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by themafia 65 days ago
In the Shuttle they would use command averaging. All four computers would get access to an actuator which would tie into a manifold which delivered power to the flight control surface. If one disagreed then you'd get 25% less command authority to that element.
1 comments
JumpCrisscross 65 days ago
> In the Shuttle they would use command averaging

I think the Shuttle, operating only in LEO, had more margin for error. Averaging a deep-space burn calculation is basically the same as killing the crew.

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Cthulhu_ 64 days ago
Sure, but these maneuvers aren't done realtime and aren't as time-sensitive; a burn is calculated and triple checked well in advance. If there was an error, there's always time to correct it.

In the case of moon landings, the only truly time-critical maneuvers are the ones right before landing... and unfortunately, a lot of fairly recent moon probes have failed due to incorrect calculations, sensor measurements, logic errors, etc.

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themafia 64 days ago
The GNC loop runs several times per second. The desired output will consequently be increased by the working computers to achieve the target. The computer does not "dead reckon" anything.

Travelling through Max-Q in Earth atmosphere on ascent is far more dangerous.

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JumpCrisscross 64 days ago
> Travelling through Max-Q in Earth atmosphere on ascent is far more dangerous

Fair enough. I don't know enough about Orion's architecture to guess at propellant reserves, and how life-or-death each burn actually is.

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KennyBlanken 64 days ago
Fucking up the re-entry burn or thruster actuation during the burn for re-entry = loss of vehicle/crew

Improper control surface actuation during re-entry = loss of vehicle/crew

Also, rocket engines that are powered by the combustion of their fuel and oxidizer (the exhaust gasses of which drive the main pumps) have a very specific startup sequence. For example, if any of the combustion chambers have a mix of oxygen and hydrogen too close to stochiometric when the igniters fire, you get an explosion, not a burn. Not too dissimilar from what happens in car engines when you get detonation (which is very different from knocking. Detonation melts holes in stuff.)

Startup initially is open-loop with no feedback or adjustment based on sensors and then at some point the computer switches over to closed loop control. It starts with hydrogen first. The sparklers? Those aren't for igniting the engine, that's done by igniters inside the combustion chamber(s). The sparklers are to ignite all the hydrogen that is pushed out the nozzle initially so there's a very fuel-rich environment in the engine and it doesn't go kaboom.

If things go wrong - such as a valve not opening as fast as it should, or not being opened the right amount at the right time - the engine goes kaboom. This happened to a bunch of engines during development and testing.

But Artemis has basically the same engines, so...shrug

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