4–91 A rocket design team has a need to detect the temperatures in a rocket motor. The combustion chamber is that part of a thrust chamber where the combustion of the propellant takes place. The combustion temperature is much higher than the melting points of most chamber wall materials and reaches a temperature of 3000 K. Therefore, it is necessary either to cool these walls or to stop rocket operation before the critical wall areas become too hot. If the heat transfer is too high and thus the wall temperatures become locally too high, the thrust chamber will fail. The most critical wall regions are at and near the nozzle throat and at the nozzle exit. There are two thermocouples located in a rocket engine, namely, one at the throat and the other at the end of the nozzle exit.

Your task is to design an emergency shutoff of the rocket engine if either temperature exceeds 600 K. Shutoff is done by sending a 5-VDC signal to a fuel controller that will immediately stop the flow of fuel and abort the rocket. Use a unipolar comparator powered by a 0- and 5-V supply. Set the comparator reference voltage at 600 K as 1 V. Use the R-type thermocouple shown in Figure P4–91 because of its superior accuracy and stability, especially in a reducing environment as is found in rocket engines.

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7 7 9 JAM Output voltage (mV) in 5 4 3 2 0 Platinum-platinum rhodium 13% 100 200 300 400 500 600 700 Temperature (C) Type R FIGURE P4-91