Rockets Falling from Orbit: The Saturn V That Launched NASA’s Skylab

In recent years, the uncontrolled reentries of the spent 22-metric ton core stages of China’s new Long March 5B heavy lift launch vehicle, used to orbit space station components, have made the news as well as met with complaints from the international community. While there are legitimate concerns about the random reentry of such a large object from orbit (which have been overblown at times by the press with clickbait-worthy headlines), this is hardly the first time citizens of our fair planet have had to deal with this issue.

China’s Long March 5B rocket lifts off from the Wenchang launch base on Hainan Island on July 24, 2022 carrying the Wentian module for China’s Tiangong space station. The 22-ton core stage would make an uncontrolled reentry six days later. (CASC)

During the Apollo era, NASA regularly left the upper stages from many of their Saturn rocket launches in low Earth orbit to make uncontrolled reentries after their orbits decayed. While the masses of most of these spent stages, and any attached payloads, were about half that of the core stage of the Long March 5B, one stage in particular was the largest ever abandoned to its fate in low orbit: The spent S-II stage of the SA-513 Saturn V used to launch NASA’s Skylab Orbital Workshop (OWS) into orbit on May 14, 1973 which had twice the orbital mass of the Long March 5B core stage of so much concern today.

Background

The Saturn V rocket was developed by NASA during the 1960s to support the Apollo program to land astronauts on the Moon. This three-stage launch vehicle was the largest rocket developed and successfully launched during this era (see “The Largest Launch Vehicles Through History”). First flown in November 1967 (see “Apollo 4: The First Flight of the Saturn V”), a total of 13 of these 2,900-metric ton behemoths were launched during the following 5½ years.

Cutaway diagram showing the major components of the Saturn V. Click on image to enlarge. (MSFC/NASA)

The first stage of the Saturn V was designated the S-IC stage with Boeing selected as its prime contractor. The S-IC was a cylinder with a diameter of ten meters and a length of 42 meters carrying 2,077 metric tons of RP-1 and liquid oxygen (LOX). Its five F-1 engines built by Rocketdyne (which was then a division of North American Aviation and is today part of Aerojet-Rocketdyne) generated 35,100 kilonewtons of thrust at lift off in its later versions. The purpose of the S-IC was to get the huge rocket off of the ground and start it on its way during a nominal 168-second burn that would get the ascending rocket to an altitude of about 62 kilometers and a speed of 2.7 kilometers per second during a typical Apollo lunar mission launch.

Cutaway diagram showing the major components of the S-IC stage. Click on image to enlarge. (MSFC/NASA)

The second stage of the Saturn V was designated the S-II stage with North American Aviation as its prime contractor (which merged with Rockwell in March 1967 and subsequently with Boeing 29 years later). Unlike the S-IC and most earlier rockets, the S-II burned the high-energy combination of liquid hydrogen and LOX which yields about half again as much thrust as a like mass of more conventional propellants. With the same ten meter diameter as the S-IC stage, this stage was 24.8 meters long, had an empty mass of 40 metric tons, and carried 456 metric tons of cryogenic propellant. Its five Rocketdyne J-2 engines generated a total of about 5,141 kilonewtons of thrust in its final versions. With a nominal burn time of 421 seconds, the S-II provided most of the energy to drive the rocket and its payload towards Earth orbit during a typical Apollo lunar mission with burnout occurring at an altitude of 188 kilometers and a speed of 6.8 kilometers per second. Travelling just short of orbital velocity, these stages would come down harmlessly downrange over the ocean after they finished their task.

Cutaway diagram showing the major components of the S-II stage. Click on image to enlarge. (MSFC/NASA)

The third stage of the Saturn V, with the Douglas Aircraft Company as the prime contractor (which, after decades of mergers, is now also part of Boeing), was designated the S-IVB stage. It had a diameter of 6.6 meters, a length of 17.8 meters and was connected to the S-II stage by a tapered interstage section. It carried 108 metric tons of cryogenic propellants for a single, restartable J-2 engine which would burn briefly during ascent with a thrust of 1,033 kilonewtons on its later version to place itself and its payload into a temporary Earth parking orbit and then later reignite to push on towards the Moon. The S-IVB stage also included a pair of auxiliary propulsion system (APS) modules each with a trio of hypergolic-fueled 670-newton engines which provided roll control during the burn of the J-2 as well as attitude control along all three axes while coasting in orbit. Each APS also included a 310-newton ullage engine to help settle the propellants in their tanks prior to reigniting the J-2 engine.

