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Measure projectile velocity

A place to ask general spud cannon related questions.
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Measure projectile velocity

Unread postAuthor: Rock » Fri Aug 31, 2007 7:06 am

I've found a cheap way to measure the projectile velocity.
I used:
2 switches
10m cable
stop watch

Looking at the old stopwatch, it has a resolution of 0.01seconds i had the idea for this.
I soldered two wires to the start stop button and connected the two switches in parallel.
One switch is attached to the tip of the barrel ( This is the tricky part.) and is activated by the projectile in the moment it leaves the barrel. The other switch is placed at the target behind a board or something that will be pushed on the switch when the projectile hits it.
The switches, botch activated by the projectile, start and stop the stopwatch, so the activation should be fast enough for a quite exact result.
My disance is 10m (easy to calculate with).
Made two pneumatic shots at ~130psi and got these results
0,26s
and
0,24s
Now divide distance by time to get the velocity:
d/t = v
10m/0,24s = 41,66m/s ~150km/h

worked very good this time but is won'T work for every launcher and is quite a lot of work to do and in some parts tricky.
Maybe i can add photos later.
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Unread postAuthor: paaiyan » Fri Aug 31, 2007 7:51 am

Hmm, that's actually a pretty good idea. The only problem I can think of is the first switch might slow the projectile down some.
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Unread postAuthor: jimmy101 » Fri Aug 31, 2007 11:51 am

This is a good way to make a crude chrono. You can use any old digital watch with a timer function.

This approach does have some limitations however.

1. The resolution of most cheapo digital watches is 0.01 seconds. In order to get a result accurate to +/- 10% the total time interval has to be at least 0.1 second (ten times the resolution). Rock's result, with a dT of ~0.25sec, are accurate to +/-0.01 sec, about a 4% error, which is OK.

2. Since the time resolution is not very good, you need to put the second switch a long way from the gun. The resulting velocity is the average velocity over that distance, not the muzzle velocity. Rock's average velocity of 42m/s is only 134FPS.

3. With a gun that fires at say 500 FPS (152m/s), and to get an accuracy of 10%, the second switch would have to be (500 fps)(0.01second)*10 = 50 feet (15m) from the muzzle. To get 2% accuracy the second switch would have to be at 250 feet (76m).

4. Since most spud gun projectiles are not very aerodynamic they will slow down a lot in 30' (~10m) or 50' or 250'.

If you can find a chrono with a resolution of 0.001 seconds things would work much better. You could get 10% accuracy at 500 FPS with the target 5 feet from the barrel. The measured velocity would be a much better estimate of the muzzle velocity.

Overall, a cheap and easy way to get a rough estimate of the performance of a spud gun.
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Unread postAuthor: n3rdkw » Sun Sep 02, 2007 7:47 pm

A quite accurate way of measuring muzzle velocity is actually quite simple.
All you need is any stop watch accurate to 0.01 seconds and any way of pointing your cannon straight up.
For most cannons, vertically mounting the cannon needs 2 strings. Attach them to the cannon at a 90 degree angle like such:

Code: Select all
    |
-- O


Make the strings dangle down and make sure it's parallel to the barrel, that way, it is absolutely vertical.
Then synchronize firing and start of timing. Usually this could easily be done by pressing both start button and the igniter together with one finger.
Then when the spud falls to the ground, stop the watch and take that time as T

Cut T in half to get the time it takes to go from muzzle to the top of its vertical flight.

Since Acceleration = change in speed / time and since the speed at the top of its trajectory is zero.
Plug in acceleration and time, you can calculate change in speed, which should be muzzle velocity.
Granted, this doesn't factor in wind resistance.

-------------------------------------

Another simple method:
Film it.
Position target, gun, and camera in an equilateral triangle with parameter of no longer than 30 meters
And then just fire the gun at the target.
Upload the movie file onto your computer, spread out the time, which should be accurate 1/30 seconds.
Then just find the amount of time it took to travel from ignition to hitting the target.
Look at the audio file on a timeline is most important actually. The explosion of gas (if you're using combustion) comes after the click of igniter. And the audio file is more accurate than the video file. The spike in sound should indicate very accurately the exit of muzzle and the striking of target.
Hopefully the distance b/w gun and target is short to keep air-resistance factor at minimum.

