Jeff Findley wrote:

In article ,
says...

I'm confused. The program to replace the RD-180 is focused on engines
with around 400,000 lb thrust at sea level. This focuses them on the
AR-1 (kerosene/LOC) and BE-4 (methane/LOX). Why are they not looking
at the RS-25 (LH2/LOX with similar thrust) or the Raptor engine
(methane/LOX)?


RS-25 is hella expensive and ULA already knows that LH2/LOX produces a
large, expensive, vehicle (e.g. Delta IV). So that's right out since
Delta IV is already flying (no development costs there). But do note
that ULA really wants to ax Delta IV in favor of Atlas V due to its high
cost.


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


Raptor (methane/LOX) isn't "fully baked" yet (BE-4 is ahead of it).


That sounds wrong to me. SpaceX test fired a full up Raptor engine
(albeit a lower thrust developmental engine) at their Texas facility
last year. The BE-4 has never been test fired and they didn't even
have a full engine put together until this year.



Seems like we're getting a lot of different engines when it might be
more efficient to settle on just a couple.


AR-1 is a "backup" engine at this point since it's so far behind BE-4 in
both schedule and (estimated) per unit price. But, AR-1 is about the
right size for two of them to be a "drop-in" replacement for RD-180 on
Atlas V. So, if ULA stumbles on Vulcan, an AR-1 engined Atlas V might
be a good stop-gap measure.


Aerojet Rocketdyne says they can start delivering AR1 engines in 2019,
so the finish line isn't all that far behind BE-4. Blue Origin says
the BE-4 will cost 60% of what an AR1 costs (at $12.5 million each);
so BE-4 engines are only around $7.5 million each? The government is
paying a lot of money to develop AR1, so I'd bet on it being pushed
for use somewhere. And AR1 does have the advantage of not needing a
bunch of new infrastructure to handle fueling and such.


At any rate, Aerojet Rocketdyne is being paid good money to develop AR-
1. Even if it meets the same fate as J-2X, they're getting money now
which helps keep the company alive.


How many billion dollars of taxpayer money are we going to spend
developing engines that never get used? Around $1.5 billion for AR1.
Around $1.2 billion for RS-25 (which only gets used if SLS keeps
flying). Another $1.2 billion for J-2X. Meanwhile Merlin engines
used on Falcon 9 cost around $1.2 million each with engines in the
Raptor/BE-4 class going for $7.5 million each? Meanwhile the entire
development budget for New Glenn is around $2.5 billion and what
little public data there is puts development costs for Raptor engines
in the hundreds of millions of dollars (vice billions) and I expect
BE-4 development is similar. What that says is that private companies
developing engines mostly on their own nickel is looking to be an
order of magnitude cheaper than traditional contracted engine
development programs...


--
"The reasonable man adapts himself to the world; the unreasonable
man persists in trying to adapt the world to himself. Therefore,
all progress depends on the unreasonable man."
--George Bernard Shaw