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The Hindu
The Hindu
T.S. Subramanian

An anomaly for two seconds led to SSLV mission’s underperformance: ISRO chief

S. Somanath, chairman, Indian Space Research Organisation (ISRO), is a bold man and if he is disappointed that ISRO’s Small Satellite Launch Vehicle (SSLV), in its debut mission from Sriharikota on August 7, put the Earth Observation Satellite (EOS-2) and students’ AzaadiSAT in the wrong orbit, he does not show it. ISRO said the SSLV put the satellites into a 356 km X 76 km elliptical orbit instead of a 356-km circular orbit. “Satellites are no longer usable. Issue is reasonably identified. Failure of a logic to identify a sensor failure and go for salvage action caused the deviation”, ISRO explained.

The SSLV is a three-stage rocket and all its stages are powered by solid propellants. It is designed to put a 500-kg satellite into orbit.

Editorial | Space to learn: On the failure of ISRO’s maiden small satellite launch vehicle mission

In an interview in Chennai, Dr. Somanath said “an anomaly” for two seconds in one of the accelerometers in the launch vehicle led to the mission’s underperformance. He explained at length what went wrong and answered a range of questions.

Dr. Somanath took over as ISRO chairman on January 14, 2022. He is Secretary, Department of Space and Chairman, Space Commission. He was director, Vikram Sarabhai Space Centre (VSSC), Thiruvananthapuram, before he became ISRO Chairman. He is a reputed rocket technologist and played a key role in the development of ISRO’s GSLV-MkIII which weighs 640 tonnes. Excerpts:

Where was this logic which did not identify the sensor failure? Was it in the newly introduced velocity trimming module?

It is not the logic of the velocity trimming module. This is a logic of the launch vehicle. When something is not working very well, the rocket will have a salvage option. Basically, we look at the various failures which are possible in a rocket. We are prepared for failures, what we should do with failures. One such condition is called the accelerometer failure. There are accelerometers and sensors around which measure the rocket’s acceleration in the forward direction. Supposing one of these accelerometers is a failure, what should we do next? There are algorithms sitting in the side. The accelerometer can fail just after lift-off… Still, it is designed to put the satellite in the correct orbit.

In this case, what happened is that the measurement of the accelerometer showed some anomaly just at the point of the separation of the second stage. When it showed such an anomaly, the internal computer felt that the accelerometer had failed. Then it triggered something called the salvaging operation.

What it will do next is instead of closed loop guidance, it will initiate what is called the open loop guidance. That is, from that point [of failure] to salvaging, it will work in an open loop guidance. There is already a path written in the computer that you should go through this path to reach the satellite. … So it does not look for acceleration data but [the rocket] will simply go in that path. Once it is going like that, the ability to put the satellite in the correct orbit is slightly diminished.

So what happens is that at the end of the next stage firing, the computer believes/assumes that “I cannot go further but somehow separate the satellite after the burning of this motor is concerned.” This stage is not a liquid stage [that is, not fuelled by liquid propellants]. It is a solid rocket [fired by solid propellants].

But the solid motor cannot be stopped midway. The computer will wait until the firing of the solid motor, currently firing, is completed. The anomaly happened at the end of the second stage. So the computer wants the third stage firing to complete. As soon as it is completed, the satellites are injected into orbit. It did that.

It did what it is supposed to do. But in that process, it was short of a small velocity. But this shortage is very dear to the whole system. If the velocity [with which the satellites should be injected into orbit] is 7.3 km per second, it has almost reached 7.2 km per second or something. It is now short of 40, 50 or 60 metres per second. This is 7,300 metres per second versus 7,200 metres per second. This difference is very important because in that orbit, from a height of 356 km… the perigee will come down to the extent that its height will be less than the atmospheric height. So the height of the perigee we are getting now is 76 km.

