and 3000V, higher than those in the previous summary of SCR specs, which were typically 1200V. The turn off times of these higher voltage SCR's are now and which are not suitable for mass drivers as they are presently configured. This larger turn off time is due to the thicker silicon wafer needed to standoff this higher voltage which increases internal capacitance. Suggestions in the past have been made to reduce SCR mass by eliminating the case and using the bare silicon wafer. This could result in a factor of 267 to 1348 reduction in the specific mass. However, discussions with researchers at Westinghouse and National Electronics have stated that the lifetime would be seriously reduced and that some of the ratings would be reduced due to the change in electrrical connection method. Other comments to be made concerning the use of SCR's in mass drivers are that fuses will be required in series with the SCR to remove it from the drive circuit in the event of SCR failure. The dominant failure mode is for it to short which would result in the shorting out of an entire sector (observed in mass driver two operation). If SCR's are stacked for voltage, which is presently required in current mass driver designs, voltage equalizing networks will be required placing a resistive load across the sector capacitor. Also, RC snubber networks will also be required to reduce the chance of dV/dt falsely turning on the SCR. These added circuit elements and series/parallel stacking requirements have greatly increased the mass and complexity of the SCR switch previously envisioned. However, this has not ruled them out of being used in mass drivers since certain stationary launcher devices could still use them and maybe some of the large asteroid retreival missions.
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