While the MOM spacecraft “Mangalyaan” is in orbit around Earth, its systems and scientific instruments are being tested to prepare for the critical burn at 0036 IST on 1st December (1906 GMT 30th November) to inject it into a Mars trajectory. The colour camera has been turned on Earth during testing and ISRO has released this photograph.
The next critical burn (the seventh) has to to use the 440 Newton liquid engine again to impart a delta-v of nearly 648 m/s and the engine will have its longest burn time of 1,351 seconds. Three mid course corrections on the spacecraft’s journey to Mars are planned for and the fuel carried has to also cater for finally braking to enter into Mars orbit.
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.
…. 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.
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.
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.
……. 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. ……
Up to (925,000) 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 (400,000) 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.
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