On August 7, 2022, the maiden mission with the Small Satellite Launch Vehicle-D1 (SSLV-D1) of the Indian Space Research Organisation (ISRO) failed, which was launched from the Satish Dhawan Space Centre, Sriharikota. The SSLV-D1 rocket, costing about 56 crore, is capable of carrying satellites weighing 500 kg or less. It is intended for the country’s commercial and strategic needs.
What was the Mission?
The SSLV-D1 rocket carried two satellites onboard, the Earth observation satellite (EOS-02), weighing about 135 kg and a co-passenger satellite AzaadiSAT, which weighed about 8 kg. The mission was to place the EOS-02 in a circular low-Earth orbit, at a height of about 356 km above the Equator, and inclined at an angle of 37 degrees. However, the satellites were placed into a 356 km × 76 km elliptical orbit. Therefore, the satellites became unusable as satellites that orbit the Earth are mostly placed in circular orbits. Earth satellites are used for imaging the Earth. So, it is easier if the satellite has a fixed distance from the Earth. On the contrary, if the satellite is placed in an elliptical orbit, its distance from the Earth keeps changing. That’s why keeping the cameras focused becomes much complicated.
About SSLV-D1
The rocket launchers, Polar Satellite Launch Vehicle (PSLV) and Geosynchronous Satellite Launch Vehicle (GSLV), are also developed by ISRO and they are much powerful and could carry huge loads. Placing an EOS in a low Earth orbit does not need such power horses. Therefore, ISRO developed SSLV to cater the launch of up to 500 kg satellites to low Earth orbits on ‘launch-on-demand’ basis. SSLV is configured with three solid stages—87 tonne, 7.7 tonne, and 4.5 tonne. The satellite insertion into the intended orbit is achieved through a liquid propulsion-based velocity trimming module. SSLV is capable of launching mini, micro, or nanosatellites (10 kg–500 kg mass) to a 500 km planar orbit. SSLV provides low-cost access to Space on demand basis. It offers low turn-around time, flexibility in accommodating multiple satellites, launch-on-demand feasibility, minimal launch infrastructure requirements, etc. The SSLV-D1 is a 34m tall, 2m wide vehicle with a lift-off mass of 120 tonnes. ISRO has marketed the SSLV-D1 as the one with the quickest turn-around time. It takes just 72 hours to prepare SSLV-D1 for launch as against over 60 days needed for PSLV.
About EOS-2 and AzaadiSAT
The EOS-2 had advanced optical remote sensing operations. It would have operated in the infrared region and could have served many purposes, such as imaging for climate studies, keeping an eye on Earth, among others. The AzaadiSAT was a collection of 75 tiny payloads weighing about 50 grams each, developed by a team of 750 girl students ‘Space Kidz India’ from rural regions across the country to mark the 75th anniversary of Indian Independence. (Space Kidz India is an aerospace organisation that aims to create basic understanding and knowledge of space among students in government schools.) AzaadiSAT is a CubeSat. It included an Ultra High Frequency-Very High Frequency (UHF-VHF) transponder working in ham radio frequency to enable voice and data transmission for amateur radio operators. Also, a solid-state positive-intrinsic-negative (PIN) diode-based radiation counter was installed to measure the ionising radiation in its orbit. Further, a long-range transponder and a selfie camera were included as well.
Reasons for Failure
According to ISRO Chairman S. Somanath, while the three solid fuel-based propulsion stages worked normally and detached smoothly to raise the remaining stages through the determined trajectory, the only problem was that the onboard computer which determined that the accelerometer had failed, owing to an irregularity in the accelerometer. As a result, it activated a process, known as the salvage operation, which shifted the satellite into an incorrect orbit. In the terminal stage, there was a malfunction of a sensor, which caused the rocket place the satellites into elliptical orbit instead of circular orbit. The satellites are no longer functional. There were absolutely no issues with the rocket’s aerodynamics and all the control systems worked very well.
To place a satellite in the orbit, the SSLV-D1 uses three solid fuel-based stages and a liquid fuel-based velocity trimming module (VTM). SSLV -D1 was designed keeping in mind the quick turnaround time needed for commercial launches. The VTM was meant to burn for 20 seconds at 653 seconds following the launch, according to the launch profile. However, it barely burned for 0.1 second, depriving the rocket of the necessary height boost. After the VTM burn, both the satellites separated from the vehicle and were placed into a wrong orbit—an elliptical orbit. (An elliptical orbit is defined by its long and short axes, just as a circle is defined by its radius.) The short axis of the elliptical orbit achieved was small, and the height at which the satellites were above the Earth, was only about 76 km. At this height, the atmospheric drag hinders the progress of the satellite. Unless adequate thrust is applied to overcome the drag, it loses height and falls towards the Earth due to gravity, and may eventually burn up because of friction.
Conclusion
Presently, rocket technology has progressed to such a stage that even if the course of the rocket alters from its planned course, there are sensors that give feedback, containing the information, to a system. With this information, a course correction is triggered to restore the trajectory of the rocket. Also, there are many sensors. If one or two sensors fail, there would be others that take over and affect the course correction. However, in this case, either redundant sensors were not built-in, or they did not work due to some technical glitch.
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