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macona
05-09-2009, 12:52 AM
Here some pics of that remote control t-shirt launcher that was in the pic with the injection molding machine at TechShop.

It was built with the kids at Tigard High School Robotics. 9 barrels, can fire 3 shirts in each barrel. The elevation is with an old seat adjuster from a chrysler town and country that was a warranty reject.

http://www.youtube.com/v/Qsid0C5lErU

More Pics at: http://www.flickr.com/photos/techshoppdx/


http://farm4.static.flickr.com/3408/3514796298_ef33f290d4_b.jpg


http://farm4.static.flickr.com/3560/3513989465_cfc2994dc7_b.jpg

Ryobiguy
05-09-2009, 01:44 AM
Here's a soundtrack for viewing those photos, salute to the t-shirt launcher inventor: http://www.youtube.com/watch?v=1b3pJYfv5eg

-Matt

P.S. How many feet per second do the shirts go?

macona
05-09-2009, 01:58 AM
That audio is great! :D

Not sure what the muzzle velocity is. Thought about rigging up some sort of chronograph.

It will launch a standard shirt about 150 feet. Pepsi cans much further!

Evan
05-09-2009, 04:09 AM
How about a wet t shirt? Tomato perhaps? :D

You don't need a chronograph. Set the launcher to fire parallel to the ground at a height of one meter and fire something that has negligible retarding effect by the air such as the Pepsi can. Measure the distance until it hits the ground in meters. Multiply by 2.214 and you have the muzzle velocity in meters per second.

You can then extrapolate by the ratio of masses from the tee shirt and the pepsi can the approximate muzzle velocity of the tee shirt.

Doc Nickel
05-09-2009, 05:32 AM
Or find any decent paintball field. They'll typically have either some radar-based chronies or at least a breakbeam light-sensing type.

We chronied a potato cannon many years ago- the typical PVC and hairspray style. It'd lob a potato about 200 yards, but at "only" around 150fps muzzle velocity. (That was using a high-end radar chronograph.)

I'd wager this shirt cannon is in that same ballpark.

... Are those socket-wrench U-joints on the driveshafts...? :D

Doc.

macona
05-09-2009, 05:44 AM
yep, they are.

If they break home depot will replace them! ;)

Your Old Dog
05-09-2009, 06:31 AM
lIt shoots at 180 fps. (according to the bud audio)

Evan
05-09-2009, 09:09 AM
It doesn't matter what device you use to throw something, the velocity will be the same for the same angle and range if air resistance is neglected. At these low velocities something dense like a soda can or a tomato will have the same trajectory if you throw it, kick it, bat it or shoot it. Since the attraction of gravity is the same in all cases basic grade 9 calculus applies. That's why it can be boiled down to such a simple term if you fix the other variables as I offered above.

BTW, if you don't want to use metric then place the device 39 inches above the ground and measure the distance in feet. Then multiply by 7.26 for feet per second.

JCHannum
05-09-2009, 10:49 AM
That is Ballistics 101, and while interesting, it is of only passing value. To be meaningful, the range must be dead level and the starting trajectory ie. the bore of the barrel, must be parallel to the ground of the range. Any elevation or depression will skew the results. If these requirements are satisfied, it still only yields an average velocity, not muzzle velocity.

mochinist
05-09-2009, 10:54 AM
That is Ballistics 101, and while interesting, it is of only passing value. To be meaningful, the range must be dead level and the starting trajectory ie. the bore of the barrel, must be parallel to the ground of the range. Any elevation or depression will skew the results. If these requirements are satisfied, it still only yields an average velocity, not muzzle velocity.you guys should debate it for the next thirty pages

Evan
05-09-2009, 11:10 AM
That is Ballistics 101, and while interesting, it is of only passing value.

I learned ballistics in grade 9 as one application of calculus. Calculus is the mathematics of motion over time.



