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:
FV – Fill & Vent valve
LF – Liquid Filter
FD – Fill & Drain valve
PT – Pressure Transducer
TP – Test Port valve
Reference:
Integral User Manual,
INT-MA-AI-0001
2002, Vol.1 Issue 6