# Aerospace Calculating the descent of a rocket landing

1. Apr 3, 2013

### alphasection

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. Apr 3, 2013

### Staff: Mentor

Welcome to PF!

There are too many variables, not to mention the variable of personal taste, to create such an equation.

3. Apr 3, 2013

### 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.

4. Apr 3, 2013

### alphasection

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
5. Apr 4, 2013

### 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.