On the morning of September 24, a liquid-propellant engine aboard India's Mars Orbiter will need to fire so that the spacecraft can go into orbit around the Red Planet.
This engine was originally developed by the Indian Space Research Organisation (ISRO) for its communication satellites.
The engine, known as the Liquid Apogee Motor (LAM), is carried on such satellites so that they can be moved from the temporary orbit in which launch vehicles place them to the appropriate operational location.
Time and again, the LAM has performed its crucial task efficiently. It also propelled India’s Chandrayaan-1 lunar probe to the Moon and hurled the Mars Orbiter beyond Earth’s clutches.
Such a liquid-propellant engine was first considered when ISRO decided to build an experimental communication satellite, APPLE (an abbreviation for the ‘Ariane Passenger Payload Experiment’). The Europeans had offered to carry this satellite for free on their newly-developed Ariane launch vehicle. However, in the end, a solid-propellant motor derived from the fourth stage of India's first launcher, the SLV-3 was used for APPLE’s orbit-changing operation. (APPLE went into orbit in June 1981).
Nevertheless, a liquid-propellant engine, producing 50 kg of thrust, was developed and tested on the ground, according to K. Sivaramakrishnan Nair, who was with ISRO at the time and led the early development of the LAM. The engine ran on red fuming nitric acid and hydrazine, with pressurised gas pushing the propellants from their tanks into the thrust chamber where they burn.
But when it came to the Insat-2 satellites, the first operational communication satellites to be designed and built within the country, ISRO wanted a liquid-propellant engine. Such an engine can be shut off once the spacecraft achieves the desired velocity and fire several times if required, allowing a change of orbit to be achieved with considerable precision. A solid-propellant motor, on the other hand, once fired, will continue to operate till the propellants are exhausted.
The U.S.-built Insat-1 satellites used a variant of a liquid-propellant engine that originally been developed for the Apollo programme which landed humans on the Moon. ISRO decided to develop a similar sort of engine and propulsion system for Insat-2 satellites as well.
The Indian LAM took about 8 years to develop, according to Mr. Nair. Several years went into establishing a suitable high-altitude test facility needed to demonstrate the engine's satisfactory operation in the airless conditions of space.
The engine generates a thrust of about 45 kg. Pressurised helium gas drives the propellants, monomethyl hydrazine and a nitrogen tetroxide mixture, from the tanks to the thrust chamber.
Another important development that took place was of a smaller liquid-propellant engine, known as a thruster. A spacecraft carries several thrusters that are typically fired in short bursts to correct its orientation and any drift in the orbit.
The thruster developed for the Insat-2 satellites produces around 2 kg of thrust. As with the Insat-1 series, a unified propellant system feeds the LAM as well as the thrusters.
When this propulsion system went into space for the first time aboard the Insat-2A in July 1992, a malfunction tested the nerves of ISRO’s scientists and engineers assembled at the Master Control Facility in Hassan in Karnataka.
A valve that was supposed to regulate the pressure of helium gas flowing into the propellant tanks failed. The gas pressure in the tanks soared, raising the spectre of a catastrophic explosion that would have destroyed the satellite.
The only option was to fire the LAM for a prolonged period, one of the senior scientists at Hassan later told this correspondent. But no one knew if the liquid engine would take the higher temperature produced by more propellants than usual burning in the thrust chamber. To their immense relief, the engine worked without a hitch, thereby saving the mission.
Since then, this liquid engine and associated propulsion system has been used in over two dozen ISRO-built spacecraft, including its communication, weather and navigation satellites.
The same system was adopted for the Chandrayaan-1 lunar probe and subsequently for the Mars Orbiter Mission. On the Chandrayaan-1 launched six years back, the engine was operated ten times in the course of three weeks to take the spacecraft from Earth orbit to its operational path around the Moon.
In the case of the Mars probe, the engine was fired seven times to propel the spacecraft beyond Earth’s gravitational hold. But after putting the spacecraft on course to Mars on December 1 last year, the engine had to remain idle for nearly 300 days. During such a lengthy coasting phase, valves in the propellant lines that were exposed to propellants might leak and not function properly.
ISRO has therefore put a second set of propellant lines for the liquid engine. One set of propellant lines was closed off after the engine fired to take the spacecraft out of Earth orbit. The other set will be opened when the time comes for the engine to fire again later this month.
Tests had been carried out on the ground to make sure that the engine could be restarted after such prolonged coasting, according to the ISRO chairman, K. Radhakrishnan. To check the engine's performance, it will be fired for about four seconds on September 22.
Then, on September 24, the engine will be fired for around 24 minutes to reduce the spacecraft's velocity by about 1.1 km per second and place it in orbit around Mars, Dr. Radhakrishnan told this correspondent.
