Saturday, February 6, 2021

Why Did SN9 Explode? Elon Musk - "We were too dumb"

I think it's refreshing when a chief engineer says, in effect, "why didn't I think of that?"  As reported on Teslarati and elsewhere, in the aftermath of the SN9's test flight and belly flop impact last Monday, Elon Musk went back to Twitter, where this exchange took place. 


It's very easy for an engineering team to get stuck in the trap of, "we've always done it that way," and that has the predictable side effect of making people stop thinking of whether there's a better way or not.  It's a cliche in Continuous Improvement classes to say if you're doing something because you've always done it that way, you should stop and reexamine it. SpaceX has only been around about 15 years, but given their successes with the way they revolutionized orbital boosters, I can imagine a certain amount of that attitude might have set in.  In this case, something along the lines of, "we know how to land boosters." 

Back at the start of the year, in a previous discussion about landing the Starship Super Heavy booster, we talked about the details of thrust to weight ratios, and how even engines that can be throttled don't get the designers out of all the traps lying in wait.  These engines don't have a zero to 100% range - there's some minimum they can't throttle back below.  
When the Falcon is returning to land, at the very last moments, a single Merlin engine can't be throttled back far enough to allow the booster to hover. They have to adjust the engine to reach zero velocity at zero height, which requires amazingly accurate knowledge of the booster's position in the real world.
If they're going to light all three engines, why not just leave them on all the way to the ground?  The Starship prototypes weigh far less than operational Starships will. They don't want the thrust to weight ratio to get too high; they want to slow it down, not let it climb again.   

While a three-engine burn all the way to touchdown appears to be extremely risky or impossible for present-day Starships, Musk implied that there was nothing preventing SpaceX from reigniting all three engines during the initial flip and landing burn and using that time to determine the health of all three engines. If all three were healthy, Starship would shut down one for a soft landing. If one engine failed to restart or lost thrust shortly after ignition, the other two would already be active and able to take over.

Musk says that Starship SN10, already at the launch pad and likely days away from its first tests, will attempt to adopt that approach on an upcoming test flight expected as few as 2-3 weeks from now.
Admitting when someone, especially an outsider, has a better idea of how to do something you need to do is a sign of a very healthy mindset.  I suppose it's also a sign that the Chief Engineer isn't worried about being fired for not having thought of it! 



13 comments:

  1. Being able to admit guilt/stupidity is a trait to be revered.

    I'm wondering why they're not launching with extra weight, but that's probably a few Starships down the way, hopefully.

    For some strange reason I don't see Boeing or LockMart taking suggestions from common commoners.

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    1. Sometimes they don't take suggestions from employees, either!

      DAMHIK.....

      And yeah, GO SPACEX!

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    2. I'm sure Boeing and the others have a corporate policy that permits suggestions to move up the chain of command for thorough review and consideration. I hear the 1995 suggestion to design reusable rocket boosters has made it to the VP level...

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    3. Just blew coffee everywhere.....

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    4. Thanks Beans, Karl and drjim. I got a good laugh from that. Fortunately I had finished my cup of coffee.

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  2. I'm trapped in bureaucratic hell as far as my job goes. It's a first world problem: I'm getting paid incredible amounts of money, and I work from home, but the things I work on are trivial and they're wedged in between endless status meetings and nonsense. The sheer waste of time is galling.

    This is the sort of leadership I wish I had had. (sans 70 hour weeks which I can't do for health reasons) People trying to accomplish things who are serious about accomplishing: You know they are serious because of behavior like this. Even the subjects they focus their discussions on reveal this.

    So much of my life has been gratuitously wasted.

    -anonymous engineer

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  3. Robert Zimmerman of "Behind the Black", published some comments by ULA CEO Tony Bruno made during a webinar this past Thursday. Seems Bruno wants the Government to start regulating the space launch business a whole lot more. I am sure one of Bruno's top targets is SpaceX as they are beating the pants off of ULA in the terms of reduced launch costs and launcher development. These are mainly due to SpaceX's being a more agile company without all of the "old school" aerospace baggage.

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    1. I tend to think some of the "Old School Aerospace Baggage" is the amount of retirement benefits that get paid out.

      And any large, old organization tends to have higher amounts of overhead compared to a young company.

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    2. This is the kind of stuff I've been expecting. The only reason SLS exists is that congress critters get something out of it. An easy example is that the rocket is worked on near Huntsville, AL and Richard Shelby can sell jobs into his state when he runs for re-election. I'm sure it goes for enough senators and rep.s that it keeps the old boy company programs alive.

      The Ho Jo administration is nothing if not career cronyism.

      It's like the Government Employees unions: "we'll give you the contract if you contribute to our re-election campaigns" and the money flows like a conveyor belt.

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  4. Or, and this is just me mumbling as an outsider, what about a set of landing engines, which could also be used for ascent?

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  5. Of interest may be this writeup about the SpaceX launch systems.

    https://www.allaboutcircuits.com/news/spacex-systems-launching-landing-falcon-9-rockets/

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    1. There's a video that I watched in the last year that goes into some detail on how they've dropped the position error during development of the landing system until now it's almost routine to put the booster right in the middle of the SpaceX logo on the drone ship. 10 minutes but easy watching.

      https://youtu.be/Wn5HxXKQOjw

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