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Force required to lift a water propelled rocket

  1. Feb 21, 2009 #1
    Hello, I need to figure out the force I require to lift the water rocket that I'm building. It weighs about three ounces or about .0850 kilograms to possibly make it easier. How much force will I need to provide lift-off. Would I take the acceleration due to gravity (9.8) times the weight of the object (which i think is correct), divide the weight by gravity, or neither.
     
  2. jcsd
  3. Feb 21, 2009 #2
    Follow up- When the water is coming out of the rocket at a force of 100 psi, is it a sufficient amount to provide lift-off?
     
  4. Feb 22, 2009 #3

    russ_watters

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    Staff: Mentor

    What you might do is select a desired acceleration (say, 3 g's), then use f=ma to calculate the force, remembering to add the 1g it has while sitting still. Also, does your mass include the mass of the water?

    For the force available, it is just the pressure times the exit area of the nozzle tines the efficiency of the nozzle (try 50%).
     
  5. Feb 23, 2009 #4
    no, not the water that is why the answer wouldn't be one hundred percent accurate, i did not take into consideration the weight of the water. But, the water is constantly leaving the bottle and the weight will go down slowly causing it to be almost impossible to calculate a very accurate answer to the question because of this variable, correct?
     
  6. Feb 23, 2009 #5
    Oh, wait nevermind i forgot that the question was how much force needed for lift, not how high it would go.
     
  7. Feb 23, 2009 #6
    Now I understand your answer. Thank you very much. This will be a very big help.
     
  8. Feb 23, 2009 #7
    How do I find the efficiency of the nozzle? By the effeciency of the nozzle, what do you mean?
     
  9. Feb 23, 2009 #8
    One last quick question, how can I find the amount of pressure leaving the nozzle? In order to have the pressure escaping equal to the ambient pressure, I need to find out what the amount of pressure would be, in order to know if my nozzle is a sufficient one. The ambient pressure would be 100 psi in my case right?
     
  10. Feb 23, 2009 #9
    Here is a formula that can help you figure out how much pressure you need in order for the rocket to lift of the pad.
    Fnet = m * Vexhaust = m * Ve - act + Ae(Pe - Pamb)

    m = exhaust gas mass flow
    Vexhaust = effective exhaust velocity
    Ve - act = actual jet velocity at nozzle exit plane
    Ae = flow area at nozzle exit plane
    Pe = static pressure at nozzle exit plane
    Pamb = ambient (or atmospheric) pressure

    Subtract the Fnet from the weight of the rocket with the water as well which will look like this:
    Fnet - the weight of the rocket = the thrust that you will need to lift the rocket.

    You can then figure out the acceleration using Newton's Second Law which is:
    Fnet = mass * acceleration
    or
    acceleration = Fnet/mass
     
  11. Feb 24, 2009 #10
    Well the mass of the rocket now is .2225 kg. I found the area of my nozzle which would be .2124 cm^2. Then i used the formula F= P*A to find the thrust force of my rocket which was 689.48 kpa that im using, and the area of the nozzle which was .2124 cm^2, to get the answer of 146.4456 N. That is what I predict will be the thrust force of my rocket. Is this correct?
     
  12. Feb 24, 2009 #11
    And as far as the Newtons needed to lift the bottle, I used the formula F=M*A(or G). I got a Force of 2.1805 Newtons needed to lift the bottle without water. I can not use this answer yet because I need to find what my mass would be with water. But, am i on the right track and have the right formula's?
     
  13. Feb 24, 2009 #12
    Would a 2 cm nozzle work better?
     
  14. Feb 24, 2009 #13
    In terms of area.
     
  15. Feb 24, 2009 #14

    russ_watters

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    Staff: Mentor

    Looks fine - nozzle efficiency is tough to predict, but 50% is probably a reasonable estimate.

    Mass with water is going to be arbitrary. Not enough and it'll empty quickly, too much and you'll be lifting too much weight. You'll need to do some testing to find the right balance.
     
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