Even after over four decades, I still remember May 25, 1973. I was eleven years old at the time and within weeks of finishing fifth grade. It was a Friday and the last school day before the long Memorial Day weekend. It was also the day NASA launched the delayed Skylab 2 mission. While I had managed to watch television coverage of the launches of many earlier Apollo missions because they happened during weekends or school vacations and so on, I was going to miss this launch because of school. But what really made this day memorable for me was my father: he had taken the day off from work to prepare for our departure for a long-weekend trip to visit relatives as soon as my siblings and I got home from school. Knowing I would miss the launch, my father made a cassette tape recording of the television news coverage for me to listen to when I had time (this was a decade before my parents got their first video recorder so an audio recording was the best that could be done back then).

While I was excited to listen to the launch and subsequently followed the details of this first mission to the Skylab space station, it would be years and even decades before I would learn many of the more interesting details of this mission. One of those details involved the Saturn IB launch vehicle for the Skylab 2 (or SL-2) mission designated SA-206. Unlike the usual straightforward story of construction, test and launch of many rockets of this time, SA-206 turned out to have a much different story involving cancelled missions and years of storage waiting to finally fly.

Initial Plans

In addition to providing another evolutionary step in the development of technologies required for building heavy lift rockets, the purpose of the Saturn IB was to launch Apollo hardware into low Earth orbit for initial flight testing. The Saturn IB was a substantially improved version of the Saturn I originally developed by the team led by famed rocket engineer Wernher von Braun and based at NASA’s Marshall Space Flight Center (MSFC) in Huntsville, Alabama. The first stage of this new launch vehicle, built by Chrysler and designated S-IB, was an updated version of the S-I stage successfully flown ten times between 1961 and 1965 during the Saturn I test flight program (see “The Last Launch of the Saturn I”). Like the S-I stage, the S-IB structure consisted of a set of eight 1.8-meter in diameter tanks holding LOX and RP-1 derived from the proven Redstone rocket clustered around a single 2.7-meter LOX tank adapted from the Jupiter rocket. A new set of swept-back fins as well as a host of other changes to the hardware and fabrication of the S-IB made it 9,000 kilograms lighter than the older S-I. The initial batch of S-IB stages were powered by eight improved Rocketdyne H-1 engines whose total liftoff thrust was increased from 6,683 to 7,120 kilonewtons.

By far the most substantial difference between the Saturn I and Saturn IB was the second stage built by the Douglas Aircraft Company and designated S-IVB. Instead of six Pratt & Whitney RL-10 engines which produced a total of 400 kilonewtons of thrust on the S-IV stage used by the Saturn I, the new and larger S-IVB stage employed a single Rocketdyne J-2 engine to produce 890 kilonewtons of thrust using the same high energy propellant combination of liquid hydrogen and LOX burned by the S-IV. In addition to increasing the payload capability of the Saturn IB so that it could carry the CSM (Command-Service Module) or LM (Lunar Module) into Earth orbit for initial test flights, the first launches also allowed flight testing of the nearly identical version of the S-IVB stage that would be employed as the third stage of the Saturn V which would send Apollo to the Moon.

On top of the S-IVB stage was the Instrument Unit (IU) which automatically controlled all aspects of the Saturn IB during its flight. Built by IBM, the IU incorporated the latest innovations in miniaturized electronics. A tapered Spacecraft Launch Adapter (SLA) consisting of four panels connected the S-IVB stage to the Apollo CSM. During flights of the Saturn V, the LM would also be housed inside this adapter. The Saturn IB, without its payload, was 43.2 meters tall and was capable of placing up to 17 metric tons into low Earth orbit initially.

