Calculating the descent of a rocket landing

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Discussion Overview

The discussion revolves around calculating the descent time of a rocket landing on various celestial bodies, focusing on the effects of gravity and thrust. Participants explore the complexities of creating a universal formula for rocket landing times and thrust requirements for achieving specific speeds.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant expresses confusion about calculating the time for a rocket to land safely, seeking a formula that accounts for gravity and thrust across different celestial bodies.
  • Another participant suggests that the multitude of variables makes it challenging to create a single equation for rocket descent.
  • A different viewpoint proposes considering the problem from a time-reversed perspective, akin to a launching rocket, while emphasizing the need for assumptions about the rocket's characteristics.
  • The original poster refines their question to focus on determining the thrust required to propel a 180 kg object to Mach 1 at sea level, questioning the relationship between thrust and altitude gain.
  • One participant challenges the notion of relating thrust directly to speed, explaining that thrust is more closely tied to acceleration, and that constant thrust results in constant velocity unless it exceeds gravitational forces.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the feasibility of creating a universal formula for rocket descent or the specific thrust requirements for achieving desired speeds. Multiple competing views and uncertainties remain regarding the relationship between thrust, acceleration, and velocity.

Contextual Notes

Limitations include the dependence on various assumptions about the rocket's properties, the neglect of air drag, and the complexity of the variables involved in rocket dynamics.

Who May Find This Useful

This discussion may be of interest to those studying rocketry, aerospace engineering, or physics, particularly in relation to thrust calculations and rocket landing dynamics.

alphasection
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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.
 
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There are too many variables, not to mention the variable of personal taste, to create such an equation.
 
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.
 
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:
so i want to know how much thrust I need to reach certain speeds
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.
 

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