Explained by: Parveen Kaswan ( Author is an Aerospace Engineer and holds a Masters Degree in Engineering Designs from Indian Institute of Science, Bangalore )
As we know India is now ready to become the 1st country in Asia to have a successful Mars mission. On November 5th India, with the help of its work horse Polar Satellite Launch Vehicle (PSLV), put the spacecraft carrying Mars Orbiter into an elliptical orbit around the earth. Here the orbit was successively raised number of times through series of manoeuvres, technically Hohmann transfer method . After getting the required amount of velocity (more than the escape velocity) the slingshot was provided to the spacecraft which put it in Mars Transfer Trajectory on Dec 1st 2013.
Now after some 300 days of journey, spacecraft is near its final destination which is an elliptical orbit around the Mars. In between its road to mars many path correction actions were taken. Now to insert the spacecraft into the orbit of Mars, the velocity of the vehicle to be reduced. This operation is full of complexities. One of them is the behaviour of MOM’s main engine called as Liquid Apogee Motor (LAM), which is off during the whole journey of the vehicle. Now the engine which was idle for more than 300 days in such extreme conditions need to be re-ignited. Though the engine was earlier used in Moon mission and various studies were conducted but it is the idling time which is much more in this mission. It is a 440 Newton capacity engine which uses Unsymmetrical Dimethylhydrazine as fuel and Mixed Oxides of Nitrogen as oxidiser and for the Mars mission special design modifications are done to take care for the complexities involved. Two sets of Propellent lines were provided with the engine, first set is sealed after the first trajectory correction manoeuvre (and the trans Martian insertion). This will prevent the leakage of the precious fuel during the flight.
Now at the time of Mars orbital insertion second propellent lines will be opened. For various Trajectory Correction Manoeuvres eight 22 newton thrusters are also provided. The final trajectory correction will be done on 22nd september with the help of LAM. Though the initial idea was to do it by thrusters only but now we are going to do it with LAM. It will also help in checking the performance of LAM after such a long time and according to that planning for final insertion can be finalised. They will have some 24 hours to modify and re-plan the mission for insertion according to the status report of LAM. For trajectory correction it will burn for somewhat around 4 seconds which will result in supplying a velocity change of 2.142 metres/sec.
Why there is a need to reduce the velocity: Every satellite needs a specific velocity to orbit around a planet, in this case Mars. Now during its journey the spacecraft was in orbit around the sun or in the Hohman Transfer Orbit. The velocity needed to orbit the Mars is less than the velocity during the transfer orbit. To do that we need to reduce the velocity of vehicle and it should be done at a low altitude so that we can also get help form Oberth effect (it is like getting more profit from Kinetic Energy).
The interesting thing is, at the time of insertion signals will take around twelve and half minutes to travel to earth, now every action needs a planning keeping in mind this lag. The whole process will take place other side of Mars which will result in no communication with the spacecraft during the orbital insertion.
The main engine will burn for around 24 minutes and 14 seconds during the insertion which will slow down the vehicle by 1.0987 Km/sec so that it can be captured by Mars orbit. If unfortunately there is some problem in the main engine ISRO is already prepared for an alternate plan which is – it will switch itself to eight 22 newton thrusters so that they can supply as much ‘change in velocity’ as they can. After the 27 minutes and 38 seconds of the LAM burning, Mars Orbiter will reappear for communication, in between there will be no connection with the vehicle. As planned some 40 Kg of propellent will remain for the rest of the mission.
Now the mission has many objectives and most important are:
a)Technological Objectives: to demonstrate the capabilities of India in terms of planning, designing and executing various cutting edge processes involved in a mission like this.
b)Scientific Objectives: exploration of Mars surface with various on board instruments on the orbiter.
And if everything will go as planned, which definitely will go, India will be among the few countries to achieve this milestone. With many cutting edge instruments it will help in finding the presence of methane in the atmosphere of Mars, which will help in further research about presence of life. It will also establish the fact that now India is very well capable in undertaking major space expeditions and it will help in proving capabilities of our national space programme.