Physics of trajectories
A familiar example of a trajectory is the path of a projectile such as a thrown ball or rock. In a greatly simplified model the object moves only under the influence of a uniform homogenous gravitational force fieldForce field (physics)
Originally a term coined by Michael Faraday to provide an intuitive paradigm, but theoretical construct , for the behavior of electromagnetic fields, the term force field refers to the Line of force one object exerts on another object or a collection of other objects....

This is the focus of the discipline of ballistics
Ballistics is the science of mechanics that deals with the flight, behavior, and effects of projectiles, especially bullets, gravity bombs, rockets, or the like; the science or art of designing and accelerating projectiles so as to achieve a desired performance....

Newton's theory later developed into the branch of theoretical physics.
Theoretical physics employs mathematical models and abstractions of physics in an attempt to explain experimental data taken of the natural world....
known as classical mechanics.

Classical mechanics is used for describing the motion of macroscopic objects, from projectiles to parts of machinery, as well as astronomical objects, such as spacecraft, planets, stars, and galaxies....

. It employs the mathematics of differential calculus.
Differential calculus, a field in mathematics, is the study of how functions change when their inputs change. The primary object of study in differential calculus is the derivative....
(which was, in fact, also initiated by Newton, in his youth).
http://www.absoluteastronomy.com/topics/Trajectory


To be meaningful, the range must be dead level and the starting trajectory ie. the bore of the barrel, must be parallel to the ground of the range. Any elevation or depression will skew the results. If these requirements are satisfied, it still only yields an average velocity, not muzzle velocity.
I believe I mentioned that. I used fewer words though.
This is a Tee Shirt launcher, not a precision target rifle. The results obtained will be accurate to within ten percent or so without taking any special measures. That is good enough. Repeatability will be considerably better as long as the same setup is maintained.

As I said, the effects of air resistance at these low velocities are negligible on an object that is reasonably massive compared to it's volume. The terminal velocity of a full Pepsi can will be somewhere over 100 miles per hour which is approximately 146 fps.

JCHannum
05-09-2009, 12:50 PM
The terminal velocity of a full Pepsi can will be somewhere over 100 miles per hour which is approximately 146 fps.

Based on what information?

lazlo
05-09-2009, 01:05 PM
you guys should debate it for the next thirty pages

Wait, are we done panicking about Swine Flu yet? :D

Jerry: Great Job! That looks like a blast to build and operate!

Liger Zero
05-09-2009, 01:08 PM
you guys should debate it for the next thirty pages


*passes over popcorn*

Beer is the cooler under the mill.

lazlo
05-09-2009, 01:20 PM
Beer is the cooler under the mill.

Ah, so that's what the door in a turret mill's base is for! :D

Liger Zero
05-09-2009, 01:21 PM
Ah, so that's what the door in a turret mill's base is for! :D

That and hiding the Diamond-Like Substance Ear-Rings you want to surprise your wife with. :D

davidh
05-09-2009, 04:43 PM
yep, they are.

If they break home depot will replace them! ;)

please explain a bit :
co2 or air or oxy?
what regulated pressure at end of chamber?
is that an accumulator on top of the barrels ?

drive motors ?
why are wheels mounted on independent a-frames ? is it power tilting ? what am i missing there ?

it looks so interesting i can;t hardly keep from getting excited.

Doc Nickel
05-09-2009, 05:25 PM
co2 or air or oxy?

-That's a SCUBA tank, compressed air. Looks like an older steel tank, so probably 2216psi. Could be 3K.


what regulated pressure at end of chamber?

-Wild guess based roughly on similar applications, about 100 psi. It's the volume that does it, not the pressure.


is that an accumulator on top of the barrels ?

-Yes. Looks like it's common to all the barrels. I wonder what the recharge rate is? That regulator doesn't look particularly high-flow. :)


why are wheels mounted on independent a-frames ? is it power tilting ?

-I get the impression it was somebody's robot chassis that got repurposed to carry the shirt cannon. But that's just a guess.

