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  #31  
Old May 15th 17, 09:48 AM posted to sci.space.policy
Fred J. McCall[_3_]
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JF Mezei wrote:

On 2017-05-14 17:45, Fred J. McCall wrote:

If you do almost a total rebuild after every flight, things last a lot
longer.


But then the engines are not truly reusable. IF SSMEs *really* need a
total rebuild between flights when operated commercially (not NASA),
then they are worthless engines for today's environment.


They're not as bad now as they originally were, but they still require
substantial work after every flight.


So the question becomes: if SSMEs/RS25 were operated by an outfit such
as SpaceX, how much work would then need between flights ?


It doesn't matter who operates them. The engines are what they are.
They don't magically get better because they change owners.



No, that's not why it 'won'. It 'won' because it costs less to
OPERATE, not because it costs less to BUY.


Exactly. My point was that just because SSME/RS25 may be more expensive
to buy does not mean that overall cost of ownership has to be higher.


Then you stated your point poorly, which I assume is why you removed
your original statement about it having to do with cost of engines.


**IF** the engines are truly re-usable, needing fewer to launch stuff
may translate into competitive total cost of ownership due to lower
maintenance.


**IF** there is magic pixie dust dropping out of unicorn asses...


Having said that:


Sea level:
Merlin: 845.2 kN 470kg
SSME: 1859 kN 3500kg

So to get the same thrust as 1 SSME, you need 2.199 Merlin engines which
would weight 1,033kg instead of 3500kg.


That is basically 2 tonnes of extra weight for each SSME engine's thrust.

Would LH2/LOX fuel needed for each SSME engine running for 8 minutes end
up being at least 2 tonnes lighter than equivalent Kerosene/LOX ?


Thrust is proportional to mass out the ass, but ISP matters.


If at the end of the day, the SSMEs end up being far heavier for amount
of thrust they generate, then I guess it really is a no brainer that
they have no place in modern rocketry.


SSMEs have significantly higher ISP than a Merlin engine. That means
they require less mass flow rate to get the same thrust. However, you
have to worry about tank size and drag because of the lower density of
LH2 as a fuel. Then there's the cost and manufacturing complexity.
SpaceX can build 200+ Merlin engines a year. RS-25 takes a lot longer
and making it really reusable with no rebuild required would be
somewhere between really expensive and impossible, given how hard it
pushes performance. I think Merlin engines cost less than $2 million
each. A fully reusable RS-25 (let's say 10 flights with nothing but
inspection and minor repair) is going to cost well upwards of $60
million each. Maintenance per flight will be some percentage of the
cost of the engine, so SSME will always be significantly more
expensive to fly. You could probably throw away a Merlin engine after
a couple of flights and still be competitive with a 'reusable' RS-25.


--
"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
  #32  
Old May 15th 17, 06:43 PM posted to sci.space.policy
Jeff Findley[_6_]
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In article om,
says...

On 2017-05-12 14:55, Fred J. McCall wrote:

Yet SpaceX ITS has more payload and costs 10% of SLS.


In fairness, SpaceX ITS is vapourware. SLS is hardware (albeit not ready).


ITS isn't fully developed yet. But I wouldn't call it "vapourware"
since SpaceX is already developing hardware intended for ITS (e.g. the
Raptor LOX/methane engine and the composite tanks).


However, banning NASA from such "pushing of limits" would be wrong.

NASA doesn't do that anymore.


It should.


It won't because it's now a "mature" government agency that has been
mandated by Congress to build SLS/Orion even though there is no use for
it yet.

Imagine for a second that they productized SSMEs into a TRULY reusable
rocket with enough engines to not need SRBs and which could land itself.
Despite higher production cost, wouldn't it becoem commercially viable
due to re-usability ?


You're talking about reusable SSTO then. NASA abandoned that when it
failed utterly on the X-33 program. Also, even as a reusable first
stage it's doubtful since SSMEs burn LOX/liquid hydrogen. Liquid
hydrogen is a p.i.t.a. to work with since it's deeply cryogenic. Using
it drives costs up (which is why ULA wants to dump Delta IV in favor of
Atlas V and then Vulcan (which will not use liquid hydrogen in its first
stage).

