Controlling a Spaceship

Any story of a lunar landing must of course begin with the LM, among the strangest and most interesting craft to fly in the twentieth century (figure 8.1). ''Fly'' in this case is a loose term—the LM never had to work within the earth's atmosphere, to fly in air. The command module carried a sleek, aerodynamic shape around the moon because it had to push through the air on the way up and burn through it on the way down. Not so with the LM, whose odd and seemingly random protrusions kept it

Figure 8.1

LM major equipment locations. (Grumman Aerospace Corporation, ''Apollo News Reference," Bethpage, N.Y., n.d. approx. 1970, GN-11.)

Figure 8.1

LM major equipment locations. (Grumman Aerospace Corporation, ''Apollo News Reference," Bethpage, N.Y., n.d. approx. 1970, GN-11.)

from resembling the fast beauty of aircraft or missiles. Still today it is the only human-occupied vehicle built to work entirely outside the earthly environment. The LM's exterior was pure function, each bulge and wrinkle reflecting a specific fuel tank, radar receiver, or human task. A practiced eye can read it like a text and see in the LM an expression of post-Cubist engineering art.

Like its earthly counterparts, the LM was a multistage rocket, but unlike them the flight down came first and the flight up came later. The two-stage LM consisted of an octagonal descent stage, which carried a large rocket engine, power supplies, and the landing gear, coupled to an ascent stage, which carried a smaller rocket engine, the pressurized space for the human crew, life support equipment, flight controls, and the Apollo computer. The entire system would land on the moon. When the astronauts had completed their lunar duties, the descent stage would become the launch pad for the ascent stage, which would separate, rise, and rendezvous with the CSM. In an emergency abort on the way down, the astronauts could separate the descent stage and return in the top half, or abort with the entire craft. Four clusters of reaction jets, located on the ascent stage, controlled the attitude of the vehicle—a particularly challenging control task for the computer, because from landing to ascent the mass of the LM would change by nearly a factor of ten.

Engineering the LM has come to be known as one of the extraordinary engineering projects of its era. The project overall has been well chronicled by its chief engineer, Tom Kelly, and by his boss Joe Gavin.2 Grumman built the craft in its Bethpage, Long Island, facility, and, like the IL, served as systems integrator for a variety of subcontractors.

Grumman was an old Navy aircraft shop—its engineers were used to working with pilots. Howard Sherman, for example, designed much of the human interface for the LM, and had strong feelings about pilot's roles: ''The designers are more used to airplanes and the pilots are a pain in the ass in airplanes from their point of view.'' Sherman felt the deference shown by the Grumman engineers to astronaut input did not always produce the best solutions.3 Gavin recalls a difficult time getting the astronauts to accept the digital controls in the LM.4

Where the MIT team measured its success according to accuracy achieved, Grumman held light weight as its key engineering value. Still, Grumman's navy heritage, of building robust airframes to survive hard landings on aircraft carriers, showed up in the very structure of the LM. Its heavy landing gear dominates, designed to withstand a drop to the moon's then-unknown surface. Nearly every other feature, however, was new for Grumman (as they would have been for any engineers). Initially envisioned with a round cabin where the crew was to sit as in a helicopter, the LM eventually had no seats at all;the crew stood as they flew, restrained by tensioned cables. This posture allowed the windows to be reduced to small triangles, finely tuned to the astronaut's field of view, saving tens of pounds of weight in glass. Technicians shaved

Figure 8.2a

LM guidance components MIT view (a) and Grumman view (b). (Hand, ''MIT's Role in Project Apollo, Vol. III,'' 52; Grumman Aerospace Corporation, ''Apollo News Reference,'' Bethpage, N.Y., n.d. approx. 1970, GN-17.)

Figure 8.2a

LM guidance components MIT view (a) and Grumman view (b). (Hand, ''MIT's Role in Project Apollo, Vol. III,'' 52; Grumman Aerospace Corporation, ''Apollo News Reference,'' Bethpage, N.Y., n.d. approx. 1970, GN-17.)

critical ounces by whittling the LM structure to paper-thin tolerances, even chemically etching the fuel lines, for instance, to the smallest possible thickness.5 The LM's walls were so thin that the pressurized upper stage resembled an inflated balloon as much as a rigid structure.

Since the LM was intended to carry a human crew, its relationship to the occupants was critical. Grumman Engineering Manager Tom Kelly remembered NASA's Chris Kraft convincing Grumman that ''the crew's time and energy was the most precious commodity on the mission.''6 Could the astronauts stand firmly enough to operate

Figure 8.2b

(continued)

Figure 8.2b

(continued)

the craft as it rocked back and forth? Could they egress the hatch while wearing bulky space suits without getting stuck or snagged? A crewman could not, for example, carry an incapacitated companion up the ladder into the LM—any accident must leave an astronaut able to climb a ladder himself, otherwise he could not return. And, of course, how would the pilots control the vehicle? What sorts of sticks, levers, buttons, and dials would they need? How much should the computer control? At the highest level of design, the LM had to match the capabilities of the crew and the shape of the descent trajectory (figure 8.2a, b).

The first LM with people on board flew on Apollo 9, in March 1969. This test of the vehicle in earth orbit exercised its numerous computerized and backup flight modes. Inside the LM, astronauts Rusty Schweikart and Jim McDivitt separated from the command module and flew about sixty miles away as a test, separated the descent stage, and then returned for the first Apollo rendezvous. They fired the descent and ascent engines two times each, and tested the ''lifeboat mode'' of the LM (which would become critically important on Apollo 13), using its engine to power back from the moon. Particularly complex was testing the digital autopilot's behavior. It had to account for three major modes: (1) a docked configuration, the CSM and LM together; (2) the full LM with the descent stage; and (3) and the small LM ascent stage on its own. Each of these had different dynamics, mass, and handling qualities affected by their odd geometries, bending moments, and fuel slosh. After exercising them all, the crew reported that ''the autopilot is the optimum control device for performing the entire lunar mission.''7

While Schweikart and McDivitt were testing the LM, David Scott remained behind in the command module. After they returned, in further tests Scott got to ''fly'' the command module by hand, with the main engine on, following needles on the display panel, experiencing true hand-piloting (Gemini style) of a spaceship for a few minutes. In addition, Scott was familiar enough with the computer to be creative with it, asking it to do new things (causing engineer Jim Nevins some concern, because the software always had bugs in it and had been certified only for performing standard procedures). Scott took the coordinates for Jupiter from a star chart; he entered them into the computer, which pointed the telescope directly at the planet. At the end of the mission the command module jettisoned the empty LM. Scott uploaded the orbital parameters of the LM from the ground and asked the computer to calculate the sight angle for the telescope. The computer pointed the optics and Scott was able to see and track the discarded LM in the crosshairs as it swung around its death orbit 2,500 miles away.8

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