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.
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.
Beowolf rocket theory (Score:5, Funny)
Re:Beowolf rocket theory (Score:5, Funny)
Obligatory XKCD [xkcd.com]
Re: (Score:2)
* you may not achieve orbit, you probably won't put it where you want, and you'll send hours and hours and hours fiddling with things till you actually dock with your target. Then you realize you forgot to watch your fuel gauge and don't have enough fuel to get home. Then decide to launch a rescue mission, repeat. Soon you'll have a nice impromptu space station going
Re:Beowolf rocket theory (Score:5, Funny)
Re: (Score:2)
Not if the fail catastrophically (Score:3)
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,
Re:Not if the fail catastrophically (Score:5, Informative)
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.
Re:Not if the fail catastrophically (Score:5, Insightful)
If he meant fail catastrophically, he would have said fail catastrophically, you fucking troll.
Pot Kettle Black.
If one fails catastrophically you basically have a very large bomb on your hands, of course you've had it at that point. If you're going to chime in here, at least try to advance the discussion with something that's worthwhile.
Let me explain it in simple steps.
The point is that it's worth considering that the chances of one of 31 smaller engines failing could be larger than the chances of one larger engine failing catastrophically. Note I am not saying that it is, the engineering considerations could make larger ones more prone to failure, I'm just saying that it's worth considering. There have been a number of catastrophic rocket engine failures in space history, so it is certainly a possibility. Also, this is different from the data-centre analogy as it is very unlikely that a server failing will destroy the whole data centre.
Probability of failure (Score:5, Insightful)
Re:Probability of failure (Score:4, Insightful)
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.
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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
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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
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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
Computer analogies don't work well here (Score:2)
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
Re:Computer analogies don't work well here (Score:5, Interesting)
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].
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None of the N-1 explosions killed anybody. Maybe you're thinking of the Nedelin disaster [wikipedia.org], where the pad explosion of an R-16 killed about a hundred people. The Soviet Union didn't have just one space program, it had three, and the N-1 and R-16 were from different programs.
Electric car analogy (Score:3)
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?
if it goes down, the whole system goes down (Score:2, Insightful)
"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.
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Tesla battery (Score:3)
what about the center core? (Score:2)
Re:what about the center core? (Score:5, Informative)
They said that failure was because of lack of fuel, so more engines wouldn't help that.
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Actually I think the problem was running out of TEA-TEB ignition fuel.
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Correct. Essentially, they had plenty of candles, but ran out of "matches" to light them.
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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?
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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.
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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
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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
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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.
Re:what about the center core? (Score:5, Informative)
IIRC it was a chemical that starts up the engines that ran out TEA-TEB (Triethylaluminum-Triethylborane) So they could not fire the others
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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)
If nothing else, this shows Elon knows nothing about mainframe computers.
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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.
Rhetorical points (Score:2)
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?
If I'm not mistaken (Score:2)
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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.
Mainframes! (Score:2)
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)
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.
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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)
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]
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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.
All this debate, and yet...... (Score:3)
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.
It's More Complicated. . . (Score:2)
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
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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.
Next Bigger Falcon Heavy (Score:2)
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!"
New engine/rocket lift capacity? (Score:3)
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).
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Answered my own question. The new launch vehicle will have a proposed payload to LEO of 165t with additional features above and beyond those of Falcon9 and Falcon Heavy. Details from wikipedia available here." [wikipedia.org].
Easiest way to explain why: (Score:2)
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.
Re:No shit Sherlock (Score:5, Informative)
Of course, it has to be actual redundancy. The Soviet N1 moon rocket had a problem that when its engines failed, they tended to take out adjacent engines. You have to be absolutely sure that failures aren't going to spread (pieces of shattering turbopumps, fires, backpressure, etc), or you're actually making the problem worse.
Of course, everyone working on rockets today knows the lessons of the N1 and it'd be incompetence not to exhaustively test for resilience against cascading failures.
Beyond redundancy, one neat thing about engine clusters is that you can create a virtual aerospike effect to some degree.
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>Beyond redundancy, one neat thing about engine clusters is that you can create a virtual aerospike effect to some degree.
Not exclusively with really a lot of engines. More smaller nozzles need less length to fully expand the exhaust. And such effect is becoming noticeable even with 2 nozzles.
Re:No shit Sherlock (Score:5, Informative)
Actually, they had an engine control computer called KORD. While simple, it was an electronic computer. It was fed 4 types of measurements from each of the engines, and based on a simple algorithm would decide if they were out of acceptable operating parameters, and if so issue a command to shut down the offending engine (in theory, before a catastrophic failure) and its opposing counterpart. It would then ramp up the good engines to compensate for the loss of the dead engines (they defaulted to operating at 75% throttle to allow for this, as well as to reduce stress on the poorly tested engines).
Computer controls based on sensors was a new thing for the team, and the difficulties in filtering out bad data came back to haunt them in the first flight; it misinterpreted pyro noise as a turbopump spinning out of control and shut down a pair of engines, then interpreted pogo and a different engine failure as all of the engines going bad - and shut down the engines for all of the stages, so they couldn't even test the upper stages.
KORD also turned out to have too long of a response time to prevent catastrophic failures in engines, which was one of the things the design team was counting on to overcome the known poor reliability of the engines. KORD's rushed schedule also left it with poor debugging and too few safety checks. On flight two, in addition to still being too overaggressive on engine shutdowns in general (in response to very real engine failures), it caused its widespread shutdowns while the rocket was still over the pad, rather than trying to keep it going long enough to clear the pad. The pad explosion was one of the largest manmade non-nuclear explosions in history and set the N1 project back a year and a half.
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"KORD's rushed schedule also left it with poor debugging and too few safety checks."
Just like modern Agile CI/CD projects.
Re:No shit Sherlock (Score:5, Interesting)
Re:No shit Sherlock (Score:5, Funny)
Re:No shit Sherlock (Score:5, Funny)
Yes. Redundancy is always good.
Yes. Redundancy is always good.
Re:No shit Sherlock (Score:4, Funny)
Yes. Redundancy is always good.
Yes. Redundancy is always good.
Yes. Redundancy is always good.
Re: (Score:2, Funny)
Yes. Redundancy is always good.
Yes. Redundancy is always good.
Yes. Redundancy is always good.
Yes. Redundancy is always good.
:^))
(Let's see how much levels Slashdot can take before crashing
Re: (Score:2)
Yes. Redundancy is always good.
Yes. Redundancy is always good.
Yes. Redundancy is always good.
Yes. Redundancy is always good.
(Let's see how much levels Slashdot can take before crashing :^))
Yes. Redundancy is always good.
(Let's see how much levels Slashdot can take before crashing :^))
Yes. Redundancy is always good. (Let's see how much levels Slashdot can take before crashing :^))
Yes. Redundancy is always good. (Let's see how many levels Slashdot can take before crashing :^))
Re: (Score:3)
Yes. Redundancy is always good.
Yes. Redundancy is always good.
Yes. Redundancy is always good.
Yes. Redundancy is always good.
(Let's see how much levels Slashdot can take before crashing :^))
Yes. Redundancy is always good.
(Let's see how much levels Slashdot can take before crashing :^))
Yes. Redundancy is always good. (Let's see how much levels Slashdot can take before crashing :^))
Yes. Redundancy is always good. (Let's see how many levels Slashdot can take before crashing :^))
Yes. Redundancy is always good. (Let's see how many levels Slashdot can take before crashing :^))
Re: (Score:2, Redundant)
Indeed. Redundancy is always good.
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Indeed. Redundancy is always good.
Usually, but not always. With redundancy comes increased complexity, weight, and cost. Also, if you're mitigating low reliability with redundancy, you're adding additional failure points. This isn't a great concern with computers that operate in a controlled, benign environment. It's a much larger concern in a violent environment where the operating equipment has to make up for the thrust or operate longer to compensate for the failed units.
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Whoosh!
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So if redundancy is good, why doesn't the upper stage have more than one engine?
Probably because the reliability analysis says it isn't needed.
Getting back to the first stage: redundancy is an interesting problem for this application, compared to, say, an aircraft autopilot.
When an autopilot fails, the redundant backup takes over fairly seamlessly, and it doesn't have to work any harder than the failed unit it's replacing. Its failure rate is no higher that it was before the first unit failed. When an engine fails, the redundant units have to work harder to take up the load of the fail
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Well, redundancy has to be balanced with other operational requirements. Redundancy has costs--both to pay for the redundant equipment, and to accommodate it (e.g. find a space for it, provide power to move it, etc.). Additionally, the value of redundancy is dependant on the cost of failure.
If the mantra is "redundancy is always good" then SpaceX should probably start launching rockets with 2,000 engines, and I should add 13 more power supplies to my desktop computer.
Re:No shit Sherlock (Score:5, Insightful)
To a certain extent, SpaceX's architectural approach of many engines has arguably reduced costs. By making more copies of single engine design, the cost per engine has dropped significantly.. The manufacturing reliability is better, too. (What would the failure rate of a Model T have been if Ford was only building one per week? Building lots of something continuously brings you up the learning curve faster, reduces mistakes, and forces you to invest in tooling and fixturing that ensures each step is successful and repeatable.)
In this case, I think it is likely that the cost/kg and the reliability of a 9-engine rocket is better than a rocket that had a single engine of comparable power.
As you say - there are limits to this approach. (I'd call it modularity, rather than redundancy.) The efficiency of rocket engines doesn't scale down well and, as you point out, requires a build up of all the ancillary equipment.
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If the mantra is "redundancy is always good" then SpaceX should probably start launching rockets with 2,000 engines, and I should add 13 more power supplies to my desktop computer.
Yeah, that isn't how it works, of course. This is how it works:
How many engines do I need to be working at the end of the mission? How many engines with a failure rate of (n failures)/(unit of time) do I need to ensure with a probability of (minimum acceptable probability of success) that I have enough engines working at the end of the mission?
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If the mantra is "redundancy is always good" then SpaceX should probably start launching rockets with 2,000 engines, and I should add 13 more power supplies to my desktop computer.
Yeah, that isn't how it works, of course. This is how it works:
How many engines do I need to be working at the end of the mission? How many engines with a failure rate of (n failures)/(unit of time) do I need to ensure with a probability of (minimum acceptable probability of success) that I have enough engines working at the end of the mission?
And beyond that:
Is the cost of providing those additional engines lower than (cost of losing the rocket)*(probability of failure)?
All of which is a far cry from "redundancy is always good".
Re:No shit Sherlock (Score:5, Insightful)
Yes, but at the same time it's more complex. I think today we have the tech for such a thing, but if we look back at the Soviet who tried (first? don't quote me) this approach with the N1 moon LV - well it failed miserably. There are a lot more risks and much more complexity, which is to the credit of SpaceX!
So saying "Duh, it's obvious" is a bit shortsighted. Redundancy and scaling is hard, especially when you're talking about a rocket. Pumps, fuel, precooling, spin up, and all that are non-trivial. Even if you take like 9 engines in a square, if one fails, yeah you have 8 remaining, but the thrust is reduced by 1/9, the LV is now unbalanced especially if more than 1 goes down on the same side (e.g. engines themselves must compensate with gimbal or vernier needs to do this, I'd like to see the software to control this, surely a beast!!), to achieve the same final delta-v you need to burn the engines longer, so they need to be rated for much longer burn time if you still want to make it to orbit, which means more chances of failures, engines overheating, and then if an engine explodes it could take out others with shrapnel, and I go on...
And that does not take into account the R&D necessary to relight the engines twice AFTER the initial burn for the re-entry and landing! So yeah, quite an R&D achievement for SpaceX to have such reliability!
Mad props.
Re:No shit Sherlock (Score:5, Informative)
The N1 failed through a combination of lack of money, lack of political will and losing the space race.
The plan was to skip building a test stand for the first stage (which would be large and expensive, to cope with 5000 tons of thrust, and take lots of time to buil). Instead, they'd do test flights, fully expecting a number of initial failures. 14 test flights were planned.
After the Apollo 11 landing, the urgency was lost and funding slowed. The last straw was appointing Valentin Glushko as head of the Soviet space program. He was a known opponent of the N-1, favoring his own design.
