The atmospheric phase of all space missions is challenging for both ascending rockets and descending space capsules.
As the rocket lifts off from the earth and heads towards its orbit, it accelerates slowly through the atmosphere to keep the mechanical loads on the vehicle to a minimum. However, a small portion directed back towards the module will still be intense enough to melt it if it is not protected by a robust thermal protection system (TPS). A TPS is predominantly classified into three types depending on how it removes heat: ablative, radiative, and heat sink. An ablative TPS is a single-use TPS that removes heat energy by sacrificing its layers through chemical and physical processes. Specifically, the TPS will absorb extreme quantities of thermal energy and itself chemically decompose into a protective layer of solid char and outgassing vapours. This process physically carries heat away from the module as the material burns off. In addition, the escaping gases create a cooler boundary layer that acts as a buffer, blocking intense heat from being transferred into the module. For a simple metaphor, it is like a block of wax that absorbs heat by melting and shedding its outer layers. Carbon phenolic and silica phenolic are good examples of an ablative TPS. India’s maiden re-entry mission, the ‘Space Capsule Recovery Experiment’ (SRE), used a carbon phenolic ablative to protect the module’s nose cap, where heat flux was the highest. The Crew Dragon capsule of SpaceX uses an ablative named phenolic-impregnated carbon ablator, or PICA, a lightweight carbon fibre matrix filled with a phenolic resin. A radiative TPS setup works by absorbing the extreme heat of re-entry, then releasing it back into space as electromagnetic radiation, primarily in the infrared spectrum but also as visible light when it is extremely hot. This is like a traditional clay tandoor oven, where the clay walls absorb heat from the burning charcoal and radiate it back as infrared energy to bake the food.
When a space capsule, like the Gaganyaan crew module, re-enters from its orbit around the earth, it will hit the atmosphere at a blistering speed of 7,500-8,000 m/s. More than 99% of this kinetic energy will be dissipated into the atmosphere as heat energy. The LVM-3/CARE mission that the Indian Space Research Organisation (ISRO) conducted in 2014 successfully demonstrated crew module reentry using an ablative TPS, establishing the foundational technology that is now being used in the Gaganyaan programme.