Cutaway diagram showing the major components of the S-IVB stage. Click on image to enlarge. (MSFC/NASA)

The Saturn V was topped off by the Instrument Unit (IU), with IBM as the prime contractor, which controlled all three stages of the launch vehicle during all aspects of flight. Based on earlier work for the Saturn I and IB rockets, the IU incorporated the latest innovations in miniaturized electronics. The total height of the Saturn V with its Apollo payload attached was 111 meters and had a liftoff mass of around 2,900 metric tons.

Cutaway diagram showing the major components of the Saturn V Instrument Unit (IU). Click on image to enlarge. (MSFC/NASA)

The Skylab Mission

In 1969, NASA approved the development of the Skylab space station as the sole survivor of a series of proposals using Apollo hardware for new missions. Based on earlier studies NASA had performed, it had been decided that the Skylab OWS would be built using a surplus S-IVB stage to provide the habitable volume for a crew of three ferried to the station using a modified Apollo CSM launched into Earth orbit on a Saturn IB. With a total mass of about 90 metric tons, the Skylab OWS would be placed into orbit using a two-stage version of the Saturn V. Unlike earlier Saturn V missions, the S-II second stage would be inserted into Earth orbit along with the OWS, which had no significant propulsion system of its own.

Diagram showing the major components of the Skylab Orbital Workshop (OWS) as it was originally configured. Click on image to enlarge. (NASA/MSFC)

NASA-funded studies had been performed for a two-stage version of the Saturn V known as the Saturn INT-21. In this configuration, the S-II stage and IU used to control the launch vehicle (which was normally carried on top of the S-IVB stage) were to be modified so that the IU would be carried on the top of the S-II. Additional modifications to the S-II stage would have provided it with a simple attitude control system for use after the stage’s five J-2 engines were shutdown. With proper venting of residual cryogenic propellants while the IU controlled the modified S-II stage, the spent stage could be directed towards a reentry over the Pacific Ocean once it had delivered its payload into orbit. Knowing the danger posed by  these large derelict stages, a procedure was put into place by NASA to dispose of the spent S-IVB second stage of the Saturn IB rockets used to launch Skylab’s Apollo-based CSM ferries in order to minimize the risk that these ten-metric ton stages would make an uncontrolled reentry (see “SA-206: The Odyssey of a Saturn IB”).

This is the expended S-IVB-207 over Europe after it had successfully placed the Skylab 3 CSM into low Earth orbit on July 28, 1973. The stage was purposely deorbited over the Pacific Ocean shortly afterwards by venting its residual propellants in order to minimize the chances it could come down over populated areas. (NASA)

Unfortunately, the modifications needed to create the Saturn INT-21 variant were not in the budget and could not be easily accommodated in the Skylab development schedule. Instead, the IU would remain mounted on top of the modified S-IVB stage that was the Skylab OWS leaving the spent S-II stage without any means of control once it delivered its payload into orbit. The S-II-13 second stage of the SA-513 launch vehicle assigned to orbit the Skylab OWS did receive a number of modifications for its mission. Many of these centered on the installation of nonpropulsive venting systems to allow residual cryogenic propellants and helium pressurant to be dumped overboard in order to maintain its distance from the OWS and minimize the risk of a catastrophic breakup of the stage once in orbit. Otherwise, the S-II-13 stage would be left to make an uncontrolled reentry after its orbit decayed months after its launch.

Stacking of the Skylab OWS on top of S-II-13 on September 29, 1972 inside the Vehicle Assembly Building (VAB). (NASA)

The last Saturn V to fly, the 2,856-metric ton SA-513, lifted off from LC-39A at the Kennedy Space Center at 1:30:00 PM EDT on May 14, 1973. The ascending rocket broke the sound barrier 61 seconds after launch and almost immediately encountered a problem. Less than two seconds later, aerodynamic forces ripped micrometeoroid/sun shield from the side of the OWS. The event unlatched both main solar arrays on the station with debris striking the sides of the launch vehicle. When maximum dynamic pressure was reached 73.5 seconds after launch at an altitude of 12.0 kilometers, the aerodynamic forces on the ascending stack began to slacken.

