ISRO releases detailed account of what went wrong with 1st development flight of SSLV

The Indian Space Research Organisation (ISRO) on Wednesday gave a detailed account of what went wrong with the first development flight of the Small Satellite Launch Vehicle (SSLV-D1) last year and why the mission could not be accomplished.

The Bengaluru-headquartered national space agency said considering the clear identification of the cause of the flight anomaly and suggested corrective actions, the next development flight (SSLV-D2) is planned to be executed complying to the recommendations.

"The second developmental flight of SSLV (SSLV-D2/EOS-07 Mission) is scheduled in the first quarter of 2023 and will launch a total payload mass of about 334 kg including EOS-07 satellite and two co-passenger satellites," ISRO said in an update.

The SSLV is designed to be affordable and amenable to industry production and aimed to function as a launch-on-demand platform for mini, micro or nanosatellites.

It is a three-stage vehicle with all solid propulsion stages and a 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, and minimal launch infrastructure requirements.

The SSLV-D1 lifted off from Satish Dhawan Space Centre (SDSC) in Sriharikota on August 7, 2022.

The objective of the mission was to inject the EOS-02 satellite of ISRO into a circular orbit of 356.2 km with an inclination of 37.21 degree. 'Azaadisat', a student satellite, was also on board the mission.

However, the spacecraft were injected into a highly elliptical unstable orbit due to a shortfall in velocity, leading to their decay and de-orbiting immediately, in spite of the normal performance of all solid propulsion stages, the space agency's update said.

The orbit achieved was 360.56 km x 75.66 km with an inclination of 36.56 degrees.

Initial investigations with the flight data indicated that the lift-off of SSLV-D1 was normal along with a normal performance of all solid propulsion stages. However, the mission could not be achieved due to an anomaly during the second stage (SS2) separation triggering a mission salvage mode (a procedure adopted to attempt minimum stabilised orbital conditions for the spacecraft in case of an anomaly in the vehicle system).

"Subsequent 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 SS2 separation, that affected the Inertial Navigation System (INS), resulting in declaring the sensors faulty by the logic in Fault Detection and Isolation software," ISRO said.

SSLV uses a newly-developed inertial navigation system -- MEMS Inertial Navigation System or MINS-6S -- consisting of six MEMS gyros (for measuring rotation rates) and six Ceramic Servo Accelerometers (for measuring accelerations) assembled in a vibration-isolated temperature-controlled assembly.

The system is also designed with an in-built NavIC receiver and also acts as the MINS navigation computer processor for running Inertial Navigation and Aided Navigation software. A novel algorithm estimates the error in the attitude (orientation) introduced due to the MEMS gyros, position and velocity of the MEMS INS, using the NavIC data and corrects them, so that mission accuracy is achieved.

Knowing the health of the accelerometer is of paramount importance for the functioning of MINS. The failure detection logic identifies degraded accelerometer (one or all of the six) and isolates the same for improved mission performance, it was noted by the space agency.

During the SS2 separation event, all six accelerometers inside the MINS package experienced measurement saturation due to high vibration levels for a short duration. The accelerometers got saturated at different time instants, within the 20 milliseconds (ms) interval of data sampling resulting in different acceleration values being measured by each sensor.

This resulted in high residue values (difference among them) beyond a specified limit for a duration of two seconds. The software implementing the fault detection and isolation assessed that the sensor outputs crossed the pre-set threshold limit and raised the alert/flag for the salvage mission mode initiation, which is a safe approach for the mission.

However, the accelerometers were found functioning well after this transient event. The salvage mission mode got fully executed without the support of the accelerometer data and injected the satellite to an unstable orbit due to lower injection velocity (around 56 m/s less than the required 7693 m/s). Though this is as programmed and expected, if kept observing for a longer duration, the residue among the sensors would be within limit and failure logic would not have been executed, ISRO said.

The vibrations at the satellite interface as measured during the flight, were well within the pre-flight test specifications except during the SS2 separation event when the Equipment Bay and Satellite interface had a dominant vibration response in the first axial mode.

The shock response measured at EB during the SS2 separation exceeded the expectations and ground test levels both in low and high frequency as well as in time duration, it was pointed out.

Due to this shock, excitation at the MINS sensors was persistent for more than 10 millisecond (ms) duration which was not expected. It is generally observed that shock from such events lasts for about two ms, whereas here a shock of 2-3 ms duration and subsequent oscillations lasting for more than 10 ms was observed.

Further, based on the flight telemetry data, all the accelerometers were found functioning normally after the transient event till the end of the mission, indicating no damage to them. However, due to mission salvage, data from these accelerometers were not used for further mission execution.

Though the salvage mode was initiated with the purpose of saving the mission, it could not inject the satellites to a safe orbit. The third stage, SS3 ignition was commanded by the sequence programme. Subsequently, the vehicle was guided through time-based open-loop mode steering without feedback, as the accelerometer data was declared faulty.

At the end of SS3 burnout, the satellites were separated safely as programmed.

There is no knowledge of the actual velocity of the vehicle as well since the velocity is computed from accelerometer data. VTM ignition was bypassed as programmed, since it could be a deterrent to the success of the salvage option in some cases.

The shortage of about 56 m/s at the end of SS3 burnout in final velocity (due to cumulative deficiency in the performance of all propulsion stages) and loss in pointing accuracy due to sensor errors, the targeted orbit could not be achieved. This indicates that execution of salvage option in all situations need not always lead to successful placement of satellites in an orbit, ISRO reasoned.

The SSLV-D1 was the first developmental mission of this new launch vehicle. The objective of development missions is to prove the launch vehicle design and architecture and to bring out any residual unknowns not identified in the qualifications tests and analysis during its development journey, ISRO noted.

"SSLV-D1 mission demonstrated the satisfactory integrated performance of SSLV in all its systems including its flight through the aerodynamic regime, which is an accomplishment by itself," the space agency added.