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Comment by wilg | Hacker News Reader

a basic mistake huh?

In a way, yes. Many of the systems on these satellites rely on math and physics we've only recently mastered, high speed radio telecommunications and all that goes with it is truly magic, a modern marvel.

Inertial stuff however, that's all basic Newtonian physics, we've had that mastered for a bit longer.

Side note/fun fact - many satellites use tape measures for this purpose, literally tape measures. They're called gravity gradient booms.

So, I get your point, it's not a simple mistake compared to me forgetting my keys this morning, but it is a simple mistake in the context of a satellite.

my main point is that its silly to assume the satellite designers at SpaceX made some kind of remedial physics error as opposed to a more subtle error that is the result of engineering or design tradeoffs

I dont know, I'm a designer like that and I make far more remedial errors than errors that were carefully weighed and considered.

That's fair enough, but in that light I feel like the parent commenter should be less incredulous.

> many satellites use tape measures for this purpose, literally tape measures. They're called gravity gradient booms.

Could you elaborate a bit? What part of a satellite is a tape measure useful for? How is it useful?

An example:

* https://tigersats.princeton.edu/cubesatkit

An unrolled tape measure is stiff so that the satellite/weight system will align up and down. As skinny things tend to do in orbit.

It's a bit like a keel for a (water) ship. You lower it and the gravitational pull on it will be slightly higher down where it is than up where the satellite is. Or you raise it up above where the satellite is -- same idea.

The "tape measure" is a mass at the end of a tether (the "tape") and they are of course not "literally tape measures".

https://en.wikipedia.org/wiki/Gravity-gradient_stabilization

They are literally tape measures. Source: I know engineers at Lockheed martin..

Manufacing that tape is a highly specialized big factory scale production process. It doesn't make sense to build a special tape factory just for the 100 feet of it they need for satalites annually. The tape many of them use is sourced from the same suppliers as tape measures for these reasons.

Fascinating, thanks!

One of the qualities of aerospace is that we get less forgiving of people making the same mistake over time. If someone mixed up metric and imperial units on a mars lander again they’d be vilified.

> a basic mistake huh?

Yes, a basic mistake. I'm not sure what you're getting at with your comment, but this is literally the kind of thing that should be caught very early on, probably just by updating a few parameters (or even just one) in the attitude control sim and observing the outputs.

If this claim is true (a big "if", admittedly), this is a breakdown in the kind of fundamental systems engineering that they teach to undergrads building cubesats.

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