# Mars lander set spinning.

forty
Before the final rocket stage the Mars lander was set spinning with an angular velocity directed along the engine force vector.

I am completely lost to why they did this. By spinning the lander do they speed it up? Using the right hand rule the angular velocity vector points in the direction of motion (if i interpreted "angular velocity directed along the engine force vector." correctly).

Any help would be greatly appreciated.

## Answers and Replies

Homework Helper
Before the final rocket stage the Mars lander was set spinning with an angular velocity directed along the engine force vector.

I am completely lost to why they did this. By spinning the lander do they speed it up? Using the right hand rule the angular velocity vector points in the direction of motion (if i interpreted "angular velocity directed along the engine force vector." correctly).

Any help would be greatly appreciated.

Angular velocity vectors point along the axis of rotation. So if you were watching the lander from above, you would see a small rocket engine firing (or pulsing in burst) to set the lander rotating either clockwise or counter-clockwise.

forty
And why is that necessary for them to set the lander on the right trajectory? can't they do that without it spinning?

jandl
Think about what happens when you try to change the direction of the angular momentum. If you want a real life example of this, think of a time that you've held a gyroscope in your hands. This should provide you some clues as to what is happening.

forty
Does it help provide 'stability' ? I'm just trying to use the idea of a gyroscope :S

Homework Helper
Does it help provide 'stability' ? I'm just trying to use the idea of a gyroscope :S

It is for stability and to maintain the spacecraft in a (reasonably) fixed orientation. This is important if the main rocket engine is going to change the velocity of the descending lander in a predictable way.

forty
So from what I've gathered, before the final rockets are fired it is set spinning, to help its stability and maintain a fixed orientation while the last stage rockets are firing. The spinning is then stopped (by some weights being fired out to either side -> increasing moment of inertia, these weights are then detached). Then on its decent it is set spinning for the same reasons as before?

Please tell me I'm on the right direction. Rotations just aren't my thing >_>

Homework Helper
I would suppose that is correct. (I'm not clear about why they bother stopping the rotation -- "despinning", as they call it -- if they need to have it spinning again later.)

forty
Well the exact question I'm trying to answer is as follows:

just before the final rocket engine was fired to set the Mars lander on its trajectory to Mars, which required a high-precision velocity force vector, the lander was set spinning with an angular velocity directed along the engine force vector. Describe in words why you think it was necessary for the engine to fire while the lander was spinning. Also describe how the spin was stopped after the engine fired by launching two small weights transverse to the angular velocity vector on the end of long wires which were later jettisoned.

I can explain the 2nd part of the question in regards to the despin but as for why it needs to spin for it to have a high-precision velocity vector stumps me. The only real thing i can think of is that it provides stability, or rather an increase in a resistance to change of its velocity kind of like an increase in inertia.