Hacker Remix

Can SpaceX land a rocket with 1/2 cm accuracy?

136 points by scottshambaugh 2 days ago | 161 comments

krisoft 2 days ago

> Bill likely misspoke or was talking about control error.

Mixing up control errors with absolute errors is a very common form of miscommunication in robotics.

I work with relatively big robots and often my colleagues would say something like this "During the test we had 0.5m cross track error, so we did X, Y, Z ...".

And I always ask them for clarification. Were they looking at the robot and seeing that it is half a meter off where it should be, or were they looking at a screen and seeing that the robot thinks it is half a meter off from where it wants to be? Because those are two very different situations. And both can be described with the same words. (And sometimes it can be both, or just one of them.)

meindnoch 2 days ago

The robot knows where it is at all times. It knows this because it knows where it isn't. By subtracting where it is from where it isn't, or where it isn't from where it is (whichever is greater), it obtains a difference, or deviation.

mucle6 2 days ago

Reference: https://www.youtube.com/watch?v=bZe5J8SVCYQ

killjoywashere 2 days ago

This voice sounds like something that Mark Farina should be dubbing into his next album. But it's the first time I've heard this bit. Where did it come from? Is this a classic in engineering circles of some shit Rockwell actually sold to the military?

yetihehe 1 day ago

It was already dubbed into a song. https://www.youtube.com/watch?v=_LjN3UclYzU

rcxdude 2 days ago

It's from an old air force training video. Best guess I'be heard it that it's an unsuccessful attempt to explain Kalman filters (or something similar) in layman's terms.

It's definitely floated around for a while, but it grew in popularity in the past few years.

lacrosse_tannin 2 days ago

this sounds like it's read directly out of the inscrutable text book for the one control systems class i had to take.

hedora 2 days ago

I think this was called "error.wav" when I first saw it sneaking around a campus network.

ls612 1 day ago

I thought it was a Cave Johnson reference before seeing this.

scottshambaugh 2 days ago

> whichever is greater

This always stuck out in an otherwise excellent bit, because you should definitely _not_ be taking the absolute value of your control error.

m4rtink 2 days ago

By this point I automatically even read it by that voice. :P

rascul 2 days ago

How does the robot know where it isn't?

amelius 2 days ago

> or was talking about control error.

Control error is defined as the difference between desired value and measured value. So this is pretty good?

Even if they use some crude method to obtain position (e.g. gps), they can still easily refine that using e.g. triangulation using cameras around the landing platform.

krisoft 2 days ago

> So this is pretty good?

Not sure what you are talking about. If you are asking if 0.5cm is good controller error for an orbital class launcher on landing? Yes, it is extremely good. Without doubt.

If you are asking about my tangential story where there is confusion between total error vs controller error then no, it is not good. Confusion is never good. Especially if the system is not within the total error budget. Because to improve it you need to know if you are dealing with measurement error or controller error.

> Even if they use some crude method to obtain position (e.g. gps), they can still easily refine that using e.g. triangulation using cameras around the landing platform.

Sure. I doubt that their total error is within 0.5cm, but both of their landings were extremely succesfull.

thot_experiment 2 days ago

Most of this article feels like it's discussing irrelevant methods, you only need GPS to get it close (well for what they're doing they don't need GPS at all, though I'm sure it's used), we have much much more accurate ways of measuring the positions of things from a fixed reference point, 0.5cm deviation on your positional measurement is trivially achievable with optical systems. Why is the author spending paragraphs discussing IMU accuracy when we're trying to line up a rocket with a tower. You care about the rocket's relative position to the tower, you can put your measurement equipment on the tower, you don't need to worry about how accurate your accelerometers are.

I assume they are doing something much more clever/hardened, but you can trivially achieve much greater spatial accuracy with a Vive Tracking Puck for like $100.

MadnessASAP 2 days ago

Certainly lab equipment can measure distances well below 1 um fairly easily, I could manage 1 um in my garage. The issue is that the conditions at time of catch are VERY dynamic and not at all lab-like.

Your positioning system needs to acquire a fix at least 100m out in variable atmospheric conditions on a rocket undergoing heavy acceleration and dumping all kinds of heat, smoke and vibrations into itself, the atmosphere, and everything around it.

In addition having a fix on your tracking device is only half the game, not you have to figure out where the rest of the rocket is in relation to your tracking device. Which again, vibrations, temperature and manufacturing all have an effect.

