Apollo 13 Rescue Exercise

Requires Adult Supervision

“Making a Square Peg Fit into a Round Hole”

By Jerry Woodfill

Apollo 13 Warning System Engineer

Click here for recent reflections about what led to requiring "duct tape" to rescue Apollo 13.


The Testimony:


            For the Apollo 13 crew, the dilemma was that the square command ship’s CO2 filters would not fit into the round rescue ship’s filter barrel.  These had to work to clean the cabin atmosphere of Carbon Dioxide gas.   The lander’s  round filters were two in number.   Each had a life of approximately 24 hours with two men on board.   Since there were now three men, that life would be somewhat shortened.   The round filters were housed in two separate barrels in the lander.   One barrel was plumbed into the cabin’s environmental control system.   The other barrel  simply stowed the second cartridge.   When the first filter was consumed, the crew simply interchanged the filters in the barrels.                                              

            At the appointed time of Congress’s prayer edict, the first round filter would need to be changed based on the 24 hour criteria.    This would leave only 24 hours to deal with the challenge and solve the problem of  “making a square peg fit into a round hole.”  Unfortunately, three more days journey would be needed at that time to bring Apollo 13 home.  This would be at least 48 hours short of the lander’s filters’ capacity.   

            My recollection of the threat, besides the earlier meeting with Don Arabian, was Don’s voice bellowing from his throne  in the mission evaluation room  that Tuesday,  “I need those guys to come up with an answer on the CO2 thing!”  He was referring to the “tiger team”  led by Ed Smylie, the crew systems manager working the problem.     But, at 9:08 PM, April 14, 1970, an explosion of prayer stormed heaven in behalf of Apollo 13.  These intercessions had been requested earlier that day by the a proclamation of the United State Congress.                                                      

            Several years later, I was speaking to a group of Christians in a small Texas community near Dallas.    My topic was, of course, the impact of prayer on the rescue of  Apollo 13.   After the meeting, a woman asked to speak with me.  She began,  “Our church had been praying that week for Apollo 13.   I recall something that was in my mind about farmers trying to make pigs go into a barrel.  It was difficult understanding what that had to do with Apollo 13, but I prayed anyway that the farmers would succeed.”


            I responded,  “What you remember is exactly what we faced that evening…it had to do with a square ‘pig’  filter not fitting into a round barrel.   Our engineers were your farmers, those pigs were the square command ship’s carbon dioxide filters, and the barrel would be the cylinder containing the round CO2 filters in the rescue ship.”  I believe Congress’ proclamation led to such a “vision” and prayer.  The team benefited from that prayer.  In a back room at  the Manned Spacecraft Center, they assembled a barn yard of space components, items known to be onboard Apollo 13:  Moon rock plastic bags, suit hoses, card board checklist covers and bungee cords.    With the square filters, these were components which might comprise a “witty invention” capable of  making those square filters work in the round barrel system. 

            The Bible speaks of God knowing the number of hairs on our heads.  The average number of hair follicles at any one time on a person is 100,000. Each hair follicles can grow about 20 hairs in a person’s lifetime.  So God knows about 2,000,000  of our hairs.  Now the number of parts in the command module was about 2,000,000.  Isn’t it reasonable to conclude that God knows each of those parts as well as our hairs. But He cares much more about the men on board than their hair so that He can impart wisdom to fix any of those millions of parts to make that filtering system work.


            While the nation prayed,  Smylie and his team conceived a configuration that might be successful.   The concept seemed to evolve as all looked on.   It was to attach a suit hose into a port which blew air through the hose into an astronaut’s space suit.   If the space suit was eliminated and, instead, the output of the hose somehow attached to the square filter,  perhaps, the crew could be saved.    This, in effect, would bypass the barrel.    The air blown through the filter by the suit fan would have no carbon dioxide as it reentered the cabin atmosphere.                              

            The challenge was attaching the hose into a funnel-like device having a small round inlet hole for the suit hose and a much larger square outlet attached and surrounding the square filter.   But the funnel would most likely leak.  Added to that difficulty was the hose and plastic Moon bags tended to collapse restricting the air flow through the filter.  The thought came, “Use cardboard log book covers to support the plastic.” It worked!  But more importantly, “How could the funnel be fashioned to prevent leaking?”   Of course…the solution to every conceivable knotty problem has got to be…DUCT TAPE!  And so it was.   Someone,  providentially, had stowed the miracle substance onboard.                                                                          

            Using the plastic Moon rock bags, cardboard, suit hoses, and  duct tape, the device was attached to that filter.   Soon the CO2 levels had descended to safe levels.   God had made a way of fitting a square peg into a round hole because the nation prayed that Tuesday evening in April of 1970. When the  Master Engineer of the Universe, the Lord Jesus Christ, was called as a problem solver, He never fails.   Having read my account, try the exercise described below.  Of course, it would be good to pray first for God’s help.   In this way, you will understand how very helpful He was to save our astronauts that week in April of 1970.

