▲ On January 16, 2003, the space shuttle Columbia took off for the last time. Source: Wikipedia.
Which is more difficult, going on a trip or returning from a trip? Personally, I find it more difficult when I return from a trip. Experientially, but simply thinking about it, it is natural to feel more tired because the return trip is the process of using most of the energy for travel and returning the way you walked with a flabby body. But what if the return trip was a life-threatening process?
Atmospheric re-entry is the last step in the process of departing from Earth and staying in Earth orbit or returning after completing a mission on another planet such as the Moon or Mars. Passing through the atmosphere, the layer of air surrounding the earth, and arriving safely on the ground is the final step in space travel. Re-entering the atmosphere is more difficult than leaving the Earth through the atmosphere in the first place. In other words, it is more dangerous. For example, on February 1, 2003, the US space shuttle Columbia disintegrated in the air at an altitude of 60 km while re-entering the earth’s atmosphere, and about 12,000 fragments were poured into the earth. All seven crew members died in the disaster. The cause of the accident was a small hole in the wing of the Columbia. It was later revealed that the hole was formed when a fragment fell from the fuel tank of the propellant used when Columbia departed from Earth and penetrated the Columbia’s left wing. As Columbia re-entered the atmosphere after completing its mission in space, ultra-high temperature air of up to 1500 degrees was introduced into the hole, melting the inside of the wing, and the space shuttle body lost its center and was enveloped in heat and disintegrated in the air.
Re-entry to the atmosphere starts with the difficulty of getting the angle to enter the atmosphere. For a pointed shape like the space shuttle, set the entry angle to 1-2 degrees, and for a flat shape, set it to 5-6 degrees. If the angle of entry is lower than necessary, it will not enter the atmosphere and will be ejected like a water swallow. If the angle of entry is higher than necessary, the entry process will be accelerated and the spacecraft can be destroyed beyond its designed temperature rise.
To safely pass through the atmosphere, two main factors must be controlled. speed and heat These two things have a mutual correlation, but only when each of them is solved can safely reach the ground. Upon re-entry into the atmosphere, the velocity is approximately 8.9 km/s. To land safely on the ground, a tremendous amount of deceleration is required. In the atmosphere, the air is thinner the farther from the ground, but air friction heat becomes a problem from the time it enters the atmosphere. The reason is not heat from collisions with air particles, but shock waves. This shock wave causes an adiabatic compression effect, resulting in extremely high temperatures. Here, adiabatic refers to the accumulation of heat and compression refers to a rise in pressure. Upon reentry to the atmosphere, collisions with the air do not occur sporadically, but occur almost continuously, and the closer the air gets to the ground, the denser the air becomes, and the situation becomes more severe. As a result, the heat continues to accumulate without a chance to escape. As the pressure rises, heat also builds up. This is the opposite of the principle of a refrigerator.
▲ Space Shuttle Discovery approaching the International Space Station on July 28, 2005. The black ceramic tiles on the floor are clearly visible. Source: Wikimedia.
It takes about 3 hours from orbit to the actual ground. Normally, for two and a half hours, the rocket engine is reverse-injected to reduce speed and adjust the trajectory to lower altitude. At this time, due to the heat generated by air friction, the air layer on the surface of the spacecraft gradually enters a plasma state, and the spacecraft communicates with the ground center. A black-out phenomenon occurs in which communication is cut off. Plasma is an ionized state in which atomic nuclei and free electrons are separated and wander, and is the fourth state of matter that is neither solid nor liquid nor gas. The frequency of the plasma generated by air friction heat is in gigahertz units, and communication is cut off because radio waves for communication in megahertz units with relatively low frequencies are shielded or reflected. From the section below 100 km above the ground, the deceleration effect due to air resistance occurs simultaneously with the plasma generation, and the plasma generation ends when it reaches 50 km above the ground. Then, along with the deceleration due to air resistance, the parachute is opened and the speed is further reduced to land on the ground.
A spaceship thermal protection system is used to protect the spacecraft from air friction heat generated upon re-entry into the atmosphere. There are many elements to the protection system, but the core is the insulation material. Insulation here means blocking the heat between the air friction heat and the interior of the spacecraft. The heat-resistant material used at this time is called a fluxing agent. Heat-resistant materials for insulation are largely divided into materials such as ablator, which is a disposable flux, and reusable C/C (Carbon Fiber Reinforced Carbon Composite) or ceramic tile. Disposable ablator is a heat-resistant material made by filling carbon fiber with phenolic resin and then heating and pressurizing it to make the necessary shape. The thermal decomposition gas generated at this time and the oxidizing gas generated as the fibers and resin remaining without being decomposed become a carbonized layer play a role in blocking some of the air friction heat. C/C is a material that combines carbon in a complex way. It has excellent strength and rigidity even at high temperatures and has a large thermal emissivity, so it can dissipate air frictional heat by radiation. Ceramic tiles are made by mixing and fixing silica/alumina fibers with a boron compound as a binder, burning them, and coating the surface with a layer having high emissivity, which has excellent heat insulation and heat resistance. The C/C is used for the space shuttle’s nose cap and main wing front, and ceramic tiles are used for the space shuttle’s wings or underside.
Looking at the process of re-entry into the atmosphere, we realize that it is more difficult to return than to leave the earth. I feel this process every time I use an airplane. When the plane lands, it is more tense than when it takes off. Still, why would I get on a plane again? Let’s ask the question. Of course, airplanes aren’t the only means of transportation on Earth, but isn’t it because I have curiosity and longing to experience new places that I’m taking a plane again? Even if there is a difference in the degree of challenge of earthlings risking their lives toward space, I wonder if they are on the same line.
Young-Doo Lee Ph.D.
[ⓒ 울산저널i. 무단전재-재배포 금지]
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