Water Rocket Car MkX – Hydrostatic Mishaps

Its always a good idea to hydrostatic test a new setup before a launch day. Considering we hadnt launched these bottles for a few months, it was an especially a good idea to test them first.

The bottles were filled totally with water and pressurized .. no leaks ….  good .. right up to 120psi then … the last bottle ruptured. This was a rip across the middle of the bottle. Pic below

Ruptured Bottle - ripped

Whilst this was not totally unexpected, it was a bit disappointing but easily repaired after a trip to the supermarket for another 2.25L bottle. Also some of the water went into the electronics section so it was left in the sun to dry while we went to the shops.

The new bottle was cleaned and connected into the rocket car and a 40psi air test to check for leaks around the bottle cap. No leaks so the bottles were filled with water to hydrostatic test again.

Again no leaks up to 120psi .. then .. the last bottle (the brand new one) ruptured …

New bottle ruptured

This time spilling about 3L of water directly into the electronic bay .. hmmm .. not good :(
A close look at the ruptured bottle showed it let go right at the thick end of the bottle. Click pic for higher resolution photo

New Bottle ruptured - close up

This is the thickest part of the bottle, so it was either a manufacturing flaw in the bottle or it pressed up against something to make a weak point and ruptured there.

It could have been either that caused the rupture, but when looking closer at the setup, its possible the new bottle alignment guides we installed could have caused this. The way they were setup meant that when the bottles expanded under pressure all three could only moved towards the front of the car. So its possible that all three expanded causing the bottle that ruptured to press agaisnt the back of the servo and possibly cause a pressure spot.

We repositioned and realigned the bottle guides to allow the bottles to expand in both directions and slid all three bottles back in the chassis about another 10mm. This should now allow the bottles to expand without hitting the servo.

Another downside of dumping 3L + of water into the electronics section is that the r/c wouldn’t  work. The electronics bay was pulled apart and dried and all connections checked with a multimeter. It appeared the on/off switch enclosure was still very wet and was shorting, so it was replaced. Another was soldered in and remounted. It is more convenient to access now.

New on/off switch

By the time the electronics was put back together and an unpowered test of the steering done, it was too late to do another hydrostatic test. We will do one tomorrow and hopefully get to do a launch.

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This entry was posted on Sunday, April 11th, 2010 at 12:07 am 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 “Water Rocket Car MkX – Hydrostatic Mishaps”

  1. George Says:

    Hi Todd,

    Interesting about the bottle failures at the base. From previous experiments we’ve done, we found that the 2.25L bottles stretch in length 8.5mm @120psi , so a stack of 3 of them is likely to stretch three times that. They only stretch 2mm @ 60psi.

    I bet it was an unexpected boom when it went off. :(


    – George

  2. todd Says:

    Many thanks for that info George. We decided to play it safe and moved the 3 bottle stack back another 20mm towards the rear of the rocket car.

    The noise wasn’t too load just sounded like a waterfall gurgling .. straight into the electronics though :( ..