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Apollo 13 Rescue Exercise

Requires Adult Supervision

“Making a Square Peg Fit into a Round Hole”

The YouTube demonstration is much superior to the exercise posted below which I created years ago. Watch the video and collect the materials described in it for your class. (Note: The hair blower would work well with the vacuum cleaner hose rather than using a handheld vacuum cleaner. Of course, the cold air setting should be selected on the hair blower. Also, the simulated square CO2 filter in the video was constructed from a one foot square AC filter bought at a grocery store. By cutting it into four square pieces and stacking them using, of course, duct tape or masking tape to hold the stack together, I fashioned the demonstration Apollo 13 square CO2 filter. )

Click here for a YouTube clip from one of the SKYPE shows which includes Jerry's demonstation constructing the Apollo 13 CO2 filter apparatus using DUCT TAPE that saved the crew from death. (The above photo is from the show.)


The following exercise is not nearly as good as the demonstration I put on YouTube.

Click here to watch a video of the Apollo 13 crew assembling the modified CO2 filter using duct tape.

Materials:  Roll of 1 inch wide masking tape,  Hair Blower, Plastic Turkey Oven Bags,

8 ½” by 11” card stock,  cloth fabric, roll of  garden weed mesh, discarded shoe (or any other similar) box,  discarded garden hose, scissors.  All the items (except for garden mesh, hose and box) are likely available at a hardware or grocery store. Hand held hair dryers are always available at home.


Left Photo of First Built Version of Filter, Right Photo  is Astronaut upgrade





  1. Examine the above photos for clues about fashioning your version of the Apollo 13 “jury-rig” filter from the list of materials.
  2. Read the air-ground instructions given the Apollo 13 astronauts for added information about making your filter.  They are listed below.
  3. Using the instructions, pictures, and materials build a square filter with a configuration which plumbs air from the blow dryer through the hose and ultimately the filter without leaking.



                                        DANGER:  BE CAREFUL

  1. ATTENTION: Be sure to select the warm rather than hot setting for the blower when you switch on the hair dryer.  Use your hand to test whether your design and finished approach is “air-tight”. 
  2. Examine the photos above.   What improvement is evident in the photo at the right of the improved version made by the Apollo 13 crew? The left photo shows the original version built by the crew from the instructions radioed by mission control, then constructed by the astronauts.


For the Instructor:


            The sides of the shoe (or similar) box serve as the frame/structure of the filter.   Cut out the center bottom of the show box after removing the shoe box lid.  Cut the plastic weed screen to cover the top  open end of the shoe box, then attach the mesh using masking tape around the top of the shoe box holding the mesh taunt and taping the mesh to the sides of the box as you might wrap a gift.  [The mesh serves as the filtering material.  Explain to the students the principle involved, i.e., having contaminated air pass through the fine mesh as though it is LiOH crystals for removing CO2 from the cabin atmosphere.  Explain the purpose of the blower being a substitute for the Apollo 13 suit fan.] 

            Have the students construct both the square filter as well as the modified apparatus. Their device would have enabled the Apollo 13 crew to avoid using the round lander filters in the barrel-like containers shown in the above right photo.

            Also, have the students fashion the means for conducting the dryer’s air flow through the garden hose into their filter system. [Prior to class, you will need to cut the garden hose into one – two foot lengths.]  Note the version the ground controllers built and the later improved version built by the Apollo 13 astronauts differ.  Likewise, will student’s designs and finished products differ.  Have a selected team of students evaluate the completed devices and give reasons for strengths and weaknesses of each.  

            Lastly:  Simply provide this description of the activity to the students and have them draw, explain, or write a narrative description of how to build their square filter apparatus without actually constructing it.  Let them pretend they are mission control instructing the Apollo 13 crew via radio in a step-by-step procedures how it is done. Then, ask a separate team of students to copy down the instructions and attempt to assemble (draw) the filter as did the Apollo 13 astronauts without having a picture or photo of it. (This is certainly the least involved of the approaches.  No materials required other than imagination and ingenuity.)


