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Does 6x + a 20 foot barrel = Supersonic?

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Unread postAuthor: MrCrowley » Fri Aug 17, 2007 8:17 pm

No, a chrono has an in-built computer if you like, the most common ones have two rods verticle in the air in the shape of a V, they connect at the base to a small computer type box thing which calculates the velocity, which then shows up on a LCD display or something.
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Unread postAuthor: noname » Fri Aug 17, 2007 8:59 pm

MrCrowley, I am a paintballer and am fully aware that not all chronographs need to be hooked up to a computer. I'm pretty sure the homemade chronograph that jimmy has a page on needs to be hooked up to a computer but I'm not sure, because it's been a while since I've seen the page.
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Unread postAuthor: MrCrowley » Fri Aug 17, 2007 9:25 pm

Ah my bad, I thought you meant if you had a proper one not a homemade one. You can hook the laptop/computer up outside if you have extention cords, or if this is the microphone homemade chrono you can just have the microphone on a long cord.
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Last edited by MrCrowley on Fri Aug 17, 2007 10:35 pm, edited 1 time in total.
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Unread postAuthor: noname » Fri Aug 17, 2007 9:58 pm

A homemade chrono is just not going to happen. My friend has one, I'll go over there sometime.
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Unread postAuthor: dongfang » Sun Aug 19, 2007 9:53 am

noname,

Nice pipeline, eeer, cannon.

Yes quit speculating and start measuring.

I wanted to make a complete and documented presentation before saying anything, but anyway:

Get some thin copper wire.

Get a length of pipe, 1 ft or so, large enough to shoot through (maybe 2 x your barrel diam).

Wind 100 turns at each end of the pipe. Wind nice and tight (not distributed across more than a few mm).

Wind a 200 turn coil on a length of pipe. This will be the trigger coil; it goes on the barrel of the gun.

Put a 1 kOhm resistor in series with each coil. Put all the coil-resistor combinations in parallel. Connect to the mike input of a pc sound card.

Get an old hard disk drive, blast it with the gun, and get a fragment of the magnets from the arm assy. About 2 x 2 x 2 mm. Build fragment into projectile, so that one smooth side faces forwards and one backwards.

Get a sound card oscilloscope program for your computer. Place 2 pipes in front Hook up everything.

The only hurdle is in getting the oscilloscope to trigger, so the data recording will start at the right time. That´s what the trigger coil is for.

Make a few test runs where you just drop the projectile through a pipe that goes through all the coils. When measuring for real, turn down sensitivity to 1/10 or so - voltages will get a lot higher.

I assume it´s obvious to everybody how to read the graph on the oscilloscope ;)

Regards
Soren
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Unread postAuthor: noname » Sun Aug 19, 2007 9:56 am

I have no idea where to find any of the stuff, how much it will cost, or anything about anything electronic, other than I can't get anything to work. Even if I do get everything right, it won't. I'll drop in on my friend this week.
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Unread postAuthor: userjjb » Mon Aug 20, 2007 6:00 am

If I had to make a WAG on muzzle velocity, I'd say somehwere in the range of 1500-1600 fps from what I've seen out of similiar hybrids, and some rough models in GGDT based on your specs. We don't have much data from hybrids at higher mixes though, so we might all be suprised.
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Unread postAuthor: Marco321 » Mon Aug 20, 2007 6:11 am

Hey
Nice gun, i don't want to be on the wrong side of that beast

I had a thought about getting an idea of it goes supersonic. If it does, it means it should hit the target before the sound reaches the target. So, if you could rig up a camera with the speaker inline with the target, you can film the shot. Then replay it in super slow motion and see if the target gets hit before you hear the bang on the camera.
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Unread postAuthor: jackssmirkingrevenge » Mon Aug 20, 2007 6:23 am

I think the penetration figures alone are an indication of supersonic velocity.

Not exactly scientific, but perhaps you could do something like this and see how the performance compares? Of course different projectile diameter and weight, paper density etc. make it impossible to determine velocity but at least you can see how the performance compares to firearm rounds.
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Unread postAuthor: jimmy101 » Tue Aug 21, 2007 10:45 am

dongfang wrote:noname,

Get some thin copper wire.

Get a length of pipe, 1 ft or so, large enough to shoot through (maybe 2 x your barrel diam).

Wind 100 turns at each end of the pipe. Wind nice and tight (not distributed across more than a few mm).

Wind a 200 turn coil on a length of pipe. This will be the trigger coil; it goes on the barrel of the gun.

Put a 1 kOhm resistor in series with each coil. Put all the coil-resistor combinations in parallel. Connect to the mike input of a pc sound card.

Get an old hard disk drive, blast it with the gun, and get a fragment of the magnets from the arm assy. About 2 x 2 x 2 mm. Build fragment into projectile, so that one smooth side faces forwards and one backwards.

