[HN Gopher] Can SpaceX land a rocket with 1/2 cm accuracy?
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Can SpaceX land a rocket with 1/2 cm accuracy?
Author : scottshambaugh
Score : 122 points
Date : 2024-10-21 15:27 UTC (7 hours ago)
(HTM) web link (theshamblog.com)
(TXT) w3m dump (theshamblog.com)
| sebzim4500 wrote:
| 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.
| magicalhippo wrote:
| 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)
| beerandt wrote:
| 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.
| scottshambaugh wrote:
| I'd be very interested in the systems advertising that! I have
| not seen that even for stationary surveying equipment. I think
| it's also important to distinguish between RMS error which is
| often the better topline spec that companies give you, vs the
| 95% confidence error which is the more relevant one for flight
| reliability.
| londons_explore wrote:
| If you want to land 99.9% of the time, it really is te 99.9%
| circle you should be looking at...
|
| Or in fact, you need _even better_ than that, since you don
| 't want your whole error budget used up by the GNSS system.
| michaelt wrote:
| In the _specific case of a docking-type manoeuvre_ presumably
| you only need the highest accuracy when you 're getting very
| close to the target.
|
| No reason you couldn't use RTK GPS for <10cm accuracy for
| most of the flight, then in the last few meters of landing
| switch over to to high-precision, short-range tracking - like
| optically tracking a marker on the grabbing arm.
|
| For other specific cases - like bridge monitoring - there are
| reports of 2-3 mm precision [1]. Of course, bridge monitoring
| has quite distinctive requirements; a 5Hz vibration component
| and a 0.0001 Hz thermal expansion component. So there's a lot
| of potential to average over lots of readings to reduce
| noise.
|
| [1] https://www.sciencedirect.com/science/article/abs/pii/S02
| 632...
| stavros wrote:
| What's absolute position? Isn't all position relative?
| westmeal wrote:
| https://www.youtube.com/watch?v=bZe5J8SVCYQ
| Maxatar wrote:
| Either I'm very stupid or that video is some kind of epic
| trolling.
| mianosm wrote:
| Epic trolling, you're sane.
| adastra22 wrote:
| Absolute position is global lat/long coordinates. Relative
| position is "I'm 0.5cm from the middle of the peg."
| willglynn wrote:
| Global lat/long coordinates are defined in terms of
| coordinate systems like WGS84 or ITRF2020, which are
| themselves the result of relative measurements between
| reference stations.
|
| The earth's crust floats on top of liquid rock. This
| matters at relevant length and time scales; in most places,
| these effects alone are on the order of millimeters per
| year. One reason why it's better to use NAD83 over WGS84 in
| North America is that NAD83 latitudes and longitudes move
| with the North American plate.
|
| Positions _are_ relative, and the closer you can put your
| datum, the less drift you'll accumulate.
| adastra22 wrote:
| There is a literal, autistic sense in which you are
| correct. But there is a practical, pragmatic distinction
| between measurements that we call absolute versus those
| we call relative, and pedantic correctness misses the
| point.
| bmicraft wrote:
| 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.
| cubefox wrote:
| 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.
| tocs3 wrote:
| I would have thought the vibration from the engines would produce
| error of greater that 1/2 cm. Still, it seamed to have worked
| well. So, there you go.
| mbell wrote:
| Armchair aerospaceing here, but it feels like he's a whole class
| of positioning sensors in this analysis. It seems to be you only
| need GPS and related absolute positioning systems to get you
| close to the tower. At that point, what you care about is the
| relative positioning of the tower and the booster. I would think
| this can be done very accurately with a host of options: cameras,
| radar, lasers, ILS style systems, etc, etc.
| echoangle wrote:
| Not really, the Gyros described in the post are also essential.
| If you don't know the attitude of the vehicle, you can't point
| the engines in the direction your control algorithm says you
| should to hit the target.
|
| Edit: I think I misunderstood the comment. Yes, you can use the
| absolute methods for rough guidance and then use relative
| positioning for the final approach. The article has a line
| about why the author doesn't think that's likely though.
| candiddevmike wrote:
| Does SpaceX even use traditional GPS? I'd assume with something
| like Starlink they would be able to employ something more
| precise/fit for purpose.
| andrewmcwatters wrote:
| GNSS RTK is incredibly accurate these days. By the time that
| you're close enough to the landing zone, you're close enough
| to get positioning down to centimeters on consumer grade
| hardware, which the article points out.
