Backward firing catapults

Our team is working on a backward-firing catapult for worlds.

There doesn’t seem to be many robots using back-facing catapults this year, since they usually require 4 pistons instead of 2. What are some tips you’ve found on making them better?

As on all types of catapults/launchers, the longer the throwing arm, the better. To get the greatest throwing distance, the release angle needs to be 45 degrees and the pistons should be perpendicular to the throwing arms just before this angle. My team actually uses 5 rams on our backwards launcher, with 3 tanks (2 for worlds probably), and we never run out of full-field launches in a match. The throwing arms should be as light and stiff as possible too.

a back launcher definitely doesn’t require 4 pistons.

Yeah, 4 pistons isn’t required. I had a 2 piston backwards launcher that could make full field shots.

I have a forward launcher now because for my robot, it was too hard to rock back and forth to launch. 2 is definitely possible though.

We’ve actually found that you get more launches with more pistons. It has to do with using air pressure more effectively longer. For example: 2 pistons at 100 psi < 4 pistons at 100 psi and 2 pistons at 75 psi < 4 pistons at 75 psi. You get more powerful launches longer. Launch power doesn’t equalize between 2 and 4 pistons until past the 15th launch.

Here’s a video of 5003’s back launcher. I hope that helps.

Trinity III Thrower

Definitely a good direction to go, as this launcher was quite impressive - it could launch over blocking robots (lift up) mere inches away.

5003’s catapult was probably one of the most impressive catapults I have seen. It was very effective and probably helped them and their alliance during the U.S. Open. So you can’t go wrong taking that into consideration.

I find that using 2 pistons is already enough power to fire the beach ball over the barriers from hanging to goal zone, but if you want the extra field shots use 4 pistons. Using 2 pistons is half the amount of shots, using 4 pistons is more than that. You would need to add rubber bands on the back of your back catapult if you use 2 pistons to have enough power to fire over the barrier, but if you use 4 pistons its not needed. You would need some momentum to get the ball over the barrier. Adding like 5 rubber-bands to assist the back catapult when using 2 pistons works well in my opinion but experiment and see what works best for you. Using 3 tanks is enough for the backwards catapult in my opinion!

Mounting the pistons is another factor, you need to mount them so they will stop at 45 degrees where it will have the most power :slight_smile:

I found that I make full field shots with 4 pistons, but I couldn’t make it over the barrier with two pistons- it seemed like to me that they didn’t have enough force to clear the 12" barrier without rubber bands. (I would like to not have to power the down-strokes, because that makes them essentially use the same amount of air as a 4-piston launcher)

On a related note, how could one make double-acting solenoids act as single acting solenoids? I’ve been connecting the b-ports together on two double acting solenoids, but I was wondering if there was a more effective way to “cap” the b-port.

For this catapult, I’m hoping to get 3-4 full field shots from about two tanks. Does 5003’s launcher use three single-acting pistons? It is indeed impressive :slight_smile:

5003’s backwards launcher uses 3 double acting cylinders, but they are using them as single acting pistons. They changed something at US Open and they can launch a lot farther now.

I think 400X had a backwards catapult at the Open, it was super effective

It may not require 4 pistons, but with a back firing catapult and 4 pistons it is possible to clear the barrier along with any ball sitting on the barrier without raising the arm. To me this is the greatest advantage to having a back firing catapult if you have the pistons available.

It can also be done with a front-firing launcher (cough 323x/z cough). Kennedy (5003) told me that his launcher didn’t get over the bar with 2 pistons (without raising), and when he switched to 3 pistons, it worked great.

For some reason, this is what I have seen over multiple tournaments including US Open.

Front Launchers use an even number of cylinders (2 or 4)
Back Launcher use an odd number of cylinders (3 or 5)

You don’t necessarily need to raise the arm to fire over the barrier, some robots can fire the beach ball over the barrier without raising the arm on 2 pistons. Your pistons need to be like straight up almost… close to 90 degrees is most efficient! :cool:

Personally, I found that placing the pistons at near 90 degrees to the ground didn’t work as well as having them at about 45 degrees to the ground. My catapult has similar specifications (range of motion of catapult, angle of pistons relative to the ground and the catapult arm, approximate length of launching arm) as 5003’s catapult, yet it cannot launch large balls as well (with 3 pistons, it can barely pass the barrier at 100 psi). Is there any other factor that I’m missing out on that might be hindering the catapult?

On a related note, what’s the best way to make double-acting solenoid act like single-acting solenoids?

  1. Use two solenoids connect both B ports together with tubing(my fav method)
  2. Take 1inch of tubing, bend it in half, zip tie it hard. Plug into B port.

Just figured I would jump in kind of late here.

We spent an insane amount of time tweaking our catapult throughout this season and ended up going to the Open with probably the fifth version of it. One of the things not usually brought up is how the large ball is cupped by the arm when thrown. With our current thrower we put more time into designing how the ball would be held by the arm than how we were actually going to throw it. We have two bent prongs on the end to stop the ball from slipping off of the arm until just before it reaches the end of its range of motion. The ability to control where that pesky weight within the large ball comes to a rest before it is thrown in crucial. If the ball is pulled in and the weight is resting out in front of the tip of the arm, then you will have trouble getting the ball to not roll off of the arm before it is supposed to. I will try and post a few images of how we achieved a fairly consistent centering device when I get the chance!

We just used S/A solenoids and hooked up only the bottom input on the D/A cylinders. Really the only reason we did this was to:

  1. Not us S/A cylinders because of the opposing force of the return spring
  2. Not use air to lower the arm. Gravity works fine!

The mass thing did help- it hadn’t occurred to me that the mass might be in different parts of the ball when we load it on the the robot.

Will connecting the b-ports together waste any air or damage the solenoid? I figured that if there is any delay in activating either one of the solenoid, some of air might leak out of the deactivated side.

Naw, it’s fine, besides, I’ve done it before and there is no way for damage to occur anyhow. Air being wasted is also something not to worry about.