Why didn't Apollo 13 complete at least one lunar orbit?

I'll just add (Stuart basically already said this) that there was much concern about the "big engine" on the service module. Nobody knew exactly what damage had occurred to the SM except that they had clearly suffered the explosion of an oxygen tank. read more

Apollo used a least energy course to the moon, in which its orbit around Earth was elongated to the point the moon's gravity would capture the spacecraft and pull it around into a stretched-out figure eight. read more

Apollo 13 was supposed to land in the Fra Mauro area. An explosion on board forced Apollo 13 to circle the moon without landing. The Fra Mauro site was reassigned to Apollo 14. read more

Apollo 13 was the seventh manned mission in the Apollo space program and the third intended to land on the Moon. The craft was launched on April 11, 1970, at 14:13 EST (19:13 UTC) from the Kennedy Space Center, Florida, but the lunar landing was aborted after an oxygen tank exploded two days later, crippling the Service Module (SM) upon which the Command Module (CM) had depended. read more