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Registered Member #902
Joined: Sun Jul 15 2007, 08:17PM
Location: Pacific Northwest USA
Posts: 1042
hello all, last year I conceived a method to accelerate objects using vacuum pressure rather than compressed air. The reason being that one, the chamber is not under internal pressure, so if it ruptures it is a more controlled implosion rather than an explosion, and two that it does not need a tank once pumped down.
the basic idea is this: a barrel, with a proper fitting projectile, fitted with a vacuum capable valve behind the loaded projectile, and a fragile, yet vacuum capable, glass seal on the exit portion of the barrel. There are two vacuum pump fittings, one in front of the projectile and one behind it, to ensure that any trapped air behind the projectile won't expand and move it farther up the barrel. The projectile is snug enough so that when air is let in it will not allow air around it significantly, but will move with relatively little resistance.
To fire the cannon (once the vacuum is pulled, at which point the pump can be removed as long as the vacuum is maintained), one would open the rear valve. This would allow ambient air pressure ( 1.01x10^5 N/m^2) to act on the projectile, and in order to fill the vacuum, accelerate the projectile to forward. The projectile would them break the glass seal and exit the barrel.
Today the idea returned to me, so I decided to do some math regarding the potential, in ideal circumstances of course, for such a device minus factoring in the Force required to break the glass seal on the end of the barrel. I was quite astonished by the results!
I did the math for two barrel Area sizes, 1 m^2 and 0.2552 m^2 (radius 0.285 meters) - while these are a little large, the smaller value is somewhat practical and the larger I did really when I wanted to first crunch the numbers so I made it easy
area calculated using: pi * r^2 Force on Projectile calculated using: Pressure * Area Standard Air Pressure: 1.01x10^5 N/m^2 Final Velocity Calculated using: square root of (2 * Acceleration * Barrel Length) Acceleration calculated using: F = mass * acceleration
1 meter barrel => Vf = 116.043 m/s 5 meter barrel => Vf = 259.480 m/s 20 meter barrel => Vf = 518.960 m/s 50 meter barrel => Vf = 820.549 m/s 100 meter barrel => Vf = 1.160 km/s
10 kilogram projectile: Acceleration: 10.1 km/s^2
1 meter barrel => Vf = 142.127 m/s 5 meter barrel => Vf = 317.805 m/s 20 meter barrel => Vf = 635.6099 m/s 50 meter barrel => Vf = 1.005 km/s 100 meter barrel => Vf = 1.421 km/s
1 meter barrel => Vf = 58.623 m/s 5 meter barrel => Vf = 131.086 m/s 10 meter barrel => Vf = 185.383 m/s 20 meter barrel => Vf = 262.172 m/s 50 meter barrel => Vf = 414.530 m/s 100 meter barrel => Vf = 586.234 m/s
1 meter barrel => Vf = 71.799 m/s 5 meter barrel => Vf = 160.547 m/s 10 meter barrel => Vf = 227.047 m/s 20 meter barrel => Vf = 321.093 m/s 50 meter barrel => Vf = 507.693 m/s 100 meter barrel => Vf = 717.986 m/s
yes, some of those barrel lengths do read one hundred meters, they obviously would not be a portable cannon but rather for a testing site or similar complex, where normally you would use rockets or explosives to move something fast enough - I figure if air pressure can do it without excessively large air tanks it might be of use somehow and it seems logical that a barrel size of one square meter would be of use since usually those sleds they use are very large
also I used these numbers because they are simple numbers 9before calculations) and to get a feel for the magnitude of air pressure.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Hi Jimmy.
That was a good computation exercise, but beware of drawing conclusions (such as astronomical velocities) from crazy, unrealistic assumptions. For example: your 15 kg, 1 m^2 projectile, if made from a dense ballistic metal, would be about 1 mm long -- a sliding diaphragm.
Another orders-of-magnitude error is the expectation that ambient air pressure will apply to a rapidly receding projectile deep in a long tube. In your model, an arbitrarily light projectile could have arbitrarily high acceleration and ultimate velocity. A more realistic model would give a limiting velocity comparable to the speed of sound in air (330 m/s?) Search the Internet for "shock tube" (the aerospace instrument, not the plastic fuse used with explosives).
Your model would be reasonably accurate on the scale of a low power spud gun, and I think I've seen such vacuum guns (?) on YouTube. The front closure can be a cork, or just a stiff card held in place by suction. For rear closure you could try a triggered burst diaphragm, or mechanically restrain a close fitting projectile, instead of contriving a valve. Can't wait to see what you come up with.
Registered Member #902
Joined: Sun Jul 15 2007, 08:17PM
Location: Pacific Northwest USA
Posts: 1042
Klugesmith wrote ...
Hi Jimmy.
That was a good computation exercise, but beware of drawing conclusions (such as astronomical velocities) from crazy, unrealistic assumptions. For example: your 15 kg, 1 m^2 projectile, if made from a dense ballistic metal, would be about 1 mm long -- a sliding diaphragm.
