Compared to rockets, re-entering modules have a unique and unforgiving set of challenges
| Photo Credit: PTI
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. 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. 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).
Why is re-entry challenging?
Compared to rockets, re-entering modules have a unique and unforgiving set of challenges. Foremost among them is that once the descent begins, there is no provision to abort the mission. Another constraint is the limited role for the crew to intervene and correct any system non-conformance. This is because atmospheric descent is incredibly fast and the deceleration forces change constantly. Human response times are simply too high to manually correct any sudden system abnormality. Hence, all the systems should be made robust enough to withstand the scorching conditions of re-entry. During re-entry, the exterior of the Gaganyaan crew module will encounter temperatures as high as 1,800° C in some regions. To withstand this heat, the module will rely on its protective external heatshield. Despite being just 30-35 mm thick, this TPS will perform the critical task of maintaining the module’s structural integrity by keeping its temperature safely below 150° C.
How does TPS protect the crew module?
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.
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.
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.
A radiative TPS will remain intact and withstand the heat without melting or degrading, making it ideal for reusable re-entry vehicles.
Third, a heat sink TPS protects itself by absorbing the heat energy like a giant sponge and raising its own temperature, but without melting or changing its phase in any other way. This is what happens when you pour hot tea into a copper cup. The cup will absorb the heat from the tea and become hot — but it also stays solid. In fact, copper and aluminium are good examples of heat-sink TPS setups.
Why does the crew module use an ablative TPS?
The Gaganyaan crew module uses an ablative TPS because it is a proven, highly robust solution tailored to the module’s single-use design philosophy. An ablative TPS can withstand an extreme thermal load without requiring complex or delicate surface maintenance.
Ablative heat shields can easily handle fluctuating heat loads to protect the structure underneath. Radiative systems are less forgiving, as any design errors can quickly cause dangerous overheating. So by avoiding expensive manufacturing, specialised inspection and complex installation processes associated with a reusable radiative TPS, ISRO is opting for a safer, more cost-effective choice.
Built on ISRO’s rich heritage in ablative technology and lessons learned from the SRE and CARE missions, the Gaganyaan crew module is engineered to safely bring India’s astronauts home through the inferno of reentry — with its first uncrewed test flight launching shortly.
(Unnikrishnan Nair S. is former director, VSSC and IIST; founding director, HSFC; and an expert in launch vehicle systems, orbital re-entry, and human spaceflight technologies. He is currently the Dr Sarabhai Professor at VSSC)
Published – August 21, 2026 08:30 am IST
