INTEGRAL Picture Of the Month
June 2020

INTEGRAL POM
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INTEGRAL recovery from its 8th Emergency Safe Attitude Mode

The picture shows representation of INTEGRAL's Reaction Control Subsystem (RCS); it is part of the Attitude and Orbit Control Subsystem (AOCS). The RCS provides for the storage of spacecraft propellant, eight rocket motors, and the pipes and valves necessary to regulate propellant flow from the tanks to the correct thrusters. The subsystem also has pressure transducers and temperature sensors. The tanks contain anhydrous monopropellant hydrazine (N2H4) and nitrogen pressurant separated by a membrane. The tank is connected to two thruster blocks (RCS Branch A and RCS Branch B) via two Latching Valves (LV1, LV2), to enable either section to be isolated during operations. During normal operations RCS-A is used.

Why this diagram? On 16 May 2020, an anomaly and entry into Emergency Safe Attitude Mode (ESAM) occurred, during a Reaction Wheel Bias, and triggered by an imbalance in Reaction Control Thruster (RCT) commanding. While still being in safe mode and under thruster control via the redundant Reaction Control System RCS-B, on 17 May an unexpected de-pointing of the spacecraft took place (which was automatically compensated within 10 min), coincident with a sudden drop in fuel system pressure from 5.4 to about 5 bar. Afterwards, the recovery from ESAM took place successfully, reenabling attitude control through reaction wheels (i.e., avoiding further  use of thrusters) and staying on RCS-B. This was the 8th ESAM anomaly encountered over the mission lifetime.

In order to check thruster performance, on 19 May an RCS-B Thruster Torque calibration was performed, showing irregular impulse strength and about 50% reduced thrust. On 22 May, a wheel speed adjustment on RCS-B had to be performed, which was done manually and with a modified procedure changing the speeds step by step while closely monitoring the thruster performance. These successful operations demonstrated that wheel speeds can be controlled with the reduced thruster performance. In order to check also the RCS-A behaviour, another Thruster Torque calibration took place on 25 May, showing similar results to RCS-B. Reaction control being possible both from RCS-A and RCS-B, and switching back the control to the nominal RCS-A, full redundancy of the spacecraft has been restored since. 

The root-cause of the anomaly is still under investigation. The drop in pressure might be related to a loss of nitrogen (the tank pressurant), i.e., by gas bubbles leaving the system via the thrusters. There are no indications for a significant loss of the hydrazine. The RCS seems to operate now at much lower pressure (note that the thrusters can operate down to at least 3.5 bar according to specs) and with possibly some nitrogen contamination.   At the end of May all instruments (except OMC, due to operational constraints) are taking data again, in staring mode at stable attitude under reaction wheel control. The spacecraft is operable and wheel speeds can be controlled. A return to normal science operations with pointed observations including Targets of Opportunities is foreseen for ~mid June.

Further updates are provided in the Latest News section on the ISOC webpage.

List of abbreviations on the graph, not appearing in the text: Reference:
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