|
| The actual question is literal: Can SpaceX land a rocket with
| sub 1 cm (1/2 cm) accuracy? GNSS RTK can get you down to a
| couple of centimeters, but getting more granular resolution
| than this isn't reliably possible with current professional
| grade technologies.
|
| I'm personally unsure if the military has greater resolution
| than what's possible with RTK or w.r.t. military use GPS, but
| I would not be surprised if they did. If that's the case,
| NASA would most likely have access to it, I would assume. But
| the article specifically calls this out saying that it's not
| accurate enough to surpass the resolution of using RTK.
|
| What's really cool about these questions is that the same
| problem space is applicable to self-driving cars and SLAM, if
| you're into that sort of thing. Lane detection, etc.
| moralestapia wrote:
| >you're close enough to get positioning down to centimeters
| on consumer grade hardware
|
| But in realtime? (single-digit second latency, at least)
| andrewmcwatters wrote:
| Yes, for the purposes of landing speeds. In fact, at
| vertical aircraft landing speeds, your time-step to
| position D is more accurate than automotive SLAM.
| moralestapia wrote:
| Nice, TIL. Thanks!
| polishdude20 wrote:
| I mean rtk accuracy is considering rtk in isolation. You
| can get better accuracy if you combine rtk with other
| methods such as an IMU.
| mensetmanusman wrote:
| Wow, I can't believe I never realized that SpaceX could sell
| access to a positioning system far better than GPS...
| asdfman123 wrote:
| Right? They land fighter jets on carriers with a light signal
| that projects out from the ship at a particular angle. It seems
| very easy to do something like this with some form of
| electromagnetic radiation. Or have some way for the tower to
| detect the exact position and communicate with the rocket.
|
| I understand engineering is complicated but this honestly seems
| like the easiest part of the problem to solve.
|
| It's more likely that SpaceX determined they didn't need super
| tight tolerances and called it a day.
| scottshambaugh wrote:
| > SpaceX determined they didn't need super tight tolerances
| and called it a day
|
| Yup! This is my conclusion in the article - the landing box
| for the Super Heavy booster is 5x13x18 meters on each side,
| with 5-15 degrees of angular tolerance in each of the vehicle
| axes. So the margins are big enough that you don't need
| millimeter level precision for the rocket position.
| asdfman123 wrote:
| > Could you use other real-time distance measurements like
| laser rangefinding or visual processing? I don't think so
|
| This is the part I question though. Seems like an org as
| well motivated as SpaceX could easily solve that if it was
| necessary.
| scottshambaugh wrote:
| My take is that it would probably be possible with enough
| effort, but there isn't an easy solution. And if you
| don't need it then the best part is no part. :)
| psunavy03 wrote:
| Carrier landings are accomplished using a combination of
| indications, and the meatball is only one of three primary
| tools. If you are not flying an on-speed angle of attack AND
| lined up on the centerline of the landing area, the meatball
| position is invalid to a degree proportional to the degree to
| which those other inputs are off.
|
| The meatball Fresnel lens is canted slightly side-to-side,
| and only places the hook in the right spot at a given angle
| of attack. Which is a design compromise necessitated by
| having to allow multiple types of aircraft with multiple
| hook-to-eye distances to land on the same aircraft carrier
| while using a visual input in one location (the cockpit) to
| properly place a device in another location (the hook point)
| with high precision.
|
| Source: I've done it.
|
| So just as it is not "very easy" to trap on board the boat
| with "just" a light signal, I would assume landing a
| building-sized booster has a similar if not bigger list of
| potential "gotchas."
| asdfman123 wrote:
| Very easy compared to all the other hard problems SpaceX
| has to solve, yes.
| WalterBright wrote:
| I suspect the real problem is wind. A last moment gust could
| push the booster far enough away that it cannot recover.
| ortusdux wrote:
| I wonder if they will paint pin alignment marks on the grabber
| arms?
| chasd00 wrote:
| If they did that and hit them consistently.. talk about rubbing
| salt in the wounds of the rest of the industry. It would be
| like a sports team running up the scoreboard on an obviously
| beaten opponent. Super heavy hitting X's painted on the
| chopsticks right in the middle would be border line
| unsportsman-like heh.
| lostdog wrote:
| You can do even better with radar. If you place a set of radar
| reflectors around the tower at known locations, then you can
| detect them from the booster and triangulate the distances to a
| precise position. Plus, radar gives you relative velocities, so
| your speed and roll rate estimatimates get even more precise. I
| bet you could get down to millimeters with a setup like this.
