1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Variable mass calculation of thrust for a water rocket

  1. Feb 5, 2015 #1
    1. The problem statement, all variables and given/known data

    I am trying to derive the formula for time varying thrust given that I know the initial mass/volume of water in a water rocket. Knowns will include initial pressure, initial volume of water, and nozzle cross sectional area.

    2. Relevant equations

    Bernoullis equation: (p/ρg) + (2V2/g) + z = c

    Isentropic expansion: pvγ = c

    3. The attempt at a solution

    I derived the formula for the exit velocity of the water from the nozzle using the Bernoullis equation,
    https://www.dropbox.com/s/gnatjasgb2fjx5i/Photo%202015-02-05%2C%203%2050%2055%20PM.jpg?dl=0
    https://www.dropbox.com/s/gnatjasgb2fjx5i/Photo 2015-02-05, 3 50 55 PM.jpg?dl=0

    I know that I am supposed to derive a differential equation for the velocity and pressure but I cant seem to setup the equation using the isentropic flow formula.

    any help will be appreciated. thank you.
     

    Attached Files:

  2. jcsd
  3. Feb 5, 2015 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Welcome to PF;
    this is something that is normally done by taking a small dm ejected in time dt, and working out the dp that results.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Variable mass calculation of thrust for a water rocket
  1. Thrust calculation (Replies: 1)

  2. Water/Air Rocket (Replies: 0)

  3. Water mass calculation (Replies: 1)

Loading...