You know the basics of a Saturn V launch. The S-IVB third stage with the Apollo spacecraft, with the Lunar Module inside the Spacecraft-Lunar Module Adapter (SLA) with the Command/Service Module on top, is hurled into earth parking orbit for tests and checks of the vehicle before the J-2 engine fires again to send the spacecraft towards the moon.

You’ve seen how the CSM later separates from the S-IVB, then docks with the LM to pull it out of the spent S-IVB stage. But animations and films like Apollo 13 oversimplify the process. You might not know how the LM was serviced and outfitted, as needed, prior to launch–when the spacecraft had been fully assembled with the Saturn and on the pad, over 240 feet in the air.

Let’s take it all apart, starting at pre-launch.

Before Liftoff

The Apollo spacecraft assembly (the CSM and LM) are, logically, the last items placed atop the Saturn V as it was stacked, stage by stage, in the Vertical Assembly Building.

The Lunar Module is electrically connected to the Saturn V through an umbilical that allows the launch crews to make electrical checks on the spacecraft before launch while it rests inside the SLA, below the Command/Service Module atop the Saturn.

MSS S68-55424-orig
The Mobile Service Structure cocoons the Apollo spacecraft stack as its moves into place. (NASA)

One cool bit that’s not seen in most space documentaries: There is last-minute work often done on the LM, while it’s inside the small confines of the SLA. To make this happen, the SLA has an access hatch, located directly in front of where the LM egress hatch rests in the SLA. This hatch was generally used in concert with any other work on the Saturn V, while the large Mobile Service Structure was in place, enveloping the upper stage and the entire Apollo spacecraft.

But there’s just empty air between the SLA walls and the LM.

SLA interior platforms with LM
Depiction of the work platforms placed around the Lunar Module inside the SLA. (Credit: Jonathan Ward)

So temporary scaffolding is placed within the SLA, around the LM on two levels, to allow adjustments. It’s removed, of course, prior to launch. Workmen could reach almost any part of the LM, despite the tight fit.

(The illustration is from Jonathan Ward, author of Countdown to a Moon Launch and the new book, Bringing Columbia Home, with Mike Leinbach.)

The Lunar Module has two hatches, one topside for use with the CSM, and another, the “mouth” of the LM, for egress to the lunar surface on landing. This hatch currently has the drogue, the conical element of the Apollo docking system, resting atop it. It’s a simple device, with a handle and lock, as well as one of two pressure dump valves on the spacecraft. A second vent is also on the egress hatch.

At launch, the upper hatch pressure dump valve on the top hatch has been left open.

“But that will vent the LM atmosphere! The SLA isn’t pressurized,” you say. That’s right. More on that in a moment.

Hours before launch, the MSS is moved away.  The Saturn V is fueled, the astronauts board the Command Module, and the Saturn V hums to life, soon rising into the sky.

CSM Separation

After launch, and all checks are complete in parking orbit after about two orbits, the S-IVB J-2 engine is fired for about 5 minutes to send the CSM/LM into a translunar trajectory.

The Command Module Pilot now assumes the full piloting role of the spacecraft in the left-most seat (the Commander sat there at launch).  The crew have been working a long checklist of procedures prior to CSM separation, including pressurizing the Command Module cabin to a slightly higher level than the normal 5 PSI: About 5.7 PSI.

CM-LV Sep - HeroicImages
This is the only button in this section used in a normal mission. All other buttons would used at some element in an abort. (Reproduction courtesy of

When all preparations are complete, the CMP pushes the CM/LV SEP switch. That activates a chain of pyrotechnic charges around the base of the Service Module, and along the four seams of the SLA.

CSM Separation
As the SLA panels float away, the high-gain antenna is also deployed. (NASA)

From there, springs also come into play to push the panels clear and away. The SLA jettison also disconnects an umbilical that monitored and controlled the Saturn vehicle from the Command Module.

Apollo 7/S-IVB Rendezvous in space
Apollo 7’s S-IVB 200-series stage, empty with a test target. The crew waved off a planned docking simulation as one of the SLA panels did not fully extend. (NASA)

The complete jettisoning of the SLA panels was enacted after Apollo 7’s view of their LM-less S-IVB showed that the SLA panels opened rather askew in a way that could jeopardize a LM docking.

