13 MORE Things That Saved Apollo 13, part 8: The Indestructible S-Band/Hi-Gain Antenna

The explosion of a liquid oxygen tank in Apollo 13’s Service Module violently propelled debris and a 13-foot (4 meter) outer panel of the SM out into space.

Later, the crew saw the damage when they jettisoned the SM prior to reentering Earth’s atmosphere. Commander Jim Lovell described the scene:

“There’s one whole side of the spacecraft missing!” Lovell radioed to Mission Control. “Right by the high-gain antenna, the whole panel is blown out, almost from the base to the engine.”

The panel was likely blasted outward and rearward, toward the deep space S-Band radio antenna. The antenna was attached to the outer edge of the module’s rear base via a meter-long strut, and was used for both telemetry and voice communications.

NASA engineer Jerry Woodfill feels this hi-gain antenna was surely struck by the panel and/or schrapnel ejected by the oxygen tank explosion.

“That deep space radio communication was maintained during and after the explosion was almost miraculous,” Woodfill said. “Such a blow should have destroyed that hi-gain antenna. Those of us who watched the telemetry display monitors saw only a momentary flickering of the telemetry, but after a few flickers we continued to receive data.”

Woodfill said it was as though a boxer had taken a devastating punch and continued to stand unfazed.

This video of the severely damaged Apollo 13 service module was taken by the crew after it was jettisoned.

If instead, the antenna had been destroyed, the loss of data would have resulted in an impaired ability to analyze the situation and communicate with the crew.

The moments following the explosion are seared into Woodfill’s memory. On the night of April 13, 1970, 27-year-old Woodfill sat at his console in the Mission Evaluation Room (MER) in Building 45 at Johnson Space Center — next door to Mission Control in Building 30 — monitoring the caution and warning system.

“Because I was watching the command ship’s telemetry on a monitor at the moment of the explosion, both the words heard in my headset, “Houston, we’ve had a problem” and the scene I saw of the video monitor have not been forgotten” Woodfill said. “Seconds before I heard the audio of Jack Swigert’s call, I watched the video screen flicker several times.

To this day, Woodfill said he cannot understand how it continued to function following the explosion.

“As an engineer, I have studied the basics of simple machines,” he said. “The concept of the lever arm dictates that when an explosive blow strikes a structure atop an arm, the arm must bend back about its attachment to the supporting structure. In this case, that structure was the command ship’s supply module, the Service Module. Later photos by the crew (below) showed the antenna intact and the conical reflector dishes present with their center probes intact. In my mind, the entire assembly simply should have been severed altogether.”

The Unified S-band (USB) system was a tracking and communication system that combined television, telemetry, command, tracking and ranging into a single system. The high-gain antenna consisted of an 11-inch-diagonal wide-beam horn flanked by an array of four 31-inch-diameter parabolic reflectors. Its multifunctional system simplified operations, and its construction saved on weight.

And obviously, it was very durable.

Woodfill reiterated how important it was that the antenna survived the explosion.

“Later on it wasn’t needed, as the crew used the Lunar Module communication system,” said Woodfill, “but having that initial continuous communication was one of the things that was very important.”

And later those in Mission Control and the MER were be able to go back and look at the data that had been transmitted to Earth during that very crucial period of the mission, to help understand what had actually occurred.

“It was critical to have that data in those first moments of the explosion to analyze what had happened,” Woodfill said. “Uninterrupted communication was essential to investigating the status of the vehicle. While it may be true that the backup omni-antenna might have provided temporary communication, based on my analysis, the omni-antenna would not have served as ably during the time of greatest initial peril. In fact, to configure its use with the NASA world-wide tracking network would have caused an unfortunate delay.”

Here are some zoomed-in photos taken by the crew of Apollo 13 after the explosion of the S-Band/hi-gain antenna, and Woodfill has noted the parts of the antenna. They show the explosion failed to sever the hi-gain antenna mast and the conical dish receivers as well as the rectangular antenna, and the center probes of the conical dishes appear intact. Considering the force of the explosion, this is remarkable.

Previous articles in this series:

Introduction

Part 1: The Failed Oxygen Quantity Sensor

Part 2: Simultaneous Presence of Kranz and Lunney at the Onset of the Rescue

Part 3: Detuning the Saturn V’s 3rd Stage Radio

Part 4: Early Entry into the Lander

Part 5: The CO2 Partial Pressure Sensor

Part 6: The Mysterious Longer-Than-Expected Communications Blackout

Part 7: Isolating the Surge Tank

Find all the original “13 Things That Saved Apollo 13″ (published in 2010) at this link.