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What is the fundamental difference between a construction of n engines vs one engine with n combustion chambers?

For example, Energia rocket used four boosters, each with a 4-chamber RD-170 engine, and four separate single-chamber RD-0120 engines for its core stage.

I guess one difference may be a pump - for designs that utilize one, instead of using compressed neutral gas for displacing the fuel, but in designs that don't use turbopumps I fail to see any difference...

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    $\begingroup$ Are there any examples of pressure-fed multiple chamber engines? $\endgroup$ Nov 30, 2015 at 12:33
  • $\begingroup$ @OrganicMarble: ...or are they considered multiple engines and not multiple chambers due to lack of a common pump? $\endgroup$
    – SF.
    Nov 30, 2015 at 13:26
  • $\begingroup$ The link says the RD-170 had 4 chambers sharing one pump, and it was done to increase combustion stability, which was failing in larger chambers. Maybe that is the underlying design driver in such cases. $\endgroup$
    – kim holder
    Nov 30, 2015 at 15:06
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    $\begingroup$ I don't know of any unidirectional clustered pressure-fed chambers, but in theory they could share control, valve, and thrust-vectoring hardware and maybe be thought of as a single engine if you squint just right. For RCS thrusters, a cluster of pressure-fed chambers pointing in different directions built as an integrated unit is common, but that's probably a "module"; I found an Apollo SM schematic that called out the RCS as 16 engines rather than 4 modules/clusters/pods/units. $\endgroup$ Nov 30, 2015 at 17:17
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    $\begingroup$ @OrganicMarble Found one 5 and a half years later: the XLR-11 used on the Bell X-1. $\endgroup$ May 6, 2021 at 23:09

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The only potential differences I can see in this would be in gimbal range and the ability to turn off certain engines. The falcon 9 uses 9 individual engines which allows it to turn off as many as it needs to in flight to either land, re-enter the atmosphere or in case one engine breaks. A multiple chamber single engine could do this with some modification however at the point where you are putting separate on the fuel lines it would essentially become individual engines. For gimballing purposes, using one pump system would only really work if all of the nozzles are connected and gimbal together. Again; if they were to gimbal separately the pump system would likely need modification which would essentially result in individual engines.

The only real advantage of using a single pump system is that it would ensure a stable fuel flow to each chamber.

"Several Soviet and Russian rocket engines use the approach of clustering small combustion chambers around a single turbine and pump. During the early 1950s, many Soviet engine designers, including Valentin P. Glushko, faced problems of combustion instability while designing bigger thrust chambers. At that time they solved the problem by using a cluster of smaller thrust chambers." (Wikipedia, 2017). Due to the size of large thrust chambers, combustion may happen unevenly within a single chamber due to the ability for pressure fronts to more significantly move. The use of a single pump ensures that all chambers can be operated as one, and decreases weight, complexity, and parts.

Using a single large pump system over multiple chambers also likely helps keep the cost and complication down in the rd-170. This design however is not widely used today in currently planned and used launch systems. The only system currently using something like this to my knowledge (please correct me if I'm wrong) is the RD-180 used on the Atlas launch vehicle (https://en.wikipedia.org/wiki/File:Rd180schematic.png).

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    $\begingroup$ How does using a single pump ensure a stable fuel flow? Do you have references to back up any of your assertions? $\endgroup$ Aug 22, 2017 at 23:22
  • $\begingroup$ @OrganicMarble During the early 1950s, many Soviet engine designers, including Valentin P. Glushko, faced problems of combustion instability while designing bigger thrust chambers. At that time they solved the problem by using a cluster of smaller thrust chambers (Wikipedia, 2017). en.wikipedia.org/wiki/RD-170 $\endgroup$
    – user20529
    Aug 23, 2017 at 3:00
  • $\begingroup$ The Black Arrow first stage had a single engine with 8 nozzles that pivoted in 4 pairs to provide pitch yaw and roll. Sylon C2's SABRE 3 engines were designed as (in each nacelle) a pair of dual chamber engines, each nozzle gimballing separately but in unison $\endgroup$
    – user20636
    Aug 25, 2017 at 9:18

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