The first flight of the Saturn IB was the Apollo AS-201 mission launched on February 26, 1966. While the Saturn IB designated SA-201 successfully sent the unmanned CSM-009 (Command-Service Module number 009) on its intended 37-minute suborbital flight, problems encountered with the spacecraft’s propulsion system forced a postponement of the AS-202 mission as corrective measures were taken (see “The First Flight of the Apollo-Saturn IB”). In the mean time, the AS-203 mission was launched out of the originally intended sequence on July 5. The objectives of the AS-203 mission, which did not fly with an Apollo spacecraft, concentrated on testing various design features of the SA-203’s S-IVB stage during orbital flight before it was flown on the Saturn V (see “AS-203: NASA’s Odd Apollo Mission”). The next unmanned Apollo-Saturn IB mission, AS-202, launched CSM-011 on a 93-minute suborbital test flight on August 25 which ended with a splashdown in the Pacific Ocean. The spacecraft met its mission objectives and the CM successfully executed a double-skip reentry profile certifying the CSM for manned orbital flight (see “AS-202: The Last Test Flight Before Apollo 1”).

By the conclusion of the AS-202 mission, NASA’s plans for the next round of Apollo test flights had been set. First up would be the AS-204 mission which would make the first orbital test flight of the Apollo CSM with a crew of three on board. The feat would be repeated with a second CSM orbital test flight designated AS-205. Next up would be AS-206 employing the SA-206 Saturn IB for the first unmanned test flight of the LM. Since the Saturn V would not be making its first of two planned test flights until sometime during the second half of 1967, the first crewed test flight of the LM would require a pair of Saturn IB launches as part of a mission designated AS-207/208. Saturn IB SA-207 would launch the first crewed flight of the upgraded Block II CSM followed by an unmanned launch of the first crew-rated LM on SA-208. The two spacecraft would then rendezvous and dock to begin manned testing of the LM.

Preparing for Flight

Assembly of the components for SA-206 by its various contractors had begun in earnest by late 1965. Work on clustering the propellant tanks for the rocket’s first stage, designated S-IB-6, was begun by Chrysler personnel on October 25 at NASA’s MAF (Michoud Assembly Facility) in Louisiana. The S-IB-6 was the first in the series to include a number of changes to resolve problems uncovered earlier as well as improve the launch vehicle’s overall performance so that eventually up to 21 metric tons could be placed into low Earth orbit (see “The Largest Launch Vehicles through History”). The most noticeable of these changes was that all of the propellant tanks on the newer S-IB stages were painted white instead of alternating between white and black. It had been found that the black-painted tanks on earlier S-IB models experienced thermal distortions when exposed to the Florida sun and painting the tanks all white would eliminate the issue. The S-IB-6 and subsequent S-IB stages also include uprated H-1 engines which increased the total liftoff thrust by 2.3% to 7,285 kilonewtons. Installation of these improved engines started on December 31 and was completed on January 28, 1966.

The second stage of SA-206, designated S-IVB-206, was assembled at the Douglas Aircraft facility in Huntington Beach, California. By late September 1965, work had begun joining the various elements of the stage’s propellant tanks with the installation of insulation panels completed on January 14, 1966. The stage’s J-2 engine was installed on March 31 and post-manufacturing checkout was completed on May 26. With the checkout of S-IB-6 completed at MAF earlier on May 3, the two stages of SA-206 were ready for final testing.

S-IB-6 was shipped from MAF by barge on May 19, 1966 and arrived at MSFC for its testing nine days later. An initial abbreviated static test firing took place on June 23 followed by a full-duration firing six days later in Static Test Tower East at MSFC. A problem was found with one of the inboard engines, #108, and it was replaced with a new engine on July 1. The S-IB-6 stage was shipped back to MAF on July 8 for post-static test checkout. S-IVB-206 left Huntington Beach on June 30 and was shipped via air by Super Guppy to Douglas’ SACTO (Sacramento Test Operations) facility in central California. A problem uncovered by the static test firing on August 19 forced the replacement of the J-2 engine’s LOX turbopump. A second abbreviated test firing on September 14 verified the performance of the J-2 and its new turbopump. Additional testing culminating with a post static test checkout were completed by November 4.

The First Assignments

By the time SA-206’s stages were undergoing their final tests, the rocket had been assigned its mission. In the first quarter of 1967, it would launch the first LM flight article, LM-1, for the unmanned Apollo AS-206 mission. For this flight, LM-1 would ride into orbit inside of the SLA capped by a nose cone in place of the CSM – the rocket was incapable of orbiting both the LM and CSM into low Earth orbit even with its improvements. The S-IVB-206 second stage of SA-206 was the first piece of mission hardware to arrive at Cape Kennedy (which reverted to its original name of Cape Canaveral in 1973) on December 14, 1966 with the S-IB-6 first stage arriving by barge from MAF four days later.