| aeyes wrote:
| Is this really viable given all the electromagnetic
| interference the rocket motor exhaust plumes are generating?
| generuso wrote:
| Falcon-9 uses radar altimeters for determining vertical
| "distance to go" during landing.
|
| While a sideways position error of even ten meters is not
| fatal, it is critical for the rocket to be quite close to
| zero altitude when deceleration brings the velocity to zero.
| (Any residual error must be dealt with by the shock
| absorbers, and their capability is modest.)
| gibsonf1 wrote:
| Um, what about using starlink to measure position as option 3!
| tonyarkles wrote:
| Starlink as GNSS is definitely a thought that intrigues me and
| clearly other people too:
| https://www.technologyreview.com/2022/10/21/1062001/spacex-s...
|
| The tricky parts (that we don't really know as non-SpaceX
| employees) are:
|
| - how accurate is the clock onboard the satellites? Given that
| it's likely an OFDM signal the timing is probably pretty good,
| but given that they're launching zillions of them they probably
| don't all have atomic clocks onboard
|
| - how accurately is SpaceX tracking their orbits? Kind of a
| similar answer here... they're doing beamforming to the ground
| terminals, so it has to be pretty good but we don't really know
| how good.
|
| - how many SVs are actually visible at a time? We need a
| minimum of four but the more the better. If there's lots
| visible we can somewhat work around the first two issues
| statistically but if there's a limited number than the orbit
| and clocks need to be super accurate.
| perihelions wrote:
| Also--how accurately do those satellites track their own
| position? Unlike the high-orbit GNSS constellations, LEO
| satellites would bounce around a bit from orbit to orbit, as
| they're relatively close to the earth and sensitive to uneven
| distributions of mass. I don't know the exact magnitudes, but
| I understand they're large by GNSS standards.
| generuso wrote:
| Starlink satellites use on-board GPS receivers for
| extremely accurate (centimeter level) measurements of their
| position. The orbits which SpaceX reports to the world (for
| collision avoidance) are based on these measurements.
| mclau156 wrote:
| Could use more simulation data with NVIDIA Omniverse and thrust
| vector control
| varispeed wrote:
| And here is me trying to find sheet metal fab that could make a
| simple enclosure that matches the design and is not warped or
| scratched.
|
| Impossible apparently.
| fxtentacle wrote:
| Most hackerspaces have a laser cutter strong enough for a few
| mm of steel sheet. So chances are, you could make it yourself.
| dingaling wrote:
| I suspect that finding a proximate hacker space, let alone
| one with such equipment, is even more of a challenge.
| Spunkie wrote:
| I think they were simply speaking of the final pads the rocket
| rests on top of the chopsticks have 5cm of error either
| direction.
|
| But judging from the bouncing the rocket did when in the
| chopsticks the error for positioning into the initial catch
| position is much larger in all directions. The chopsticks coming
| closed around the rocket do the heavy lifting for final alignment
| to that 5cm I imagine.
| mclau156 wrote:
| A lot of people think it landed on the large grid fins, this is
| not true it actually landed on much smaller landing pegs
| nordsieck wrote:
| This is more or less easy to see depending on the video you
| watch. Here's a good one that demonstrates it very clearly.
|
| https://www.youtube.com/watch?v=ExV6PHRM8eI
| stronglikedan wrote:
| I was impressed before. Now I'm doubly impressed after having
| watched that. Thanks.
| Culonavirus wrote:
| Landing on the grid fins would be a really bad idea. Even
| though they're car-sized, they're not load bearing and "only"
| made of steel (not titanium etc. .. just yet). Starship's
| Raptors blast during hot staging is enough to bend them on the
| top.
| https://www.reddit.com/r/SpaceXLounge/comments/1g3bi7s/grid_...
| m4rtink wrote:
| They need to be able to handle some forces but indeed likely
| not an equivalent of half or even quarter of the booster
| landing weight.
| cubefox wrote:
| Though the original plan was indeed to land it on the
| (reinforced?) grid fins:
| https://x.com/elonmusk/status/1344327757916868608
|
| I actually think there is some old Starbase tour interview
| where a SpaceX guy implied it was Musk's idea, though I could
| be misremembering. Catching the booster kind of makes sense,
| since they needed the tower arms anyway for stacking and
| unstacking.
| whitehexagon wrote:
| Interesting, are there more than 4? because I was also amazed
| that the rocket was rotated at exactly the right angle to be
| caught by them. But maybe that is the 'easier' challenge when
| you are hovering with such accuracy.
