|MadSci Network: Engineering|
Dear User, I am afraid that your question somewhat exceeds the time I can afford to spend on engineering a "super" bottle rocket, so forgive me if I would not go into too much detail. I will try to outline some general ideas which may be of use. First of all, I will assume that the total weight of the rocket with the fuel (water) should be under 300g. This places a cap on how much total thrust can you expect in an ideal case. I suspect that the first place to invest time in optimizing would be the engine - namely the size of the nozzle opening and the pressure of air above the liquid. Also, if you maximize the pressure you might want to use concentrated saline solution instead of water, especially if you are *not* limited by fuel weight, but even if you are, expelling more mass per second creates larger thrust. Two bottles may be utilized in sequence i.e. glue them together with epoxy (i advise to be very generous with epoxy and to use some kind of cloth tape soaked in epoxy rather than just plain glue to ensure that the thing won't blow up in your face). You then get a total volume of 3 L which means that in addition to water (saline) you can pack in extra compressed air which can also create thrust. I think coke bottles are good to 3 atm at least, be careful not to exceed the allowed presure. Filling the smaller bottle with heium won't do much good - simply because the volume is not enough to create sufficient lift. Parachute - by all means ! Just make sure that the surface area is enough. I leave it to you to design a good parachute opening system - there are several ways to do this. Adding a parachute is a very good idea from the point of view of safety - less chance that the rocket will injure someone. Generally, streamlining your design would help, but make sure that you do not add too much extra weight. A simple nose cone, glued and sanded (polished) to the main body should reduce the drag. Also make sure that your fins are sturdy and are glued well, because a good bottle rocket develops enough thrust to rip poorly glued fins off. As always, be accurate and align your rocket accurately. If you have the option of testing the rocket's stability in a "wind tunnel" i.e. for example by hanging it in a strong draft, or out of the car window (be careful) it may help to improve performance. Hope it helps. A.G.E.
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