What Led to Needing Duct Tape to Save Apollo 13’s Crew?


by: Jerry Woodfill, the Former NASA Apollo 11 and Apollo 13 Spacecraft Warning System Engineer



Hollywood’s Apollo 13 movie is, perhaps,  best known for  engineers constructing a duct-taped-jury-rigged Carbon Dioxide filter.   While I never participated in that heroic “lego-like” building exercise, watching actor Tom Hanks recognize the threat reminded me of the role I’d played.  Though not memorialized by Hollywood, it, arguably, might be cited as crucial.  The following should encourage  STEM practitioners, like myself, to pursue perseverance and resourcefulness.  What prompted my recollection were the images below.                                                                                                     


To summarize the scene:  The Warning System turns on the Carbon Dioxide caution light.  Immediately, ground control asks, “Jim, could you check your CO2 gauge for us?”  The pair of photos, from the movie, capture the event.



                                      Carbon Dioxide Caution Light       Carbon Dioxide Gauge


Even though my system, caution and warning,  ignited the warning light alerting the crew, that’s not what gives me the most satisfaction.  Here’s why:  I’d dreamed of being recruited by NASA as, perhaps, a flight controller, or a communication, radar, even a guidance and navigation engineer.   After all, I had a BS Degree from Rice University in Electrical Engineering.                                                            


But my only offer came from a Manned Spacecraft Center instrumentation engineering section. The group dealt with meters, switches gauges, and things called transducers.  These devices were responsible for measurements on board the spacecraft.   And, then, sadly, my journeyman assignment became the switches and gauges -  certainly not the engineering currency leading to promotion.                            


As the Apollo program progressed, my role became more diminished. A pair of more senior engineers in the section replaced me.  I was no longer responsible, even for the switches and gauges. As a token responsibility, I was given the spacecraft metal panels which housed the switches and gauges. Even a mechanical engineer would not find such a job exciting, and, to think,  I had once studied field theory, quantum electronics and other heady disciplines as a Rice electrical engineering candidate.                     


Added discouragement came on a NASA trip to California.  At a meeting, one of the engineers from a west coast concern commented on their organization’s difficulty creating an instrumentation engineering group.  His comment was, “No one wants to be an instrumentation engineer, thinking it is a dead-end assignment, best avoided if one wants to be promoted.”                                                         


It seemed, unbeknownst to me, that instrumentation was looked upon as a sort of “menial servant” whose lowly job was servicing end users such as radar, communications, even guidance computers.  In fact, the users could just as readily incorporate instrumentation in their devices. Then, there would be no need for an autonomous group of instrumentation guys.               So how did these rather drab prospects lead to the Apollo 13 rescue?   At the onset of Apollo, the NASA Manned Spacecraft Center spacecraft system organizations cloned those at the spaceships’ makers, (North American Rockwell, maker of the Command Module and Grumman, the lander’s manufacturer)   For some reason, never known to me, besides the switches, gauges, and metal panels Rockwell combined in the same instrumentation category was another item.  It was an electronic device called the Caution and Warning Detection Unit or CDU for short.                                                                                                       

I suppose my supervisor, observing my distraught demeanor, mercifully assigned the alarm electronics unit to me.   As a result, I became the Command Module Caution and Warning Project Engineer.  This led to inheriting the Lunar Lander Caution and Warning when a colleague left NASA for more pay with an aerospace contractor.                                                                                            


I had been frustrated with being assigned lone items like a switch, gauge or panel, even a “warning detection unit.”   But soon, I realized no one was responsible for the entire warning system on either the Command Module or the Lunar Lander.   So I chose to assume the role.  (No one else wanted the job.)  As such I visited with a dozen or more managers of items which the warning system monitored for failure.   Their ideas along with my acquired expertise about how best to alert astronauts to system failures profited me.  It earned me respect with upper Apollo managers, flight controllers, astronauts and engineers.                                                                                                                                        


