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I was watching First Man (2018) and noticed that just before the launch of Armstrong's Gemini mission the capsule's altimeter read zero.

My understanding of the Titan II GLV is that it's 109ft tall so I would expect the altimeter to read this if set to "aerodrome" pressure level (QFE) or something different if set to area pressure (QNH). Unfortunately I don't have a Cape Kennedy METAR from the launch date available to check!

My thinking is this was a mistake in the movie. Surely it must be important for the astronauts to set the pressure in case of an abort? Or maybe not given the distance the rocket can travel whilst it is still possible to abort the pressure setting would be wrong anyway by the time of landing.

It would certainly be strange for a pilot not to set the altimeter before commencing flight.

Another consideration here is the mechanism to deploy the parachutes. Was it based on the barometric pressure or a radio altimeter?

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    $\begingroup$ Titans did not launch from Houston. $\endgroup$ – Organic Marble Mar 27 at 17:56
  • $\begingroup$ Air pressure altimeters are precise to about some tens metres or about 100 feet. You should not expect them to show 109 feet exactly. $\endgroup$ – Uwe Mar 27 at 18:26
  • $\begingroup$ @uw3 The Gemini capsule altimeter appears to be a Kollsman altimeter as used in regular aviation. It has a knob to adjust the base pressure altitude; rotating that knob changes the indicated altitude. I would expect the astronaut to rotate the knob until the altimeter read whatever was considered zero for the launch (0, or 109, or 109 + launch site altitude MSL, or... whatever they decided to use. Those knobs are sensitive enough to cause the altimeter to read pretty much whatever you want it to read when you are on the ground. $\endgroup$ – Wayne Conrad Mar 27 at 18:44
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    $\begingroup$ @WayneConrad If you set the altimeter to 109 feet (the scale does not resolve single feet) and wait 15 minutes for the air temperature or pressure to change, you get some 10 feet more or less. $\endgroup$ – Uwe Mar 27 at 19:11
  • $\begingroup$ @uwe Yeah, more or less. It's how we used to set the altimeter at an uncontrolled airfield I flew at. There was a sign on the hanger with the field altitude MSL. You turned the knob until the altimeter read that altitude, then you took off. Fly again later, you might have to adjust it again. $\endgroup$ – Wayne Conrad Mar 27 at 19:42
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According to The Smithsonian, the pressure altimeter was intended primarily as a landing aid. In the event of an abort during takeoff, parachute deployment would be based on speed, not altitude.

Rockets move incredibly fast. From launch, Gemini traveled 50 nautical miles horizontally in two and a half minutes, and achieved an altitude of 210,000 feet. So, you were right when you said that the pressure at any abort landing site would be wildly different than the one at takeoff.

A normal landing is about the only time the astronaut would even look at the altimeter, to know when to deploy the parachutes. The drogue chute was to be deployed between 40,000 to 50,000 feet, and the main chute at 10,600 feet. At those altitudes, the actual distance from sea level isn't really a concern. Absolute air pressure is more important, to make sure that the slipstream doesn't tear the parachutes apart, so I'd be surprised if the altimeter wasn't set to 29.92 for the landing (couldn't find any references on that). After that point, about the only use for an altimeter would be to warn when the capsule was about to impact the water, and looking out the window is a lot more reliable method for that.

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