|
| I keep finding myself watching the catch every few days, and it
| does not tire to impress.
| krisoft wrote:
| > 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 wrote:
| 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 wrote:
| Reference: https://www.youtube.com/watch?v=bZe5J8SVCYQ
| hedora wrote:
| I think this was called "error.wav" when I first saw it
| sneaking around a campus network.
| killjoywashere wrote:
| 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?
| rcxdude wrote:
| 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 wrote:
| this sounds like it's read directly out of the inscrutable
| text book for the one control systems class i had to take.
| exe34 wrote:
| this only works if the retroencabulator is properly
| calibrated.
| m4rtink wrote:
| By this point I automatically even read it by that voice. :P
| scottshambaugh wrote:
| > 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.
| amelius wrote:
| > 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 wrote:
| > 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.
| kabdib wrote:
| I wonder how much of a problem crosswinds are. Not a lot of mass,
| and that big can has a lot of sail area.
| weard_beard wrote:
| From what I understand this is the primary difference and
| problem with Falcon 9 vs. Super Heavy.
|
| From the Article:
|
| "Why can't SpaceX do a catch with a Falcon 9?
|
| -It does not have separate landing propellant tanks, so
| propellant slosh will disturb its trajectory. The Super Heavy
| booster has dedicated central header tanks for landing
| propellant, so there should be minimal propellant slosh to
| disturb the vehicle attitude.
|
| -It lands with a single engine which cannot throttle low enough
| to hover the vehicle, and as such must perform a "hoverslam"
| maneuver to bring the vehicle to a stop right on the ground.
| While the Super Heavy booster must perform most of a hoverslam
| maneuver to slow down just before coming in to the tower, it
| can hover for the final fine positioning.
|
| -Because it lands with a single engine, roll control is minimal
| close to touchdown when the airspeed is low and the grid fins
| can impart minimal torque, and is limited to its weaker cold-
| gas thrusters. The Super Heavy booster can control roll with
| its 3 engines all the way to the ground.
|
| -Falcon 9 has no engine-out capability for landing. SpaceX has
| not confirmed it for the Super Heavy booster, but I believe one
| engine out is likely possible (more on this later).
|
| -It is smaller with a lower moment of inertia. Rockets get more
| stable and easier to control the larger they are, much like
| it's easier to balance a broom on your finger than a pencil.
|
| -It is smaller, and so thanks to the cubed-square law has a
| higher area:mass ratio. This means that it will be more
| affected by wind gusts that might blow it off course."
| therealfiona wrote:
| Maybe if the rocket knows where it is because it knows where it
| isn't.
|
| That is what they told us in missile maintenance school.
|
| And gyros have gotten a lot better. Especially if you're throwing
| money at the issue like you know those folks are.
| blackoil wrote:
| Rocket alone need not be this accurate as grabber arms should
| also do some maneuvering to get the final accuracy.
| LooseMarmoset wrote:
| You can see the landing arms adjusting in the video as well.
|
| His choice of stainless steel is panning out well here - I
| doubt if aluminum or composite body structure would hold up as
| well to the "grab" forces from even minor misalignment. A
| composite structure would likely be entirely compromised by a
| big scrape.
|
| It would be interesting to see a test where the landing speeds
| were deliberately too high - how much deceleration can the arms
| handle safely?
|
| I think the chopstick mechanism is probably the best possible
| catch mechanism for such a tall object. The booster will be
| suspended from the top, which means the booster isn't subject
| to tipover as it would be if landing legs were involved. We've
| already seen this many times in the Falcon 9 booster series.
|
| I can't see chopsticks ever working from a droneship, though -
| too much induced rotation for chopsticks to compensate.
|
| As an alternative to chopsticks, a catch 'sleeve' might be
| possible, though it would magnify alignment errors
| considerably.
| slashdave wrote:
| 0.5cm isn't even well defined, considering that the rocket itself
| is probably out of round by larger tolerances, not to mention
| issues of thermal expansion.
| varjag wrote:
| Yup it's surface amplitude with the engines running is probably
| more than that.
| chasd00 wrote:
| is "surface amplitude" vibration? I could see vibration being
| graphed over time as a wave with an amplitude and frequency.
| I can't really comprehend the size of super heavy combined
| with the energy density of just one turbo pump on one raptor
| engine (let alone 33x2) and then the precision of control
| needed to catch the whole thing with chopsticks. Not many
| things do i admit are just beyond me period but this is for
| sure.