LOX/methane or LOX/kerosene would be a far better choice.

The shuttle engines lasted far more than 10 flights which is what SpaceX
expects its engines to last.


With lots of tear-downs and refurbishment. Merlin is designed to fly 10
flights without being torn down for refurbishment. After it is
refurbished, it should be good for 10 more flights. In the end, SpaceX
hopes to get 100 flights out of each Falcon first stage.

Look at aviation: the 777 won over the 747 because it has 2 expensive
engines instead of 4 medium price engines. Maintenance becomes a huge
issue for cost savings when you have half the number of engines to
inspect/maintain in your fleet.

Imagine if SSMEs were truly re-usable and you could launch to space with
half the number of engines needed by SpaceX,


Who cares about "number of engines"?!? I care about launch costs. Do
you have any idea how much an SSME costs to build and maintain? I
didn't think so.

snip crap about Aerojet Rocketdyne


It all comes down to economics. If Rocketdyne were told it would compete
against SpaceX for lifting cargo to LEO, how much more cost efficient
would production of SSME/RS25 become ?


Not very. They're very "high strung", high chamber pressure, staged
combustion, LOX/LH2 engines. There the most complex US made liquid
fueled rocket engine ever to fly. That's a *terrible* place to start if
your goal is to lower costs.

Aerojet Rocketdyne doesn't know how to compete. It knows how to build
engines to the specs in the US Government contracts they get. They have
zero interest in building engines no one wants to buy. And no one wants
to buy their engines, if they don't have to, because they are expensive.

Jeff
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These posts do not reflect the opinions of my family, friends,
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  #33  
Old May 15th 17, 06:54 PM posted to sci.space.policy
Jeff Findley[_6_]
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In article m,
says...

On 2017-05-15 04:48, Fred J. McCall wrote:

SSMEs have significantly higher ISP than a Merlin engine. That means
they require less mass flow rate to get the same thrust.


How significant is the savings in fuel mass ?


Who cares? Cost is what matters. LH2 is less dense than kerosene, so
it takes *much* bigger insulated tanks to hold it. Besides the SSMEs
costing more, the vehicle ends up costing more.

snipped math that is over-simplified

In my mind, just because NASA required major engine refurb between
flights doesn't mean that it actually needed it.


Perhaps, but do you want your engines to work or to go "kaboom" every
once in a while? SSMEs were very reliable on the shuttle because they
were inspected after each flight and were torn down for refurbishment so
often. Do you really want an SSME turbo-pump to go "kaboom" every once
in a while?

more SSME love fest snipped

The SSME was a good engine, for what it was. But times have changed.
Requirements have changed. If you want low cost reusable stages to get
to orbit, throwing money at the SSME is throwing good money after bad.

Liquid hydrogen is a terrible choice for a first stage engine. So, SSME
is out on first principles. The space shuttle was a very odd duck which
required very odd engines. No one else, besides SLS, wants them.

Jeff
--
All opinions posted by me on Usenet News are mine, and mine alone.
These posts do not reflect the opinions of my family, friends,
employer, or any organization that I am a member of.
  #34  
Old May 15th 17, 07:09 PM posted to sci.space.policy
Fred J. McCall[_3_]
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Default RD-180 relplacement

JF Mezei wrote:

On 2017-05-15 04:48, Fred J. McCall wrote:

SSMEs have significantly higher ISP than a Merlin engine. That means
they require less mass flow rate to get the same thrust.


How significant is the savings in fuel mass ?

Since, for equal thrust, the SSME is 2 tonnes heavier than equivalent
thrust on Merlins, does this mean that the LH2/LOX needed to accelerate
to same speed at MECO would be at least 2 tonnes less per SSME engine ?

Or put it this way: do SSMEs truly have a performance advantage over
Merlins or are they just bigger engines?


LH2/LOX engines like SSME have a performance advantage over RP-1/LOX
engines. The Merlin has higher thrust/weight but the SSME can burn a
lot longer for a given amount of fuel.


