What Really Happens During Atmospheric Reentry?

[walterdennisclark]

You should use the word compression more. When air can't get out of its own way it crowds together. When air compresses, it gets hot. At Mach 25 the air is so compressed that it is so hot the air is no longer transparent and its heat allows it to glow. It is the hot air that gets the heat shield hot.
That's the important thing you should have mentioned. It is NOT the heat shield that heats the air.

[ThunderGun]

Every time I watch Amy's channel I learn something new.

[mizzyroro]

Amy it's not fair they are giving you a hard time about the science. You have repeatedly told us you are not a scientist but a historian. Don't let them get to you. We love you and your work.

[PassiveSmoking]

The phenomena described here is known as the "ideal gas law" in scientific circles. The ideal gas law describes how a mathematically ideal compressible fluid (a gas) behaves as its pressure changes.

To cut a long story short, the ideal gas law states that when you pressurise the ideal gas, its temperature increases. Conversely, when you decompress a pressurised ideal gas, its temperature drops. The more violent or prolonged the compression or decompression, the more pronounced the temperature change.

Actual guesses don't behave exactly the same as the ideal gas, but for all practical purposes the behaviour of real gasses is close enough.

This applies to aircraft and spacecraft moving faster than the speed of sound, because in subsonic flight the gas is capable of flowing smoothly around the object that's passing through it (subject to aerodynamics, of course). But when you hit supersonic speeds, the gas compresses more than it flows, forming a bow shock of compressed gas. The compression heats the gas which in turn dumps its heat into the aircraft. At hypersonic speeds the heating is so extreme that the gas breaks down into a plasma, just as Amy describes in the video.

You don't have to travel at hypersonic speeds in order to experience the ideal gas law for yourself though! All you need is an aerosol and/or a bicycle pump. Just hook the pump up to a bicycle tire and inflate it as you normally would. After a while, you'll notice that the barrel of the pump is getting quite hot. If you pump vigorously enough you might even be able to get the pump hot enough to be quite uncomfortable to touch.

As for the aerosol, just spray it constantly for a minute or so. The gas in the can decompresses and you'll feel the can cool down in your hand. Condensation and even ice can start to form on the can's surface if you do it for long enough.

[antoyal]

Air compression is why Quicksilver's crowd-pleasing scene in his first X-Men movie was completely, utterly unbelievable. Seeing him move that quickly in a closed room without his immediately destroying the entire building in a massive explosion should be like seeing Spider-Man lift a skyscraper one-handed. Your brain should say "nah ah, no way" but instead people just think, "Wow, he's fast." It goes to show that our intuitions are seemingly hard-coded for low-speed systems that our ancestors encountered on a day-to-day basis, and why people's intuitions have even more problems with relativistic velocities.

[timheavrin]

Amy, for someone clearly too young to remember Mercury, Gemini, and Apollo you do a remarkable job explaining how things work. I was around during those 3 programs. But you fill in the gaps. Tell those folks who get annoyed with you if they think can do a better job than you then they can give it their best shot. Great job young lady. Keep up the good work.

[upandatom]

Wow I never knew that was how a sonic boom worked that's so cool! Thanks for the awesome vid Amy, you never disappoint :)

[MrHwsmp]

One thing that people frequently misunderstand is that most of the heating is NOT caused by the _friction_ of the air passing over the capsule. What actually happens is that the air/plasma in the supersonic shockwave ahead of the capsule gets very, very hot, and that heat _radiates_ back onto the capsule.

[vividthespis]

The amount of patience you have for people in the comments section is nothing short of Mother Theresa level. Not only on this video but in general. It’s appreciated.

[joeyrovito]

I really do appreciate how you explain things and how you cover things in your show it's really nice it's rare thank you

[OverlandOne]

In the early days, (Mercury, Gemini, Apollo) this super heating of the air near the capsule would cause an ionization blackout, which would block all radio transmissions both to and from the capsule. This made for some exciting times for the ground controllers during re-entry. I really enjoy your videos.

[jdvaillancourt]

Hi Amy, I really enjoy the work you do! Just to clarify, the Concorde was technically the 2nd supersonic passenger aircraft to fly, the first being the Tupelov Tu-144 "Concordski" (ok, not the official name, but I prefer it :D ) although the Concorde did beat the Russian plane into service.

[mattigch]

Great video, your content is always so interesting to watch. Keep up the great work!

[jasond]

Thanks for another great vid Amy. Keep them coming!

