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Old October 3rd 17, 12:58 AM posted to sci.space.policy
Fred J. McCall[_3_]
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Default NASA is teaming up with Russia to put a new space station near the moon. Here's why.

JF Mezei wrote:

On 2017-10-02 05:58, Jeff Findley wrote:

Only for the center lower stage. It has to be different than the
boosters, which are essentially Falcon 9 first stages with nose cones
instead of their own upper stage.


Musk said that it _all_ had to be re-engineered. Consider that the
boosters don't push a payload up, they push a payload attached it to its
side.

Carbon fiber tanks will be something new for SpaceX, so it's a risk.
The question is, how big of a risk?


My concern with the 1 big tank design is that for a long duration
flight, a failure of the one tank is sayonara for everyone.


It's always 'one big tank'. Using a ****load of little tanks is a
great way to build a vehicle that is too heavy to fly.


But today, "efficiency" isn't the design metric, it's the eventual cost
per pound to orbit that's the design metric.


Aren't smaller engines more reliable (or easier to make more reliable)?
If so, it makes sense to use multiple smaller engines.


It's not the size of the engine that determines reliability. Rather
it is how hard a particular engine pushes the envelope to get better
power/weight. ****loads of little engines require more plumbing, so
you would have to push them harder to get similar performance to fewer
larger engines.


Howewer, in commercial aviation, the reverse is now true. The 777 has
won over the 747 mainly because owning a plane with 2 engines costs a
lot less than one with 4 (as engines are costly to buy and maintain).

I wonder if rockets will also eventually adopt the "fewer but bigger
engines" mentality to cut maintenance costs.


They did. Now we're headed in the other direction. Saturn V used
five large engines on the first stage. Falcon Heavy uses 27. The
primary advantage of having more engines is that you get some degree
of 'engine out' capability. The primary disadvantage is that there
are more engines to keep synchronized and more for something to go
wrong with.


more existing engines on the next design than to have to design and
build new engines, then both Musk and Bezos will do so, even if it
complicates the plumbing, structure, control systems, and etc.


But again, once you have more then 3 engines, does using 3 5 or 9 make
things that far more complex? Isn't there a lot of "copy/paste" done on
the engine mount designs once you are beyond 3 engines ?


Think plumbing and directional control. Also think about history. The
first stage of the Saturn V had 5 engines. The first stage of the
corresponding Russian rocket used 30 engines. N-1 went 4 attempted
launches with 0 successes. More engines means more chance that
something will go wrong with one or more of them.

--
"Insisting on perfect safety is for people who don't have the balls to
live in the real world."
-- Mary Shafer, NASA Dryden