The launch of Skylab on the last Saturn V to fly on May 14, 1973. (NASA)

The S-IC stage shutdown the last of its F-1 engines on schedule 158.2 seconds after launch at an altitude of 85.2 kilometers some 85.7 kilometers downrange at an inertial velocity of 2,801 meters per second. Just 1.7 seconds later, the S-IC stage was jettisoned followed by the ignition of all five J-2 engines on the S-II stage. The now spent S-IC stage reached a peak altitude of about 206 kilometers before falling back to Earth impacting the Atlantic Ocean some 870 kilometers downrange about 11 minutes after liftoff.

Diagram of ascent ground track for SA-513 launching the Skylab space station. Click on image to enlarge. (NASA)

Unfortunately, the impact of the debris from the Skylab space station had damaged the wiring to fire the pyrotechnic devices meant to separate the interstage at the bottom of the S-II-13 after its engines had powered up. The rocket continued to ascend more or less on course despite the additional mass. The only major anomaly was an increase in engine bay temperatures but the stage’s propulsion system continued to operate as planned. The center engine of the S-II-13 stage shut down 314 seconds after launch to lessen the acceleration loads on the OWS as well as reduce the risk of experiencing excessive longitudinal vibrations, an effect called “pogo”. The troubled S-II-13 stage shut down its last four J-2 engines within a fraction of a second of the scheduled time some 589.0 seconds after launch at an altitude of 442.1 kilometers, 1,811 downrange travelling at a speed of 7,642 meters per seconds. Two seconds later, the Skylab OWS separated from its now spent second stage. To add to the list of problems NASA’s first space station would have to overcome, the firing of the S-II stage’s retrorockets ripped one of the unlatch solar arrays from the side of the OWS as it was placed into its initial 431.5 by 433.8-kilometer orbit with an inclination of 50.03°. NASA officials would postpone the launch of the first crew to Skylab until they could diagnose the extent of the damage to the station and formulate a repair plan.

A view of the damaged Skylab space station taken by the Skylab 2 crew during their fly-around inspection on May 25, 1973 showing its missing micrometeoroid/sun shield and solar array. (NASA)

The now inert S-II-13 stage, which had received the international designation of 1973-27B once in orbit, opened its vents to allow almost 18 metric tons of residual propellant and other consumables to be dumped overboard to safe the stage. Passive tracking by C-band radars at Merritt Island, Bermuda, and Carnarvon for portions of the second and sixth revolutions of S-II-13 showed it to be in a 371.2 by 432.4-kilometer orbit with an inclination of 50.04° two hours after launch with little likelihood that it would further threaten the Skylab OWS. Because of continued venting, the stage was in a 372.3 by 443.5-kilometer orbit by its sixth revolution. Active tracking of S-II-13 ceased thereafter as the now empty stage, with its interstage still attached, continued in orbit.

The orbit of the 45-metric ton S-II-13 finally decayed on January 11, 1975 after 606 days in space. With barely a mention from the press (unlike today’s events with smaller rocket stages), the S-II-13 stage had reentered over the Indian Ocean causing no harm. It would be another 4½ years before the 90-metric ton Skylab space station fell from orbit on July 11, 1979. In contrast, this event received much media attention as the reentering space station spread debris over Australia.

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Related Video

Here is a video of the launch of Skylab 1 on May 14, 1973.

Here is a NASA-produced documentary, Skylab: The First 40 Days, covering the launch of the Skylab workshop and the efforts to save the space station.

Related Reading

“The Largest Launch Vehicles Through History”, Drew Ex Machina, February 19, 2018 [Post]

“SA-206: The Odyssey of a Saturn IB”, Drew Ex Machina, May 23, 2018 [Post]

General References

Alan Lawrie & Robert Godwin, Saturn V The Complete Manufacturing and Testing Records, Apogee Books, 2005

“Saturn V Launch Vehicle Flight Evaluation Report – SA-513 Skylab 1”, NASA MSFC MPR-SAT-FE-73-4, August 1, 1973

“Apollo/Saturn V Postflight Trajectory – SA-513 Skylab 1 Mission”, Boeing D5-15560-13, August 14, 1973