"Jeff Findley" wrote in message
...

In article , lid says...

Jeff Findley wrote:

One of the issues with very big engines is combustion instability. That
was a huge issue during F-1 engine development. So, making many smaller
engines is a way to avoid having to develop very large engines.

There's also the Russian solution, RD-170 is an "F-1 class" engine
with four combustion chambers because they deemed it too hard to solve
the combustion stability. The Russians have a number of multi-chamber
engines in that lineage including the dual-chamber RD-180 used in the
Atlas V.

It MAY... be that combustion instability may be less of an issue these
days with all the advances in CFD but no one seems to want to go down
that path. Either way I suspect that with modern computer control it's
better to build and use multiple Raptor/BE4 sized engines unless the
rocket is bigger than even Super Heavy!

Agreed, that's an option. One set of turbopumps driving multiple
combustion chambers and nozzles.

Or does growing beyond a certain size introduce fluid dynamic problems
of how liquid methane and o2 behave in pipes, behave as they flow
through large turbine with much mroe "space" to build, cause bubbles
and
cavitate etc) ?

Would it be fair to state that the Raptors are sized to be easy to
build? (so they need more).

That was the philosophy with Merlin. Also, commonality between the
second stage engines and the first stage engines naturally means you
need far more engines for the first stage. Thanks rocket equation!

Musk has said that SpaceX is "tracking to well under $1M for V1.0" and
has a goal of $250k for V2.0 (250 ton thrust-optimized engine.

It's always been the plan that Raptor should be producable in large
numbers and that the per-unit cost should be lower than the smaller
Merlin, AFAIK they've actually below the much smaller Merlin's cost?
The benefit of starting with a clean sheet and thinking how to build
it during the entire process I guess.

They're still tweaking the design a lot but they're already producing
a lot of engines (SN20 was on Starship SN4 when it was consumed by
fire after ground equipment issues).

Yep. Designing for mass production is a good thing. Starship and Super
Heavy are also intended to be designed for mass production as well.
Musk envisions sending a "fleet" of Starships to Mars every couple years
when the orbital mechanics makes the trip "affordable" in terms of
delta-V and travel time.

Still, not sure that the Merlin size was choosen for ease of
production reasons. Fundamentally only SpaceX knows ALL the reasons
for their decisions, it's not impossible it could have been involved
but there's so many other reasons why it makes sense for SpaceX to
build Raptor around that size that I doubt it.

I find it fascinating that there's so many rocket engines around the
1.6-2.4 MN (MegaNewton) range - Raptor, BE-4, RD-191, NK-33, RS-25 to
name some well-known ones. And the per *chamber* thrust for RD-170/180
(Energia and Atlas V respectively) is also in that range.

I can only think of one liquid fuelled rocket engine with more thrust
per chamber, the F-1 with 6.7 MN in a single chamber (RD-170 is more
powerful but needs 4 chambers). I discussed earlier why that might not
be a size path others may want to go down.

Yep. For some reason that size seems like a good "stopping point".
These days it seems easier to just add more engines to a design rather
than try to design an engine bigger than that.

I think multiple engines gives more reliability, etc.

But I can definitely see a time, much like with jetliners where 3 and 4
engines became popular for a variety of reasons but now we are looking at
engines like the GE GENx that are larger than the 737 body, that rockets may
move in that direction.
That said, it took decades and literally millions of hours of flights before
we got there with jets, so I won't hold my breath for rockets.


One thing that was mentioned that the relatively smaller size of Merlin
made it easier and cheaper to test. When you're doing hundreds of test
firings, your engine needs to be small enough that the tests are
practical to do in the middle of Texas. I'm sure there is some truth to
that.

Eh, Texas is always bragging about size.

Jeff

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