Back to the 777/747 comparison:

GE 90-115B: thrust: 513kN weight: 8,762kg * 2 = 1026kN at 17,524kg

CF6-50 : thrust: 240kN weight: 4104kg * 4 = 960kN at 16,416


Interestingly, the thrust to weight ratio is the same for the twin
engine vs quad. But maintenance costs and purchase cost (buying 2 more
expensive instead of 4 less expensive) make the twin engine far more
attractive.


Again, that's not it. The purchase price of the jets has nothing to
do with aircraft choices. It's all about operating costs and the 777
costs about 70% of what a 747 costs to operate.


So, in an era where re-use becomes common, the maintenance cost between
flights starts to matter if you need fewer engines to launch same payload.


So, no. the big driver for aircraft operating costs is fuel
consumption and the 777 is that much better than the 747.


So what starts to matter now is whether SSMEs could ever become
commercially competitive if it was unhindered by anything "NASA" and was
actually re-usable with little turn around costs.


In an alternate universe where unicorns **** magic pixie dust anything
is possible. However, in the real world a higher performance engine
like RS-25 is always going to require more maintenance than an engine
that doesn't push performance so hard like the Merlin. They will cost
more to build because they push performance so hard and they will cost
more to maintain because that maintenance is going to be some
percentage of the cost basis of the engine. For rockets the cost of
fuel is almost irrelevant to the cost of operation, so trying to
compare to aircraft is comparing apples and aardvarks.


In my mind, just because NASA required major engine refurb between
flights doesn't mean that it actually needed it.


Don't be an idiot. They wouldn't have done it if it didn't need
doing. If that's how things look in your mind, you're out of it.


In the history of jet planes, it took quite a while for engine
performance/reliability to allow the drop from 4 to 3 and then 2 engines
on wide body jets.


Apples and aardvarks.


I can see it being easier for SpaceX to go with "many" smaller/simpler
engines first just as early jets had many engines smaller engines.

But that does not mean that eventually, fewer larger rocket engines will
end up being more economical, just as it happened with jet aircraft.


Apples and aardvarks.


To this end, some real R&D effort to turn the SSMEs into truly re-usable
low maintenance engines would be interesting especially if those engines
offer performance advantage despite their heavier weight.


And who do you think is going to pay for that? The entire life of the
RS-25 engine program was about reducing maintenance requirements and
they apparently have decided that that is a mug's game and are now
going in the other direction to lower the cost of the engines. You
just want to go that one step further and invoke magic and
unobtainium. High performance engines like RS-25 will ALWAYS be 'less
reusable' than lower performance engines like Merlin or Raptor. Just
look at chamber pressures. RS-25 runs twice the chamber pressures of
something like Merlin, with all that implies for pumps, wear, etc.


--
"Some people get lost in thought because it's such unfamiliar
territory."
--G. Behn
  #35  
Old May 15th 17, 09:22 PM posted to sci.space.policy
Jeff Findley[_6_]
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Posts: 2,307
Default RD-180 relplacement

In article ,
says...
And who do you think is going to pay for that? The entire life of the
RS-25 engine program was about reducing maintenance requirements and
they apparently have decided that that is a mug's game and are now
going in the other direction to lower the cost of the engines. You
just want to go that one step further and invoke magic and
unobtainium. High performance engines like RS-25 will ALWAYS be 'less
reusable' than lower performance engines like Merlin or Raptor. Just
look at chamber pressures. RS-25 runs twice the chamber pressures of
something like Merlin, with all that implies for pumps, wear, etc.


Excellent point. To put this in terms JF might understand:

The SSME is like the engine in an Indy car or F-1 car. They are
extremely high revving engines designed for maximum acceleration and
high sustained speeds. They might be good for more than one race, but
why take a chance of your engine blowing up? So, they're torn down far
more often than what your average car owner would consider "reasonable".

For an average American production car like a Dodge Challenger Hellcat
(which has a 700+ horsepower Hemi which isn't very high revving) would
be considered "fast". The Dodge Challenger Hellcat is also fairly
reasonably priced considering its performance. And do note that the
average Hellcat owner is never, ever going to push their engine anywhere
near as hard as an Indy or F-1 engine is pushed, even if they take it to
the local drag strip on the weekends and do a few 1/4 mile runs every
month.

