THE precise and flawless manoeuvre with which the Indian Space Research Organisation’s (ISRO) Mars orbiter was eased into its designated orbit around the red planet has surprised even ISRO scientists. This is evidence of the meticulous preparation, checking, extensive simulations and ground-testing of all the on-board systems including the main thruster engine 440-Newton Liquid Apogee Motor (LAM), and the accuracy of the command algorithms and the associated software that has gone in to give the satellite complete on-board autonomy of operations in this crucial phase.
The achievement is even more remarkable because, notwithstanding its modest mission objectives and corresponding relatively small-sized spacecraft, such a complex mission was put together in just 18 months. The mission objectives required the development of 22 new software modules, modification of 42 modules and usage of 19 existing modules, a process that began only in November 2012 after the project was sanctioned in July.
This total autonomy was essential to the mission given the Earth-Mars distance of about 224 million kilometres at the time of Mars capture to a maximum of 375 million km after six months. This means there would be a communication delay of 25 to 42 minutes given the time that an electromagnetic signal will take for its round trip. So any real-time intervention would be impossible. Further, the geometry of Mars, the earth and the satellite during Mars Orbit Insertion (MOI) was such that the manoeuvre would have to be performed when there was an occultation of the satellite by the planet when the satellite would be out of visibility for ground systems and there would be a total communication blackout.
Complete autonomy means that all the commands that are uploaded into the on-board computer are time-tagged and are sitting in the spacecraft’s command processor, and any major problem in the ground-to-satellite link will not affect the firing, pointed out K. Radhakrishnan, Chairman, ISRO. Spacecraft autonomy is also essential during the lifetime of the satellite because, as the satellite goes round in its Martian orbit, eclipses, whiteouts and blackouts will recur. “We essentially had a suitcase model of the satellite system on the ground and whatever operations are to be carried out in space have been performed and tested on the ground. The ground tests involved as many as 120 parameters,” Radhakrishnan said a week before the MOI firing, on September 24.
The final Martian orbit achieved by the MOI manoeuvre on September 24 has a 427-km periapsis (shortest distance from Mars) and a 76,993.6 km apoapsis (the farthest distance) as against the planned 423 km × 80,000 km orbit. “This is, in fact, a better orbit than what we had aimed for, because a smaller apoapsis orbit is more stable,” pointed out Koteswara Rao, ISRO’s Scientific Secretary. “But these orbit parameters are based on measurements on points on only a part of the orbit. We will have the exact parameters after it completes one full orbit,” he cautioned.
But, more significantly, the burn duration of the main thruster LAM during the MOI was reduced from the 24.14 minutes planned earlier to 23.15 seconds following the successful test of LAM firing on September 22 after the spacecraft had entered the sphere of influence (SOI) of Mars’ gravity. This test-firing, which was for a very short duration of four seconds, consuming only about 0.5 kilograms of the fuel, not only obviated the need for Plan B (which would have involved firing only the eight 22-N smaller thrusters for a longer duration) but also reduced the actual firing duration required for the MOI.
The final burn duration was only 23.08 seconds, a difference of nearly a minute from the planned duration. This duration is actually determined by the on-board accelerometer itself, which shuts off the engine automatically once the required change in velocity—actually a braking velocity to slow down the spacecraft to enable its capture into the Martian orbit—is realised. As against the targeted 1,098.7 m/s, the operation achieved a velocity change of 1,099 m/s. That is indeed an amazing precision. A difference of one minute in the burn time also means a significant gain in terms of the on-board fuel saved.
Before the MOI, the quantity of effectively available on-board fuel was 281 kg, of which about 250 kg was expected to be consumed during the LAM firing. But that difference of one minute has meant a fuel saving of about 10 kg and this can, in principle, increase the spacecraft life beyond the targeted six months. “We achieved a completely unexpected efficiency of 99.6 per cent in the LAM performance during the test firing,” Radhakrishnan said. “Normally, one does expect a performance degradation of about 2 per cent when you restart after leaving it idle for as long a duration as 300 days. Even our simulations had indicated that. But to our surprise, we got such high efficiency that we decided to reduce the burn time during the MOI,” he said. And even the final firing seems to have gone off with equal efficiency.
