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Cryogenic stage full-duration test on January 19

T.S. Subramanian

This test at Mahendragiri will confirm India's position in the global league possessing a highly complex technology

The indigenously developed cryogenic engine at the Liquid Propulsion Systems Centre of ISRO at Mahendragiri in Tamil Nadu. — Photo: A. Shaikmohideen

Chennai: Standing tall, strapped to a towering Main Test Stand in the midst of jungles is India's very own cryogenic stage at the Liquid Propulsion Systems Centre (LPSC) at Mahendragiri, 30 km from Nagercoil town in Tamil Nadu. When this indigenous cryogenic stage fires for 720 seconds on January 19, pouring forth flames and shaking the Western Ghats around, it will confirm India's position as the sixth country to possess this highly complex technology. The others which already have it are the United States, Russia, Europe, Japan and China.

The firing is called a full-duration test because the Indian Space Research Organisation's Geo-synchronous Satellite Launch Vehicle (GSLV) with a cryogenic stage will also fire for 720 seconds in actual flight before injecting a satellite weighing more than 2,500 kg into an orbit of 36,000 km by 180 km.

ISRO Chairman G. Madhavan Nair called the cryogenic technology ``highly complex'' and ``exotic.''

A big jump

The LPSC engineers, led by Director R.V. Perumal, are looking forward to the 720-second test, which will mark a big jump from the 50-second successful firing done on October 28, 2006. If the test is successful, a GSLV with India's own cryogenic stage will lift off from Sriharikota this year itself. It will make the country totally self-reliant in every department of rocket technology: its launch vehicles can put any satellite in any type of orbit.

India's dependence on foreign launchers to put satellites weighing more than 2,000 kg in orbit will stop. The four GSLVs launched from Sriharikota so far carried Russian cryogenic stages.

A series of tests leading to the full-duration test has been completed at Mahendragiri. This includes trial filling of cryogenic propellants, controlling and monitoring their pressure, and integrating the cryogenic stage with the equipment bay in flight configuration.

Mr. Perumal said: "We have completed the development of the cryogenic engine and we have now taken up the qualification of the cryogenic stage. This cryogenic stage is totally ours in design and development of the engine with tanks, all control components for pressurisation, filling of cryogenic propellants and other operations."

Four cryogenic engines were already tested at Mahendragiri for a cumulative 6,000 seconds. "What we are basically trying to do is to fly [test] an indigenous cryogenic stage which has 12 tonnes of propellants and can produce a thrust of 7.5 tonnes." A cryogenic engine is powered by cryogenic propellants — liquid hydrogen at minus 252 degrees Celsius and liquid oxygen at minus 183 degrees Celsius. A cryogenic engine is at the heart of a stage. It is the engine that gives the thrust to the stage to fly. A cryogenic stage consists of not only the engine but also control systems, intricate wiring and electronic equipment.

Multidisciplinary task

Development of a cryogenic stage is a multidisciplinary task. Handling, storing and pumping the cryogenic fluids at these very low temperatures call for advanced technology because they are extremely volatile. If they are stored in tanks or pumped through pipes made of ordinary metals, the metals will become brittle. The lubricants will solidify. So, the LPSC developed new alloys to store liquid hydrogen and liquid oxygen and pump them through pipes. But there was another problem.

The pipes ferrying the cryogenic fluids were made of different metals and had to be welded together. Thus, pipes made of aluminium and stainless steel, aluminium and titanium, copper and nickel were fused. "Many dissimilar metals have to be permanently joined together," Mr. Perumal said.

How demanding and intricate the welding was could be gauged from the fact that the diameter of these pipes ranged from just six to 42 mm!

Hydrogen, being light, easily leaks. The bimetallic joints, therefore, called for ingenuity. The ISRO went for welded joints instead of bolted joints, which are used for liquid propellants.

According to B.N. Suresh, Director, Vikram Sarabhai Space Centre (VSSC), Thiruvananthapuram, testing a cryogenic stage on the ground is more difficult than a test in flight because of atmospheric pressure on the Earth. "Apart from the complexity of the propulsion system, you must make sure that all the indigenous elements work harmoniously."

Although four engines had been tested for a total of 6,000 seconds, many new elements came into play when it came to a stage test.

The VSSC has developed a "next-generation electronic system" for the cryogenic stage and the full-duration would qualify it. The VSSC has also developed an advanced mission computer for the stage and "our own chip for the indigenous computer," Dr. Suresh said.

Facing a challenge

In January 1991, the Soviet space agency, Glavkosmos, reached an agreement with the ISRO to supply not only cryogenic engines but also transfer technology. However, the U.S. administration, quoting Missile Technology Control Regime rules and claiming that cryogenic engines could be used to launch missiles, arm-twisted Russia in 1992/93 not to transfer the cryogenic technology. The ISRO, then, accepted the challenge of developing the technology on its own.

Mr. Perumal said: "... We went ahead and made our own engines. It was a challenge. As we started testing the engines, we started learning more and more, and we did a lot of analysis in the modelling of the engine and its process. At the end of it, we have our engine today."

According to Mohammed Muslim, Project Director, Cryogenic Upper Stage Project, LPSC, the ISRO did even better than the Russians in predicting the engine's control requirements. "Our deviation was just one-fourth of the Russians," he said. Perumal chipped in: "It shows we have our own feel for the engine."

Machining of impellers was difficult, Mr. Muslim said. Very intricate castings were fabricated. ISRO engineers came up with pyrovalves at low temperatures and special insulation material with multilayers.

While the ISRO had some prior information on the engine, it had to start everything from scratch for the stage.

Cryogenic technology is "basically a difficult technology" because the propellants have to be stored at very low temperatures and "so there must be appropriate choice of materials" (alloys) to keep them in tanks or pass them through pipes. These propellants tend to absorb the heat in the atmosphere and their bulk goes up. Their pressure increases. They evaporate very fast. Before the propellants are pumped into pipes, all hardware must be conditioned and brought to the same temperature as that of the propellants. The tanks and the engines should be chilled before ignition. Special ignition systems should be developed for liquid hydrogen and liquid oxygen.

The ISRO has fabricated tanks made of high-strength aluminium alloy to store the cryogenic propellants. These tanks weigh 100 kg less than the Russian tanks. Ninety-five per cent of the materials indigenised came from MIDHANI, Hyderabad.

A. Gnana Gandhi, Director, Directorate of Quality Assurance and Reliability, ISRO headquarters, Bangalore, pointed out that even when the engine was firing, it had to be kept at a low temperature and an ingenious way of regenerative cooling was employed. In this process, liquid hydrogen was made to pass through small orifices drilled in the nozzle valves. Mr. Gnana Gandhi called it "a complex bracing technology development."

Development of booster high-speed turbo-pumps for liquid hydrogen was another big challenge. As hydrogen has low density, the turbo-pumps have to operate at very high speeds — at 40,000 revolutions a minute. In other words, the pumps developed at the LPSC rotate 700 times every second.

Leak monitoring device

Since they rotate at such phenomenal speed, proper cooling techniques have been invented keep the bearings cool. As hydrogen leaks easily, and flames are not visible when it burns, the LPSC built its own leak-monitoring devices.

They have been installed all round the Main Test Stand.

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