There were two versions of the S-IVB stage. The 200-series stages used on the Saturn I-B were notably different from the 500-series stage used on the Saturn V in that the 200s couldn’t be restarted. There was little fuel left in its tanks for its earth-orbit ascent as the 200-series was the second of two stages. The 200-series was not outfitted with extra helium tanks needed to pressurize the fuel and oxidizer tanks needed for a restart.

Despite the intended purpose of using the Saturn I-B to perform earth-orbit tests of both the CSM and LM, dramatically increasing weights of both spacecraft during development quickly outpaced the maximum payload capacity of the Saturn I and I-B. Ultimately, the I-B could lift a lightly fueled CSM or a LM, but not together.

So, if you’re looking at photos of the Skylab and Apollo-Soyuz missions, which used four of the six remaining Saturn I-Bs, you’ll notice that the SLA panels are still connected to the S-IVB on CSM separation. The 200-series S-IVB stages were mothballed for 7 years and not modified to jettison the panels as the 500-series.

Docking and Pressurization

Back to that pressure dump valve in the LM upper hatch. This valve’s central use is in the depressurization of the LM cabin once the astronauts have their Portable Life Support System backpacks and spacesuits prior to opening the lower egress hatch for a moonwalk. On launch, this valve is intentionally left open.

LM tunnel hatch lm10-co20
The pressure dump valve, left center. This is the view from inside the LM cabin; the valve is also controllable from the tunnel once docked. (NASA)

The logic is pretty sound. The Lunar Module has very limited resources to pressurize and maintain a cabin atmosphere of about 5 pounds per square inch (PSI) of oxygen. The first “H” missions (11-14) had enough for two repressurizations or so. While inside the SLA, the LM cabin is simply exposed to relatively normal air at normal atmospheric pressure, which includes nitrogen. That’s fine while workers prepare the LM. In space, it’s a different matter.

As part of the changes after the Apollo 1 fire, using normal air at normal atmospheric pressure provided better fire prevention and suppression, but wasn’t preferable in space. Prior to boarding the Saturn, the astronauts don their suits and pre-breathe for hours in pure oxygen to remove nitrogen from their bodies to avoid decompression sickness (the “bends”). They enter the CM with the cabin filled with air, but the astronauts are connected to CM oxygen hoses and remain in their pressurized suits. As the CM ascends, the CM cabin is vented to space and then refilled with oxygen at a safer 5 PSI, also removing the nitrogen.

The LM, however, hasn’t a means to automatically vent its air-filled cabin. Even if it could vent and then repressurize, it would use some of its limited oxygen supply, reducing moonwalk time.

So the LM pressure dump on its tunnel hatch is deliberately open at launch, letting the LM cabin vent to vacuum on ascent.

After CSM separation from the SLA, the CSM turns around using its reaction control jets. It slowly approaches to dock with the Lunar Module.

The pneumatically-extendable probe’s three latches grip the inner ring of the drogue. From there, the probe is retracted and tunnel latches activate to form a hard dock. (NASA)

The probe/drogue system was one of the few elements in Apollo where there wasn’t a backup. Docking consists of two phases. The probe’s three small latches grab the interior of the drogue–a soft dock. Retracting the probe, pulling itself toward the drogue, brings forward twelve latches inside the docking ring that hold the LM firmly against the CM tunnel to form an airtight seal–hard dock. Only on Apollo 14 did this part become seriously problematic. It took over two hours to complete due to an unexplained malfunction in the probe latches, threatening the lunar landing.

Unlike seen in the Apollo 13 film, probe retraction is much slower: About eight seconds. During retraction, the crew disables their thrusters on the CSM. There’s a little “play” in the probe that allows the CSM to rotate and swivel along the probe tip. Thrusters are off as there might have been some minor offset of angle of the probe to the drogue, causing the spacecraft to be pulled in slightly off-center. Keeping the CSM’s (and S-IVB’s) thrusters off prevented the spacecraft’s guidance from sensing a false problem and firing jets that could cause damage to the probe or the LM. The limited angles of the probe-drogue connection would align everything as the docking ring connected and the tunnel latches took hold.

