In article ,
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On 2020-06-15 20:25, Greg (Strider) Moore wrote:

I think multiple engines gives more reliability, etc.

Greater maintenance cost. Greater odds of an engine failing.

Possibly greater maintenance costs. Smaller engines are easier to work
on because most of the parts to pull and inspect wouldn't be so heavy as
to need cranes and hoists to remove.

A single engine plane as 37 times fewer chances of an engine failure
than a plane with 37 of the same engines. Or course, when you lose your
single engine, the impact is a tad more significant.

Early trans-Atlantic airliners had four engines. Why? Because early
turbofan engines were not so reliable back then and having four engines
meant you could lose one or two of them and still get to an airport.

Modern trans-Atlantic airliners have only two engines. Why? Because
modern turbofan engines are crazy reliable and losing one means you can
still get to an airport.

Guess where engine reliability is for liquid fueled rocket engines?
It's therefore advantageous to have extra engines so that an engine out
won't lead to loss of mission or loss of vehicle. Note that even the
space shuttle could handle the loss of an SSME during some parts of the
flight (if it's early enough, it will trigger an abort).

The type of engine failure is also important. Software detecting
something and performing a clean shutdown in flight is quite different
from a turbopump breaking apart and sending sharpnel all over the place,
damaging nearby engines and likely the tank above. So big kaboom.

That's why on Falcon 9 there are literally barriers between the engines
meant to contain an engine failure. I do not know if Starship and Super
Heavy will continue this practice or not. During the entire history of
Falcon 9, only *one* Merlin has "lost chamber pressure" during flight,
which meant it underwent some level of "kaboom". That flight completed
the primary mission. Redundancy worked even in this case.

Having 37 greater chances of en engine shutdown in flight just means
reduced thrust, so you brun longer so minor inconvenience with no real
impact compated to a single engine shutting down in flight.

Actually, when you have 37 engines, you can throttle up the remaining
engines a tiny bit and get the same thrust. The SSMEs had a bit of
reserve power to throttle up in an emergency, but because there were
only three of them, an engine out could not be fully compensated for by
the remaining two engines. When you have 37 engines, losing one of them
isn't nearly as big of a deal.

But having 37 greater chances of uncontained engine failure in flight is
37 time more dangerous than the single engine.

Again, that's why the Falcon 9 puts barriers between the engines. Super
Booster could do the same.

And you now have to introduce the issue of whether the smaller mass
produced engines are more reliable than the fewer handcrafted huge mega
engines.

I'd go with the mass produced version. Mass production typically means
automating many tasks that a human could screw up on a hand built
version. Your machines will never come in on a Morning hung over from
partying on the weekend.

In this thred, early on, it was pointed to me that having larger engines
introduced fuel flow issues with some rough combustion (forget exact
terminilogy). If this ends up with more complex engine and/or lwoer

You're missing the big point. A bigger engine is more likely to fail
because they're much harder to get right. Solutions to things like
combustion instability means more complexity. More complexity means
more things to go wrong. Smaller, simpler, engines are the way to go.

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.

The 3 or 4 engines were originally needed because of the power needed to
fly that jet since available engines didn't have the thrust to do the job.

Again, it wasn't just that, it was the reliability issue as well. It
wasn't until fairly recently that two engine aircraft were certified for
trans-ocean flights. They had to prove that modern turbofan engines
were up to the task in terms of reliability. That took a lot of effort
to certify to the FAA that the twin-jets were truly reliable enough.

What SpaceX is doing with engine re-usability has many similarities to
aviation.

https://en.wikipedia.org/wiki/ETOPS#History

Initially, all aircraft were restricted to routes where they were no
more than 100nm from an alternate airport in case they lost an engine
(irrespective of number of engines). (this was roughly 60 minutes on
piston propellor aircraft crossing Atlantic)

That's what I've been saying. I'm snipping the rest of your post
because I see a lot of rambling, but not much of a point.

Jeff
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