[Izayuukan]

Actually there was another supersonic passenger jet, the Tupolev 144. It was inferior to the Concorde in most ways and didn't last anywhere nearly as long in commercial service. The 144 does however hold the distinction of breaking Mach 2 before its more famous rival.

[casnimot]

I saw the wood heat shield video, too. Being a woodworker myself, I'm not too surprised someone actually used it, even if only experimentally.

[kosta]

Right, I've got to pull you up on the shockwaves too! The "sound barrier" as you used it (as in the aircraft gets through it... and presumably comes out on the other side? There's a little ambiguity here...) is the transonic flight range. This occurs when an aircraft (or other object) flies fast enough for certain parts of the flow structure to reach or exceed the speed of sound (i.e. break the sound barrier). This happens because displacing the fluid also accelerates it around the object.

When these pockets of air go supersonic they radically change the forces acting on the surfaces at those particular spots. Often these changes result in motion which causes the conditions that formed the shocks to temporarily subside and then once the extra forces and moments are gone and the body has returned to normal - reappear. This is the aerodynamic basis for wing flutter in transonic aircraft (though flutter is also a structural problem).

This instability of flow in the transonic range is what causes aircraft to violently shake. The shape of the shockwaves at that stage represents the greek letter lambda, hence their name - lambda shocks. They look more or less the same way regardless of which surface they occupy (be it wing, control surface or engine nacelle) as long as flow is attached under normal conditions.

The front of this lambda shock is fairly weak, it's a curved shockwave behaving mostly like an oblique shock. Oblique shocks are generated by sharp or blunt bodies and as a result are at an angle to the flow direction (hence oblique). The shockwaves are fairly weak because very little change occurs inside them, usually resulting in supersonic flow at both ends when traveling faster. Since these shocks are fairly weak, they're of no concern at that stage.

The back of the lambda shock consists of a normal (as in perpendicular to the flow direction) shockwave emanating from the body and transitioning into an oblique shock that curves forwards. The two subdivisions are in truth just one shockwave, but since the behaviour is so different at different locations it makes our lives easier to split it up. That normal shock can become quite a problem for an aircraft. Depending on how much curvature is present, it can grow until it comprises the entire aft end of the lambda shock. This is a problem because a normal shock forcibly reduces the Mach number behind it to a subsonic value and the faster the oncoming air is going - the slower it comes out of the other end. This makes them very strong and generally means they're bad news. When these shocks get stronger they begin to mess with the boundary layer and at a certain point they're enough to cause total flow separation right at the shock.

This phenomenon is known as shock stall and it does several things. First it adds drag. The lambda shock does that anyway, but separation behind it makes it worse. Second - it dramatically reduces the wing's lifting potential and changes its pitching moment (thus causing flutter when strong enough). Finally, the separated boundary layer creates an area of very low dynamic pressure behind it (fancy way of saying slow air I guess) which means that any control surfaces behind that shock (and therefore in that wake) are effectively useless. This is why supersonic military aircraft tend to use whole-body elevons, where the elevators in the tail section comprise the whole horizontal stabiliser and can be moved independently for roll control.

This problem is particularly pronounced with higher thickness and camber aerofoils, which is why transonic flight was a very major problem in the early days of high speed aviation. This problem is in fact what brought an end to some very early high speed flight testing done by the Russians. Yes, they went transonic first. It happened using a demonstration aircraft fitted with rockets for extra propulsion, but they abandoned it after a pilot engaged the rockets in a dive, went transonic, lost elevator control and crashed into the ground. It was deemed too dangerous and all the equipment was mothballed.

The solution was I believe formalised by NASA in the form of supercritical aerofoils. These are flatter and thinner, generating weaker shocks and therefore delaying the onset of shock stall and flutter. The Boeing 777 was the first US produced airliner to use a supercritical aerofoil which gave it an edge in speed.

[brucegoodwin]

Keep going Amy! You are getting smarter about the physics and explaining it. Science rocks.

[mattscatterty]

Ooo! Amy, I would LOVE to see a video on the potential heat-shield requirements of a mission to Europa! It's by far my favourite moon, and I think it to be the most beautiful as well. I know it has a very thin atmosphere, but Mars does as well and it requires a heat-shield, so I'm curious how a mission to Europa would compare!

[witscheman]

A little about the "bow shock" Amy mentioned: I think most people would pronounce bow like the bow of a ship since many scientists who study sonic shockwaves start by learning the physics of a bow wave that a ship produces when a vessel travels faster than the wave speed of water.