Day 25: Aerothermodynamics: The Science Behind Surviving Atmospheric Re-entry
Although we don’t feel it hitting our skin as we walk around here on Earth, air is made up of molecules that have to be pushed out of the way for us to move. When trying to move a large object at significantly high speeds through the dense atmosphere challenges start to arise.
The speed of sound changes based on altitude: given that sound is the propagation of vibration waves between air molecules, sound travels faster through the denser atmosphere of the Troposphere than in less dense air in the Stratosphere because there are less molecules to propagate vibration waves through at those altitudes. For this reason, when trying to propel an object such as an airplane at speeds close to speed of sound, it becomes dependent on the speed-to-altitude ratio, that is, the higher above the Earth a plane is moving the faster they can move because there is less atmosphere to push out of the way.
This ratio is expressed as “Mach”, where Mach 1 is the exact speed of sound at sea level (~343 m/s). This is the reason that at Mach 1 the coefficient of drag (ie. the measurement of surface friction with a substance, in our case air molecules) is at its highest, you are pushing the most air (remember that Mach 1 can change depending on altitude, so say the sea level Mach 1 is 343 m/s but Mach 1 at 40,000ft is 294 m/s). When you break the sound “barrier” you begin to transfer the friction energy into heat energy and that’s where atmospheric re-entry– aerothermodynamics– comes into play. (In reality the sonic “barrier” is not a barrier per se, but a bunched up group of molecules that cannot move out of the nose of the aircraft in time until they do at once, which is the sonic boom that is heard when the “barrier” is broken). Aerodynamics is a complex field of physics that contains many formulas and other denotations, but that is the essence of it.
Re-entry deals with these principles, but given the other factors involved with barreling back from low Earth orbit, re-entry falls under Aerothermodynamic Principles: the study of the exchange of heat between solids and gases on aircraft flying through the air at very high speeds. When a craft such as the Shuttle or the Apollo Capsule are re-entering Earth’s atmosphere, they are doing so at speeds far greater than Mach 1 (you are essentially trying to kill off the energy that took you into orbit in the first place, and the atmosphere acts as a “brake” to slow down the craft at orbital speeds). When a spacecraft enters the atmosphere, they are hitting air particles so fast that they literally break apart the atomic bonds that hold them together. This breaking of bonds releases energy in the form of an electrically charged plasma (the “fire” you see surrounding reentering capsules in many videos).
This plasma is at an incredible temperature of upwards of 5000*C, and the heat of re-entry is so great that spacecraft need special thermal protection to protect the integrity of the craft during re-entry. These come in the form of either ablative, like those on capsules, or Insulators, like the tiles on the Space Shuttle. The ablative heat shields are made of carbon-ceramic composites and are designed to burn away slowly in layers (thus ablative) while taking away thermal energy as it breaks apart. These are rated for temperatures of re-entry, meaning they have sufficient layers to re-enter the atmosphere safely. The Thermal Insulation Tiles on the Space Shuttle’s Thermal Protection System (TPS, aka heat shield) are silica and carbon-carbon composite materials designed to retain heat energy very slowly, and thus release that energy very slowly as well. The tiles on the nose (who received the highest temperatures) were composed of the carbon-carbon composite, while other mainly silica composite tiles were used throughout the body of the spacecraft. Silica and carbon both act as incredible insulators because of their capacity for heat absorption.
As the Shuttle entered Earth’s atmosphere, the tiles would absorb heat of the plasma and hold it, until eventually releasing it into the surrounding atmosphere while the Shuttle glided to the runway. These tiles were so effective at heat-retention, in fact, that during a demonstration of their product a Lockheed Martin Engineer oven-baked one of these tiles to 2500*F and picked it up with his very own hands to demonstrate to NASA their incredible insulation properties. Never more clear was the importance of re-entry heat shields then during the STS-107 Space Shuttle Columbia disaster of 2003, a topic I will cover in future essays.
