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Starship Flight 5: Launch and booster catch [video]

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Re: Starship Flight 5: Launch and booster catch [video]

#591

Post by m4rtink »

arijo wrote: post #323
dtquad wrote: post #183

Earlier quoted context omitted.

MIMO and nonlinear control theories are probably some of the hardest topics in all of engineering. SpaceX control system also has to compensate for the fuel moving inside their rockets so the control algorithms probably involve some kind of fast numerical fluid simulation. Another interesting thing SpaceX is doing is to use consumer-grade chips in triple redundancy configurations instead of using $100,000+ radiation-…

> Another interesting thing SpaceX is doing is to use consumer-grade chips in triple redundancy configurations instead of using $100,000+ radiation-hardened aerospace/defense grade chips. This has been known in the high availability and safety systems industry for a while and a good book to learn these reliability engineering techniques is "Reliability Evaluation of Engineering Systems". The book is available on amaz…

From an article for this I remember one more interesting side effects of this approach - the flight computer ends up as a generic x86/ARM board that the engineers can just have on their desk during development. Previously the dev boards would use the same rad hard chips and would be as expensive and scarse as flight hardware, resulting in much harder development & engineers having much less experience with the real hardware.

Re: Starship Flight 5: Launch and booster catch [video]

#592

Post by r3d0c »

Yacovlewis wrote: post #571
magicalhippo wrote: post #354

Earlier quoted context omitted.

First off, the booster was going about 1250 km/h when it started its landing burn, it relied purely on drag to get it slowed down to that speed. Going by the telemetry of the seconds before the landing burn and noting the speed vs time, it seems drag was around 40 m/s^2 when it was going at around 3000 km/h. Since drag depends on velocity squared though, it had reduced to just above 10 m/s^2 just before the engines l…

What kind of work do you do that you're able to understand this stuff? Genuinely curious. I'm way beyond my depth trying to follow

this is ap hs physics or uni 1st year physics

Re: Starship Flight 5: Launch and booster catch [video]

#593

Post by magicalhippo »

Yacovlewis wrote: post #571
magicalhippo wrote: post #354

Earlier quoted context omitted.

First off, the booster was going about 1250 km/h when it started its landing burn, it relied purely on drag to get it slowed down to that speed. Going by the telemetry of the seconds before the landing burn and noting the speed vs time, it seems drag was around 40 m/s^2 when it was going at around 3000 km/h. Since drag depends on velocity squared though, it had reduced to just above 10 m/s^2 just before the engines l…

What kind of work do you do that you're able to understand this stuff? Genuinely curious. I'm way beyond my depth trying to follow

Just a programmer, though been interested in physics since I was a teen and did take a bachelor degree in simulation (mainly physics).

Long ago though so rusty, $dayjob doesn't involve any advanced math at all.

edit: To expand, the "rough spreadsheet integration" was just the Euler method[1] assuming a constant acceleration. So

    v(t+dt) = v(t) + a * dt
The acceleration comes from F=ma as mentioned, where F is the force of the engines (Newtons), m is the mass of the rocket (kg) and a is the acceleration (m/s^2). Solving for a we get a = F/m and we get

    v(t+dt) = v(t) + F/m(t) * dt
To make things easy I assumed the weight of the rocket was constant at each timestep, but if we take dt to be small enough it's a decent enough approximation. For each timestep I also updated the mass using the estimated mass flow:

   m(t+dt) = m(t) - 650 * dt
I started with m(0) = 377000 kg, v(0) = 1250 km/h = 347 m/s, and a constant -31850000 N force from the engines.

Using dt = 0.1 seconds, I got almost exactly 4 seconds until the velocity reached zero.

[1]: https://en.wikipedia.org/wiki/Euler_method

Re: Starship Flight 5: Launch and booster catch [video]

#594

Post by creer »

XorNot wrote: post #96
ptek wrote: post #52

Very inspiring, glad I woke up to watch it. Congratulations to the SpaceX team. This is what competition looks like. It’s funny how last decade SpaceX had to sue the government to get more launches.

