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T. S. Subramanian
Mars Orbiter tests have shown our ability to predict: ISRO chairman
After the Polar Satellite Launch Vehicle (PSLV-C25) put India’s Mars Orbiter into a perfect earth-bound orbit on November 5, it has been a smooth journey so far for the spacecraft. The Indian Space Research
Organisation (ISRO) boosted the Mars Orbiter’s apogee in six complex manoeuvres executed between November 7 and 16. ISRO did this by giving commands from the ground to the spacecraft’s propulsion system, called 440 Newton engine, to fire. A crucial event of the trans-Mars injection of the spacecraft will take place on December 1 by a prolonged firing of the 440 Newton engine.
In this context, The Hindu met K. Radhakrishnan, ISRO Chairman, on November 18 in his office at ISRO headquarters, Bangalore, for his assessment of what has been achieved so far in India’s Mars Orbiter Mission, what lies ahead, the complexity of the mission, the spacecraft’s autonomy to take decisions on its own when there is an emergency etc.. Excerpts from the interview with Dr. Radhakrishnan, who is also Chairman, Space Commission and Secretary, Department of Space:
How do you assess what has been achieved so far in ISRO’s Mars Orbiter Mission?
After the launch of the PSLV-C25 on November 5, the separation of the Mars Orbiter from the launch vehicle was smooth and the injection of the spacecraft into the earth-bound orbit was precise. During the last few days, we have been raising the spacecraft’s orbit, specifically its apogee in steps. The first orbit-raising manoeuvre was done in the early hours of November 7. Till now, we have completed six manoeuvres including a supplementary one. Currently, the spacecraft’s apogee is 1,92,915 km.
In the early hours of December 1, around 00.36 hours, we have the trans-Mars injection of our Mars spacecraft. On that day, we are going to use the 440 Newton liquid engine again to impart a delta-v, that is, an incremental velocity of nearly 648 metres a second to the spacecraft and the engine will burn for 1,351 seconds. It is crucial in the sense that we need to give the exact velocity required to take the spacecraft from the earth-orbit, passing through the sphere of influence of the earth which extends up to 9.25 lakh km from the earth, cruise through the long helio-centric phase, then get into the sphere of influence of Mars, and on its arrival near Mars on September 24, 2014, it has to be put into 376 km plus or minus 50 km above Mars at that point of time. On the same day, the next crucial operation of the spacecraft’s Mars orbit insertion has to take place.
When this running of the 440 Newton liquid engine takes place on December 1, we also have eight numbers of 22 Newton control thrusters firing.
What will these control thrusters do?
There are two tasks for them. One is the spacecraft’s attitude control. Secondly, if it is required, they will aid the 440 Newton thrusters to augment its thrust-level. Both the functions will be performed and the Mars spacecraft will then be moving towards the helio-centric orbit. Then on December 11, we plan to have one small firing for mid-course correction of the spacecraft. There may be one more mid-course correction during the helio-centric phase, and subsequently, a fortnight before the spacecraft’s arrival near Mars, there will be one more mid-course correction. So there will be three mid-course corrections between December 1, 2013 and September 24, 2014.
What is the purpose of these mid-course corrections of the Mars spacecraft’s trajectory?
With the velocity imparted to the spacecraft on December 1, 2013, we will have an estimate of its expected position on September 24, 2014. We will be continuously tracking the spacecraft and if there are deviations vis-à-vis the end goal, we will make the corrections. So December 1 will be a crucial operation. The spacecraft’s propulsion system, i.e., the 440 Newton liquid engine, will complete its first phase of operations on December 1. It has to be re-started for its operation on September 24. There is thus a long gap.
How confident are you that you can re-start the 440 Newton engine after it has hibernated in deep space for about 300 days during the spacecraft’s voyage?
We have been using the 440 Newton engine for our Geo-synchronous Satellite - GSAT- missions where the spacecraft’s orbit has to be raised about a week after its launch. In the case of Chandrayaan-1, we had to restart the operation after a fortnight. For that, we had qualified the liquid engine in 2008 to restart after one month.
