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I am wondering what kind of ability NASA had during the Gemini or Apollo program era to control the vehicle from the ground.

From reading some capcom transcripts and looking through some random procedures, the ground is always relaying commands to the astronauts by voice, and expecting the astronauts to respond by flipping switches, e.g. requesting "O2 tank fan heater to OFF".

But say the astronauts were asleep, did the ground controllers have any kind of remote control of the vehicle?

Was telemetry always a passive reading of vehicle state?

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  • $\begingroup$ Related: Could an Apollo LM land unmanned? $\endgroup$ – a CVn Jun 30 '17 at 9:01
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    $\begingroup$ It's reasonable to assume that remote control was possible up to a point. Remote piloting - firing main engines, attitude thrusters, for sure. Remote operation of auxiliary functions like O2 tank fans and heaters (associated with long term power supply and life support), probably not. $\endgroup$ – Anthony X Jul 1 '17 at 0:57
  • $\begingroup$ I'm also curious to hear of any info describing what would take precedence should a conflict arise... could the ground control lock the crew out of any systems? I know they were carefully screened for that kind of thing, I am just interested in the technical possibilities. $\endgroup$ – Innovine Jul 1 '17 at 7:09
  • $\begingroup$ Consider that Apollo-era technology was primitive by current standards.Reliability and weight saving would have favored simplicity in design. The crew is an invaluable resource to correct mechanical faults/failures which may occur during the mission, so where ever feasible, things would have been as accessible as possible to the crew for inspection or repair. That would have given the crew ultimate authority over everything within their reach. Remote or automatic control would have been to support or supplement crew action. Overriding crew action would have been impractical and undesirable. $\endgroup$ – Anthony X Jul 1 '17 at 16:16
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Backing up a bit to give design comments from Project Mercury:

The Mercury astronauts had taken part in the development of their spacecraft, and insisted that manual control, and a window, be elements of its design. As a result, spacecraft movement and other functions could be controlled three ways: remotely from the ground when passing over a ground station, automatically guided by onboard instruments, or manually by the astronaut, who could replace or override the two other methods. Experience validated the astronauts' insistence on manual controls. Without them, Gordon Cooper's manual reentry during the last flight would not have been possible

Furthermore, astronaut group 1 and group 2 both required test pilot experience, and group 3 allowed military fighter jet experience instead of test pilot experience. Group 4 brought in scientists and gave them pilot training if needed, so overall the astronaut pool for Gemini and Apollo was made up of people that were expected to be able to fly the craft, and considering the astronauts were leaning on the design of the craft to make it user-pilotable they expected to be doing the flying: Project Gemini

It became known as a "pilot's spacecraft" due to its assortment of jet fighter-like features, in no small part due to Gus Grissom's influence over the design...

Given the features included for manually piloting and the fact that the person flying the craft was always a pilot, I'd guess that the pilot doing all the flying was the method preferred by the people on the craft. I'd have sworn I'd read that somewhere too, but it's been a while and I'm not finding that reference.

That said, the Apollo unmanned test flights demonstrate that everything necessary could be controlled from the ground:

  • Apollo 4: Launched unmanned on a Saturn V into a circular parking orbit, tested re-igniting the S-IVB to get to an elliptical orbit, separated the command module and used the service module engine to adjust the orbit twice. Landed 16 km from the target landing site.
  • Apollo 5: Launched the lunar module unmanned aboard a Saturn IB. The computer aborted the planned maneuver after 4 seconds due to a miscommunication about launch configuration, so

    The ground controllers moved to an alternate plan to fire the descent engine manually two more times. They then performed the "fire in the hole" test and another ascent engine burn.

  • Apollo 6: Launched aboard a Saturn V but had performance issues and ended up in an elliptical orbit instead of the planned circular one. The S-IVB failed to restart so they used the service module to raise the orbit, which left them short on fuel for finishing out the planned tests. Landed 80 km from the planned touchdown point.

Furthermore, this answer discusses remotely controlling the lunar module to crash it into the moon on most lunar missions after transferring the astronauts back to the command module.

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    $\begingroup$ The Apollo Service Module was also remotely commanded (or preprogrammed, not sure which) to maneuver away from the Command Module after separation, just prior to reentry. $\endgroup$ – Russell Borogove Jun 29 '17 at 19:30
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    $\begingroup$ Also, if they were remotely flyable, there are many cases from listening to capcom where the astronauts miss certain requests which have to be repeated, the apollo 13 evac from the CSM being a fine example. Why bother the astronauts with multiple requests to turn the failing tank fans on and off and switch radio antennas when they were clearly busy with transferring the guidance between vehicles? Why this, if the ground could have simply carried it out? The fact that they could fly unmanned missions doesn't explain much about what occurs on the manned missions, in my mind at least. $\endgroup$ – Innovine Jun 29 '17 at 21:37
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    $\begingroup$ My mistake, I read control as fly/maneuver, not full control of every aspect of the craft. I'll update my answer tonight from home. Short answer is that there were certainly things that couldn't be done from the ground but I'll try to research the extent of that later. $\endgroup$ – 1337joe Jun 29 '17 at 21:59
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    $\begingroup$ Apollo 5 was pressurized (considering they replaced the LM windows with aluminum plates due to an issue with the windows blowing out when the craft was pressurized in a vacuum). 4 and 6 were using Block I Command and Service Modules (instead of Block II like the manned flights), but those followed AS-201 which verified the capability of the environmental control system. $\endgroup$ – 1337joe Jun 29 '17 at 22:13
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    $\begingroup$ @RussellBorogove: Turns out that was hardwired. The SM has no computers; a simple state circuit would activate the RCS in translate aft mode once detached for any reason and would continue to translate aft until out of fuel or the fuel cells died (different tanks). $\endgroup$ – Joshua Jan 6 at 23:00
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NASA SP-97, "Proceedings of the Apollo Unified S-band Technical Conference" contains a paper titled "Apollo Digital Command System".

Although this paper concentrates on the mechanics of how the command system operates, it does give some examples of commands, and more importantly, the types of command that could be sent. These examples are headed CSM/LEM indicating that both modules could be commanded to.

  • The Real Time Commands (RTC) are commands that are known and identified before the launch. These commands are the on/off variety that are used to control the spacecraft systems. For example, dump tape recorder playback, ON or OFF, C-band radar beacon, ON or OFF, telemetry mode select, ONE or TWO.
  • The Apollo guidance navigational computer commands will provide updated information to the spacecraft computer. This will enable the spacecraft computer program to be updated or varied due to the new information developed during the mission.
  • The Central Timing Equipment will receive correct timing data whenever there are indications that the spacecraft timing system is not accurate.

They also discuss RTC and stored-program commanding to the 3rd stage (S-IVB).

I had wished for a comprehensive list of RTCs but did not find one. But it's clear that the ground could directly control a subset of CSM/LEM systems, upload new program data to the onboard computers, and update the spacecraft clocks. And they had these capabilities on the 3rd stage as well.

Unsurprisingly since so much of Shuttle design was rooted in Apollo design, this setup is very similar to that of Shuttle.

A different paper "Command and Communication System" in the same volume gives an overview of the command system usage to the 3rd stage. Briefly summarized, it states that there would be no commanding to the 3rd stage during ascent, that it would be used to checkout the systems and update the computers in parking orbit, and it would be used continuously during the lunar injection burn.

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As you can read here https://en.wikipedia.org/wiki/Apollo_program there were six unmanned missions before the manned missions. The remote control ability was necessary to do enough unmanned test flights before the risk of a manned flight could be rated.

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