Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Calculating the descent of a rocket landing

  1. Apr 3, 2013 #1
    Hello, I've been working on calculating the time that a rocket would take to reach ground level, safely. But some things are getting me confused, also I would like to create a formula for this so i can just input values no matter what planet or celestial body you're landing on. To put it in clearer terms:

    I want to know how long a rocket would take to land safely(eta) at a landing pad going vertically down, taking into account the gravity and the upward thrust that a rocket engine would create.

    Thank you, if i'm missing something than tell me.
     
  2. jcsd
  3. Apr 3, 2013 #2

    russ_watters

    User Avatar

    Staff: Mentor

    Welcome to PF!

    There are too many variables, not to mention the variable of personal taste, to create such an equation.
     
  4. Apr 3, 2013 #3

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    If you can neglect air drag, it is easier to consider the time-reversed process: A launching rocket.
    You have to add several assumptions about the rocket to calculate anything.
     
  5. Apr 3, 2013 #4
    Thank you!

    Hmm... You're right that there is too many variables. This question was just to understand how much thrust is needed to lift an object at whatever speed I want, or to decelerate any object with a rocket engine fixed to it. Ok a (hopefully) simpler question:

    How much thrust is needed to propel 180kg to mach 1 at sea level, when I say propel I mean vertically up, I tried to convert the force the object exerts (which came out to be ~1773.8(N)). If I push upwards, the opposite direction of gravity, with a rocket at this force I will not gain any altitude (am I right with this?) so i want to know how much thrust I need to reach certain speeds. For instance I want to go 5 m/s weighing only 180 kg, how much thrust would I need?
     
    Last edited: Apr 3, 2013
  6. Apr 4, 2013 #5

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    That is not a meaningful question - thrust is related to acceleration, not to speed.
    If you have 1774N of thrust (in vertical direction), your velocity is constant - it can be zero, it can be supersonic, or anything else. If you have more thrust, the rocket can accelerate, and reach any velocity if it has enough time.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook