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Space Software Hardware Technology

Elon Musk Explains Why SpaceX Prefers Clusters of Small Engines (arstechnica.com) 240

An anonymous reader quotes a report from Ars Technica: The company's development of the Falcon 9 rocket, with nine engines, had given Musk confidence that SpaceX could scale up to 27 engines in flight, and he believed this was a better overall solution for the thrust needed to escape Earth's gravity. To explain why, the former computer scientist used a computer metaphor. "It's sort of like the way modern computer systems are set up," Musk said. "With Google or Amazon they have large numbers of small computers, such that if one of the computers goes down it doesn't really affect your use of Google or Amazon. That's different from the old model of the mainframe approach, when you have one big mainframe and if it goes down, the whole system goes down."

For computers, Musk said, using large numbers of small computers ends up being a more efficient, smarter, and faster approach than using a few larger, more powerful computers. So it was with rocket engines. "It's better to use a large number of small engines," Musk said. With the Falcon Heavy rocket, he added, up to half a dozen engines could fail and the rocket would still make it to orbit. The flight of the Falcon Heavy likely bodes well for SpaceX's next rocket, the much larger Big Falcon Rocket (or BFR), now being designed at the company's Hawthorne, California-based headquarters. This booster will use 31 engines, four more than the Falcon Heavy. But it will also use larger, more powerful engines. The proposed Raptor engine has 380,000 pounds of thrust at sea level, compared to 190,000 pounds of thrust for the Merlin 1-D engine.

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Elon Musk Explains Why SpaceX Prefers Clusters of Small Engines

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  • by OffTheLip ( 636691 ) on Friday February 09, 2018 @09:04AM (#56094501)
    Seems like a good idea to me but I'm no rocket scientist.
  • by Chrisq ( 894406 ) on Friday February 09, 2018 @09:24AM (#56094585)

    up to half a dozen engines could fail and the rocket would still make it to orbit

    Not if the fail catastrophically. If one blows up you've had it. This is probably more likely than a computer failing and burning down your data centre, so a factor worth considering,

    • by hackertourist ( 2202674 ) on Friday February 09, 2018 @09:46AM (#56094687)

      The rocket is built to contain engine explosions. We don't know if that'll be effective for all engine failures, but they've already had at least one engine failure on a F9 flight without consequences for the mission.

  • by trybywrench ( 584843 ) on Friday February 09, 2018 @09:30AM (#56094605)
    Obviously SpaceX has calculated this but Id like to see a graph of the probability of flight failure of a rocket with 5 big engines and a rocket of 31 small engines. The more engines the higher the chance one will not work but also the higher the redundancy. The fewer engines the less chance one will not work but also the greater the chance one going out dooms the flight.
    • by Kjella ( 173770 ) on Friday February 09, 2018 @11:24AM (#56095121) Homepage

      I'm sure you'd like it but there's two critical numbers you lack, one is what degree of off-balance power configuration is possible while still having a stable rocket. The other is the probability of a cascading engine failure, engines going out is not the biggest problem it's taking the rocket out with them. So you have to know the exact nature of the engines, just the number of them doesn't say much.

    • by green1 ( 322787 )

      depends how over-engineered it was.

      If the rocket is designed to reach orbit with 20 engines, but has 31, then 11 can fail and it still works, but if it was designed to reach orbit with 31 engines, and has 31 engines, then even a single failure means no go. Meanwhile that rocket with 5 may only need 3 to reach orbit, in which case 2 could die and it would be fine, or it could need all 5, and then if even one goes it doesn't get there.

      Now many companies and organizations would put in whatever the minimum numb

      • The number of engines a rocket has means absolutely nothing if you don't also know the minimum number required to achieve the goal.

        There's a lot of flexibility in the system.

        A rocket launches a wide variety of payloads to a wide variety of orbits. That means that for 95% of the launches, the rocket will have spare capacity. And even then, most satellites have their own thrusters and fuel so they can adjust their orbit. A failed engine may mean that the satellite will be put in a lower orbit, and needs more of its own fuel to become operational. Another option is for SpaceX to cancel the landing (if that was planned) and use all remain

    • A simple, idealized model isn't hard. Suppose a 99% chance of success (no explode) per engine.

      The raw probability of a vehicle NOT having an engine failure is just P^n in this case, which means that you have a 99% chance of a problem-free flight with one engine, but a 95.1% chance with 5 engines, and only a 91% chance with 9 engines... all the way down to 76% chance of no failure at 27 engines. Thankfully, real rocket engines are much better than 99% reliable.

      So, the chance of at least one failure increases

  • Do you have to shut down computers opposite to the one that fails to maintain "computing balance"? Can a computer that fails blow up and take a few adjacent computers with him? Can a failing computer cause a cascading effect by sending bogus signals through the network that makes other computers fail? Does a failing computer fundamentally alter your mission profile to the point that you have to change the computations for ALL other computers?

    Maybe we should stay with car analogies. They aren't any better, b

    • by hackertourist ( 2202674 ) on Friday February 09, 2018 @09:51AM (#56094695)

      Do you have to shut down computers opposite to the one that fails to maintain "computing balance"?

      The Soviets did that on the N-1 because it allowed them to install the engines without gimbaling hardware, simplifying the design. The F9 does have gimbals, so it doesn't need to shut down the opposing engine.

      Does a failing computer fundamentally alter your mission profile to the point that you have to change the computations for ALL other computers?

      So what? That's what computers are really good at [cbsnews.com].

    • This preference for engine clusters is like using a stuff-ton of laptop cells instead of a much smaller number of automotive cells in the Tesla battery pack?

  • by Anonymous Coward

    "That's different from the old model of the mainframe approach, when you have one big mainframe and if it goes down, the whole system goes down." Except that mainframe doesn't and AWS does.

    • mainframes went down a lot more than ppl realized. And with AWS, it continues working just fine, even when you lose a VM.
  • by burtosis ( 1124179 ) on Friday February 09, 2018 @09:37AM (#56094653)
    It's basically for the same reasons as tesla batteries are made from thousands of the small lithium cells that are already mature.
  • It would be interesting to know why this engine redundancy wasn't leveraged to save the center core of the Falcon Heavy when it attempted to land on the drone ship. They claim two of three engines failed to fire. If so, why wasn't the system programmed to automatically try to fire two alternate engines in that failure mode? Unless the failures where of a more catastrophic nature of course...
    • by bws111 ( 1216812 ) on Friday February 09, 2018 @10:26AM (#56094821)

      They said that failure was because of lack of fuel, so more engines wouldn't help that.

      • Actually I think the problem was running out of TEA-TEB ignition fuel.

        • by sconeu ( 64226 )

          Correct. Essentially, they had plenty of candles, but ran out of "matches" to light them.

          • by darronb ( 217897 )

            I wonder if that was just a screw up... The center engine had enough to relight. They usually land with just the center engine. They had only just tested a 3 engine burn not so many days before with the previous F9 launch. Maybe someone forgot the ignition fuel for the additional engines?

            • No, it was the fact that it doesn't necessarily relight when you try it, and each try uses up a charge. Eventually you run out.

              • by darronb ( 217897 )

                Huh. A bit troubling... What's the interval for these "tries"? Does this happen often?

                What doesn't make sense is that TWO of the engines had this problem. Unless the relight fuel is somehow a shared resource... it was the fact that it was two engines in an unusual use case that contributed heavily to my hypothetical that it was a planning problem.

                I guess the real question here is what's the fix? More fuel/attempt? More "charges" as you call them? Tweaking with the flow dynamics? Entirely new reli

                • TEA-TEB is pretty nasty stuff, it spontaneously combusts in the presence of oxygen. So, you just dump some in to the combustion chamber and it should get hot enough to light the LOX and RP-1. It has to be stored under nitrogen to prevent it from going boom in the air.

                  I don't know if the TEA-TEB is shared between the Merlin motors.

                  I am not sure if the mechanism for restarting a Merlin 1D, whether or not it injects a set amount, or it keeps dumping it in until the engine lights up.

                  The Falcon stage is falling

    • I should mode you up, but am going to reply to others, so no sense.
      I had not thought about it until you mention it, but that is 1 hell of a good question/idea. I suspect that they never thought to do that. Yet, it makes great sense, other than being out of balance. Still being out of balance, simply means more work for fins and RTS.
    • by Rxke ( 644923 ) on Friday February 09, 2018 @10:39AM (#56094871) Homepage

      IIRC it was a chemical that starts up the engines that ran out TEA-TEB (Triethylaluminum-Triethylborane) So they could not fire the others

    • Not all engines are equipped with the chemicals needed for an air start, IIRC. Or there's two tanks that feed all the engines. Either way, they ran out of TEB, there was no engine failure.

  • Mainframes? (Score:3, Insightful)

    by bws111 ( 1216812 ) on Friday February 09, 2018 @10:09AM (#56094739)

    If nothing else, this shows Elon knows nothing about mainframe computers.

    • Or, you know, one of the founders of what would become Paypal might know a thing or two about the trade offs of distributed computing vs. more monolithic systems.
      But a reporter who writes an 'article' based on a single oversimplified analogy quote might not hit the nuances you're looking for.

    • If nothing else, this shows Elon knows nothing about mainframe computers.

      I think that comment says more about you than it does about Elon. Do you seriously think Elon doesn't get that it's an imperfect analogy used to make a rhetorical point?

  • Even the Russians used multiple smaller engines in their space program. And if I'm not mistaken the Saturn V used five of them.
    • This is the point of the article. Those five engines were huge. The five F1 engines of first stage of Saturn V were more powerful than the 27 Merlin 1D of Falcon Heavy. The Space Shuttle's RS-25 is also more than 2x bigger than the Merlin.

      The Smithsonian has part of a mockup of a F1 filling one of their rooms. Highly suggest visiting it or some other place with an F1 on display.

  • That's different from the old model of the mainframe approach, when you have one big mainframe and if it goes down, the whole system goes down

    In the case of the mainframe the redundancy is build in. You don't have to use 100's of mainframes because they almost never go down. I've been working on mainframes for the last 20 years, and I can count on one hand the number of times the mainframe was down in a production environment in that period.

  • More engines (Score:5, Insightful)

    by EnsilZah ( 575600 ) <EnsilZah AT Gmail DOT com> on Friday February 09, 2018 @12:17PM (#56095403)

    I don't think this article is particularly newsworthy for anyone who's familiar with the subject or even stopped for a few seconds to think about it.

    But here are a few points why multiple smaller engines are better in this case.

    * Mass production makes things cheaper and sometimes better.
    If you look at the cost of an engine, the raw materials are a pretty small percentage of the total cost, It's more about the manpower and tolerances required.
    If you have a guy performing a certain task maybe once every two months, he'll be slower and less proficient at doing it than say every few days.
    And overall, economies of scale make more engines cheaper to manufacture.

    * Redundancy, as mentioned already in the thread.
    SpaceX has already lost one of the engines on one of the earlier flights and continued to complete the mission.
    They have walls between them that prevent the explosion of one from damaging the next.
    And it's only going to improve for Falcon Heavy and BFR, they'd be able to lost multiple engines and compensate for the imbalance.

    * Telemetry collection.
    You get to build up a history of past performance a lot faster with ten engines than you do with one or two.
    After 54 flights, SpaceX has gathered operational telemetry on 486 first stage engine firings and 54 second stage ones (Not including all the test firings).

    * Throttling, maneuverability, unique thrust characteristics.
    In the early stages of landing R&D SpaceX had a rocket called the Grasshopper, which was modified Falcon 9, that was able to fly up, hover, and then land.
    Most larger engines would not be able to throttle low enough to maintain a hover.
    This one is just a guess, but I imagine it's much easier and faster to gimbal a smaller engine, and you don't have to put the smaller, cheaper gimbaling hardware on all the engines.
    It seems that the exhaust from the multiple engines behaves somewhat similarly to an Aerospike engine, which gives the configuration some extra efficiency.

    That's all I could think of, but I'm sure there are more reasons.

    • That's all I could think of, but I'm sure there are more reasons.

      Tooling. Smaller engine parts have a much better chance of fitting in standard lathes and mills.

  • OTRAG (Score:5, Interesting)

    by Jodka ( 520060 ) on Friday February 09, 2018 @12:19PM (#56095419)

    The most extreme example of this sort of thing ever attempted was OTRAG [wikipedia.org].

    John Carmack had some interesting things to say about that at his now-defunct Armadillo Aerospace website, some of which have been preserved at Wikipedia here. [wikipedia.org]

    "I have been corresponding with Lutz [Kayser] for a few months now, and I have learned quite a few things. I seriously considered an OTRAG style massive-cluster-of-cheap-modules orbital design back when we had 98% peroxide (assumed to be a biprop with kerosene), and I have always considered it one of the viable routes to significant reduction in orbital launch costs. After really going over the trades and details with Lutz, I am quite convinced that this is the lowest development cost route to significant orbital capability. Eventually, reusable stages will take over, but I actually think that we can make it all the way to orbit on our current budget by following this path. The individual modules are less complicated than our current vehicles, and I am becoming more and more fond of high production methods over hand crafter prototypes."

    • by Jodka ( 520060 )

      Here [archive.org] is the full archived version of Carmack's blog post about OTRAG, including photos of an injector assembly which he was gifted.

  • by mschuyler ( 197441 ) on Friday February 09, 2018 @02:16PM (#56096161) Homepage Journal

    All this debate from Slashdot rocket scientists over whether Musk is properly designing his rockets, whether he "understands" the finer points of his (imperfect!) metaphor, whether he really understands computers at all, and yet...

    he launches rockets (and lands them) again and again and again and again.

  • Bob Truax argued that the most cost-effective way to build a big rocket was using one huge engine, which is what was planned for the Sea Dragon booster. He said cost is driven not by size but by parts count. More engines equals more parts that have to be produced, inventoried, tested, assembled, etc., and that leads to higher cost.

    However. . . The way SpaceX are returning their boosters to earth wouldn't work with one huge engine. There would be no feasible way to throttle it down enough for the return

    • He said cost is driven not by size but by parts count

      Once you start landing and reusing rockets, the cost starts to move in different areas. Not size or parts count, but effort required to prepare for relaunch.

  • The Chief Rocket Scientist of SpaceX took Elan's message about redundancy to heart.

    He announced the next Bigger Falcon Heavy lunch vehicle will be made entirely of the rocket engines sold to high school science projects. 3.2 million of these rocket motors glued together will form the launch vehicle.

    He said, "We know Elon got his start by building an electric car by duct taping 8000 laptop batteries together. Same thing here, back to the basics. man!"

  • by Last_Available_Usern ( 756093 ) on Friday February 09, 2018 @04:46PM (#56097077)
    The flight of the Falcon Heavy likely bodes well for SpaceX's next rocket, the much larger Big Falcon Rocket (or BFR), now being designed at the company's Hawthorne, California-based headquarters. This booster will use 31 engines, four more than the Falcon Heavy. But it will also use larger, more powerful engines. The proposed Raptor engine has 380,000 pounds of thrust at sea level, compared to 190,000 pounds of thrust for the Merlin 1-D engine.

    Does anyone know what this means relative to the lift capacity of this new rocket they're working on? The Falcon Heavy was already a huge leap over the competition and this doubles the thrust with a few more engines (understanding that some of that thrust is going to come at the cost of carrying additional fuel too).
  • Weight of Rocketdyne F-1 engine on Saturn V (moon rocket): 18,000 pounds, thrust:1,500,000 pounds

    Weight of Merlin 1-D engine on Falcon 9: 1,000 pounds, thrust: 190,000 pounds

    The specific impulse of a Merlin engine is 282 seconds, the specific impulse on an F-1 engine is 263 seconds.

    TL;DR:
    18 Merlin engines weigh the same as one F-1 moon rocket engine but only 8 of them are needed to provide the same thrust.

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