The Sun Compass: Getting back Home

Dave Scott and I have talked about the importance of the Sun compass for these first forays on the lunar rover. It is a common misconception that if the rover malfunctioned and ceased to run, then all one would have to do is follow the rover tracks back to the LM. That would be all right if Dave and Jim drove in a straight line to and from the LM. In reality, their EVA travels took them on indirect routes from the LM. A 1970 Bellcomm report states that if the surface was flat, a 6-foot moonwalker could see the LM from approximately 6 km away. Unfortunately, the Moon is not flat and has obstructions such as ridges, rocks, and crater rims that can block the LM from view. If a two-meter hill ran along the traveler's route between the rover and the LM, then he might pass the LM without seeing it.

As stated, a moonwalker previously could, hypothetically, average one kilometer in twenty minutes while the rover can drive the same distance in five minutes. The distance traveled plays a significant role in determining the length the explorers can go to allow them time to walk back to the LM. Most of the lunar rover EVAs were driven to different points of interest along various routes that did not return directly to the LM. Figure 1 shows a lunar rover EVA terrain map Dave used to design an illustration to show the different EVA routes. Dave focused on the EVA 2 route to Hadley Delta and Spur Crater. The course runs South for three kilometers to the mountain and then turns East for two kilometers. The route essentially forms a right triangle.

Fig 1: Dave Scott drew this map example

Dave's chart points out the issue of using a visual sighting on a landmark from two different observation points on the EVA. The route for the second EVA took Dave and Jim to the slope of Hadley Delta, and then they turned East to drive along the slope toward Spur Crater. If Dave were to make a sighting on a distance northern peak at the position of his first turn to the East and then make a similar observation at the end of his route at Station 5, he would get two different bearings back to the LM. The visual bearing from Station 5/Front Crater at the end of the outbound track would take him approximately 700 meters to the East of the LM. If there were obstacles between the LM and the lunar rover, then Dave and Jim would fail to see the LM in the distance. A mistake like that could catastrophically deplete their supplies of Oxygen, Water and Electrical power,

Fig 2: Dave Scott's example with the right triangle for reference     

The diagram in figure 2 includes a graphic of a right triangle (the route forms an obtuse triangle, but let's keep it simple) for comparison with Dave's example. The vertical leg of the triangle shows the rover’s journey South towards the slope of Hadley Delta. The vertical leg is the first viewpoint of a peak in the distance beyond Mount Hadley. The base leg of the triangle shows the eastward travel from the point of the rover’s turn eastward to its farthest stop at Station #5 near Front Crater. The hypotenuse represents the second viewpoint of the distant peak. Dave was explaining to me that he could have taken a visual heading at each point and come up with a different route, so just visually sighting the peak and walking directly at the mountain from Station #5 would have the crew pass by the LM by 700m to 1km to the East. Dave said that the terrain was undulating and if he and Jim were below a ridge, then they might have passed by the LM and not known it. The Sun compass would allow Dave to find the correct heading directly back to the LM.

                                                                                                                                                         Fig 2A: Apollo 15 Lunar Traverse Geometry diagram by TRW

During the continued research of the Sun compass a series of MSC Internal Notes (papers) by MSC Mission Planning and Analysis Division were found in the NASA archives. These papers showed that TRW and NASA tested the accuracy of the Sun compass in Houston prior to the launch of Apollo 15. In one specific paper titled, "Analysis of the Sun Compass Position Determination Accuracy for Apollo 15 Lunar Surface Traverses" dated July 26, 1971 provides a diagram in figure 2A that shows how accurate the Sun compass would be during a direct walkback to the lunar module from various locations on all three EVAs. The paper drew the following conclusions that there were few stations on EVA1 and EVA3 and no stations on EVA2 for a direct walking return, that Dave and Jim would need an initial position from the LRV navigation system, and that a return to the LM can be done without significant detour from a straight line to the LM. While deviations from the direct return were a requirement, they would be within the Sun compass parameters.

Now, why is knowing the proper compass heading back to the LM so important? The answer essentially boils down to oxygen and water. How much of O2 and H2O remains in the PLSS at the time of the rover failure will determine the best and fastest track back to safety. The PLSS (Personal Life Support System) provided O2 and H20 for up to eight hours, and the OPS (Oxygen Purge System) provided thirty minutes of high-pressure emergency O2 needed if the PLSS failed. Those supply reservoirs placed time and distance limits on all manned lunar EVAs with the most extended lunar surface EVA being 7 hours, 36 minutes and 56 seconds during Apollo 17’s second EVA. Apollo 17 also traveled 7.6 miles which is still the longest distance from the LM.

Fig 3: Apollo 15 Lunar Surface Procedures Walkback estimate page

The Apollo 15 Lunar Surface Procedures, final edition, printed on July 9, 1971, provides insight into NASA’s thinking on time and distance on each LRV EVA. As Figure 3, “Table 3.6-4” shows, the EVA plan estimated a three kilometer per hour walking pace for the moonwalkers. They predicted that Dave and Jim would need two and a half hours to walk back to the LM from Station 5/Front Crater and still have a thirty-minute reserve left in their PLSS. Based on the effort that Al Shepard and Edgar Mitchell put into their second EVA to Cone Crater, that continuous three km pace would have been optimistic. Therefore, the best possible route would have been to follow the most direct path to the LM. That would require Dave and Jim to take a compass bearing on a distance landmark (Mountain to the North beyond Mount Hadley) and use that bearing to figure the most direct heading to the LM some three and a half miles in the distance. Although the fastest way home was a straight line, one look at the topographical map that Dave used shows that the South Complex was directly in their path to the LM. In all probability, the moonwalkers would have to change direction to circle and bypass the craters in the cluster. The cluster and the area had ridges and rims all along the course, so the sun compass would be needed to make changes in their heading back to the LM to allow for the fastest possible route to the safety of the LM.

 

Comments

[Susan Roy]

thank you for taking the time to write this!