On April 11th, the third manned lunar landing mission was launched from Cape Canaveral, Florida, carrying astronauts James A. Lovell, John L. Swigert, and Fred W. Haise. The mission was headed for a landing on the Fra Mauro highlands of the moon. However, two days into the mission, disaster struck 200,000 miles from Earth when oxygen tank No. 2 blew up in the spacecraft. Swigert reported to mission control on Earth, “Houston, we’ve had a problem here,” and it was discovered that the normal supply of oxygen, electricity, light, and water had been disrupted. The landing mission was aborted, and the astronauts and controllers on Earth scrambled to come up with emergency procedures. The crippled spacecraft continued to the moon, circled it, and began a long, cold journey back to Earth.
In fact, this was the second of two incidents that Apollo 13 encountered on its flight. The mission began with a little-known smaller incident: during the second-stage boost, the centre (inboard) engine shut down two minutes early. The four outboard engines burned longer to compensate, and the vehicle continued to a successful orbit. The shutdown was determined to be due to dangerous pogo oscillations that threatened to tear the second stage apart. The engine experienced 68 g vibrations at 16 hertz, flexing the thrust frame by 3 inches (76 mm). The engine shutdown was triggered by sensed thrust chamber pressure fluctuations. Smaller pogo oscillations had been seen on previous Titan and Saturn flights (notably Apollo 6), but on Apollo 13 they were amplified by an unexpected interaction with turbopump cavitation. Later missions implemented anti-pogo modifications that had been under development. These included addition of a helium gas reservoir to the centre engine liquid oxygen line to dampen pressure oscillations, an automatic cutoff as a backup, and simplification of the propellant valves of all five second-stage engines.
Following the oxygen tank incident, the astronauts and mission control were faced with enormous logistical problems in stabilising the spacecraft and its air supply, as well as providing enough energy to the damaged fuel cells to allow successful reentry into Earth’s atmosphere. Navigation was another problem, and Apollo 13‘s course was repeatedly corrected with dramatic and untested manoeuvres. On April 17th, tragedy turned to triumph as the Apollo 13 astronauts touched down safely in the Pacific Ocean.
As a joke following Apollo 13’s successful splashdown, Grumman Aerospace Corporation pilot Sam Greenberg (who had helped with the strategy for re-routing power from the LM to the crippled CM) issued a tongue-in-cheek invoice for $400,540.05 to North American Rockwell, Pratt and Whitney, and Beech Aircraft, prime and subcontractors for the Command/Service Module (CSM), for “towing” the crippled ship most of the way to the Moon and back. The figure was based on an estimated 400,001 miles (643,739 km) at $1.00 per mile, plus $4.00 for the first mile. An extra $536.05 was included for battery charging, oxygen, and an “additional guest in room” (Swigert). A 20% “commercial discount”, as well as a further 2% discount if North American were to pay in cash, reduced the total to $312,421.24. North American declined payment, noting that they had ferried three previous Grumman LMs to the Moon (Apollo 10, Apollo 11 and Apollo 12) with no such reciprocal charges. They saw no reason to pay for Apollo 13.