The Braking Phase

The landing began with the braking phase at 50,000 feet. Why did the LM orbit ten miles high before descending? The moon has no atmosphere, so a vehicle could theoretically orbit very close to the ground. In practice, however, that would not be safe: one would need a virtually perfect, circular orbit to be sure not to intersect the terrain. Such perfection was not achievable in the real world—tracking, guidance, and a variety of other factors introduced uncertainties into the orbit. The same held true for the terrain. It's one thing to orbit down low above a perfect sphere, but quite another if a hill or a mountain pops up in the way. NASA engineers decided that the uncertainty in the orbit and the guidance was about 15,000 feet, and the uncertainty in their

Figure 8.7 c

Checklist timeline for Apollo 12 from PDI to landing, showing flow of commands down left side and then across to middle column. p

Tables are values of acceptable descent rates (h dot and delta h) versus altitude. (Annotations by the author from Apollo 12 Timeline S

Book, http://history.nasa.gov/alsj/a12/a12LM_Timeline.html [accessed January 5, 2007].) 8

Till. KUAH H Den

Mr^je iJI l exxx tbis DPS i

Figure 8.7 c

Checklist timeline for Apollo 12 from PDI to landing, showing flow of commands down left side and then across to middle column. p

Tables are values of acceptable descent rates (h dot and delta h) versus altitude. (Annotations by the author from Apollo 12 Timeline S

Book, http://history.nasa.gov/alsj/a12/a12LM_Timeline.html [accessed January 5, 2007].) 8

knowledge of the moon's terrain was about 20,000 feet. Thus they chose 50,000 feet as the preferred orbit to speed along without fear of hitting anything.24

The LM attained this orbit in two different ways. In the early flights, after the command and lunar modules undocked, the LM fired its engine on the far side of the moon, just about opposite from the intended landing area. On the later missions the vehicles remained docked, and the CSM fired its engine to bring them both to the lower orbit, thus saving fuel and weight on the LM (the CSM could then return to a higher orbit during the lunar stay). Either procedure brought the LM to an elliptical orbit of about sixty by ten miles. Both burns occurred out of sight of the ground (and hence in communications blackout), which introduced some anxiety, for an error of a few seconds in the burn could cause the LM to intercept the moon prematurely and crash.

The landing sequence really began when the two spacecraft reemerged from behind the moon, reacquired signals from Houston, and verified that the burn had succeeded and the new orbit was within acceptable limits. At this point, nicknamed AOS for ''acquisition of signal,'' the LM zoomed over the moon at nearly 4,000 miles per hour. The astronauts were wearing their spacesuits with the helmets and gloves removed, and they copied a series of vectors read up by voice to the ground, known as ''PADs'' (pre-advisory data). These data provided them up-to-the-minute reference data in the event of an abort with lost communications.

If all checked out in the 50,000 foot orbit, the astronauts began the descent procedure, initiating the braking phase with an event called PDI or ''powered descent initiation.'' Typing ''V37,'' pressing ENTER, and then ''63 ENTER'' initiated P63. This program calculated the ignition time based on the navigation state vector and the landing target, and then prepared to fire the descent engine when the LM reached its ten-mile perilune (the low point of its orbit). The DSKY then displayed the time to ignition, among a few other variables, for the astronaut to review. Depressing the PRO(ceed) button on the keyboard advanced to the next step. The computer then replied with a VERB/NOUN message asking the astronaut if he would like to align the inertial platform, to which he ordinarily answered no by pressing the enter key (it should have been aligned before separation). Initial requests to approve every calculation in the computer proved impractical, but the PRO button allowed the astronauts to give final approval before major activities.25

Next the computer maneuvered the LM to the initial attitude for the PDI firing, though not before presenting the angles for the astronaut to review, and waiting for a PRO keypress again. Once in this position, the displays briefly went blank, about thirty-five seconds before the burn, to indicate the computer was getting ready to fire the engine. Five seconds before the burn, the astronaut again had to review the display and press PRO before the engine fired. At this point the crew could reject the burn or delay it by up to five seconds. Any longer delay would mean going around for another orbit before trying again.

The burn began with the RCS thrusters firing for a few seconds for ''ullage'' to push all the fuel to the bottom of the tanks. Then the main engine began firing, marking the official moment of PDI. When the burn began, the DSKY's three-line display presented inertial velocity, the rate of descent, and altitude. The computer also started a clock, and later events until landing were referenced to this moment.