Jeff
--
All opinions posted by me on Usenet News are mine, and mine alone.
These posts do not reflect the opinions of my family, friends,
employer, or any organization that I am a member of.
  #36  
Old May 15th 17, 09:39 PM posted to sci.space.policy
Jeff Findley[_6_]
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Posts: 2,307
Default RD-180 relplacement

In article om,
says...

On 2017-05-15 13:43, Jeff Findley wrote:

stage it's doubtful since SSMEs burn LOX/liquid hydrogen. Liquid
hydrogen is a p.i.t.a. to work with since it's deeply cryogenic.


Is LH2 a show stopper argument for any modern engine?


It's not a show stopper, but it's going to make for more expensive
engines (bigger turbopump needed for its lower density), a more
expensive stage (bigger tanks which need cryogenic insulation), and more
expensive operations (due to its cryogenic nature).

LH2 is best reserved for upper stages of launch vehicles which need a
"high energy upper stage". For a lower stage, it's just not worth the
added cost.

The reason I ask is that until the last couple of years, there has never
been any reason for outfits such as Rocketdyne or Boeing to work to
lower costs since NASA contnued to hand them pork money, and satellite
launch market was served only by expensive rockets (with Soyuz starting
to make a dent). Since then, SpaceX has been born and "old" companies
would/should fear for their survival with their old expensive launch
services.


So, in my mind, it is possible that the SSMEs are currently
uncompetitive simply because Rocketdyne never really tried. So the
question is whether it would be possible to turn SSMEs into something
competive if they realised their life depeneded on it.

But if the use of LH2 precludes competitive low maintenance engines,
then I guess there is no point in trying to make SSMEs competitive.


Bingo! We have a winner!

In a context where NASA is supposed to do R&D/science, *IF* NASA
developped new materials or construction techniques to make LH2 engines
truly reliable and re-usable, could LH2 engines ever be competitive?
(thrust/weight/ISP, cost of fuel, tanks etc) ?


Not really. LH2 is always going to be a p.i.t.a. to work with and
really isn't going to be suitable for lower stages.

Do note that both the Saturn IB and the Saturn V had LOX/kerosene first
stages. Note the relatively tiny size of the kerosene tank on the
Saturn V first stage. See how much smaller it is compared to the LOX
tank? Now look at the 2nd stage of the Saturn V. That small tank is
the LOX tank. Note how much *bigger* the LH2 tank is. If Saturn V used
LOX/liquid hydrogen for the first stage, it would have been huge. The
VAB, MLPs, and etc. would all have needed to be much taller and/or
wider.

Or has kerosene or even methane zipped by LH2 are are so far ahead that
there is absolutely no point in researching LH2 engines anymore ?


LH2 is still arguably a good choice for upper stages due to its superior
ISP. But upper stages are necessarily far smaller than lower stages, so
that helps to keep their cost down, at least somewhat.

Liquid methane will be interesting. It's not a "deep" cryogenic like
liquid hydrogen. So in terms of temperature, handing it isn't much
different than handling LOX, which you'll need to handle anyway. As for
density, liquid methane is between kerosene and liquid hydrogen. One
advantage of liquid methane would be that you could use it in both the
lower and upper stage which simplifies a few things. For example, you
can use the same engines on the lower and upper stage assuming you put a
large expansion nozzle on the upper stage engine. This is what SpaceX
plans to do with their next generation, fully reusable, TSTO.

Also, how come LH2 was chosen for the Shuttle if it already presented
serious challenges for re-usability and kerosene was already used by
others?


Because of the "performance uber alles" mindset of the former ICBM
designers who led the design of the thing. The engines had to burn from
LEO to orbit, so they wanted the highest ISP possible. But to get that
to fit in the relatively small area of the shuttle's engine compartment,
they needed smallish sized engines. So to compensate for both issues,
they designed the SSME as a staged combustion engine with an extremely
high chamber pressure. In other words, it's very "high strung".

But even if you make a LOX/LH2 engine less "high strung" like the RS-68
used by Delta IV first stage, they will still be more expensive due to
the lower density (bigger turbopump required) and cryogenic
temperatures. On top of that, you still have to deal with liquid
hydrogen's very low density and deep cryogenic temperature. So the
stage is still going to be more expensive than a "less efficient"
LOX/kerosene or LOX/methane stage which burns more fuel.

