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Homework Help: Minimum thrust of engines at lift-off

  1. Feb 28, 2010 #1
    1. The problem statement, all variables and given/known data

    Hi there

    Rocket has a total mass of 3.4 x 10^6 kg at lift-off. The first stage contains 2.0 x 10^6 kg of propellants which is consumed at a rate of 1.4 x 10^4 kg s

    Find the minimum thrust of the engines at lift-off?

    I would have thought that you needed one more variable to be able to solve this?

    2. Relevant equations

    3. The attempt at a solution

    I used the mass flow x escape velocity and ended up with 15.7 x 10^6 N. It takes 142 sec for the first stage to finish its burn.

    I just dont think that this is right - as the next part states that if this thrust is sustained - find the time over which the first stage operates and the acceleration of the vehicle at that time. (Neglect air friction - and suume whole flight is vertical.

    Help needed please. Cheers
    Last edited: Feb 28, 2010
  2. jcsd
  3. Mar 1, 2010 #2
    Any thoughts?

  4. Sep 15, 2010 #3
    Wouldn't the minimum thrust just be m*g? In order to get the rocket moving, you would need at least 3.3*10^7 N of force.
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