Re: OT: Repeatably lobbing "projectiles"

Trebuchet is probably the method of choice. There is a bloke round here with one big enough to lob a watermelon about 100m. It is an impressive beast and flat packs into his pickup traler.

One to throw a tennis ball 20ft would be a bit dangerous close to but very reproducible if that is what you want.

That is a less efficient system because a lot of kinetic energy is left in the spring after it has lost contact with the projectile. The bowstring method transfers much more of the energy.. The Roman ballista and catapulta used a flip-over arm to bring the heavy beam to rest and transfer most of its energy to the projectile.

All those methods were designed during an energy-scarce era, something we need to re-visit and learn from.

That is a problem because if the 'catch' (of whatever type) moves to one side, it has the potential to introduce a variable factor. If it holds onto the projectile along the axis of projection by friction, you have a different variable factor as it releases.

When faced with a similar problem in the past, I used a thin piece of cotton to retain the projectile and snipped or burned it through to cause the release, but this isn't really suitable for your application.

If you don't mind a slighly variable delay in the starting time, could you retain the projectile with a clamp and a lead rod aligned on the centre of mass. Releasing the clamp would put tension on the lead rod which would 'neck' through and give a clean release. That method was used for time-delay fuses in WWII.

Another release mechanism (which I have seen used in air rifles) is to have three concentric components. In the centre is a rod attached to the mechanism with a 'waist' machined in it. Arounf that is a sleeve with three hole drilled radially at 120-degree intervals. In the holes are ball bearingsa which engage with the waist when they move inwards. Around the outside is a tube which keeps the ball bearing pressed inwards.

When the sleeve is slid off, the balls move outwards and release the inner rod. Because they are symmestrical, no unwanted displacing thrust is applied to the rod.

Ratchet mechanism and count the number of clicks or stepper motor.

Speed of firing might be its main weakness. The thing depends on moving a heavy counterweight up slowly to store the energy and letting it go in a single pulse with mechanical amplification of the lever and sling.

Reproducibility was as I recall quite good.

The spring is moving as fast as the projectile when it separates from the projectile. The same objection applies to a bow and arrow, but there it is the relatively light bow-string which is moving as fast as the projectile, and the relatively heavy bow moves a lot more slowly than the string.

One other thought you might be able to buy a tennis practice serve machine off the shelf with high reproducibility.

But one end has considerable mass and is moving, so it has kinetic energy.

Have you seen the devices for throwing clay pigeons that are 'cocked' by a crank on a lorry windscreen wiper motor tensioning a chest expander spring. All very off-the-shelf stuff but not particularly accurate or efficient. (but bloody frightening when you are working on one and it goes off unexpectedly).

Yes, that's exactly the same idea. [...]

In an air rifle it is allre-set by a mechanical linkage that operates from 'breaking' the barrel to reload, with small auxiliary springs that pull all the components into place automatically. The user is unaware of any complication.

Not exactly off-the-shelf, but have you thought about a linear motor? That would be eminently controllable as the speed of the projectile could be measured and corrected as it accelerates.

That's not amplification, that's impedance matching.

A bow and arrow arrangement with two bow strings, one slightly longer than the other. The shorter string is notched into the projectile and the longer string is held back by a 'rolling nut' in a similar way to a mediaeval crossbow. The displacement caused by the rolling nut releasing the longer string will not be communicated to the shorter string or the projectile.

Boring, but think 'sling'. david and Goliath etc.

Rotating arm, center-fed ~ counterbalanced.

axle tilt, fixed length arm (rpm), angle of release,

Golfers would go for it.

RL

The tennis ball thrower seems promising if the projectile weight is close to right.

Another thought would be a crossbow-like mechanism with a motorized winding mechanism. Range could be adjusted by moving the limbs to vary the preload on the string. Bulk can be reduced by using a reverse-draw layout.

We used a pulsed coil accelerating an aluminum ring which then hit the valve poppet to drive it off of its seat. \[electronics content

A similar approach could be applied to a crossbow or catapult sear.

Thanks for reading,

bob prohaska

Apparently the technical term for the winding mechanism of a crossbow is "Crannequin", which sounds as though it could be quite naughty - so it is a word we ought to use more often.

It sounds like you need a steel-spring crossbow mechanism as the starting point.

.

Joe

Apologies if I missed it, but what are the masses and accelerations in play?

bob prohaska

It was.

Hammerless valves would start to leak millisconds before reaching full flow.

The idea was to place a shaped "puff" of gas between two electrodes, then apply high voltage to initiate an arc in the gas and compress it. The experiment was called a Z pinch. Since gas expands at the speed of sound the puffing operation had to be finished in a few tens of microseconds. Otherwise that gas would go places unwanted and cause trouble.

My point was that if your launcher's trajectory is disturbed by the release mechanism, whacking the release out of the way will reduce the disturbance..

Thanks for reading, bob prohaska

Per projectile? All the same? Are they fragile? Same aerodynamics, or variable? Grams per cc, or cc's per gram?

What's the permissible error in position and landing speed?

That's a mortar or howitzer.

Now you're talking projectiles dominated by aerodynamic drag. Does your launcher need to take air movement into account?

Are you thinking of something like

One could build a version to launch Nerf balls. Note that it used a hammer to operate the gas valve and compensate at least partly for pressure changes. You'd want a very low pressure for 20 feet. Perhaps you want a toned-down potato cannon (I think that was mentioned previously by someone).

hth,

bob prohaska

A long cylinder with recesses in its surface into which the projectiles are loaded. It is spun rapidly inside a loose-fitting tube and withdrawn at a steady speed. The end of the tube where the projectiles come out is shaped as a scroll with a lip so that the projectiles become free at a definite angle of rotation.

The speed of rotation could be used to control the velocity and rotating the outer tube to vary the position of the lip would change the launch angle. The whole thing could be made with a screw thread traversing mechanism like the very earliest experimental Edison tinfoil phonographs.

I'd look at the way people throw pétanque boules. I imagine they've developed an accurate technique.

Yes, repeatability is the first requirement for accuracy.

Not sure I understand what you're getting at here. On the face of it that statement seems mistaken.

Perhaps the most clever feature of the Giradoni mechanism is pitting the momentum of the hammer _against_ the air pressure bearing on the release valve. As the pressure goes down, the valve opens farther, at least partially compensating for the lower pressure.

Since you will have control of the exact pressure that particular compensation isn't needed. Still, if the valve opens differently on timescales comparable to the departure time of the projectile repeatability will suffer and with it accuracy. Opening the valve poppet before the projectile moves is one way to help repeatability. For the velocities you envision that shouldn't be difficult.

A pneumatic potato cannon seems like a good starting point.

bob prohaska

You are interacting with "Don Y". If you go review other postings where "Don Y" has initiated a thread like this one you will find that the "Don Y" poster quite effectively trolls the group every single time. The initial post is always ambigious and seriously lacking in necessary details, such that 234+ wrong possibilities could be seen by readers. And as others in the group request details to try to gain an understanding from the vague, detail lacking initial post, they are slowly tricked out over the course of days and plural postings from "Don Y", intermixed with numerous random asides to maximize confusion. Meanwhile, as the supposed details are trickled out, the apparent requirements needed also shift as posters appear to gain any understanding in order to keep the responders in the dark and allow the "Don Y" trolling articles to continue to troll everyone who responds.