LAT Thermal Control System Thermal Control System TCS

LAT Thermal Control System • Thermal Control System (TCS) controls temperatures within the LAT while operating – Uses actively controlled Radiators and variableconductance heat pipes (VCHP’s) for rejecting heat – Handles varying environmental heat loads – Maintains LAT internal detectors at stable operating temperatures • TCS responsible for maintaining LAT within its survival temperature limits when off or in an offnominal condition – Uses thermostatically-controlled (TBD) make-up heaters on the Grid – Includes automatic shut-off of Radiators to minimize radiative heat leak

Thermal Control System Modes • Off – LAT is off and all buses are un-powered. This occurs before and during launch. On-orbit, this mode would open the VCHP’s up, activating the Radiators and providing enormous heat-rejection capability to a LAT which is drawing no current. This would lead to relatively fast cool-down of the LAT. – Corresponds with LAT Off mode. • Pre-deploy – Immediately after fairing separation, the surv-1 bus needs to be powered. This will activate the VCHP survival heaters and isolate the Radiators from the LAT. This should happen immediately, but there may be some time cushion, since the folded-up solar arrays will serve as a radiation shield for the Radiators. When the surv-1 bus is activated, the heaters are powered, since the keep-alive switch is already in the engaged state. – Corresponds with LAT Pre-deploy mode. • Survival Ops – A maximum of TBD minutes after solar array deployment, the surv-2 bus must be powered. This will energize the make-up heaters on the Grid. If the Grid temperature is out of its operating range, thermal switches will engage, and the heaters will heat the Grid. – TCS is in “Survival Ops” whenever LAT enters “survival” mode: when LAT loses control of TCS because SIU is powered off, when SC pulls the plug, or if SIU or PDU goes south. • TCS Safe – The LAT SIU and PDU have been powered. The TCS enters this mode once the SIU confirms that all thermistors are functioning as expected and that the power to the VCHP TCS heaters off the LAT bus is available. This mode is entered once the VCHP TCS heaters are fully activated and the VCHP survival heaters are turned off. Note that this means that the keep-alive switch controlling the VCHP survival heaters is reset by manual/software intervention, not just at SIU power-up.

Thermal Control System Modes (cont. ) • Thaw – TCS enters this mode if thermistors indicate that Radiator VCHP runs could be frozen. Controlled thaw routine of frozen ammonia in VCHP’s is run in this mode. Thaw parameters are set-able, and thaw routine and exiting can be manually shortened from the IOC, if desired. Any problem encountered during thaw drops TCS back into “TCS Safe” mode. • TCS Operating – TCS is functioning fully, except that LAT temperatures may be out of operating range. VCHP’s/Radiators are functioning fully, with feedback control functioning fully. For standard turn-on scenarios, the Radiators will be almost completely closed until the LAT warms up. While in this mode, LAT temperatures will enter operating range and all overrides can be reset. Also, make-up heaters on the LAT may or may not be on in this mode. However, they must be off to go to next higher mode. • TCS Standby – TCS is fully functioning and all temperatures are in operating range. Rate-of-change parameters may be out of specifications in this mode, especially during warm-up and any configuration change in the instrument power. • Nominal – All temperatures and rates of change are within nominal operating range. TCS sends green light to SIU for going to “LAT Observing” mode.

Observatory, LAT, and TCS Power-up

Observatory, LAT, and TCS Power-up (cont. )