Weapon research

[Remote-Controlled Dave]

People who have spinning type weapons, I just want to do a bit of research.
How much do the discs have to weigh to cause damage/throw an opponent in the air? I suppose it depends on the power in the motor and other things, but surely theres a bandwidth of successful weights for such a weapon?
I'm just interested, might have a play around before next AWS.

[leo-rcc]

The shape of a disk defines also its effectivity. DBA disk is narrow and high diameter, while Katjusha is thicker and a little smaller. in the end it all depends how much kinetic energy you can tranfer at once. heavy outer diameters and a reasonable amount of rpm helps.

[Andrew_Hibberd]

Kwijebo's disc is 100g, however you need to get as much weight as possible at the rim, it will take longer to spin up however it will transfer more energy.

i think this is too much weight in the disc, some others have a range from 25g to 65g.

I built kwijebo arround the disc, however a better approach would be to build an ant then see how much weight you have left over.

[BeligerAnt]

It's all about inertia.
To increase inertia, concentrate the mass towards the rim (for a given size of disc).
Inertia depends on disc diameter (and mass as above).

From this point it is possible to get completely sucked into the maths and calculate (theoretically) how far your disc will throw your opponent - only to find out that they knock you out of the arena first!

The downside is the bigger the inertia, the longer the disc takes to spin up. (Since inertia is resistance to change - it doesn't want to start to spin, but once it's spinning, it don't wanna stop!)

[Rhys]

Randar had a 20g disc with 80mm diameter. Not that good in comparison to others but it does the job. New one will be 35g and 85mm diameter.

[BeligerAnt]

The inertia of a disc is I = 0.5 * M * R^2

Where M is mass, R is radius, and ^2 is "to the power of 2" or "squared"

The kinetic energy stored in the disc is E = 0.5 * I * w^2

Where I is inertia and w (omega) is angular velocity.

Note mass is in kg, radius is in metres(!) and w is in radians/sec

To get more ant-friendly units, I = 0.5 * 10^-9 * m * r^2
with m in grams, r in mm

w = 2 * pi * (revs per sec) or
w = (pi / 30) * (revs per min)

So E = ((pi^2)/1800) * I * RPM^2

Or, E = 2.74 * 10^-12 * m * r^2 * RPM^2

So Randar's 20g 80mm (diameter) disc at 10000rpm would store about 8.77 Joules - enough to throw a 150g robot some distance!

Since r and RPM are squared, they provide a bigger increase in energy than simply increasing the mass. Concentrating the mass towards the edge of the disc increases the inertia towards a theoretical limit of M * R^2 (i.e. twice as much as a plain disc)

[Cavecrusher]

8.77 Joule, hmmm.

Using the old DBA specs (35gr, 90mm, 16krpm) i get 46.755 Joules

Using the new DBA specs (42gr, 60mm, 30krpm) i get 93.29 Joules, but since the mass is more distributed towards the outer ring it's more like 100 Joule,

Doing some simple calculations E=m*g*h (m=0.150, g=9.81) we get h= 68 meter, lol

[leo-rcc]

In theory Katjuscha is also in that range, but because i cant really hit when I spin that fast it will be less then that.

Having a lot of KE is nice but you need to transfer it first. The new DBA's disk will be much better at that than mine.