Once the satellite moves in such an elliptical orbit, there is an atmospheric drag and this drag will make the satellite come down very fast. Within 20 minutes or so, the satellite will lose its orbit. So this is what really happened.

The issue we faced here is everything in the rocket worked. All the stages worked. All the propulsions worked. All the sequences worked. The only issue here was that due to an anomaly in the accelerometer, the computer inside decided that the accelerometer had failed, hence “I should save the vehicle.” So it triggered a salvaging option which put the satellite into a wrong orbit. So there was really no issue with the accelerometers. They were healthy even afterwards.

Why the computer found that the accelerometers had a small issue is something we do not understand. There could be an actual problem in the sensor. Or there could be a logic problem in the sensor. But this is a standard system that exists in all ISRO rockets. Wherever there is an accelerometer of this nature, it will do exactly like this. So there is nothing wrong in the way it did it. But the sensor did this for this particular time.

Whenever the rocket stages are separating, there will be a transient. A small jerk will be there. In this jerky thing [this time], the accelerometer level slightly exceeded. So we should relook whether the approach to isolate or not to isolate can be suppressed.

You don’t have to declare it a failure. You can wait for a little more time and then have a relook at it. We have thought about it.

For two seconds this anomaly existed. After two seconds, it came back to normal. Unfortunately, the computer declared that it was a failure within those two seconds.

Two seconds?

There is an unknown thing residing in this whole algorithm. Why two seconds? If it had been three seconds, the mission would not have been like this. If the level of isolation was a little higher, it would not have happened. There could be many reasons. We have to understand why such things happen. There could be a hardware failure, a software glitch, an external trigger or the shock of the transient was slightly higher than what we expected because it is a new rocket. Whatever we have qualified is for a new rocket. In a new rocket, the behaviour of this type will be different. Probably, if we had changed the threshold element, the mission could have been saved.

So it was not because the SSLV was carrying satellites, weighing about 500 kg together, which were heavier than it could carry?

No. Absolutely no issue with the vehicle’s aerodynamics. Control systems, all worked very well.

Is the vehicle’s configuration alright?


In the last PSLV flight in July 2022, ISRO performed novel experiments using the fourth stage of the launch vehicle. The fourth stage did not come down after putting the three Singaporean satellites in orbit. You used it as a platform in orbit to conduct some experiments.

It was not the first time we did it. We had done it earlier with the PSLV’s fourth stage.

Yes, you had done it earlier.

The fourth stage requires power. In a rocket, the power comes only from the battery. In a satellite, the power comes from the solar panels. You need a computer which is running in a regular rocket. That computer is a rocket initiation computer. It cannot do this when the rocket is in orbit. So another computer is required. We switch over from the launch computer to the orbit computer. It requires low power and can be used in orbit.

The rocket takes its reference from the earth. The satellite takes its reference when it is in orbit. So we have star sensors to help it to find its position by itself.

Lastly, what is important is we made the platform [the fourth stage] a controlled one. Earlier, we made an uncontrolled one. It is now capable of changing its orientation by looking at the star sensors. It can receive commands from the earth. We have a commanding capability from the ground to turn, switch on, switch off and make the platform look in different directions. It can be commanded in different orientations. So this is the beauty of what we did.

With this capability, we can host payloads. So we hosted payloads on the platform from others as well as from ISRO.

Chandrayaan-3 will not have an orbiter.

No. It will have an orbiter. We cannot go to the moon without having an orbiter in the current configuration. The orbiter is there. We call it propulsion module this time because the earlier orbiter had different payloads. So what we are doing now is we are not doing any experiments using the orbiter’s payloads. The orbiter’s only job is to take the lander up to the moon’s orbit.

The orbiter in Chandrayaan- 3 will not have any payloads?

It will not have any payloads. The payloads are only in the lander. For the lander to communicate with the Earth, we need the orbiter. Otherwise, the lander cannot communicate directly with the Earth. First, the lander will communicate with the orbiter, the mother ship, and the mother ship will communicate with the Earth. That is why the mother ship, which will be orbiting around the moon, is necessary.

