If you ever want to watch me cry, you have a couple of options: you can show me a very cute puppy, play a certain type of sad song from a particular set of Broadway musicals (see: Finale from Les Miserables, For Good from Wicked, and It’s Quiet Uptown from Hamilton), or – if all else fails – you can show me the re-entry scene from the 1995 movie Apollo 13.
Atmospheric re-entry is fascinating. Re-entry (or as the cool nerds call it, ‘entry, descent, and landing’, or EDL) is the series of events that happen when a spacecraft has decided it’s been in space for long enough and it’s time to come back home to Earth, or in some more difficult cases, land on another planetary body.
Here’s where I try to enamor you with why I think re-entry is probably the coolest part of space travel, and why it can be so difficult.
To think about the problems, let’s start with a plane – the SR-71 Blackbird. This Lockheed Martin jet is one of the fastest vehicles ever to fly, and was developed in the 1960s as a supersonic spy plane. A quick preface: The term ‘mach’ refers to the speed of sound at a given set of conditions; the speed of sound can vary significantly at different parts of the atmosphere due to varying air density and temperatures. (These factors dictate how much “stuff” the sound needs to travel through) The SR-71 reportedly topped out at about mach 3, or 2,200 mph.
Now the main problems from supersonic flight arise from the heating. At top speeds, the SR-71 would get hot. Like really, really hot. The pilot would have to wear firefighter-proof gloves, and even then could only touch the wall of the cockpit for a few moments at top speeds before the gloves started to burn.
Apollo re-entry speeds, on the other hand, topped out at a toasty mach 20. If you’ve ever seen the Apollo 13 movie, you may remember the crew’s tense discussion about the re-entry ‘window’; essentially, if they came in too shallow, the spacecraft would skip off the atmospheric interface and be flung back into space. If they were too steep, the vehicle would burn up. There was a relatively small target they could hit in order to get the desired ‘skip re-entry’ trajectory, and the crew of Apollo 13 had to find this ‘sweet spot’ without the aid from their guidance computer.
At lunar re-entry speeds, the temperatures experienced on the outer layer of the heat shield were close to 5000 degrees Fahrenheit. So how did engineers protect their astronauts while they rode a giant fireball back to Earth? The answer: TPS.
‘TPS’ stands for ‘Thermal Protective System’, and is colloquially called a heat shield. If you’ve ever seen the Space Shuttle, it’s the set of black tiles on the bottom of the spacecraft.
These tiles have some really, really unique properties. For the sake of continuity, I’m going to focus on the Apollo command module TPS, which looked a bit more like this:
As you can see, it looked a bit like something you’d find at the bottom of your barbecue grill. This is because while the Shuttle was designed to be reused dozens of times a year, the Apollo command module was single-use, and coated in something called AVCOAT.
AVCOAT, or more specifically, AVCOAT 5025-39, is the code for the material designed by the Avco company for the Apollo missions. It was later changed out for the shuttle, but has now made a comeback for next-generation manned capsules and is currently used on the Orion capsule. AVCOAT is a material made in a honeycomb matrix from an epoxy novalac resin with ‘special additives.’
The most important property of this material was that it was ablative, which meant that it gradually melted off of the spacecraft during re-entry. This seems like a bad thing, right? Generally, yes, you wouldn’t want bits of your spacecraft being vaporized as you plummet to Earth. But this is actually how the material protected the astronauts inside. I like to compare it to the process of sweating. When the human body gets too hot, it starts to sweat – glands on your skin release water, which in turn take some of the heat with it as it evaporates.
Ablation is kind of like that, and it occurs mainly at two levels in an ablative material. TPS materials use convective and radiative heat flow in order to manage the temperature of the spacecraft, a process which is illustrated below. The outer layer of the material chars, melts, and sublimes away while the inner bulk of the TPS starts to undergo pyrolysis and expel product gasses.
This way, the TPS material is able to vent off heat by melting away as well as by mixing the hot shock gases around the craft with the cooler product gases from the pyrolysis reaction. It is important to note that beyond these two layers, there is still enough TPS material to insulate the spacecraft from the temperatures experienced by the outer layers.
Re-entry was by no means a comfortable process – sure, ablative TPS material like AVCOAT keeps the spacecraft cool long enough for the drag forces to slow it down to a more manageable mach 2.
And from that point on, hopefully you packed your supersonic parachutes. If not – hello, ground!