Thread by @Chethan_Dash

#Thread #Update #SSLV

Here We Go: @isro, which couldn’t put put satellites into their desired orbit in its maiden launch attempt of #SSLV yesterday, is studying the various salvage options the rocket can employ to achieve a different result. 1/n

https://twitter.com/Chethan_Dash/status/1556493541437632512

While an expert committee has begun its investigation into Sunday’s mission and would eventually make a set of recommendations, preliminary findings show that the velocity trimming module (VTM) — the terminal stage of the rocket — could not ignite. 2/n

After a smooth lift-off, 1st three stages performed nominally. Mission entered terminal stage where VTM was to ignite & place sats into desired orbit. For this, onboard comps needed info from accelerometers (sensors), based on which VTM's workload would be decided. 3/n

Here, there was an anomaly, which @isro said was caused by a “logic in the rocket to identify a sensor failure and go for a salvage option”. 4/n

I spoke with @isro chief Somanath to understand what transpired in the last leg of the mission. He said: “So far, we’ve learnt that there was no major failure of accelerometers. There are multiple accelerometers… 5/n

...When they show values that don’t match with each other, onboard computers assume there’s a failure & observe accelerometers for 2 secs to check if they become alright. In 2 secs, if values don’t come back to an acceptable level, it’ll declare the accelerometers unsafe… 6/n

…If you use accelerometers that aren’t correct and do the mission, it won’t go in the intended path. Now, assuming that for two seconds the values were not matching, the computer declared a failure… 7/n

…When this happens, there’s an onboard stored prog, a pre-defined path that’s followed. SSLV went on the actual path. Finally when it reached (365km) orbit, the logic did not allow the VTM to fire and make corrections. Hence, the orbit was not achieved,” Somanath explained. 8/n

VTM could not fire because the logic had kicked in after onboard comps didn't receive info from accelerometers (sensors). “And because it had already reached a safe orbit, the salvage option employed decided it would be safe to release the satellites,” one scientist said. 9/n

As part of the detailed study that will now follow, Somanath said, Isro will look at “various salvage options that could have been carried out and which one would be best suited.” 10/n

“When we design a logic, there are values given based on which it gets triggered. Here, there was a trigger that worked and the logic did not allow the VTM to be ignited… 11/n

…This is a minor issue that can be reworked, overall, the findings so far remain that all the new systems worked well and the overall architecture also performed well nominally,” Somanath said. n/n