This video is supported by the SciShow Space pin!

You can find cool rocket  pins at DFTBA.com/SciShow. And the pin this month features  the rocket from this video! [♪ INTRO] The Saturn V rocket was the superpowered  icon of America’s space program.

At launch, it was taller than the Statue  of Liberty, weighed in at 2.8 million kilograms, and burned through 18  metric tons of fuel per second. Now the engineers who designed this behemoth didn’t start at the bottom with  the most powerful rocket stage. They started with the top stage, called the S-IVB.

It had the very special responsibility  of taking Apollo astronauts from the Earth to the Moon. But that wasn’t its only job over the years. From a retrofitted space station to a makeshift moonquake generator, it played a lot of roles.

So let’s celebrate the S-

IVB:  the third stage of the Saturn V, but the first in our hearts. Like all rockets, the Saturn  V was a massive balancing act. In order to ferry humans all the way  to the Moon, it needed a lot of fuel. We’re talking millions of  liters across all three stages.

But all that fuel made it even harder to push an already very heavy rocket into space. So to help lighten the load,  engineers turned to the lightest, energy-dense fuel they could get their hands on. Liquid hydrogen.

The original S-IV stage, which was  designed as the fourth and final stage of a rocket that NASA never  built, used a cluster of six liquid hydrogen and liquid oxygen-fueled engines. But the version that went inside the  Saturn V, the S-IVB, used one big engine. And unlike previous rocket stages,  which used clusters of small fuel tanks to carry all of their propellant,  both the S-IV and S-IVB packed their new-fangled hydrogen fuel into one large tank.

All 250-ish thousand liters of it. But all that hydrogen came with a challenge. It had to stay cold.

Very cold. If liquid hydrogen gets above  negative 252 degrees Celsius… about 20 degrees above absolute zero…it will boil. And because a gas takes up  more space than a liquid, too much hydrogen boiling  away would rupture the tank.

So to prevent an explosion, engineers  installed vents to release excess gas. That’s a relief. And to keep the fuel chilly, they  developed precisely measured, sanded, and hand-placed insulation tiles that  they could put on the inside of the tank.

If they had tried to rely on  pre-existing, external insulation… the stuff they’d used on older fuel tank clusters… they would have lost a lot more hydrogen to boil off while just trying to cool the tank down! And that would have been an  unacceptable amount of waste. So it took a lot of time, money, and effort, but it was worth it to make a rocket stage  that could put astronauts on the Moon.

Between 1966 and 1968, engineers  tested the S-IVB on uncrewed missions, both as the second stage atop  the two-stage Saturn IB rocket, and the third stage of the Saturn V. When it was time to add humans, Apollo 7 packed three astronauts on top of a Saturn IB. And for Apollo 8 onwards, astronauts were at least hurled around the Moon by a Saturn V.

Just look at that thing. They were crammed in right there at the tippy top, in the glorified tin can called  the Command and Service Module. Within the first 10 minutes of launch,  the Saturn V’s first and second stages had used up all their fuel, and were  ejected so they couldn’t act as dead weight.

And that’s where the S-IVB entered the scene. With the first burn of its  engine, the astronauts were pushed into a stable orbit about  160 kilometers above the Earth. And after a second burn, they  were on their way to the Moon. 40 minutes later, it was time to say  goodbye to their final rocket stage.

The crew popped the Command and Service  module off of the S-IVB, flipped their tin can around, docked with the Lunar  Module, and continued on their way.   The S-IVB, meanwhile, used the last  of its propellant to redirect itself and avoid a future collision with the  people it worked so hard to get out there. During Apollos 11 and 12, the S-IVB was  supposed to slingshot around the Moon and enter a predictable orbit around the Sun, but that's a little bit harder than it sounds. Apollo 11’s went basically as  planned, but Apollo 12’s wound up in a really wide orbit around Earth for a few years,   and we lost track of it in 1971.

But back in 2002, an amateur astronomer spotted an unidentified not-quite-flying object. While we haven’t totally pinned down its identity, there’s a pretty decent  chance it's our missing S-IVB. It’s covered in the same titanium dioxide paint.

It’s about the same size. And by calculating its orbit  and rewinding the simulations, the path it’s traveled basically  runs smack into the Earth. After the Apollo 12 mishap,  scientists came up with a more productive role for S-IVBs to play.

They would intentionally crash them into the Moon. See, astronauts had left  seismographs on the lunar surface to measure seismic activity, like moonquakes. And you know what causes seismic activity?

A big chunk of rocket crashing into the Moon. Because scientists knew the  size and mass of each S-IVB, and approximately where they all  crashed, they could use the impacts to not only calibrate the seismographs,  but use how those seismic waves were traveling through the Moon  to learn what it’s made of! And it looks to be made of a solid, iron-rich  core surrounded by a liquid iron shell, a partially molten mantle, and a crust  that’s way thinner on the side of the Moon that’s facing us, than it  is on the side that doesn’t.

The Apollo program came to an end in 1972, leaving NASA with three unused Saturn Vs. Now they wouldn’t fit in the  attic, so in 1973, one of them was used to launch Skylab,  the US’s first space station. And its S-IVB was swapped out  for a highly-modified sibling.

Instead of getting humans to the Moon,  this S-IVB stayed in Earth’s orbit for six years, and had the very  important job of serving as Skylab’s living quarters,  working space, and waste tank. And these days, the three  S-IVBs that were once Moon-bound are on display at museums around the U. S.

And NASA is on the way to  send humans back to the Moon, just using a very different rocket. In 2022, the Space Launch System successfully launched an Orion capsule  around the Moon and back. But that system doesn’t really  have a successor to the S-IVB.

And maybe that’s for the best. After all, if we’re going to  keep sending people to the Moon, maybe we shouldn’t continue smashing parts of our fancy new rockets into the surface. At least intentionally.

I mean, those are really expensive rockets! A much more affordable option  would be to smash a smaller representation of the rocket into  the dirt outside of your home. Which you can do now because  we’ve made a pin with S-IVB on it available this month at DFTBA.com/SciShow!

You can get your very own today. And if you’d rather use your  pin in more conventional ways, see jackets and pinboards. Each month on SciShow, when  we talk about another rocket, we’ll make another pin to celebrate it.

But once the month is over, we’re  moving on to the next rocket, so it’s a limited time deal. To grab this month's pin,  head to DFTBA.com/SciShow. And thank you for your support. [♪ OUTRO]