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Musk explains why SpaceX prefers clusters of small engines

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Re: Musk explains why SpaceX prefers clusters of small engines

#41

Post by ygra »

grondilu wrote: post #19

Would it then make sense to plan for even more smaller engines? Like while BFR is meant to have 31 Raptor engines, could it have 60 Merlin engines instead or something?

One goal of BFR is Mars, from which you can only get back if you can produce fuel on the ground. Which kinda rules out RP-1 as propellant. Hence the need for a methane engine.

Unrelated to the number of engines here, obviously; just another point to consider. And if they end up designing a new engine they could just as well apply what they have learned in the meantime. Merlin is a very conservative design, favouring simplicity and cost over thrust. If you plan for re-using your rocket a thousand times, cost of an engine isn't that much of an issue anymore and you can prioritise other aspects.

Re: Musk explains why SpaceX prefers clusters of small engines

#42

Post by physcab »

bunderbunder wrote: post #36
vannevar wrote: post #22

Earlier quoted context omitted.

There are economies of scale and reliability when you're building large numbers of something. Yes. One of the biggest predictors of reliability in aerospace systems in general is time in operation---the longer you've actually run something, the closer it approaches the upper limit of reliability for that component. Running 27 small copies gives you 27x the time in operation vs a single big-ass engine. The downside is…

How does one quantify the risk of cascading failure? My initial instinct, informed by computing, is to say that it's easier to avoid cascading failures in the system that is composed of more, smaller parts. All other things being equal, in a rocket with 5 engines, if one of them fails, then each of the remaining ones needs to pick up 1/4 of the slack to compensate. In a rocket with 9 engines, not only would each of t…

The difference between aerospace and computing is that aerospace is heavily materials dependent. Getting a bad pixel in a monitor is rare but when it happens it’s not catastrophic. When you’re producing a ton of parts you have to also maintain quality. Not doing so will introduce impurities which will alter the material properties. An alloy with different material properties could potentially burn, fracture, expand etc more differently than represented in their acceptable failure models.

Re: Musk explains why SpaceX prefers clusters of small engines

#43

Post by gizmo385 »

marktangotango wrote: post #34
jlebrech wrote: post #21

and soon it'll be clusters of boosters, there's space to fit another 4 in a hexagonal pattern. you could lift a mini hexagonal mars base fully assembled that way, land it and land the next one quite close.

In the post Falcon Heavy press conference, he specifically mentioned scaling up to a Super Heavy with two additional side boosters, four in all. Sounded like they had designed for that, from the way he just threw it out there. Seems odd to me, since FH is an interim vehicle until BFR comes on line in 5-10 years.

> Seems odd to me, since FH is an interim vehicle until BFR comes on line in 5-10 years.

I thought they were targeting launches much sooner than 5-10 years out for the BFR?

Re: Musk explains why SpaceX prefers clusters of small engines

#44

Post by sehugg »

vannevar wrote: post #22
mhandley wrote: post #12

There are two other reasons which SpaceX has mentioned before: - When you're landing a rocket, you need to be able to throttle down quite low. Even a single Merlin 1-D engine, throttled down, is too much thrust to be able to hover with a nearly empty booster. It's really hard to get stable combustion at very low throttle settings. Having only one engine out of nine running for landing makes this much more manageable.…

There are economies of scale and reliability when you're building large numbers of something. Yes. One of the biggest predictors of reliability in aerospace systems in general is time in operation---the longer you've actually run something, the closer it approaches the upper limit of reliability for that component. Running 27 small copies gives you 27x the time in operation vs a single big-ass engine. The downside is…

Yes, one of the poorly-understood problems with early NASA missions was "pogo", a propellant vibration problem which affects the entire vehicle: https://www.popsci.com/how-little-vibrations-break-big-rocke...

Re: Musk explains why SpaceX prefers clusters of small engines

#45

Post by bryananderson »

SpuriousSignals wrote: post #4

It all depends on how an engine fails. A failure to provide thrust is survivable with more engines. But a vulnerability that causes a fuel line to go boom, taking out the whole rocket, is made more likely by having more engines.

