Posts Tagged ‘robinson coupling’


Triple Bottle Reinforced Stack – Hydrostatic Test

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.

Hydrostatic Test of reinforced stack

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.

177psi or 12bar

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.

Posted by on August 20th, 2010 Comments Off

Water Rocket Mist Attachment – Static Test

An interesting development we saw on 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.

20mm Conduit Cap with 10mm ID nut - no glue yet

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.

Conduit connected to threaded rod (robinson coupling rod)

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.

Conduit cut to size of bottle

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.

Conduit section with cap and 2 holes drilled near the base

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.

Robinson coupling ready for mist attachment

Mist attachment connected to robinson coupling

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

2 Bottles with robinson coupling and mist attachment - 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


– 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

Posted by on July 27th, 2010 5 Comments

How to Make a Robinson Coupling

I have found a supplier of threaded lamp rods, washers and nuts that fits the bottle caps perfectly. If you are in Australia then Beal Pritchett Pty. Ltd. in St Leonards is the place to go :)

I wanted to make sure the parts all fit correctly so I put one set together, it made sense to make a video so it may be of some assistance to others trying to join bottles.

Parts used are

– 1 x 30mm x 10mm OD. threaded lamp rod
– 2 x 14mm OD nuts
– 2 x 18mm metal flat washers
– 1 x bicycle inner tube for 3 rubber gaskets
– 2 bottles – 1.25L in this case
– 1 bottle cap – standard 28mm bottle cap
– Threadlock – (optional)

Tools used
– 2 socket wrenches
– 2 x 14mm sockets
– 1 x shorts socket extension
– 2 x long socket extension
– Metal coat hanger
– Electric Drill
– 3mm & 10mm drill bits
– 10cent coin (22mm diameter)
– Sizzors

The video is below, expert camerawork by Mark :)

Posted by on May 30th, 2010 2 Comments

Water Rocket Mk VI – More testing

Robinson Coupling Update

We had a few test runs on the front driveway at 60psi but there were leaks, so back onto the workbench to strengthen the coupling on on MkVI. To be truthful, we should have had the additional washers in the robinson coupling configuration in the first place, but previously with 1.25L bottles and 130psi max we didnt see any leaks without the metal washers. Due to the increased volume and pressures we are working at now, we realize these are indeed needed.

We have inserted another metal washer at on the bottle cap side of the coupling so now we have one behind the nut at each end as described by the diagram below.

robinson coupling exploded diagram

Robinson coupling exploded diagram

The MkVI with the 2 x 2.25L bottles and above coupling configuration was hydrostatic tested at 140psi for 1 minute without any leaks. So its all ready to go for next weekends test at our new launch / track site.

Initial Test Runs – low pressure (100psi)

We have performed some initial test runs at 100psi (due to space limits at home driveway) to see how MkVI performed. No video unfortunately as it was just adhoc testing to empty the bottles after the hydrostatic tests. Initial  observations are

– Thrust lasts a lot longer than the 1 bottle config
– During pressurization some water is forced into the second bottle
– All the water is not expelled from the second bottle after launch
– The 100psi pressure launched MKVI to a distance of 38m which is similar distance to 1 bottle at 140psi.

Our driveway is long but narrow, so we really threaded the needle to get this distance which means directional stability is good.

Posted by on August 4th, 2009 Comments Off