Transposition and docking

zhy987

New member
Help me understand transposition and docking.

this procedure occurs after TLI right?

so the stack is travelling towards the moon at thousands of miles an hour right?
the CSM separates, so now the CSM and the stack are travelling separately but in the same direction.
the CSM performs a 180 degree turn…and the CSM is still travelling towards the moon.

Does the CSM actually engage its thrusters and change direction towards the stack that is travelling toward it? Why doesn’t it just slow down a bit (while still travelling towards the moon) and allow the stack to catch up to it and then dock?

im having trouble understanding how the CSM does a 180 and then flies towards the stack…is the CSM actually travelling away from the moon during that procedure? So it’s travelling thousands of miles an hour towards the moon, rotates, STOPs, then flies away from the moon to engage the LM Which is travelling thousands of miles an hour towards the moon?

Apollo 13 the movie makes it look like all the objects are just floating in space and not moving at any velocity.

Xyon

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Help me understand transposition and docking.

this procedure occurs after TLI right?
Yes.
so the stack is travelling towards the moon at thousands of miles an hour right?
Yes.
the CSM separates, so now the CSM and the stack are travelling separately but in the same direction.
Yes.
the CSM performs a 180 degree turn…and the CSM is still travelling towards the moon.
Yes.
Does the CSM actually engage its thrusters and change direction towards the stack that is travelling toward it? Why doesn’t it just slow down a bit (while still travelling towards the moon) and allow the stack to catch up to it and then dock?
It does both! There are no "brakes" on the spacecraft. To slow down a bit, it has to do a 180 flip both to align with the LM and so that the thrusters will impart a small amount of thrust in the opposite direction to their original heading.

It's all about relative velocity. Both the CM and the LM at this point are separate objects travelling at a similar (but not identical, because of the forces imparted during separation) velocity towards the Moon. The CM, being the controllable one, thrusts towards the LM. This reduces very slightly its overall velocity towards the Moon, but increases its velocity relative to the LM. Now both objects are still travelling towards the Moon, but they're also travelling towards each other!

GLS

Well-known member
Orbiter Contributor
Hi! Welcome to the forum!

Basically, when you have objects flying close to each other, forget the "they are travelling at xyz m/s" and focus on their relative velocity, because that is what matters to have them closing up, keeping their distance, or flying apart.

On the Apollo T&D the CSM separated and thrusted forward a bit, to move away from the S-IVB/LM stack. I don't know the actual numbers but let's say the CSM thrusted forward 1 m/s. You now have the S-IVB/LM moving towards the Moon at, say 10 km/s, and the CSM at 10.001 km/s, but what matters here is the 1 m/s relative speed between them.
The CSM rotating 180º doesn't matter for the speed, and so they will keep that 1 m/s relative speed and continue to drift apart.
Then the CSM thrusts toward the S-IVB/LM stack, so against the existing relative speed. Let's say you want to dock at 0.1 m/s, so you have to thrust to null the 1 m/s that is separating the vehicles and then add 0.1 m/s to them start closing in. So now the S-IVB/LM continues towards the Moon at 10 km/s and the CSM is now at 9.9999 km/s.

Relative velocity is the name of the game.

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