With the original November 1966 delivery date of LM-1 already missed as Grumman continued to struggle with a range of issues during the spacecraft’s final assembly and testing, Apollo program managers decided to start preparations for the AS-206 mission with the hope of launching it as soon as possible after the long-delayed AS-204 mission now known as Apollo 1 (see “The Future That Never Came: The Unflown Mission of Apollo 1”). With Apollo 1 and its SA-204 rocket occupying the pad at Launch Complex 34 (LC-34), S-IB-6 was erected on Pad B of Launch Complex 37 (LC-37B) on January 22, 1967. The S-IVB-206 stage was added to the stack on January 23 for the beginning of prelaunch testing.

But as preparations for the launch of AS-206 were starting in earnest at LC-37B, the Apollo program encountered its biggest setback. On January 27, 1967 astronauts Gus Grissom, Ed White and Roger Chaffee died in a fire which swept through their capsule during what should have been a routine countdown rehearsal for the upcoming Apollo 1 mission. As the space agency and the country tried to come to grips with the tragic loss of the Apollo 1 crew, the program’s aggressive test flight schedule was placed on hold as the cause of the fire was investigated.

With the continuing delays in the delivery of LM-1, Apollo program officials decided on March 20, 1967 to switch launch vehicles for the first LM test flight. SA-204, which was undamaged by the Apollo 1 accident, would now be used to orbit LM-1 and SA-206 was subsequently removed from LC-37B. With plans for future Apollo flight testing uncertain, S-IB-6 left the Cape by barge on April 3 for storage at MAF. Likewise, S-IVB-206 was shipped back to SACTO ten days later to be placed into storage until Apollo program managers could work out a new schedule.

Within six months, it had been decided that SA-206 would be used to support contingency plans to launch test flights of the LM either by orbiting a second unmanned LM test flight (if LM-1 failed to meet its mission objectives) or supporting the first manned LM test flight as part of a dual-launch mission (if the Saturn V was not crew-rated in time). S-IB-6 was pulled from storage on October 16 to begin modifications and checkout. S-IVB-206 was removed from storage at SACTO on November 12 to begin its post-storage checkout.

In the end, the schedule contingencies which would have required SA-206 never materialized. The Apollo 5 mission launched on January 22, 1968 was able to meet its objectives testing LM-1 so a second unmanned test flight with LM-2 was no longer needed (see “Apollo 5: The First Flight of the Lunar Module”). Earlier on November 7, 1967, the Saturn V operated nearly perfectly on its first flight for the unmanned Apollo 4 mission (see “Apollo 4: First Flight of the Saturn V”). Although problems were encountered with the second Saturn V during the Apollo 6 mission launched on April 4, 1968 (see “Apollo 6: The Saturn V That Almost Failed”), Apollo program officials felt that their Moon rocket would be available in time to support manned orbital test flights of the CSM/LM in early 1969. S-IVB-206 was placed back into long-term storage on January 11, 1968 while S-IB-6 returned to storage on December 18.

Assignment: Skylab

Even before the launch of the first Apollo test flights, NASA had started examining other scientific missions which could use Apollo hardware as part of a follow-on effort to the first Moon landings which received the name of the Apollo Application Program in August 1965. Eventually these studies evolved into the Skylab program where a two-stage version of the Saturn V would launch an orbiting workshop built from a surplus S-IVB stage. The Saturn IB would be used to launch a CSM to ferry crews to the new space station. McDonnell Douglas, which had been formed during a corporate merger in 1967, began conversion of S-IVB-212 (built for Saturn IB SA-212) and S-IVB-515 (from Saturn V SA-515) into workshops after they received a contract from NASA in August 1969.