Re:No shit Sherlock (Score:5, Interesting)
A lot of the Soviet plans were based around the expectation of failures. All of their (numerous) Venus missions, for example, were launched in pairs. The idea was that the incremental cost was low but the initial costs high, so you might as well send two. And if both work, you collect two separate datasets, from different locations. Usually when one failed they pretended it was an experimental or military launch - for example, Venera 4's twin was Kosmos-167, while Venera 7's was Kosmos-359.
It's hard to call one approach the right approach and one the wrong approach. The Soviet approach certainly paid dividends on Venus, but their Mars programme was a miserable failure compared to the US.
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It's hard to call one approach the right approach and one the wrong approach.
If there are humans sitting on top of your rocket, one is clearly the right approach. (depending on the value you place on your astronauts' lives, I guess)
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Please jump back to January 26, 1967, and warn NASA. Hindsight is always 20/20.
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Absolutely right, I agree with you and also stand by what I said. I will add all other sorts of concerns for the N1 like them being unable to test the assemble stages beforehand due to size/transportation issues, and such.
I'm not saying it was a bad approach, it was pretty smart of them to attempt it (and I've heard the control program for the engine was probably the most awesome piece of software/hardware at that time) - but it was too soon/was a gamble. It took longer, caused delays, then that allowed the
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No not always.
Even in terms of computer science, there are algorithms that cannot be done in parallel. These algorithms need a faster CPU core, not more cores or more computers to get the job done faster.
In terms of having multiple rocket engines. The question to bring up, Will the extra size and weight of each engine be less efficient then having just one engine.
Granted smaller engines allows for easier scaling to larger rockets, and if they are setup so if one goes out they don't cascade to a massive fail
Re: (Score:2)
Additionally, does the control system for the rocket respond fast enough if an random engine fails. Now what about 2? Or 3? Or 4, grouped together? A sudden change in thrust vector will arise if an engine fails, and the overall control system has to be able to account for that kind of failure throughout the entire flight - or the flight is a failure.
Often, adding parallel stages will result in lower overall system stability. Yes, you have redundant parts, but if it throws your system for a loop for lon
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Redundancy is always good
No! There are two extreme cases. One, where the probability of total system failure is the sum of the probabilities of failure of components (bad), the other where it is the product (good)
1% chance of failure + 1% chance of failure = 2% chance of total failure
1% chance of failure * 1% chance of failure = %0.01 chance of total failure
There here are also intermediate cases, for example when failure of redundant components have some probability greater than 0% and less than 100% of bleeding over into the o
Calculating probability of failure (Score:4, Insightful)
No! There are two extreme cases. One, where the probability of total system failure is the sum of the probabilities of failure of components (bad), the other where it is the product (good) 1% chance of failure + 1% chance of failure = 2% chance of total failure
You can't sum chance of failure like that. That's not how the math of it works. (think about it - if you take that to it's logical conclusion with 200 failure modes each at 1% chance of failure you can end up with a >100% chance of failure which isn't possible) First you have to determine whether the failure modes are genuinely independent or not. But even if you have two completely independent failure modes with a 1% chance of Failure A and a 1% chance of Failure B, the total chance of Failure is NOT F(A)+F(B) = 2% because there is a probability of both failure occurring simultaneously so the real probability will be less than 2%.
1% chance of failure * 1% chance of failure = %0.01 chance of total failure
It doesn't work like that either unless those failure modes are such that both have to occur for a failure to occur.
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Well, calculated your way: 1% + 1% = 1.99% rather than 2%.. Typically you want the overall probability of failure to be a small number. If your probabilities of failure are so high that it makes a significant difference, then you don't have a failsafe system anyway.
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There's s difference between redundancy and having lots of small engines aggregated together. If the reliability of an engine is the same between large and small engine, then clearly it's far better to have one large engine than two engines working together. To my mind redundancy means I have two units, but i only use one until it fails, then I can use the other. Redundancy is built into the SpaceX design to be sure (it can reach orbit with only 7 or 8 engines firing), but it's not quite the same thing a
Re:Big Falcon Rocket (Score:5, Funny)
I thought the F in BFR stood for something else than Falcon...?
Yes but Falcon doesn't get censored in interviews
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Fictional?
Fake?
Faux?
Fishy?
Farcical?
Fanciful?
Fantasy?
Far-fetched?
Folly?
Fruitless?
Frustrating?
Fireball?