|
| /raptor 3 pumps look like you could hold them in your hands
| but iirc they deliver over 100k horsepower each.
| slashdave wrote:
| The larger size makes control much easier.
| cryptonector wrote:
| It's probably defined by the catch points as that's what
| matters: whether the catch points end up where they belong
| (good) or not (disaster). The catch points are not "out of
| round".
| slashdave wrote:
| That's worse, considering that the control points are a huge
| distance away. Just flex alone has to be huge.
| cryptonector wrote:
| I should have mentioned that the catch points are on one
| long bar that goes atop the booster. They are _not_ simply
| attached to the sides of the booster. Booster shell flexing
| does not affect the catch points at all.
| nine_k wrote:
| I would suspect it's some average of many points that the
| control systems try to track, because the rocket is not a
| point, and the press wants to quote a single easy number.
|
| I'd expect that the rocket has a ton of sensors, and a ton of
| passive and semi-active tracking devices all over the body.
|
| E.g. I'd put a bunch of NFC-type responders in a number of key
| positions, responding at different frequencies. Then a typical
| sweeping-frequency radar pulse would activate them all, and the
| response time and the Doppler shift would tell about positions
| and speeds of many points on the rocket. I'd do a similar thing
| with reflectors and IR/optical tracking.
|
| All these points should follow some reasonable trajectory for
| some point the top of the rocket, near the chopsticks, would
| move towards some desired catch location point. Probably this
| motion is where "with precision of whatever cm" relates to.
| burningChrome wrote:
| Just a few years ago, it was a huge leap just have a rocket
| return and land, now we're pushing the actual accuracy of how
| well it can land?
|
| Anybody else thinking this quite the time to be alive?
| stagger87 wrote:
| Well, to be fair, the engineers working on this system have
| been thinking about the accuracy this whole time even if you
| haven't. Even on the first go, it still had to land on a
| relatively small pad floating in the ocean.
| golemotron wrote:
| The fact that someone tried is significant. We've had 50+
| years of not trying.
| jerf wrote:
| As impressive as the space efforts in the 60s and 70s were,
| I've often thought that they were a false start created by
| a war-like impetus to show off. Tech-wise, we really
| weren't ready for a space age. The sort of control systems
| that make this sort of outcome possible haven't been around
| for all that long, really, especially if you mark them from
| being economical and not just "it technically existed in a
| lab somewhere". Plus if you really dig into how these
| rockets are built and maintained, you see a lot of other
| technologies that have not been around for that many
| decades, like, practical and reliable 3D printing, and
| computing simulations that have more computational power
| per second than the entire computing world could scrape
| together in a year in the 1960s, and those are just the
| highlights, not the exhaustive list.
|
| A lot of people are like "we got to the moon in the 1960s,
| where's the progress we should have had since then?" but I
| see the 1960s as the bizarre exception rather than the
| thing that should be used to set the rule. There was no way
| the space age was going to happen then, in an era where
| you're almost sitting there counting each bit of RAM you
| can afford to send into space. The true Space Age is just
| dawning now, and it's still early in the dawn; we still
| have to have massive international cooperation to put a
| single space station up, we can't do something as basic as
| refuel in orbit, we just barely started having people in
| space for commercial rather than governmental reasons...
| it's just the beginning.
| johndunne wrote:
| I think the 60's showed how much humans can achieve in
| terms of innovating with very little (in terms of tech).
| Now, we're seeing how much can be achieved with a whole
| lot more. And, I agree, the space age really does feel
| like it's only just heating up. Very exciting time!
| bobetomi wrote:
| I think it's not so much that we weren't ready for a
| space age tech-wise, but that the the reason we have so
| much of our technology today is because of investments
| made in the 1960s. NASA had basically unlimited money to
| throw at every technical challenge in the way of landing
| a human on the moon.
|
| The apollo program drove the need for more computational
| power, more memory, better guidance and navigation and
| control systems, better materials, experiments to better
| understand many phenomena, etc. And after the apollo
| program ended, the contractors that developed those
| technologies on NASA contracts could just commercialize
| them. And the data from experiments, on materials,
| aerodynamics, combustion, and so on, that is publicly
| available has made engineering so much cheaper and
| easier.
| ars wrote:
| I feel like he's limiting himself to just 2 positioning methods.
|
| There are so many other methods that the lander can use to know
| where the tower is.
