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Mars Orbiter Mission is on stable trajectory: ISRO chief

R. Ramachandran caught up with the ISRO chairman Dr. K. Radhakrishnan ahead of the crucial manoeuvre of Mars Orbit Insertion (MOI) of ISRO’s Mars Orbiter Mission (MOM)

Excerpts from the interview:

What is the current state of preparation for Mars Orbit Insertion (MOI) manoeuvre on September 24?

On September 14th and 15th we uploaded the commands for the MOI. During the uploading process we verified them and after uploading everything we again went through the reading of all those commands. These are the commands for Plan A of the MOI manoeuvre. The idea is that all the commands are sitting in the spacecraft’s command processor and any major problem in the ground to satellite link will not affect the firing. The commands are time-tagged.

Plan A involves the main thruster engine, the 440 Newton Liquid Apogee Motor (LAM), and eight small thrusters (22 N) working together for nearly 24 min. That is the nominal plan. We have also uploaded a possible Plan B if it has to be exercised.

What would Plan B be?

Plan B would be the firing of small thrusters alone for a longer duration. But that would be decided after September 22 if it is essential. On September 22 we are going to have a test firing of the main LAM for nearly 4 sec. LAM was used for the initial six orbit raising (around the earth) operations as well as the Trans-Mars Injection (TMI) manoeuvre on December 1, 2013. After that it has not been used for nearly 10 months. The two Mars Transfer Trajectory (MTT) mid-course corrections that were done on December 11, 2013, and June 11, 2014, were done only with the small thrusters. Small thrusters are working; there is no issue. What we are now looking at is the working of the main engine. [See Fig. 1 for the Mars Orbiter Mission Trajectory so far.]

So what steps have been taken to ensure that?

If you look at the fluid circuit all the flow lines coming from the fuel and oxidizer tanks to the main engine have two sets; each flow line has a second parallel path as well. Normally we have only one circuit. What is involved in the process here is, [after the initial operations] the first set was closed. Now the second set has to be energized. What we have to establish is that without any interruption fluid will pass through the second set of flow lines and reach the main engine. For the thrusters, however, fuel and oxidizer are tapped from a different point in the circuit and these flow lines were never closed and were always working as the thrusters were required for MTT corrections. Since we know from the corrections done that these Attitude and Orbit Correction (AOC) thrusters are working, these will always be available.

As for Plan A two things have to happen for the firing to take place. The solenoid coil igniter (there is a redundant second coil as well) above the main engine has to work and the flow lines have to be clear for the fuel and oxidizer to reach the engine simultaneously. Otherwise everything will be stuck.

Now there is some pressure in both the fuel and oxidizer tanks. We have seen on the ground that with this amount of pressure the entire operation works. Before the launch of the orbiter itself in 2013 we had the live test of an identical LAM engine. This LAM engine is the same as that we have been using for our Geostationary Orbit missions and this was also used in the Chandrayaan-1 mission after a gap. Now also we have seen that on the ground the engine operates after a long gap. Hardware identical to what is in the orbiter has been tested after a year-long gap. So, on the ground we have live tested for the restart of the engine after a long gap. This was actually completed last month.

Secondly we have also simulated the D-day firing with the propulsion parameters on that day – the tank ullage volume, the pressure, the temperature etc. – for nearly 2000 sec, which is more than the 1500 sec or so required for actual firing, and we have seen how it works. So that guarantee is also there. This was also done last month.

If there is a problem even then, these thrusters will be fired for a longer duration of 90 min. But the middle point of the firing arc will remain the same; only that we have to start early. But we will not get the same performance with Plan B. It will not be the same planned orbit.

The basic issue is you have to see that the pressure in the tanks is sufficient for the entire operation; it should not fall below a critical value of 11-12 bar. [1 bar is about one atmosphere pressure.] If it goes below this level you will not get the required performance. Right now the pressurization system above the propellant tanks – the pressurant tank (with very high pressure) and the two pressure regulators – have been isolated which normally come into the pictures only if the pressure falls below 11 bar. It was a conscious thought out decision to shut the system out and we find that there is no need for them as the pressure now in the tanks is being is at the optimum level of 16.5 bar. And we have tested on the ground the fuel-oxidizer flow performance in the ‘blow down’ mode [where the pressure is allowed to drop gradually from 16 bar] and we have seen that the pressure does not fall below the critical level even after the full firing of about 2000 sec as against 1500 sec during the actual firing. So if the flow takes place there is no issue. But if required at any point the pressurization system can be activated.

There was a planned trajectory correction on September 14. But this doesn’t seem to have been done. Why?

Now when the spacecraft will arrive in the vicinity of Mars, we have been talking about a distance of 500 km plus or minus something like 60 km. If we see the way it is going now the distance will be about 720 km. Our daily calculations also indicate that the trajectory is stable…

How did this gap of 200-odd kilometers arise? I thought you were maintaining the error margin of 50 km all through and that is why the planned April correction could wait to be carried out instead in June.