During the last two years, considering the specific requirements of our Mars Orbiter Mission for re-starting the spacecraft’s 440 Newton engine after it has idled for about 300 days, we had done these two actions. One is we have provided a set of parallel circuits for the propellants’ flow-lines and also provided redundancy in the form of a latch-valve. So what essentially happens is that one portion of the fluid circuit will be closed after December 1. The parallel path will be energised for the operation in September 2014.
Secondly, we had fired the liquid engine in a special test facility established at the Liquid Propulsion Systems Centre at Mahendragiri, Tamil Nadu, for its performance after the re-start. It has been found to be within the specifications. The performance degradation of the engine to restart after such a prolonged period has been only around two per cent and it is well within limits. In the spacecraft’s orbit-raising manoeuvres, during its trans-Mars injection and its insertion into the Martian orbit, the firing of the liquid engine is done in a closed loop mode. Here, a precision accelerometer is used to estimate the incremental velocity added as the liquid engine burns and when the accelerometer gives a feedback that the required incremental velocity added to the spacecraft has been achieved, the burning of the liquid engine is automatically terminated. So, minor variations in the performance of the liquid engine will not matter because we are cutting off its burning based on the delta-v that is achieved. That is why we call it closed loop of firing.
In the absence of such an arrangement, the liquid engine would have been commanded to fire for a given period and any variation in its performance would have resulted in a variation [in the incremental velocity]. But here, what you have to achieve is the incremental velocity. When the programmed incremental velocity is achieved, the engine is cut off.
So what I am trying to convey here is that minor variations in the performance of the liquid engine is not going to affect the mission. If the engine has to work for a few seconds more or a few seconds less, it will be decided by the computer.
In the PSLV itself, we have the closed loop guidance system where the rocket’s fourth stage burning is terminated, based on the conditions achieved. That arrangement is there in the Mars Orbiter’s propulsion system.
Up to 9.25 lakh km from the Earth, the spacecraft will be in the sphere of influence of the Earth. Subsequently, it will be moving into the helio-centric phase of its flight. It is a long one, where you have to look at the influence of other planets and the Moon and then the solar radiation pressure acting on the spacecraft. That pressure varies with respect to time because the geometry of the sun and the spacecraft matters here. This is something we have not done so far and this helio-centric phase of the flight is new to us.
In Chandrayaan-1, we had travelled up to four lakh km, which was well within the sphere of influence of the Earth. But here for the first time, we are moving out of the sphere of influence of the earth. So how the spacecraft will behave during the helio-centric flight of 680 million km along the arc is new to us. Then the spacecraft gets into the sphere of influence of Mars which is nearly six lakh km from Mars.
From our understanding of the Mars gravity model, the influence of the atmosphere of Mars, the influence of the two satellites of Mars and the solar radiation pressure there on the spacecraft are very important. This is also a new thing that we are attempting.
So the navigation of the Mars spacecraft from the orbit of the Earth to the orbit of Mars, passing through all these three phases, is a new knowledge that we are acquiring and validating during the next 300 and odd days.
You have stressed that the centrepiece of our Mars Orbiter is its autonomy. Can you explain how it can take decisions on its own when there are emergencies?
Since long distances are involved in this mission, there will be a delay in the signal from the ground reaching the spacecraft and vice versa. This delay could be of the order of six minutes to 20 minutes one way. In the spacecraft, we have provided redundancies for the critical components and sub-systems. In a normal situation, the ground controllers assess the performance of its systems and give commands from the ground for the switch-over from the primary system to the redundant system [if there is an emergency]. In this particular case, the spacecraft itself has to assess the performance of its systems and this is called Fault Detection, Isolation and Reconfiguration - FDIR.
Secondly, when we need to operate a scientific instrument on board the spacecraft, a chain of commands has to be sent to the spacecraft for reconfiguring it both in terms of its orientation and selection of its various sub-systems and components for the specific payload operation.