| lysace wrote:
| Yes, that seems like a very big hand-wavy assumption. This
| paragraph is quite...something:
|
| > Could you use other real-time distance measurements like
| laser rangefinding or visual processing? I don't think so - the
| surface of the vehicle is too irregular to get a reliable fix
| point, especially while it is moving, and these are vulnerable
| to smoke/fog/gas/ambient lighting. Technologies like Ultra
| Wideband are vulnerable to multipath reflections and
| attenuation by the booster's steel walls, and aren't more
| accurate than RTK anyway.
|
| That is not exactly an exhaustive list of methods to locate an
| object.
|
| I have no idea whether 0.5 cm precision is feasible or even
| needed, but this part felt a bit off.
| generuso wrote:
| As the article explains, with a well designed procedure, the
| required navigation accuracy is quite modest. Even the latest
| consumer IMUs and GPS would do, and SpaceX is using even slightly
| more accurate "tactical grade" units, typical for all launch
| vehicles.
|
| Good article. It is nice how it goes through all the points
| systematically.
| cubefox wrote:
| There is also an interesting analysis about the control
| engineering perspective:
|
| https://youtube.com/watch?v=QHikx6kVvAo
|
| It talks about how real time control system algorithms work
| with algorithms like like PID and MPC. I assume the SpaceX
| solution is likely one of the most advanced control systems in
| the world.
| mensetmanusman wrote:
| The designers of the raptor engine should get a Nobel prize in
| chemistry for combustion physics.
| paulsutter wrote:
| The key was the full scale combustion simulator software that
| SpaceX developed to design the Raptor.
|
| This talk was given at NVIDIA GTC 2015 and inspired me to go
| into manufacturing
|
| https://www.youtube.com/watch?v=vYA0f6R5KAI
|
| title: "GPUs To Mars: Full Scale Simulation of SpaceX's Mars
| Rocket Engine"
| thot_experiment wrote:
| This talk is FANTASTIC and has been an inspiration for me for
| years as well. Highly HIGHLY recommend it.
| paulsutter wrote:
| The accuracy that counts is for the gap between the arms and the
| booster, which could well be 1cm since they can measure that with
| sensors as the booster descends between the arms, and the arms
| can be controlled accurately.
| cryptonector wrote:
| > Half a centimeter landing accuracy is not possible, and Bill
| likely misspoke or was talking about control error.
|
| Maybe the 1/2 cm accuracy refers to the final position of the
| booster's catch points on the arms after they've closed, after
| the booster's engines are off, and after the booster settled, and
| maybe they mean lateral accuracy. I would forgive them for that
| because that's the accuracy that _actually_ matters here.
|
| If the catch points were off then that might spell disaster, so
| the catch points' landing accuracy _including_ the help of the
| catch arms is what matters.
| HarHarVeryFunny wrote:
| The catch/lifting points may look small, but actually protrude
| from the side of the booster by 2-3 feet. Note that the booster
| is actually in a hover at the point the arms close in to touch
| it, so as long as it's vertical rotational axis is right (there
| are only 2 pins - one on either side), the positioning of the
| pins on the catching arms is basically guaranteed.
| cryptonector wrote:
| Right, so if SpaceX meant that they had an error of only .5cm
| maybe they meant that the error on the booster rotation angle
| was small enough to produce only a .5cm error at the catch
| points. Since they weren't specific, it's hard to know what
| they meant.
|
| The booster rotation angle error and the catch point
| placement error were much too small to detect with the naked
| eye on the published videos. Every other measure of accuracy
| was clearly within tolerances -- and also hard to discern
| with the naked eye.
|
| As amazing as .5cm accuracy sounds, if SpaceX meant catch
| point placement error, then it's quite as impressive because
| that only implies everything was within tolerance _and_ only
| the booster rotation angle error need have been impressively
| near-zero measure. That's... still amazing, honestly. If you
| can get the booster rotation angle error near zero then you
| can get the other errors way down too.
| thot_experiment wrote:
| 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.
| cubefox wrote:
| 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 wrote:
| The buoys were not trivial devices. See
| https://x.com/CosmicalChief/status/1626333723514834944
| cubefox wrote:
| Still, lasers on a buoy?
| mikewarot wrote:
| If it's positioning relative to the chopsticks, I'm sure it's
| possible to know where you are within a centimeter, even with all
| the rocket exhaust flying around. That's what DGPS is all about.