Firebomb?
Firework?
Flack?
Fragmenting?
Flameout?
Failing?
Falling?
Free-falling?
Flipping?
Flopping?
Floppy?
Flawed?
Fiasco?
Finicky?
Faulty?
-Fatalities?
Funerary?
Fanart?
Fanservice?
Fuckup?
(Just kidding of course. I think SpaceX is great and they'll get there eventually, although I seriously doubt their BFR timeline, and expect plenty of fireworks en route to their desired reliability level)
Re:Big Falcon Rocket (Score:5, Funny)
Us Gen Xers just say it: big fucking rocket...
Wow Grandpa, that's so badass. You're using that .. word .. like it's just a word. I'm terribly impressed.
when kids are in ear shot.
Friendly reminder, millenials aren't kids anymore.
Re:Big Falcon Rocket (Score:5, Informative)
Friendly reminder, millenials aren't kids anymore.
Yes they are. Get off my lawn!
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She was likely just uselessly seeking attention. After all, she's gen X.
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Gen *Y* is the Millennial generation. Gen Z is the "little shits" turning 18 now.
Boomer 1946-1964
Gen X 1965-1981
Gen Y/Millennial 1982-1998
Gen Z/Post-Millennial 1999-2016(?)
Give or take a couple of years on either end of the ranges depending on what specific cultural element you are judging the generations by. (The defining characteristics of a post-gen-z generation aren't yet known, so we don't know when to draw a line on gen z.)
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That is why you have sensors on the engines to make sure that WE shut them down before a RUD.
As such, the chances of being forced to shut down 1 is higher, but, with the massive number of sensors, it is very small chance of not making it to orbit.
Re:I call BS (Score:5, Insightful)
Damn straight! And while we're at it we should get back to single piston car and truck engines. Those Europeans are screwed with their finicky 12 cylinder sports cars or even (gasp!) 16 cylinders!!! It's madness I say!
Simplify it all to a more efficient single cylinder engine. And don't even get me started on all those crappy WWII airplane engine designs...
Has my point been made? No? Sometimes the cost and/or efficiency of the engine is not the biggest consideration in a project.
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"pay a substantial weigh penalty for the buttressing and gimballing"
Which is why they're doing a lot to optimize the design, including the octoweb (I imagine they'll do something similar on BFR) & not gimballing all of the engines.
"Soviets went with smaller but more numerous engines"
No, the Soviets went with their design because they lacked the manufacturing infrastructure that the US had and building that infrastructure would have taken time. As this was a race time is something the Soviets didn't rea
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Another reason for 9 small engines is that they need to land the booster, and the required thrust for a landing is very small due to almost empty fuel tanks. Throttling down a huge engine to such a small thrust makes for a very difficult (and probably suboptimal) design. Also, you'd need an engine pattern so that you can put one engine in the center, so 3 in a circle isn't an option either.
Also, with 9 engines on the booster, a single engine is the right size for the 2nd stage, which reduces cost even more.
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The drawbacks of the old school of rocket design far outweigh any advantages going into the future.
Reliability is more complicated than part counts (Score:4, Insightful)
1) more engines is more parts, which is generally a recipe for more failures
Only if you hold everything else in the system constant which is clearly not the case in most real world systems. To use a car example, modern cars have a LOT more parts in them than cars from 40 years ago but they also are demonstrably more reliable. Same with jet engines. Modern ones are more complex and with (usually) more parts but they also are more reliable. The relationship between number of parts and reliability is not a simple linear one. Many of those added parts actually contribute to the reliability of the overall system.
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No, ULA uses Russian RD-180 and RS-68 on their rockets. SpaceX's Merlin is a newly designed engine that is similar to some that NASA have used in the past.
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I love the fact that they include "pricing" on the SpaceX website, like your just buying a refrigerator...
Or a flamethrower.
you know, for the children (Score:2)
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It sells very well.
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I love the fact that they include "pricing" on the SpaceX website, like your just buying a refrigerator...
I clicked the shop button, but I couldn't add one to my shopping basket.
Exactly! (Score:2)
It took the US and the USSR years to solve instability problems with engines larger than the V2 rockets.
Also, why would anyone trust the opinion of a software developer with regard to hardware development?