BBC's Water Rocket-Vehicle Contest

jmichaelg writes: "The BBC is running a rocket contest to see who can build the fastest 2 liter water bottle propelled vehicle. The idea is you use a bicycle pump to pressurize the bottle to drive the vehicle. There are pressure limits (no more than 70 psi) so dry ice is out of the question. Gotta use a bike pump. Fastest car to go 20 meters is the winner. If you're going to play, you had better get going ... contest ends on Nov 6." Bonus points if you're riding it at the time.

Comments

Boyscouts

[MxTxL]

And you thought you escaped racing little model cars when you got out of boyscouts.

The easiest way to win:

[Omerna]

Since the vehicle can have a moving start, just give it a good kick. This will easily move it 20 meters. (Well, maybe not easily, so get a little propulsion from the bottle too).

=)

[Myuu]

Finally, something to use those old school NASA programs on!

Low weight low rolling resistance is the way to go

[Anton Anatopopov]

I would like to enter this, I have a keen interest in amateur hydraulics, and I think I have a chance.

My vehicle would probably be a three-wheeler made primarily from balsa-wood, with ceramic skateboard bearings (coated with a thin film of latex for wheels. I figure if I can minimise the weight of this thing, and keep the rolling resistance down then I am in with a good chance of winning

I just hope they let American Residents enter the comp. I cannot wait to get going on this.

A few thoughts on design

[SClitheroe]

Chassis - The car that wins is going to have very carefully measured and constructed alignment. Getting the vehicle to run 20 meters straight, and with the thrust exactly aligned (in whichever direction the particular design calls for) is key. You can't afford to waste any of those thrust newtons on trying to drive the car sideways, or having the car trade forward velocity for lateral movement.

Water/Air ratio - if you run out of air too quickly, you are hauling excess water all the way down the track. Similarly, if you design the vehicle such that air can escape freely once the water level falls below a certain point, you are wasting power.

"Gearing" - one has to wonder if a direct-drive design (air pushing water out the back) is the most efficient. If you used hydraulic principles, you could in effect "gear down" the high pressure, resulting in high-torque that could drive over-size wheels, similar to the way that a hydraulic lift works. The winning design is going to have to find a tradeoff between quick accelleration/coasting and continued power for the duration of the track. Could the careful design of a nozzle accomplish the same thing? (/me thinks back to the model rocket days)

Multiple pressure vessels - this is just fanciful thinking, but one of the techniques steam locomotives used was to use the high pressure air in one set of cylinders, and then re-use the resulting lower pressure to drive a second set of cylinders. Extrapolating from this, I wonder if having two pressure vessels, totalling the legal limit, and firing at different times (ie. one for acceleration, one for maintaining speed) might be feasible.

I guess, though, in the end, the simplest, lightest design will win. Having a good chassis is still key though.

My experience with this

[cr@ckwhore]

We did something similar in my high school chemistry class several years ago. We were allowed to use chemical reactions as a source for the propulsion. We had a 2 lane "water drag track", and were given 1 soda bottle, and some basic supplies, like rubber bands, balloons, soda straws, etc.

The easiest rig to build, and the most common, was simply the soda bottle with a rubber stopper in the opening. The rubber stopper had a hole in it, which would function as an outlet... the bottle would then be filled with the infamous vinegar and baking soda mix. This provided enough fuel for long lasting propulsion, while being simple. One of the biggest problems with this design however, is the outlet on the bottle was usually above water. To acheive optimal propulsion with this method, the best thing is to make sure the jet is below the water line.

Another method (and the winning method) was to completely seal the bottle with a full rubber stopper. The bottle was filled with the infamous (and bad smelling) baking soda & vinegar mix, and promptly plugged with the stopper. After about 2 minutes of building pressure, the stopper would fire out of the bottle, propelling it forward with great velocity. There were some minor problems keeping the boat in the water, but that was the winning design.

Fancy designs don't work... some over-acheivers were inclined to build more complex designs, incorporating a lot of internal parts, etc. Go for the K.I.S.S theory! (Keep It Simple Stupid).

Hee Hee!

[gnovos]

As it doesn't mention *water* anywhere, just for kicks I would fill the bottle up with a liquid right on the verge of vaporizing at room temperature when kept at a pressure less than 70psi. As long as the car is kept cool and pressurised, it will stay in liquid form, but as soon as it starts slushing around and depressurising, the liquid will begin to vaporize and ramp up the pressure. Additionally, since you don't actually have to have the liquid escaping (as long as the liquid remains, you will have a very hefty supply of gas), only the gas, you will be under acceleration for the entire trip!