Demystifying the PSLV rocket science

The Polar Satellite Launch Vehicle, popularly known as the PSLV, is one of the highly successful projects of the Indian Space Research Organisation (ISRO), and one that has proved to be commercially viable too. Of the 21 launch missions, including last week’s PSLV C-21, there has been only one complete failure (the first one), and this record is a commendable one for the scale of work and challenges to be faced in space.

Like all science expeditions, space missions too come with a certain degree of uncertainty, countering which is quite a challenge. In the face of these risks, ISRO has gained a reputation as one of the most reliable carriers of artificial satellites and payloads from various countries on its PSLV.

So, how has ISRO achieved this reliability? The PSLVs, like all launch vehicles, are programmed and controlled to launch satellites into their respective orbits with precision. Without precision, satellites will be off their orbits. High-performing and reliable control systems deliver this precision. Control systems are intelligent feedback mechanisms, which, in case of rockets like the PSLV, steer them in space and keep them aligned to their flight path.

Degrees of freedom

To get any moving object to twist and turn on any of its axes, understanding the aerodynamics of the object is very important.

Intelligent aerodynamic design allows slowing down and steering of aircraft within the atmosphere; design elements such as the wings and rudders play an important role in this control. With space rockets like the PSLV, aerodynamic control measures are ineffective due to the absence of atmosphere, and they rely primarily on attitude control by manoeuvring the spacecraft about its centre of mass.

For complete control in space, the three ‘degrees of freedom’ of the spacecraft – pitch, yaw and roll — must be manipulated. Pitch indicates how high or low the rocket should point, yaw is for sideways control on the horizontal plane, and roll is simply the control of roll of the rocket. By controlling these three elements, any spacecraft can be steered to its path precisely.

Control systems

The intelligence of a space rocket lies in its control systems. Control systems use various sensors, such as magnetometers and gyroscopes, to sense physical conditions. Based on information gathered from these sensors, the flight computer initiates corrections in attitude of the craft using different actuating mechanisms.

The PSLV, being a four-stage launch vehicle, uses different control mechanisms at every stage to steer the rocket capsule. The initial take-off stage uses thrust vectoring mechanism, by tilting the engine exhaust nozzle in the required direction. With accurate calibration, thrust vectoring is effective in controlling the pitch and yaw of the rocket. At later stages, another interesting mechanism used for attitude control is the engine gimbal, which enables control of pitch and yaw of the spacecraft.

The roll of the craft is controlled using reaction control systems that deploy small thrusters on the body of the rocket, which coordinate in opposite or same directions as necessary to set the mild rolling of the rocket as it ascends, instilling stability in flight.

The tasks involved in attitude determination and control of systems, such as sensing, sending feedback and actuating the thrusters or other controllers, involve elaborate calculations. These systems are modelled mathematically and are implemented using software programmes on the flight computers. Microcontroller-based attitude determination and control systems with a range of sensors and actuators interfaced to them have made implementing these powerful control mechanisms precise by replacing the less flexible and cruder analogue feedback mechanisms.