Gaganyaan is India’s maiden crewed space mission, designed to carry Indian astronauts to space and safely return them back to earth. Here, the rocket — Human-rated Launch Vehicle Mark (HLVM) 3 — will inject the orbital module (OM) into the desired orbit. The Gaganyaan astronauts will be onboard the OM.
What does the OM contain?
The OM consists of two sections, the crew module and the service module, connected by a joint. The crew module serves as the crew habitat while the service module provides on-orbit support to the OM.
After the orbital phase, the propulsion system in the service module will fire its thrusters to de-orbit the OM, then the service module will separate from the crew module by severing the joint with a redundant mechanism.
While both modules descend to the earth, the crew module — which is designed to survive the intense thermo-structural loads of re-entry — will decelerate by aero-braking and safely splash down in sea. Meanwhile, the service module will burn up.
Some crewed spacecraft, such as Russia’s Soyuz and China’s Shenzhou, use a three-module configuration. The third module provides extra living and working space for the crew while in orbit. It also houses the docking mechanism, cargo, and basic life-support facilities, including the toilet. Like the service module, this third module will also separate during descent and will be destroyed during re-entry.
Which configuration is best for re-entry?
The crew module design must balance several critical objectives, including maximise the internal volume, manage the aerodynamic lift and drag generated during atmospheric flight, be as easy as possible to fabricate, maintain aerodynamic and hydrodynamic stability, and stabilise the module dynamically at low speeds.
Because no single configuration can satisfy all these requirements simultaneously, the final shape of the crew module will depend on which design attributes are prioritised. To minimise launch and re-entry mass, engineers strip the crew module down to essential landing systems, directly reducing the required size and mass of both the heatshield and the parachutes.
A spherical configuration offers the highest possible internal volume with the lowest structural mass. This is because a sphere has minimum surface area required to enclose a given volume. The design closest to a sphere was the Soviet Union’s Vostok module, in which Yuri Gagarin made the historic first trip to space.
However, because a perfect sphere creates no aerodynamic lift, it falls straight down through the atmosphere, like a dropped stone. This subjects the crew to painfully high g-forces.
Consequently, a sphere-cone combination is the preferred configuration for a re-entering body. Its blunt base generates a detached shockwave that pushes intense frictional heat away from the spacecraft, while its conical body provides the aerodynamic stability and lift necessary for a controlled, survivable descent.
The Gaganyaan crew module has a sphere-cone configuration.
Why aren’t all re-entry modules mono-stable?
Aerodynamic and hydrodynamic mono-stabilities are highly preferred features for a re-entering body.
A module is aerodynamically monostable if it maintains only one stable attitude while flying through the atmosphere — like a shuttle cock. Similarly, hydrodynamic mono-stability ensures the module will self-right itself and float in a single, stable orientation after splashdown.
Mono-stability is controlled by the module’s aerodynamic shape and the location of its centre of gravity. As the centre of gravity is dictated by the packaging of internal subsystems, system engineers often lack the flexibility to achieve the required location of the centre of gravity.
As a result, in practice, modules frequently have more than one stable orientation. The Gaganyaan crew module, for instance, has two distinct stable aerodynamic positions as well as two stable hydrodynamic positions.
The undesired attitude is managed by firing control thrusters during flight through the atmosphere and by deploying a gas-based up-righting system upon splashdown in sea.
What is dynamic instability during re-entry?
Dynamic instability is a critical condition experienced by a re-entry module, resulting in rapidly growing and uncontrolled oscillations as it decelerates through the atmosphere. Just like a traditional kite without a tail wobbles and spins wildly out of control because it lacks stability, the crew module can experience dangerous, self-growing swings that make it tumble if it is not controlled.
How smoothly the crew module falls through the atmosphere depends strongly on its shape, mass, and the way air flows around it. The module shakes and wobbles the most as it approaches the speed of sound, where bouncing shockwaves and swirling air violently whip it around. To stop the module from spinning dangerously out of control, it must either use small control thrusters to steady itself or deploy parachutes before this instability grows too large.
Ultimately, balancing all of these competing traits, from its blunt aerodynamic shape and mass to its fight against dynamic instability, is what transforms the module design into a masterfully engineered lifeboat capable of bringing astronauts safely back to the earth.
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. Currently Dr Sarabhai Professor at VSSC.
Published – July 15, 2026 09:15 am IST
