Archive for August, 2009

 

How Much Air is in my water rocket ?

Its a good question … How much air is inside that water rocket ??? The easiest way to think about it is .. what volume of standard pressure air is being squeezed into the water rocket when its pressured to a particular pressure.

The attached calculator is an excel spreadsheet which will work this out for you. The volume of (Standard pressure) air in the bottle when its pressurized is related to

– The volume of the bottle you are pressurizing
– The amount of water in the bottle
– The temperature of the air under pressure (The more you squeeze in the hotter it gets)
– The air temperature of the day you are doing the launch

The equations we used to derive the formula are included in the bottom of the spreadsheet and are from the Combined Gas Law. As we are not super cooling or super heating the air, these equations will approximate pretty closely the correct amounts

The equations use the following units – for volume (Litres), pressure (psi) and temperature (Kelvins). There are converters there if you use other units of measurement. Just fill in the Green squares in the spreadsheet and you can go wrong.

Download the excel file using the link below.

Volume of Air in Rocket Calculator

The default increase in temperature we have used is 22 degrees Celsius. This is based on the Day 64 Thermal Test results that George over at Air Command has on his website. This information and videos are here. Thanks George :)

Its interesting to experiment with changing Air Temperature in the bottle. You can see that but cooling the air you can get more in at the same pressure. More Air equals more thrust ….

Posted by on August 27th, 2009 Comments Off

Water Rocket Car MkVII – Launch Day 2

Here is the video .. more to come

Posted by on August 24th, 2009 2 Comments

Water Rocket Car MkVII – Launch Day

I think the video sums it up pretty well. We had about 1 hour to do a few tests of the MkVII water rocket car so we went down to our launch track and had 2 runs.

It certainly takes a lot more effort to get the 6.75L bottles up to 120psi. We calculated there is about 39.8 L of (standard pressure) air squeezed inside the 6.75L bottles.

First Test – 1500ml water and 120psi

This wasnt a bad start. The veer to the right was most likely a wheel alignment issue. It was a lot faster than previous launches and even after 70m managed to hit the gutter with enough velocity to cause it to roll and land on its head. We did a few unpowered pushes to get the wheels aligned a bit better.

Second Test – 1500ml water and 120psi + foam

This test was a bit of a disaster. We must have over-compensated on the wheel alignment as it just veered constantly off to the left and hit the gutter at full thrust. The impact sheered off the front wheel and bent the rear metal L bracket holding the rear wheel. It still had enough speed after the crash to make it all the way across the road. The front wheel was a good 25m away from the main car.

Damage

The damage wasnt too bad. The front wheel needs replacing and the rear L bracket holding the rear wheel needs straightening. The bottles themselves are fine. The fins are bent a bit but just needs straightening. We will be able to have it fixed for a few more tests this coming weekend.

Posted by on August 19th, 2009 Comments Off

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.

Posted by on August 19th, 2009 Comments Off