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Old October 22nd 17, 10:04 AM posted to sci.space.policy
Fred J. McCall[_3_]
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Default Were liquid boosters on Shuttle ever realistic?

JF Mezei wrote:

On 2017-10-21 00:36, Fred J. McCall wrote:

reduced stage size. A smaller booster stage means much less
aerodynamic drag as the vehicle lifts off from near sea-level and
accelerates up through the more dense (thicker) part of the atmosphere
near the earth. The result of a smaller booster stage is it allows a
more efficient ascent through the thickest part of the atmosphere
which helps improve the net mass lifted to orbit."


While this sounds logical, the problem with the text is that it lacks
numbers or orders or magnitudes.

What is the cost of aerodynamics, and what are the weight savings from
using more efficient fuel?


Jesus, Mayfly, do SOME of your own homework. The fuel weight part is
easy once you know Isp.


And the cost of aerodynamics can vary depending on whether you use a
short and fast booster or long and skinny one.


You're going to eat yourself alive on dry weight if you make a 'long
and skinny' booster, since it takes a lot more structure to keep the
thing from breaking into pieces.


And in the case of the shuttle, what if say 25% of a booster' fuel came
from a taller (but same diametre) ET, allowing for smaller aerodynakic
drag on the smaller boster tanks ?


What if it was all powered by magic unicorn farts? You just
complicated the **** out of the engineering, what with the taller
tank, needing to worry about crossfeeding the boosters during PART of
the flight, etc.


The problem with the expert arguments as stated above is that they can
be used as spin to pusgh for one architecture without providing numbers
to support that suggestion because the numebrs may or may not give the
proposed solution such an edge.


So stop asking and believe what you like.


Also remember that if the boosters need less fuel mass, it means that
you need fewer booster engines and that also lowers fuel consumption and
thus lowers aerodynamic drag.


Uh, no, because you aren't using the same engines (or even engines
with the same thrust).


With regards to cost of engines, I am not sure it is fair to compare the
costs of the SpaceX merlin engines with cost of engines by bloated
companies who life comfortably on pork.


You're an idiot.


If SpaceX had built both types, then yeah, you could compare costs since
they both would have been made by a lean and mean company. It could be
that the cost difference would be huge, or minimal. We don't know
because that hasn't happened.


And why do you think that is?


As a newbie into the rocket business, SpaceX likely started with the
simplest and cheapest, and that means RP1. What is not known is how far
is LH2 from being competitive if it were done by SpaceX or another
company that is as lean and mean as SpaceX.


There's a reason SpaceX has no interest in LH2/LOX engines. If they
wanted to go 'simplest' they'd be using hypergolic fuels (although
they're a little ugly to handle during loading), since those engines
are MUCH simpler (which is why Draco and Super Draco use hypergolic
fuels). Note that SpaceX's next big engine is CH4/LOX rather than
LH2/LOX.

An LH2/LOX engine will always cost a lot more than an RP1/LOX engine.
This is obvious with very little thought, which I guess explains why
you don't get it. The most expensive parts of the engine are the
turbopumps. To get the same thrust out of a lower density fuel
(actually a lower density exhaust, but it correlates) you must pump a
higher volume of fuel in a given time. This means that your
turbopumps need to be bigger and more powerful (and thus more
expensive). In addition, with LH2 engines you're pumping a deeply
cryogenic fuel, which means the materials your pump is made out of
need to account for that as well as standing up to the higher power
required.

Musk is a bright guy, but he can't change the physical laws of
material science. A Merlin engine costs $2 million and change, as
near as can be determined. RD-180, which is a more aggressive design
with higher operating pressures and over four times the thrust costs
around $20 million. Before the Russians jacked up the prices they
were $10 million. That's actually for two engines hooked together,
though, so call it around $5 million or so per engine. So even a
simple LH2/LOX engine costs at least twice as much.


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