The 12V solenoid included with Hypertek ground support equipment was originally used to control nitrous flow to race cars. Therefore, it has a tendency to heat up, and can burn out, particularly as hybrid motors get larger and larger, and take longer to fill. The solenoid only requires high power when it first opens against the 800 psi or so pressure. Once open, it needs very little power to remain open. The following simple circuit will allow full power for a fraction of a second, and then reduce it to hold the solenoid open for the duration of the fill.

I used a capacitor rated at 50V. The value and voltage are not critical. In this solution, the resistors are bypassed as the capacitor charges, allowing full current to open the solenoid. The cap charges in less than a second, forcing the voltage drop across the resistors, reducing the current to less than 10% of the original. There have been no failures of this system in the last 12 or so launches.