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Homework Help: Physics Bottle Rocket

  1. Jun 5, 2012 #1
    I'm currently working on a bottle rocket for my physics class. Instead of taking the assigned experiment, a friend and I have chosen a side experiment to work on.

    The experiment: Create a rocket using a 2L bottle as a compression chamber and compare the time required to reach the apex of its flight using different 0, 10, 20, ..., 60% water and compressed air @90psi.

    Now I've created a sketch of what I, using various forms of research, believe to be a good start to my rocket. I've gone with a parabolic nose cone which is recommended for subsonic flight and what I feel is a good fin location. The mass of the rocket comes in at 175g dry. What I need help with is some of the finer details. I cannot change the size of the nozzle which must stay at 22mm to fit the launcher. The rocket is launched straight up. I'll include a sketch.

    What can I do in order to make the rocket fly higher?
  2. jcsd
  3. Jun 5, 2012 #2


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    I would suggest that you put the ideal amount of liquid in the rocket.

    If the bottle was basically full of water [and thus heavy] with very little compressed air, I don't expect the "rocket would rise very far.

    If the bottle had only 5 ml of water - and a whole lot of air - the water would be expelled almost instantly, followed by the air, with the "rocket" again not rising very far.

    You experiment proposed to find the ideal mix, I suggest you conduct the experiment and see what the ideal mix is, rather than worry about what theoretically should happen (if only you could eliminate air resistance).

    It may be that only after you have conducted your experiment, that there is an indication of a percentage you have not actually tried.

    You could then try that suggested mix to see if if was higher than anything else you tried.

    Perhaps even slightly increase then decrease the mix to see that the maximum was correctly established.

  4. Jun 5, 2012 #3
    We did a prelim test today using 30% water which yielded about 5.5 seconds of total flight time from launch to crash. is there any way to find altitude using calculus? Im aware it will be difficult due to the variables but a close guess would suffice.
  5. Jun 5, 2012 #4


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    You said you fired it vertically.

    Did it travel vertically?

    I would be trying to find the time the rocket took to fall back to Earth, as you should know how far an object falls in a given time - with air resistance hopefully not being a factor.

    I would use a video camera for timing, as stopping and starting a Stopwatch brings in significant errors when measuring small time intervals. You can also play back carefully to check when maximum height occurred.

    Unfortunately, I suspect this light, large bottle will be severely affected by air resistance while falling back to Earth.
  6. Jun 5, 2012 #5
    It isn't just just a bottle. When I hit 10 posts Ill post up my design.
  7. Jun 5, 2012 #6
    This is my sketch.


    it is 2 bottles joined together. (Still seperated for a 2L compression chamber) It looks almost exactly like that but with a more rounded nosecone.
  8. Jun 6, 2012 #7
    So I did a few more tests today and our best rocket reached its apex at 5.5 seconds. Is that going to be about the max or is there any way to squeeze more time out of it?
  9. Jun 7, 2012 #8


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    I don't know that anyone knows a theoretical answer. You still haven't said how far up your rocket was after 5.5 seconds???
  10. Jun 7, 2012 #9
    A teacher I know who has done this with 2L bottles for many years says it generally works best around 2/3 to 3/4 full of water. You were at just 30% water-did you vary the water amount a lot?

    Looks quite cool, by the way.
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