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The primary focus of the NASA space program during the 1960's was on the goal of safely landing astronauts on the Moon before the end of the decade. It was a mammoth undertaking that involved all of the NASA centers. Dryden Flight Research Center made a number of contributions to the NASA space program during the 1960's.
For example, the X-15 rocket plane pioneered flight controls used in space, the Paresev studied the concept of paraglider landings for a space vehicle, and the lifting bodies explored the use of wingless spacecraft that could glide to a precise landing; but it was the tiny, Lunar Landing Research Vehicle (LLRV) that had the most direct impact on the Apollo missions to the Moon.
All of the time and expense lavished upon the Apollo lunar missions ultimately hinged upon the last few minutes before the lander touched the lunar surface. This was a daunting fact when the program began in 1961. The first landing would be an entirely new experience for any astronaut, and it had to be perfect. NASA chose three approaches for lunar landing training: (1) An electronic flight simulator, (2) an outdoor, lunar-landing-type vehicle that "flew" suspended from a large gantry and employed hydrogen-peroxide powered attitude-control and thruster rockets as well as a cable system for control, and (3) the free-flying training vehicles, which evolved from the Lunar Landing Research Vehicle.
The lunar lander, called a Lunar Excursion Module, or Lunar Module (LM), was designed for vertical landing and takeoff, and was able to briefly hover and fly horizontally before landing. At first glance it seemed that a helicopter could be used to simulate flying the LM, but early test flights proved that it was not even close. Helicopters, or any vertical takeoff and landing (VTOL) aircraft, are subject to the influences of winds, air temperature, and the Earth's gravity. In order to simulate flying near the Moon, the flight vehicle had to automatically nullify the effects of nature so it would behave as if it were operating in a vacuum, and it had to respond as if it were subject to the much lighter lunar gravity.
Ideas for this unique type of flying machine began circulating at Dryden in early 1961. By the end of the year, it awarded a study contract to Bell Aerosystems. Bell was the only firm in the United States that had a great deal of experience developing VTOL aircraft using jet lift for takeoff and landing, and was coincidentally doing its own research on a free-flying simulator when it was approached by NASA. On February 1, 1963 Bell received a contract for the design and fabrication of two LLRV's. They were delivered to Dryden during April 1964.
The LLRV had a tubular metal framework with sheet-metal truss construction surrounding a 4200 lb-thrust, General Electric CF700-2V turbojet engine. The jet was attached to a large movable-gimbal arrangement. The gimbal permitted the engine and the framework to move independently. Propulsion and flight control were accomplished using a combination of the jet engine and rocket thrusters. It had no aerodynamic control surfaces.
The jet was controlled automatically to simulate flight within the lunar environment. Aerodynamic drag forces were opposed with vectored thrust so the vehicle would respond as though in a vacuum, and the jet supported five-sixths of the vehicle's weight to simulate lunar gravity.
Flight control of the LLRV was mainly done with small, hydrogen-peroxide rockets. Two sets of eight rockets were mounted around the vehicle to make it turn, or pitch up and pitch down. For added safety, each set was independent of the other, and the pilot could use either one or both.
After the jet engine had throttled-up enough to simulate lunar gravity, the LLRV's vertical movements were controlled by two rockets mounted in the center, next to the jet. There were also six emergency lift rockets that could also be used in the event of a jet engine failure.
The pilot's controls and flight instruments were in a forward-mounted cockpit that hung about six feet above the ground. The pilot sat in a rocket-propelled, Weber ejection seat. The LLRV was flown using a conventional stick and "rudder" pedal system for controlling attitude and yaw. Control inputs were sent to the thrusters through direct electronic signals, without the use of mechanical linkages.
The LLRV was able to duplicate the "feel" of the LM's controls, and it was equipped with some LM instrumentation, such as a radar altimeter, a Doppler radar for measuring velocity, and an accelerometer that gave indications in units of lunar gravity, from -1 to +10.
Fuel constraints limited most flights to about ten minutes, but a lunar-landing-simulation maneuver could be done in nearly two minutes. The task called for lift-off with the turbojet gimbal locked in position (VTOL mode). The pilot would take the craft to an altitude of about 200 feet while moving to a ground marker 400 feet ahead. This could take about 8 seconds. At that point, the pilot would change heading 90 degrees, and maintain a hover. Then the VTOL mode was disengaged, and the LLRV was flown in lunar mode: the jet's gimbal was unlocked, and lunar flight conditions were automatically simulated. A system of gyros and hydraulic servomechanisms kept the jet essentially vertical with respect to the ground, regardless of the vehicle's attitude.
The pilot continued the hover by using thrust from the lift-rockets. The vehicle would then be pitched-down to begin traveling forward, as it descended to a landing marker 800 feet ahead. The sink rates for the descent were kept to less than ten feet per second.
Approaching the marker, the pilot would use a pitch-up maneuver to slow the LLRV's horizontal speed, then establish a hover about ten feet over the landing spot, and make any final corrections before touching-down.
Pilots described the feeling of flying in the lunar mode as one of "slow motion" compared to Earth mode VTOL operation. Large attitude angles were required to start or stop horizontal flight, while a lot of lead-time was needed to slow the vehicle over a specific spot. The pilots were forced to operate at much steeper attitudes, and for longer duration, than required for conventional VTOL operation.
By mid-1966, NASA had accumulated enough data from the LLRV flight program at Dryden to give Bell a contract to deliver three new vehicles, known as Lunar Landing Training Vehicles. These were sent directly to the Manned Spacecraft Center (now, Johnson Space Center) in Houston, Texas, where they were used to train the Apollo astronauts. The original LLRV's were modified as LLTV's and used in Houston as well.
Donald "Deke" Slayton, then NASA's astronaut chief, said there was no other way to simulate a Moon landing except by flying the LLTV.