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From: Don Y <blockedofcourse@foo.invalid>
Newsgroups: sci.electronics.design
Subject: Re: OT: Repeatably lobbing "projectiles"
Date: Thu, 21 Nov 2024 22:53:54 -0700
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On 11/21/2024 9:24 PM, bp@www.zefox.net wrote:
> Don Y <blockedofcourse@foo.invalid> wrote:
>> On 11/21/2024 5:52 PM, bp@www.zefox.net wrote:
>>> Don Y <blockedofcourse@foo.invalid> wrote:
>>>>
>>>> Ideally, I would like the mass to STOP at the destination (not bounce
>>>> or roll away) but that's a secondary goal.
>>>
>>> Apologies if I missed it, but what are the masses and accelerations in play?
>>
>> The masses are negotiable.  As is the acceleration (as long as you
>> can overcome gravity).
>>
> Per projectile? All the same? Are they fragile? Same aerodynamics,
> or variable?  Grams per cc, or cc's per gram?

You don't care if the projectile FLOATS to the destination.
As long as it isn't disturbed by minor air currents, etc.

>> The goal is to PLACE a "thing" (projectile) at a specific location
>> relative to the launcher.  And, to have it reach that location
>> with minimal interaction with anything located between the launcher
>> and destination.
> 
> What's the permissible error in position and landing speed?

Landing speed depends on what the projectile is made of.
E.g., you wouldn't want it to "bounce" past the targeted
location.  I have seriously considered "water balloons"
because they "disintegrate" on impact and the water
disipates.  By contrast, hurling ROCKS would result in
a bunch of rocks "persisting" in the arena.

>> E.g., If something was located immediately adjacent to the
>> launcher, the projectile's path should go OVER the obstacle
>> and not expect to be able to pass THROUGH it.
>
> That's a mortar or howitzer.

Yes.  Or, any launch mechanism that goes UP before OVER.
Contrast with a machine gun that goes THROUGH.

>> Imagine using a nerf gun to lob those little sponge balls
>> up, over obstacles -- but, being able to precisely control
>> the range so they land where planned (no farther nor shorter)
> 
> Now you're talking projectiles dominated by aerodynamic drag.
> Does your launcher need to take air movement into account?

No.  But, the design choice should have tolerance for small
air currents.  If necessary, it will be operated indoors.
Expecting it to work outdoors in an area that may experience
high/variable winds would be a taller order.

>> But, to do so completely under automation, without any human
>> intervention.
> 
> Are you thinking of something like
> https://en.wikipedia.org/wiki/Girardoni_air_rifle
> ?

Overkill as it doesn't need to be that portable.

But, conceptually, I think this is the basis for a possible
approach.

Imagine a small (battery powered) compressor feeding a
container that has a valve on its outlet.  A pressure transducer
to monitor the current pressure -- which will change at a rate
that is defined by the capabilities of the compressor and
the volume of the container.

So, a machine could (when commanded to "prepare to fire")
activate the compressor and monitor pressure until it
reaches the desired set point.  If the "fire" command is
delayed, it could reactivate the compressor to handle any
leakage from the container (or, backwards through the
compressor while OFF).

When commanded, the valve on the outlet -- which feeds into
the projectile launcher -- can be "cleanly" opened (hence my
interest in your example) to allow the compressed air to
rapidly exit the container, propelling the projectile out.

Shortly thereafter, a new projectile could be "loaded"
in anticipation of a future command to "prepare to fire",
etc.

> 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).

The point is finding the right combination of mechanism
and projectile to get repeatability from the system.
Most "guns" just want to achieve a "maximum" (range,
load, etc.)

Imagine a nerf gun that could launch a watermelon 100 meters.
It would be a flop in THAT market!

    "Fred, your son just broke Mr Wilson's picture window,
    again.  That;s the third time this month..."