At the moment of PDI the LM pointed backward, orbiting the moon in a ''feet first'' position;the computer fired the descent engine ahead as the spacecraft pointed back along its path. For twenty-six seconds, the engine stayed at 10 percent thrust to allow the computer to trim the engine gimbals a bit to be sure it would fire through the center of gravity and not impart any additional motion to the vehicle besides slowing it down (the astronauts could barely feel this gentle thrust).

Once trimmed, after about thirty seconds, the thrust climbed to 100 percent, for about seven and a half minutes as the LM covered nearly 250 miles and slowed its velocity from 5,500 to 600 feet per second (from 3,750 to 410 miles per hour), and from an altitude of 50,000 feet down to under 10,000. Now, at any time, the crew could abort by pressing buttons marked ABORT (to abort using the descent stage) or ABORT STAGE (to jettison the descent stage and abort with the ascent stage only). In an abort, the crew workload would rise dramatically as they selected and commanded one of numerous different trajectories to make their way back for a rendezvous.

If everything went well, however, workload was much lower, as the major job of the crew during the braking phase was to monitor the guidance system, the primary PNGS navigation and the backup AGS. They also had a third source: as the LM traversed the front face of the moon, ground-based systems had excellent tracking, monitored closely by ground controllers. Divergence in the PNGS and AGS solutions would indicate a problem. In case of a discrepancy, the ground-based solution could provide an additional ''vote'' to decide which was most accurate and whether to proceed.

During the PDI burn, the LM could face either up or down. On Apollo 11 the astronauts faced down, giving themselves a visual confirmation of their height above the terrain. They then turned themselves over (actually ''yawed around'') with the hand controller, or the computer would have turned them over at a predetermined time, about three minutes into PDI at about 40,000 feet in altitude. Either way, they were now coming in on their backs, feet pointed toward the landing area, looking up toward earth to enable the landing radar to see the ground. About four minutes after PDI, the flight director in Houston took a survey of his team and cleared the LM ''go'' for landing (an exercise repeated at about 3,000 feet).

Around this time the landing radar began detecting the lunar terrain, a critical moment. A light marked ''altitude'' went out when the radar detected range, and ''velocity'' went when the Doppler signals came in to measure velocity (usually a few minutes later). Until this point, the navigation solutions were inertial, based on motions of the spacecraft measured by accelerometers, calibrated by star and landmark sightings, and updated with data from earth tracking. Now the landing radar provided literal ground truth, the moment when the LM navigated as much with data from the lunar surface as from the stars. It was the first direct sensing of the two quantities that really mattered: where was the spacecraft relative to the lunar surface? How fast was it moving? Without these critical numbers, the guidance solution could be off by several miles in altitude; without them an astronaut would likely not be able to eyeball his way down.

When the landing radar locked in, the LMP then entered VERB 16 and NOUN 68, which displayed DELTAH, the difference between the radar data and the onboard state estimator. If they were close, say within 10,000 feet at 20,000 feet altitude, it provided an excellent check of the overall system's accuracy.26 (This measure assumed a perfectly round moon. If the terrain were changing rapidly, as it would on later missions in mountainous regions, then the altimeter reading would change drastically even as the spacecraft descended gradually. Later missions actually included a rough model of the underlying terrain in the computer to account for these variations.) If the radar data looked good, that is if DELTAH was not too large, the crew ''accepted'' it with a VERB 57. The computer thus incorporated the radar altitude and velocity in a weighted average with the inertial estimates and issued a new navigation solution, one that should converge within a few seconds to approximately the landing radar altitude. When it occurred as planned, the appearance of the radar data relieved significant tension;the LM was now locked into its target, the moon. It also gave them a new primary means to measure their altitude, descent rate and along-track velocity, displayed on two tape meters and an x-y display, respectively.27 Incorporating the radar data and the computer's convergence on a solution was a major milestone in the landing.

If the radar did not come in, or if it came in with too much of a difference from the PNGS, or the new solution did not converge, mission rules demanded an abort. The LMP compared the H and DELTAH numbers with a printed chart on the timeline and checked it against the chart, as well as a variety of other parameters. He was operating like a computer himself, comparing values from the instruments against the ''dead man's curve'' (NASA encouraged the crews to use the safer-sounding ''abort boundary''). For each altitude, there was an acceptable DELTAH and descent rate that would allow a safe abort; if those limits were exceeded (i.e., if the LM were descending too fast for a given altitude), the LM could not recover in case the descent engine failed, so an abort would be mandated. Other problems might be indicated by program alarms, each with a number to indicate the problem. For instance, 1406 indicated that a guidance computation was failing, and 1410 indicated an overflow in the guidance equations.

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