Remember, fuel is cheap (much less than 1% of launch costs), so making
your first stage a bit bigger due to less "efficient" engines really
doesn't cost you much, except for a few more engines and more tankage to
hold the extra fuel and oxidizer.

Jeff
--
All opinions posted by me on Usenet News are mine, and mine alone.
These posts do not reflect the opinions of my family, friends,
employer, or any organization that I am a member of.
  #38  
Old May 16th 17, 12:41 AM posted to sci.space.policy
Alain Fournier[_3_]
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Default RD-180 relplacement

ON May/15/2017 at 4:39 PM, Jeff Findley wrote :
In article om,
says...

On 2017-05-15 13:43, Jeff Findley wrote:

stage it's doubtful since SSMEs burn LOX/liquid hydrogen. Liquid
hydrogen is a p.i.t.a. to work with since it's deeply cryogenic.


Is LH2 a show stopper argument for any modern engine?


It's not a show stopper, but it's going to make for more expensive
engines (bigger turbopump needed for its lower density), a more
expensive stage (bigger tanks which need cryogenic insulation), and more
expensive operations (due to its cryogenic nature).

LH2 is best reserved for upper stages of launch vehicles which need a
"high energy upper stage". For a lower stage, it's just not worth the
added cost.


Just to expand a little on that. It is good to have light fuel for upper
stages. For lower stages, what is more important is to have high thrust.
Once you are in low Earth orbit, if you want to escape Earth, you can
use a low thrust engine and burn for a long time, so you don't need
high thrust. If you aren't quite in LEO yet but are close to orbital
speed, you can get away with not too much thrust and somewhat long
burn times. If you are on the launch pad and you have a low thrust
engine, you aren't going anywhere.

You can have a high thrust LOX/LH2 engine, but it will cost you one
way or another. It is much easier/cheaper to have a high thrust engine
with a higher density fuel than LH2. And the added weight of the fuel
is a much milder problem for the first stage because that weight will
in large part be burnt before reaching too high a speed.

For the first stage of a rocket, higher density fuels are clearly more
favourable. For the second stage, SpaceX chose to stick with high
density. I think that is a wise choice, but the advantage is much
smaller. It isn't impossible that someone would someday develop
technology for using LH2 that would make that more favourable for
a second stage.


Alain Fournier

  #39  
Old May 16th 17, 02:02 AM posted to sci.space.policy
Fred J. McCall[_3_]
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Default RD-180 relplacement

JF Mezei wrote:

On 2017-05-15 13:43, Jeff Findley wrote:

stage it's doubtful since SSMEs burn LOX/liquid hydrogen. Liquid
hydrogen is a p.i.t.a. to work with since it's deeply cryogenic.


Is LH2 a show stopper argument for any modern engine?


For a first stage it isn't a great choice. It can make sense for
upper stages.


The reason I ask is that until the last couple of years, there has never
been any reason for outfits such as Rocketdyne or Boeing to work to
lower costs since NASA contnued to hand them pork money, and satellite
launch market was served only by expensive rockets (with Soyuz starting
to make a dent). Since then, SpaceX has been born and "old" companies
would/should fear for their survival with their old expensive launch
services.


Most of them are and you will note that they're all moving away from
LH2 first stages.


So, in my mind, it is possible that the SSMEs are currently
uncompetitive simply because Rocketdyne never really tried. So the
question is whether it would be possible to turn SSMEs into something
competive if they realised their life depeneded on it.


You need to put some knowledge in your mind so you don't have space
for such silly notions. The RS-25D is the most complex and
sophisticated bleeding edge performance engine ever built by anyone
anywhere. The fallout of that is high costs and high maintenance if
you're going to reuse them.


But if the use of LH2 precludes competitive low maintenance engines,
then I guess there is no point in trying to make SSMEs competitive.


Tankage, deeply cryogenic fuel, high chamber pressures (which means
high wear on the engine and pumps) are all going to make RS-25
uncompetitive with 'milder' engines using less cryogenic and denser
fuels.


