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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.
... so what about a bit of JET A and a little jet motor? You use the pop bottle for the fuel.
Where does it say you can use water? Oh, anything goes. So, if the Jet A idea doesn't pass, what about using good old coca-cola instead of water? That should give it an extra kick (without exceeding the pressure limits, you should get a bit of extra pressure over time once the soda starts bouncing around... Pump it up to, oh, 60 psi. Let it rip. The CO2 released from the soda as the car bounces around should keep the pressure up there for longer than just air and water alone....)
Nevermind.
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.
Jet-A doesn't burn all that well in liquid form, it is kinda hard to get started. It is kerosine after all. Once you do get it started it burns nice and hot with lots of black smoke, it won't explode unless you atomize it. Gasoline is much more explosive than Jet Fuel in liquid form.
Now I hope and pray that I will But today I am still, just a bill