Wow that was lengthy



Will a moderator please move this topic to the HOW-TO section?
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Unread postAuthor: jimmy101 » Mon Sep 03, 2007 12:48 pm

n3rdkw wrote:A quite accurate way of measuring muzzle velocity is actually quite simple.
All you need is any stop watch accurate to 0.01 seconds and any way of pointing your cannon straight up.
For most cannons, vertically mounting the cannon needs 2 strings. Attach them to the cannon at a 90 degree angle like such:

Code: Select all
    |
-- O


Make the strings dangle down and make sure it's parallel to the barrel, that way, it is absolutely vertical.
Then synchronize firing and start of timing. Usually this could easily be done by pressing both start button and the igniter together with one finger.
Then when the spud falls to the ground, stop the watch and take that time as T

Cut T in half to get the time it takes to go from muzzle to the top of its vertical flight.

Since Acceleration = change in speed / time and since the speed at the top of its trajectory is zero.
Plug in acceleration and time, you can calculate change in speed, which should be muzzle velocity.
Granted, this doesn't factor in wind resistance.



This is commonly called the "hang-time" method of estimating the muzzle velocity of a gun. It is extremely inaccurate for a spud gun for a couple of reasons. This method can easily be off by a factor of two or more in the estimated muzzle velocity.

The most important reason is that the time to fall back to earth from apogee is not the same as the time from firing to apogee. Or, to put it another way, the teminal velocity is not the same as the muzzle velocity because the terminal velocity is limited by wind resistance, but the muzzle velocity is not. The terminal velocity of a typical spud is somewhere in the vicinity of 100~150 fps. Since the muzzle velocity of a even a smallish spud gun is a lot greater than 100 FPS this technique fails miserably.

To get a reasonably accurate estimate of the muzzle velocity from the hang time you have to know what the mass and coefficient of drag is of the spud. Joanna's trajectory calculator should be of help.

You might also read this

BTW, the hang time is pretty insensitive to the launch angle, 90 +/- 5 degrees is sufficiently accurate.
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Unread postAuthor: jimmy101 » Mon Sep 03, 2007 12:58 pm

n3rdkw:
For your second method, all you need is the audio recording of the gun firing.

For simplicity, the mic should be equidistant from the muzzle and target, it does not have to be part of a equalateral triangle.

The flight time can be extracted from the audio recording as the difference in time between "bang" and "splat". The distance to the target divided by the flight time is the average velocity over that distance. If the distance is small (a couple feet) then this gives a good estimate of the muzzle velocity.

A video recording doesn't have anywhere near the needed time resolution for measuring muzzle velocity.
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Unread postAuthor: Spuddin » Mon Sep 03, 2007 1:12 pm

jimmy u brought up a good point about the video recording. i have tried using my digicam which takes 20 frames p/s and setting it up on a tripod and filming my canon firing. i then watched the vid (.mov) and moved the slider bar manually from one frame to the next to get 2 frames, then measured how far the projectile traveled in feet in between the 2 frames, did some math and got the velocity but i dont know if it was accurate. has anyone else tried this?
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Unread postAuthor: jimmy101 » Mon Sep 03, 2007 5:25 pm

Spuddin: Yep that'll work but as you probably found out it is not real percise and kind of tricky.

You need to use a very fast shutter speed on the camera (a movie camera's shutter speed is not related to its frame rate, except the shutter speed has to be less than 1/frame rate). The fast shutter speed is needed so the spud isn't just a feet long blur in the frames.

You need a good reference for measuring the distance. The distance scale in the frame is not constant, or even linear.
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Unread postAuthor: boilingleadbath » Mon Sep 03, 2007 8:14 pm

To get a accurate result from the hang time test, you need to know:

1) The hang time. Accurately, because the scenario is very sensitive.
2) And either:
a - The Cd, diameter, and mass of the projectile
b- ...or the apogee of the projectile.

Neither is terribly easy to measure.

