Factual error: When Peevey gets stuck in the truck, Cliff fires up the rocket pack and simply pushes it clear. If the rocket pack can push out enough thrust to move a 2 1/2 ton truck, Cliff's body can't take that kind of stress. He stretches his arms out, starts pushing, and sets off the rocket, putting himself between the truck and an accelerating rocket demonstrably capable of moving it. The truck is stationary so has 2 1/2 tons of inertia. The rocket instantly accelerates it to an appreciable speed. Human arms cannot take that kind of stress. He's going to be crushed.
Factual error: There are numerous references throughout the film to "Deutschemarks" as the currency of Nazi Germany. However, these were not introduced until after the war; I think the currency they actually used were called "Reichmarks".
Factual error: The only ways to steer a rocket are to gimbal the nozzle or put steerable vanes in the exhaust. You cannot, ever, put a rudder on the front of a speeding aircraft.
Answer: The tether was designed to allow Lothar to be lowered from the zepellin and recovered later without requiring it to land (very tricky for an airship) hence it was hundreds of feet long.