A descent to success

ISRO's very first attempt to bring back an orbiting satellite to the earth turns out to be a success.

A TENSE atmosphere, mixed with expectation, prevailed at the Spacecraft Control Centre (SCC) of the ISRO Telemetry, Tracking and Command Network (ISTRAC) at Bangalore on January 22 morning. Equally tense was the atmosphere at the Mission Control Centre (MCC), a few hundred kilometres away, at Sriharikota. It was for the first time in the history of the Indian Space Research Organisation (ISRO) that its engineers were managing an event of this magnitude - bringing back a satellite that they had put in orbit 12 days earlier with the help of a Polar Satellite Launch Vehicle (PSLV-C7) from Sriharikota. The shuttle-cock shaped satellite, called the Space Capsule Recovery Experiment (SRE-I), was to splash down in the Bay of Bengal, about 140 km east of Sriharikota and the Coast Guard was to recover it.

The tension gave way to joy and applause erupted when the first manoeuvre of those controlling the SRE-I, which was in space at an altitude of 635 km, had ended in success. The satellite was perfectly oriented. The thrusters aboard the satellite were fired to de-boost it at 9.00 a.m. as it passed over the ground station at Saskatoon in Canada, which had a braking effect on the satellite's velocity. After the thrusters finished their firing, commands were beamed to the satellite from the SCC to re-orient it so that when it re-entered the earth's atmosphere, it would encounter least resistance.

As the SRE-I decelerated and started plunging towards the earth's atmosphere, it coasted on a course in an arc of about 18,000 km, which took it halfway across the globe. It moved from the Equator to the North Polar region. Then the SCC steered it towards the south, and as it passed over ISRO's ground station at Lucknow, its re-entry phase began at an altitude of 100 km at 9.37 a.m. As expected, a communication black-out ensued from the SRE. At the end of the blackout, ground stations started receiving telemetry signals from the satellite again.

The satellite's velocity was about 29,000 km an hour at the time of re-entry. It was protected from the intense heat generated by the friction with the atmosphere by the carbon phenolic ablative material and silica tiles on its outer surface. The nose part of the satellite experienced a scorching 2,000{+0} Celsius, and the rear 1,400{+0}Celsius. As the capsule dived deeper towards the earth, its velocity increased but its shuttle-cock shape took care of its descent. When it came over Sriharikota at a height of five km over the Bay of Bengal, its orange-coloured pilot and drogue parachutes deployed with precision at the fixed time one after another. At this height, the satellite's aerodynamic braking had considerably reduced its velocity to 363 km an hour. The deployment of the pilot and drogue parachutes further reduced its velocity to 170 km an hour. As the main parachute, orange in colour, opened at 2 km above the Bay of Bengal, the satellite slowly came down and gently splashed down in the waters at 9.46 a.m. Its velocity then was 43 km an hour. Immediately, a triggering device on board the satellite activated its flotation system, which ballooned up like a bulb. The flotation system kept the 555-kg SRE-I, made of mild steel, afloat.

The mission was not yet fully over and ISRO top officials were circumspect. The Coast Guard personnel had to locate the SRE-I and recover it. Soon after the Coast Guard received word that the satellite had touched down on the waters, a helicopter took off from the Coast Guard vessel "Sarang", which was anchored many kilometres away. The "Sarang" had 16 ISRO personnel and divers. About 30 minutes after the SRE-I landed, the helicopter spotted it bobbing in the waters. The helicopter did some dye-tests to see whether there was any harmful fallout from the SRE-I. Then the divers went into the waters and took underwater photographs to see whether the satellite was safe. It was later towed and put on board the Sarang.

An ISRO engineer, who did not want to be named, said: "Nothing symbolises the ascent of India in science and technology as the return of the SRE-I." ISRO chairman G. Madhavan Nair called it "fantastic" and "a historic moment for ISRO." B.N. Suresh, Director, Vikram Sarabhai Space Centre (VSSC), Thiruvananthapuram, said: "The accuracy with which the SRE-I returned to the earth was unbelievable because it came down within 15 km downgrade and less than six km laterally." Madhavan Nair pointed out that the deviation in landing was just 20 km from the designated area.