Being the next available Saturn IB after SA-205 had been used to launch the manned Apollo 7 mission into orbit on October 11, 1968 (see “Apollo 7: Rise of the Phoenix“), SA-206 was assigned to launch the first Skylab crew. S-IB-6 was removed from storage at MAF on October 18, 1971 to begin preparations for its flight. Since SA-206 was originally configured to launch an unmanned payload, certain systems needed to be reworked to support crewed launches as well as modifications made to improve the reliability of various subsystems. This work was finished on February 12, 1972 with the final checkout running through June 6. S-IB-6 was shipped by barge to Kennedy Space Center (KSC) on August 17. In the meantime, S-IVB-206 had been removed from storage in California and was shipped to the Cape on June 23, 1971 for temporary storage there. The stage was removed from six-months of storage at KSC on April 17, 1972 and began a series of checks culminating in a propulsion subsystem checkout completed on August 21.

But before SA-206 was used to launch the first crew to Skylab for the SL-2 mission, the rocket was needed to serve another purpose first. Since LC-34 and LC-37 had been deactivated on January 1, 1969 and were no longer available to launch the Apollo-Saturn IB (see “From Apollo to Orion: Space Launch Complex 37”), provisions needed to be made for launches from LC-39 which had been used to conduct Apollo-Saturn V missions. After much study, Mobile Launcher 1 (ML-1) was modified to support launching the Saturn IB. The most noticeable change was the addition of a 39-meter pedestal on the launch platform of ML-1. Since the Apollo-Saturn IB and Apollo-Saturn V were essentially identical from the base of their S-IVB stages upwards through the Apollo spacecraft, this pedestal or “milk stool” raised the Saturn IB high enough so that the existing facilities could be used on ML-1 to access the Apollo/S-IVB. This minimized the modifications which were needed for ML-1 to support Apollo-Saturn IB launches. The first task for SA-206 was to perform a facilities fit check and verify the changes made to ML-1.

On August 31, 1972, S-IB-6 was lifted on top of the pedestal on ML-1 located in High Bay 1 of the VAB (Vehicle Assembly Building) at LC-39 to begin the rocket’s second stacking over five years after its first. The S-IVB-206 stage was added to the stack on September 5 followed two days later by the addition of IU-206 which had arrived at KSC on August 24. For this facilities check, a boilerplate model of the CSM was employed instead of actual flight hardware. BP-30 (Boilerplate number 30), which was originally BP-18 used earlier in the Apollo program for structural testing and then refurbished to serve as a dummy payload for a possible third unmanned Saturn V test flight (see “Apollo 8: Where No One Has Gone Before“), was mounted on top of SA-206 on September 5. Except for just a stub used in place of the full-size LES (Launch Escape System), the SA-206/BP-30 combination emulated the Apollo-Saturn IB configuration exactly. On January 9, 1973, SA-206/BP-30 was rolled out to pad 39B to begin facilities checks. With propellant loading and other systems checks completed by January 30, the rocket was rolled back to the VAB on February 2 to begin preparations for the SL-2 launch.

Launch of Skylab 2

BP-30 was destacked on February 20, 1973 and CSM-116 with SLA-6A attached was fixed to the top of SA-206. With the addition of the LES on February 24, the rocket and payload were ready for the rollout to LC-39B two days later. Starting with the Flight Readiness Test on April 5 and ending with the successful conclusion of the Countdown Demonstration Test on May 4, all systems were checked in preparation for the launch scheduled on May 15. Unfortunately, the Skylab Workshop was damaged during its launch on May 14 forcing the SL-2 launch to be postponed. After ten days of hectic effort to develop and prepare to implement a recovery plan, Skylab 2 was finally ready for launch.

At 9:00:00 AM EDT on May 25, 1973, SA-206 lifted off from LC-39B carrying NASA astronauts Charles Conrad, Joseph Kerwin and Paul Weitz for the start of what was hoped to be a 28-day stay on Skylab. Ten seconds after launch, the ascending rocket turned to an azimuth of 47.58° east of north and began its pitch over program to start moving downrange – the furthest north of any previous American manned launch. SA-206 broke the sound barrier 60.5 seconds after liftoff and experienced maximum dynamic pressure 15 seconds later at an altitude of 12.84 kilometers.