|
| It's still wildly un-nerving to me that there's no publicly
| stated option other than the chopsticks for landing(edit: some
| future passenger craft). Imagine if you've got enough fuel to
| avoid slamming into the ground, and a nice big ocean, or a lake
| sufficiently deep... couldn't a water landing happen and let
| future passengers survive?
| pedrocr wrote:
| This is not the passenger vehicle. That's the second stage,
| this is the first stage booster. They've also landed the second
| stage but that did a water landing from orbit which in many
| ways is even more impressive.
| rkagerer wrote:
| 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 wrote:
| 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...
| nurettin wrote:
| Not a US taxpayer, so I don't really have a stake in this, but
| I'm curious as to where SpaceX spending is compared to their NASA
| contract milestones.
| sfblah wrote:
| Take a look at videos on YouTube by ThunderF00t. SpaceX is
| pretty problematic. Currently, around 80% of SpaceX's rockets
| are dedicated to launching Starlink satellites. This is because
| they've already soaked up all the demand for satellite
| launches, and things like starship don't really have a reason
| for existing. Also, in spite of massive improvements in
| technology since the 1960s, they're vastly underperforming the
| track record of the Apollo program.
|
| My view is that Tesla, SpaceX, etc. are just cults of
| personality. Sure, they've produced a non-zero amount of
| technological progress, but for the most part they're leading
| us to dead ends.
| cubefox wrote:
| > Also, in spite of massive improvements in technology since
| the 1960s, they're vastly underperforming the track record of
| the Apollo program.
|
| The SpaceX budget is several orders of magnitude smaller than
| the Apollo budget. Also, the Apollo era rockets were entirely
| unable to launch a large constellation like Starlink at
| reasonable cost like Falcon and Starship.
|
| > Sure, they've produced a non-zero amount of technological
| progress, but for the most part they're leading us to dead
| ends.
|
| Not sure whether you are serious. Assuming you are, what
| would be the better alternative then?
| sfblah wrote:
| It's not several orders of magnitude smaller. According to
| ChatGPT, SpaceX has spent $20-30B as of 2023, compared to
| $160-170B in 2023 dollars. That's less than one order of
| magnitude, and consider that SpaceX is starting with all
| the knowledge gained from Apollo, plus 50 years of
| technological progress.
|
| To your second question, the better alternative is not to
| send a bunch of junk into space. Aside from low-orbit
| satellites, space is a waste of time and a distraction, and
| increasingly appears just to be a way to trick people into
| ignoring disastrous fiscal and monetary policies while
| enriching one person.
| minetest2048 wrote:
| Even if thats true, who else can launch satellites at SpaceX
| price and regularity? This is an actual question, when Falcon
| 9 stage 2 exploded several months ago, my cubesats got
| delayed
|
| I know several space startups that's currently limited by
| launch prices
| sfblah wrote:
| Who cares? Since the 1960s, space launching has never been
| limited by supply. It's limited by demand. That's why 80%
| of SpaceX's cargo is for satellites to support their
| marginally useful Starlink business. Like Hyperloop, "space
| startups" are just dressed up snake oil.
|
| Imagine if there were hundreds of "startups" aiming to sink
| stuff for some reason to the bottom of the Mariana Trench.
| Sure, doing that is difficult and fun to think about from a
| science fiction perspective, but the bar to show how such a
| thing could be useful is and should be quite high.
|
| I asked GPT to give me the 5 most compelling space startups
| created since 2020. Here's what it gave me:
|
| * Space refueling
|
| * Getting things back from space more cheaply
|
| * Removing space debris
|
| * Getting things to space more cheaply
|
| * Microgravity life sciences
|
| Of these, only the last one is an actual product. The
| others are just picks and shovels for nonexistent products.
| And, I don't see why microgravity life sciences requires
| the ability to deploy unlimited quantities of crap to low-
| Earth orbit.
| mardifoufs wrote:
| Thunderf00t has basically 0 credibility when it comes to
| SpaceX. He predicted that starlink couldn't ever work, for
| example. In fact, he even thought that this landing would
| fail.
|
| Like he might have good point hidden somewhere, the issue is
| that we know that he will reflexively "debunk" anything
| SpaceX does, and predict failure every time. Meaning that his
| takes on SpaceX are beyond useless.
| sfblah wrote:
| https://www.google.com/search?q=ad+hominem+attack
|
| Edit: Just out of curiosity, I looked up his video about
| Starlink, and his point was it's a bad business, not that
| it's physically impossible to do it:
|
| https://www.youtube.com/watch?v=zaUCDZ9d09Y
|
| I'd actually love it if you could find a single thing he
| says in that video that's provably false. I'm quite
| confident history will judge Elon Musk properly, as the
| "Too Big To Fail" version of Theranos and Elizabeth Holmes.
