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Comment by lokedhs | Hacker News Reader

Running it on a timer actually works quite well. I don't know how this particular system actually works, but for example the Saturn V launch vehicle handled the initial navigation for several minutes from takeoff by executing previously calculated commands based on launch time, weather conditions etc.

Also, if I have understood things correctly, the lunar lander used a timer based approach to keep the rotation of the vehicle aligned with the surface of the moon as it orbited.

Since the orbital dynamics are very well understood and there really isn't anything that can affect the motion of the spacecraft other than engine burns, there doesn't seem to be any good reason to make it more complicated.

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> Since the orbital dynamics are very well understood and there really isn't anything that can affect the motion of the spacecraft other than engine burns

This is wrong.

1) Atmospheric drag (unless you're in really high orbits)

2) Wind

3) Uneven gravitational field. Our planet is not a perfect sphere. The moon is even less so.

4) Solar pressure (though very minor and kinda evens out in orbit).

Which of these can't you calculate ahead of the launch? Numbers 1, 3, and 4 seem like you could calculate it ahead of time. And maybe number 2 as well, if you do it relatively close to launch time.

They use models. No model is perfect, they all have uncertainties.

Which is why they use pilots too

The unevenness of the gravitational field on Earth is for the most part ignorable. It isn't like maneuvers are precise enough that one could account for it in the calculations anyway.

Space is empty enough and engines imprecise enough that for orbital stuff you have enough margins to correctly predict and plan a trajectory ahead of time, and just do correction burns later if necessary.

On the Moon the gravitational field is indeed uneven enough to matter, but exactly one orbiter really cares about it (LRO iirc has a specific orbit where the unevenness mostly cancels out), most simply rely on occasional station keeping burns.

When in stable orbit, I buy that argument.

But many things can go slightly wrong when firing all engines fighting your way up the atmosphere.

That is true. In the Saturn V case, the guidance computer checked the actual location of the spacecraft and issued corrections that had accumulated due to precisely that.

Atlas V burn was nominal, this error didn't happen until the capsule was out of the atmosphere.

And even more things can go very wrong with more complicated control systems.

>Running it on a timer actually works quite well.

It would seem we have an existence proof now saying that the above statement is incorrect.

Because one specific implementation seems to have failed a test run?

Let's compare the number of instances a timer has caused an issue and the number of instances it hasn't to determine the actual rate before we make a broad assumption about a field we have no experience in due to a single test case, yes?

Yeah I’m a programmer like most of us here and I’ve seen timers abused quite a lot. They sometimes are appropriate, but more often, they are good examples of an antipattern.

The wording “it would seem” should have tipped you off that no assumptions are being made, but I guess it didn’t work.

And even if we were to make a comparison as you suggest and derive some ratio, that’s not super useful without comparing it to the ratio for some alternative approach that does not use timers.

A man-rated launch vehicle failing to reach orbit seems like very good reason.

The complexity of adding a remote man it the loop creates way more room for error than running a timer. They just fucked up plain and simple.

This is like saying that because the airplane crashed because the wing was incorrectly designed and popped off, everyone should fly biplanes...

> This is like saying that because the airplane crashed because the wing > was incorrectly designed and popped off, everyone should fly biplanes...

We do that. Not with the wings, but with the engines. When jetliner engines were less reliable, we required 3 or more engines on flights that included long sections without passing near an airport (mostly transoceanic flights). Now that some engines are more reliable, we have ETOPS certifications for jetliners using two high-reliablility engines. And yes, there still are two, because no matter how reliable the engines may be, one _may_ fail.

What is backing up that CST-100 timer?

https://en.wikipedia.org/wiki/ETOPS

It did reach orbit. They're just not at the orbit they want to be at.

The man-related launch vehicle worked flawlessly. This was a capsule failure. They’re built by different entities.

I thought the space shuttle was the same. They took wind measurements before launch and the ascent was hard coded based on that data.

The Space Shuttle was always hand flown, not automated.

Launch was automated. Landing was manual but not for technical reasons.

Didn't they do one automated landing? I seem to remember reading that.

They could do it but the pilots wanted something to do so they got to fly manually. The Russians did unmanned launch and landing with their space shuttle Buran.

Wind?!

Onnplanet earth, where those things are launched their is an atmosphere with wind. That wind impacts where exactly the rocket leaves the atmosphere and deploys the space ship.

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