HHWRSA

We have had our triple bottle stack of reinforced bottles ready for a while to hydrostatic test and finally got around to it this week.

Each bottle in the triple stack is reinforced using this method.  We had previously hydrostatic tested individual reinforced bottles to 180psi but not a stack of them. The 3 bottles in the stack are held together using robinson couplings.

Our goal was to hydro test to 180psi. The bottles were filled with water, safety glasses were put on and being well back we pumped up to 180psi. This was a pic we took which says 177psi but we did put a few more psi in it to be sure we got to the 180psi. We left it for about 30 seconds before releasing the pressure with the abort valve.

We didnt see any leaks which was good, so it was a successful test.

The triple stack has been fitted to the water rocket car ready for a test run. Based on the success of the last launch day we will probably need to fit a parachute to the car to slow it down, so we will turn our attention to that next.

This launch day was to test the mist rocket attachment in a horizontal configuration as well as try out the camera that was mounted to the water rocket car.

Here is the video

Both of the runs went well. The rubber tyres on the back have improved the stability again so this seems the way to go.

The mist attachment worked ok, but there was a lot of water left in the bottles, probably 500ml out of 1.5L. So we will need to improve this. Also filling the second bottle without getting water back in the first or filling the third bottle was difficult, so we will need to find a way to improve that.

The actual distances we achieved were very good. This is primarily due to this being the first successful full run we have done with the lighter chassis. The second run at 120psi could have gone at least another 20m if we had enough road. The 144.5m (458ft) will stand as our current record, for the moment anyway. We did have a bit of wind on the day which may have assisted a little but not significantly

The camera mounted on the car worked well, we did some test runs previously with the camera pointed backwards towards the nozzle but it just got covered in water from the launch which made for a pretty uneventful video.

We are off to do a water rocket  demo at one of the State High Schools tomorrow, so we are all ready for that. We are keen to share a bit of WRcar  knowledge with the students and see what they can come up with.

An interesting development we saw on www.wra2.org from Jelo and Thunderrockets was a device called a mist rocket. This is an alternate method of mixing the air and water into the exhaust plume which produces a single air/water thrust and not the normal water then air pulse of a standard water rocket. Also when flown vertically there was a distinctive jet sound.

We though we would give this a try for our water rocket car.

The design uses a PVC pipe to funnel water from the second bottle to the nozzle. The air pressure in the top and bottle bottles are equal. The air in the top bottle pushes the water down the pipe and the air pressure in the bottom bottle forces air through two (2) small holes to mix air with the water just prior to escaping from the nozzle.

This is how we built it and tested it.

First we purchased some 20mm electrical conduit and a conduit cap. The 20mm conduit just fits in the 22mm bottle throat. A 10mm hole is drilled in the centre of the conduit cap and a 10mm internal diameter (ID) nut from our robinson couplings is carefully glued (with 24hr araldite) into the base of the cap, making sure no glue gets on the threads.

The conduit cap is then glued (24hr araldite again) to a section of 20mm conduit (longer than the bottle at this stage) and left for 2-3 days to set properly. Once this is set, this section should screw easily onto a standard 10mm robinson coupling thread.

The next step is to cut the conduit so that it fits just inside the nozzle cap. The section is screwed onto the robinson coupling between two bottles then marked, unscrewed again and cut to size. The conduit shouldn’t protrude past the bottle lip as this will cause the bottle lip not to seat properly against the rubber washer in the nozzle and the bottle wont hold pressure.

The conduit section then needs 2 holes drilled near the base of the rod. we used 6mm holes, these are to let air from the bottom bottle mix with water from the top, also if any water does get in the bottom bottle it allows it to escape.

The completed conduit (now referred to as mist attachment) is then inserted into the bottle and screwed onto the robinson coupling. Note that about 10mm of thread was needed on the threaded rod to catch on the nut threads.

The the nozzle is screwed into place and its ready to test / launch. Here is the pic ready to test

The handy thing with this attachment is that it can be added and removed quite easily for test or launch.

Mist Attachment Test

The test we conducted was a vertical static test with 1L of water (no foam) and 100psi. The bottles used are 2.25L bottles. We were interested to see the following

- If we could reproduce the jet sound that thunderrockets produced on launch
- Examine the exhaust plume to see if it generate a good air / water mix
- Examine if there was any distinct air pulse after the water was ejected from the bottles

We did find that filling the top bottle a little more challenging to ensure water didn’t get in the bottom bottle. We used a small section of hose connected to a funnel to get the water past the 2 holes in the mist attachment, then gently pumped the top bottle.  Here is a video of the test

Conclusion

- We didn’t get the jet sound, even after 2 separate static tests – its a possibility that the rocket needs to be flying through the air to generate the sound, or possibly the holes were too big.
- The exhaust plume definitely had a good mix of water and air and produced a spray similar to a foam launch
- There was no distinct air pulse after the water was ejected
- An interesting observation was that there was minimal (a small amount) splash back in the bottle at the end of the thrust phase.

We will try this test again in a horizontal configuration to suit our water rocket car