Monday, July 20, 2009
Even Apollo Required Mundane Fixes
I wrote in my last blog about the new images of the moon which show the Apollo landing sites and even the astronauts, footprints on the moon. The photos also show the scientific experiments that were placed there to monitor the geophysical makeup of the moon. These experiments were collectively called the Apollo Lunar Surface Experiment Package (or ALSEP). Each Apollo mission carried a set of experiments but the mix of experiments was different on every flight. [The image to the left was from Apollo 16 and you can see the Lunar Rover in the background next to the Lunar Lander.]
I began working for Bendix Aerospace Systems Division in Ann Arbor, Michigan in 1970, right after graduating from the University of Michigan. Bendix was the prime contractor for the ALSEP experiments. The Apollo 11 and 12 missions had already successfully landed on the moon by the time I joined Bendix. Shortly after I started, the Apollo 13 disaster occurred (April, 1970) and it set back subsequent missions by more than a year. I was involved in engineering efforts on all the subsequent Apollo science packages (14 through 17).
A little background about the experiments: ALSEP was designed to be a package of experiments that shared a common communication and power source. The Central Station housed all of the communications electronics and telemetry systems. It is the tall, gold structure in the photo below. The antenna for transmitting data is the pole-like object on top of the Central Station. The gold is actually metal-coated mylar-plastic film and it was used to control the thermal environment inside the Central Station. Keep in mind that the moon in the daytime is over 200 degrees F and it is -200 degrees F at night. That is a tough environment and controlling the temperature in the electronics in all of the experiments was a major challenge.
The little gray, finned object to the left and behind the Central Station is the Radio Thermal Generator or RTG. It is the power source for all of the experiments. Electricity is generated by converting the heat from a Plutonium-238 fuel cartridge inside the RTG. As an aside, one of the Apollo 12 astronauts said that he could feel the heat from the Plutonium cartridge all the way inside his space suit when he was loading the cartridge into the RTG. That's hot (in more ways than one)!
The silver disk-like object in the foreground is actually a lunar seismometer, part of something called the Passive Seismic Experiment or PSE (the program just loved acronyms). It was designed to measure "moonquakes". Actually, you are not looking at the seismometer but at the thermal shroud (think blanket) which covered the seismometer. The instrument was so sensitive that if it had been directly exposed to the lunar day and night, it would have "creaked" due to thermal expansion and contraction. Even with the thermal shroud, it still creaked a little and buggered up the data.
All of the experiments (and there are three more that are out of the picture) were connected to the Central Station by cables. You can just make these out in the photo as copper/gold-colored ribbons in the lunar soil. They really were ribbons, by the way, ribbons that were about three inches wide, each of which carried multiple wires.
By the time I got to Bendix, the system and the experiments had already been designed (the work started in 1966). Bendix was in the process of building the final instrument packages and testing them to make sure they were ready for their missions. As a young engineer, my job was to work on problems that came up from experiences on previous missions. One of those experiences happened on the Apollo 16 mission. Astronaut John Young was deploying some of the experiments when his boot became entangled in one of the flat-ribbon cables. With the bulk of his suit and the changed sensations of lunar gravity, he was completely unaware that he had snagged a cable. When he moved, he accidentally tore the cable out of the experiment (it was an experiment for measuring heat flow) and the experiment was wrecked. Several million dollars gone in an instant. You can see this happen in the movie below if you start watching around the 11:36 mark. (The whole movie tells the story of the Apollo 16 mission. It is worth watching.) [movie from Internet Archive]
I was at work the afternoon that the ALSEP deployment was being broadcast live from the moon. We were all in a conference room watching a television and we could see Young get tangled up in the cable and everyone in the room started yelling at the set as though Young could hear us. There was a palpable gasp that went up from the group when we saw the cable snap. We knew before Young did that it was not going to be repairable. It is just not possible to put a broken cable back together on the surface of the moon.
Not surprisingly, shortly after the mission, orders came down from NASA to look at ways to prevent this from happening again. Despite the best of planning, nobody had really thought about strengthening the cable connections. Each of us has tripped over an extension cord or laptop power cable sometime in our lives. Usually, a plug pops out of its receptacle but sometimes plugs can be damaged or broken. Same here. But there were no plugs to come undone (the cables were hardwired in place). The cable snapped. My job was to design a better cable strain relief that would withstand the impact of a skipping astronaut in full gear. We couldn't change the basic cables so we had to find a way to spread the load out. A combination of well-rounded mounting brackets and reinforcing fiber tapes seemed the best way to get the job done.
But how do you test something like this to know it will work? You can't go to the moon to do it. We rigged up a test system in the lab and enlisted our "test astronaut" to see he could break the cables in a deliberate accident. What you see in the first picture below is the ALSEP package of experiments (a non-flight model) in the form that it is stowed in the Lunar Lander and before it is unpacked. Attached to the front of the package is our redesigned test cable with a heavy lace to put around the test astronaut's leg. Yours Truly is standing with my back to the camera in the stripped shirt.
It didn't seem like a fair test to have the high friction between the bottom of the ALSEP package and the floor of the lab so we put the package on a greased plate to decrease the friction and more closely simulate the friction in the lunar dust.
The next photo shows the test in progress with the astronaut putting his full weight into the cable. Two assistants stood on either side of him to catch him if he fell. You can see the cable snap taut.
The last photo shows me examining the cable stress relief after the test. It worked; the cable was intact and fully functional. This wasn't the only testing but it was the most convincing. We retrofitted all of the cables on the Apollo 17 mission with the new strain reliefs. Of course, the astronauts were now extremely mindful of getting tangled in the cables and nothing untoward happened on that mission.
Complex missions like the Apollo Program were staffed by tens of thousands of engineers doing the same sort of unglamorous, day-to-day jobs like fixing cables. Nothing is perfectly designed the first time. Not every contingency can be planned for. (If you want to see the ultimate example of this, rent the movie Apollo 13). To tell the truth, many days were pretty mundane but I still feel proud of the fact that I was there and had a tiny part in the Apollo missions. We have so few things in life that command the kind of vision that the Apollo Program did. It brought out the best passions in so many people. As Georg Hegel, the German philosopher said, "We may affirm absolutely nothing great in the world that has been accomplished without passion." Apollo proved that to me. I wish there were a comparable challenge to stir the dreams of the next generation.
Postscript: The ALSEP packages went on sending back data until September 30, 1977 when for the reason of budget cuts, the experiments from all the flights were powered off to save costs on the earth-side monitoring station.
P.S.S. Today mark's the 40th anniversary of the first lunar landing on the Apollo 11 mission.