Atmospheric entry

(aerospace engineering) The penetration of any planetary atmosphere by any object from outer space; specifically, the penetration of the earth's atmosphere by a crewed or uncrewed capsule or spacecraft.

The motion of a body traveling from space through the atmosphere surrounding a planet. Entry bodies can be natural bodies, such as meteors or comets, or vehicles, such as ballistic missiles or the space shuttle orbiter. Entry begins when the body reaches the sensible atmosphere (defined as 400,000 ft or 122 km altitude for Earth).

The primary forces acting on an entry body are aerodynamic lift and drag, gravity, propulsion, and centrifugal acceleration. Of particular concern to the designer of entry vehicles is the control of the trajectory to minimize the effects of heating on the thermal protection system and aerodynamic loading on the vehicle structure. From Newton's law of motion, the forces on the vehicle determine the resulting trajectory as the body traverses the atmosphere.

Aerocapture vehicles use the atmosphere to deplete energy prior to orbit capture in order to significantly reduce the amount of propellant required and thus reduce the mass requirements.

For controllable vehicles, the concept of trajectory control refers to the management of the kinetic and potential energy so as to maneuver from the initial entry conditions to the desired final conditions with due regard to system constraints and dispersions. One manner of accomplishing this is to establish a guidance corridor (see illustration). Initially, the flight path is steep enough to prevent skipping out of the atmosphere but shallow enough to keep the maximum temperature and structural load factor within limits. Later in flight, the total heat load into the structure, which increases with time of flight, becomes a constraint.

Guidance corridor. Constraints on the trajectory, which determine the boundaries of the corridor, are indicated.

The most dominant aerodynamic force is the vehicle drag, which provides the deceleration. High-drag bodies are characterized by large, blunt reference profiles. The lift force is perpendicular to the drag force, works perpendicular to the velocity vector, and is the primary force vector for trajectory control. The ratio of lift to drag (L/D) determines the amount of trajectory control available. For the space shuttle this ratio is 1.1, while for the Apollo entry capsule this ratio was 0.3. See also Aerodynamic force.

Vehicles entering the atmosphere experience heat transferred from the hot air surrounding the spacecraft to the colder wall of the spacecraft. The transfer of heat or energy is accomplished by conduction, radiation, and convection. A vehicle traveling at supersonic or hypersonic velocities deflects the air and forms a shock wave. The air between the detached bow shock and the vehicle is heated to very high temperatures by molecular collisions converting the kinetic energy to thermal energy. Approximately 97–98% of this energy flows past the vehicle into the free stream. The remaining 2% has to be managed by the thermal protection system on the spacecraft. See also Aerothermodynamics; Hypersonic flight; Shock wave; Supersonic flight.

The flow within the thin boundary layer next to the surface of the vehicle can be either laminar or turbulent. Turbulent flow has faster-moving particles and higher rates of heat transfer to the surface than laminar flow. Thus, it is desirable for entry vehicles to maintain laminar flow as long as possible to minimize the surface temperature. See also Fluid flow.

Ablator materials, which were used on Mercury, Gemini, and Apollo spacecraft, accommodated the convective heating through absorption, vaporization, and the resultant char layer that reradiated the heat to the atmosphere. Ablators, however, are not reusable. See also Nose cone.

The reuse requirement for the shuttle orbiter necessitated development of new concepts of thermal protection. One concept is an external low-density insulator that can withstand high temperatures for multiple orbiter entries for at least 100 flights. The insulator is a tile fabricated from high-purity silica fibers reinforced with silica binder.

The temperature on the wing leading edges and on the nose of the shuttle orbiter was predicted to exceed 2300°F (1533 K). A reinforced carbon-carbon system was developed to withstand up to 3000°F (1922 K). Reinforced carbon-carbon is a laminate of woven graphite cloth with a carbon binder and a silicon carbide coating which prevents oxidation.