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
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