Aug 17, 2024
Re-entry: Hot Enough to Boil Steel
One thing is going to space; another is coming back. The art of entering the Earth's atmosphere is something scientists and engineers are still working on perfecting, as temperatures from the hot plasma hitting the vehicle can reach up to 3800°C (7000°F), although in most cases they only reach about 1500-2700°C (2700-5000°F).
By having a heatshield, you eliminate the extra complexity and fuel consumption of only relying on propulsively slowing down and landing your rocket stage, which is why many have utilized this technology in the past and many are pursuing it now. But how do they protect against the extremely high temperatures, and what can we learn from previous attempts? Will it ever be possible to develop a rapidly reusable heatshield? What alternatives exist? Let's find out!
The First Atmospheric Entry
Well, the first entry was probably done by some meteor a few billion years ago. The first human-made entry, however, was performed by the German MW 18014 A-4 rocket, also known as the V2 rocket, which reached an apogee of 176 km in 1944. The first manned re-entry was made when the Soviet cosmonaut Yuri Gagarin returned from his 108-minute long flight around the globe and re-entered in the Vostok 3KA's descent module. At a 7-kilometer altitude, he ejected and parachuted to the ground.
Let It Burn!
Before taking a look at other re-entry vehicles, we have to understand the two main types of heatshields and how they work their magic. The first one is the ablative heatshield. Descent modules such as the Vostok, Voskhod, Apollo, Dragon, Shenzhou, Soyuz, Mercury, Gemini, and Orion have all used ablative shielding for their missions.
When the module is coming in hot at hypersonic speeds of up to 11 km/s, the surrounding air molecules have a lot of kinetic energy relative to the vehicle. Knowing the mass of the Apollo Command Module is 5557 kg, we can make out a rough number of 340 GJ of kinetic energy at its top speed.
When the air molecules hit the vehicle, carrying this tremendous amount of energy with them, the electrons get knocked out of their orbits as a result of the extra kinetic energy from vibrations, making them jump to higher orbits until the electrostatic potential can't hold them and a plasma is created.
The vibrations cause higher temperatures, as temperature is proportional to the kinetic energy per particle. The ablative heatshield absorbs the thermal energy and then burns away, thereby "burning the kinetic energy away" and slowing down the vessel while also thermally protecting it—truly genius engineering.
The ablative material is typically made of either silica or carbon phenolics. Silica phenolics are normally cheaper than carbon phenolics, as the silica fabric is only half the cost of its carbon equivalent. The silica fabric makes up over 95% of the heatshield, as the phenolics only act as resin, holding the fabric sheets together.
The capsule often has a blunt shape facing the heat. It's made in this way to mitigate the shock waves coming from the supersonic air hitting the vehicle. By having a blunt surface instead of a sharper surface with a low ballistic coefficient, you prevent oblique shock waves from forming that create more pressure, and you create a boundary layer around it. This boundary layer works as a shield layer, protecting against the shock waves that will form.
In the boundary layer, a stagnation point will form, which is where the velocity of the fluid is zero, and thus the pressure is the highest in this region according to the Bernoulli principle. This is bad, but it's better than shock waves hitting the vehicle.
The blunt shape also helps prevent the incoming air from further accelerating the vehicle by pushing the molecules to the sides instead of behind the capsule. The denser boundary layer plasma will also convect more heat away.
As seen in the image above, amorphous carbon is charred after undergoing intense heating, while the "cracks" are formed after the resin burns away, which also happens on re-entry. In the case of the Dragon capsule's PICA-X, the pieces are shaped into tiles that fit the spacecraft.
Filler materials such as hollow glass spheres are still being researched, but they provide lower thermal conductivity, lower density, and better thermal stability. Hollow glass spheres are small chambers or "bubbles" inside the heatshield filled with an inert gas and are only about 50 microns in size.
Again, Again, Again!
Let's say you make it through re-entry and you had such a good time that you want to do it again immediately. You look at your capsule, only to find out that all the resin has burned away and your silica is all charred. It will take months to replace, and you cannot wait that long. You need to do it again as soon as possible. Well, I have good news for you.
Reusable heatshields, a.k.a. refractory insulation, also work by converting energy, but not entirely in the same way as the ablative does. While the ablative gets rid of the kinetic and thermal energy by burning it away, the reusable heatshield radiates it away. So to treat the two heatshields fairly, let's go over the conversion of energy again, this time for the reusable one.
The kinetic energy from the incoming air makes everything vibrate on a subatomic level, thereby creating thermal energy. The thermal energy is then emitted away by the heatshield in the form of thermal radiation, although the hot plasma around the surface also convects a significant amount away, as plasma makes an excellent heat conductor because of its free electrons.
