Quick Test Firing
I took a little time to mess around with the barrel bends again. I somewhat accidentally discovered that the PVC glue bonds strong enough to press fit barrels and risers into the barrel bends. This will eliminate two of the metal parts I had planned to use. I also did some test fires through the barrel bends. Everything worked great except the guns seemed to be firing a little soft. I will probably have to wait until the weekend to investigate more.
- Blog Topics:
- Brian's blog
- Login or register to post comments
Comments
Re: Quick Test Firing
Well those barrels look to be VERY short which would not help their final velocity at all. How tight is the fit of the bearing in the barrels and such? Assuming the junctions between the risers, bends and barrels are smooth, you should get excellent energy transfer through the bends since they are a perfect circular arc. In other words, the motion of the bearing can be modeled like a pendulum in that section. However it would be interesting to make some parabolic bends and see how the transfer goes.
Re: Quick Test Firing
The ID is about 1/4" for the 7/32" rounds - which is not bad (Lion Cub's barrels are substancially larger). Because of the gap I was guessing that the barrels did little more than straiten out the trajectory. You could be right though... they might help the round along a bit too.
I also had the breech pins way too low and no other rounds in the cannons. This is known to reduce their effectiveness.
The last like cause is not having sealed magazines is causing too much presure drop. I might have to go with a little bit bigger accumulators.
Re: Quick Test Firing
From my eperience with potato guns, I know that barrel length has an important role in the exit velocity. Basicly, if the barrel is too short, then the round doesn't spend enough time in the barrel being accelerated by the gasses and so much energy is wasted. On the other hand, if the barrel is too long then the round will actually be slowed by friction between the round and the barrel and some other factors. IIRC you need to do isentropic expansion calculations to determine what a good barrel length is. I also know there is a nice pneumatic spudgun calculator out there somewhere that makes this sort of analysis pretty easy and which you might be able to use to analyze your guns. I do think your barrel diameter is probably excellent.
I would try longer barrels and fix the other obvious issues before messing with those "cute" accumulators.
One thing that occured to me this morning is that you might be able to use your rotary table to make "rifled" barrels. Can you mount the rotary table to your mill vertically?
Re: Quick Test Firing
I took one of battleship Lion's cannons out and mounted it in the same test hardness. They still hit very hard (as some of you can testify). So the problem is not the test setup.
I also compared accumulator volumes... Lion's are about 6 cu in compared to Nassau's 5 cu in. Lion fires three 1/4" rounds at a time to Nassau's two 7/32" rounds. So I don't think the 1 cu in is the problem but it might be.
To test if the 1 cu in is likely to be the problem I obstructed the feed tubes to almost eliminate the gas leaking into the magazines. If the 1 cu in was enough to make the difference I would expect that eliminating the massive waste up the feed tubes would have corrected it. It didn't. In fact, there was no noticable difference in how hard the cannons fired. This seems to rule out the lack of a sealed magazine as the source of the problem. It also strongly suggests that the slightly smaller accumulator isn't the problem either.
So it looks likely that it is either barrel length or the tight arc. It looks like proving this will be my project next weekend.
Re: Quick Test Firing
What is the ratio of the length of Lion's barrels to that of the barrels on Nassau? What pressure are the accumulators set to?
Re: Quick Test Firing
Lion's barrels are about twice as long as Nassau's. Presure is probably around 90-100 PSI in the test setup right now.
Re: Quick Test Firing
Looks like the gold star goes to Drew. I attached a rough cut 3" barrel (same size I will need when everything is installed) and the power went up dramatically. I am surprised that the gas keeps imparting so much energy to the bearing even after the bend. I really thought all of the leaks would rob enough volume by then. This is really great news for me because it looks like the cannons will work just fine.
I will get some test foam this weekend or early next week and see if I can punch through it with the regulators max'd out. If not then I might try adding the extra cubic inch of volume and see how much that helps.