Archive for July, 2009

 

Water Rocket Car Hydrostatic Test

In order to test the bottles and the connections we setup a hydrostatic test to see what pressure the bottles would take and if there were any leaks. A hydrostatic test is the safest way to test bottles as the water is not compressible so there is not much stored energy in the bottle at high pressures.

The hydrostatic setup is basically our launch setup with the abort valve pipe going into a bucket, or in the case a container to hold the overflowing water. In our setup once the desired pressure is reached and the test complete, the abort valve is turned and the water is sent out to the bucket for collection. The wrocket car never leaves the launcher

Hydrostatic Test Setup

Hydrostatic Test Setup

The wrocket bottles are full totally with water.

Wrocket Car Full of Water

Wrocket Car Full of Water

We pressurized to 60psi and held with no leaks. Approaching 120psi there was a leak from the robinson coupling and water was escaping. IT was more than a few drops so we will take the bottles apart and fix it. We are currently only using a washer at one end, so we will add another at the other end. We wil lalso use a bi tof locktite on one of the nuts to hold it in place on the threaded rod.

Posted by on July 30th, 2009 Comments Off

Water Rocket Car MkVI

After our latest successful tests of having the wrocket car going straight, its time to work on increasing the speed. There are two ways to do this

– reduce weight
– increase thrust

We decided to work on both ….. some design work has started on a new lighter longer bodied wrocket car. Hopefully this will see the light of day in a few weeks. While this is in the works, we have added another 2.5L bottle to the existing wrocket car to increase thrust on the existing car …..  and here she is

2 Bottle Wrocket Car

2 Bottle Wrocket Car

The modifications we have made are as follows

– Added second 2.25L bottle with an enhanced Robinson coupling
– Changed the position of the bike computer
– Installed a smaller stopper between the first and second bottles
– Used stronger cable ties to hold the wrocket down
– Added a nosecone to reduce aerodynamic drag
– Moved the front wheel angle supports to allow space for the second bottle

The enhanced robinson coupling uses an additional washer to assist in sealing a bigger area between the bottles.

Stronger Robinson Coupling

Stronger Robinson Coupling

Robinson Coupling

Robinson Coupling

The bottle connection is made as per our previous howto post showing how to make these couplings except for an additional washer. We tested the coupling up to 60psi without any leaks. We will do a hydrostatic test up to 140psi to see if the coupling will hold the intended launch pressure.

The bike computer has just been moved to a vacant spot to get out of the way for the second bottle. The nosecone was made from the top section of a 2.25L bottle with a half ping pong ball glued (with PL Premium) in place of the spout.

Front View

Front View

A smaller stopper is screwed in place betweeen the bottles. This is used to arrest the forward motion of the bottles on the wrocket itself. The base of the rear bottle on launch hits up against this stopper and the cable ties then hold it in place.

New smaller stopper

New smaller stopper

Here is a view through the nozzle showing the coupling. Just visible in the centre of the coupling hole is the molded join of the bottom of the far bottle. Click on the pic below for a larger view.

Nozzle View

Nozzle View

It will be interesting to see how the water and air mix going from one bottle to another in a horizontal position as well as how much water is left in both bottles. We will do some tests but have a solution in mind to keep the first bottle full of water and leave the second for air only .. we will see if we need it or not after or next test day.

Posted by on July 28th, 2009 Comments Off

Water Rocket Car MkV

This weekend we got some better testing with the new fins. Here is the video

First Test – 1500ml water and 120psi

This test went well. Pretty straight. The slight veer to the right was a wheel alignment issue as it was slightly off. An unpowered test went to the right as well

Second Test – 1500ml water and 130psi

The wheels were aligned a little better for this test. We hadnt been able to keep the wrocket car straight at 130psi with the 2.25L bottle without the fins. So this would be a good test to see if the fins were making a difference. One of the cable ties that tie the bottles down to the wrocket car base snapped at this pressure. Although this time we took a few extra cable ties to do running repairs between launches. This was one of our best launches yet, straight down the middle and out onto the road …. Seems the fins are working :)

Third Test – 1500ml water and 140psi

This was a test again to see if the extra thrust would spin the wrocket car. You can see it did veer right a little but then continued going straight which is what we wanted. Previous high psi launches where the car has started to veer under high thrust caused a spin. Here the fins arrested this and the rest of the way it went straight.

Forth & Fifth Tests – 600ml water and 120psi (no ton video)

We ran a forth and fifth test with less water, but the nozzle developed a leak. We think it was damaged when it went down the drive and bottomed out hitting the road from the third test run. We tried resetting the seal in the nozzle but it kept leaking at 120psi so we just launched at that pressure. The runs went pretty straight as well and actually had slightly higher peak speeds at 20.7kmph.

Posted by on July 28th, 2009 Comments Off

Water Rocket Car Mk IV

We managed to design and fit two rear fins to the back of the water rocket car. These are to try to improve rear stability under high thrust. There are two fins due to the fact that it was difficult to mount a single fin big enough on top of the bottle.

The fins are made out of 7mm plywood and are 250mm x 200mm in size (approx). Each weigh approx 130g. They are attached to the body of the wrocket car front and rear with small “L” brackets and screws. They are very solid and there is no flex in the fins.

This pic is the cardboard template and the first fin

Cardboard Fin Template + Plywood Fin

Cardboard Fin Template + Plywood Fin

The fins are notched out around the metal bracket that attaches to wheels. You can see the rear L bracket. There is 50mm overhang of the fin past the rear of the wrocket car

Left Rear Attachment (wheel removed)

Left Rear Attachment (wheel removed)

Here is the attachment at the other end of the fin

Fin front attachment

Fin front attachment

Here is a pic of the fins in place without the bottle

Fins attached minus bottle

Fins attached minus bottle

Rear shot of Fins minus Bottle

Rear shot of Fins minus Bottle

And finally with bottle attached

Bottle attached + fins

Bottle attached + fins

Rear shot of Fins + Bottle

Rear shot of Fins + Bottle

For the launch we found a 15kg (approx .. its small but heavy) rock nearby and sat it on the launcher to stop the recoil. We only had time for 3 launches due to fading light. Here is the video

First Test – 1485ml water and 120psi (not on video)

The first launch is not on the video. The wrocket car arced off to the right. We didn’t do a wheel alignment after a test at home to check the bike computer. That test ended up crashing into a rock and flipping. So we did an unpowered test and it was obvious that a wheel alignment was required. After a quick wheel alignment we fueled up and tested again

Second Test – 1485ml water and 120psi ( on Video)

The second test was again at 120psi and 1485ml (2/3 full). This one went straight out of the launcher and continued straight. There was a bit of deviation right at the end of the thrust phase but not very much. This launch was measured at 20.2km/hr. Not super fast, but a good test of the bike computer. The rock worked well as the launcher didn’t recoil at all. The slow motion shows the pretty straight launch

Third Test – 1485ml water and 130psi (not on video)

After the second launch and refill to 2/3 full, we put the car on the launcher only to see the rear wheel nut was very loose. I tightened it by hand it was holding but wobbly. we decide to launch it anyway and see what happened. At launch the rear wobbled a bit and as soon as the car started heading left it was uncorrectable and it ended up doing a 180 degree turn and in the bushes.

We will do some more work before next weekends tests

Posted by on July 19th, 2009 Comments Off