Archive for August, 2009

 

Water Rocket Car MkVI – Launch Day

A glorious Saturday afternoon , a great way to forget the worries of the week, testing water rockets cars with my fearless 8 year old water rocket assistant. Today was our first test day at our new found launch site. A newly formed industrial complex about 15 minutes from home. There are no buildings up on the site yet, so we had the place to ourselves . .oh and the L plate learn to drivers who we had a good chuckle at between launches as they ran up the gutters practicing 3 point turns ….

Here is a pic of the road surface

Looking back to the launch position

Looking back to the launch position

We had to use a petrol blower to clean the small rocks off the road , we cleaned a stretch of about 80m which took about 30 minutes. A perfect test track.

We had about 7 launches. We tested water only and a foam (water + bubble bath) mixture in the bottles to see which produced better thrust. Here is the video of the launch day

And here are the results of the launches

Test Pressure Fuel Used Distance Fuel remaining after launch
120 psi Water Only (1.5L) 30m (*big crash *) not measured
125psi Water Only (1.5L) 60m 300ml
125psi Foam (1.5L) 75m 100ml
140psi Foam (1.5L) 55m (* small crash *) bubbles only
140psi Foam (1.5L) 76m bubbles only
140psi Water Only (1.5L) 70m 100ml
125psi Water Only (600ml) approx 35m empty

The foam did make a bit of a mess … gooey I think the description was …..

Mess after a foam launch

Mess after a foam launch

The fill stand came in very useful. It allows us to fill the bottles in the car without needing to hold onto it. This pic is the water rocket car in the stand filled with the foam mixture. A blue strawberry flavour ..

Water Rocket Car in the fill stand

Water Rocket Car in the fill stand

Conclusion
- We were quite happy with the performance of the MkVI water rocket car. Our previous record of 38m was well and truly bettered by our best launch of 76m. It probably would have gone further as it was starting to go uphill the last 15m.

– We didnt have any rear end swapping in this car, so the rear fins are still doing their job.

– We did have the launcher blown backwards and to the right on a 140psi launch, so we needed to find a second 15-20kg rock to hold the launcher down. This seemed to work.

– The thicker cable ties worked well, none snapped even at 140psi

– Its pretty tough this car, after slamming the gutter when the front left wheel stuck and barrel rolling at least twice .. we just dusted it off and put in back on the launcher ….

Posted by on August 10th, 2009 Comments Off

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

Water Rocket Car Fill Stand

We found it was annoying balancing the wrocket car vertically while we fill it. Especially in the two bottle design where the nosecone extends past the front of the wrocket car. The solution was to build a quick Fill Stand. This took all of 15 minutes to put together and makes it much easier to fill. The wrocket car just sits vertically in the stand so we can fill it without holding it.

Stand - Side view

Stand - Side view

The stand is just made from offcuts of wood and there is enough space for the nosecone not too hit the ground even when the bottles are totally full.

Wrocket Car in the stand - front view

Wrocket Car in the stand - front view

Wrocket car in stand - side view

Wrocket car in stand - side view

Nosecone not touching the ground

Nosecone not touching the ground

The small piece of wood between the bottles has 2 purposes. The first is during a launch to stop the bottles flying off without the car and the second is in the stand to support the weight of the bottles

Close up of Bottle support

Close up of Bottle support

We slightly changed this support from the previous one as we found that during a test the smaller support actually caused the lower bottle to cave in when we had a crash (dont have a pic of it). This new design spreads the load across a larger area of the bottom of the bottle so this should not occur in future crashes.

Posted by on August 1st, 2009 Comments Off

Small Diameter Nozzle Failures

Testing new ideas plays a big part in water rocket development. While trying to keep the wrocket car straight the idea of reducing the diameter of the nozzle was an obvious choice to even out the thrust. So we tested a few different ways to create a smaller diameter nozzle.

Its often said you learn more by failing at something .. and whilst most of these attempts failed it has  taken us in the right direction to accomplish the result. We decided to post the details of these failures anyway to show the different approaches to solving the problem

1. Glued on Nozzle

We tried to glue the nozzle onto the outside of the bottle cap using 2 part Araldite and put a 6mm diameter hole in the bottle cap. This would in effect create a 6mm diameter nozzle as compared to the normal 9mm diameter of the gardena nozzle. There is a rubber seal between the nozzle and the bottle cap. Both the nozzle cap and the nozzle were sandpapered to create a rough surface for the glue to bond to. Here are some pics

Nozzle Parts before glueing

Nozzle Parts before glueing

The araldite was applied to the nozzle, both sides of the gasket and the bottle cap and left for 30hours to dry.

Held in place for 30 hours

Held in place for 30 hours

Here is the nozzle and bottle top glued.

Glued Nozzle

Glued Nozzle

View of the 6mm nozzle

6mm nozzle

6mm nozzle

It was attached to a 1.5L bottle and pressure tested

6mm Nozzle of 1.5L Bottle

6mm Nozzle of 1.5L Bottle

The results was the nozzle failed at 80psi. It was an explosive failure not just a leak, the bottle flew off and the nozzle section was left in the launcher. Here is a pic of the nozzle pieces after the failure

Failed Connection

Failed Connection

Reason for Failure:

The most likely cause of this failure was the rubber gasket itself. It looks like it failed in the rubber material itself possible as it was being stretched due to the air pressure. Its possible the glue may have bonded better to the nozzle and bottle cap directly without the gasket, however the bottle cap lid is slightly curved so we thought we would need a gasket.

2. Nozzle Filled with Araldite and Drilled out

Another method we used to try to obtain a smaller diameter nozzle was filling the nozzle itself with Araldite, waiting until it was set then drilling out a 6mm hole. The nozzle sidewalls were sanded prior to filing with Araldite.

Nozzle filled with araldite

Nozzle filled with Araldite

This didnt work out either as the whole section of Araldite turned and lost grip with the sidewalls of the nozzle when it was drilled with a 6mm drill bit. The slug of Araldite just fell out.

3. Nozzle filled with Araldite + 6mm drill bit

This attempt used the same process as above by filling the nozzle with Araldite but this time having a 6mm drill bit placed down the centre of the nozzle until it was nearly dry. The idea was to pull it out just before it was set. This didnt work out either as the Araldite went off quite quickly and it was too late to remove the drill bit. Again the whole Araldite slug came out as we puled out the drill bit.

4. Metal gasket inside bottle cap.

The obvious answer was to put a thin metal gasket with a diameter just smaller than the bottle cap inside the bottle cap itself. IT is placed between the nozzle and the rubber gasket, with of course a 6mm diameter hole through the centre. We tried this first with a 10c coin and drilled a 6mm hole through the centre of it. For non Australians this coin fits fine into the bottle cap but it is probably 2mm thick. This was too much and there wasnt enough thread left to do up the nozzle on the bottle.

We are currently searching for an alternative for this metal gasket, something in the order of 1mm thick or less and just under the diameter of the inside nozzle. This we think will be the best solution and as soon as we have one built and tested we will post some pics.

Having done all this preliminary work, its possible now that we may not need a reduced diamter nozzle as the rear fins have provided much better stability, however for distance record attempts it may come in handy :)

Posted by on August 1st, 2009 Comments Off