On February 3, 2023, the Indian Space Research Organization (ISRO) announced the results of an investigation into the failure of the first launch of the new rocket “SSLV” in August last year, which was caused by abnormal vibrations of the aircraft.
ISRO hopes to refurbish the spacecraft and launch a second satellite within the next one to two months.
Causes of launch failures and countermeasures
SSLV (Small Satellite Launch Vehicle) is a new rocket developed by ISRO.As the name suggests, it is a small rocket aimed at launching small and microsatellites.
The rocket is 34m in length and 2m in diameter, and has four stages. This final stage is called “VTM (Velocity Trimming Module)” and is equipped with a small rocket engine for the purpose of speed regulation.
It has a launch capacity of 500 kg into low earth orbit at an altitude of 500 km, and is specialized in launching small and micro satellites, for which demand has been increasing in recent years. ISRO said that it was developed with the goals of low cost, short launch intervals, flexibility to accommodate multiple satellite launches and various customer needs, and a small launch infrastructure.
SSLV was first launched on August 7, 2022. The mission name was “SSLV-D1”, where D stands for Developmental or Demonstration, which itself was for development and demonstration purposes. Although it is the first launch and a launch as part of a development test, it carries ISRO’s earth observation satellite “EOS-01” and one microsatellite and puts it into an orbit with an altitude of 356.2 km and an orbital inclination of 37.21 degrees. I was aiming for
After taking off from the Satish Dhawan Space Center in southern India, SSLV-D1 continued to fly smoothly for a while, but before long the rocket failed to reach its planned orbit and was launched. ended in failure.
About half a year later, on February 3, ISRO announced the situation at the time of the launch failure, the results of the investigation into the cause, and countermeasures.
It said the anomaly occurred during the separation of the second and third stages. At this time, unexpected vibrations occurred for a short time in the rocket’s equipment mounting area. This temporarily saturated the readings of all accelerometers in the rocket’s navigation system.
Although there was actually no problem with the accelerometer itself, the “FDI (Fault Detection and Isolation)” system, which detects and diagnoses failures installed in SSLV, determined that an abnormality had occurred in the accelerometer. Ordered to enter “Mission Salvage Mode”. In this mode, when an abnormality is detected in the rocket, the flight is continued without using that part (or replacing it with another sensor, etc.), in this case without using the accelerometer. An order was issued to continue the flight. Such a system called FDI (or “FDIR”, which is a combination of Recovery and Reconfiguration), is a general one that is installed on other rockets and satellites.
The SSLV then continued to fly without data from the accelerometers, separating the 3rd and 4th stages, starting and ending burning of the 4th stage, and separating the satellites.
However, in the absence of accelerometer data, the rocket flies using a pre-set time-based guidance scheme, so errors in the MEMS gyros also introduce errors in the vehicle’s attitude reference. Also, since the speed is calculated from the accelerometer data, the flight continued without knowing the actual speed.
As a result, the speed was insufficient at the end of the burn of the third stage, and the accuracy of the attitude control was also insufficient, so it was not possible to get on the target orbit. According to ISRO, the required speed for this mission was 7693m/s, but 56m/s was insufficient. Although the fourth stage of the rocket and the satellite finally reached an orbit at an altitude of 360.56 km x 75.66 km, the perigee altitude entered the atmosphere, so the rocket and satellite returned to the atmosphere without going around the earth. It broke in and burned up.
In response to this, ISRO plans to change the separation mechanism of the second and third stages, which caused more vibration than expected, to a mechanism with less impact. This low-impact separation mechanism is also used to separate the third and fourth stages.
In addition, the logic of the FDI system was reviewed and changed to take a more realistic approach based on the flight data of Unit 1. In the event of a suspected sensor failure, such as this one, more time was taken to recheck before entering salvage mode.
In addition, the structural design of the equipment mounting part and satellite mounting part has been improved to suppress vibration. In addition, in the event that an abnormality occurs in the sensor of the inertial flight system, a mode has been added to fly using data from the satellite positioning system “NavIC”, which is independently developed by India.
Also, in the conventional salvage mode, the 4th stage VTM was not included as part of the system, but it was modified to include it. As a result, if it is found that the combustion up to the third stage is insufficient for the speed, as in this case, it is possible to devise the injection by VTM and make it possible to fly to the minimum trajectory.
“The launch of SSLV-D1 was part of the development of this new rocket,” ISRO said in a statement. And I believe that we were able to demonstrate the performance of all systems through flight.”
Preparations for the launch of the second test vehicle, which has undergone the aforementioned measures and repairs, have already begun, and it is planned to be launched in the first quarter of this year (by March 2023). This No. 2 satellite will carry a payload of 334 kg in total mass, consisting of ISRO’s earth observation satellite “EOS-07” and two microsatellites, and will be launched into a low earth orbit at an altitude of 450 km and an orbital inclination of 37.2 degrees.