totm nov 2023 SpaceX Discussion Thread

[magnemoe]

1 hour ago, Delta dart said:

All the other tanks sprung leaks before exploding so will this one have a spectacular explosion or is it done?

They probably used nitrogen so no explosion outside of over pressure blast. 

[Flavio hc16]

47 minutes ago, tater said:

 

as i said, he wasn't being sarcastic, he was genuine in saying that they were "dumb"

[JoeSchmuckatelli]

54 minutes ago, Flavio hc16 said:

as i said, he wasn't being sarcastic, he was genuine in saying that they were "dumb"

You know - it had to be a decision at some level.  You have to assume that they at least discussed the merit of the plan... Which almost worked for Sn8.  So I don't think it was oversight, or 'dumb' out of hand - but one of those things that in retrospect you decide was dumb - even though at the time it seemed like a reasonable course of action. 

 

I forgot which tweet it was - but I think that lighting 3 has it's own potential downside... So I hope Sn10 is illuminating, if not quite as bright as Sn9 was there for a moment 

Edited February 4, 2021 by JoeSchmuckatelli

[mikegarrison]

52 minutes ago, JoeSchmuckatelli said:

You know - it had to be a decision at some level.  You have to assume that they at least discussed the merit of the plan...

I wouldn't assume that. Lots of times there are things that seem obvious in hindsight that nobody thinks of before.

I'm still not sure whether this "light 3" idea makes sense anyway. They have *very* *little* *time* to recognize a problem and react to it. Do they really have time to check the status of the engines and then shut one down if the other two are good? Ideally they should try to find a way to buy themselves more time.

And then there is British Midland flight 92, where a fan blade broke in the left engine, disrupting the air conditioning and filling the cabin with smoke. The pilots believed this indicated a fault in the right engine, since earlier models of the 737 ventilated the cabin from the right, and they were unaware that the -400 used a different system. The pilots mistakenly shut down the functioning engine. They selected full thrust from the malfunctioning one and this increased its fuel supply, causing it to catch fire. Of the 126 people aboard, 47 died and 74 sustained serious injuries. (Quoted from Wikipedia)

Any system where you start three and then shut down one is potentially vulnerable to something going wrong which causes you to shut down the wrong one.

Edited February 4, 2021 by mikegarrison

[Brotoro]

Has SpaceX said yet what caused the failure? I presume it was an engine failure (because of the little blast that was big enough to rip off the thermal blankets, and the fire at the top of the engine where there should be no fire).

[tater]

27 minutes ago, mikegarrison said:

Any system where you start three and then shut down one is potentially vulnerable to something going wrong which causes you to shut down the wrong one.

yeah, I suppose it introduces another failure mode. I guess the question is if the chance of that is countered by an increase in survivability. <shrug>

[Clamp-o-Tron]

41 minutes ago, tater said:

yeah, I suppose it introduces another failure mode. I guess the question is if the chance of that is countered by an increase in survivability. <shrug>

Everything you add or adds a failure mode of it’s own. It’s just a question of whether that risk is lesser, or can be managed better.

[sevenperforce]

8 hours ago, tater said:

Airliners can as you pointed out, even have survivable events with all engines out—not a thing ever with Starship. So engines in fact need to be substantially more reliable than the jet engines used in airliners (though bird strikes/etc are likely less of a concern—hmm, wonder if any Merlins have ever ingested a bird coming down?

Pretty sure sure anything that ends up anywhere near the business end of a Merlin -- or a Raptor -- will end up atomized.

 

5 hours ago, kerbiloid said:

They should use the decommissioned tanks for a probabilistic sacrifice.

Say, probability of a tank explosion is 1:1.
They should explode a decommissioned tank, so the probability gets full up, and the other tank won't explode in flight.

Spoiler

 

5 hours ago, tater said:

Might be the only time I quote myself, but in this case ^^^...

Not to challenge your brilliance but my brilliant 8-year-old said the same thing as soon as I told him why the rocket went all asplodey.

[sevenperforce]

1 hour ago, tater said:

yeah, I suppose it introduces another failure mode. I guess the question is if the chance of that is countered by an increase in survivability. <shrug>

Let's see, the equation is something like:

ForThreeEngineStartup: Risk = RiskOfBadRestart * 3 * RiskThatBadRestartCascades

ForTwoEngineStartup: Risk = RiskOfBadRestart * 2

So if the risk of a bad restart cascading to otherwise-healthy engines is greater than 2/3, you should only light 2. If it is less than 2/3, you should light all 3.

 

[paul_c]

Those equations don't look right

Shouldn't it be something like (1-risk of bad restart) ^ 2

Also, yes there is a risk of shutting down the wrong engine, or a fault cascading from a bad restart to a bad shutdown to another engine etc but these types of things can be engineered away. For example you could group the engines into "red" engines, "green" engines and "blue" engines then colour code all the wiring bundles as appropriate; or even develop a system of unique notches in the wiring harnesses so that a miswiring of a signal is unlikely, etc etc.

I dare say they looked at it and decided the end point was engine reliability of xxx so the disadvantages of lighting more engines than needed outweighed the (temporary maybe, only during testing and early on) advantages of it. And let's not forget, this isn't KSP where you can operate a slider to dial down the engine thrust to 0.5%, these things have a limited operating window and engineering to widen that is expensive.

Similar to how transcontinental airplanes tended to have 4 engines in the past, then the trijet made economic sense, to nowadays where 4 engine planes are uneconomic and its bit twins doing the routes.

[tater]

1 hour ago, sevenperforce said:

Pretty sure sure anything that ends up anywhere near the business end of a Merlin -- or a Raptor -- will end up atomized.

I was thinking before restart for the landing burn, though SS has the engines not ingesting stuff as they start horizontal.

