Space Forum / Space Flight / September 2005

One thing that might be easier than propellant crossfeed is
pressurization crossfeed: the strap-ons have helium pressurant
tanks that also pressurize the core stage.  The core stage
pressurant system is deleted.  After the strap-ons blow off
the core tanks are blowdown.  It's the Atlas idea again.

This screws up their idea of identical airframes, of course,
unless the pressurant system is easy to unbolt.

One easy way to get the core to continue burning past strap-on
separation would be to delete 4 engines in the core but fill
the tanks completely.  This reduces lift-off thrust by 17%,
but you get an extra half stage.

..it also means, btw, that that core stage reenters really
fast compared to a normal Falcon 5 or 9 launch.

Damon> For cost reasons, I would think the upper stage design
Damon> would be the same across all models.

This is the part I don't get.  Why not vary the length of the
tanks?  If the engines are so much more expensive than the
airframe, what's the harm of having idle Falcon 9 airframes
while the Falcon 5's are launching?  *Especially* if they are
thinking that they'll have non-reusable pricing for the
first couple of years.

Damon> Wonder if they are using milled wall on the Merlins?

Apparently the latest engine failure was due to a manufacturing
flaw in the ablative combustion chamber.  No milled wall or
tubes in there.

Damon> A Merlin in vacuum does 100,000 lbs/thrust; I believe
Damon> that's plenty for an upper stage in this weight class.

I think they've downrated that a bit.  I haven't run the math
on the F9, or especially the F9-S9, but the second stage in a
LEO insertion usually has about 1/7 the thrust of the first
stage.  That says F5 has an overly heavy engine (so they'll
just have to burn more gas :), F9 has a slightly small engine
(but maybe staging ratios are larger for LOX-kero upper stages),
and a single Merlin is way too small for F9-S9 to LEO.  F9-S9
to GEO could use a single Merlin though.

Damon> SpaceX plans a much bigger F-1 class engine for really
Damon> heavy lift vehicles.

Here's an interesting choice for them: what size engine?

Assume that you want engine-out capability on both first and
second stages of the heavy lifter.  That suggests 3 or 4
engines.  Unless the thing is insanely big, or you are doing
two new engines, you'll want 3 to 5 Merlins on the upper
stage.  That means the lower stage engines are around 7x the
size of a Merlin -- half the size of an F-1.

It's amazing to think the Saturn 5 second stage had about
the same thrust as a Falcon 9 will have.

Damon> I'm interested in their launch facility design for
Damon> Falcon 9 launchers; how will the vehicle be integrated
Damon> and transported to the pad?

Is there any fundamental problem with driving the stages to
the launch area seperately, assembling the vehicle on a rail
and then tilting it up?  Is the issue that the tilt-up
vehicle is no longer going to fit on the back of a semi or
on public roads?  The Army tools around with M1A1s on flatbed
trailers.  Would an unfueled F9 first stage weigh more than
an M1A1 tank (~120,000 pounds)?

Hmm... how do you move a 5.2 meter payload fairing to a
launch site?  I suspect that doesn't even qualify as a "wide
load".