Space Transportation System (STS) Orbiter Radiator System Temperature and Pressure Drop Control

Expert Analysis

The STS Orbiter Radiator System includes a Flow Control Assembly which maintains control of the temperature of the liquid Refrigerant 21 flowing from the radiator system to the Freon Coolant Loop. The Flow Control Assembly nominally controls the Freon Coolant Loop inlet temperature to 38°F + / - 2°F. The Flow Control Assembly also contains an emergency radiator bypass mode which will stop radiator system fluid flow if the radiator exit temperature becomes less than 33°F + / - 0.5°F. This prevents freezing temperatures from reaching the cabin heat exchanger which contains water.

Well into the Shuttle Orbiter design phase, two challenging requirements were added to the Flow Control Assembly. The first challenge consisted of an added requirement for a rapid radiator inlet temperature transient that must be accommodated. An oil cooler heat exchanger upstream of the radiator would cause the radiator inlet temperature to decrease at a rate of 2.5°F / second when oil flow in the heat exchanger was activated. This rapid change could cause the radiator control temperature (at 38°F) to dive below the emergency activation temperature 33°F in a few seconds. A modification of the control laws was required to provide a rapid control response to the inlet temperature changes. The original linear control (valve rate of movement proportional to the sensed temperature error) was modified to add a nonlinear control component (valve rate of movement was proportional to the sensed temperature error plus the rate of change of the temperature error). The control gains were also tailored to achieve the desired rapid control. Testing demonstrated that the control performance exceeded the customer needs following the change.

The second challenge resulted from the fact that the radiator system pressure drop would vary with position of the radiator bypass valve causing flow variations in the entire Freon Coolant Loop. This was found to cause problems with the pump module and other components in the Freon Coolant Loop. The customer added the requirement to maintain a near-constant radiator pressure drop regardless of the temperature control valve position and resulting radiator flow. A two-fold solution was found for this problem. First the radiator system was balanced by sizing the bypass line to provide the same pressure drop at full bypass as the pressure drop in the radiator at full radiator flow. Second, a requirement was placed on purchased temperature control valve to tailor the pressure drop as a function of valve position, adding a pressure drop profile for the intermediate valve positions. These design additions provided the flow/pressure drop characteristics needed by the customer.

This expert was the principle engineer, performing all analyses and directing all testing on this effort. An electrical engineer modified the electronic controller to achieve the required control laws.

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