OT: Apollo Splash Down Module

OT: Apollo Splash Down Module

There might be some metalworking in here, but that is not the question. This is more of an aerodynamic question.

Would the splash down module self correct in free fall r did it "have to be" oriented properly. I don't mean if it had massive out of control inertial spin/tumble, but rather if it just happened to be out of correct orientation when it started to hit significant atmospheric drag.

Long gone are the days when I could ask my grandfather a dumb question like this and he could show off by asking the eggheads over at Lewis before responding.

Anything I read on this topic is long forgotten. I've haven't been a space nerd in a long time.

Reply to
Bob La Londe
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It is designed to orient itself correctly solely from aerodynamic forces. Which is actually easy to do, and far more reliable than any active controller could ever be.

Civilian aircraft are also designed to be aerodynamically stable, so they won't do anything dramatic if the pilot lets go of the controls.

Modern fighter aircraft are designed to be aerodynamically unstable, rendered stable by a fast-acting control system, the reason being to improve maneuverability, which is useful in dogfights, and outmaneuvering anti-aircraft missiles.

With an unstable aircraft, if the control computer is out for more than something like a tenth of a second, control cannot be recovered, and the wings are likely to soon tear off. A very bad day.

Joe Gwinn

Reply to
Joe Gwinn

OT: Apollo Splash Down Module

There might be some metalworking in here, but that is not the question. This is more of an aerodynamic question.

Would the splash down module self correct in free fall r did it "have to be" oriented properly. I don't mean if it had massive out of control inertial spin/tumble, but rather if it just happened to be out of correct orientation when it started to hit significant atmospheric drag.

Long gone are the days when I could ask my grandfather a dumb question like this and he could show off by asking the eggheads over at Lewis before responding.

Anything I read on this topic is long forgotten. I've haven't been a space nerd in a long time.

Bob La Londe

--------------------------------

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"The Apollo command module is designed with an offset center of gravity to provide a trimmed entry angle of attack (with the heat shield forward)."

In contrast Columbia lost its necessary active stability after the punctured wing's hydraulic lines burned through. It apparently entered a flat spin, the damaged wing came off, followed by the payload bay doors which exposed the aluminum structure to burn through at the joint between the payload bay and the crew cabin. The freed front end tumbled until its tile-protected outer shell was blown off by shock wave pressure entering at its rear, then the exposed aluminum cabin burned up in about 10-15 seconds. Based on stored mission shoulder patches found early in the debris track they think the pressurized cabin shifted and was punctured in the storage compartment holding the patches by part of the outer shell frame, and since the crew hadn't lowered their face shields (intentionally?) to pressurize their suits they likely passed out very rapidly in the near vacuum before it burned.

If they had survived down to SR-71 flight conditions they could have bailed out and parachuted down, but not at Mach 18.

I knew an excellent NASA source at Mitre until he retired. He explained in detail how the Apollo 11 flight control computer he helped design became overburdened by routines they kept adding to it and unforeseen flight conditions, until the loop watchdog timer briefly lit its fault light at the end of each cycle before being reset. The programmers hadn't thought to extend the hardware timeout, and by then he was on another project.

I built a lab demo of a proposed DS5 laser comm crosslink for TDRSS, partly in my basement, otherwise my space program involvement was on ground terminals for Milstar.

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It's an upgrade to an early, encrypted predecessor of of Starlink. The US military has had a global secure communications network similar to wired and wireless Internet since the 1960's (with Teletypes and card readers), and I maintained it in the early 70's.

Reply to
Jim Wilkins

Civilian aircraft are also designed to be aerodynamically stable, so they won't do anything dramatic if the pilot lets go of the controls.

Modern fighter aircraft are designed to be aerodynamically unstable, rendered stable by a fast-acting control system, the reason being to improve maneuverability, which is useful in dogfights, and outmaneuvering anti-aircraft missiles.

With an unstable aircraft, if the control computer is out for more than something like a tenth of a second, control cannot be recovered, and the wings are likely to soon tear off. A very bad day.

Joe Gwinn

----------------------

AF447, the French airliner that fell into the South Atlantic, had a brief problem with pitot icing in a thunderstorm that told the autopilot the plane was flying too slowly, somehow resulting in a sharp pull-up that zoomed the plane up into a real stall. Afterwards the plane fell in a flat stall that confused the flight crew who saw normal RPMs which should mean normal power from the engines although they weren't taking in enough air to produce much thrust.

It seems no one tests the stability of $200 million airliners (without ejection seats) in stall conditions, they rely on simulations and didn't actually know what would happen. The plane stayed relatively level in pitch but rolled considerably and was difficult though not impossible to correct, as it did have a little forward airspeed. The stall warning was disabled below flight speed to avoid being set off by ground wind. When the pilot nosed down as he should have to gain forward airspeed the stall warning became enabled and sounded, thoroughly confusing them. The fell to their deaths in a perfectly good airplane that was doing something they didn't understand.

My hang gliding instructor had a model aircraft, and a borrowed homebrew analog computer that modeled the flight equations for it, that was stable only in certain sizes (he said). It was merely a 14" long straight wing with a dowel extending from the center of the leading edge. At the flying hill he showed me that it would glide back and forth in the updraft, recovering nicely from upsets by turbulence. At home he released it to follow a phugoid oscillation, with the computer output needle following its changing angle of attack.

I've been wondering if I would ever find a good excuse to write "phugoid" in a posting. Thanks.

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Reply to
Jim Wilkins

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