A successful recovery

The test conducted over the Pulicat lake signals that the Indian Space Research Organisation is on track to launching the Space Capsule Recovery Experiment, which will enable India to join a select group of countries with the knowhow.

in Sriharikota

AS the helicopter circled at a height of 5 km over the Pulicat lake, off Sriharikota island in Andhra Pradesh, the spectacle that unfolded was stunting. A contraption dropped out of the helicopter and a parachute spread out from it. Moments later, another parachute opened and swayed in the wind. Finally, a third parachute, bright orange in colour, unfolded, and from it dangled a "till-box" (capsule) weighing about 500 kg. As the winds gusted over the vast expanse of the lake, the parachute swayed back and forth, and quietly touched down on the dry lake-bed.The parachute and the capsule were recovered quickly by Indian Space Research Organisation (ISRO) teams.

The air-drop experiment, held on August 19, signalled that ISRO is on track to launching a recoverable satellite next year on board the Polar Satellite Launch Vehicle (PSLV).

The operation began around 6.30 a.m. A number of camera stations belonging to ISRO had been set up on the road from Sullurpeta to Sriharikota, which cuts through the waters of the lake.

As the helicopter took off from Sriharikota, a cameraman's walkie-talkie erupted into life. "The SRE [Space Capsule Recovery Experiment] monitor has been connected to the helicopter. Any moment it can take off," a voice alerted the cameraman. In a few moments, the helicopter was air-borne. It flew in a huge circle over the lake. As it completed the second circle, it dropped the till-box.

This was the third test over the lake and all were successful. The previous two were conducted on June 23 and 28. ISRO had earlier conducted air-drop tests of the capsule several times over land near Agra in Uttar Pradesh. The next recovery experiment, from the sea, is scheduled to be conducted in a few months' time.

THE recoverable satellite is to be called SRE. The satellite will stay in space for many days, and its payloads will be used to conduct micro-gravity experiments. ISRO has selected experiments in quasicrystal growth and biomemetic material synthesis. The SRE is to land in the Bay of Bengal and to be retrieved by Coast Guard ships. It will have ultra-high frequency beacons, floatation systems and water dyemarkers to aid its recovery.

India will join a select club of countries when it launches a recoverable satellite. Only the United States, Russia, the European Union and China possess this frontier technology. A recoverable satellite has to be de-orbited and protected from intense heat when it re-enters the earth's atmosphere. To prevent the satellite from being destroyed, it has to be coated with composites. It should also have closed loop guidance and control systems.

The SRE is to fall into the Bay of Bengal within a specified area. The speed of the descent should be staggered. The three parachutes should deploy with milli-second accuracy.

The SRE will help cut down costs because the systems retrieved can be reused. Besides, they are crucial to ISRO's plans to build a reusable launch vehicle (RLV). A technology demonstrator of the RLV will be flight-tested in 2008. The RLV's booster and upper stages will fall into the sea and will be recovered to be used again. The vehicle itself will taxi to a halt like an aeroplane on the runway of one of the Indian Air Force's (IAF) aerodromes.

ACCORDING to Dr. B.N. Suresh, Director, Vikram Sarabhai Space Centre (VSSC), Thiruvananthapuram, ISRO "got all the data" from the latest test. "All the results were as expected. There was no problem anywhere. The data are promising. We are in the process of getting into the details of sea recovery," he said.

The VSSC designed the capsule and manages the project. The Satish Dhawan Space Centre at Sriharikota looks after the capsule recovery operations. The IAF provides the helicopters. The Aerial Delivery Research and Development Establishment (ADRDE), Agra, provides the parachutes. ISRO's Telemetry, Tracking and Command Network plays an important role as well.

The Pulicat tests were meant to check the terminal phase of the descent. They were qualification tests to check the deployment of the parachutes. "This is basically mission-level testing to check all the critical operations," Suresh said. The critical operations included de-orbiting the satellite through propulsion after its payloads had done their experiments, then making it enter the atmosphere with the closed loop guidance and control systems, and making it soft-land on the sea within a small area. For this, atmospheric and wind disturbances should be contained. "You are basically bringing down the velocity step by step from the orbital velocity to touch-down velocity," Suresh said.

The VSSC engineers connected with the SRE explained that parachutes were an important means of breaking the descent of the satellite. The parachutes form an important component of the terminal phase of the flight from a height of about 5 km. "Our aim is to reduce the speed of the descent from 100 miles [160 km] a second to 12 miles [19.2] a second. The main parachute can land safely on the sea surface at 12 miles a second and can be recovered," said an engineer. To reduce the speed in stages, three parachutes are deployed in sequence. First a small parachute opens, then a drop chute unfolds, and finally the main chute gets into action. When the drop chute is in action, the descent speed of the recoverable satellite is reduced from 100 miles a second to 50 miles a second. The main parachute further reduces it to 12 miles a second to cushion the impact of the landing at sea.

The SRE has four major hardware elements: aerothermo structure; spacecraft platform; deceleration and floatation system, and payloads for conducting experiments in micro-gravity. There will be flight electronics on board the satellite. Triggering systems will deploy the parachutes one after another at precise intervals. The flight electronics will sense the altitude and deploy the parachutes. The parachutes, pyro-devices, avionics packages of triggering units, telemetry and tracking systems, sensors for measuring the altitude and so on are placed inside the capsule (satellite). The floatation system will keep the satellite afloat. Its beacons will signal that it has landed. Dyemarkers will make it visible.