science tumbled

Damn Interesting is back, with the story of the one and only manned descent to the depths of the Mariana trench, the deepest point on the Earth’s surface, in 1960:

Partway into their long descent, Walsh and Piccard grew alarmed when they discovered they were no longer able to raise the mother ship on the sonar/hydrophone communication system, even after repeated attempts. The two men were thus left truly isolated from the outside world. Curled up in the cramped, cold, and dimly-lit sphere, the adventurers continued their hours-long downward journey with only one another’s voices and the occasional pop or groan from the Trieste’s strained hull to punctuate the anxious silence.

At approximately four hours into their descent–several thousand feet above the sea floor–a sharp clang sounded through the pressure sphere and the vehicle shuddered violently. Once their wincing subsided, the men did what they could to inspect the craft and its condition. It seemed that the water pressure at this never-before-encountered depth–six tons per square inch–had cracked the outer pane of the lucite window. For the moment the vehicle itself remained watertight, but the damage was worrisome. The Trieste was outfitted with a few safety systems; for instance, the ballast doors were held closed by electromagnets, so in the event of electrical failure the doors would fall open and drop the ballast, causing the vehicle to rise to the surface. But such systems would be of no help to the men inside if the 1,000 atmospheres of pressure crushed their delicate passenger compartment. Moreover, no other vehicle in existence was capable of reaching such depths, which meant that if her float tank became compromised there was no chance of rescue. Nevertheless, the stalwart scientists opted to press on.

About three quarters of an hour later, the bathyscaphe Trieste made history as its hull came to a gentle rest on the silty floor of the Challenger Deep abyss. The Trieste and her crew had spent four hours and forty-eight minutes in transit. The bathyscaphe instrumentation indicated a depth of 37,798 feet and external pressure of 1,099 atmospheres–approximately eight tons per square inch.

Maybe you’re all tired of articles about the moon landing, but I found this description of the sheer dumb luck involved in the moon lander’s computer making a successful descent fascinating:

Allan’s descent program needed a routine to accurately estimate the new thrust level, which could be accomplished by reading the “delta-V” (change in velocity) measured by the LM’s accelerometers. He wrote a short routine that took into consideration, i.e., compensated for, the engine’s lag time, which TRW’s “interface control document,” full of useful information for the programmers, said was 0.3 seconds. It took 0.3 seconds for the LM’s descent engine to achieve whatever thrust level the computer might request. The final version of the thrust routine, which was put into the LM, was written by Allan’s friend Don Eyles. Eyles was sufficiently enthusiastic about the programming challenge that he found a way of writing it which required compensating for only 0.2 of the 0.3 seconds. The IBM 360 simulator showed Eyles’ program worked beautifully. His routine was aboard Apollos 11 and 12 which landed successfully. However, telemetry transmitted during the landings later showed something to be very wrong. The engines were surging up and down in thrust level, and were barley stable. A guy at Johnson Space Center called Allan and informed him that the LM’s engine was not a 0.3-second-lag engine after all. It had been improved some time before Apollo 11’s launch such as to lower the lag time to only 0.075 seconds. Correction of this item in the interface control document had simply been overlooked. Once this discrepancy was discovered, the IBM 360 simulator was reprogrammed to properly simulate the actual, faster engine. Running on the simulator, Don Eyle’s thrust program, with the 0.2-second compensation, exhibited the surging that had occurred on the real flights. But here’s the most interesting fact: the simulator also showed that had Allan Klumpp chose to “correct” Don Eyles’ program by compensating for the full 0.3 seconds that was printed in the document, the LM would have been unstable and Apollo 11 would never have been able to land. By pure luck, Don Eyles was creative enough to write the thrust routine in a way that kept the LM just inside the stability envelope and allowed successful landings!

Allan’s descent program called “P64” periodically computed a polynomial function to describe the optimum descent trajectory. This polynomial would smoothly merge the LM’s current position and velocity vectors into the target point position and velocity vector. The “target point” for P64 was just above the landing point (When the LM reached the target point with a small vertical descent rate, P64 would cease execution and the landing phase would be handled by a program called “P66”). The computer would then make the LM fly the trajectory, which would be recomputed every 2 seconds. An opportunity for disaster presented itself here. Many sci.space.tech readers may know enough mathematics to understand the undesirable “wiggles” that can be generated by high-order polynomial curve fits. Under conceivable circumstances, the polynomial function computed by P64 could droop down, go beneath the lunar surface, rise out of the surface, then descend to the target point! If such a trajectory were computed during a real landing, and the LM were allowed to follow it, the LM would crash. There was no logic coded in to detect this situation and prevent it. No programming solution was ever found. An example scenario where this disaster could have happened follows. If the LM was off course, away from the terrain model stored in the computer, and flying over a deep crater, the landing radar would fool the computer into thinking the LM was higher relative to the mean surface than it previously assumed. This could cause a newly computed polynomial trajectory to “droop” down sharply, unintentionally intersect the real lunar surface, then rise back out of the surface, inviting the LM to crash! Allan said this problem could also be caused by an astronaut retargeting the landing point beyond the fuel range.