Suresh pointed out that the entire mission, although complicated on several counts, "was an impeccable success". First, it was a multi-mission because on January 10, the PSLV-C7 had lifted off with four satellites, including two which belonged to India. The others were Cartosat-II and the SRE-I. LAPAN-TUBSAT, jointly built by Indonesia and the Technical University of Berlin, and Pehuensat-I of Argentina. This was the first time that ISRO orbited four satellites with a single rocket.

Although ISRO had orbited three satellites each in its PSLV missions in 1999 and 2001, the four-in-one mission was difficult because four of the two satellites were heavier. So the PSLV-C7 used a device called the Dual Launch Adopter to put in orbit the four satellites and all of them went into accurate orbits one after another at a height of 635 km each. Cartosat-II, the LAPAN-TUBSAT and Pehuensat are working well. Cartosat-II has sent down some "beautiful images" of India.

But the real challenge lay not only in building the SRE-I but bringing it back safely to the earth. The magnitude of the challenge could be understood from the fact that the first 12 Discoverer missions of the North American Space Administration (NASA) in 1959 and 1960 to bring back the satellite or recover the films on board the capsule failed.

Only the 13th Discoverer mission was a success. But ISRO succeeded in its very first attempt to bring back an orbiting satellite. Only the United States, Russia, the European Union and China possess this technology.

What is the significance of the success of the SRE-I? It is the first stepping stone towards India's plans to send an Indian into space and bring him/her back safely to the earth. If an astronaut or satellite were to be brought back safely to the earth from space, a country must master the re-entry technology. After the SRE-I success, Madhavan Nair said, "This is a humble step towards sending an Indian into space."

The technology of composites that protect a space capsule on re-entry is a closely held secret. Besides, the satellite should re-enter the atmosphere at the right angle. The India-made thermal tiles pasted on the outer surface of the SRE-I worked so well during the re-entry phase that Dr. Suresh remarked, "The module looked as fresh as it was when we gave it for integration with the PSLV for the launch." Thus, the SRE-I is a forerunner to ISRO mastering the re-entry technology.

It is also a pre-cursor to recovery technology. Recovering the satellite after it falls into the sea is an art in itself. Several drop-tests were conducted on land at Agra, in the Pulicat lake, off Sriharikota island and then in the Bay of Bengal to learn the technology. The Coast Guard trained with ISRO for a few years in recovering the "till boxes" from the Pulicat lake and the Bay of Bengal. After the Coast Guard recovered the SRE-I, the ISRO chairman said: "People call the mission a failure unless the capsule is recovered."

The SRE-I is a technological forerunner to India building a reusable launch vehicle (RLV), on which the VSSC is already working. An RLV, like the U.S. space shuttle, reduces the cost of space transportation because the systems recovered can be re-used. The VSSC is working on an RLV technology demonstrator. The RLV's booster and upper stages will jettison into the sea from where they will be recovered and used again. The vehicle itself will taxi like an aircraft on the runway of one of the stations of the Indian Air Force.

The SRE-I tested a host of other technologies that included navigation, guidance and control systems, hypersonic aero-thermo dynamics, management of communication black-out, deceleration and flotation systems.

Two experiments were conducted on board the SRE-I with the help of payloads. They related to growth of crystals and metals in micro-gravity.

Several organisations came together to make the mission a success. The VSSC and the ISRO Satellite Centre, Bangalore, built the SRE-I. The SDSC, Sriharikota, was in charge of the recovery operations. The Aerial Delivery Research and Development Establishment, Agra, provided the parachutes. The SRE was tracked, its health monitored and commands given to it from ISTRAC, Bangalore, and other ground stations at Lucknow, Mauritius, Biak in Indonesia, Bearslake in Russia, Saskatoon in Canada, Svalbard in Sweden and Transo in Norway. The parachutes, the pyro-devices, avionic instruments for triggering the deployment of parachutes and the flotation system, sequencer for making the parachutes open with split-second accuracy, telemetry and tracking systems were housed inside the satellite.