Another orders-of-magnitude error is the expectation that ambient air pressure will apply to a rapidly receding projectile deep in a long tube. In your model, an arbitrarily light projectile could have arbitrarily high acceleration and ultimate velocity. A more realistic model would give a limiting velocity comparable to the speed of sound in air (330 m/s?) Search the Internet for "shock tube" (the aerospace instrument, not the plastic fuse used with explosives).
Your model would be reasonably accurate on the scale of a low power spud gun, and I think I've seen such vacuum guns (?) on YouTube. The front closure can be a cork, or just a stiff card held in place by suction. For rear closure you could try a triggered burst diaphragm, or mechanically restrain a close fitting projectile, instead of contriving a valve. Can't wait to see what you come up with.
Rich
I figured that such extreme numbers were bound to be impractical, hence I I stressed ideal circumstances, and I really just wanted to have simple numbers to work with (and as for the projectile in the full square meter, i would only expect something like a mostly hollow cylinder of aluminum or something - not really a ballistics projectile, but something to launch)
and I'll check around on youtube for it, and a spud gun sized version would be cool - and that way I won't have to have a pressurized PVC tube, but then again an evacuated PVC pipe might not be a good idea, so maybe iron pipe?
and your point on the flow of air through the long tube is a good one, however with a larger tube (again with theoretical, not practical) I don't think that would be an issue, but with longer and thinner barrels I can see that being a problem
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
DaJJHman wrote ...
... but then again an evacuated PVC pipe might not be a good idea, so maybe iron pipe? and your point on the flow of air through the long tube is a good one, however with a larger tube (again with theoretical, not practical) I don't think that would be an issue, but with longer and thinner barrels I can see that being a problem
I see no safety problem with evacuated PVC tube. Spudguns use -internal- pressures of a few atmospheres.
On the other point: it's not an issue of resistance to flow. No matter how large the diameter, the energy of expanding gas has to accelerate the mass of the gas itself -- that's where the speed of sound comes in. [edit] I think the best rocket motors, with engineered converging/diverging nozzles, achieve only a few times the speed of sound in the exhaust gas.
Registered Member #902
Joined: Sun Jul 15 2007, 08:17PM
Location: Pacific Northwest USA
Posts: 1042
Klugesmith wrote ...
DaJJHman wrote ...
... but then again an evacuated PVC pipe might not be a good idea, so maybe iron pipe? and your point on the flow of air through the long tube is a good one, however with a larger tube (again with theoretical, not practical) I don't think that would be an issue, but with longer and thinner barrels I can see that being a problem
I see no safety problem with evacuated PVC tube.
On the other point: it's not an issue of resistance to flow. No matter how large the diameter, the energy of expanding gas has to accelerate the mass of the gas itself -- that's where the speed of sound comes in.
I see, I didn't realize that air flow was limited by the speed of sound. But is it limited in actual speed of travel by the speed of sound, or does it affect it in another way? I assume it would be calculable?
also as for the PVC, I refer to the danger if it breaks under a vacuum because if it does, the result is less violent than a PVC pipe with compressed gas, but still not nice - maybe I can just use some plastic netting to keep things in if something happens?
EDIT: after ten pages on youtube, I have yet to find a video of this under "vacuum gun" - was the one you are thinking of under a different title? - all I keep finding are people shooting vacuums, or people making fake guns from vacuums...
EDIT 2: even on Google all I can find so far is an article from the New York Times, from 1882 and there is not a coherent description of operation that lets me know if it is a similar design, or something completely different...
Registered Member #902
Joined: Sun Jul 15 2007, 08:17PM
Location: Pacific Northwest USA
Posts: 1042
Klugesmith wrote ...
Sorry, wrong name. Note that there is theoretically no recoil force on the barrel.
thanks for the links, I might see if I can convince the school to let me use their vacuum pump, or maybe I'll save up $100 and get one from Harbor Freight since a spud gun sized device won't need a very deep vacuum for testing purposes
a non-valve trigger might be a little tricky, but I'll see what I can figure out, maybe just a diaphragm that will last until a decent vacuum is attained, and have the vacuum hose at the front of the barrel
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
I see, I didn't realize that air flow was limited by the speed of sound. But is it limited in actual speed of travel by the speed of sound, or does it affect it in another way? I assume it would be calculable?
The air can't move faster than the speed of sound into the barrel because that is how fast pressure changes travel (sound is just pressure changes). The speed of sound depends on the temperature and since the air expands as it moves through the restricted opening it will cool down a bit so the speed of sound will be lower.
The only realistic ways to go faster is to change the gas or heat it up.
Registered Member #193
Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022
Bjorn's right. Another way to look at is that the speed of sound is roughly the mean speed oc the gas molecules. If the projectile is already going that fast, the molecules can't catch it up to push on it.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
It's more or less the same as a regular spudgun pressurized to 15psi, or whatever atmospheric pressure is these days.
One square meter is about 1600 square inches, so if you put 15psi behind it, that's a force of 24,000 pounds, which is where your incredible acceleration figures come from.
Doesn't matter if it's atmospheric pressure behind and a vacuum in front, or 2 atmospheres (30psi absolute = 15psi gauge) behind, and 1 atmosphere (15psi absolute = 0psi gauge) in front. In both cases the differential is 15psi and the net force 24,000lbs.
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Vacuum Acceleration Launcher