Doc.

mochinist
05-09-2009, 06:47 PM
*passes over popcorn*

Beer is the cooler under the mill.what kind of mormon are you? my in laws look at me like I'm the devil when I pop open a beer.

lazlo
05-09-2009, 06:48 PM
-Wild guess based roughly on similar applications, about 100 psi. It's the volume that does it, not the pressure.

He's got a SCUBA K-valve yoke and first stage regulator on the tank, so that's 150 PSI coming out into the line through the air gauge. I don't' see any other valves downstream, so I'm guessing 150 PSI is going into the accumulator.

Liger Zero
05-09-2009, 07:30 PM
what kind of mormon are you? my in laws look at me like I'm the devil when I pop open a beer.

One that grew up. ;) Once I learned the truth about the "word of wisdom" (the prohbition against smoking, alcohol and coffee) I kind of relaxed a bit.

Really one or two every so often don't hurt and it's often "necessary" in certain social situations... same with coffee.

You go into an interview and they offer you a cuppa. "No thanks" makes them give you the ":confused:" look and if you explain that it's because of some silly rule, you get the :eek: treatment.

*shrug* A couple of beers is minor, wait until God reads the rest of my record. :D

Evan
05-09-2009, 07:33 PM
Based on what information?

Ask a sky diver. It has about the same density as a human body but less aerodynamic resistance. You work it out.

JCHannum
05-09-2009, 08:46 PM
That's what I thought you were referring to, but I cannot understand why you brought it up.

A skydiver, or any free falling object will reach terminal velocity when the resistance of the air provides enough drag to negate the acceleration of gravity. The object will continue to fall, but at a fixed speed. This is why a feather will take longer to fall a given distance than a BB shot. A skydiver can control his terminal velocity by the position he assumes, spread eagle vs a tight ball.

An interesting fact, but it has absolutely no bearing on the velocity calculations you have presented.

Evan
05-09-2009, 09:09 PM
Simple, the can won't slow down much below muzzle velocity because of air resistance. It's in free fall once fired and the muzzle velocity is lower than the terminal velocity.

macona
05-09-2009, 10:20 PM
Like Doc said. Steel SCUBA cylinder. Filled at a local dive shop.

The tube on top is the accumulator. Wrapped with a kevlar-carbon fiber composite. The regulator has been adjusted down to 100PSI. Fill time between shots is about 3 seconds. The controller is a VEX system and is fully programmable so they have it set up with a delay between firing shots.

Originally the robot under the launcher was being built for the robot wars thing. But they went under so that never happened. I think it was built to have suspension or something but now its just solid. There is a bit of give in the go-cart tires.

I did very little on it. I had built the extruded aluminum frame for it last year. I did supply the seat mechanism and cut it loose from the rest. The kids made the mount. The night before they were supposed to show it off I was up till 1 fixing the wiring.

They had all the solenoids wired in a 3x3 multiplex. That dont work so well with coils. Send power to one and it goes wherever. So I soldered in a bunch of diodes to isolate the valves from each other. That did the trick.

JCHannum
05-09-2009, 11:11 PM
Simple, the can won't slow down much below muzzle velocity because of air resistance. It's in free fall once fired and the muzzle velocity is lower than the terminal velocity.

What?

Muzzle velocity is the velocity of a projectile at the muzzle and has nothing at all to do with gravity or terminal velocity. There are two components involved that are separate. The projectile is travelling downrange at a velocity imparted by some external force. At the same time, gravity is acting upon it to pull it to the ground. These two forces acting together create the somewhat arc shaped trajectory. The higher the muzzle velocity, the flatter the arc and the farther the projectile will travel before striking the ground.

Muzzle velocity is a variable while the time of flight will be approximately constant as it is determined by the time it takes gravity to pull the projectile to earth assuming the level and parallel conditions above exist. This will hold true in a vacuum, in open air many other variables will effect the outcome.