Within 10 minutes of a Saturn V launch, the first two stages had fallen away as the spacecraft settled into Earth orbit. Within another 10 minutes, both stages had crashed into the Atlantic Ocean. They weren’t recovered for reuse; their jobs were done before the half hour mark on any lunar flight. But the third stage of the Saturn V lived on, following crews all the way to the Moon. Once there is was also left to crash, but this time it was smashed for science. 

From S-IV to S-IVB

The S-IVB stage has its roots in the earlier S-IV stage. The S-IV was conceived in the late 1950s as the fourth stage of a rocket then known as the C-4, the C-series being the one that eventually morphed into the Saturn family we know and love. When the C-4 rocket was scrapped the S-IV took on a new life as the second stage of the Saturn I rocket. In this role, it used a cluster of six engines to propel a small payload into low Earth orbit.

As the Saturn rocket family evolved so too did the upper stage. The first stages got bigger in diameter and so too did the S-IV. It also got a new powerhouse, replacing the six engines for a single engine of a different design. Now a totally new beast, it was rechristened the S-IVB. This new upper stage became, contrary to its name, the second stage of the Saturn IB rocket and the third stage of the Saturn V. And it was no longer just a booster stage to punt a payload into orbit. Now the S-IVB would be responsible not only for helping get the crew into orbit, it would be the stage that would also send them to the Moon.

Translunar Injection, aka Getting to the Moon

When the an Apollo crew reached orbit before heading off to the Moon, they did so with the S-IVB still attached and with the Lunar Module still safely encased in its adapter section. The spacecraft stayed like this while the crew paused in orbit, giving them and Houston a chance to make sure everything was working smoothly before getting a GO for the translunar injection burn that would propel them out of Earth orbit and on a trajectory to the Moon.

Because it was actively firing when the crew got on a path to the Moon, the S-IVB was going right along with them. Not to mention it was carrying the lunar module so the crew really needed to keep it around.  About a half hour after the TLI burn (exact time varied with each mission so this is a ballpark number for simplicity’s sake) the crew separated the command-service module from the S-IVB. Then came transposition and docking, the tricky maneuver wherein the command module pilot turned the spacecraft around, docked with the lunar module, and pulled it out of its adapter. Only once the spacecraft were docked did the crew separate from the S-IVB once and for all.

But the separation maneuver didn’t significantly change the trajectory of the spent rocket stage. It was still following them to the Moon, so controllers on the ground worked their magic to send it to the Moon with purpose. The S-IVB’s Auxiliary Propulsion System was fired remotely and any remaining propellants dumped, putting it on a trajectory a safe distance from the crew and towards a specific goal. The S-IVBs from Apollos 8, 10, and 11 were put into a heliocentric orbit. The S-IVB from Apollo 12 was put into Earth orbit.

Starting with Apollo 13, the S-IVB’s had a third life. Scientists devised a way to use the spent rocket stages to learn more about the Moon. And this means we need to look at what the astronauts left up there.

Science on the Surface of the Moon

Among all the science instruments astronauts used on the Moon was the Apollo Lunar Surface Experiment Package or ALSEP, though in the case of Apollo 11 it was an EASEP, the Early Apollo Surface Experiment Package. This was a suite of instruments designed to work for at least a year on the surface, gathering data and sending it back to Earth for analysis. These packages were deployed near each landing site, a necessity since the astronauts had to carry (or drive) them from the lunar module for manual deployment.

As part of Apollo 11’s EASEP was the Passive Seismic Experiment Package. This consisted of four seismometers powered by two panels of solar cells, three long-period seismometers and one short-period vertical seismometer to measure meteorite impacts and moonquakes. Apollos 12, 14, 15, 16’s ALSEPs included the Lunar Passive Seismic Experiment designed to measure sub-surface properties and small vibrations from moonquakes, man-made explosions, and spacecraft impacts…  see where we’re going with this yet?

The instruments vibrated in response to movement of the ground, say, in a moonquake. This aced on a central lever inside the instrument with a mass on one end that caused it to vibrate in sympathy. This vibration was detected electronically and measured by seismometers, and the signal was sent back to Earth magnified by a factor of 10 million. The data was now available to scientists to study in tandem with other surface experiment results.

Because each mission left one of these instruments at the various landing sites, by the end of the Apollo program geologists had a network set up covering a decent amount of the surface that allowed them to locate moonquakes in three dimensions; an event picked up by three stations allowed them to determine the time and location of the strike. By studying how the shock waves moved through the Moon from one site to another they could learn about the Moon’s composition and internal structure.

Smashing for Science

So back to the S-IVBs. Apollo 13 lost one of the second stage engines early in the launch so that mission’s S-IVB had to fire 9 seconds longer than planned to compensate. After the crew recovered the lunar module from the S-IVB, the auxiliary propulsive system burned for 217 seconds to put it on a path towards the Moon. It impacted almost three days later on April 14, 1970.

Apollo 14’s S-IVB was also sent into a lunar trajectory, impacting on 4 February, 1971. As was Apollo 15’s S-IVB. It impacted the Moon on 29 July, 1971, fairly close to Apollo 14’s ALSEP. Apollo 16’s upper stage had some problems with the auxiliary propulsion system so it could only fire once and tracking it became pretty difficult. No one knows exactly where it landed but based on seismic data from the Apollo 12, 14 and 16 seismic stations the estimate is it hit on 19 April, 1972. The final S-IVB from Apollo 17 hit the Moon on December 10, 1972.

The impacts all provided known data points. Because the masses and velocities were known, these were seismic events that not only yielded good data they helped scientists calibrate the network of instruments on the Moon. Having a known point of comparison for subsequent seismic data allowed geologists to determine the masses and velocities of other impacts measured by the ALSEPs.

And it wasn’t just S-IVB stages. The spent ascent stages of the lunar modules were also smashed into the Moon for data points!

Sources: Stages to Saturn (NASA SP-4206); Apollo Lunar Surface Experiment Package manual; NSSDC.