Training for a Landing

Like all designs on paper, the landing trajectory, related computer programs, and crew procedures seem elegant, even perfect when viewed in isolation. But long before they were put into practice, a critical question arose: how to match the human operator to the trajectory? That is, how do you train people to perform the actual task of landing the spacecraft? As the most uncertain part of the Apollo flights, the landings called for the most preparation.

Again NASA turned to simulation. Indeed the breadth and variety of simulations for Apollo exceeded anything in previous programs. During Mercury and Gemini, crews spent about a third of their training time in simulators; by Apollo it was closer to half.39 The earlier programs had four distinct simulators each. Apollo had no less than eleven. These included procedure simulators (for both CSM and LM), translation and docking rigs, centrifuges, and partial-gravity machines to simulate lunar walking.

By far the most important, however, were the ''mission simulators,'' one for the CSM and one for the LM, which aimed to replicate as much of the mission as possible with high fidelity. In addition to accurately reproducing switches and indicators from the two vehicles' control panels (according to computer models of the systems' dynamics), the mission simulators included thruster noises, cabin decompression, and the sounds of firing pyrotechnics. Each comprised a huge ''train wreck'' of computers, spacecraft hardware, film projectors, models, and analog video equipment.40 They were supplied by the spacecraft vendors themselves, often out of parts rejected for flight hardware, and replicated conditions inside the craft and the appearance and behavior of the controls (although they were ''fixed base,'' meaning they did not move).

Both Grumman and North American subcontracted the job of creating mission simulators to the Link Company (a division of General Instrument), which had produced the famous ''Link trainers'' of World War II. Link delivered one set of mission simulators to Houston and another to the Kennedy Space Center (actually two for the CSM and one for the LM) to allow the astronauts intensive training right up until launch day. The mission simulators were run by enormous digital computers, several for each, with extensive optical systems to synthesize views outside the windows during missions. Data lines connected the computers to mission control, so ''integrated simulations'' could be performed that included not only the vehicles themselves but also the flight directors and ground support teams.

Staying current with the rapidly developing spacecraft proved a constant challenge, and the simulators themselves faced overwhelming problems of configuration control. Trying to simulate the behaviors of the Apollo computers proved difficult because the software changed so frequently. Instead, an entire mainframe was dedicated to simulating the computer itself, which ran the actual flight software. When the IL released the code for rope manufacture, it also sent a copy to NASA for loading directly into the simulator. This third-party verification helped build confidence in the code and also revealed some errors.41

Simulator instructors worked long hours to get the crews ready. Simulator availability became a pacing item in crew training. Michael Collins called simulator training ''the very heart and soul of the NASA system.'' One was not ready to fly until having proven one's skill on the simulator.42 He might have added that the spirit of the simulator, buried inside code in the computer, would accompany the astronauts on their journeys.

The ''lunar mission simulator,'' or LMS, ran off three mainframe computers, and included five tons of glass—lenses, mirrors, and projectors to accurately recreate the scenes of a landing. Astronauts could practice sighting landmarks, entering data into the computers, and simulate landings from about 12,000 feet to touchdown. As the pilots ''flew,'' computer models of the LM's motions directed a small camera above a physical model of the lunar terrain, sixteen feet in diameter at 1:2,000 scale

(actually, the terrain model was mounted upside down, so the camera looked up at it from below). Craftsmen made the models from Surveyor and Orbiter spacecraft images, and later updated them with data from the early Apollo missions. Their three-dimensional models recreated specific landing sites with a resolution of ten feet. A network of servos ''flew'' the camera over the diorama, right down to the point of landing. Accurately creating these scenes proved a particularly difficult problem—errors in one of the dioramas caused David Scott to become disoriented during his landing on Apollo 15.

A history of simulation technology in the space program has yet to be written, but it would show how the creation of virtual reality preceded, rather than responded to, the creation of real-time computer graphics. In fact, simulations during the Apollo program became so sophisticated that visual representation became their weakest link.43 Yet the pilot's vision in the critical final moments was to be the central human function in the landings.

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