At TMI we took one route that should take us to about 500 km from Mars. To achieve this only we make those minor trajectory corrections. In the original plan we had provided for four corrections. The first was December 11, 2013; the second one was scheduled for April, the third in August and the fourth in September. We did not do the correction in April because it was felt that the spacecraft trajectory was steady and did not need any correction then. We then did the second correction on June 11. The question then was whether we required one in August. We found that there was no need for it because, without the August correction, the trajectory was going to be about only 700 km away from Mars.

[An altitude of] 700 km is a good number because as long as you don't come too close to about 200 km or it goes farther to about 1100 km so that it escapes Mars altogether, we are safe actually. So we didn't want to tinker unless it was essential and in any case September 14 option was available. And if we had done that we would have fired the small thrusters and that correction would have already brought the orbiter to about 500 km. But we did not do the September correction and instead what we have decided to do is to restart the LAM on September 22 when the orbiter will enter the sphere of influence of Mars. This firing, which will last only for about 4 sec, will impart some correction to the trajectory enough to bring the trajectory approximately to 500 km. So instead of firing small thrusters for a longer duration, and earlier, we will be doing it with the main engine on the 22nd for 4 sec.

Why four seconds? There are two considerations. There is a minimum time of firing necessary and four seconds are good enough to get at least 3 or 4 good points to know that the fluid flow is proper and also to measure the acceleration imparted correctly. So we will measure once in 520 milliseconds. The velocity change imparted will be about 2.14 m/s.

The second aspect is the correction from 720 km to about 550 km. The question is when you can do this. If you do it too early, it will give you a different number. And if you do it too close – you can do the previous day too – the problem is your ability or the time required to get into Plan B would be touch and go. That is why we have chosen to do this [test firing] 41 hrs before the actual MOI [that is, at 1430 hrs on the 22nd].

How have you determined the timing of this test firing?

In that time frame [of 41 hrs] we are fully within the IDSN [Indian Deep Space Network] visibility. Actually it will be in the IDSN visibility range for about seven hours from then and so there is sufficient window to do the operation on that day. There is no issue there. As I mentioned, when we fire the LAM, there are two coils in the solenoid [igniter]. First we will try firing the solenoid Coil 1. If Coil 1 works and we get a good indication of the acceleration, then we are through. If it does not work we will fire Coil 2 after a gap of 1 hr. All these operations will be tested on the 22nd. If both the coils do not work, then we don't depend on LAM.

As I said before, the centre point of the firing arc is important for the orbit. If we are going to use Plan B we have fire for more time and so we have to start the process early but the mid point will remain the same. If everything goes well as per Plan A, the nominal orbit [Fig. 2] will have an apoapsis of 80,000 km, a periapsis of 423 km and an orbital period of about 3.2 Earth days [or 76.8 hrs]. If Plan B has to be used, the apoapsis will come to about 0.27 million km. But we have to ensure that the periapsis does not come closer and so we will have to make some small corrections to the orbit.

The braking velocity required during MOI is stated to be about 1.1 km/s. What is the present velocity of the spacecraft which basically determines the burn time of the LAM?

When we speak of velocity of the spacecraft it has to be with respect one of the three celestial objects involved in the mission: the Earth, the Sun and Mars. We don’t talk about the Earth any more. With respect to the Sun, it is 22.57 km/s [81,252 km/hr]. With respect to Mars, when the burn starts the orbiter is speeding at 5.127 km/s at an altitude of 1847 km from Mars. When the burn ends after 24 min, its velocity will be 4.316 km/s and the altitude will be 973 km. During the burn the lowest altitude that the spacecraft will reach is 462 km. This reduction will not be the same as the negative velocity of 1.1 km/s that is imparted because as it moves the velocity increases because of Mars’s influence and the geometry of the trajectory.

What are the key steps involved in the MOI manoeuvre?

Before we start operations on September 24th, the orientation of the spacecraft has to be changed so that its thrust vector is in the appropriate direction to bring down its velocity. This condition has to be achieved for starting the LAM firing. The time given for the reorienting the spacecraft (rotating and changing the direction) and convergence is 21 min. So the reorientation begins at T-21 min. [Fig. 3]

When we get into this changed orientation, two things happen: one, there is an eclipse (of the Sun) [This occurs at T-5 min 13 sec; See MOI timeline]. Because of the eclipse there will be no power generation and so the batteries should have sufficient power to keep the operations going. Second, there will be an occultation taking place between the Earth and the spacecraft during the firing period [This occurs post-firing at T+4.3 min]. So there is no communication after about 4 mins of the start of LAM firing; there will be no signal coming to us in real time for about 20 mins. The thruster firing is only 1 or 0. So you will know whether firing has taken place or not and the required acceleration has to follow. Only that you will not be getting the signal. Once the braking velocity of 1098 m/s is achieved the accelerometer automatically terminates the firing. Then it is just physics and the orbiter should be in the correct orbit.

