Archive for April, 2010

 

Water Rocket Car MkX – Launch Day

The Easter break allowed a chance to actually do some work on the water rocket car and to upgrade the car to test some stability ideas we have been working on. These include

– outrigger rear wheels
– rear wheels rear of the nozzle
– larger rear wing
– bottle alignment guides

All of these design changes added a bit of weight to the car but we wanted to test for stability rather than just increase distance. What we were after was a straighter launch

We also had a small update to the launcher with the addition of a 20L container to hold 20L of water to hold the launcher down and reduce recoil. It also doubles as a good water supply for rocket fuel :)

Here is the video of the launch day

The longest distance recorded was 87m but that was uphill so over 100m on the flat isnt too much trouble even with the extra weight. We were happy with the results of the stability upgrades with the last 120psi run going straight up the middle of the road.

We will do some more work on this model before we build the next one, there are a few more things to test yet. Hopefully the next update wont take soo long

Posted by on April 12th, 2010 Comments Off

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.

Posted by on April 11th, 2010 2 Comments

Water Rocket Car MkX – Thrust Alignment

Another small update today on the car. This update is to ensure that all thrust generated is directed out the back of the car and not slightly sideways. Removing any small sideways thrust vector ensures that all of the thrust goes toward propelling the car in the right direction and not fighting the tyres to push the car offline.

Thrust Vector Problem

The bottles are held in place with cable ties that need to be slightly loose under normal atmospheric pressure. At launch pressure (90-120psi) the bottles expand and then the cable ties hold the bottles tightly in place. If the cable ties are too tight at normal pressure they can break at launch pressure. Having the bottles slightly loose makes it difficult to know if the bottles are pointing where we want it too under pressure.

To ensure the bottles (and therefor thrust ) are pointing directly out the back of the car, two extra pieces were added at the necks of the first and second bottle. The extra pieces hold the bottles in the exact location and keep them aligned to a centre marker at the back of the car.

Bottle alignment guides - rear 2

Bottle alignment guide - rear

Bottle alignment guides - all 3

The bottles under normal pressure and launch pressure (hydrostatic test) now point directly out the back of the car.

These extra pieces add some extra weight to the car but we will fix that in a later design if this works.

Posted by on April 10th, 2010 Comments Off

Water Rocket Car MkX – Stability Update

Its been a while since we have done any work on the water rocket car. We have put some thought towards a new design but also how the new concepts could adopted to the existing car. The Easter break gave us a chance to actually make some of these changes to the car.

The main changes are
- outrigger rear wheels
– rear wheels rear of the nozzle
– larger rear wing

The larger rear wing was necessary as the ejecting water spray would have hit the uprights of the current wing. The first two modifications are the ones we want to test out. These changes are designed to make the car more stable directly after launch and help it track straighter. We found under the highest thrust at launch we were constantly trying to catch the car with steering input as it usually didnt launch in a straight line.

To add these changes to the existing car and still keep the 3 bottles, required an extension be added to the rear of the car. A separate section was cut out and glued + screwed to the rear of the car. Also some of the existing rear end was removed to just reduce weight (this was later put back on just cause it looks cool).

New Rear Section + Wheel Mounts

The centre section of the new addition was removed to save weight as it wasn’t needed structurally. The existing wing was disassembled and a new centre section made up and reassembled. The wing is 50% wider than the previous wing. The wing uprights are screwed into place in the photo below.

Rear Wing Uprights

The wing itself was then screwed into position and set on the middle downforce position. The wheels have been put on and bottles sat in position  (the bottles are not all strapped down in the photos below).

Longer Bodied MkX

Longer Bodies MkX - Front

Note the size of the area under the wing for the spray to get out without hitting the rear components of the car. This car is quite long now, measuring in at 1.35m

We hope to be able to give this a test run next weekend.

Posted by on April 6th, 2010 2 Comments