One of the systems which the lander’s warning system monitored was environmental control.  Obviously included was alerting both ground control and the onboard astronauts to the level of carbon dioxide gas in the cabin atmosphere.   This led me to convene a NASA-Grumman team to consider how best to warn of Carbon Dioxide and other threats.   We needed to determine at what threshold level should the warning system ring an alarm.  (Now likely, my  relationship to Apollo 13’s duct tape filters is beginning to surface.  But it’s not  the warning system that is most important.)            While I’d like to suggest that the CO2 alarm light on board the lander was equal to that “Rube Goldberg” filter building activity, it wasn’t.                                                                                                                              

My system had two categories of alarms, one, a yellow light for caution when the astronaut could invoke a backup plan to avoid a catastrophic event, and the other, an amber warning indication of imminent life-threatening failure.  Because onboard CO2 content rises slowly, the alarm system simply served to advise and caution the crew to change filters.  We’d set the threshold or “trip-level” of the alarm system electronics to do so.                                                                                                                                 


Attempting to distance myself from instrumentation by assuming a warning system perspective, I’d become conscious of the end-to-end nature of sounding an alarm.  All the components must work, starting with the CO2 sensor.  The signal must pass from there through the transmitting electronics,  wiring, ultimately reaching my warning system “brain” known as the Caution and Warning Electronics Assembly (CWEA).   


Now, what I once had deemed trivial was altogether essential for saving the lives of an Apollo astronaut crew.  Yes, instrumentation was just as important as any advanced system aboard the command ship or the lunar lander.          


Then came the unexpected.   The engineer responsible for that “trivial” CO2 sensor left NASA.   The sensor’s job was simply to convert the content of carbon dioxide into an electrical voltage, a signal transmitted to all - my warning system, the ground controllers, and the cabin gauge.               I had an unsettling thought, “If it doesn’t work, no one would be aware that the crew is suffocating on their own breath.”  Forget about a warning system.   Who cares about duct taped square filters fitting into round barrels?                                                                                                                 


The presence of that lethal gas can only be detected by one thing, an instrumentation transducer.  Immediately, I became an ex-officio expert on a device I’d loathed at the onset of my career.   At once, I collected available data about its performance, even the transducer’s calibration curves.  They related the content of carbon dioxide to the overall cabin pressure.                                                                                    


It had to work, and, if it didn’t function altogether correctly, I’d have information available to those analyzing its performance.                                                                                                  


And so it was: The Apollo 13 Mission Engineering Manager summoned   me to his office just hours after the explosion.  He wanted to discuss my warning system ringing carbon dioxide alarms.                    


“Sir, here is the story,” I explained, placing before him the calibration curves of the CO2 Partial Pressure Transducer. “Let’s see what the instrumentation CO2 transducer is telling us about the threat to the crew?”                                                                                                                                             


And, I thought, Without it, likely, no one would have known  the crew was in grave danger, leave alone how to save them.  Instrumentation engineering wasn’t a bad career choice after all. 



For nearly 50 years, Jerry Woodfill has been employed by NASA in Houston. He holds BAEE and BSEE degrees from Rice University which he attended on a basketball scholarship. At the onset of the lunar landing program, he managed the spacecraft warning systems so that he was monitoring spacecraft Eagle's descent when Neil Armstrong landed on the Moon. Likewise, on April 13, 1970, Jerry was monitoring Apollo 13's warning system when the vehicle exploded. His system was the first alert of the life-threatening malfunction depicted in the Tom Hanks-Ron Howard movie APOLLO 13. Universal Studios designated Jerry as a national spokesman to the media for the release of the film Apollo 13, The IMAX Experience.  For his role in the rescue of Apollo 13, he shared the Presidential Medal of Freedom as a member of the Apollo 13 Mission Operations Team. 


For Complimentary NASA Programs:

The Rescue of Apollo 13 or

Failure Is Not an Option, Steps to Success

call Jerry at 281-483-6331