The heatshield tiles on the Space Shuttle's Orbiter had an upper layer of fine borosilicate glass, a material with very low thermal expansion and, consequently, a high resilience to thermal shocks. These shocks occur when materials expand differently as a result of a rapid temperature change and then come into contact with each other. This causes mechanical stress on the parts and can result in a crack.
If your capsule enters the Earth's atmosphere at hypersonic speeds, you can imagine the temperature will surely rise very quickly, and because of that, you really want to have a material resilient to thermal shocks to take the worst heating.
After a comparatively thin layer of borosilicate glass, a thicker part was made of silica fibers. Because of the fiber structure, the tiles were very light and consisted of 90% air, which may be visible in the image above. By having a gas take up most of the space, the heat has a hard time being conducted through, because of the low density of molecules and consequently low heat transfer + you save a lot of mass.
In some of Starship's heatshield tiles, there are also "pockets" of just air. However, there is a sweet spot where the air takes up a good part of the volume, but not enough to start circulating around and thereby convecting the heat to the spacecraft's surface. Usually, this part is about 1.5 cm thick, although it may vary.
Besides being a poor heat conductor, silica, or silicon dioxide, is very heat-resistant too, with a melting point of 1600–1700 °C, depending on its configuration. Silica on its own has the chemical formula SiO2, but silica is often in a structure with other silica molecules and has the formula SiO4.
Since there are four oxygen atoms, each wanting two extra electrons to satisfy the octet rule, and one silicon atom wanting four, they make SiO4 as seen in the image above, where the silicon forms four strong covalent bonds. This, however, does not satisfy all the electrons' needs, as they still need one more electron each. This is where the structure we talked about comes into play.
Each oxygen atom then borrows an electron from a silicon atom in another molecule beside it to satisfy the octet rule. This is what makes the silica so heat-resistant. Each silicon atom has four covalent bonds in its own molecule, each with a bond energy of 621,7 kJ/mol, meaning that 621,7 kJ of thermal energy would be needed to break one mole of silica, and all the silica molecules are connected in one big structure.
This high bond energy comes from the fact that the difference between oxygen and silicon electronegativity (the tendency to attract electrons) is 1.7, which is pretty high.
Whoa, that was a bunch of chemistry. Now back to the macroscopic world!
The Future of Re-entry
The Space Shuttle's Orbiter had about 24,300 tiles, and as you may know, it had some trouble keeping them all attached. On STS-27, a large TPS (Thermal Protection System) tile was lost due to debris from the SRB's hitting it. Since the Orbiter's frame was made of aluminum, the re-entry plasma would have burned through and caused another catastrophic failure if it had not been for the steel plate the antenna rested on.
This was just one of the numerous times when the Space Shuttle was at risk of total failure due to missing tiles, so how are aerospace companies planning on mitigating this problem in the future?
Dreamchaser
Sierra Space's Tenacity and its cargo module, Shooting Star, are currently undergoing final testing and launch preparations in the Space Systems Processing Facility, where components going to the ISS are staged. Tenacity will be the first Dreamchaser to fly, and its inaugural flight to the ISS is currently expected by the end of 2024, atop ULA's Vulcan Centaur.
Dreamchaser will be able to deliver over 6 tons of cargo to the Internal Space Station and an additional 4.5 tons with the Shooting Star attached. They will berth with the ISS and separate when undocked from the station. The Shooting Star will burn up in the atmosphere, but Dreamchaser will re-enter.
Dreamchaser also uses a reusable heatshield, but with larger tiles than what was used on the Space Shuttle's Orbiter. This gives each tile a larger surface area to attach to, making them less vulnerable to falling off. It also means fewer points of failure compared to the Orbiter, as Dreamchaser only uses around 2000 tiles.
In addition, SNC engineers have used newer manufacturing technologies than what was used during the Shuttle program to make the tiles lighter, stronger, and cheaper—the three magic words in the aerospace industry.
Starship
SpaceX's Starship, like the Shuttle, has had problems keeping its tiles attached during flight, static fires, and simply resting in the rocket garden. During the first integrated test flight on April 20, 2023, many tiles were seen falling off, and even though SpaceX's silica-based TUFROC material is similar to that of Shuttle's, SpaceX has taken a lot of measures to fix the attachment problem.
First, we need to understand how the tiles are attached. SpaceX uses a combination of pins, adhesive, and a secondary containment material, as well as a newly added feature, to do this.
On the first four flights, the majority of the tank section was covered with pinned tiles as well as a white blanket behind. In the days leading up to the fourth flight, SpaceX CEO Elon Musk stated the following on X:
This is a matter of execution, rather than ideas. Unless we make the heat shield relatively heavy, as is the case with our Dragon capsule, where reliability is paramount, we will only discover the weak points by flying.
