Lunar Landing Research Vehicle - Lunar Sim Mode

Lunar Sim Mode

There were two distinct modes of flight for the LLTV. The basic mode was with the gimbaled engine fixed to the body so that it always pointed downward in relation to the body. But in the gimbaled "Lunar Sim Mode", the engine was allowed to swivel and was kept pointing downward to the earth. This allowed the vehicle to tilt at the far greater angles that would be typical of hovering and maneuvering above the lunar surface. Despite its ungainly appearance, the LLRV was equipped with an astonishingly sophisticated array of early sensor and computational hardware. The system had no specific name, but the effect it produced was called "Lunar Sim Mode". This was the highest degree of hardware-based simulation, and was the purpose of the whole project. This was not a system to unburden the pilot, such as an autopilot does, nor was it meant to introduce any sort of safety or economy. The system's sole intention was to project the illusion of piloting the Lunar Module. So, Lunar Sim Mode can be thought of as a mixture of stability augmentation, recalculation of vertical acceleration according to the lunar gravity constant, all followed by accompanied instantaneous corrective action. The LLRV's Lunar Sim Mode even was able to counter-correct wind gusts within milliseconds, as they definitely would have disturbed the impression of a missing atmosphere. Sensor input for the Lunar Sim Mode was the Doppler radar. The visually significant sign of an engaged Lunar Sim Mode was the free-gimbaled turbofan, always strictly pointing downward toward the ground, regardless the LLRV's current attitude. This unique aircraft represents one of the few hardware simulators that ever became airborne.

Donald "Deke" Slayton, then NASA's chief astronaut, later said there was no other way to simulate a moon landing except by flying the LLRV.