|Abort Mode One-Alpha. Image credit: NASA.
Although man-rated, Saturn V rockets experienced four close calls. The first occurred on 4 April 1968, during the unmanned Apollo 6 test flight, when instability in the rocket’s fiery exhaust produced violent fore-and-aft shaking known as "pogo." Two of the five J-2 engines in the rocket’s S-II second stage shut down and pieces broke away from the streamlined shroud linking the Apollo Command and Service Module (CSM) to its S-IVB third stage. The CSM comprised the conical Command Module (CM), which carried the crew, and the Service Module (SM) which included electricity-generating fuel cells and the CSM's main engine, the Service Propulsion System (SPS). The Apollo 6 S-IVB's single J-2 engine under-performed, placing the stage and CSM into a lopsided orbit, then refused to restart.
Had the Apollo 6 CSM carried astronauts, pogo might have injured them; even if they had reached orbit unscathed, the S-IVB engine failure would have scrubbed their moon mission. As it was, flight controllers separated the unmanned CSM from the crippled S-IVB stage and used its SPS as a backup engine for completing the mission's Earth-atmosphere reentry test.
The Saturn V's IBM-built Instrument Unit — its ring-shaped electronic brain, located atop its S-IVB third stage — soldiered on without a hiccup, however, safely guiding the giant rocket into Earth parking orbit. The Apollo 12 crew of Charles Conrad, Alan Bean, and Richard Gordon carried out a successful lunar landing mission and returned to Earth on 24 November. During the mission, Conrad reported seeing dark discoloration on the umbilical housing linking the CM and SM, but it remains uncertain whether this was a scorch mark left by lightning since discoloration has been noted on at least one other CSM umbilical housing (Apollo 15).
|NASA would rename the "Uprated Saturn I" (right) depicted in this 1966 illustration the Saturn IB. Image credit: NASA.
|Image credit: NASA.
The final Saturn V to fly, intended originally for Apollo 20 but launched unmanned with the Skylab Orbital Workshop (OWS) on top in place of an S-IVB stage and the Apollo CSM and LM spacecraft, survived a close call on 14 May 1973. A design flaw caused Skylab's meteoroid shield to tear loose 63 seconds into the flight. As the disintegrating shield tumbled down the length of the accelerating rocket, it tore at least one hole in the interstage adapter that linked the OWS to the S-II second stage and apparently damaged the system for separating the ring-shaped interstage adapter that linked the S-II with the S-IC first stage. This meant that the 18-foot-long adapter did not separate from the S-II three minutes and 11 seconds into the flight as planned. The S-II stage had excess capacity, however, so dutifully hauled its unplanned five-ton cargo into Earth orbit.
Apollo 12 might easily have ended in a Launch Escape System (LES) abort. The image at the top of this post shows the LES in action during Pad Abort Test-2 on 29 June 1965. The LES was a 33-foot-tall tower containing three solid-fueled rocket motors. The largest was the Launch Escape Motor, which had four exhaust nozzles. The tower stood atop the Boost Protective Cover (BPC), a conical shell that covered the CM.
There were four successive abort modes during Saturn V ascent to Earth orbit. As the Saturn V climbed toward space, the aerodynamic environment around it changed - the air grew thinner, the rocket moved faster, and increasingly it tilted so that it flew parallel to Earth's surface. As the environment changed, the abort modes changed to compensate.
Abort Mode One was in effect on the launch pad, during S-IC first-stage operation, and during the 30 seconds following S-IC separation, by which time the Saturn V would have reached an altitude of about 56 miles. Had it occurred, the Apollo 12 abort would have taken place during the first part of Abort Mode One. Known as Abort Mode One-Alpha, it took effect 45 minutes before scheduled launch and continued until about 42 seconds after liftoff, by which time the rocket would have climbed nearly vertically to an altitude of 3000 meters (9800 feet).