I suppose the combustion chamber is not nearly as delicate as turbine blades at high RPM.

 

1 hour ago, sevenperforce said:

Not to challenge your brilliance but my brilliant 8-year-old said the same thing as soon as I told him why the rocket went all asplodey.

It's pretty self evident if the point of multiple engines is redundancy. I mean, they don't even need the SL engines except for landing.

[JoeSchmuckatelli]

Automated system on the ship should be able to get 'lever arm' measurements and then rate each for priority, from most to least. 

I presume the system can also tell whether each rocket has had good relight or not.

So if all 3 relight* appropriately, it just does an if-then process to shut down the one with the least lever arm. 

 

*If 2 relight, it quits feeding the 3d for safety and lands with the 2 good regardless of the lever arm measures

[RealKerbal3x]

 

[Elthy]

SpaceX still has a problem, even if they try the three engine burn next time. If the issue was with the fuel supply another engine wouldnt help, if the problem was in the engine itself then they seem to have issues with Raptors reliability. The second one is propably worse, since those are way more complex to develop/modify.

[RealKerbal3x]

6 minutes ago, Elthy said:

SpaceX still has a problem, even if they try the three engine burn next time. If the issue was with the fuel supply another engine wouldnt help, if the problem was in the engine itself then they seem to have issues with Raptors reliability. The second one is propably worse, since those are way more complex to develop/modify.

From Elon's tweets it really seemed like the issue was Raptor reliability. One of SN9's engines kept running perfectly up until impact anyway, so if there was something wrong with the propellant supply it would have to be localised to the plumbing for the specific engine that failed, and that seems less likely than an engine issue to me.

[zolotiyeruki]

5 minutes ago, Elthy said:

SpaceX still has a problem, even if they try the three engine burn next time. If the issue was with the fuel supply another engine wouldnt help, if the problem was in the engine itself then they seem to have issues with Raptors reliability. The second one is propably worse, since those are way more complex to develop/modify.

It quickly becomes a question of whether it makes more sense to improve the design to prevent failures due to external factors, or increase the redundancy to mitigate the failures.  In the case of Sn8, you definitely want to improve the fuel system so as to not starve your engines.  If a very speedy bird flies up the engine bell, that might be something you address by lighting three engines instead of two.

[RealKerbal3x]

A Raptor for SN10 has arrived! 

 

[Le Lynx]

cool

[JoeSchmuckatelli]

59 minutes ago, RealKerbal3x said:

A Raptor for SN10 has arrived! 

 

In this image of 10, the cone shows a different color band of metal... Does that indicate a different type of steel? 

[RealKerbal3x]

3 minutes ago, JoeSchmuckatelli said:

In this image of 10, the cone shows a different color band of metal... Does that indicate a different type of steel? 

Maybe, we don't know for sure.

[JoeSchmuckatelli]

1 hour ago, zolotiyeruki said:

If a very speedy bird flies up the engine bell, that might be something you address by lighting three engines instead of two.

I don't believe Rocket Martins are native to South Texas. 

 

Although I have heard of an influx of non-native unladen African Swallows... 

[Codraroll]

1 hour ago, zolotiyeruki said:

If a very speedy bird flies up the engine bell, that might be something you address by lighting three engines instead of two.

I'm no ornitologist, but would this realistically be an issue? Birds have evolved to fear predators coming from above, after all. At bird-flight altitudes, a falling Starship or Falcon would go engine-first only when at (or near) terminal velocity, while travelling nearly vertically. And they are big. It would be easier for a bird to spot and dodge a falling rocket than a diving bird of prey, and they generally do that successfully most of the time. A bird allowing a falling rocket to get close enough to strike one of the engines would have to be very inexperienced, very absent-minded, or possibly suicidal. 

[sevenperforce]

13 hours ago, paul_c said:

Those equations don't look right

Shouldn't it be something like (1-risk of bad restart) ^ 2

Yeah, you're right.

1 hour ago, JoeSchmuckatelli said:

Automated system on the ship should be able to get 'lever arm' measurements and then rate each for priority, from most to least. 

Lever arm is just differential torque from a particular engine combination. With the kick-flip, you ideally want to use the engines which are balanced on either side of the CoM (B & C below), because using one side engine and the "top" engine (A) is going to produce a yaw moment, which will have to be corrected with gimbal but then induces a roll moment, and...well, it can be done, but it's messy.

So the decision tree looks something like this:

1. Relight C
2. Relight B
3. Relight A
4. Initiate kick-flip
5. Check Relight C
6. Check Relight B
7. IF(C_Relight = OK || B_Relight = OK), Cutoff A
8. ELSE Check Relight A
9. IF(C_Relight = OK || A_Relight = OK), Cutoff B
10. ELSEIF(B_Relight = OK || A_Relight = OK), Cutoff C

21 minutes ago, Codraroll said:

I'm no ornitologist, but would this realistically be an issue? Birds have evolved to fear predators coming from above, after all. At bird-flight altitudes, a falling Starship or Falcon would go engine-first only when at (or near) terminal velocity, while travelling nearly vertically. And they are big. It would be easier for a bird to spot and dodge a falling rocket than a diving bird of prey, and they generally do that successfully most of the time. A bird allowing a falling rocket to get close enough to strike one of the engines would have to be very inexperienced, very absent-minded, or possibly suicidal. 

Birds are also known for being, well, fluffy.

I imagine the supersonic shockwave from a re-entering booster would tend to swat them away before they even made physical contact.

[paul_c]

Bird strikes are normally due to a flock of birds. With so many of them, most of them get away. I imagine rocket launch facilities use similar tactics to airports to control the bird population.

[RealKerbal3x]

Look at those heat tiles on SN11!