Hello,
I have made a forward firing catapult for the robot that my team and I have made. It is powered by two pneumatic es and it does actuate pretty good it just is not powerful enough. I think it needs more acceleration but I am not quite sure how to do that. if any one can give me any suggestions that will make it stronger will be greatly appreciated.
Zach
You’ll have more luck if you post some pictures and a video so folks can see what is specifically wrong with your design.
One problem with your catapult could be the angle of the pistons. You want the pistons to be as close to 90 degrees to the catapult arm as you can. Also check for friction.
That being said, post pictures please. 
Okay I thought it would be a little difficult to answer the question without any pictures that is my fault.:o I will post pictures as soon as i get the robot back. The robot is at school right now.
Another design aspect to consider is your use of elastics (like rubber bands). Our catapult works primarily due to the quick pull of the rubber bands, meaning the pistons are mainly used to keep the catapult down when not firing.
I have thought about that. I just did not know if the rubber bands would keep the catapult form going down, I will just have to test to see how man rubber bands will allow more acceleration and allow it to still go down.
Make sure that the psi in your reservoirs is good. If you have 30 psi vs. 100 psi, that would make a huge difference in your power and performance.
I know it’s not my thread or anything but given we are in the subject… is there a way to make my single acting piston catapult better? I cant make it launch with enough force right now.
You should move the angle that the pistons push at closer to 90 degrees as your current setup only has that angle when fully extended, which is good, but it can’t overcome the primary force due to the angle when the catapult is in resting position.
At what PSI are you firing at usually.
The big issue I see that you are having is what Andriod (Ruiqi) was saying above.
You want the piston to act perpendicular to the lever to use your power most efficiently. What you have now is actually pushing most of its power into the joint rather than into launching the object. Right now you have something like a 15 degree angle when a 90 degree is optimal.
How to fix this would be to lower the location that the piston is mounted at.
The best way to think about it is as 12 pounds of force in either direction on a double acting piston.If you rubber band 2 pounds of force trying to extend the piston you then have 14 pound extension power and 10 pound retraction power. Rubber bands help store the energy from the opposite stroke. Obviously 2 pounds of extension isn’t what you would ever get because of elastics having power based off of position and was only used to illustrate a point.
This principle can be amped up to almost double the power of a double acting piston.
-Tubing might help. 
-Less friction. Make sure everything rotates freely.
-Better angle of firing. Having the pistons that close to parallel to the catapult arm is wasting energy.
https://vexforum.com/attachment.php?attachmentid=7960&stc=1&d=1389206429
Green = piston, Black = catapult arm, Light Green = piston (extended), Gray = catapult arm (extended)
You also don’t want your catapult to go that far forward. Mount your pistons lower and further back on the catapult arm to reduce the angle change.
Alright thanks Ill try modifiying a bit, testing and post some more pics to see what could be fixed afterwards.
Can someone please post a diagram of how the piston should be oriented? We have a double action piston rear catapult capable of throwing a big ball 4-6 feet and it starts out in the extended state pretty much parallel to the throwing arm and ends up perpendicular at the joint. If that can be re-oriented to get better action we would be delighted.
It should start perpendicular if possible.
https://vexforum.com/attachment.php?attachmentid=7960&d=1389206429
Old vs. New
How many launches would I get from 2 tanks with 2 double acting pistons? Assuming it starts at 100 psi?
It depends how your catapult is build. If it’s built to maximum efficiency, I’d say you could get about 8-10 full field launches.
Is it possible for 15 or a few more if it is Rubber band assisted?
I get 5 shots on one tank with two pistons. The rubber band assist, without the retractive power of the double acting cylinder will not get you much power, with the double acting cylinder retracting though, would use twice the air for twice the power. You would then only shoot really far on the first shot and then very short on the second and acutally get less shots, if you were using one tank that is. Best without rubber band assist on two tanks is 10 launches. Or succeed in building a one piston catapult(possible) and get 15+launches on two tanks.