Can you handle Physics 11?

Hey NBR, I've got a little project for school I think you guys can help me with. I'm doing the infamous "Bottle Rocket Project", but simplified for kids that couldn't get the hang of a parachute.

I am building some form of vehicle that will be rocket propelled and it has to go farther than 30 other vehicles using an identical system.

Everyone is using a 2 liter bottle that will have water pumped into it until it reaches a certain PSI and the bottle is then unlatched and the rocket is launched.
We all must use the same PSI, and the "fuel" tank must be able to handle the unknown PSI without bursting. The fitting at the end must also be that of a two liter bottle. Other than that there are no rules.

Basically I'm taking a two liter pop bottle, and shooting it at a 45 degree angle using only water pressure for acceleration. I want to make it go as far as possible, what should I do?

Everyone is encouraged to stick to the old project (where we would try and go the highest/ get the most time in the air) and make "rockets", but I'm thinking I should add a wing and make it more of a "glider".

So far I've got 2 pop bottle cut up and taped to the bottom of a two liter to add length and more of an aerodynamic nose. I also made a skirt out of a two liter bottle to the top of the full bottle for length.

I don't know if I should simply add a few plastic fins to save weight and go for a harder acceleration and just hope it flies straight, or add a big wing to give it some lift, but reduce acceleration. Also would shorter or longer be better in either case. Someones gotta know what they're doing here.

For starters, don't launch it from a conveyor belt.

I can see we're off to a good start :P

A few ideas already given by family:

-make a smaller opening for the water to get out of, creating a longer period of time for the bottle to accelerate. Unfortunatly this will not work do to the fitting on the launcher.

-crush some seltzer into the tank to react with the water and give it more of a push. This could work, but the teacher has yet to give us the PSI he's going to use which could mean I will burst the tank, and it may be seen as cheating wich would give me a failing grade.

Engineering says follow the KISS rule. All the rockets will launch at the same pressure, so don't screw with it too much. You want all the force at the start. All of it! Ghetto rockets have a habit of turning or tumbing as soon as they get into the air, so if you're still waiting on some thrust after it tumbles theres a good change you could just power into the ground, wasting otherwise good impulse.

Focus on making it not tumble and keeping low weight to get the most out of your propulsion. If you want hangtime, launch it vertical, then figure out some fall arrest that doesn't add to much weight.

I'd be lazy and just stick to putting a small nose cone and some little fins on, or just launch a bare bones bottle.

Bigger Fins= More resistance More stability
Smaller Fins= Less resistance Less Stability

Make sure that when you make the fins that they are of exact equal size and are equally spaced when put on the rocket

Since you said you wanted to make it more of a glider, I would suggest using either just 2 large fins/wings or 2 large wings and 2 small fins if stability is and issue with just the 2 large wings

Well you don't want the bottle to rotate in the air (drag) so you could add weight to the front of the bottle or a thin rope to the rear to keep it stable in the air as the liquid drains from the opening. Apart from that I don't think there is too much you can do apart from testing various bottles at equal PSI to see if one has more propulsion over another?

I was thinking of taping something huge to the top sorta like this

non? I also got Styrofoam i could carve wings out of, otherwise just do the standard four fins?

If you tape that to the top it'll most likely do a big loop the loop and hit you in the back of the head:)

Do fins work? I doubt a pop bottle is long enough to have any directional stability from a few fins?

Yeah thought so. You've gotta add length to the bottle to improve directional stability - http://www.youtube.com/watch?v=4wMU7sG9Bws[HTML_REMOVED]feature=related

If something like that doesn't work with your mounting system the string or small rope idea I mentioned above should still work.

Mythbusters Season 8, Episode 5. Propane tank rocket.

You need a gun.

is the amount of water controlled and do you know what psi is actually going to be used? If you know the psi, that will affect the best amount of water to use, as the higher the psi, the more water you can get away with to generate more thrust. But lower psi will lower the optimal level of water as it is heavy, so you get diminishing returns.

Aside from the fins and nose cone adaptations, longer body for stability, I would look more into making the exit hole smaller (it should still be able to screw on to whatever attachment you are using while generating more thrust).

One last thing, if you can control the angle of release, 45 degrees is optimal in a zero friction environment, I remember that for javelin, the optimal release angle was actually 42 degrees to get it further as the 45 results in shorter distance travelled due to air resistance/drag.

is the amount of water controlled and do you know what psi is actually going to be used? If you know the psi, that will affect the best amount of water to use, as the higher the psi, the more water you can get away with to generate more thrust. But lower psi will lower the optimal level of water as it is heavy, so you get diminishing returns.

Aside from the fins and nose cone adaptations, longer body for stability, I would look more into making the exit hole smaller (it should still be able to screw on to whatever attachment you are using while generating more thrust).

One last thing, if you can control the angle of release, 45 degrees is optimal in a zero friction environment, I remember that for javelin, the optimal release angle was actually 42 degrees to get it further as the 45 results in shorter distance traveled due to air resistance/drag.

PSI and water will be the same all around for all students, but we have not been given the exact numbers of how much, and I don't think we'll know until launch day. The hole needs to be big enough to allow the pipe he uses to fill the rocket in, which is the exact size of the 2L pop bottle opening, meaning no modifications can be added there. Also the angle is set to his pipe, which I believe is 45 degrees. The teacher wants the project to be all about aerodynamics, and ingenuity

thin rope to the rear to keep it stable in the air

^ this.. the rope should act like a streamer and keep that arse end of the bottle where it should be.. also a whole lot easier to get right than sticking fins on it and having it go all whacky in flight after you realize they are not the same size/weight as your bottle goes off and causes the rest of your classmates to dive for cover ( which is quite funny btw)

and take all stickers or labels off the bottle.

I'd do nothing to it….don't overcomplicate things.The outcome of any mod is unknown. You have a 1 in 3 chance of it improving the flight.

Although,if you want to get badassed,build an accumulator that you could apply to the water just after takeoff…..prof gives you 50 psi? Charge the accumulator to 80 psi and give it a little boost just as the pressure drop occurs.