Posts Tagged ‘rocket’


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

Water Rocket Car MkXI – The Green Hornet

A lot of work has gone into reworking the rear of the rocket car and reducing weight as much as possible. The modifications include
– New rear chassis of the car
– Low friction (Lighter) rear wheels
– Rear wheels much further back from the nozzle – reduce thrust steering
– New Rear fin design – larger wing surface
– New nozzle alignment guide
– New electronics bay

The following parts were also modified to reduce weight
– Chassis
– Nosecone
– Rear wheels
– Rear Fins
– Electronics bay
– Rear Alignment guide

As this will probably be the last major change to this chassis before we build a new one, we also gave it a nice paint-job and christened it “The Green Hornet”

Rear of the Car
The Rear section of the chassis was replaced with a new design that was lighter, had the wheels much further behind the nozzle, allowed for the new 45 degree wing design as well as a space to try a new additional nozzle / air amplifier (not pictured yet). The rear section is only glued to the front section of the chassis with wood glue. It is extremely strong, as we found out when pulling off the older rear section of chassis which was also glued on.

Rear of the Car

Rear of the Car

The rear wheels have had the rubber removed to both lighten the rear wheels and reduce friction with the road. The theory is to minimize the force required to straighten the car, ie: move the rear of the car back into alignment with the front of the car. We are trying to make it behave more like a rocket than a car as per our previous down-force rear wing design which tried to reduce thrust streering by maximizing rear grip. Testing will tell if this configuration will work or not.

The nozzle alignment guide has been lightened but still ensures the nozzle points directly out the back of the car.

Rear Fins
The new rear fins are made of corrugated plastic to reduce weight. A set was made from thin plywood first but they were to heavy and too flexible, the corrugated plastic ones with the corrugations running from the centre to the tip were actually stronger and lighter. A 45 degree piece of wood was glued to the fin using PL Premium. The wood is screwed to the chassis to secure the fins.

Rear 45 degree Fins

Front of the Car
Changes here include a new electronics bay setup with a flip top section that opens up to allow access to the servo and steering rod connections. The on/off switch has also been moved to a better location now protected within the nosecone.

New Electronics Bay & R/C Steering

The nosecone has also been lightened and of course painted green :) A hoel to access the on/off switch


Nose Cone

Chassis Weight Reduction
The new chassis is 1.40m long but a lot lighter than the previous chassis.The jigsaw and hole saw were put to good use to lighten the chassis as much as practical while still retaining its strength. Being painted white, it is easy to see the holes cut out of the chassis. Triangles were used where possible for cross-members to retain strength.

Chassis in white

Front Alignment Guide
The front alignment guide between the first and second bottle was left as is. This is the section where the rocket itself is imparting most of the force to propel the rocket car forward, without it the rocket will shoot off and leave the car where it is. So it was thought to best leave this with a larger surface area rather than reduce it.

More Pics of the Car

View from the front

View from the Rear

Rear Wheels - No rubber

Weight Saving
The car pictured above weighs in at 2.560kg which, while considerably more than a water rocket, is far below the previous version of the car which weighed in at 4.5kg. This weight reduction should improve both the maximum speed of the car as well as the distance traveled .. hopefully.

Additional Work
The new “additional nozzle / air amplifier” will be a second nozzle behind the main nozzle. The idea is to try to push in some of the surrounding air into the exhaust plume using a difference in air pressure between the exhaust plume and the air rushing past the rocket car. This will sit in the area, behind the main nozzle and in between the rear fins. It will be removable so we can test the car with it and without it.

We also need to strap the bottles down to the chassis and do a hydrostatic test to make sure we have allowed enough space for the bottles to expand under pressure with the new alignment guide.

We need to modify the launcher to increase the length of the hose from the launcher to the female gardena connector, this is due to the extra length added to the car and the nozzle now being further away from the rear wheels

Posted by on May 20th, 2010 Comments Off