Two LM questions

[Dalhousie]

Discussions on FB have raised two questions I have not been able to satisfactorily answer.

1)Why were the LM engines gimballed?

2)How often did the LM actively dock?  I  can find Apollo 9, possibly Apollo 10.

Thanks

[Zakalwe]

Why were the LM engines gimballed?

Only the Descent propulsion System motor was gimballed. The Ascent motor wasn't.

The DPS was gimballed to allow the thrust from the DPS to be aligned with the LM's CoG during the DOI manoeuvre and again during PDI. In both manoeuvres the DPS was throttled to 10%- for 15 seconds at DOI (and then at 40% for 15 seconds) and 26 seconds at PDI, which allowed the computer to sense if the DPS thrust was acting through the centre of gravity of the LM. If it wasn't , the gimbal mechanism gimballed the engine. The gimballing mechanism was too slow to be used to steering. This function was left to the RCS system.


Have a look at page MP15 of this PDF...it describes the gimballing mechanism:

[Dalhousie]

Why were the LM engines gimballed?

Only the Descent propulsion System motor was gimballed. The Ascent motor wasn't.

The DPS was gimballed to allow the thrust from the DPS to be aligned with the LM's CoG during the DOI manoeuvre and again during PDI. In both manoeuvres the DPS was throttled to 10%- for 15 seconds at DOI (and then at 40% for 15 seconds) and 26 seconds at PDI, which allowed the computer to sense if the DPS thrust was acting through the centre of gravity of the LM. If it wasn't , the gimbal mechanism gimballed the engine. The gimballing mechanism was too slow to be used to steering. This function was left to the RCS system.


Have a look at page MP15 of this PDF...it describes the gimballing mechanism:

Thanks.

But why gimbal at all, if the RCS provided faster control?

[bknight]

Why were the LM engines gimballed?

Only the Descent propulsion System motor was gimballed. The Ascent motor wasn't.

The DPS was gimballed to allow the thrust from the DPS to be aligned with the LM's CoG during the DOI manoeuvre and again during PDI. In both manoeuvres the DPS was throttled to 10%- for 15 seconds at DOI (and then at 40% for 15 seconds) and 26 seconds at PDI, which allowed the computer to sense if the DPS thrust was acting through the centre of gravity of the LM. If it wasn't , the gimbal mechanism gimballed the engine. The gimballing mechanism was too slow to be used to steering. This function was left to the RCS system.


Have a look at page MP15 of this PDF...it describes the gimballing mechanism:

Thanks.

But why gimbal at all, if the RCS provided faster control?

The documentation indicates "and is gimbaled (can be tilted) for
thrust vector control."  I suspect it may be for larger corrections, but that is speculation on my part.

[raven]

Really neat little document.
It really tells its information well to someone who only has fairly minimal technical knowledge. It also answers a question niggling in the back of my mind from watching 'Apollo 13': What the heck a 'talkback' was.

[ka9q]

What the heck a 'talkback' was.

Talkbacks are important with mechanical systems. You can't rely on the electrical command going to the device because the valve, relay, actuator, whatever can fail to physically move into the commanded position.

This lesson was learned the hard way during the Three Mile Island nuclear accident.

[ka9q]

But why gimbal at all, if the RCS provided faster control?

Because it saved RCS fuel and accommodated greater center-of-mass offsets.

The ascent propulsion system did depend on the RCS because the ascent engine wasn't gimbaled. That made it simpler and more reliable, important because there was no backup (unlike the DPS, which had the APS as its backup). The much lower mass and especially the moments of inertia of the ascent stage made reliance on the RCS for c.m. offsets much more practical. Also, the RCS and APS propellant supplies could be cross-fed to ensure that the RCS would have plenty of propellant. There were no propellant feeds between the ascent and descent stages.

[Dalhousie]

But why gimbal at all, if the RCS provided faster control?

Because it saved RCS fuel and accommodated greater center-of-mass offsets.

The ascent propulsion system did depend on the RCS because the ascent engine wasn't gimbaled. That made it simpler and more reliable, important because there was no backup (unlike the DPS, which had the APS as its backup). The much lower mass and especially the moments of inertia of the ascent stage made reliance on the RCS for c.m. offsets much more practical. Also, the RCS and APS propellant supplies could be cross-fed to ensure that the RCS would have plenty of propellant. There were no propellant feeds between the ascent and descent stages.

These are the droids I am looking for, thank you!

[bknight]

...

This lesson was learned the hard way during the Three Mile Island nuclear accident.

Hijack, How so?  I've not read much about TMI in the last 20 years.

[ka9q]

This lesson was learned the hard way during the Three Mile Island nuclear accident.

Hijack, How so?  I've not read much about TMI in the last 20 years.

A relief valve on the primary coolant loop stuck open instead of reclosing when it should have, allowing too much coolant to escape. The operators didn't realize this for several hours because the light on the console indicated the electrical command to the valve (to close), not the actual valve state.

And that's why you want talkbacks for critical mechanical controls.

A very common example of a talkback are the landing gear status lights on aircraft. You don't assume the gear are up or down just because that's what the control switch says.

The Wikipedia article on TMI has full details.
 

[Bob B.]

2) How often did the LM actively dock?  I  can find Apollo 9, possibly Apollo 10.

During any docking there is an active vehicle (the one that does the maneuvering) and a passive vehicle.  I assume your question is referring to when and how often was the LM the active vehicle.  There were only two dockings on each mission.  The first was the transposition and docking maneuver, which was performed with the CSM as the active vehicle.  This is obvious because the LM was unmanned at the time and not powered up.  The second docking occurred after the LM ascended from the lunar surface and met up with the CSM in lunar orbit.  During this docking it was the LM that was the active vehicle.  In an emergency, the CSM could take over as the active vehicle.

[Count Zero]

I thought that the LM was the active vehicle maneuvering to rendezvous with the passive CM, but once rendezvous was achieved the LM stabilized and the CM became the active vehicle for docking.  Is the wrong?

[Bob B.]

I thought that the LM was the active vehicle maneuvering to rendezvous with the passive CM, but once rendezvous was achieved the LM stabilized and the CM became the active vehicle for docking.  Is the wrong?

That's not how I understand it.  Everything I've read says the LM was the active vehicle (for example, this page).  If I'm wrong about that, then hopefully somebody else will chime in and correct me.  It's been my belief that it was the LM that performed all the maneuvering, though I assume it was the CMP who activated probe retraction.

[Allan F]

Everything I have read points to the CSM being passive, until physical contact was made. All the film of the docking procedures I have seen points to the LM maneuvering, showing all sides to the camera in the CM, before proceeding to docking.

[VQ]

Everything I have read points to the CSM being passive, until physical contact was made. All the film of the docking procedures I have seen points to the LM maneuvering, showing all sides to the camera in the CM, before proceeding to docking.

That would make sense; why waste CSM RCS propellant if the LM, having finished its mission, still had fuel that was about to be ditched?