JF Mezei wrote:

On 2017-05-10 05:30, Fred J. McCall wrote:

So why aren't they using something other than RS-25 on SLS?


Because NASA got 16 SSMEs for free when they retired the shuttle.


Not a reasonable reason to bet the long term farm on that engine.


With regards to production costs, considering new tooling is being built
with 21st century tech to produce new SSMes, shoudln't production costs
go way down?


Not nearly as much as you think. The expense of producing the engine
is largely because of the engine.


Back in the 1970s, "state of the art" aircraft engines were roughly
45,000lbs of thrust.

Today, modern CAD/CAM makes ~115,000lbs aircraft engines commercially
viable and in fact, the 777 has won huge market share with those engines
compared to older generation qhich required quad engines on planes.


Uh, what does that have to do with anything? Yes, we could build
bigger and cheaper rocket engines than the RS-25 (and both SpaceX and
Blue Origin are doing so). But those engines aren't RS-25s.


Isn't it likely that while producing SSMEs in the 1970s was way beyond
commercial "state of the art" and thus very expensive, modern CAD/CAM
would be well capable of automating production of parts with the
required tolerances for those engines?


No. RS-25 is a high performance engine (still).


Or put another way: are SSMEs still beyond what modern tooling can
build? Or is it more a question of Rocketdyne just taking money from
NASA and not caring about making this a viable product ?


Engines like RS-25 are still going to require a lot of 'touch labor'.


--
"Millions for defense, but not one cent for tribute."
-- Charles Pinckney