Introduction
Small Satellite Launch Vehicle (SSLV) of ISRO is designed for affordable launch of nano, micro and mini satellites. The key features of SSLV are launch on demand facility, low cost and flexibility to accomodate various satellites. In this article we will look at the failure cause of first mission of SSLV.
Courtsey – ISRO
SSLV is a three stage launch vehicle with 3 solid propulsion stages and liquid propulsion based Velocity Trimming Module (VTM). The lift off weight accounts to nearly 120 tonnes. It can carry satellite of mass 500 kg till the orbit of 500km. SSLV is 2m in diameter and 34m in height.
ISRO launched SSLV-D1 (first mission of SSLV) on 7th August 2022. The objective of the mission was to inject EOS-02 satellite of ISRO into circular orbit of 356.2 km at an inclination of 37.21 degrees. Along with the same, another satellite named Azaadisat (satellite developed by students) was accommodated.
However, the mission could not be achieved due to an anomaly in second stage separation.
Cause of Failure
SSLV has three solid stages, once started we can not control any of it. After this there is Velocity Trimming Module(VTM). The first three stages make it possible to move the satellite almost to its orbit, VTM is used for precisely controlling the satellite till its intended orbit.
The problem started during the separation of the second stage. During the separation of the second stage, there were vibration disturbance on the Electronic Bay (EB) which affected Inertial Navigation System. EB deck is present on the fourth stage(VTM stage) that contains all the electronic systems and navigation system for the satellite.
Due to the high vibration disturbances the navigation system got saturated. (Saturation occurs in a sensor when the signal is larger than the range of sensor). This was then detected by FDI(Fault Detection and Isolation) software and it resulted in raising the flag for salvage mission mode initiation.
The salvage mission mode got executed without the support of navigation system and injected the satellite into unstable orbit due to lower injection velocity and compromising the mission.
Inertial Navigation System
SSLV uses MEMS (Micro electromechanical systems) Inertial Navigation System. It contained 6 MEMS Gyro (for measuring rotation rates) and 6 Accelerometers (for measuring accelerations). It helps us to know how the system is moving at a particular instant with respect to the starting point.
Generally, MINS (MEMS Inertial Navigation System) has some damping, but due to high vibrations saturation occured in the sensors. The problem in the sensor was detected by the FDI and it isolated this sensors from the system.
How did FDI detect the problem?
Along with MINS, NavIC receiver is also present. An algorithm estimates error in the attitude (orientation) introduced due to the MEMS Gyros, position & velocity of the MEMS INS, using the NavIC data and corrects them, so that mission accuracy is achieved. Now, at time of SS2 separation all the 6 accelerometers got saturated. This resulted in different acceleration values measured by each sensor. So, the difference between MINS and NavIC data kept on increasing. The time given for correction was 2 seconds, but it reached beyond the same and so the FDI raised the flag and salvage mode was initiated.
This was executed as programmed, but if the data of MINS were looked again after some time, the errors in the sensors would have been within limit and failure logic would not have been executed.
Vibration during separation
The vibration at satellite interface as measured during the flight and pre-flight were with specifications except during SS2 separation. The shock response observed at the EB deck were higher than expected in both time domain and frequency domain.
The accelerometers were found to be working fine even after the transient event, but as the mission entered into salvage mode, the data from the accelerometers was not used for further mission.
Salvage Action for SSLV
Now that we know the cause of failure, ISRO has taken several actions so that future missions can be achieved successfully.
- The new system is placed for separating third stage from second stage, which is proven to be generating less shock. The new system uses Marman band system for separation and springs for giving axial separation velocity. It is used in place of Circular expanding bellow system which shear the rivets and give axial separation velocity.
- The FDI logic is modified to check for longer duration before engaging into salvage mode. It is modified to handle the shock events.
- Modifications in the EB deck and satellite deck were done to increase the frequency of the structure.
- In case of failure of inertial system sensors, mission will progress using NavIC data in closed loop guidance scheme.
Future Mission
After all the changes, SSLV-D2 was successfully launched on February 10, 2023 at 09:18 hours IST from the first launch pad at SDSC SHAR, Sriharikota.
This detailed explanation is provided for understanding of the general audience.
Source – https://www.isro.gov.in/mission_SSLV_D1_summary_D2.html