MikoInSpaceBlog

Thrust to Weight Ratio (TWR) is a dimensionless ratio of thrust to weight of a rocket, jet engine, propeller engine, or a vehicle propelled by such an engine that is an indicator of the performance of the engine or vehicle.

In rockets, it indicates how fast you can go with the weight and thrust of your rocket.

A higher TWR means that you can go faster, while a lower TWR means you go slower.

On Earth

During liftoff you need a TWR of atleast 1.00 to overcome the atmosphere and gravitational forces.

In Space

While in space, TWR doesn’t really matter due to there not being an atmosphere and gravitational forces being much lower or in some cases even nonexistant.

A low TWR in space just means that you will go slow, which in many use cases does not matter at all.

Even if you cannot execute a burn due to your vessel being way to slow, you can still, in most cases like changing apoapsis or periapsis, break up the burn into many different burns.

How to Calculate TWR

Thrust to Weight Ratio

TWR is calculated by dividing the Thrust by the Weight (TWR=T/W)

Weight

Weight is calcuated by multiplying Mass and Acceleration due to Gravity (W=m⋅g)

For Earth, The Acceleration due to Gravity would be 9.81 m/s²

Thrust

Thrust is usually indicated by the Rocket Engines Manufacturer, it is usually measured in Newtons (N)

It can be calculated, but calculating it requires some more advanced math that I’m not gonna go into detail with.

Here is an article from NASA on how to calculate thrust.

Example

Here is a generic example of a TWR calculation:

TWR= 735,750N/800,000N ≈ 1.09

(Note: To have a TWR > 1.00, thrust must be greater than the weight, so double-check the numbers to ensure accuracy)

This has been an MIS Blog Post.