The action of reentering the earth’s atmosphere after space travel.
Since 1830, small groups of scientists have studied meteors, natural objects entering Earth’s atmosphere at high speed. Their studies have ranged from chemical analyses of recovered meteors to speculations about the physical changes that might take place during the meteor’s high-speed passage through the atmosphere. However, when the technology became available to send manufactured objects outside the atmosphere, various engineering disciplines such as aerothermodynamics, high-temperature materials science, and trajectory analysis were developed.
A craft that is built to withstand reentry into the earth’s atmosphere is known as a reentry body (RB). The engineering requirements that must be met by any reentry body design depend upon the purpose of the reentry body. For example, a reentry body containing an astronaut must be able to soft-land. Such a reentry body must not only survive the environment of passage through the atmosphere, but also impact the earth at a very low vertical speed. For a reentry body such as the space shuttle, there is the additional requirement that the landing occur at a specific location. In other cases, such as that of the early Mercury capsule, a soft landing in the ocean was all that was required. For military weapons, a soft landing may be of no importance.
The most fundamental aspect of reentry bodies is their overall material requirements. In physics, the unit of measurement for energy is the joule. About 1,054 joules are required to raise the temperature of 1 pound of water 1 degree Fahrenheit. A material is vaporized when it passes from a solid to a gas. About 60,000,000 joules are required to vaporize 1 kilogram (2.2 pounds) of carbon. Nearly all other materials require less energy to be vaporized. A little more than 2,000,000 joules are required to vaporize 1 kilogram of water, for example. The kinetic energy, or the energy of motion, of a typical reentry body just entering the atmosphere might be 30,000,000 joules. Therefore, if all the reentry body’s kinetic energy were converted to heat or thermal energy, the entire reentry body would vanish, unless it were made entirely of carbon.
The fact that many reentry bodies survive to make soft landings indicates that a significant amount of energy is dissipated in some way other than the vaporization of the reentry body itself. Some of the kinetic energy, turned into heat, is radiated or conducted into the air surrounding the reentry body. This energy changes the chemical makeup of the air by changing its molecules. The process of changing a molecule by the application of high temperature is called disassociation.
The amount of heat absorbed by the reentry body and the amount absorbed by dissociation depends upon the reentry body’s speed; it also depends upon the reentry body’s altitude, because the gases that make up the atmosphere vary with altitude. In addition, the reentry body’s shape also strongly influences the amount of heat it absorbs. A blunt shape is very effective in directing the heat away from the reentry body.
The accompanying figure illustrates some of the important parts of the flow about a typical reentry body. This figure assumes that the observer is stationary with respect to the reentry body. Consequently, the flow of air is moving from the left to the right. An important way of describing such flows is with the term Mach number, or the ratio of the speed of the flow to the speed of sound at the point in flow. The speed of sound varies with the temperature and the temperature varies throughout the flow region. The speed of the flow approaching the reentry body is much greater than the speed of sound. A very strong shock wave, identified in the accompanying figure by the term “bow shock,” is formed. This shock wave is detached from the reentry body and stands ahead of the reentry body into the oncoming flow. The distance between the shock wave and the body is called the standoff distance.
In region A, between the reentry body and the bow shock wave, the speed of the flow is less than the speed of sound. The stagnation point is where the flow is brought to rest. In the vicinity of the stagnation point, the heat flow to the reentry body is the greatest. Therefore, the reentry body is often covered with a carbon heat shield in region A. Much of the flow in region A passes into region B. The flow that passes into region B increases in speed, finally equaling the speed of sound. The line called the sonic line shows where this transition from a speed lower than to a speed higher than the speed of sound occurs. In region A the speed is less than the speed of sound, in region B the speed is greater than the speed of sound. Some of the flow in region A goes into what is called the boundary layer.
Somewhere beyond the maximum thickness of the reentry body is a series of weak pressure waves called expansion waves, which bring the flow from the higher-pressure region B to the lower-pressure region C. The pressure in region C is slightly higher than that external to the bow shock wave. The region outside of the bow shock wave is unaffected by the presence of the reentry body until it encounters the bow shock.
The body shown in the accompanying figure has the shape of a teardrop with the blunt end facing the oncoming flow. A very thin layer forms around the reentry body where the friction of the air becomes important. This layer is called the boundary layer when it is in contact with the reentry body and the shear layer when it continues past the reentry body. Fluid friction comes about when adjacent layers of air have greatly different speeds. Fluid friction is illustrated by the following simple experiment: If one rubs the heel of one’s hand rapidly along the surface of a desk, one becomes aware of a warmth in that part of the hand contacting the desk. The desk acts as one layer of fluid, and the heel of the hand acts as an adjoining layer. Because one layer is moving rapidly relative to the other, heat is generated in much the same way as in reentry. In this experiment, the mechanical energy of forcing the hand over the desk against friction is converted into heat energy, raising the temperature of the outside of the hand. The region where fluid friction is important is known as viscid, and the region where friction is unimportant is known as inviscid.