The idea of the September 22 test firing itself was quite innovative. It was actually a two-in-one operation: one to carry out a trajectory correction manoeuvre (TCM) of bringing the altitude of the final orbit down to the designated value of around 500 km from the 720-odd km that the spacecraft would have achieved if this firing had not succeeded and the spacecraft had gone along in its trajectory; two, to test the performance of the main engine for the crucial D-day operation. This TCM, which was otherwise scheduled to be carried out on September 14, was not done with this two-birds-with-one-stone operation in mind. You could argue that that there was a risk of not getting the correct altitude if the LAM had failed in the test. But, if the LAM had failed, in any case an optimum orbit with thrusters alone would not have been possible. So why not this? So went the scientists’ logic and it was indeed remarkable thinking. As Radhakrishnan pointed out, the most crucial firing for the orbiter was the Trans-Mars Injection (TMI) manoeuvre. At TMI, the route taken, with four TCMs using the small thrusters, was projected to take the spacecraft to about 500± (50-60) km away from Mars (the periapsis). The four TCMs that ISRO had originally planned were: TCM-1, which was performed on December 11, 2013; TCM-2 scheduled for April; TCM-3 for August and TCM-4 for September.
But ISRO did not have to do the TCM-2 in April because it was felt that the spacecraft trajectory was steady and did not need any correction at that time. TCM-2 was subsequently done on June 11. TCM-2 was also important from another perspective. Except for the engine itself, this manoeuvre had all other aspects involved in the MOI manoeuvre: reorienting the spacecraft, loss of telemetry, and so on. The question then was whether TCM-3 in August was required. It was found that there was no need for it because, without TCM-3, the trajectory was going to be about only 720 km away from Mars. Finally, TCM-4 was reconfigured to be a twin operation with the LAM itself instead of the thrusters. The success of TCM-2 already had given confidence to ISRO scientists that the MOI could be performed without any problem if LAM worked. And the modified TCM-4 proved that LAM will work more efficiently than they had imagined.
Providing a parallel circuit of flow lines for the propellants, a feature not used in other inter-planetary spacecraft—which may have been the reason for the high rate of failures in such missions—was really an innovative solution to a potentially serious problem. The ground test of the D-day firing with the propulsion parameters on D-day—the tank ullage volume, the pressure, the temperature, and so on—and with a new set of flow lines but with the main engine that had gone through some firings, had been conducted successfully for nearly 2,000 seconds, which was more than the targeted duration of about 1,500 seconds for actual firing. This test, which was done only in August, had also indicated that the engine should work after being idle for 10 months, giving the scientists added confidence in its success.
One of the important elements in propellant flow is the pressure in the fuel and oxidiser tanks. Basically, the pressure in the tanks should last the entire operation, MOI and the additional minor on-orbit corrections to the satellite later during its lifetime.
The pressure should not fall below a critical value of 11-12 bar. [One bar is about one atmospheric pressure.] There is a pressurisation system—including the pressurant tank (helium under very high pressure) and the two pressure regulators—sitting above the engine to regulate the pressure in the propellant tanks, which normally come into the pictures only if the pressure falls below 11 bar. But an unexpected leak in one of the pressure regulators required the pressurisation system to be completely isolated from the engine. This was a well-thought-out decision because the ground tests for full 2,000 seconds had shown that the engine performed well with the pressure (of about 16.5 bar) available in the tanks as measured before the test firing without requiring to bring the pressurisation system into play. Both the test burn and the MOI burn firing were therefore performed in the so-called “blow down” mode (in which the pressure is allowed to drop naturally as the engine consumes the propellant). The final pressure as measured after the MOI firing was 11.5 bar in the fuel tank and 12 bar in the oxidiser rank, which, according to Koteswara Rao, are more than optimum for the minor on-orbit corrections that will be required during the mission lifetime of six months.
Camera switched on Only the on-board colour camera has been switched on now after MOM entered the Martian orbit. According to Radhakrishnan, these pictures have been compared with archived pictures taken from lower altitudes by the Mariner and Viking missions of NASA. “They compare well,” he said. The other instruments will be calibrated and checked over the next one week to 10 days one by one, according to Koteswara Rao. Science will begin after that with appropriate instruments being switched on when required, depending upon the altitude, illumination condition, and so on. “Our scientists are right now discussing what should be the schedule of operations for observing the comet Siding Spring on October 19,” said Radhakrishnan.
The success of the mission so far has proved beyond doubt ISRO’s capability in building reliable systems that can endure journeys of over hundreds of millions of kilometres in hostile space environment and its deep-space communication and navigation capabilities on inter-planetary scales. Innovation and ingenuity in the conceptualisation and designing of the on-board systems have also contributed greatly to the success. And to have been able to achieve this in its maiden attempt when other, more experienced, space-faring nations failed is indeed commendable. With this achievement, ISRO will command more respect in the world of space technology and industry.