In the event of a catastrophic Saturn V failure while Abort Mode One-Alpha was in effect, the 155,000-pound-thrust Launch Escape Motor would have pulled the BPC and CM free of the SM, which would have remained mounted on the doomed rocket. Meanwhile, the small side-mounted solid-propellant rocket motor near the LES's nose, the Pitch Control Motor, would have ignited to push the LES-BPC-CM combination eastward, toward the Atlantic and well clear of the Saturn V. The CM would then have dropped free of the BPC and deployed its three large parachutes to descend gently into the Atlantic within sight of Kennedy Space Center.
|27 April 1972: The Apollo 16 CM descends to a splashdown in the Pacific Ocean after an 11-day voyage to the moon. A CM descending into the Atlantic after an LES abort would have appeared very similar. Image credit: NASA.
High and Fletcher found that calculating the characteristics of launch pad failures was not an exact science, in large part because there were so many variables to be taken into account, and also because no rocket as large as the Saturn V had ever exploded. They explained that "many of the [fireball] parameters may defy an accurate theoretical treatment."
For their analysis, they assumed that all propellants in the exploding rocket would contribute to forming a fireball. This would occur, they explained, because "large overpressures from detonations and the intense heat from both detonations and burning would cause failure of any propellant tanks not initially involved." If a Saturn V exploded on the pad at launch, 5,492,000 pounds of RP-1 refined kerosene, liquid oxygen (LOX), and liquid hydrogen would contribute to its fireball. For a Saturn IB pad explosion, 1,110,000 pounds of RP-1, LOX, and liquid hydrogen would fuel its fireball.
High and Fletcher wrote that the fireball from a Saturn rocket launch pad failure would expand in a "nearly fixed location." For the Saturn V, the fireball would expand to a diameter of 1408 feet. The Saturn IB fireball would expand to 844 feet. The fireballs would thus completely engulf the Saturn launch pads. For both rockets, fireball surface temperature would attain 2500° Fahrenheit, and heat would be felt up to a mile from the launch pad.
A fireball would begin to rise when it reached its maximum diameter. Fireball ascent would commence about 20 seconds after a Saturn V launch pad explosion and about 10 seconds after a Saturn IB explosion, High and Fletcher calculated. The Saturn V fireball would reach an altitude of about 300 feet in 15 seconds, while the Saturn IB fireball would climb 300 feet in 11 seconds. The Saturn V fireball would persist at its maximum diameter for 34 seconds, while the Saturn IB fireball would last for 20 seconds. The fireball would then begin to cool and dissipate.
Though they assumed for their calculations that all propellants in an exploding Saturn rocket would contribute to its fireball, High and Fletcher wrote that some would likely be "spilled on the ground, creating residual pools which [would] burn for relatively long periods of time." This was, they judged, especially likely if a launch pad failure began with the rupture of the fuel tank in the Saturn V's S-IC first stage. The ruptured tank would spill RP-1 onto the pad, then the oxidizer tank located above it would rupture and mix liquid oxygen with the burning fuel, triggering an explosion. They added that "the residual fire and extreme heat of the fireball [would] prevent approach to the ground area enveloped by the fireball for an unknown period."
Estimation of Fireball from Saturn Vehicles Following Failure on Launch Pad, NASA Program Apollo Working Paper No. 1181, R. High and R. Fletcher, NASA Manned Spacecraft Center, Houston, Texas, 3 August 1965.
Skylab 1 Investigation Report, Hearing Before the Subcommittee on Manned Space Flight of the Committee on Science and Astronautics, U.S. House of Representatives, Ninety-Third Congress, First Session, 1 August 1973, U.S. Government Printing Office, 1973.
Apollo Experience Report - Launch Escape Propulsion Subsystem, NASA Technical Note D-7083, N. Townsend, NASA, March 1973, pp. 1-7.
Where No Man Has Gone Before: A History of Apollo Lunar Exploration Missions, W. David Compton, NASA SP-4214, 1989, pp. 177-178.
How Apollo Flew to the Moon, W. David Woods, Springer-Praxis, 2008, pp. 69-73.
A Forgotten Rocket: The Saturn IB
"Assuming Everything Goes Perfectly Well in the Apollo Program. . ." (1967)