The Douglas Aircraft Company originally manufactured the F5D Skylancer for the United States Navy as an all-purpose fighter-interceptor based on an aircraft carrier, with an arresting hook below the afterburner to assist in landing and wing tips that mechanically folded up for parking. Douglas built only four Skylancers before the Navy canceled the program, and NACA, later NASA, acquired all four; two exist today. Speed brakes on the top and bottom of each wing near the fuselage slowed the aircraft by increasing drag, the aerodynamic force that opposes an aircraft's motion, and during the abort launch procedure Neil Armstrong created, the pilot engaged the speed brakes to slow the plane to simulate emergency power. The Skylancer's delta wing formed a triangular shape intended to reduce drag and increase lift, and because it had a lift-to-drag ratio similar to the Dyna-Soar X-20 spaceplane, NASA chose it for the abort launch procedure. Pilots steered the delta-winged aircraft with elevons and a rudder: the F5D's electronic and hydraulic elevons controlled pitch and roll, and the hydraulic rudder controlled yaw.