It may look like a primitive amusement park ride, but the so-called "Iron Cross" was the first simulator built to test the
use of reaction controls for maneuvering flight vehicles in the upper atmosphere and space.

In 1956 the X-2 reached an altitude of over 125,000 feet (where the air pressure is low enough to render aerodynamic flight controls almost useless). Also at that time, the hypersonic X-15 was on the drawing boards, and it was expected to fly twice
that high. The human space program was not yet underway, but it was obvious that piloted space flight was not too far ahead.
Clearly the time had come to develop an effective, non-aerodynamic flight control system. Work began on "reaction control," which used small rockets that push the vehicle about the three axes. Thrusters act with a force in one direction, making the vehicle react with a movement in the opposite direction.

It was recognized that the earliest flight evaluation of reaction controls would occur with supersonic research aircraft; but it was also believed that ground-based simulation studies should come first. A two-phase study began at Dryden. Phase one developed a fixed-base setup with an analog computer to solve the equations of motion. Phase two put it together with a mechanical, moving simulator with pilot controls. The simulator was made of metal I-beams joined at the center, and dubbed the Iron Cross.

The I-beams pivoted about a center pedestal. A pilot's station at the end of one beam came with a seat, control stick and a rudimentary display that showed bank angle, pitch, yaw, and angle of attack. The other ends of the beams each had jet nozzles that expended high-pressure nitrogen gas to provide the reaction forces.

The Iron Cross had a mass distribution which matched the inertia ratios of the X-1B research airplane, which was slated to become the first aircraft testing reaction controls in flight.

The pilot's control stick was unusual for the time because it provided control about three axes: roll, pitch, and yaw. There were
no rudder pedals to control yaw; it was instead controlled by lateral movements of the pilot's thumb on the top of the stick.

Use of the analog computer and Iron Cross showed that control techniques were somewhat different than with aerodynamic controls. They warned that perfectly trimmed flight would be difficult to maintain manually, and it was very easy to over control. No conclusive difference was established between ease of control with full-on, full-off controls and proportional jet controls, but pilots tended to use an on-off ("bang-bang") technique even when they had proportional controls. This later became the
preferred technique as pilots used reaction controls on the X-15.

Reaction controls were tested on only a few flights with the X-1B because it was retired from flight status after it developed
fatigue cracks in the propellant tank. Subsequently, a hydrogen-peroxide reaction control system was designed for an F-104 fighter which eventually tested the system at very high altitudes.

Reaction controls were next used on the X-15, and finally on all of the NASA piloted space vehicles.

Dryden Home > Collections > Movies Home > Iron Cross Attitude Simulator