Exactly - engine-out failures are far more common than boom failures, so it makes sense to have lots of small engines.

The Soviet N-1 failures had more to do with rushed, shoddy engineering in a desperate attempt to catch up to Apollo.

Re: Musk explains why SpaceX prefers clusters of small engines

#46

Post by philwelch »

marktangotango wrote: post #34
jlebrech wrote: post #21

and soon it'll be clusters of boosters, there's space to fit another 4 in a hexagonal pattern. you could lift a mini hexagonal mars base fully assembled that way, land it and land the next one quite close.

In the post Falcon Heavy press conference, he specifically mentioned scaling up to a Super Heavy with two additional side boosters, four in all. Sounded like they had designed for that, from the way he just threw it out there. Seems odd to me, since FH is an interim vehicle until BFR comes on line in 5-10 years.

It's important not to get too intoxicated with your own success. If BFR is as successful as their Falcon series, it'll be ready in 5-10 years. If there are unanticipated challenges with BFR (after all, this is literally rocket science), having a lower-risk approach mitigates that risk.

Re: Musk explains why SpaceX prefers clusters of small engines

#47

Post by Robotbeat »

marktangotango wrote: post #33
mhandley wrote: post #12

There are two other reasons which SpaceX has mentioned before: - When you're landing a rocket, you need to be able to throttle down quite low. Even a single Merlin 1-D engine, throttled down, is too much thrust to be able to hover with a nearly empty booster. It's really hard to get stable combustion at very low throttle settings. Having only one engine out of nine running for landing makes this much more manageable.…

The landing case makes me wonder about the wisdom of Blue Origins choice of going with a larger engine, the BE-4 with 550,000 lbf for the New Glen vehicle. Much more thrust than a Falcon 9 landing on a single M1D. Surely they've run the numbers and find it viable though.

Blue Origin also has worse mass fraction on the booster stage than SpaceX does, which means they don't need to throttle down as much. If you look at New Shepard as a preview to New Glenn, there are a whole bunch of aerosurfaces which add dry mass, essentially ballast. New Glenn will add big side fins, which will help increase lift, drag, and provide more ballast.

Also, the New Glenn booster will have 7 engines. Not so different from Falcon 9's 9 engines per booster.

Overall, New Glenn is a good design (and will eventually have a reusable 2nd stage). If SpaceX stopped with Falcon 9 and Falcon Heavy, New Glenn could easily give SpaceX a run for their money. Luckily, SpaceX won't stop with Falcon Heavy. BFR, once they finish getting Block 5 out the door in a couple months and crew Dragon flown sometime by around the end of the year, will be almost their sole engineering/development effort (Starlink being kind of a separate division).

Of course, Blue Origin also won't sit still with New Glenn. They'll make the upper stage reusable, then also add a hydrogen/oxygen kick stage for very high energy payloads, and then they'll be working on the New Armstrong monster.

SpaceX is super far ahead, but luckily Bezos is so rich that I expect both companies now to deliver on their equally grand visions.

Re: Musk explains why SpaceX prefers clusters of small engines

#48

Post by neltnerb »

bunderbunder wrote: post #36
vannevar wrote: post #22

Earlier quoted context omitted.

There are economies of scale and reliability when you're building large numbers of something. Yes. One of the biggest predictors of reliability in aerospace systems in general is time in operation---the longer you've actually run something, the closer it approaches the upper limit of reliability for that component. Running 27 small copies gives you 27x the time in operation vs a single big-ass engine. The downside is…

How does one quantify the risk of cascading failure? My initial instinct, informed by computing, is to say that it's easier to avoid cascading failures in the system that is composed of more, smaller parts. All other things being equal, in a rocket with 5 engines, if one of them fails, then each of the remaining ones needs to pick up 1/4 of the slack to compensate. In a rocket with 9 engines, not only would each of t…

To answer the first question, you use a fault tree analysis to predict potential failure starting points (like a broken component) and then describe how those failures will propagate through the system.

https://en.wikipedia.org/wiki/Fault_tree_analysis

For an example, say I'm building a system that needs to hold a block of aluminum at 550C, 99% of the time. Okay, so you add a thermocouple and a heater to it, easy.

