3 pipes through slits cut long ways in the pipe. The springs will be attached to some point forward in the flanking pipes and to the cross bar, which will be used to pull the springs back and, once released, will launch the dart/ball in the center tube.

My research so far - the spring rates of springs loaded in parallel are additive. With 5x 10-in*lb springs, we can produce 667 newtons of force when pulled back 3 inches (springs in question are 5.5 in long) . That will produce an acceleration of 303 m*s^2 on a 1 lb (2.2 kg) albeit for only a fraction of a second.

http://www.amazon.com/Extension-Spring-5-625-Long-080-count/dp/B001292S08/ref=sr_1_1?ie=UTF8&s=industrial&qid=1257718057&sr=1-1 These are the springs we're looking at. We'll need to use an even number of springs, so either 4 or 6 (or 8 or 10).

So, what do you guys think? Practical? Will we be able to get any distance? Any suggestions?

A ROUGH picture is shown below. === are spring tubes, | are handles, and left arrows indicate direction in which springs will be stretched, and the right arrow is a dart flying out of the center launch tube.

XXXXXXXXX     ------>


just a correction on paragraph 2, sentence 3: 1lb isn't 2.2 kg. It's the other way around (1kg is around 2 lb)

This all looks quite reasonable, and you've really done a good job with the quantitative analysis. 

You probably want to go with a ball (like a tennis or squash ball) rather than a dart, for two reasons:  a lightweight dart is likely to tumble in the air, rather than flying straight; and if it does fly straight, it constitutes a substantial danger to anyone who walks downrange (like the guy walking his dog that you didn't know about).

30g acceleration out the barrel is pretty decent.  If you know the duration of the throw (is it likely to be tenths of a second or milliseconds), then you can estimate the muzzle velocity from v = at, and from that, plus a 45-degree ballistic trajectory, estimate your range. 

For example, assume 1/10th second for the launch, then you get a muzzle velocity of 30 m/s; at 45-degrees, that's 21 m/s vertical component, so about 2.1 seconds to reach maximum height, 4.2 seconds full travel.  You ought to get about 120 meters out of the shot.

My son & I are going to attempt something similar - how did this work out for you?

You might want to repost this comment at the top level, so the original author (as opposed to myself, who merely replied) will see it.

Thank you for the complement! Of course you're totally correct about estimating distance, but I have no way to measure how long it takes the springs to release their energy. It's definitely a fraction of a second though. I like the tennis ball idea. Not only do they weigh ~2 oz., which is 1/8 of the weight of the dart mentioned earlier, but they can be lit on fire. Accidentally, of course.

I just realized something (it's been too many years since I've done homework problems in classical physics :-).  You don't need to know the snap-time of the spring to solve the muzzle velocity problem.  Conservation of energy does it for you.

You know the spring constant (10 lb/inch; what Amazon writes on their Web site is the wrong unit), so you can compute the potential energy (= 1/2 kd2) stored in the springs when they're stretched a distance d.  Neglecting friction in the tube, all that P.E. is converted to the initial kinetic energy of the projectile, so  1/2 kd2 = 1/2 mv2.  Solve for v = d*sqrt(k/m) (make sure you include g to convert weight back to mass :-) and you have the muzzle velocity.

u would need some big sprngs, my spring bb gun cant pop a balloon at point bank range

The problem with springs is that, even the best of them, eventually start to compress and hold that shape (lose their tensile strength)

You're right, but the springs we're considering are sold in 5 or 10 packs for pretty cheap. We'll probably have to make sure they're replaceable.

Depending on how you are going to support the 3 barrels (especially the center one) with a slit through them lengthwise, and depending on the length and gauge of the barrels, you may find a few problems with it drooping and jamming.

That's a good point, we'll probably have to bolt them together at the front and back. Long front bolts, with the head inside the launch tubes through the spring tubes, could be used to hold the front end of the springs in the spring tube. A single long bolt through the spring tubes and launch tube in back would stabilize it a lot, and shouldn't get in the way of anything. Thanks, I'll make sure to take that into consideration.


8 years ago

The plan is to use extension springs, by the way.


8 years ago

This project sounds like good times. I'm not sure how far you are looking to launch the darts, but I might suggest making them heavier than 1 lb . You'd of course have to increase either the number of springs or the distance you pulled them back to get the same distance. I'm not sure what the standard spring is rated for, but I suspect it would be easier to find springs that can be pulled farther back than to it would be to engineer more than four around your central column.