Heat Shields - Things Kerbal Space Program Doesn't Teach

alloyed scott manley here today we're gonna learn what Kerbal space program and doesn't teach you about atmospheric entry heating that is the science of aerothermodynamics a fusion of aerodynamics and thermodynamics something which will probably scare everybody but I mean it's very simple what we're doing is working it converting the kinetic energy of a spacecraft or our space probe into a thermal energy as it slows down in the atmosphere now the kinetic energy of an object in orbit it's moving a bit seven point eight kilometers per second and roughly that works out to thirteen mega joules per kilogram the energy of explosives like TNT is something like four mega joules per kilogram so stirring mega joules per kilograms pretty much enough to vaporize anything that we can imagine the good news is that lots of that energy doesn't actually go into the spacecraft it goes into the atmosphere instead which is why we get those wonderful glowing re-entry Trails spacecraft are designed to absorb as little of that heat as possible to avoid cooking the interior and the sensitive hardware inside a spacecraft might absorb say one to five percent of the total kinetic energy in the form of heat the science for this really got going in the 1950s as they were developing missile technology the scientists involved came up with analytical equations which did a pretty good job of modeling the heat flow but it's such a deep problem that doing a high-resolution modeling of a modern vehicle with a modern simulation would challenge even the most powerful supercomputers available to us many sources state that friction with the atmosphere is the mechanism which leads to atmospheric heating and while this is true for aircraft moving at supersonic speeds for spacecraft moving at hypersonic speeds it's not the case a completely different mechanism is involved when a capsule moves the atmosphere at hypersonic speeds the atmosphere cannot get out of the way fast enough and a shock wave forms worth of lost if the incoming air rapidly changes to match that of the vehicle and this results in extremely high pressures and by extension temperatures this shock wave forms a short distance from the body surface and is where most of the heating of reentry is actually generated in this cross-section you can see that the boundary layer is significantly cooler than the shockwave which is of course a great thing for the surface of the vehicle reducing the temperature that is directly exposed to so it's desirable to keep the shockwave as far away from the body as possible now those analytical solutions that the boffins came up with in the 1950s they are showed that for a spherical body the distance to the superheated shock front is roughly proportional to the radius of the sphere and this is generally true of any surface the less it curves the more it pushes the shock wave away from the surface and this is why space planes favor blunt shapes compare the sharp edges of the x-15 and the sr-71 with the blunt nose of the space shuttle the temperatures and pressures inside the shock front are easily high enough to cause the atmosphere to ionize and disassociate into its atomic species making this atmosphere glow with the heat this hot plasma emits significant quantities of thermal radiation which will keep the surface and this hot plasma will actually flow around the entire vehicle and the heat from this will actually heat the backside of the vehicle as well and this means there's different heating regimes on both sides of the spacecraft on the front a lot of the heat is coming from the contact so the you will want to actually emit as much heat as possible so that's why the bottom of the Space Shuttle was black to maximize emission of heat whereas the top well that is only getting heated by the thermal radiation from the plasma read radiating onto it and that's why the top of this Space Shuttle was white to maximize the amount of heat that was reflected away now as you go to higher Aintree speeds such as returning from the moon or from interplanetary space the thermal radiation from this glowing plasma inside the shock wave actually becomes an even more efficient way of heating the surface so that instead of being dominated by the convection the radiation becomes more important moreover those disassociated atoms and ions are of course chemically reactive and they will undergo reactions with each other as they flow towards the spacecraft or they can actually have reactions on the surface of the spacecraft with the surface acting as a catalyst and in some cases they can chemically attack the heat shield and say oxidize it so and this is important right the you know atmosphere that you're flying through the chemistry of the atmosphere needs to be taken account off when you're designing the heat shield a heat shield maybe your design safe for Mars or Venus with carbon dioxide dominated atmospheres and it might not work as well in the oxygen nitrogen atmosphere of the earth so yeah heat shields or thermal protection systems they cover a whole range of technologies and approaches to the problem probably the easiest design to understand is the space shuttles heat shield with its patchwork of tiles thermal blankets and reinforced carbon-carbon components the materials were able to handle high temperatures on the surface and had very low thermal conductivity so what they did was they stopped the heat conducting inwards towards the aluminium skin and structure the tiles were a low density foam or sponge made with silica glass fibers and sealed with a glass coating the result is a material able to handle skin temperatures of a bit 1,200 Celsius while minimizing the thermal conduction