The S-IB-6 stage operated nearly flawlessly with the four inboard H-1 engines shutting down 138.7 seconds after launch and the remaining four outboard engines following 3.6 seconds later upon LOX depletion. With the S-IB engine shutdown coming only 1.36 seconds later than planned at a velocity of 2,040.49 meters per second, Skylab 2 was travelling just 7.07 meters per second faster than planned. The S-IVB-206 separated from the spent first stage 143.7 seconds after launch at an altitude of 59.34 kilometers and ignited its J-2 engine to continue the ascent to orbit. The spent S-IB-6 flew along a ballistic path and impacted 493 kilometers downrange 535 seconds after launch at 31.55° North, 76.76° West off the southeast American Atlantic coast.

The S-IVB-206 stage continued the push into orbit with its J-2 engine shutting down 586.21 seconds after liftoff which was only 3.7 seconds later than planned. SA-206 had successfully placed Skylab 2 into a 150.0 by 352.1 kilometer orbit with an inclination of 50.0° – the highest inclination orbit for an American manned spaceflight until that time. The apogee was 6.3 kilometers higher than planned owing to an excess velocity of 1.8 meters per second at second stage engine cutoff. And with an initial in-orbit mass of 16,332 kilograms, CSM-116 was the heaviest object ever orbited by a Saturn IB up until that time. The Skylab 2 CSM separated from S-IVB-206 16 minutes after launch to continue its rendezvous with the Skylab Workshop. Ultimately the crew of Skylab 2 was able to repair the space station and complete a record-setting 28-day mission in orbit.

Meanwhile, S-IVB-206 (now with the COSPAR satellite designation of 1973-032B) still had to complete its mission. In order to minimize the risk from space debris, it was planned to deorbit the spent stage using its Auxiliary Propulsion System (APS) to stabilize its attitude while residual cryogenic propellants were dumped through the J-2 engine. Properly timed, the impulse from this retrograde maneuver would be sufficient to bring the spent S-IVB stage down over an uninhabited stretch of the Pacific Ocean in a controlled fashion.

During its fourth revolution, S-IVB-206 used its APS to reorient the spent stage for its retrograde maneuver. At five hours, 24 minutes and 20 seconds after launch, S-IVB-206 began a 460-second dump of its remaining LOX resulting in a 23 meter per second change in velocity. About 30 seconds after the LOX dump, the dumping of residual liquid hydrogen began. The 125-second event shaved another 4.6 meters per second off of the stage’s velocity. Attitude control was lost about 418 seconds later when one of the two APS modules ran out of propellant. S-IVB-206 reentered the Earth’s atmosphere and its remains impacted the Pacific Ocean northwest of Hawaii almost exactly six hours after launch ending the eight-year odyssey of SA-206 with a fully successful mission.

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Related Video

Here is part of CBS New coverage of the launch of Skylab 2 on May 25, 1973:

Here is a NASA documentary about the launch of Skylab 1 and mission of Skylab 2 entitled Skylab: The First 40 Days:

Related Reading

“Apollo 5: The First Flight of the Lunar Module”, Drew Ex Machina, January 22, 2018 [Post]

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

“Christmas 1973 on Skylab”, Drew Ex Machina, December 24, 2014 [Post]

General References

Roger E. Bilstein, Stages to Saturn: A Technological History of the Apollo/Saturn Launch Vehicles, University Press of Florida, 2003

Alan Lawrie, Saturn I/IB The Complete Manufacturing and Testing Records, Apogee Books, 2008

Final Flight Performance Prediction for Saturn AS-206 (Mission 276) Propulsion System S-IB-6 Stage, TR-P&VE-68-68, Chrysler Corporation Space Division, June 15, 1968

Skylab Saturn IB Flight Manual, MSFC-MAN-206, Marshall Space Flight Center, September 30, 1972

Skylab News Reference, NASA, March 1973

Skylab 2 Post Launch Report, RCS 76-0000-00048, Kennedy Space Center, June 21, 1973

Saturn IB Launch Vehicle Flight Evaluation Report – SA-206 Skylab-2, MPR-SAT-FE-73-3, Marshall Space Flight Center, July 23, 1973