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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]

#351

Post by timschmidt »

cubefox wrote: post #285
adt2bt wrote: post #99

Earlier quoted context omitted.

Think of the word ‘reusable’ in this case as less a binary descriptor but more of a scale of reusability. Yes, both systems are reusable, but there are key differences in the refurbishment of the systems that partly explains the cost difference. It took more labor, resources and time to refurbish the shuttle. Also consider rapid reusability was a stretch goal when it was being designed, but we have come a loooong way…

> Yes, both systems are reusable, but there are key differences in the refurbishment of the systems that partly explains the cost difference. It took more labor, resources and time to refurbish the shuttle. Starship uses essentially the same ceramic heat shield tiles as the Space Shuttle, so the fact the Shuttle had so much trouble with refurbishment doesn't mean that SpaceX has solved these refurbishment issues with…

Shuttle's tiles were each unique. Starship is mostly clad with identical hexagonal tiles which can be mass produced and eventually refurbished by machine. A robot already welds on the tile fittings.

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

#352

Post by philistine »

shirro wrote: post #199
teractiveodular wrote: post #19

It's difficult to overstate how important the milestone of catching the booster is. Now we have a reusable rocket an order of magnitude larger than anything we've had before, and the cost of kg to orbit just nosedived.

Second stage reuse seems the far more challenging problem. Other companies should have reusable boosters soon but if significant amounts of Starship continue to ablate on the way down they could be faced with a disposable Starship competing with smaller and cheaper second stages that are well sized for typical payloads. We already knew boosters can be flown back to launch sites reliably with high accuracy. We don't k…

> Other companies should have reusable boosters soon

You're way too optimistic. Starship will deliver commercial payloads, with SpaceX phasing out Falcon 9 outside of ISS launches, before anyone has a reusable Falcon 9 equivalent.

It pains me to say this, but SpaceX is in a class all its own.

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

#353

Post by SAI_Peregrinus »

systemvoltage wrote: post #263
nomilk wrote: post #225

Earlier quoted context omitted.

I'd never considered a 70m tall chunk of mostly metal "agile", but the way it's controlled into the arms so precisely is something to behold.

Do you know what the entire booster is hanging off of? It can't be the fins. How is the load transferred to the arms?

The grid fins are what it's hanging off of.

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

#354

Post by magicalhippo »

drexlspivey wrote: post #230
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.

The booster was falling at 4500 Km/h 30 seconds before the catch with 2-3% fuel left. How is that amount of fuel remotely enough to stop the downward momentum?

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 lit at 1250 km/h, and so would quickly become negligible once the engines lit.

Going by Wikipedia, the Super Heavy[1] has 3400000 kg of fuel at launch, so 3% of that is about 102000 kg. For the landing burn, it used 13 Raptor v3 engines[2] to scrub speed. Each Raptor flows about 650 kg/s max, so 3% fuel is enough for about 12 seconds for the 13 engines.

The empty mass of the Super Heavy is about 275000 kg, so about 377000 kg before the landing burn with 3% fuel.

Using the sea-level vs vacuum performance of the Raptor v2 engines, one can estimate that each Raptor v3 produces about 2.45 NM of force at sea-level. So 13 of them would produce about 31.85 MN of force.

Using Newton's second law, F=ma, this gives an initial deceleration of about 84 m/s^2 and about 104 m/s^2 when empty. If we do a rough spreadsheet integration, we get that a burn of roughly 4 seconds is needed to scrub the speed assuming no other forces.

Now, comparing this with reality, the full 13 engines were lit for a little over 5 seconds.

In my simplified calculations I was assuming full throttle the whole way, which obviously isn't realistic, and I also assumed 3% fuel. So over all I think that's a pretty decent estimation.

[1]: https://en.wikipedia.org/wiki/SpaceX_Super_Heavy#Engines

[2]: https://en.wikipedia.org/wiki/SpaceX_Raptor

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

#355

Post by arijo »

brcmthrowaway wrote: post #349
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.

Do the control algorithms use AI?

Actually this is a good question - there's a lot of control engineering research on Data-Driven Control Systems.

Check Steve Brunton youtube channel, he is one of the leaders in this area: https://youtu.be/gb_C9LcjDSI?si=xUjqUZ9-0MIFohX6

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