The FAA needs more funding at this point. They're not staffed for the rate of launch requests which commercial space has grown too.

It seemed to not be FAA here, but the several other agencies that all had 60 days to respond. FAA fast-tracked it but couldn't fast track it past the other guys' 60 day allowed time.

Re: Starship Flight 5: Launch and booster catch [video]

#595

Post by bgnn »

mlsu wrote: post #481
dtquad wrote: post #183

Earlier quoted context omitted.

MIMO and nonlinear control theories are probably some of the hardest topics in all of engineering. SpaceX control system also has to compensate for the fuel moving inside their rockets so the control algorithms probably involve some kind of fast numerical fluid simulation. Another interesting thing SpaceX is doing is to use consumer-grade chips in triple redundancy configurations instead of using $100,000+ radiation-…

I know MPC takes a LOT of compute power. It's not like a finely tuned PID loop or even a cascade of PID loops, computationally. Does anyone know (or have educated speculation on) what kind of hardware is running these algorithms? Like, do they have a linux machine that's running the control loops? Are we talking megabytes, gigabytes of SRAM? I would think no -- you would definitely need hard real time for something l…

It would be a real time OS running it for sure. Which one I don't know.

Re: Starship Flight 5: Launch and booster catch [video]

#596

Post by _flux »

DiggyJohnson wrote: post #334
goodgrief99 wrote: post #330

Why don't they mount moving video cameras? I mean, the second part of the fly is pretty boring. They shows us a view from 2 static cameras. Is it so difficult to make one camera moving? With 360 degrees view. Show us the shape of the earth the full circle, show us the moon. Is it so hard?

Yes what you’re describing is nearly impossible.

I don't think they'd need to actually have moving cameras (I think this is what the poster meant) to give a 360 degree view, but 360 degree cameras (well, 180 because they're right next to an obstacle) that could be reprojected to 2d.

Re: Starship Flight 5: Launch and booster catch [video]

#597

Post by supportengineer »

robotnikman wrote: post #588

The engineers at spaceX are amazing, and I'll admit I envy their tenacity and genius. The amount of progress we have seen in rocketry and space travel in the last decade is mind blowing, I never thought I would see this in my lifetime. Edit: Decided to give it a shot and applied for a backend software engineering position at SpaceX. I'm not sure if it will be noticed, but its worth a shot. I may not be a rocket scien…

We could have had this all along.

Re: Starship Flight 5: Launch and booster catch [video]

#599

Post by mlindner »

startupsfail wrote: post #584

[flagged]

Burt Rutan, one of the most legendary aerospace engineers of all time used to do regular long presentations to people on why global warming was not man made. Some of them are archived on youtube. And there's many other cases of otherwise smart people having bonkers opinions in areas outside of what they are experts in. If anything, successful people are significantly more likely to do this. It rarely results in their downfall.

SpaceX will do just fine.

Re: Starship Flight 5: Launch and booster catch [video]

#600

Post by varenc »

pj_mukh wrote: post #392
ghc wrote: post #23

Nothing could have prepared me for how that catch looked. I was sure the rocket was careening into the tower at the last second before it straightened out. The control algorithms must be incredible for the landing system to work within those small tolerances.

I know the control algorithms are the mind-blowing part here but, does anyone have any literature about how the Rocket localized itself with respect to the chopstick arms? It must've been some combination of GPS and Radar pings to the arms? And then the onboard IMU to make sure it hits it straight.

Great question! Could just be Real-Time Kinematic (RTK) GPS like someone mentioned. Essentially the landing arms know their position very precisely and they measure the tiny errors in GPS data, and send that correction data live to the rocket in real-time as it's landing. Once the rocket gets very very close it could also just be using vision systems to zero-in on exactly where the chopsticks are.

To speculate more, they could also be using something like ultra-wide band positioning. This relies on the same time-of-flight principle as GPS but instead of using satellites in orbit to provide the precise time information you rely on various nearby ground stations. Would only be useful right at the final approach, the last couple hundred meters, but it's another way they could get very very precise position information. (fun fact: Ultra Wide band positioning is also how iPhones can locate AirTags with centimeter accuracy)

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