Get a sound card oscilloscope program for your computer. Place 2 pipes in front Hook up everything.

The only hurdle is in getting the oscilloscope to trigger, so the data recording will start at the right time. That´s what the trigger coil is for.

Make a few test runs where you just drop the projectile through a pipe that goes through all the coils. When measuring for real, turn down sensitivity to 1/10 or so - voltages will get a lot higher.

I assume it´s obvious to everybody how to read the graph on the oscilloscope ;)

Regards
Soren


Sounds like a lot of work. Heck, for a large bore barrel (bigger than 1") a pair of phototransitors is a heck of a lot cheaper and you don't need to modify the ammo.

Heck, with big ammo you could just get a pair solar cells (or even CDS photocells) for the detectors.

BTW, if anyone tries this, the "thin copper" wire needs to be insulated. Since standard insulated wire wastes too much space for the insulation, you should use magnet wire. Radioshack might have it, if not, rip apart an old moter and use the wire from the windings. If the wire is bright copper colored it is coated with lacuer and is insulated wire. You should double check with a VOM.

The transparent coating needs to be removed from the ends where you'll be making electrical connections or it won't work.
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Unread postAuthor: dongfang » Fri Aug 31, 2007 5:12 pm

Hi, Jimmy,

Heck, with big ammo you could just get a pair solar cells (or even CDS photocells) for the detectors.


Do you know how slow CDSs and solar cells are? They need dozens to 100s of milliseconds to see anything.

Yes, the nicest solution is probably a phototransistor based one. But either you will need a fancy modulated light source, or an AGC input circuit, or both. Or you will have to make major readjustments every time a cloud or a tree gets in front of the sun. I have lost patience with simple phototransistor circuits.

The coil set-up is simple; it is just .. copper! Well, lacquered copper. I got this kind of plots on the scope program last time I cronied:

Just a suggestion anyway :)

Regards
Soren
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Attachments
20mm.PNG
Oscillogram from the firing of a 20 mm sprinkler valve gun. When knowing the spacing of the coils, velocities and accelerations can be calculated. This particular shot was 150 m/s average between the last 2 coils. The low amplitude of the first pulse is because of the low velocity there; that´s why I suggest more windings on an extra, 0´th trigger coil.
20mm.PNG (31.96 KiB) Viewed 490 times
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Unread postAuthor: jimmy101 » Sat Sep 01, 2007 1:32 pm

dongfang wrote:Hi, Jimmy,

Heck, with big ammo you could just get a pair solar cells (or even CDS photocells) for the detectors.


Do you know how slow CDSs and solar cells are? They need dozens to 100s of milliseconds to see anything.

Yes, the nicest solution is probably a phototransistor based one. But either you will need a fancy modulated light source, or an AGC input circuit, or both. Or you will have to make major readjustments every time a cloud or a tree gets in front of the sun. I have lost patience with simple phototransistor circuits.

The coil set-up is simple; it is just .. copper! Well, lacquered copper. I got this kind of plots on the scope program last

time I cronied:

Just a suggestion anyway :)

Regards
Soren


Solar cell do indeed respond slowly. Thought it doesn't matter if the time constant is in the tenths of a second range, all you need is a measurable change in 0.1mS range, you certainly don't need 100% response, heck a 1% response is probably enough.

Why in the world would you need a modulated light source for a phototransistor detector? All you need is a constant source that is sufficiently brighter than ambient. Heck, point the PTs down towards the ground and have an IR LED pointing back up at it. At the range of a couple inches in the IR LED will be much brighter than the IR reflection off the ground. Passing clouds etc. won't make any difference.

With my homemade chrony I just use the bright sky as the light source. Passing clouds etc. have no affect on the performance. The only trick is that the PT will saturate in fairly low light, so there is no delta signal between the low light level and the high light level. Easy enough to correct, just put a small aperture in front of the PT(s) so the field of sky they see is small. I've found that a 1/16" or 1/32" aperture will make the PTs work, without any calibration or correction, in lighting conditions ranging from full direct sun all the way down to indirect light in my garage at night (a couple 100W bulbs).

Your magnetic based detector is cool, and clever, but it requires mod'ing the ammo (adding the magnet) and is not easily transferred from gun to gun. In addition, I don't think it will work on a metal barrel.

Here are recordings of a BBMG firing (copper barrel)
Each peak is a BB, the ROF is easy to determine:
Image
Zooming in to see the peak pairs for each BB, from which the muzzle velocity can be calculated.
Image

These recordings are of BBs which are much harder to see because they are so small. Not sure how you mod a BB so it can be detected with a coil. (You might be able to energize the coil and measure the very small voltage spike that a conductor passing through the magnetic field would produce.)
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