Earth orbit vs Lunar orbit rendevous

What was the advantage of linking up the Apollo command module and LEM in Lunar orbit rather than Earth orbit?

You mean after the LEM has been on the surface of the moon? How is it going to get into earth orbit from the moon’s surface?

No, I mean why not link them up in Earth orbit before the journey to the Moon.
Perhaps an explanation of Earth orbit rendezvous vs Lunar orbit rendezvous would be a better question.

Okay then…

My first guess is that the LEM was not designed to survive a long space flight and there were concerns about what shape it would be in when they arrived at the moon. It was relatively protected where it was stored on the way to the moon.

My second guess is that with the LEM attached to the Command Module there would be additional drag and it would take more fuel to get to the moon…

If you link up in Earth orbit you have to design the coupling so that:

  1. Astronauts can move through it.
  2. It stays air-tight
  3. It can withstand the full thrust of lunar injection burn.

If you link up in lunar orbit you don’t have to worry about #3. The stresses on the coupling during docking and crew transfer are much lower than the stresses when the big engines are on.

In fact, one of the problems in getting the Apollo 13 astronauts back safely was that they needed the life-support system of the landing module, but they couldn’t use the service module engines while it was still attached.

They did neither.

The set of third stage (S-IV), LEM, and CSM reached orbit, and a couple of hours later, the S-IV reignited to perform the trans-lunar injection (TLI) burn. This put the entire stack on a trajectory to the moon. It was then no longer in Earth orbit. Whilst on the way, the CSM separated from the LEM/S-IV pair, turned around, docked with the LEM, and then pulled it away from the S-IV. Because the entire stack was on a lunar trajectory the S-IV impacted on the moon’s surface much the same time as the mission reached lunar orbit.

The difference between Earth orbit rendezvous and lunar is much bigger. They are very different missions. EOR had a command capsule that itself would land on the moon, and take off again, and itself return to Earth. Clearly this is a much much bigger and more complex spacecraft. You need to land on the moon with a spacecraft that is capable of surviving re-entry, plus has the fuel needed no just to make it to lunar orbit from the surface, but to put you on a trans-Earth trajectory. Vastly bigger. A Saturn V was not nearly large enough to launch such a heavy craft, so one proposal was to launch in two parts, and put the system together in Earth orbit. That was the Earth orbit rendezvous. The third option was to build the Nova launcher, and launch the entire thing in one go. That was the “direct ascent” option. Lunar orbit rendezvous refers not to the joining of the LEM and CSM on the way to the moon, but to the joining of the two when the LEM leaves the moon, and they link up, so the moon walkers can go home. The rendezvous is all about two separate spacecraft finding one another in orbit and docking.

Thanks again, Francis.

If I’m answering the right question, taking all the stuff needed to get back to Earth orbit down to the Moon and then bringing it back up is less efficient.

That LEM has to land using thrusters and then launch itself off the Moon.
If it had to carry the rocket and fuel to make it back to Earth orbit the whole thing gets much bigger. It would have that weight, plus the weight of the extra fuel and engine powerneeded to carry it.
Since they only needed the LEM from lunar orbit to the Moon and back, all that extra stuff stays circling the moon.
So Lunar orbit rendezvous uses less fuel on the way down and uses a relatively small rocket to blast-off from the moon.

But if all of that was bigger, launching it off Earth would have needed a rocket even bigger then the Saturn V.

Drag?! :smack:

Why, yes.
Haven’t you seen how the space craft maneuver in Star Wars?

I thought the stack was on a free return trajectory, and after S-IV separation it was fired briefly to put it on an impact trajectory. Were the command and lunar modules really on an impact trajectory prior to the mid-course correction burn?

Have I been doing my Kerbal Space Program Apollo reenactments all wrong?

I suspect you are right. I was simplifying a bit, and haven’t checked. Certainly staying on a free return whilst performing all the tricky stuff is a much safer option. I’m not sure how many restarts the S-IV was capable of, given it isn’t a simple matter of just turning a key.

Yes, the stack was put into a free return trajectory. The command module would fire to put it into a lunar orbit, but the S-IVB obviously would not.

Not all of the S-IV stages impacted the moon. Some of them flew past and are now in heliocentric orbits. One was even mistaken for an asteroid. Wikipedia’s not really clear on if they could be restarted for a second time. For Apollo 8, it sounds like the remaining propellant was vented through the engine without actually restarting the engine, which gave it enough change in velocity to keep it from interfering with the spacecraft.

In both Direct Ascent or EOR modes, it would be necessary to land the CM with the SM and a hypothetical Landing Stage (or Lunar Descent and Lunar Ascent stages) on the moon. So, the crew would be mostly on their backs with no direct view of the lunar surface attempting to land. Television cameras? We know how well those worked on Apollo 12. And we know how Armstrong had to search for a boulder free area to land the Eagle. I just can’t see doing that in a , what, 55’ tall rocket (the CM was 13’ tall and the SM was 20’. I don’t know if that included the nozzle of the module). And landing the thing upright on an unprepared surface?

After the TLI and during the docking, the CSM, LM, and S-IV were on a free-return trajectory. After the extraction, the CSM fired a brief burn to add a couple feet per second to get away from the S-IV.

I seem to recall the reason why the lunar flights only had an orbit and a half around the Earth before the TLI was a lack of confidence in how long they could store LH2 and LO2 in the rocket before it would vent out. They didn’t do a barbecue roll until after the docking and extraction. So, I don’t know how long they’d want to put off firing the S-IV for a 3rd/final time.

Good point about landing, that is probably why NASA made the right decision.
Barbecue roll? Is that rolling the spacecraft to normalize temperature?

Yes, it is a standard term for a thermalization roll.


The coupling did have to be strong enough to withstand the lunar orbit insertion burn, though.

Another reason for linking up post-TLI is the way the stages were stacked. See this informative diagram. If they took out the part that flies down before they were done with the part that falls off third, they would have needed to put it all back together.

No, that’s not quite true. After the very big problem, they didn’t trust the part that goes along at all to do anything right. (OK, I’ll stop now.) Even if it had worked, using it might have damaged the spacecraft further.

Normally the SM’s main engine would have fired, with the LM docked to the CSM, for the lunar orbit insertion I already mentioned.