*

            It’s cold, and not just in the hibernating Command Module.  With most systems switched off to conserve power, the Lunar Module now, on April 16,  is nearly as cold, even with all three crewmembers crammed shoulder to shoulder. Jim Lovell tells the ground, “You might pass on to our friends in crew systems that lunar boots make great foot warmers.”  Later he says, “I just brought out some hot dogs and they’re nearly frozen.”

            The real action is occurring on the ground, where engineers, controllers and astronauts are developing checklists for the critical last hours of flight. 

*

            At 11:53 p.m. on April 13, 1970, two hours, thirty-five minutes after calling, “Houston, we’ve had a problem here,” Jack Swigert switched off the last of the Command Module’s systems.  The heart of the spacecraft and the only module with a heat shield, reentry systems and parachutes to return the three-man crew safely to earth, was, in effect, in an induced coma.  For the next two-and-a-half days, the crew would survive in the Lunar Module, designed to land on the moon but now transformed into a lifeboat.  However, to survive, we will have to reactivate the Command Module, something never envisioned.

            A new reentry checklist needs to be developed to awaken the command ship and steer it to a splashdown. Normally, such checklists take three months to develop.  Engineers and controllers of a Tiger Team at mission control only had three days to do so. 

            The cone-shape Command Module, composed of two million separate components, normally performed the myriad of multiple functions needed to journey to and from lunar orbit.  The capsule housed 1,550 pounds of electrical equipment, communications systems weighing 225 pounds, vital navigations systems weighing 1,100 pounds, environmental controls totaling 450 pounds and 550 pounds of recovery equipment.  Reviving all those systems wasn’t as simple as throwing a switch or even a few switches.   A precise order needed to be worked out.  And that order would be constrained by power requirements.  The module contained three re-entry batteries designed to power the craft for just the final 45 minutes of flight, after the point where the cylindrical service module with it’s large storehouse of supplies are normally jettisoned.  Now the batteries would need to power the command module for the final 2.5 hours of flight.  Turning on too much equipment early, or in an order that caused to heavy a power load, could blow the batteries.

             Planning the checklists for the final six hours of flight began just 47 hours before re-entry on April 17.  The first goal — preserve the Command Module’s batteries as long as possible by using power from the Lunar Module for the initial steps. A small team of controllers led by John Aaron made an initial estimate of what systems could be powered up in what order.  It was a rough starting point for give and take with wider working groups of systems engineers who naturally wanted priority for their subsystems.  Aaron called it a “straw man” timeline that the engineers could throw rocks at, that would evolve through many discussions and trade-offs between systems teams.  

            Aaron served as referee, closely watching the power budget, deciding the trade-offs to maintain levels.  For example, guidance controllers wanted to use the most accurate of three systems, the Primary Navigation and Guidance System for re-entry.  Aaron vetoed it, as it would consume too much power.  A less accurate system employed six small gyros. Even that was too much.  Aaron finally approved turning on three of the gyros in conjunction with the third system, which was a simple meter that measured the gravity forces on the spacecraft.  The gravity levels would indicate if the capsule was entering the atmosphere too steeply or two shallowly. 

            Another power hit centered on the heaters for the re-entry thrusters.  Fuel may have frozen in the lines.  The heaters would need to be switched on early in the power up. Effects rippled from step to step, full of ramifications downstream.  The engineers worked non-stop through several revisions of the checklists. 

            For any restart, the initial step involves establishing a start point.  Swigert first configured and verified every switch, numbering nearly 500, on the Command Module’s control panel, placing them in the launch position. 

            By noon on April 16, the day before re-entry, the timeline is finalized — and ready for one final check, a run through the integrated Lunar Module/Command Module simulator, a task that takes all afternoon.  At 6:30 p.m., the Capcom tells the crew the checklist is nearly ready to be read up to them.  Command Module Pilot Jack Swigert will need to copy down the checklist for the power-up. He is warned to gather up as much blank paper as he can find.

            After delays to make sure everyone in Mission Control has a copy of the plan, the Capcoms, fellow astronauts, the only ones who normally talk with the crew, begin the slow process of reading up each step, with Swigert writing and repeating each back to them.  His hand goes cramped writing so much.  The process takes two hours.   Then the ground reads up the checklist for powering down the Lunar Module, a much simpler sequence, to Fred Haise.  That takes less than an hour.  By 11 p.m., just 6.5 hours before the sequence of steps will begin, the read-up is complete. 

             Many unknowns remained.  Without electronics to heat them, the temperatures quickly dropped inside the module.  By the time of reentry, they would be near the freezing mark.  Could the cold cause systems to fail?  And condensation covered the control panel and other surfaces. Could moisture cause electronics to short out or cause a fire?  

            In a few hours, we’ll know.