and also being light enough for spaceflight the thermal conduction is incredibly low in these so low that there's a famous demonstration where they placed one of these tiles inside a furnace and heated it up to be red hot and they would then take it out and carefully hold it in their hands while it was still glowing hot but they were also extremely fragile you could crush them with your bare hands please don't do that if the red hot you will burn yourself and also they had a tendency to pop off the structure of the spacecraft as the as it flanks from Arizona Mike forces now early on in the air shuttles program the low temperature tiles on the top were largely replaced by thermal blankets which were more flexible and would be less likely to detach during atmospheric entry the tiles would accept the thermal energy and they would slowly conduct the heat inwards towards the aluminium skin the tile thickness varied across the vehicle to account for the differing heat Lords then some places they were up to like 12 centimeters thick where the flow of heat would be so had to be slowed for as long as possible but in places where aerodynamics dictated sharper curves like the nose and leading wing edges the shock wave would be closer in and the heating was significantly higher still and the glass base tiles were simply not enough instead the shuttle used reinforced carbon-carbon that is graphite reinforced with carbon fiber which is able to understand or to withstand temperatures of about 1,500 Celsius you might not realize it but that pencil led which is soft enough to write on paper is actually able to handle extremely high temperatures it can get as high as like 4000 Celsius making an excellent component of heat shields it does however have a tendency to oxidize when heated in an oxygen atmosphere so the carbon carbon components would be covered with a layer of silicon carbide to insulate it from the apps' ferric oxygen unlike the tiles the carbon carbon components were actually quite strong and they didn't need a structural skin beneath them to hold their shape it was however a lot heavier than the tiles which meant that they were only used in the places where it was absolutely necessary and yes the carbon carbon wing edge was strong but it wasn't strong enough to handle the impact of a chunk of foam which led to the loss of the shuttle Columbia in 2003 less well known is the case of STS 27 which was a classified mission there was debris that was lost from the starboard booster then it struck the spacecraft knocking eat I'll loose the crew were concerned and then I wanted to get images to the engineers on the ground but because it was a classified mission they had to send it via an encrypted television signal and the engineers when they looked at it they couldn't see what the astronauts were seeing and they told the astronauts they thought it was fine it was just shadows well even with a tile missing sts-27 landed successfully but the damage was evident as soon as people came to the spacecraft the survival of a vehicle is actually credited to the fact that the tile that was lost happened to sit directly under the steel mounting plate for the l-band antenna anywhere else they would have probably burned through the aluminium skin but the steel was able to handle the heat a whole lot better and therefore the crew survived now despite the problems with accidental damage the shuttles thermal protection system was designed the way it was because it was supposed to be reusable while the post-flight inspection and repair was laborious it was a lot easier than stripping and replacing the entire shield after every flight furthermore small defects and bumps would generate turbulence which would mean hot spots and hot spots if the would wear down the shield faster in those locations so this third would cause spots called thermal pitting and this code actually seriously affect the aerodynamic properties of the vehicle and you know the shuttle was not the greatest aircraft to begin with so the shuttle heat shield wasn't supposed to wear down over time unlike the Apollo capsules yes I know I'm going back in time here but the heat shield in the shuttle acted like an insulator without any chemistry or any physical changes expected unlike the heat shields that was you were used on basically every single other spacecraft most space capsules use a heat shield that's designed to ablate that is the heat causes chemical and physical changes in the material which cause parts of it to evaporate away in a gas this process actually helps in three different ways first of all the chemical reactions and phase changes will take heat in secondly the escaping gas literally will carry this heat away from the spacecraft finally as the gas escapes it will form a boundary layer that will protect the spacecraft will insulate it from the actual atmosphere so ablation is actually a very powerful mechanism to keep the spacecraft protected from atmospheric heating the Apollo capsule heat shields had to handle returned from much higher speeds than the space shuttle but its heat shield was actually a lot thinner because it was designed for only one use instead of materials that remain intact at thousands of degrees Celsius the shield is primarily made of a plastic which can't handle more than a couple of hundred degrees specifically it was made from a phenol formaldehyde resin which was supported by a honeycomb structure made of fiberglass now manufacturing this heat shield was terribly labor-intensive after laying down the fiberglass honeycomb a worker would have to go and inject the resin into each one of these little cells using something that looked like a caulk gun this understandably would take a really long time you might