In a context where NASA is supposed to do R&D/science, *IF* NASA
developped new materials or construction techniques to make LH2 engines
truly reliable and re-usable, could LH2 engines ever be competitive?
(thrust/weight/ISP, cost of fuel, tanks etc) ?


Anything that would do that for LH2 engines could equally be applied
to 'milder' engines and so they would still maintain their wear and
maintenance advantages.


Or has kerosene or even methane zipped by LH2 are are so far ahead that
there is absolutely no point in researching LH2 engines anymore ?


It's not a matter of 'zipped by'. It's a matter of those both being
less cryogenic and denser fuels.


Also, how come LH2 was chosen for the Shuttle if it already presented
serious challenges for re-usability and kerosene was already used by
others?


Because the Shuttle isn't a staged vehicle and you kind of want the
higher energy LH2 engine once you're up in near vacuum. But you can't
carry enough tankage to make that work all the way from the ground
with just LH2, so you get strap ons...


--
"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
  #40  
Old May 16th 17, 02:21 AM posted to sci.space.policy
Fred J. McCall[_3_]
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Posts: 10,018
Default RD-180 relplacement

JF Mezei wrote:

On 2017-05-15 14:09, Fred J. McCall wrote:

percentage of the cost basis of the engine. For rockets the cost of
fuel is almost irrelevant to the cost of operation, so trying to
compare to aircraft is comparing apples and aardvarks.


The numbers given by Elon Murk show cost of fuel is minimal compared to
cost of new rocket.


Cost of fuel is minimal compared to almost anything you do with a
rocket, new or reused.


However, in an environment where re-use becomes common, cost of turn
around (incl engine check/maintennce) and cost of fuel become the big
ticket items, just as is the case for commercial aircraft.


Cost of fuel is still going to be literally in the noise. Hardware
turn around and launch costs are going to drive the numbers.



And who do you think is going to pay for that? The entire life of the
RS-25 engine program was about reducing maintenance requirements


But NASA has lacked aggressive plans to "finish" the Shuttle by making
the SSMEs truly reusable. Not much point in spending mega money in R&D
to make SSMEs truly re-usable if at the end of the day, NASA management
will still require total teardown between flights.


You need to get off this nutty idea that NASA was tearing them down
and refurbishing them just for the **** of it. They made it as
reusable as they could (through four versions of the engine).


When SLS is put out of its misery, and Rocketdyne has delivered those 6
SSMEs that potentially go to museums, what is to happen to Rocketdyne if
it has no engine that is competitive? Shouldn't the company be worry
about its future with SpaceX revolutiuonalizing launch services ?


Of course it should but what do you propose they do about it? Throwing
billions of dollars into an engine that has no market (RS-25) is just
insane behaviour. They're making R-1 (RP-1/LOX) to be as
competitively priced as they can and it can't compete with BE-4
(methane/LOX) which has similar performance.


But if LH2 engines have no future, then I guess they are correct in not
spendng any more money than NASA forces them to on the SSMEs.


Big of you.



going in the other direction to lower the cost of the engines. You
just want to go that one step further and invoke magic and
unobtainium.


No, I am merely exploring whether something that was never tried before
could be possible. Or whether the SSMEs are truly hopeless in terms of
conceptually being competitive.


It WAS tried. They just couldn't bloody do it and they can't do it
now.



High performance engines like RS-25 will ALWAYS be 'less
reusable' than lower performance engines like Merlin or Raptor.


Hand waving doesn't comvince me, especially since there was never a
serious attempt to make SSMEs cheaper to make with modern manufacturing
and more re-usable wth moderns materials knowledge.


Well **** you, then. One of us is right and the other one is you. I'm
fine with that. Educate your ignorant ass.

Just as a clue, 'cheaper' means 'less reusable'. You can't do both.
On any given engine, making it 'more reusable' will mean making it
more expensive. And the idea that they just didn't bother to try to
make the RS-25 reusable is, well, STUPID.


The SLS pork to Ropcketdyne COULD have been such an attempt to turn
those expenseive engines into something that might have been
commercially viable. (Unless , again, LH2 engines are just never going
to cut it).


Unicorns and pixie dust...


--
"Ignorance is preferable to error, and he is less remote from the
truth who believes nothing than he who believes what is wrong."
-- Thomas Jefferson
 




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