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The educational activity supporting the above exercise follows.  It is simply a close approximation of the air-to-ground procedures called up to the crew in April of 1970. It was created from the original Apollo 13 radio communication for a college class assignment in Communications by George Wilson, COMM 4120, Assignment #3, 13 May 2004.  This was an exercise in communicating Instructions and Procedures.



Adaptation Procedure


Environmental System CO2 Filters


Apollo 13 Lunar Module



This procedure, which takes approximately one hour to complete, will allow the fitting of a square Odyssey Command Module (CM) scrubber filter through the Aquarius Lunar Module’s (LM) round mounted filtration hole and will modify the Environmental Systems scrubber unit. The material is for astronauts to use when CO2 scrubbers fail in the CM, all CM filters are used or in other situations where additional scrubbing of CO2 is required. Crew Systems Division assembled and tested this information.



All required equipment is contained onboard within the Apollo 13 CM and LM.


Cover to the Apollo 13 flight plan (to cover and protect the hose entry)

2 lithium-hydroxide canisters

Roll of gray duct tape

2 LCG bags

2 hoses from the red suits

2 socks

1 bungee cord (to secure the modified filtration device to the wall of the LM)


PROCEEDURE - 19 steps

#1) Place the LCG bag over the top of the square lithium-hydroxide canister. The bag must be pulled down to just over the triangular ventilator slots on the side.

 NOTE:                  Be careful not to rip the bag because there are only three bags onboard.


#2) Tear the duct tape down the middle lengthwise to double the linear length.


#3) Seal the bag to the square canister by wrapping the duct tape around the canister where the bag opening is.

* NOTE:                  The tape must seal the opening to prevent leakage of air flow.


#4) Poke or cut a hole in the middle of the top of the covering bag approximately the diameter of the hose from the red suit.


#5) Insert the hose into the hole. Secure the hose connection into the LCG bag with duct tape.

* NOTE:                  The tape must seal the opening to prevent leakage of air flow.


#6) Cover the top of the LCG bag and hose attachment with the flight plan cover in an arch. The hose will stick out of one side of the arch.


#7) Attach the two sides of the cardboard flight plan cover that make contact with the square filter using a six inch long piece of duct tape over the top. This will prevent crushing the hose and air entry on the final mounting in step #19.


#8) Wrap the entire top sides of the filter cube with a three foot (about an arms length) piece of duct tape. Repeat wrapping on the bottom of the sides of the cube.

* NOTE:                  The tape must seal the opening to prevent leakage of air flow.


#9) Secure the bag with strips of duct tape two per side running from one side, under the bottom and back up the other side. Repeat on the other side. The bottom of the cube will resemble a tic tac toe board when this step is completed. (See Figure 1 to the right)

Figure 1



#10) Stuff the sock into the ventration hole in the center of the square scrubber. This will prevent the air from bypassing the filter. Cover the hole with a couple of pieces of tape to keep it from falling out.


#11) Repeat steps #3 through #10 for the second canister. This will be the replacement when the first filter becomes saturated.


#12) Open the sensor relief valve. This will normalize the pressure and allow you to attach the hose to the intake valve.


#13) Attach the free end of the hose to the scrubber intake.


#14) Attach the end of the bungee cord to the hook above the lithium canister mounting location on the bulkhead. (See Figure 1 in step #9 above)


#15) Secure the canister to the bulkhead by hooking the other end of the bungee cord below the mounting location. (See figure 1 in step #9 above)


#16) Attach the crossover hose to the secondary air cleaner.


#17) Close the sensor relief valve opened in step 12.


#18) Set the CO2 select to secondary using the LM air cleaner selection switch on panel eleven.


#19) Engage the air cleaning scrubber fan by flipping the ACSF switch located on panel eleven.



* Check the CO2 levels on the partial pressure (marked PART PRES C02) meter on panel eleven. The level should begin to fall (safe level of partial pressure is below eight (8)). Further verification will be indicated through the amber CO2 warning light set to illuminate if the level is above ten (10).


* Reminder: CO2 levels above 15 can be fatal and will cause brain asphyxia, impaired judgment and blackouts




* If the canister seals are not correct in steps #3, #5 and #8, CO2 cleaning may not be adequate. You will recognize this if you hear a whooshing sound when the system starts. Re-tape the seal that is leaking.

* If the LCG bags get ripped air flow may leak out and fail to clean the CO2. Use the spare bag if this happens.