Lark with boosters affixed being prepared for launch 26 October 1950
Lark was launched by two solid-fuel booster engines and powered in flight by a LR2-RM-2 liquid-propellant rocket engine. Later Lark vehicles used a slightly improved LR2-RM-6 engine.
Built by Reaction Motors, a Lark engine used red fuming nitric acid (HN03 + 13% N204 and 13% H20) as fuel. This fuel was so highly corrosive and toxic that those workers handling it had to wear chemical-safe suits for protection. Housed inside the main body of the Lark, the engine was capable of producing 620 pounds of thrust.