Re: OT: Repeatably lobbing "projectiles"

Don Y <blockedofcourse@foo.invalid> wrote:

On 11/21/2024 2:53 AM, Liz Tuddenham wrote:

Bow and arrow?
 
That was actually meant seriously as the geometric arrantement of the
string acts as a variable-ratio transformer between the movement of the
propelling spring (bow) and the increasing velocity of the load (arrow).
This means that if you charge the spring with a known amount of energy,
it will always be transferred with the same high efficiency into the
load.
 
By varying the starting point of the arrow, and hence the spring
tension, you have an accurate repeatable energy input.

 
The "variable force constant launch angle" approach was
implemented on a similar principle:  pull a spring back to
a particular amount of compression and then release.
(This is how a pinball is launched, hence my inspiration)

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.

The "release" seems to be a big problem

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.


--
~ Liz Tuddenham ~
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www.poppyrecords.co.uk