Spacecraft re-entering Earth's atmosphere must navigate intense heat and friction. Primary defenses against burning up include heat shields, design, trajectory and controlled deceleration.
Spacecraft travel at speeds of 25,000 km/h (16,000 mph) on reentry. Friction between spacecraft and air molecules generates heat of 1650°C (3000°F). Some materials, like tungsten, can take the heat but most burn up.
Ballistic missiles also leave the earth's atmosphere and re-enter. At the beginning a missile's trajectory well defined. In heavier layers of atmosphere it's increasingly influenced by gravity and aerodynamic drag.
Heat Shields
The heat shield is positioned on the leading end of the spacecraft. This protective layer bear the brunt of friction and resulting heat.
Ablative Shields: As the shield encounters extreme temperatures, the outer layer vaporizes, carrying heat away from the spacecraft. Apollo spacecraft used a phenolic resin-based material, which carbonized and vaporized.
Such shields were used on the Vostok, Voskhod, Mercury, Gemini and Apollo spacecraft, and are currently used by deep-space Orion (NASA), and Soyuz, a series of Russian craft from the 1960s to present day.
Carbon phenolic is the most used heat shield in spacecraft today.
Reusable Heat Shields: Space Shuttle orbiters, before discontinuation, use shields made from thousands of ceramic tiles. The lightweight tiles radiate heat away from the spacecraft.
Details: Much of the orbiter's exterior is air trapped within nearly pure silica fibers. This refractory insulation absorbs and redirects heat back into the atmosphere.
These tiles are coated with silicon borides and borosilicate glass, with darker tiles on the lower surface. Lighter tiles are on the tail, parts of the upper wing, crew cabin surfaces, and the exterior of the payload bay doors.
The nose cap, nose landing gear doors, and leading edges are made of reinforced carbon-carbon, which is rayon infused with graphite-filled resins and coated with silicon carbide.
Upper, white areas not covered by tiles are mainly made from either Nomex felt coated with silicon-rich elastomer or beta cloth, which is woven silica fibers covered in Teflon.
Missiles keep their warheads in a re-entry vehicle to protect them from burnup. The craft uses different shield types depending on its manufacture.
Blunt Body Shape
A spacecraft has blunt, rounded front. The shape makes a shockwave ahead of the vehicle to compress and heat the air before it contacts the spacecraft itself. Heat is concentrated in the air around the shockwave, not directly on the shield.
Trajectory Management
Mission planners calculate the re-entry trajectory to optimize safety and control. Factors include:
Atmospheric Conditions: density and composition of the atmosphere varies depending on altitude and weather.
Target Landing Site: trajectory must be precisely calculated to be sure the spacecraft lands in the designated zone.
"Angle of Attack"
The angle at which the spacecraft enters the atmosphere is called the angle of attack, and it's carefully controlled. A steeper angle causes faster deceleration and higher heat. Shallower angles enable a slower descent.
Controlled Deceleration
Slowing down the spacecraft reduces kinetic energy.
Atmospheric Braking: This uses the atmosphere to slow the spacecraft. By adjusting the angle of attack and using the heat shield to manage resulting friction, spacecraft can gradually shed velocity.
Parachutes: Once the spacecraft slows sufficiently, parachutes are deployed to further reduce speed. Multiple parachutes are often used, often starting with a smaller drogue parachute to stabilize the vehicle.
Different sets of parachutes are deployed at various altitudes. The Soyuz spacecraft deploys its primary parachutes at about 10 km (6 mi) above the Earth's surface, and stabilizes descent with smaller chutes.
Before launch of the craft, engineers use wind tunnels and computer simulations to replicate extreme conditions of reentry. The tests continually refine spacecraft designs and heat shield materials.
READ: Lora Ley Adventures - Germanic Mythology Fiction Series
READ: Reiker For Hire - Victorian Detective Murder Mysteries