Once hard-docked and leak checks are done, the CMP enters the tunnel and opens a pressurization vent on the CM hatch (leaving the hatch itself closed).  Taking advantage of the slight 5.7 PSI overpressurization of the CM cabin, the tunnel as well as the LM cabin pressurizes using the comparably copious oxygen supplies aboard the CSM to a safe 5 PSI. Another benefit is to verify the LM cabin’s integrity since the pressurization should level off after a time. If the pressurization does not equalize, the LM cabin or tunnel has a leak. Repressurization takes about half an hour.

The astronauts are in no particular hurry during transposition and docking, although they can’t stay with the S-IVB too long. The depleted stage’s battery power is good for only a few hours after launch. The S-IVB’s fuel and oxidizer tanks also expand in pressure as it bakes in the sunlight, the stage being forbidden to move during the procedure.

NASA knew pretty well how much overpressure would compromise an S-IVB. Along with data from ground tests, NASA let an S-IVB’s tanks rupture somewhat deliberately on the AS-203 test flight of a Saturn I-B that looked like it was ground down like a pencil stub. To eliminate the chance of the S-IVB turning into a bomb, vents are located on opposing sides of the stage to release some of the pressure without causing the stage to move and throw off any of the LM separation work.

Enough battery power must also remain in the S-IVB for ground controllers to activate the spent stage’s auxiliary propulsion thrusters to redirect the S-IVB to a parallel course that will keep it away from the CSM/LM, eventually careening it to impact the moon. It sounds sad, but one reason the stage is destroyed this way is to get some great seismological data from any other Apollo seismology experiments left on previous missions.

LM Separation

The S-IVB itself has no power to remove the LM’s connection to the last of the SLA. That magic comes from a successful docking.

Once docking is completed and the tunnel is fully pressurized, the CMP reenters the tunnel, this time removing the CM tunnel hatch. He checks the docking latches. Only three are needed for a suitable hard dock, but if necessary, he will manually reset other latches.

The CMP also un-stows two umbilical cables.

These umbilicals provide data and electrical power to the Lunar Module itself, allowing the CSM’s fuel cells to provide power to the LM to charge and conserve its batteries. The umbilicals also connect the CM to a device called the LM/SLA sequence controller (LSSC).

The CM tunnel hatch is returned just in case. Inside the Command Module, when all is ready, the CMP throws back yet a second protective cover on a switch: S-IVB/LM SEP, located just below the DSKY.

The LM is securely bolted in four places inside the base of the SLA, resting on it’s “shoulders” above the legs, known as outriggers.

LM 2 Outrigger
A close-up of an outrigger on LM-2 at the National Air and Space Museum. (Personal photo)

In case it wasn’t clear, the SLA is more than just the four panels. The Lunar Module’s outriggers sit at the edges of the SLA circular base. The panels attach above it, covering the LM as well as the bell of the Service Propulsion System engine.

LM Separation
The pyros and guillotines that cut a service umbilical and remove the tiedowns to the LM outriggers are all powered by the CSM via the LM/SLA Sequence Controller. At LM separation, springs below the outrigger (“LM fitting”) also gently push the vehicle outward. (NASA)

In the S-IVB, the LSSC commands pyrotechnic charges to sever the holds on the LM outriggers. Springs pull away the tie-downs, rather than let loose parts fly away that could damage the LM. Below the outriggers, springs push out the LM, behaving as passive thrusters to aid separation. As necessary, the crew can also apply RCS reverse thrust to assist.

The CSM/LM may adjust their view so that they can see the S-IVB one last time through the hatch window to verify with Houston that they are sufficiently far from their old third stage.

Later, Mission Control can activate non-propulsive venting of what’s left of the fuel and oxidizer to avoid stage rupture (things can still fly very far and damage the CSM/LM), and then use its thrusters to guide the S-IVB on a parallel course to the moon and to its final fate.