The Hindu : `Space centre explosion occurred during most critical operation'

Online edition of India's National Newspaper
Wednesday, Feb 25, 2004

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National

`Space centre explosion occurred during most critical operation'

By T.S. Subramanian

CHENNAI, FEB. 24. The fire and the resultant explosion on Monday at the Satish Dhawan Space Centre, Sriharikota (SHAR), which claimed six lives, took place during "the most critical operation" after the solid propellant was cast inside the test rocket motor chamber, according to a top engineer of the Indian Space Research Organisation (ISRO).

"Any spark, any energy source or friction" during this "post-curing operation" could have led to the catastrophic fire and the explosion, he said.

The fire erupted in Building 117 in a huge facility called the Solid Propellant Booster Plant (SPROB) where the solid propellant, hydroxyl-terminated poly butadiene (HTPB), is manufactured. When this solid propellant burns in a rocket engine in flight, or catches fire by accident, it produces a temperature of about 3,000 degrees centigrade.

It is from SHAR that the ISRO's launch vehicles such as the Polar Satellite Launch Vehicle and the gigantic Geo-Synchronous Satellite Launch Vehicle are launched.

The ISRO's erstwhile SLV-3 and the Augmented Satellite Launch Vehicle were also launched from SHAR.

The engineer said the most important ingredient of HTPB, ammonium perchlorate — which resembled the common salt — was a highly explosive material. The explosion has brought down Building 117, with the roof caving in and the cranes inside reduced to molten metal.

"The post-curing operation is the most critical operation. Casting and curing were over. They were trying to transport the cast segment [rocket motor] from that building to another when the accident happened," the engineer said.

The interior of the rocket motor chamber is insulated with rubber or other material. When the propellant burns in the motor, that is when the engine keeps firing and temperatures ranging from 2,500 degrees centigrade to 3,000 degrees centigrade are generated, it does not reach the metallic outer wall of the motor because of the insulation. The engineer said, "If the temperature reaches the metallic portion of the rocket motor wall, the story is over. But you say [that the accident took place] when the finished rocket motor was being transported from one building to another. If that is the case, it is unusual. But it does not preclude the possibility [of an explosion] because the whole business is hazardous. The accident is unusual but it can happen."

The sprawling SPROB was commissioned at Sriharikota in March 1977. A few hundreds of its facilities are spread over 2,500 hectares. The processing facilities of the complex are located on an elliptical road. SPROB was designed and built with indigenous knowhow.

Apart from ammonium perchlorate (oxidiser), the solid propellant consists of fine aluminium powder (fuel) and hydroxyl terminated poly butadiene (binder). All these materials are produced indigenously.

When these chemicals are mixed, they form into a viscous slurry which is filled under vacuum conditions into the rocket motor chamber in the cast/cure facility.

Prior to this, the motor case is prepared with a thermal insulation liner.

The curing agents are added to the slurry and it forms into a solid mass. Hence, it is called a solid propellant, as distinct from a liquid propellant.

The rocket motor then undergoes non-destructive testing (NDT) using high-energy X-ray machines to detect flaws such as cracks, voids and so on.

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