Flying actual spacecraft

[peadar1987]

So I know pretty much how to fly a spacecraft in KSP, check your altitude, look at the navball, place nodes in map view and so on, but what relationship does this actually have to real life?

What sort of information did the Apollo astronauts have at their disposal? Presumably they didn't have draggable manoeuvre nodes and the like! Did they have their altitude and position relayed to them from the ground? How was their speed measured? How did they line up for manoeuvres? Was there some sort of optical wizardry that could put a marker on the navball, or did they have to just make a set of numbers match up? Was all of their navigation inertial? How did this affect their instrumentation when they transitioned from earth orbit into lunar orbit?

[R0cketC0der]

In real life most of the flight is done by computers. Also all burns except for corrections are calculated before the launch of the missions, but for corrections and emergencies the burns are calculated by mission control and then relayed to the spacecraft.

[John Crichton]

Mechjeb...a more complex version of Mechjeb

[Idobox]

Position and speed was measured by telemetry from Earth plus inertial guidance.

The didn't have draggable manoeuvre nodes, but they would enter start time, direction and duration of burn in numerical form. Something like azimuth, altitude, t-sart, t-stop, in 4 numbers.

[Nibb31]

For Apollo, the navigation was all done on the ground. The computer on board the spacecraft was called the PGNCS and was pretty basic. The PGNCS knew its position through inertial measurement and could be recalibrated with a star tracker (a sort of sextant) that the astronauts pointed to a specific list of stars.

However, the main way of knowing where they were was provided by ground stations that used radar and telemetry to locate the spacecraft. By extrapolating between two points, you can easily calculate the orbital parameters.

For manoeuvers, mission control typically sent them then equivalent of our KSP manoeuver nodes: point the spacecraft at the correct angle, then burn full trottle (the SPS wasn't throttlable) at a specific time for a specific number of seconds. These instructions were sent by voice, the astronauts typed the burn parameters into the DSKY (the AGC's control panel), and the computer handled the rest.

During Apollo 13, there was a famous moment when then they had to perform a free-return correction burn with the LM engine while the PGNCS was powered down to save power. They pointed the spacecraft in the right direction by using the star tracker and burned manually with a stopwatch.

Edited April 22, 2014 by Nibb31

[peadar1987]

Okay.

I suppose all you really need is attitude, position and velocity, everything else is just plugging those values into the right equations to predict your trajectory.

Does anyone know how they handled the "sphere of influence" change? (I know that's not how it actually works in n-body physics, but at some point you are going to change from being gravitationally dominated by the earth to being gravitationally dominated by the moon)

[Nibb31]

Okay.

I suppose all you really need is attitude, position and velocity, everything else is just plugging those values into the right equations to predict your trajectory.

Does anyone know how they handled the "sphere of influence" change? (I know that's not how it actually works in n-body physics, but at some point you are going to change from being gravitationally dominated by the earth to being gravitationally dominated by the moon)

There isn't anything to handle because SOIs don't really exist. The IMU could be switch to various referentials depending on the mission phase.

A fun tidbit is that there were red circles on FDAI (the real name for the nav ball) which corresponded to the "normal" and "antinormal" position. The astronauts had to make sure to never point the spacecraft in those directions because the IMU (inertial measurment unit) gimballs would lock up which would cause them to lose attitude reference information for the rest of the mission.

Edited April 23, 2014 by Nibb31

[peadar1987]

There isn't anything to handle because SOIs don't really exist. The IMU could be switch to various referentials depending on the mission phase.

Well, I knew that in the sense that there isn't a point when the earth's gravity abruptly stops affecting you. I suppose that the only way the spacecraft knows its orientation without star sightings and recalibrations is by inertial guidance, so you'd just set your attitude to whatever the slide rules at mission control told you to, and they'd do all of the calculations to figure out what you should point to relative to that, and there'd be no real need for the guys in the spacecraft to know their attitude relative to the moon.

And the LEM would have been far below orbital speeds, so could figure out which way was "down" just by using gravity.

A fun tidbit is that there were red circles on FDAI (the real name for the nav ball) which corresponded to the "normal" and "antinormal" position. The astronauts had to make sure to never point the spacecraft in those directions because the IMU (inertial measurment unit) gimballs would lock up which would cause them to lose attitude reference information for the rest of the mission.

There was a really good thread here a while back about exactly that. I think Gemini had four-axis gimbals which were immune to gimbal lock, but for some reason they weren't used on Apollo, perhaps to save mass?

[Aghanim]

Well radar and telemetry works for interplanetary travel, but how about navigation in interstellar travel?

[Streetwind]

Look at

for a glimpse of how Yuri Gagarin spent his time in Vostok-1. And then there's

of a US Mercury mission.

Both are of course very old craft, but it's nevertheless interesting to see.

[Moon Goddess]

Well radar and telemetry works for interplanetary travel, but how about navigation in interstellar travel?

That would probably depend on how FAR interstellar you are going.

Sol System to Alpha Centari system? Cameras would allow you to plot the location of both of those stars for orientation, parallax on various other stars should provide "altitude".

Longer distances you should be able to do the same as long as you kept a plot on a selection of stars during the whole trip.

[Nibb31]

Well radar and telemetry works for interplanetary travel, but how about navigation in interstellar travel?

Same thing. Use a star tracker and inertial measurement. Ground control can confirm position from trajectory calculations and measure the time it takes to get a message back from the spacecraft to know how far away it is.