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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 05:43:45 -0700
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On 11/21/2024 5:34 AM, Martin Brown wrote:
> On 21/11/2024 12:06, Don Y wrote:
>> On 11/21/2024 4:46 AM, Martin Brown wrote:
>>> On 21/11/2024 07:04, Don Y wrote:
>>>> We're trying to make a mechanism that will allow for the
>>>> REPEATABLE lobbing of small projectiles over short (0-20 ft)
>>>> distances.
>>>>
>>>> We've tried using a constant force with a variable launch angle
>>>> (higher for shorter).
>>>>
>>>> And, a variable force with a constant launch angle (push harder
>>>> to throw farther).
>>>
>>> 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.
>>>
>>> https://en.wikipedia.org/wiki/Trebuchet
>>>
>>> He takes it around rural summer fetes to raise money for charity (and buying 
>>> more water melons). They land with a very satisfying splat!)
>>>
>>> One to throw a tennis ball 20ft would be a bit dangerous close to but very 
>>> reproducible if that is what you want.
>>
>> But how do you RELIABLY adjust the length of throw?
>>
>> I.e., "I want THIS (repeatable object) to be tossed in a high arc
>> to land X (not Y or Z) feet from the launcher"
> 
> Ratchet mechanism and count the number of clicks or stepper motor.

But the "release" mechanism would still be under a lot of stress.
OTOH, a 'slow" release would likely have a less consequential
effect on the projectile's flight.

I had, instead, imagined changing the weight of the counterweight
so the release would be unaffected (?)

>> It is permissible to store "calibration factors" -- including
>> in *tabular* form.  But, you should be able to use any of those
>> specified ranges with a reasonable guarantee that you'll strike
>> at the correct distance (the launcher won't be able to verify the
>> *actual* distance achieved)
>>
>> The entire sequence has to happen without human intervention
>> (and, ideally, in a short enough time span that one can repeatedly
>> "get off rounds" without long delays)
> 
> 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.

I will have to make this very small (probably much less than a cubic
foot in total volume).  But, I can correspondingly alter the size
and weight of the projectile.

It also seems to give me the most flexibility in the characteristics of the
projectile (watermelon, tennis ball, water balloon, etc.)

> Reproducibility was as I recall quite good.

But I suspect the forces involved in lobbing watermelons scores of
yards swamps the characteristics of the load -- so variations
would appear as "noise".  Imagine how the same (scale) mechanism
might vary with a load like a tennis ball (over those distances).