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10 hours ago, CatastrophicFailure said:

There’s that hindsight thing again... :P

 

Looks like Elon had been playing stock KSP and was surprised by the switch to RO.

Thinking about throttle settings. With two engines that’s definitely a suicide burn. But not all suicide burns are created equal. I am guessing that the flip will be timed such that the suicide burn will be achieved at or near minimum throttle for both engines, so if one has a thrust shortfall the other can throttle up to compensate.

This is really the inherent problem with these landings...all this stuff is literally rocket science. Having three engines for redundancy is great because it allows these options but that doesn’t necessarily fix problems with timing. If something goes bad on a plane, you typically have plenty of time to come up with a solution, and there’s no mission-critical system that undergoes a bunch of transients right at touchdown. 

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Pragmatically, I think they need to get to a point where they can have the TWR <1 with good control; as well as TWR>1 obviously. And don't forget there will be a not insignificant ground effect too.

Whether this is achieved with greater reliability, 2 engines (ie redundancy), RCS assistance, depends on a number of things being engineered or developed to a point further down the line than they're at now. Things like an engine not producing thrust when commanded to, is obviously a major issue which they can't have happen too often, if at all.

Once they have the control in the 0.8 > TWR > 1.2 'zone', the landing gear issue is rendered somewhat moot. So is the post-landing explosion scenario - they could in theory, hover 5cm above the pad at TWR=1.000 and land safely with almost no fuel remaining. Obviously this is inefficient on fuel so later on, they will want to coordinate the whole de-orbit, approach, landing to use less fuel but still retain the good amount ((reserve) fuel for) of control.

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30 minutes ago, sevenperforce said:

Thinking about throttle settings. With two engines that’s definitely a suicide burn. But not all suicide burns are created equal. I am guessing that the flip will be timed such that the suicide burn will be achieved at or near minimum throttle for both engines, so if one has a thrust shortfall the other can throttle up to compensate.

Yeah, sounds reasonable. If we take 40% as Raptor's minimum thrust (and that's a figure from months ago, undoubtedly they've reduced that since then) then two engines on minimum throttle produce the same thrust as a single engine on 80% thrust. And throttling up an engine is a lot simpler than starting a different one, so this should provide engine redundancy throughout the entire flip and landing burn sequence.

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Looks like SN16's leg skirt is being scrapped. Maybe this is for new legs.

Edit: SN16's leg skirt is already attached to its aft dome. Maybe this was SN17's or something.

Edited by RealKerbal3x
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1 hour ago, sevenperforce said:

This is really the inherent problem with these landings...all this stuff is literally rocket science. Having three engines for redundancy is great because it allows these options but that doesn’t necessarily fix problems with timing. If something goes bad on a plane, you typically have plenty of time to come up with a solution, and there’s no mission-critical system that undergoes a bunch of transients right at touchdown. 

Yeah, it really is a 1 shot thing.

Makes me wonder about engine choice, to be honest. Ideally you's have the Earth landing engines such that it would nominally land with all burning, and sink rate controlled by throttle. Engine outs such that remaining engines can throttle up.

If Raptor can't throttle deep enough, I wonder if they could make a different version with less thrust for landing.

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Anyone else reading Liftoff by Eric Berger? I'm 7 chapters in and folding it hard to put down. I'd have loved to have worked for SpaceX in those early days.

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I wonder why the engine wasn't producing the requested thrust on landing, plume seems alright to me..

 

Perhaps a faulty sensor? A stuck throttle valve should prevent them from shutting the engine down right, so that wouldn't be it..

Edited by Flying dutchman
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1 hour ago, Flying dutchman said:

I wonder why the engine wasn't producing the requested thrust on landing, plume seems alright to me..

 

This.

They need to get to grips with why the engine(s) seem to work fine on a test stand, but don't once attached to a spaceship and flown in the profile they have in mind for the thing. Unlike an aeroplane or helicopter this thing is 100% reliant on the engines working properly to make a landing. Yes I know the SNs are allocated for these testing phases but there is a cost to crashing, both monetary in cleanup; possibility of damaging other kit; regulatory and reputational costs too. I would not be surprised if the FAA once again want quite a detailed investigation into the latest crash and want to see changes made - on their terms, not SpaceX's, which will be like walking through treacle compared to if SpaceX themselves could dictate the pace of advancement.

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7 hours ago, paul_c said:

They need to get to grips with why the engine(s) seem to work fine on a test stand, but don't once attached to a spaceship and flown in the profile they have in mind for the thing. 

That's why they fly.

The only site at risk belongs to SpaceX, and the people who get the say on whether SpaceX can risk that site is SpaceX. The flights haven't come even close to risking anything else, and they're equipped with flight termination systems in the event that they might. And on the contrary, I don't think SpaceX is taking any reputational damage whatsoever from these flights. 

I don't see why the FAA would have any more problem with SN10 than SN9 (or especially SN8).

Edited by RCgothic
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6 hours ago, RCgothic said:

The only site at risk belongs to SpaceX, and the people who get the say on whether SpaceX can risk that site is SpaceX.

No, this is not how it works. Everything that flies needs to have some kind of permission.

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And the FAA have no reason to deny that permission based on the flights to date or they would already have denied SN9 or 10.

There isn't more risk just because the prototypes haven't stopped crashing. 

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4 hours ago, RCgothic said:

There isn't more risk just because the prototypes haven't stopped crashing. 

The risk is some competitor's lobbyist convincing a regulator that 'something needs to be done.'  Easiest thing in the world to do if you want to throw a wrench into a competitor's works; get the administration to put on the brakes for no good reason... other than because Sen. Sidekick or Rep. Handmeout has a 'constituent' who isn't getting paid. 

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22 minutes ago, Spaceception said:

Is that heatshield patch bigger or about the same size as SN10s?

It's bigger.

14 minutes ago, JoeSchmuckatelli said:

Is there anything different about the flaps? 

 

I can't tell if it is a camera angle /shadow thing or something new 

They seem the same to me.

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15 minutes ago, JoeSchmuckatelli said:

I can't tell if it is a camera angle /shadow thing or something new 

Think they are the same, it's just the light.

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