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