Important Update on June 17, 2010!
For the latest and vastly improved launcher follow this link
then continue here to view the original.
My First Launcher
When I decided to start working on this water rocket project I wanted to begin with an original design for a dependable launcher and be satisfied with that before taking on the challenge of designing and building more than simple rockets. I just wanted to see a plain soda bottle blasted over the tree tops by a launcher that worked smoothly, was adaptable and could be easily modified and repaired if necessary. When I made that first launch I knew I wanted to continue with water rockets!
Here is my completed launcher.
That is the plastic base from a broken office chair with five nails replacing the wheels for stability. The outer PVC support pipe fits tightly into the chair base’s upright metal column after the chair’s pneumatic lift cylinder was removed. The air line is connected to a smaller diameter PVC pipe within the support column by a metal fitting and PVC elbow visible just above and to the left of the chair’s metal upright. Not visible is a long slot cut in the bottom of the PVC support pipe that allows the elbow to pass through to where it is now. This allows the pressurized inner pipe to be height adjustable and removable.
Here you see the launch tube that is used for full bore launches. It is also usable with the secondary reduced nozzle that I built that drops into place at launch. Details of how to make and install that secondary nozzle can be found in the ROCKETS section of this blog. The launch tube is reduced in outside diameter at the lower end to fit inside the pressurized PVC pipe. An O-ring around the reduced part helps keep it in place but it is designed to be easily and frequently removed. Sometimes under higher pressure it pops off when the rocket launches. I want to prevent that and will try to fix it by adding a second O-ring for more friction. Pressure is not really held by the O-ring but instead the top of the pressurized PVC pipe is smooth and seals against the lower side of a garden hose washer on the launch tube that seals against the rocket bottle’s lip. The launcher’s jaws hold them tightly together to seal.
The PVC fittings at the base of the launch tube are a threaded mating set. The top piece with male threads has slots cut into it to hold the jaws on the bottle’s flange. The lower piece is not cemented to the PVC support column like it normally would be but instead has three small bolts holding it on. One is visible on the ring of blue tape. This allows easy replacement if necessary but more importantly they hold the pressurized PVC pipe in place. The fitting and support column have three matching holes through them that are tapped for the bolt threads. The bolts are only tightened against the pressurized pipe and can not pierce it.
The threaded fittings were used because they provide easy adjustment in getting the right tension to hold down the bottle. Turning the top fitting changes the distance between the launcher’s jaws and the hose washer that the bottle seals against. Different bottles may need different distances to seal them properly. This achieves one of my goals of making a launcher that is adaptable to most bottles.
Here is a close-up of the jaws. They were made from metal corner braces with most of one side cut off. Then the short side was filed to a wide “V” notch to better grip the bottle flange. The holes in the center of the notches were already drilled in the braces and probably aren’t needed. Make sure the jaws can reach the launch tube so don’t cut them too short. Notice how the metal jaws have a slight bend in them. This allows them to pivot at the bend and provide leverage to open the jaws to release the bottle. The yellow pieces are two plastic jar lids, one up, one down, with a large hole drilled in their centers to fit tightly around the PVC fitting’s threads. They provide support for the jaws. That is a piece of heavy bungee cord tied around the jaws to hold them in place against the yellow lids. The blue ropes pull down on the long ends of the jaws to make them open.
A bottle secured by the launcher’s jaws.
This shows the ropes that pull the jaws open and the hardware that helps to guide them. To work smoothly the ropes must be pulled straight down and be even on both sides to make both jaws open simultaneously. If not the rocket will lose a lot of pressure before the late opening jaw lets go. The PVC and metal ring guiding the ropes fits snugly but easily turns to match the rotation of the threaded part at the top of the launcher.
This is the last part of the launcher mechanism. The long eye-bolt with wing nut provides tension adjustment on the ropes. The large U-bolt around the metal chair column provides a strong anchor for downward pull. It easily can be rotated around the column to match the rotation of the rest of the parts. It also supports the launch lever.
The black curved metal piece is the launch lever. The launch cord attaches with a hook to the upper end. The offset weight of the piece tends to keep it against the support column. If the lever looks familiar, it is a common part that links a garage door to the electric opener machinery. We had an extra one lying around after I had to replace the old opener. The curved shape seems to give it more leverage but I can’t explain the physics behind it.
Here is its position after a launch. I swore the first time that I did an actual launch that the pull was so light and smooth that a cat could set it off. That really helps keep the launcher steady and vertical.
This shows how there is still enough clearance to use ring style tail fins on 2 liter bottles with this launcher. Another goal for the launcher because that is the only type that I’ve used so far. This rocket uses vinyl mini blind slats to attach the ring to the rocket. The white ring is a 1 inch section of 4 inch PVC drain pipe attached to the blind slats with rubber bands. A section cut from a 2 liter bottle is forced over the PVC ring, with more rubber bands holding it on the blinds. The blinds are wedged between the bottle and another bottle section forced over its middle. This all collapses when the rocket crashes at low altitude tests and can be reassembled again usually without breakage.