The lander and the rover…

The lander and the rover are already there. The orbiting element is also there. Unlike Chandrayaan-2, the orbiter in Chandrayaan-3 will not have payloads to perform experiments. It is a mere relay station.

At what stage is ISRO’s Human Space Flight (HSF) or the Gaganyaan programme? There will be two unmanned flights first.

There are different missions planned. The first and foremost is the abort mission. So we are planning to do a test vehicle flight… There are four abort flights we are planning to do. Two of them will be done immediately. After that, we will do the first GSLV-MkIII unmanned flight. After that, we will do two more abort missions. Then we will do the second unmanned mission for the Gaganyaan. Only after these six flights, we will do the manned mission.

Have our astronauts completed their training in Russia?

They are here.

No information is available on who these Indian astronauts are. Earlier, in the 1980s, we knew that ISRO’s P. Radhakrishnan and N.C. Bhat were to fly into space on board the U.S. space shuttle. Their names were announced and information was available about who they were. But this time, we do not know who our astronauts are and whether they belong to ISRO or the IAF.

They are Indians (laughs).

They are IAF test pilots?

They are Indians. That is the most important news you should know.

Will two or three go in the first flight?

That is a decision we will take a little later. Currently, we are training four people. The choice of flying one, two or three, that decision we will take a little later.

Is their training complete?

Their training is not complete. It is a process. Until they go, the training will continue. Their practical training in Russia, theoretical training here, simulator training and flight training — all these will continue. It is part of their life.

How long will they stay in orbit? What kind of experiments will they perform?

The original intent was to keep them in orbit for at least seven days and do some experiments related to micro-gravity. This was the plan… But the experiments are they themselves and their vehicle itself is an experiment. So they will conduct psycho-physiological measurements, the whole system validation etc.

But then, seven days are a long period. It will need a lot of resources including water, oxygen and other things. It is not a very big habitat [the crew module]. It is a compact habitat. So definitely, we will not venture into one week in the first mission. It will be for a short time. The whole purpose is to show that it is possible to take people there and bring them back safely. Towards that, the duration will be fixed. It will be fixed based on various factors, how they will be able to communicate, from what location we will be able to recover them etc… The duration will be much shorter than one week in this mission.

The first SRE (Space Capsule Recovery Experiment) mission in January 2007 was a big success. You were able to bring back the satellite which was in orbit, it splashed down in the sea near Ennore, Chennai and you recovered it. ISRO brought back the crew module in the Crew Module Atmospheric Re-entry Experiment (CARE) from orbit and recovered it from the sea near the Andaman archipelago. The Reusable Launch Vehicle- Technology Demonstrator (RLV-TD) was a success. All these missions demonstrated that ISRO has mastered the re-entry technology. So you must be raring to go ahead with the Gaganyaan mission of sending our astronauts into space and bringing them back.

Yes, definitely. This work is really a high risk business. In this risky business, the only option is to continuously do tests of various nature to gain confidence. The only way we can develop confidence is by processes, by developing hardware, software testing, anomaly testing, protection against anomalies, like what you saw in this SSLV. So whatever logic we are thinking about, whether they are working or not, we have to actually put them to test. In Gaganyaan, we have to actually create conditions which are called for in the flight and test them. This is the difference between a regular rocket mission and a human space flight. So our options are we have to continue doing the tests many numbers.

Will the manned flight be in 2024?

Our attempt is to do it by the end of 2023 or in 2024.

What are the missions lined up in the coming months?

Our next mission will be a GSLV-MkIII flight. It will be a commercial mission. We are getting ready for the PSLV-C54 mission, which will launch the Oceansat. It is an earth observation satellite. We want to launch the next developmental flight of SSLV (SSLV-D2) in November. These are the immediate launches before November.

The writer is a senior journalist

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