So while yet, a vive tracking puck isn't entirely unlike the workable solution it is also entirely unsuitable as a solution and should not be used as a baseline to measure off of.

thot_experiment 21 hours ago

So? Yeah it's a challenging environment, we know that. My point is that the default way to solve this problem is to track your object from your reference point.

cubefox 2 days ago

Bill Gerstenmaier was talking about the flight test 4 landing accuracy, which landed on the open sea in the Gulf of Mexico, not on the tower like the recent test flight 5. The only thing nearby was a buoy. I'm pretty certain it didn't have advanced laser systems.

asadotzler 2 days ago

The buoys were not trivial devices. See https://x.com/CosmicalChief/status/1626333723514834944

cubefox 2 days ago

Still, lasers on a buoy?

Sammi 1 day ago

This is the part about landing a space rocket that you wonder is not technically possible?

Looking at the image I can see a dark device on top of a mast that could be anything electronic.

cubefox 1 day ago

The buoy is shaking in the water, so it likely can't aim at ship.

jameshart 22 hours ago

There’s a big cross in the middle of the landing pad that you’re trying to aim for - you don’t need advanced laser systems to get an accurate fix on where the landing pad is from the rocket - or where the rocket is form the landing pad for that matter.

sebzim4500 2 days ago

The article discusses the absolute error coming from RTK systems and claims that it won't be as low as 0.5cm, but surely the relevant metric is relative error, and I can see commercial systems advertising that level of precision.

i.e. the booster doesn't know it's actual position to within 0.5cm but it knows it's position relative to a buoy or the catch arms to that precision.

beerandt 2 days ago

Rtk already is 'relative' error- it requires one or more base stations (with either known absolute location or assumed one for relative positioning).

But survey grade gnss is a web of rabbit holes, if you want to get into it.

And there are ways to get sub mm accuracy both relative and absolute, but idk of one that would be quick enough for the required reaction time of dynamic landing via 'catching'.

But multi-centimeter (4-5) that's really easily doable is probably good enough for other systems to take over from.

magicalhippo 2 days ago

When Tim of EverydayAstronaut quoted this[1], as I recall the quote was within 0.5cm of the target landing site. So I assumed that to be relative accuracy and not absolute.

[1]: https://www.youtube.com/watch?v=pAPt5vbr-YU (don't recall timestamp, sorry)

bmicraft 2 days ago

My lawnmower (openmower) can do <2cm accuracy over GNSS. It is absolutely believable they could achieve 0.5cm on GNSS (plus rtk correction data from a fixed base station nearby) alone without measuring any relative distance using other systems.

snypher 2 days ago

I'd love to hear about 2cm accuracy uncorrected. Does it have dual GPS units?

bmicraft 1 day ago

Well, it isn't "uncorrected". It's just that you don't need any additional hardware other than a second gnss receiver on the base station and some kind of link between them.

GNSS is more than accurate enough once you know all slight errors in satellite orbits and the atmospheric distortions currently affecting the area near the base station and can correct for them.

cubefox 2 days ago

It's significantly more difficult to actually land a jumbo jet sized rocket booster with that precision than to measure its own relative position. Gerstenmaier was talking about landing accuracy. My guess is that measurement accuracy is a red herring. More likely it was a slip of the tongue (the good man is 70 years old) and he meant to say it landed with a 0.5 meter, not centimeter, accuracy relative to the buoy.

rkagerer 2 days ago

I found this video from the perspective of a landing pin interesting, especially when played back at low speed: https://youtu.be/ExV6PHRM8eI?t=17

You can see the arm comes in, then there's some side-to-side bounce (not sure how much is the rocket bouncing off vs. the arm fine-tuning its position). Just after contact seems to be made, and before the shock absorbing (or yaw-correcting) pistons drop much, there's a large flash from the engine. Is that a characteristic of engine shutoff, or was there a last-second "hover" push just before shutoff and drop? I wonder how much force the arms felt.

Another perspective showing both arms, and (as mentioned in the article) how the left one adjusted more significantly at first: https://youtu.be/JlcrNakUGVs?t=3

HarHarVeryFunny 2 days ago

Worth noting that the grid fins are about 15 feet long, and the landing pins probably 2-3 feet.

https://nextbigfuture.s3.amazonaws.com/uploads/2023/12/Scree...