Flight Reentry

Definition: The action of reentering the Earth’s atmosphere after space travel.

Significance: The study of reentry and the aerodynamics associated with the high-speed penetration of a planetary atmosphere allow humans to understand the nature of spaceflight.

History

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 occur 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.

Reentry Bodies

A craft built to withstand reentry into the Earth’s atmosphere is called 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 survive the environment of passage through the atmosphere and 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 occurs at a specific location. A soft landing in the ocean was required in other cases, such as the early Mercury capsule. 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 by 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. Slightly more than 2,000,000 joules are required to vaporize 1 kilogram of water. 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.

Many reentry bodies survive to make soft landings, indicating a significant amount of energy is dissipated in some way other than the vaporization of the reentry body itself. Some 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 absorbed by dissociation depends on the reentry body’s speed. It also depends on the reentry body’s altitude because the gases that make up the atmosphere vary with altitude. Additionally, the reentry body’s shape strongly influences the amount of heat it absorbs. A blunt shape effectively directs the heat away from the reentry body.

The accompanying figure illustrates some crucial parts of the flow of a typical reentry body. This figure assumes the observer is stationary concerning the reentry body. Consequently, the airflow is moving from left to right. An important way of describing such flows is with the term Mach number or the ratio of the flow's speed to the speed of sound at the point in the 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 powerful 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 flow speed 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 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 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 in the accompanying figure is shaped like 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. The following simple experiment illustrates fluid friction: If one rubs the heel of one’s hand rapidly along the desk's surface, 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 moves rapidly relative to the other, heat is generated much like 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 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 near 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 the interior of the reentry body.

In region C, all the flow has about the same speed, and friction effects are 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 the flow expands 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 affects 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. In some cases, the magnitude of the drag forces 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. Very blunt bodies, rather than streamlined bodies, limit the peak drag forces.

Reentry Body Control

A reentry body can be controlled by altering the trajectory or path 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 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 outside the atmosphere to limit the side forces during reentry. Expandable flares can also be used to increase drag and slow down the reentry body.

The split windward flap is a versatile control system consisting of two side-by-side flaps that resemble rectangular paddles. When the flaps are extended at equal angles, they pitch the reentry body, and when they are extended at unequal angles, they roll it.

The space shuttle is controlled much like a high-performance airplane with a rudder and a combination of elevators 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 place great demands on the vehicles’ materials and shape. Heat loads threaten to vaporize a large part of the structure. Additionally, the structure must support aerodynamic loads that 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 carefully managed to ensure integrity right down to a soft landing.

Bibliography

Baker, David. The History of Manned Space Flight. Crown, 1981.

Chen, Wanchun, et al. Steady Glide Dynamics and Guidance of Hypersonic Vehicle. Springer, 2021.

Falangas, Eric T. Performance Evaluation and Design of Flight Vehicle Control Systems. Wiley, 2016.

Martin, John J. Atmospheric Reentry. Prentice Hall, 1966.

Regan, Frank J., and Satya M. Anandakrishnan. Dynamics of Atmospheric Reentry. American Institute of Aeronautics and Astronautics, 1992.

Schouler, Marc, et al. "Survey of Flight and Numerical Data of Hypersonic Rarefied Flows encountered in Earth Orbit and Atmospheric Reentry." Progress in Aerospace Sciences, vol. 118, 2020, p. 100638, doi.org/10.1016/j.paerosci.2020.100638.

Tacchi, Bruno, et al. "Reconstruction of the Kentucky Re-Entry Universal Payload System Hypersonic Flight Trajectory." Journal of Spacecraft and Rockets, vol. 61, no. 2, 2024, pp. 438-47. doi.org/10.2514/1.A35826.