Howto build a Launch Cage with Guide-rail – Part 1

We intend on competing in the WRA water rocket dragster class, which is a new water rocket class dedicated to water rocket cars. In order to compete and submit an entry for a record, the cars pressure vessel needs to be enclosed within a wire mesh cage as part of the safety rules. This can either be on the car itself or a separate launch cage. We opted for a separate launch cage. The launch cage ensures any shrapnel from a pressure vessel that ruptures on the launcher stays within the enclosed cage and away from spectators and launchees.

This three part post describes how we designed and built our cage. Whilst of course this is not the only way it can be done, there are some things we learned along the way which may assist others who want to build a similar cage.

The Design
The design had some basic requirements that we started with

  • The entire pressure vessel of the car needed to fit within the cage
  • We needed to be able to transport our cage to our launch sites
  • Not too heavy
  • Comply with the WRA safety rules – mesh size etc
  • Ensure it would be strong and arrest shrapnel

So we started with some basic measurements – the cage would need to be 155cm long to cover the entire car + launcher in the cage. For the width we quickly determined that we would need some leeway either side of the rear wings otherwise the wings could get caught up in the cage on launch. We decided to leave 10cm (4″) either side of the wings to the edge of the cage. For the height we left 20cm (8″) of space as well.

It was fairly obvious at this stage that a launch rail would be required to keep the car straight leaving the cage so it would not get caught up in the cage itself whilst launching.  Also it seemed we needed an access door on top to allow the water rocket car to be mated to the launcher. It was going to be to far to reach in from the front of the cage to do it. We now had our  basic layout for the cage. This is the sketched up plans we used to build the cage.

Basic Design - Launch Cage + Guide Rail

Building the Frame
I tend to like to work with wood as its more forgiving of mistakes and changes. We wanted to keep the weight down but ensure the wood we used was still strong enough not to break or split if we had a rupture. We had some leftover 70mm x 18mm pine from a previous project and decided we would use that for the frame. The 70mm was a bit of an overkill so we decided to split it down the middle to make 35mm x 18mm sections. This also doubled the amount of timber we had available to use. It was a bit tedious splitting all the timber with a jigsaw but the end result was fine. Im guessing we used about 16m of the 35mmx18mm timber all up.

Construction started by making the top section and the two sides separately, the joins are nailed and glued

Basic Frame, top section (left) and one of the sides (centre)

Basic Frame, top section (left) and one of the sides (centre)

The sides were then attached to the top section and glued in place, the clamps held everything in place while the glue dried

Sides and Top section nailed and glued together

Sides and Top section nailed and glued together

The next section was the access door. This is 30cm x 50cm in size and adequate enough to reach into the cage. It is nailed and glued and attached to the frame by 2 hinges. It stops flush with the top section of the cage by two stoppers.

Access Door - Closed position

Access Door - Open position

Next we needed a way to connect the launcher without having to physically connect it to the launcher or  screw it to the cage for a launch. Even with our 20kg water ballast on the launcher, the launcher itself still recoils backwards ~10cm (4″) on launch. We didnt want the launcher ripping the mesh of the cage when it recoiled, so we made a wooden section in the cage that fits over the launcher to connect them without impeding the launchers function. When the launcher recoils on launch it will drag the entire cage with it and not stretch the mesh section of the cage, the cage can also easily be lifted off and separated from the launcher.

Launcher Attachment

Launcher Attached to Cage

Here is the water rocket car in place to check out everything fits and there is enough space for the car to clear the sides of the cage as well as test the access door to the coupling of  the car and the launcher.

Test Fit of Water rocket car in the cage

Test Fit of car / launcher connection through access door

View from behind the launcher

This is the end of the frame build. The next post will cover the guide rail and additions to the water rocket car to connect to the guide rail.

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This entry was posted on Saturday, January 22nd, 2011 at 10:58 pm and is filed under Development. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.


2 Responses to “Howto build a Launch Cage with Guide-rail – Part 1”

  1. George Says:

    Very nice work Todd, I like how you added the access door at the top. It would be interesting to see an air-only burst test of a 2L bottle inside the cage, and see how the cage behaves during the explosion. :)

  2. todd Says:

    Thanks George, Yes i would like to see a burst test in there too :) I believe we have built it strong enough to withstand a burst test. We will probably try for the dragster record first then try some burst tests :)