The air in direct contact with the reentry body must come to rest relative to the reentry body, whereas the air at a short distance from the reentry body has a speed greater than the speed of sound. Therefore, the fluid experiences a rapid change in speed over a small distance. Therefore, friction becomes a predominant part of the fluid motion in the vicinity of the reentry body. This friction force generates heat, which can cause vaporization of the surface of the reentry body.
The vaporization of the surface of the reentry body is often called ablation. The material forming the surface of the reentry body changes directly from a solid to a gas. The products of vaporization, usually compounds of carbon and oxygen, enter the flow near the body. Because vaporization requires heat energy, the reentry body is deliberately designed to sacrifice a portion of its surface to prevent heat from penetrating into the interior of the reentry body.
In region C, all the flow has about the same speed, and friction effects are rather insignificant. Just behind the reentry body is a small region where the flow seems trapped and is being pulled along by the reentry body. The shear layer from around the body comes to a small area called the neck, beyond which there is an expansion of the flow into a wake. The wake has a core region, where friction effects are significant, and an outer region, where the flow is essentially inviscid. Whereas the friction in the wake core cannot affect the reentry body, it does provide a means by which the trajectory of the reentry body can be detected from the ground.
The shape of the reentry body does affect the size of the wake and the chemical activity within the wake, and therefore the ability of a ground station to detect or track the reentry body. The flow field produces a great amount of heat, which must be controlled by selecting the shape and materials of the reentry body. In addition, the flow field produces drag. The magnitude of the drag forces in some cases can be one hundred times the weight of the reentry body. A crewed reentry body must be designed in a shape that will avoid such high drag loads. It has been found that very blunt bodies, rather than streamlined bodies, limit the peak drag forces.
A reentry body can be controlled by altering the trajectory or path that it follows as it moves through the atmosphere. The two major reasons for controlling a reentry body are first, to reduce its speed and, second, to direct it to an impact or landing site on Earth. An impact, or high-speed Earth encounter, results in destruction of the reentry body, a landing, or low-speed Earth encounter, allows recovery of the reentry body intact.
The aerodynamic forces on a reentry body are those of drag and lift. Drag, identified as the force in the direction of the velocity, tends to reduce the velocity. Lift acts at right angle to the velocity and therefore changes the direction of the velocity.
Small gas jets applied to the body, similar to those of the Mercury capsule, can make small but significant changes to the direction of the velocity. Such controls are used well outside of the atmosphere to limit the side forces during reentry. Expandable flares can also be used to increase drag and slow down the reentry body.
A versatile control system is the split windward flap. This control consists of two side-by-side flaps that have the appearance of rectangular paddles. When the flaps are extended at equal angles, they cause a pitching of the reentry body; when they are extended at unequal angle, they cause the reentry body to roll as well.
The space shuttle is controlled much like a high-performance airplane with a rudder and a combination of elevator and ailerons called elevons.
Another method of controlling a reentry body is by bending or canting its nose, or front part. The reentry body’s center of gravity may also be moved laterally by moving an object within the reentry body. Lift can then be developed in a preferred direction, similar to the way hang glider pilots move the gliders’ center of gravity, and thereby change direction, by moving their own weight.
Reentry bodies operate in flight conditions that make great demands upon the vehicles’ materials and shape. Heat loads threaten to vaporize a large part of the structure. In addition, the structure must support aerodynamic loads that can reach values as high as one hundred times the weight of the reentry body. With a human crew aboard, heat and aerodynamic loads must be very carefully managed to ensure integrity right down to a soft landing.
Baker, David. The History of Manned Space Flight. New York: Crown, 1981. A thorough history of most crewed spaceflight up to space shuttle flights, with discussion of reentry problems and engineering solutions spread throughout the book. Martin, John J. Atmospheric Reentry. Englewood Cliffs, N.J.: Prentice Hall, 1966. An engineering text with some introductory material that may be accessible to those without a strong background in physics. Regan, Frank J., and Satya M. Anandakrishnan. Dynamics of Atmospheric Reentry. Reston, Va.: American Institute of Aeronautics and Astronautics, 1992. An engineering text with overview introductory sections requiring no extensive background in mathematics or physics.
The Apollo 11 space capsule reenters Earth’s atmosphere.