The beam leaving the rods represented three hundred seventy-five million watts of energy, tightly packaged for delivery to Earth. But this was only a small fraction of the solar energy arriving at the big mirror.
The remainder, the loss, must be dumped by the black surface at the back; and to account for the loss in the rods themselves, to prevent their instantaneous slagging into useless globules of aluminum oxide, their excess loss energy must also be dumped.
A cooling bath of liquid nitrogen therefore circulated over each rod and brought the excess heat to the rear of the big lens, where it, too, could be dumped into the blackness of space beyond.
For all its size and complexity, Hot Rod was only a trifle over six per cent efficient; but that six per cent of efficiency arriving on Earth would be highly welcome to supplement the power sources that statistics said were being rapidly depleted.
The spherical shape of the mirror itself, one of the easiest possible structures to erect in space, had dictated the placement of the rods through its center since there was no single focal point for the entire mirror surface.
But it had also added a complication. From this position, the rods could have been designed to fire either straight forward or straight back.
However, due to the hollow nature of the thirty-five hundred foot laser barrel; the necessity for access to the rods from inside that barrel; and the placement of the control booth at its outside end, the firing could only be forward, straight towards the sun on which the mirror was focused.
But to be useful, the beam must be able to track an ever-moving target.
This problem had been solved by one of the largest mirror surfaces that man had ever created—flat to a quarter of a wave-length of light, and two hundred fifty feet in diameter, the beam director, from this distance looking as though it were a carelessly tossed looking-glass from milady's handbag, anchored one diameter forward of the big power balloon.