The Launch Pad

Every good rocket launch starts with a good launch pad.

Goals

Your launch pad needs to accomplish a few simple things:

  1. Support the rocket - keep in mind how tall and how heavy your rocket will be as you design your launch pad.
  2. Restrain the rocket - as soon as you start to pressurize your rocket, it will want to leap off the pad, so you need a way to hold it down until you are ready to launch.
  3. Release - When you are ready to launch, you will need a way to detach your restraints and let the rocket go.

As long as you accomplish these goals, you have a lot of flexibility in how you design your base platform.

Launcher Base

This tutorial gives a step by step of one example of a launch platform.

Parts List

Here's a list of the basic parts you'll need to build this version:

assembled parts

The tools you'll need will vary, depending on your approach, but you'll at least need something to cut the PVC with, and probably a wrench to tighten the threaded fittings. Also, it's probably a good idea to dry fit all your PVC components before you start gluing.

Steps

  1. Insert the two 4" lengths of 1" PVC in either end of the PVC Tee with the 1/2" threads: assembly step one
  2. Now add the other two PVC Tees to the assembly: assembly step two
  3. And now the 8" lengths of PVC pipe: assembly step three
  4. Now you need to add end caps so this assembly will hold air. The three PVC end caps block three of our openings: end caps
  5. For the final opening, we fill that with the PVC bushing. This allows us to attach an air hose coupler with ease: PVC bushing
  6. Once all the above fittings are glued and dried, you can attach the brass bushing for the air hose connector. Most air hose adaptors use 1/4" connections. If you can find PVC with 1/4" threads, you can save a piece here, otherwise, use a black iron or brass bushing as I have here. Wrap the threads in Teflon tape, and thread the brass bushing into the PVC. Then wrap the threads of the air adaptor and thread it into the bushing: Air connection Air connection assembled

Retention/Release Mechanism

This is certainly the most complicated part of any water rocket launcher. The key is to be able to hold the rocket in place with a relatively air-tight seal, until you are ready to launch. And then easily let go so the rocket can leave with as little resistance as possible. The mechanism used here is known as the Clark Cable Tie method. It is fairly easy to build and very reliable.

Parts List

Steps

  1. Start by laying a strip of duct tape upside down. Use a strip that is about 8" long. I like to tape the ends of the tape to the bench so it won't curl up or slide around. Upside down tape
  2. Take a cable tie and tape it perpendicular to your strip of tape somewhere in the middle of the strip. Make sure the bump of the connector end of the tie is facing up. How high to extend the tie above the tape is a matter of trial and error. In this case I have it extending 1.25" above the tape. Upside down tape
  3. Now continue laying all the ties next to each other. Make sure they are even at the top and parallel to each other. In this case I've added enough to go around a bottle without a break. You can space them further and use fewer ties, but this works. All ties in place
  4. Take the 4" sprinkler riser and thread it into the 1/2" threaded coupler. First riser
  5. Now cut the ends of the tape where you taped them to the table (this is a lot easier than trying to pull off the strips of tape you used to hold it to the table), and carefully wrap the zip ties around the riser and coupler. If you extended your ties 1.25" above the tape like I did, line up the edge of the tape exactly with the edge of the top of the threaded coupler, like you see in the picture: Wrapping the ties
  6. Use the remainder of your duct tape by wrapping it tightly around the ties several times. You can cut a few more lengths of tape and continue wrapping down to the bottom of the riser. Wrapping the tape
  7. Once the tape is wrapped tightly, use some wire cutters or scissors to remove the bottoms of the ties that extend past the threads of the riser. Your final product should look something like this: Wrapping complete
  8. Now take your hose clamps and tighten them firmly around the wrapped ties on the riser. You can then thread the riser into the base. The bumps on the ties will hold the lip around the neck of a 2-liter bottle: riser mounted on the base
  9. The 1.25" coupler slides over the zip ties. This can be raised into place when the bottle is on the launcher to hold the ties closed around the lip of the bottle. A section of bottle makes a nice spring to hold the coupler in place until the bottle is pressurized. You'll want to drill a small hole in one end of the coupler so you can tie a string to it to use to pull it down for a launch. Sleeve

The launch tube

Now that we have the ties in place to hold the rocket down, we need a launch tube that will align the bottle with the ties and keep the water from leaking out. The method we are going to use here involves a longer sprinkler riser with a shallow groove cut in it to hold an O-ring. This is a fairly simple method, but making the groove can be difficult without the right tools.

  1. Take your 18" riser and cut it so when you slide a bottle down over the riser, the end comes an inch or so shy of the bottom of the bottle. In my case, I ended up cutting the riser at almost exactly 10".
  2. I'm using a table saw to cut the groove. I raised the blade to just a hair over 1/16". It may be best to start with a shallower cut and make repetitive cuts, raising the blade a hair each time, until you get a fit that allows you to slide the bottle over the O-ring with just a bit of pressure. The saw blade
  3. I set the fence so the blade cuts the pipe a little over 1/4" from the end of the threads on the end. I then use my miter gauge to keep the pipe perpendicular to the blade, and after sliding the pipe over the highest point of the blade I slowly rotate it, taking great care to keep the end pushed tightly against the fence. Note that I'm using a thin kerf saw blade, which will cut a groove just a hair wider than my O-ring. A regular saw blade might cut a groove that is too wide. cutting the groove
  4. Place the O-ring around the pipe and roll it into the groove.
  5. Thread the riser into the coupler with the zip-ties.

Other Options

There are lots of ways to build a base for your launcher. Because PVC pipe and fittings are so common and easy to work with, there are many variations of launcher base that will work great.

Base Variations

Here's an example of a base that uses the Clark Cable-Tie method on a "U" shaped base using 2" PVC: U-shaped launcher #1 U-shaped launcher #2

And another example made from a single 2" PVC Tee with a simple wooden base: Wooden base

Release Mechanism Variations

Another area where you can be creative is the retention/release mechanism you use. There are a variety of quick-release garden hose adaptors that seem ideal for this job. This launcher uses a small PVC base mounted in some wooden blocks for stability, with a small Gardena quick-release adapter mounted on top: Gardena

Of course, the quick release can be mounted on any base. The only additional parts you would need to put a quick release on the launcher base we built above would be:

The detail photo below shows how it is all assembled: Gardena launcher detail

Note that the springs continually pull the release down, while the washers hold it up in the locked position. Releasing the rocket is as simple as pulling the washers out.