| jimrandomh wrote:
| Talking about someone's track record of predictions is
| not an ad hominem, in the context of evaluating their
| credibility with respect to the subject of those
| predictions.
| sfblah wrote:
| Actually, it is. From the dictionary:
|
| An ad hominem attack occurs when someone attacks the
| person making the argument rather than addressing the
| argument itself. For example, saying "You're always
| wrong, so you're wrong about this too" without addressing
| the current claim would be ad hominem.
|
| That's exactly what the person I responded to was doing.
| mardifoufs wrote:
| >They claim they're going to get these laser
| communications between the satellites which will make
| things faster for a long distance... [this is because
| light travels faster in a vacuum than through fiber optic
| cable you up to London a very important one for the
| Global Financial system Starlink latency is under 50
| milliseconds while the current Internet is around 70
| milliseconds] yeah Starlink can't do any of that at the
| moment.
|
| >Probably something to do with the fact that the
| satellites are hundreds of miles or kilometers apart and
| you're trying to hit a tiny moving Target from another
| moving target with a laser and then and chaining those
| together that doesn't sound very easy but they're
| promising to launch some satellites that can do it in the
| next generation [getting close to launching satellite 1.5
| which has laser interest satellite links]
|
| >Now where have I heard that before... Let's just call me
| skeptical on this one
|
| https://youtu.be/zaUCDZ9d09Y?t=1688
|
| This entire video is just the same thing over and over.
| As is usual with him, he doesn't actually make any strong
| enough claim and just goes on endless sneering. But in
| the context of the video, it is absolutely clear that
| he's saying it is just another lie and that it's
| basically impossible. Remember, this was mere _months_
| before the laser network went up. He also claimed that
| the bandwidth is never going to be usable, but it
| absolutely is.
|
| Also, that's just the fallacy fallacy. This isn't some
| sort of debate club, it makes perfect sense to discredit
| someone based on past record. For example I would
| absolutely take with a grain of salt anything Musk
| promises. It would be very dumb to just erase any priors
| everytime someone claims something. And no one does that
| expect when it's to play fallacy semantics online. I
| never attacked thunderf00t as a person, I'm attacking his
| completely bogus track record on SpaceX.
| thamer wrote:
| > I think < 10 cm accuracy is achievable
|
| If you don't know how precise GPS receivers can get with dead
| reckoning techniques, this demo of someone "drawing" onto a map
| of their driveway using a GPS receiver is very impressive:
| https://youtu.be/3tQjIHFcJVg?t=245
|
| It looks like they're getting measurements that are only a few
| inches away of the module's real position, although of course the
| conditions seem favorable with an unobstructed sky and consistent
| alignment.
|
| The module they use is a ZED-F9P by u-blox. I've used ~$50 u-blox
| GPS modules in DIY electronic projects before since they're often
| the brand you'll get when buying GPS modules, but this particular
| type with dead reckoning is _much_ more expensive. Sparkfun has
| it for $275 for example: https://www.sparkfun.com/products/16481.
| simgt wrote:
| I believe this is demonstrating the performance improvement of
| RTK [0] alone, not dead reckoning. GPS + dead reckoning is what
| phones and wearable do afaik, whereas RTK requires getting
| correction data associated to base stations nearby and seems
| mostly relevant for industrial applications (you need a
| subscription in the case of u-blox).
|
| [0] https://en.wikipedia.org/wiki/Real-
| time_kinematic_positionin...
| simgt wrote:
| I vaguely remember Musk talking about reflective paint for some
| band on the landing pad of Falcon, a long time ago.
|
| > At the most precise, an RTK positioning system could lower
| position accuracy all the way down to 2.5 cm (+1cm per km of
| distance). If SpaceX put a receiver on the launch tower or the
| ocean buoys, then the landing position could be incredibly
| accurate. But even the most advance positioning tech won't
| guarantee it down to 0.5 cm. And RTK does rely on being able to
| acquire and maintain a link between the booster and ground for
| this precision.
|
| I don't understand this last sentence. Afaik RTK correction only
| requires receiving correction frames on the booster's side, which
| can be distributed via l-band just like GPS. I suspect the
| latency constraints are also quite low as the conditions aren't
| going to change quickly near the tower with the kind of good
| weather they choose for launch.
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