WASHINGTON — The Indian Space Research Organisation (ISRO) says it has identified and corrected the problem that doomed the first flight of a small launch vehicle as it gears up for a second attempt.
ISRO said Feb. 1 that an investigation into the failed inaugural launch of its Small Satellite Launch Vehicle (SSLV) last August concluded that an unexpectedly strong shock during separation of the second stage saturated accelerometers in the rocket’s guidance system, triggering a “salvage mode” that ended up placing its payload into an unacceptably low orbit.
The SSLV’s inertial navigation system uses six accelerometers that, at the time of second stage separation, measured vibrations that where both stronger and lasted longer than expected from ground testing. That briefly saturated the accelerometers and caused the guidance system to assume that they had malfunctioned. In fact, the accelerometers were not damaged, ISRO stated, and functioned normally for the rest of the flight.
The guidance system then initiated a salvage mode to attempt to place the payloads into orbit without using data from the accelerometers. That included open-loop guidance of the rocket’s third stage without using feedback from those accelerometers. A kick stage called the Velocity Trimming Module (VTM) was not ignited, ISRO said, “since it could be a deterrent to the success of salvage option in some cases.”
That resulted in a velocity shortfall of 56 meters per second along with a loss in pointing accuracy at the time of payload separation, the agency said. The payload, the EOS-02 Earth observation satellite and the student-built AzaadiSAT smallsat, ended up in an orbit with a perigee of only 75.7 kilometers. That orbit caused them to reenter “immediately,” ISRO concluded.
ISRO outlined several corrective actions to prevent a similar problem from repeating. It is changing the stage separation system to one already used for the third stage that produces less shock. It is also shifting to a “more realistic approach” to handling accelerometer data in the guidance system, including waiting longer before concluding the accelerometers have malfunctioned and triggering the salvage mode. That salvage mode will shift to closed-loop guidance using data from NavIC, India’s regional satellite navigation system.
ISRO said that with those measures in place, it is ready to attempt a second SSLV launch. The SSLV-D2 mission will carry the EOS-07 Earth observation satellite and two smallsat secondary payloads. The launch could take place as soon as next week, although ISRO said only that the launch is scheduled for the first quarter.