Vibration disturbance for short duration led to failure of SSLV-D1 mission : ISRO

Chennai: A Vibration disturbance for a short duration during the separation of the Second Stage led to the failure of the maiden developmental flight of the Small Satellite Launch Vehicle (SSLV-D1) in August last year, the Indian Space Research Organisation (ISRO) said on Thursday.
This was revealed in Failure Analysis report of SSLV-D1 Mission submitted to the ISRO.
ISRO said the Failure Analysis Committee, which went into the failure of the first SSLV mission, said detailed analysis of the flight events and observations ranging from countdown, lift-off, propulsion performance, stage separations and satellite injection
revealed that there was a vibration disturbance for a short duration on the Equipment Bay (EB) deck during the Second Stage (SS2) separation, that affected the Inertial Navigation System (INS), resulting in declaring the sensors faulty by the logic in Fault Detection & Isolation (FDI) software.
It said SSLV is designed to be affordable and amenable to industry production and will function as a launch-on-demand platform for Mini, Micro or Nano satellites.
It is a three-stage vehicle with all solid propulsion stages and liquid propulsion-based Velocity Trimming Module (VTM) as the terminal stage.
The launcher also targets many novel features including low turnaround time, flexibility in accommodating multiple satellites, launch-on-demand, minimal launch infrastructure requirements, etc.
The first developmental flight of SSLV lifted off from Satish Dhawan Space Centre (SDSC) on August 7, 2022 at 0918 hrs. The objective of the mission (SSLV-D1/EOS-02) was to inject EOS-02 satellite of ISRO into a circular orbit of 356.2 km with an inclination of 37.21 deg.
Azaadisat, a student satellite was also accommodated in the mission,authorized by IN-SPACe.

However, the spacecraft were injected into a highly elliptical unstable orbit due to a shortfall in velocity, leading to their decay and deorbiting immediately, in spite of normal performance of all solid propulsion stages. The orbit achieved was 360.56 km x 75.66 km with an inclination of 36.56 deg.
Initial investigations with the flight data indicated that the lift-off of SSLV-D1 was normal along with normal performance of all solid propulsion stages. However, the mission could not be achieved due to an anomaly during the SS2 separation, which triggered a mission salvage mode (which is a procedure adopted to attempt minimum stabilized orbital conditions for the Spacecraft in case of an anomaly in the vehicle system).

The Failure Analysis Committee in its report has recommended Change of Separation
System, dynamic characterisation and design modification of strucutres, Usage of
NaVIC data and VTM be in loop for salvage mode.
The separation system used for separating Second stage from third stage was based
on a Circular Expanding Bellow system which shear the rivets and give axial separation
velocity. This system is replaced by well proven Marman band system for separation
and springs for giving axial separation velocity.
The new system is proven to be generating lesser shock and is already used in the
separation of third stage.
The FDI logic based on the accelerometer threshold is modified to evolve a more
realistic approach based on the data generated through system level tests, integrated
separation tests and flight. The accelerometer residue logic checking in MINS is modified
to handle transient events. Moving average window is modified so that in case of failure
identification of multiple sensors in MINS, checking for a longer duration before setting
the salvage mode is implemented.
Dynamic assessment of EB & Satellite assembly along with VTM is carried out and structural
design modified to increase the frequency of the structures. Modifications in EB deck and
Satellite deck were implemented to minimize response to the observed excitations.
Further, in case of failure of inertial system sensors, the mission will be progressing using
NavIC data in a closed loop guidance scheme.
In case of failure of inertial sensors and non-availability of NavIC data (for more than 10 sec),
an open loop steering guidance will be executed. The propulsive capability of VTM will be
considered in this salvage mode also and thrusters will be operated to ensure the minimum
required perigee for the mission.
UNI