Computer incorporates sighting and IMU angles

6) Computer determines direction to landmark and star, compares with expected direction, updates state vector and error matrix

1) Astronaut enters code into DSKYto identify landmark and star to computer

1) Astronaut enters code into DSKYto identify landmark and star to computer

6) Computer determines direction to landmark and star, compares with expected direction, updates state vector and error matrix

Figure 5.4

Illustration of astronaut use of optical system and computer to track a landmark for update of navigation state vector. (Redrawn by author from Draper et al., ''Spacecraft Navigation Guidance and Control,'' 2-74.)

North Americans did not like this scheme because it was complex, added weight, and required special seals so the pressurized cabin atmosphere would not leak around the mechanism. The astronauts saw the possibility that the optics could not be retracted in the case of a failure of the mechanism, which would compromise the heat shield on reentry. They renamed the ''deployable optics'' the ''deplorable optics.'' David Gilbert, the NASA program officer who had just taken over responsibility for guidance and control, remembered stepping into ''a hell of a squabble'' between MIT and North American over the issue. Finally, he solved it by decree—''we decided we could live with a little less field of view and ended up with a fixed optics installation.'' That is: no door, no deployable assembly, simply a fixed window for the telescope to look out the side of the spacecraft. If the sighting needed a different view, the astronauts would move the spacecraft itself to bring it around. Gilbert remembered a tough decision: ''I guess the first bad move I made against MIT.''66

Figure 5.5

The Block II Apollo guidance computer crew station in the command module. Note the eyepieces for the space sextant and scanning telescope (top), the hand controls for aligning the sights (middle), the computer itself (bottom), and the DSKY (right). The Block II computer was sealed as a single unit. (Draper Laboratories/MIT Museum.)

Figure 5.5

The Block II Apollo guidance computer crew station in the command module. Note the eyepieces for the space sextant and scanning telescope (top), the hand controls for aligning the sights (middle), the computer itself (bottom), and the DSKY (right). The Block II computer was sealed as a single unit. (Draper Laboratories/MIT Museum.)

Why was the issue so contentious? What was at stake for the MIT engineers? David Hoag explained that early on, they began looking for ways to quantify their system's performance—''how could we measure that we had done a good job in our design?'' One way was to calculate the optimal amount of energy required to get to the moon. Based on Kepler's laws for a given trajectory, you'd need to use a certain amount of energy for a certain number of velocity changes, said Hoag. ''You couldn't go to the moon with any less propulsion than that.'' The closer the actual guidance and navigation system could get to that minimum required level of energy—and thus the minimum possible fuel use—the better job the MIT engineers would be doing from an engineering point of view. Hence the deployable optics: if the optics could swivel around to a wide field of view, then the spacecraft's thrusters—and its precious maneuvering fuel, their measure of accuracy—would not have to burn to calibrate the optics.67

IL engineers' goal wasn't only to get to the moon but also to do it the best way possible, with the least amount of energy and the highest possible accuracy. Only reluctantly would they give up that goal for other considerations. In the end, they compromised on the optics (or at least accepted NASA's decree), one of many such decisions where engineers had to forgo their optimal designs for the greater goals of the project. It was a classic engineering trade-off—complexity and safety (of the deployable optics) versus a little more fuel use. Here NASA played its role as a system engineer—making decisions to arbitrate disputes between contractors, between subsystems, and between conflicting criteria of performance.

Hoag's view of these conflicts is revealing: ''One of the most frustrating things was that we couldn't design alone . . . all these things came in and it was a communications chaos really but all turned out to be pretty skillfully handled. In retrospect, the experience was pretty rough.''68 In theory, the guidance problem was mathematical, even beautiful. Even when implemented in an inertial system, it retained a sparse elegance that the IL engineers had come to love. Yet when combined with other organizations, a human operator, and a real system, the abstract purity of the guidance scheme had to adapt to the gritty realities of a large project.

Telescopes Mastery

Telescopes Mastery

Through this ebook, you are going to learn what you will need to know all about the telescopes that can provide a fun and rewarding hobby for you and your family!

Get My Free Ebook


Post a comment