imagine that a heat shield largely made of plastics would melt and flow away but it's actually a thermosetting plastic that means that polymers are essentially interlinked and won't together so that instead of melting and reforming in different shapes it literally just starts to break down and chemically what happens is various gases start to get evolved as it gets heated you'll get you know water methane carbon dioxide carbon monoxide but what's left behind actually is a significant amount of carbon so it literally turns black the low-density carbon that's left behind is actually an incredibly good insulator that will further protect the material beneath it now of course some of you have mentioned star light which is of course this mysterious compound that was famously used to protect an egg from the heat of a blowtorch this big basically works in a very similar way as it burns it creates this thin layer or this thick layer of carbon that expands out of course the stuff that was used on the Apollo heat shield had to be much more structurally sound because it's flying through the atmosphere at hypersonic speeds and where the Apollo heat shield had a density of about half a gram per cubic centimeter and the char layer that would be left behind would have dropped its dense data about 0.23 grams per cubic centimeter that's less than half but 55% of this would be the earth fiberglass was there and then 45% would be the carbon that was left a more modern version of this is the pica heat shield phenolic impregnated carbon ablator where the fiberglass honeycomb is replaced by a carbon fiber sponge or foam and the resin is more or less soaked into this structure pica was first used for the Stardust sample return capsule which still holds the record for the fastest entry into the Earth's atmosphere by any spacecraft the SpaceX have taken pica and developed it further creating something called pica X which is largely an improvement designed around manufacturing it more easily it's been used on their dragon capsules for you know 216 launches now and that's actually important because the Dragon capsule is the main mechanism by which the International Space Station returns experiments to earth know SpaceX have been reusing their dragon capsules but it's not like the Space Shuttle the heat shield damage is quite extensive and to reuse it they have to literally strip off the heat shield and put a new one on the most extreme heat shield ever used was on the Galileo atmosphere probe which flew into Jupiter's atmosphere at a healthy clip of 48 kilometres per second the spacecraft was only about 300 kilograms and half of that 450 kilograms was on the heat shield during reentry it's slaughter at rates approaching 230 G's I hate temperatures of something like 16,000 degrees Celsius and by the time it slowed down was able to deploy its parachute it had fully ablated away 82 kilograms more than half of the heat shield but looking to the future SpaceX are reportedly working on an actively cooled heat shield for their starship vehicle according to Iran's twitter feed the spacecraft will use a shiny stainless steel structure with liquid methane from the fuel tanks circulating just under the surface to keep the skin cooled during reentry only one half of the surface would need to be cooled in this way the leeward side would still get heating from the plasma but that could be reflected away largely it was also recently announced that they've licensed a new heat shield technology from NASA tough Rock that's toughened uni peace fibrous reinforced oxidation resistant composite has been demonstrated to handle sustained heating temperatures off of its 1700 Celsius appropriately enough for such a cutting edge technology it was demonstrated on the leading wing edge of the x-37b space plane tough rock combines a fibrous glass layer for insulation with a carbon carbon cap and a ceramic outer layer to protect against oxidation from the atmosphere I would guess that this material makes most sense for protecting the wings and control surfaces which probably aren't amenable to the act of cooling used elsewhere like the space shuttle the starship is designed to be reusable hopefully with a little less refurbishment than the space shuttle required unlike the space shuttle it's also designed to handle reentry speeds from interplanetary space and that's something that spacecraft have only ever done with ablation based heat shields it's a formidable engineering problem but someone has to solve it if we want to live in that sci-fi future I'm Scott Manley fly safe [Music]

Channel: Scott Manley

Views: 1,020,551

Rating: 4.9547195 out of 5

Keywords: reentry, atmosphere, chemistry, space, nasa, space shuttle, avocoat, pica, pica x, dragon, aerothermodynamics

Id: hLHo9ZM3Bis

Length: 17min 57sec (1077 seconds)

Published: Sat Dec 29 2018

Reddit Comments

[u/Need1ToStick]

Talk about living on the edge of death.

[u/Daninjaman]

Holy shit

[u/neoengel]

Damn.

Calling it the shuttle to help promote the idea of regular space flight really downplayed the absolute danger of the entire process. Even at liftoff, a crew capsule can abort and eject from the top of a rocket - but when the shuttle launched they were committed with no means of escape.

There's a NOVA episode that goes into detail about how very lightweight foam from the rocket booster damaged the wing on the liftoff of Columbia - it took some pressure from engineers to get NASA to accept that as the cause after the disaster when they were also basically ignored when they sounded the alarm upon video review of the launch.

https://youtu.be/OOBRbE7VVjg

(Please be advised, the narrator is Neil deGrasse Tyson who some may object because of current allegations against him.)