BTW, my hangtime ballistics program is nearly done... but has some work left to do - namely, I have to finish the final search algorithm.
Currently, I have coded:
1) The flight simulation portion
2) Support for the following input pairs:
Velocity and Cd to find Hang Time and Apogee
Velocity & Time -> Cd & Apogee
Velocity & Apogee -> Cd & Time
Cd & Time -> Velocity & Apogee
Cd & Apogee -> Velocity & Time

leaving:
Time & Apogee -> Velocity & Cd

...then I do a bit of idiot-proofing and it's ready to upload.
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Unread postAuthor: n3rdkw » Tue Sep 04, 2007 12:16 am

If you have someone willing to help, have them stand some distance from firing position, use a protractor and a piece of string and basic geometry to measure height. With the height, you're good to go.
If you want to be really accurate, then calculus must be applied to calculate precise air resistance.
Otherwise, simply factor air resistance as a constant.
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Unread postAuthor: jimmy101 » Tue Sep 04, 2007 11:39 am

boilingleadbath wrote:To get a accurate result from the hang time test, you need to know:

1) The hang time. Accurately, because the scenario is very sensitive.
2) And either:
a - The Cd, diameter, and mass of the projectile
b- ...or the apogee of the projectile.

Neither is terribly easy to measure.


Another way is to replace the Cd and diameter with the terminal velocity (or, the terminal velocity can be used to calculate the Cd).

I agree that the velocity calculation is very sensitive to the hang time. "One potato ...two potato..." won't cut it. You really need to get the time accurate to 0.1 second or better.

Boilinglead can't wait for you trajectory calculator (still waiting for Joanna's...hint hint)
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Unread postAuthor: jimmy101 » Tue Sep 04, 2007 11:51 am

n3rdkw wrote:...
Otherwise, simply factor air resistance as a constant.

You can't take air resistance as a constant because it isn't. The force of the air resistance is a function of the velocity of the projectile. Usually, the force is taken as being proportional to the square of the projectiles velocity (for sub-sonic rounds). A v<sup>2</sup> relationship is hardly a constant.

For a massive round moving slowly, air drag has virtually no affect. Imagine throwing a bowling ball. The ball's trajectory can be adequately calculated ignoring air drag.

For a less massive, or greater surface area projectile, air drag dominates the trajectory. Imagine throwing an air filled balloon. The trajectory cannot be calculated without taking into account air drag.

For any shape or mass shell, if the velocity is high enough, then the trajectory cannot be calculated without taking into account air drag.

Spudgun spuds, baseballs, bullets and the like fall into the "balloon" category, not the "hand thrown bowling ball" category. Mostly because of the high velocities that these projectiles are launched at.
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Unread postAuthor: Rudesill Ballistics » Thu Sep 06, 2007 2:42 am

I tried a method involving two microphones and a line splitter to test my Airsoft pistols, I read about it on a nerf site.

I put one mike next to the muzzle and the other ten feet away behind a sheet of paper (would make a loud pop when hit), then I recorded it into Audacity (any program that has waveform audio will do) and measured inbetween the peaks.

Here's a link:

http://www.seattleairsoft.com/modules/news/article.php?storyid=15
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Unread postAuthor: jimmy101 » Thu Sep 06, 2007 1:09 pm

Rudesill Ballistics: That is a very good (and easy) way to get the muzzle velocity.

You don't need two mics if the gun makes enough noise going off. Just position one mic equidistant from the muzzle and the target. For a spudgun, or a bb machine gun, there is enough noise from the round leaving the barrel to not need the second mic.

You don't need the piece of paper, just a hard surface. Angle the surface so the ricochets don't hit you in the head. :D
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Unread postAuthor: paaiyan » Thu Sep 06, 2007 1:47 pm

jimmy101 wrote:Rudesill Ballistics: That is a very good (and easy) way to get the muzzle velocity.

You don't need two mics if the gun makes enough noise going off. Just position one mic equidistant from the muzzle and the target. For a spudgun, or a bb machine gun, there is enough noise from the round leaving the barrel to not need the second mic.

You don't need the piece of paper, just a hard surface. Angle the surface so the ricochets don't hit you in the head. :D


i think if you only use one mic it would be a good idea to have the target only a short distance away from the muzzle, otherwise the split second it takes for the sound to reach the microphone might come into play and throw off your figures.
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