At the end also, we will start receiving the signal after the firing has just completed because the spacecraft has to be reoriented back. The reverse rotation takes about 10 min as compared to 21 mins for forward rotation. Once this happens we will begin to get the signal and power generation will also be okay. After that we have to get some 5-6 good points to measure the periapsis and the apoapsis so that we get the exact orbit achieved.

The TCM-2 [Trajectory Correction Manoeuvre] that we did in June was virtually a similar operation for our purpose. There was a telemetry cut off when the spacecraft was reoriented.

If the spacecraft misses being captured altogether, it will be lost forever…

Now if you are not able to achieve the velocity reduction at that time it will follow the escape trajectory [Fig. 3] and will be lost. You cannot do anything about it.

The most crucial firing for ISRO’s orbiter in my opinion was the TMI. Because that trajectory, that arc, decides the direction in which we are going. If we had taken a longer duration also it would have been a difficult situation. That we managed successfully. Here now the system has to work. That is the basic issue. And our projections of distance etc. show that they are steady. That is a good indication and we have done the necessary simulations and whatever commands that we have uploaded have also been tested on the ground with similar systems.

What is the situation with regard to the availability of on-board propellant for the operations involved?

For Plan A there is no issue. For Plan B we will be just on the brink because it has to work for a longer duration. The effective amount of fuel available now is about 285 kg and out of that we will be using about 250 kg for Plan A. But if Plan B has to be used we will come to about 281 kg. After that we only require for periapsis corrections. And at least 3 kg of effective will be left, which is sufficient for periapsis corrections. Otherwise what can happen is that after one orbit it may come very close to the Mars surface and it can get lost. So we need that amount for orbit corrections.

What about the health status of the instrumentation in the payloads? Do you have any idea about it?

We have tested it. When the spacecraft was in the earth orbit, all the instruments were tested and some measurements were also made to see how they perform. And sometime in April-May-June, we went through all the instruments once again. Instruments are in good health.

Any issue with regard to power?

As far as power is concerned, since power has to be provided by the battery, we have ensured that power is sufficient. So systems that are not required to be used at that time will not be powered. But thermal control systems have to be on power. Second aspect of the operation would be to test all those systems beforehand which are called upon during the MOI. There should not be any surprise. This is a basic principle which we have been following. All the autonomy provisions except the failsafe mode in case of major failure have been exercised and we have actually authorized the satellite for the autonomous operation of this process. So it will select systems like the appropriate processor, the appropriate gyro etc. The other thing is there should not be any man-made error during the entire operation.

How important is the thermal control in the Martian orbit?

Actually thermal control is not a very big issue like in the case of Chandrayaan-1 [orbital period of about 11 hrs]. But we still require thermal control for all the instruments. There are actually about 120 parameters in all to be tested and what we have seen from the analysis is that the predicted values and actual measurements match well. See we had 300 days for all this. That is the advantage.

NASA’s MAVEN will do its MOI two days before ISRO…

MAVEN has not used any of its on-board fuel so far…

Why? Didn’t they have trajectory corrections to do?

They have basically gone for a very different route. They have not gone straight as ISRO’s Mars Orbiter has done. They have gone up and coming back because their aim is to hit 150 km periapsis. There is a circuitous way of achieving that actually. They reached there anyway using their launch vehicle. Their apoapsis is around 6250 km and periapsis is nearly 150 km. Because of this low periapsis of 150 km they are going very cautiously. Trajectory corrections do not require much fuel anyway, only a few grams or so.

Have there been any cause for concern or hiccups in the mission so far?

As of now I should say in the entire mission from November 5 to date we have had only a couple of small issues. One was during the orbit-raising operations. One we had to abort. The second issue was related to about half an hour telemetry loss during the entire mission. This happened we were switching over from one telemetry system to another. We have three antennas -- low-gain antenna, high-gain antenna and medium-gain antenna – and this happened at the time of switching over from one to another. Otherwise we did not have any issue as far as the entire system is concerned.

What is your current reading with regard to the lifetime of the spacecraft?

If Plan A works we feel that a lifetime of six months should be possible. Because what we require are minor corrections to the orbit. A periapsis of 423 km gives us enough margin. We should only not hit something like 150 km or so. Otherwise it is okay. If it is Plan B we have to see how much less it would be. Only when corrections are required we have to be careful. Even if we have only one propellant, either the oxidizer or the fuel, it may not result in combustion but that itself will produce minor thrust. A gas outage also has an impact. So we can use it. These are all the bonuses that we have.

But there should be enough pressure to achieve that…

Yes, but that is a different story.

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Printable version | May 7, 2021 5:09:43 AM | https://www.thehindu.com/sci-tech/science/interview-with-isro-chief-dr-k-radhakrishnan-isro/article6431827.ece

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