Building a 3 bottle Water Rocket Car

This post is about the design and build of our fastest water rocket car yet MkVII. Its a 6.75L rocket using 3 x 2.25L bottles.

The design was sketched out on paper first and some basic measurements taken from the existing water rocket car. Here is a scanned version of the design ….

3 Bottle Water Rocket Design

3 Bottle Water Rocket Design

Some of the design goals included

– slimline and lower weight base
– crash bumper at the front
– more aerodynamic front
– narrower section in the middle of the base to reduce weight

This design also includes “ground effects” downforce at the rear of the rocket car, shown as the hourglass shape at the rear. The idea is that the air under the rear of the car is funneled into a smaller cross sectional area. It takes less energy for the air to speed up than heat up, so the air speeds up. Using Bernoulli’s Principal, the faster flowing air creates an area of lower pressure under the car. The higher pressure air above the car, then pushes the car onto the ground. It also includes a small diffuser at the rear which provides a space for the airflow to decelerate and expand so that the boundary between the car’s airflow and “external” airflow is less turbulent.

This initial build doesnt include the ground effects additions under the car, we will add these later

The base was measured with 3 x 2.25L bottles and drawn out on a piece of 12mm plywood and then cut out using a electric jigsaw. Here it is after being cut out and sanded next to the MkVI water rocket car.

3 Bottle Rocket Car Base - Plywood

3 Bottle Rocket Car Base - Plywood

3 Bottle Rocket Base - Front view

3 Bottle Rocket Base - Front view

The front wheels were then fitted. New more rugged metal L brackets were made up for the front wheels.For those interested these just happened to be Ethernet Switch Rack angles that I had lying around, but any L bracket would do. I cut them to size to minimize weight and drilled holes for the screws and nut to hold the wheels on.

Front Wheels Mounted

Front Wheels Mounted

The rear wheels are the same as the 2 Bottle rocket. The small pieces of wood just allow the base of the wrocket car to be closer to the ground to get a lower centre of gravity

Rocket Car - Rear Wheels

Rocket Car - Rear Wheels

The rear fins were fitted with smaller nuts and bolts to the base instead of screws and the 3 Bottles which are all connected with robinson couplings have been placed on top to check the fit and look.

Wings and bottle mockup

Wings and bottle mockup

Front View

Front View

Holes were drilled to allow three cable ties per bottle to secure the bottles to the base. The sets of holes are spaced 60mm apart (narrower than the actual bottles) and have a diameter of approx 6mm. The cable ties are threaded through, ready to secure the bottles.

Cable ties to hold down bottles

Cable ties to hold down bottles

A small wooded support block was screwed in lace between the first and second bottle to stop lateral mouvement of the bottles. The cable ties were all tightened (not too much) and trimmed.

Ready to test

Ready to test

The bottles were hydrostatic tested prior to being cable tied in place. When the support block is unscrewed and the bottles are not pressurized, they can slide out to be worked on, then put back in.

The water rocket Mk VII car is ready to take to the test track.

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This entry was posted on Wednesday, August 19th, 2009 at 11:17 pm and is filed under Development. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.

 
 

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