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One engine vs clustered engines

39 posts in this topic

6 hours ago, IGNOBIL said:

Regarding failure one example was falcon 9 3rd or 4th launch, one of the engines failed and the flight carried on nominal, you can see a dark streak in the trail!

also the quantity of production in the merlin engines reduced their price a lot, that's the reason they swapped from the krestel engine to merlin vacuum for the upper stage

Yeah, but the likelihood that any ONE engine fails will go up, but the likelihood of ALL engines failing goes down.

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On 2/23/2017 at 5:54 PM, Bill Phil said:

Yeah, but the likelihood that any ONE engine fails will go up, but the likelihood of ALL engines failing goes down.

This clearly depends on the failure mode.  The first Falcon 9 had an engine loss that was merely turned off.  The N-1 had failures which would damage nearby engines.  I'm not sure if Antares suffered a similar fate (watching the video leads me to believe that it lost all thrust after the initial explosion, but without a scale you can't really eyeball acceleration).

You basically have to design a device that mixes liquid oxygen with a fuel and convince it not to explode on failure.  This is easier said than done.  The N-1 tried to avoid this by shutting down on the first sign of failure.  In practice, this meant it typically shut down every engine shortly after lift-off, bringing down the whole thing.

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On 18/07/2016 at 4:00 PM, monstah said:

I understand this topic is in the spaceflight forum, thus talking about real life launches (with good info!). I wonder how does the logic apply to KSP, however. I've always used single big engines, and apart from the gimbal roll authority and low techtree positioning, I never found clusters to be better than a large one. They're prettier, tho.

Old post I know, but actually KSP demonstrates the case for clusters. The game doesn't have unlimited size engines, so if you're launching big things you need to use engine clusters because that's your only choice.

In real life it's not so cut and dried, but for example Falcon Heavy will use 27 engines because that's what SpaceX have. I believe that when Falcon Heavy flies, it will have more engines at liftoff than any other successful rocket in history. (N-1 had more, but went boom.)

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52 minutes ago, cantab said:

I believe that when Falcon Heavy flies, it will have more engines at liftoff than any other successful rocket in history. (N-1 had more, but went boom.)

By a big margin. Previous record is ten, jointly held by R7 derivatives (five main and five vernier engines) and CZ-5 (ten main engines).

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22 hours ago, cantab said:

Old post I know, but actually KSP demonstrates the case for clusters. The game doesn't have unlimited size engines, so if you're launching big things you need to use engine clusters because that's your only choice.

In real life it's not so cut and dried, but for example Falcon Heavy will use 27 engines because that's what SpaceX have. I believe that when Falcon Heavy flies, it will have more engines at liftoff than any other successful rocket in history. (N-1 had more, but went boom.)

Considering Space-x has launched 30 rockets (300 merlin engines of various levels of development) and never had a failure directly due to one of those engines, this sounds like a pretty safe means of launching a rocket.  Control theory (and the systems designed to execute such controls) have come a long way since the N-1.

Remember that space-x is still developing the raptor engine, so they don't think that clusters of Merlins are always the best solution.  One reason that this makes sense if you assume that their will be far more falcon 9 launches than falcon heavy, it makes sense to concentrate on the lighter rocket and trade engineering costs for manufacturing costs in the heavy one.  Blue Origin is going with a different balance, but remember that space-x was largely driven to this design due to the falcon-1 engine using a merlin engine.  Scaling the thing up to 10 merlins was the only way to fill their >$1 Billion NASA contract.

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On 7/18/2016 at 9:40 AM, radonek said:

Big engine = combustion stability issues = lots of development money to resolve.

Clustered engine = synchronization&control issues = lots of development money to resolve.

^ This. Clustered engines also have issues with plumbing and potential shock wave interference.

Best,
-Slashy

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@Red Iron Crown hinted at this earlier, but as you increase engine size, volume of the chamber is scale factor cubed, but surface area is scale factor squared. This means that you have less area on the engine for regenerative cooling (relative to its volume). This is the reason that one can only make expander cycle engines such as the RL10 so big.

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On 3/13/2017 at 9:16 AM, GoSlash27 said:

^ This. Clustered engines also have issues with plumbing and potential shock wave interference.

Best,
-Slashy

Wouldn't the Russian system of having multiple nozzles per engine have the same shock wave interference problem?  They still seemed to think it easier than one big nozzle.  Plumbing issues are pretty much the point of heart of rocket science: have to get the fuel to the turbopumps and hence to the combustion chamber.  Possibly simply reuse the same design and "simply" have a cluster of boosters.

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On 09.03.2017 at 4:14 PM, wumpus said:

Remember that space-x is still developing the raptor engine, so they don't think that clusters of Merlins are always the best solution

But they are going to have even bigger cluster of Raptors on their ultra-heavy booster.

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That was apparently a result of searching for the engine size that maximised thrust to weight ratio. (Or for the very important scientific number 42).

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5 hours ago, wumpus said:

Wouldn't the Russian system of having multiple nozzles per engine have the same shock wave interference problem?  They still seemed to think it easier than one big nozzle.

wumpus,

 Absolutely. As I understand it, they find the problems with clustered small boosters easier to overcome than the combustion instability problems associated with large boosters.

5 hours ago, wumpus said:

Plumbing issues are pretty much the point of heart of rocket science: have to get the fuel to the turbopumps and hence to the combustion chamber.  Possibly simply reuse the same design and "simply" have a cluster of boosters.

Well... none of it is "simple". Every additional line is a potential source of failure. Changes in temperature affect the flexibility of the lines and each line has a resonant frequency. If a single line fails, it affects the mixture to every associated engine (or just nozzle) with catastrophic consequences. Basically it's a "pick your poison" kind of tradeoff.

Best,
-Slashy

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On 14.3.2017 at 11:32 PM, GoSlash27 said:

 

Slash is absolutely right.

And Russians use multiple smal engines to provide Backup Systems in the fuel delivery. If one line dont work you can surpace the fuel through other engine supportlines. You loose a littel on overall outcome but all engines can fire.

SFIK the most engines a bundled in a 2-4 Cluster in theyr fuelline crossupport. But im to long away from this topic. 

Urses

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6 hours ago, Urses said:

And Russians use multiple smal engines to provide Backup Systems in the fuel delivery.

Depends on the engine - Some of what looks like multiple engines are actually *one* engine, one pump assembly with multiple chambers.

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Yes, like seen in Sojus.

For Venera Series on other hand they where multiple Pumps crosfeeded multiple chambers. Because it was a long way to come up with some inflightrepairs:D

Mostly the multiple crossfeed was Used if you has a absolutely new engine or was going for ability to prolong the survivebility of your craft.

The best way to describe this is the military work around for maintance.

You have a brocken tank, truck and car. After the maintance you have a working tank and maybe Truck but nobody know where is the car:cool:

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