Apollo 13’s Mysterious Explosion
By Jerry Woodfill
Apollo 13 Warning System Engineer
Drawing of Apollo 13 Command Module (left) and Service Module (right)
When Apollo 13’s Oxygen Tank 2 exploded, there was grave concern about damage to the Command Module’s heat shield. The structural design of the service module above has a 30 inch by 13 foot long open tunnel-like volume in the center of the module. The tunnel is much like a chimney such that gases, liquids, or particles could readily move through it toward the main engine bell at the right and the heat shield at the left. The tunnel is not sealed so that the explosive force of the burning oxygen from the exploded O2 tank 2 could escape into and around the tunnel in the direction of both the heat shield and main engine. Covering the outside of the service module oxygen tank and adjacent tanks, fuel cells, and equipment was a 13 foot long exterior panel..
Crew photos after the service module was jettisoned in preparation for the command module’s reentry via the heat shield revealed the panel was missing, blown into the vastness of space by the exploding pressure of the detonating oxygen. Damage to the plumbing from the exploded tank caused the remaining Oxygen Tank 1 to lose its oxygen as well. Likewise, there was apparent damage to the Hi Gain Antenna at the right indicating the panel had catapulted into space striking the antenna.
Concern was voiced that shrapnel from the exploding tank had entered the tunnel, and perhaps ultimately caused damage to both the heat shield and main engine. The main engine could be ignored. The lander’s descent engine could be used instead for the voyage home. Unfortunately, there was but one heat shield. It had to work.
The attachment strength of the service module panel to the structure required a considerable internal pressure of 24 pounds per square inch for severing it from the service module.. A much lower pressure was required to separate the command module with its heat shield from the service module, only 10 pound per square inch. Miraculously, the command module and service module stayed attached while the internal pressure of the explosion rocketed the panel into space.
As an educational exercise, speculate on why the panel blew and the crew capsule/service module attachment remained in tact. The experiment below helps to explain the pressure differentials involved in examining the issue.
Apollo 13 Pressure Experiment
Materials: Tall drinking glass, cardboard, pitcher of water, bucket for collecting spilled water, printed template of service module, rubber band.
Procedure: Per the materials and pictures above, cut out the template and wrap it around the water glass as shown. Attach it to the glass by a rubber band or by simply wetting the glass prior to wrapping the template around the glass. Cut out a square of card board whose edges are at least ˝” beyond the glass rim. Fill the glass to the rim with water displacing all the air. Make sure the rim of the glass is wet then place the cardboard tightly atop the glass rim with your free hand. Holding the cardboard against the glass rim, turn the glass over, i.e., rotate it toward the earth by 180 degrees. Remove your hand from the cardboard to observe that the water is held in the glass by the cardboard without assistance from your hand. (Perform the above steps over a bucket to assure spillage is contained for clean up after completing the experiment.)
Discussion: It is important to explain the concept of pressure as a force per unit area. Since the weight of a given volume of water is altogether greater than the weight of the same volume of air, it would seem that the water should flood from the glass when it is turned over. Why doesn’t that happen? The explanation is that the pressure against the cardboard results from a column of air thousands of feet high while the pressure from the column of water measures in inches. Since water has a weight of a little of .036 pounds per cubic inch, an eight inch by one square inch column of water in the glass would exert a force of about .3 pounds per square inch on the cardboard. This would result in a pressure on the “make believe” command module heat shield of .3 pounds per square inch compared to the pressure holding the shield onto the service module of 14.7 pound per square inch. The competing forces would assure that air pressure wins the push: 14.7 pounds per square inch versus .3 pound per square inch.
A similar explanation was found on the internet: (The experiment was performed by Alexander Graham Bell in 1912.)
sea level the atmospheric pressure is 101.3 Kilopascals (
Analogy to Apollo 13’s missing panel mystery:
Of course the glass, with it liquid contents, would be similar to the service module’s fuel tanks in our comparison to the actual Apollo 13 incident. And, like the outcome for Apollo 13, the fuel tank and shield (cardboard) remained attached while the pressure of the exploding oxygen tank blew the panel into space. We know that no air pressure exists in space. It is a vacuum. Therefore the force which held the vehicles together was the strength of their mechanical attachments. Two pressures were at work. Each attempted to overcome respective attachment forces: the force which attached the service module to the command capsule, and the force which attached the service module panel to the service module. Comparing them to the upside down water glass experiment helps to understand the nature of pressure forces.
Because the explosive pressure force of the Oxygen was immediately applied in great strength to the panel, this overwhelming force would be expected to blast that panel apart from the vehicle, exceeding the 24 pound per square inch attachment strength. However, venting of residual explosive oxygen into the framework of the service module could well be expected to overcome the attachment strength between the two vehicles, separating them.
Yet, it did not. Why not? Apparently, the presence of "tankage" and other structure acted to mitigate and dissipate the sudden pressure spike before it reached the interface between the vehicles. However, if a shard from the exploded O2 tank 2 had punctured any of the adjacent tanks, likely,. a secondary explosion of any of them would have propagated both the explosion and build up of pressure. In that event, certainly, the vehicles would have experienced either a fatal separation or fatal damage to the heat shield.
Incidentally, such a piece of shrapnel did fracture the plumbing between the oxygen tanks such that eventually, all the oxygen in tank one vented into space. Fifteen minutes after the explosion, Commander Jim Lovell looked through a spacecraft window and observed something venting into space, the residual oxygen from tank one. That observation spoke volumes about the seriousness of the event such that most in mission control ceased blaming the situation on instrumentation readings and acknowledged that the service module had been mortally wounded.
Forensic analysis of the above explanation is obscured by the absence of a service module for an engineering autopsy. It burned up in Earth’s atmosphere. Nevertheless, the water remained in the glass (the service module’s tanks), so to speak, the heat shield was not damaged, and the vehicles remained attached.