Right now, we are not resilient to loss of a single tile in most places, as the secondary containment material will probably not survive.
A likely cause of flight 4's burn-through of the right flap, was too wide of a gap between some of the tiles, allowing the hot plasma to make its way to the steel behind it. The white blanket (the secondary containment material) likely also serves as a filler material to mitigate this type of issue.
SpaceX also applies a gap filler to places like the upper nosecone and the aerocovers, as these are the most curved surfaces on the ship, naturally causing a larger gap. Red glue (originally blue) is also used in these places, as well as the flap hinges, seams, and other places.
Said to make its debut on flight 5, is a new black ablative, although only in certain places, like the shielded side of the flaps and on most of the tank section and nose cone. SpaceX also installed an additional layer of white felt over the ablative layer to make sure those places are as thick as where the white blanket is installed.
New tiles have also been installed on most of the ship. Elon has stated that they are twice as strong as the previous ones, most likely referring to their mechanical strength. They look just like before, but that may be a good thing. Sure, they're stronger and probably have better insulation, but their looks are the same, and that's a sign of SpaceX being pretty confident in their current design.
Orion
NASA's Orion capsule, which will take humans back to the moon in 2025, faced some problems on the Artemis I re-entry. The capsule's ablative heatshield saw more char losses than expected, as shared by the NASA Office of Inspector General.
NASA is still working on understanding the root cause of the problem, but they have recreated some of the problems at NASA's Ames facility and figured out some important aspects of the problem, according to Artemis II astronaut Victor Glover.
The higher speeds of a lunar trajectory, compared to that of a vehicle returning from LEO, makes the engineering that goes into creating the heatshield a lot harder.
Different trajectories have been proposed for Artemis II to limit the heat, but NASA has yet to decide whether to scrap the current free-return trajectory in favor of other re-entry angles.
Nova
In February 2021, Stock Space announced its 100% reusable launch vehicle, Nova. This, of course, means that the orbital second stage also has to be recovered. Stoke is planning to do this by making an RTLS (Return To Launch Site) propulsive landing, an accomplishment yet to be achieved by any orbital rocket.
The second stage's downward-facing side (the one that will take the extreme heat) is no "ordinary" heatshield, if you can call any heatshield that, as the spacecraft's propellants cryogenically cool the steel surface. Around the hot end, there will be a ring of 30 thrusters. They are pointed at an angle, creating an aerospike effect together.
The aerospike engine is a special type of rocket engine that maintains its efficiency even at high altitudes where the lack of air pressure would normally cause the flow to expand. If the ambient pressure is lower than a traditional rocket engine's exhaust, it expands, and it is said to be underexpanded.
Wait, underexpanded? That's exactly what it isn't! The wording may seem weird at first, but what it describes is how that flow has been expanded inside the nozzle. If the flow is underexpanded inside the nozzle, that is, if its pressure is too high relative to the outside, then it will go out to all sides and create thrust in directions the rocket does not gain much forward thrust from.
If the flow is overexpanded, the pressure is too low, and the exhaust will be compressed by the atmosphere, leading to flow separation in the nozzle, causing violent vibrations and a big boom, but not the kind of boom you want in your rocket engine.
What Stoke Space is trying to do is to run their cryogenic liquid hydrogen (<−253°C or −423°F) through the steel surface. This does two very beneficial things.
- It cools the surface by absorbing the re-entry heat.
- It works as an expander cycle because it vaporizes the fuel and spins the turbines, which turn the pumps, which pump more cryogenic fuel to the thrusters and also cool the steel surface even more.
This means that as the surface is heated, more cryo-propellant is automatically pumped through and cools it proportionally. More mass flow is also directed to the thrusters, which slows the stage down even more, thereby decreasing the amount of re-entry heat hitting the surface.
Conclusion
So, to summarize how heatshields do their magic—well, in many different ways, but mainly, they work by converting the thermal energy to some other form of energy or convecting it away.
There are multiple new heatshields in development from aspiring aerospace companies such as SpaceX, Stoke Space, and Sierra Space, each taking a unique approach to re-entering the Earth's atmosphere. It will be interesting to see if one type succeeds over the others or if each company has chosen the right type for their particular vehicle.
With the measures being taken by SpaceX to protect Starship with redundant shielding and attachments, it's certainly starting to seem more likely that we will see a rapidly reusable vehicle in the future that will take us to Mars and beyond. Whether that will be in the near or distant future has yet to become obvious, but with rising competition in the reusable launch market such as Stoke, Rocket Lab, Relativity, and Blue Origin, SpaceX's pace may accelerate even more than it currently is, as may the rest of the industry.
