Vex Pneumatic Catapult Design Data

Well, i finally got around to documenting some of team 599’s pneumatic catapult test data. A prototype was built that could handle one or two pistons and enough tests were done to parameterize the piston distance (energy) vs pressure.

Youtube link has an explanatory video:
http://www.youtube.com/user/vamfun#p/a/u/0/llyzwjUCHQc

Some testing was done with the Clean Sweep football (6 oz) and a bunch with two tennis balls taped together (about 4.5 oz’s)

Using ballistic equations to relate the distance to energy supplied by the piston, it was determined that at 100 psig tank pressure the pistons supply roughly .9 to .95 ft-lbs. The energy is proportional to the regulation pressure as expected and it appears that the optimal regulation pressure is 60 psig. This minimizes the number of pistons and tanks while providing about 16 shots before degraded performance occurs. The 16 shots is based upon tank depletion rates.
see [http://vamfun.wordpress.com/2009/12/19/vex-pneumatic-shot-calculation-vs-p_regulation/
for table vs regulation pressure.

I developed an excel calculator which will estimate energy and the piston / tank requirements to achieve a specified input distance with a given catapult configuration.

See Catapult Energy Clean Sweep excel zip file. This file also has a second sheet with all the parametric test data and plots for 5 configurations:
conf…sol… pistons… Other
a… 1… 2… b port open
b… 2… 2… b port open
c… 1… 1… b port open
d… 2… 2… b port connected to solenoid
e… 2… 2…b port open, pistons fired , then catapult released

It was surprising that a and b configurations delivered the same distances. I expected that the additional solenoid would improve the air flow and energy but a single valve can drive two pistons with these dynamic loads. There is a little more lag in building up pressure but the flow rates are not loading the valve significantly after that.

Connecting both ports of the two way piston to the solenoid (d) degrades the performance a few percent due to longer exhaust port lines and solenoid orifice. The prototype piston was gravity reset so an open port was possible.

Firing the piston while holding the catapult and then releasing added about 5 to 10 % to the distance.

To achieve a viable Clean Sweep robot that can match the spatula bots we would design for a 9.5 ft distance (measured in plane of release at about the height of the wall). This minimizes maneuver requirements and allows diagonal shots from the back wall. We would need 5 to 6 pistons with a regulated pressure of 60 psig. Typically a tank per piston is needed and one solenoid per 2 pistons.

Robodox 599 chose not to build a competition pneumatic catapult. We really just wanted to understand the pneumatic piston energy better.](http://vamfun.wordpress.com/2009/12/19/vex-pneumatic-shot-calculation-vs-p_regulation/ )
Catapult Energy clean sweep.zip (15.8 KB)

Great workup. Thanks!