What if the thermocouple fails?

Well, if the thermocouple fails open then the temperature will read infinity and the heater will shut down and probably produce a non-catastrophic failure.

If the thermocouple fails closed, the temperature will read room temp and the heater will blast full on until the aluminum melts at 660C, which is a catastrophic failure.

If the relay in the temperature controller fails, the furnace probably turns off but theoretically could fail on if the relay switch gets fused.

Okay, so I can see that there is an unlikely but possible chain of events that could cause a catastrophic failure. So I add a second thermocouple to act as a safety shutoff using a second redundant relay and controller if it reads a temperature above 600C.

Total probability then is estimated by either using real world performance metrics or best-guesses. I'd say the odds of a thermocouple failing in 10 years of operation at 550C is nearly 100%, so this failure will nearly certainly occur.

Or consider an LED array with 10 of them in parallel. If one blows open, the remaining 9 each get 10% more current so are more likely to fail. So your first branch of the tree might be that the odds are 10% that an LED will fail at design current within five years. That may well not qualify as a failure, especially since the other 9 LEDs are ~10% brighter due to the higher-than-spec current. But now your probability for the next failure is 20% within five years. So you do need to define different outcomes, usually by severity of impact and probability of outcome in event of a predicted possible failure point.

Re: Musk explains why SpaceX prefers clusters of small engines

#49

Post by Robotbeat »

We can mark this inaugural Falcon Heavy launch as the point when people stopped laughing at BFR.

The latest version of BFR is only about twice as much thrust as the final FH variant (which will launch in a few months with slight thrust upgrades) and around the same number of engines. Recovery, even with such a complicated bunch of stages, seems to work pretty well, validating SpaceX's knowledge of reentry and reuse.

Launching crew within about a year from now is when people will stop laughing about SpaceX sending people to Mars. https://www.youtube.com/watch?v=0qo78R_yYFA

Re: Musk explains why SpaceX prefers clusters of small engines

#50

Post by theothermkn »

aurizon wrote: post #38

A large portion of the efficiency of small engines comes from the reduction of "hoop stress". This is the linear tension in the wall of a pressure vessel(rocket engine) which varies as the square of the diameter. Twice the diameter = 4 times the pressure (hoop stress) the walls must take. A rocket is a special case of a balloon - with an expansion nozzle attached to couple the impedance of the combustion chamber to t…

> This is the linear tension in the wall of a pressure vessel(rocket engine) which varies as the square of the diameter. Twice the diameter = 4 times the pressure (hoop stress) the walls must take.

This is incorrect. Cutting the cylinder in half lengthwise and taking a unit length, we see that the cross section of the walls of the chamber (unit length x 2 x wall thickness) resists the pressure force from the contained fluid (2 x radius x unit length x pressure). Since the pressure and unit length are constant under scaling, this resulting force grows linearly with the radius. Thus, wall thickness has to grow proportionally to the radius, and there are no mass penalties from the chamber wall from either a smaller or larger engine. A similar result holds for spheres. Indeed, the particular result is independent of the geometry of the pressure vessel.

The larger immediate result is that pressure vessels scale just fine with size, up or down, and there's no benefit at either end of the length scale, in relation to contained volume.

What you do get for a smaller engine is more manageable combustion instabilities. Look at the 5-fold symmetry (odd, rather than even) of the injector baffles in the SSMEs. This is to stop a tangential oscillation mode, an important failure mode of larger engines. What you lose for smaller engines is that you have more of the fluid "close" to the walls. The boundary layers don't grow linearly, so you get more heat transfer at the boundary in proportion to the contained fluid. (Radiant transfer in the engines complicates the picture, but convective transfer is definitely proportionately worse for smaller engines.)

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