Normally, these are sent from the ground. But in the case of the Mars spacecraft, due to the long communication delay, such chains of commands are stored in the spacecraft and they are triggered based on the command from an on-board sequencer. Such commands are, therefore, based on the time-tagged commands sent through the on-board sequencer. You load them well in advance and give the command that at this particular instant, it has to start. If a firing or an operation has to take place at a particular point T, you load the commands well in advance and say that it has to start at this instant.
This is what is provided for in this mission.
The third level of autonomy is to enable the spacecraft to put itself in a “safe mode” in the event of a major anomaly and wait for the commands to be received from the ground. When we say that it has to be put in the “safe-mode”, its antenna should be pointing towards the earth and the solar panels should be in a position to receive the energy from the sun. That means the spacecraft is safe and you can send commands to it. These three levels of autonomy are provided in the spacecraft.
During the orbit-raising manoeuvres which started on November 7 and which went on till the morning of November 16, ISRO has been testing the performance of these redundant systems on the spacecraft and exercising the option of bringing them into operation.
The gyroscopes, accelerometers, star-sensors, and attitude and orbit-control electronics, attitude control thrusters, the FDIR and the thrust augmentation logic, which enables the augmentation of the thrust of the 440 Newton liquid engine by eight numbers of 22 Newton control thrusters during the critical phases of operation, were tested. The termination of the burn of the 440 Newton engine, based on the feedback from the accelerometer, was tested. The on-board sequencer, which is used to store and initiate time-tagged command, was also tested.
All these tests took place during the nine days from November 7 to November 16?
All these were tested during the orbit-raising manoeuvres. The expected orbital parameters have been achieved closely. It shows our ability to predict and we have seen that happen. Currently, the Mars Orbiter is in a highly elliptical orbit with an apogee of 1,92,915 km.
Regarding our preparedness for the spacecraft’s trans-Mars injection on December 1, as on today, we have raised the spacecraft’s apogee to the required 1.9 lakh km. We have raised its orbital inclination and other parameters to the required level. We have tested the spacecraft’s sub-systems and the provision for autonomous operations when required.
During the next ten days, we will be exercising the orbiter’s high-gain antenna and the medium-gain antenna which are required to be used when the spacecraft is far away from the Earth. During the orbiter’s Earth-phase in the coming ten days, we will be energising its scientific instruments to check their health.
What are the preparations under way for the lift-off of the Geo-synchronous Satellite Launch Vehicle (GSLV-D5) in December this year? It was to put GSAT-14 into orbit in August last. The lift-off was aborted then because of the leak of liquid propellants from the rocket’s second stage.
The GSLV-D5 was slated for launch in August. An hour and 15 minutes before the scheduled lift-off, we found a leak in the fuel tank of the rocket’s second stage. The leak was detected in time and ISRO quickly decided to call off the launch and to restore the vehicle. We had the entire restoration process done under the guidance of K. Narayana, former Director, Satish Dhawan Space Centre, Sriharikota. The GSAT-14 communication satellite, which was encapsulated in the heat-shield, had been preserved and tested periodically. The cryogenic upper stage was preserved and tested periodically. The rocket’s second stage has been re-done with a new propellant tank made of aluminium alloy 2219. We had to re-furbish the strap-on booster motors. All the components and parts which had come in contact with the leaked liquid propellant have been replaced. The electronic packages residing in the strap-on stages had to be replaced. The rocket’s first stage, which uses solid propellants, has been replaced.
The vehicle’s assembly began in the Vehicle Assembly Building (VAB) on October 18. The rocket’s first stage has been assembled completely. The four strap-on stages are ready to be assembled and they will be done this week from November 20 to 23. The second stage is also ready at Sriharikota and soon after the completion of the assembly of the four strap-on stages, we will be taking up the integration of the second stage. On December 3, we have the Mission Readiness Review (MRR) meeting. Subsequently, the assembly of the indigenous cryogenic stage will begin, followed by the assembly of the electronic bay, the spacecraft and the heat-shield. So the launch of the GSLV-D5 with the indigenous cryogenic stage is scheduled for December-end.
The GSLV-D5 is on top of our agenda. The Mars Orbiter Mission is not at the cost of our GSLV programme. The GSLV programme is going ahead full steam.