As the simplified drawing in the next slide indicates (see figure 12) each oxygen tank has an outer shell and an inner shell, arranged to provide a vacuum space to reduce heat leak, and a dome enclosing paths into the tank for transmission of fluids, and electrical power and signals. The space between the shells and the space in the dome are filled with insulating materials. Mounted in the tank are two tubular assemblies. One, called the heater tube, contains two thermostatically protected heater coils and two small fans driven by 1800 RPM motors to stir the tank contents. The other assembly, called the quantity probe, consists of a cylindrical capacitance gage used to measure electrically the quantity of fluid in the tank. The inner cylinder of this probe is connected through the top

of the tank to a fill line from the exterior of the SM and serves both as a fill and drain tube and as one plate of the capacitance gage. In addition, a temperature sensor is mounted on the outside of the quantity probe near the head. Wiring for the quantity gage, the temperature sensor, the fan motors, and the heaters passes through the head of the quantity probe, through a conduit in the dome and to a connector to the appropriate external circuits in the CSM. The routing of wires and lines from the tank through the dome is shown in slide 9 (see figure 13).

The oxygen tank, as designed, contained materials, which if ignited will burn in supercritical oxygen. These include Teflon, used, for example, to insulate the wiring, and aluminum.

Pressure in the tank is measured by a pressure gage in the supply line, and a pressure switch near this gage is provided to turn on the heaters in the oxygen tank if the pressure drops below a preselected value. This periodic addition of heat to the tank maintains the pressure at a sufficient level to satisfy the demand for oxygen as tank quantity decreases during a flight mission.

The oxygen tank is designed for a capacity of 320 pounds of supercritical oxygen at pressures ranging between 865 and 935 pounds per square inch absolute (psia). The tank is initially filled with liquid oxygen at -297°F and operates over the range from -340°F to +80°F. The term "supercritical" means that the Oxygen is maintained at a temperature and pressure which assures that it is a homogenous, single-phase fluid.

The burst pressure of the oxygen tank is about 2200 psia at -150°F, over twice the normal operating pressure at that temperature. A relief valve in the supply line leading to the fuel cells and the ECS is designed to relieve pressure in the oxygen tank at a pressure of approximately 1000 psi. The oxygen tank dome is open to the vacuum between the inner and outer tank shell and contains a rupture disc designed to blow out at about 75 psi.

As shown in figure 13, each heater coil is protected with a thermostatic switch, mounted on the heater tube, which is intended to open the heater circuit when it senses a temperature of 80°F. As I will point out later in tracing the Board's conclusions as to the cause of the accident, when the heaters were powered from a 65 volt DC supply at KSC during an improvised detanking procedure, these thermostatic switches, because they were rated at only 30 V DC, could not prevent an overheating condition of the heaters and the associated wiring. Tests conducted for the Board indicate that the heater tube assembly was probably heated to a temperature of as much as 1000°F during this detanking procedure.

THE APOLLO 13 MISSION

With this general background, I will now summarize the Apollo 13 mission. This mission, as you know, was designed to perform the third manned lunar landing. The selected site was in the hilly uplands of the Fra Mauro formation. A package of five scientific experiments was planned for emplacement on the lunar surface near the lunar module landing point. Additionally the Apollo 13 landing crew was to gather the third set of selenological samples of the lunar surface for return to earth for extensive scientific analysis. Candidate future landing sites were scheduled to be photographed from lunar orbit. The crew consisted of Captain Jame A. Lovell, Commander; Fred W. Haise, Lunar Module Pilot; and John L. Swigert, Jr., Command Module Pilot, who replaced Thomas K. Mattingly, III, who had been exposed to rubella and, after tests, found not to be immune.

Launch was on time at 2:13 p.m., EST on April 11 from the KSC Launch Complex 39A. The spacecraft was inserted into a 100-nautical mile circular earth orbit. The only significant launch phase anomaly was premature shutdown of the center engine of the S-II second stage. This anomaly, although serious, was not related to the subsequent accident. It is being investigated by the Apollo organization. As a result of this shutdown, the remaining four S-II engines burned 34 seconds longer than planned and the S-IVB third stage engine burned a few seconds longer than planned. At orbital insertion, the velocity was within 1.2 feet per second of the planned velocity. Moreover, an adequate propellant margin was maintained in the S-IVB for the translunar injection burn.

After spacecraft systems checkout in earth orbit, the S-IVB restarted for the translunar injection (TLI) burn, with shutdown coming some six minutes later. After TLI, Apollo 13 was on the planned free-return trajectory with a predicted closest approach to the lunar surface of 210 nautical miles.

The command and service module (CSM) was separated from the S-IVB about three hours into the mission, and after a brief period of station-keeping, the crew maneuvered the CSM into dock with the LM vehicle in the LM adapter atop the S-IVB stage. The S-IVB stage was separated from the docked CSM and LM shortly after four hours into the mission, and placed on a trajectory to ultimately impact the moon near the site of the seismometer emplaced by the Apollo 12 crew.

At 30:40:49 g.e.t. (ground elapsed time) a midcourse correction maneuver was made using the service module propulsion system. This maneuver took Apollo 13 off a free-return trajectory and placed it on a non-free return trajectory. A similar profile had been flown on Apollo 12. The objective of leaving a free-return trajectory is to control the arrival time at the moon to insure the proper lighting conditions at the landing site. The transfer maneuver lowered the predicted closest approach to the moon, or pericynthion altitude, from 210 to 64 nautical miles.

From launch through the first 46 hours of the mission, the performance of the oxygen tank #2 was normal, so far as telemetered data and crew observations indicate. At 46:40:02, the crew turned on the fans in oxygen tank #2 as a routine operation, and the oxygen tank #2 quantity indication changed from a normal reading to an obviously incorrect reading "off scale high" of over 100 percent. Subsequent events indicate that the cause was a short circuit which was not hazardous in this case.

At 47:54:50 and at 51:07:44 the oxygen tank #2 fans were turned on again, with no apparent adverse effects. The quantity gage continued to read "off scale high."

Following a rest period, the Apollo 13 crew began preparations for activating and powering up the lunar module for checkout. At about 53 and one-half hours