The Scarlet Lake Mystery by Harold Leland Goodwin description:
Giant rockets that probe the borders of space provide authentic scientific background for this breath-taking Rick Brant adventure. When Rick, son of a famous scientist, and his pal Scotty go to work at a secret rocket base in Nevada, only two men at the base know their real mission: to track down the saboteur who calls himself 'The Earthman'.
a steel bar at the top of the launching tower. It was spring-loaded and triggered to move right across the path of the rocket when we fired."
"What does spring-loaded mean?" Mrs. Brant asked.
"The bar was activated by a spring. The spring was under tension. The steel bar lay along one of the pieces of the frame, and was held by a latch. When the trigger withdrew the latch, the spring pushed the bar across the path of the rocket. That's what spring-loaded means in this case."
"Couldn't anyone have found the steel bar?" Scotty wanted to know.
"Yes, if anyone had looked for it. But once the launching tower was erected, there was no reason for anyone to go to the top for an inspection."
Scotty nodded his understanding.
"To go on, as soon as we found the bar and the spring mechanism we knew we'd been sabotaged. But that wasn't all. Etched on the bar was a rather good picture of a knight in armor, in the process of driving his sword through a rocket. Underneath was the ins