🚨 Decision Point #9

Recharge for Re-Entry?

GET ~112:00:00 | 30 Hours to Re-Entry

⚠️ The Situation

Re-entry — the one part the LM lifeboat can't help with — is about 30 hours away
During the post-explosion scramble, the dying Command Module ran on its re-entry batteries — battery A lost about half its charge (~20 of 40 amp-hours)
The CM's three entry batteries now hold about 99 of their ~120 amp-hours. A normal re-entry uses roughly 70–80
The math works — barely. No cushion for a longer timeline, an extra system draw, or one weak battery
In Houston, flight controller John Aaron's power team has an idea nobody designed for: push electricity backwards through the docking umbilical, letting the LM's descent batteries refill CM battery A

The Twist:

The umbilical linking the two ships was built to send power the other way — from the CM, keeping the docked LM warm on the way to the Moon. Reversing it has never been tried. Not in space. Not in a simulator. Not anywhere.

Apollo electrical system and battery connections between the LM and Command Module — Two spacecraft, one cable — never designed to carry charge from the LM to the CM

⚡ CHARGE FROM THE LM

Trickle-charge CM battery A from the LM's descent batteries — a procedure invented this week

✓ Advantages

The CM would hit re-entry with essentially full batteries — real margin for surprises
Uses hardware already connected: no spacewalk, no new wiring
The crew's brutal power-down left the LM batteries ahead of budget — power to spare
Aaron's team has already drafted the procedure and checked the numbers

✕ Disadvantages

Never designed for, never tested — anywhere
Spends LM battery power the lifeboat still needs for the next ~29 hours
Slow and inefficient: about 15 hours of charging
A wiring fault could damage both ships' electrical systems at once

🔋 FLY WITH REDUCED MARGIN

Skip the experiment — 99 amp-hours should cover a normal re-entry

✓ Advantages

No untested procedure — zero chance of an electrical fault touching two ships
Every amp-hour stays in the lifeboat, where the crew is living
On paper, ~99 amp-hours covers a normal 70–80 amp-hour re-entry

✕ Disadvantages

Only ~20 amp-hours of cushion — and nothing on this mission has gone to plan
No room for a longer re-entry timeline or an unexpected system draw
Battery A stays half-empty; one battery hiccup and the margin is gone
Once the LM is cut loose, there is no second chance to charge

🤔 WHAT SHOULD MISSION CONTROL DECIDE?

👆 Choose one of the options above 👆

NASA's Decision

✓ CHARGE FROM THE LM

"We're 20 amp-hours short on one of the entry batteries." — CAPCOM Jack Lousma to the crew, GET 111:05 (Apollo 13 Flight Journal)

About an hour later Houston gave the go, and at roughly GET 112:12 Jack Swigert started the charge.

How It Worked:

Reverse flow: the LM's descent batteries fed current backwards through the existing LM–CM umbilical into CM battery A
Trickle charge: slow and gentle — about 15 hours in all
Constant watch: Houston monitored voltage and current for any sign of a fault

Result: Success!

When Odyssey powered up for re-entry, its batteries held about 118 of 120 amp-hours — nearly what they held at liftoff. The crew hit the most dangerous minutes of the mission with power to spare.

The Hero: John Aaron

John Aaron, the EECOM (electrical systems) flight controller, led the "tiger team" that budgeted every amp of the trip home — including this improvised reverse charge. He's the same controller who saved Apollo 12 with the legendary call "SCE to AUX" after a lightning strike. You'll meet him again later in this story.

Why It Was the Right Call:

The charge cost the LM power it could afford and bought the CM margin it couldn't live without. Re-entry is exactly when you want spare electricity: computers, thrusters, and the parachute pyrotechnics all run on those three batteries. NASA traded a known surplus for a critical reserve — and it paid off.

📚 Sources for Skeptics

Would you have risked the untested charge, or flown with the thinner margin? Good — arguing trade-offs is exactly how engineers think. Don't take our word for it — here's the real record:

Apollo 13 Flight Journal — the Day 5 "Starting the Battery Charge" chapter has every radio call from this decision
Apollo 13 in Real Time — replay the whole mission with real Mission Control audio, synced to the second
Apollo 13 Mission Report (PDF) — NASA's official engineering report on the trip home