Space Forum / Space Flight / September 2003

We had a similar discussion here under "Low mass ratio SSTO" .

I think people just want to write papers and justify large RD budgets
that end up being spent on people who do not work. They need big
ticket buzzwords like SSTO or fusion reactors. They just take the
money, promise something that's hard to do like a non cylindrical fuel
tank or a SCRAM jet engine and later they just say, oh well we did not
succeed. Of course many times they do not need to say a thing because
they get lucky and the project is cancelled before they are supposed
to deliver hardware.

Of course it is easy to build SSTO.

Zoltan

A LOX/Kero SSTO would be fairly straightforward (I
hesitate to say easy only because any major project,
even just digging a big ditch (like the Suez canal)
has its own difficulties and complexities).  I
perceive several reasons why such things aren't being
built now.  First, is history.  From the beginning,
orbital rocketry was pretty difficult.  The engines
available were terrible, and were it not for the
fact that liquified gasses and the means to handle
them in bulk had been developed since well before
the turn of the 20th century it might have been even
worse.  More so, building very strong structures
which are also very light is, and has been, a
substantial challenge.  If it were not that the
aircraft industry had been working in the same
direction about a step ahead of rocketry in the early
days (since large scale aircraft preceded large scale
rocketry by only a few decades), it would certainly
have been much, much worse.  Anywho, what all that
meant (poor engines, and clunky structures, even
when both were state of the art) was that SSTO was
impossible early on.  The early liquid fueled rockets
had Isps about as high as the best solid rockets do
today.  And especially in that Isp range with respect
to orbital rocketry any significant performance hit
bites pretty hard (an Isp of ~240s leads to a mass
fraction to reach orbital speed of *twice* that as
with an Isp of ~300s (which is the lower end of the
range of Isps for current Kero engines)).  Factor in
the unfamiliarity both with building very large
rockets (e.g. Saturn V class, since you get a hefty
advantage in dry mass fraction as you scale up) and
with building very low dry mass fraction rockets and
that led to the result that staged rockets were the
only way to get to orbit.  At the time.

Here's where the second point comes in, since rocketry
was so difficult early on and seemingly so fickle that
a kind of institutionilzed cult of accumulated
superstition arose.  More like the shipwrights of old
than, say, the cpu designers of today, rocket builders
were enormously leery of from-scratch designs and
instead opted for evolving, modifying, and scaling
old designs to get what they wanted.  And that led to
a whole host of ideas becoming fixed in the head of the
industrial and government aerospace establishment,
including things like LOX/LH2 being a superior fuel than
denser fuels, a fixation on minimizing GLOW in relation
to payload size, etc.  As the rocket industry developed,
rather than getting rid of old preconceptions, mostly
they just added new ones (such as the "inherent"
advantages of winged RLVs over capsules).  That inertia
has morphed itself into the hand-built, multi-stage
launch vehicles you see today and into the idea that
SSTO is a really difficult problem.

Third and finally, and this is the important point,
the launch vehicle business is already established and
current launch vehicles meet the needs of the customers
fairly well.  Brand new vehicles, especially of different
designs, would have to develop rather quickly and mold
themselves (at leas initially) more to the common
characteristics of current launch vehicles (especially in
terms of g-loading and vibration).  And that adds a
development burden to anyone wanting to do an SSTO that
would be commercially viable with existing markets.
Though there are emerging markets which may provide
useful niches.  One of the biggest problems of SSTOs is
that they have too much thrust.  When you take a single
stage with only a small payload and you take the engines
which are designed to lift it off the ground by brute
force with fuel tanks to the brim and you keep those
same engines on when the fuel tanks are empty (i.e. when
the vehicle plus payload weighs at most 1/7th as much
(for the best LOX/LH2 engines) or more likely less than
7% as much (for really good LOX/Kero engines)), you end
up with a hell of a lot of excess gees.  In fact, for a
manned LOX/Kero SSTO, if the engines weren't throttled
down the g-load at engine burnout would be pretty close
to lethal.  High g-loads are slightly more tolerable but
not all that much better for multi-million dollar pieces
of machinery, so it's a general problem.  So a usable
SSTO needs to be able to throttle *very* deeply.  And
that almost certainly means not just individual engines
with a large thrust range but being able to cutoff
engines as the vehicle depletes its fuel.  And that
adds complexity, difficulty, and cost to the design and
construction.

For example, your Titan-II w/ NK-33 SSTO has a mass-
ratio of 17.1:1, which means that the peak thrust at
burnout would be around 17 gees, give or take.  You
give a dry weight of about 7,000 kg, and one NK-33 has
a thrust in vacuum of abour 167,000 kg-f, so that gives
a max g-force of about 24 gees.  According to this page:
http://www.spaceandtech.com/spacedata/engines/nk33_specs.shtml

The NK-33 has a throttle range of 55 - 104% (the vac.
thrust data is from astronautix.com, FYI), so that gives
a minimum g-load at burnout of 12 gees.

As I said, fairly straightforward but not easy.

There is little doubt, among those willing to actually look at the facts,
that you can get a single-stage expendable into orbit -- at least, with
little or no payload -- and have been able to do so for some time.
Another example of that (without even exploiting the dense-fuels bonuses)
is what you get if you put six SSMEs on the base of a shuttle ET; that one
actually should be able to carry quite a substantial payload.

The "reusable" part can legitimately be questioned.  The assumptions you
made about how much various things add to the dry mass are highly
debatable.  For example, assuming that wings add only 7% is awfully
optimistic.

It is very difficult to actually *resolve* disagreements about basic
assumptions when the battle is viewgraphs vs. viewgraphs.  Nothing short
of flying hardware will settle such disputes.  Claims that LOX/LH2 rockets
are inherently difficult and complex to operate were quite persistent for
years, but quietly evaporated when DC-X started flying.

The problem is that the NK-33 is a russian engine. NASA would *never* build
a craft with a russian engine, and the russians themselves lack sufficient
funding to do something interesting with their huge stockpile of fantastic
engines. They also lack the technology to build very lightweight fuel
tanks. Private (or at least semi-private) entities such as Boeing and
LockMart have used russian engines very successfully (Sea Launch and Atlas
V), but they are too conservative to develop something revolutionary.

So if you can convince someone to build a simple cylindrical design with
NK-33 engines, then SSTO with significant payload is quite doable. Reusable
SSTO should also be doable if you use vertical landing. And even a VTVL
SSTO space transport with zero payload would be very useful since you could
use it as a testbed for various technologies and get a quite significant
payload by just adding a few booster rockets.

The problem is not technical but political. This is kind of sad since most
of us s.s.t regulars really enjoy technical discussions like Kerosene vs.
LH2, VTVL vs. VTHL vs HTHL, pure rocket vs. airbreather, SSTO vs. TSTO and
so on. But such discussions will not get us closer to CATS unless somebody
can pony up the money to actually build it. NASA will not do it for us.

Please review this thread for another, more detailed, SSTO concept using
NK-33s and a wet wing.

http://www.google.com/groups?hl=en&lr=&ie=UTF-8&oe=UTF-8&selm=5d2ar5%24fs5%40new
s2.delphi.com&rnum=1

I hope pasting this will work. If not, then I will paste in the entire
article. Thanks,

BobDL

Please review this thread for another, more detailed, SSTO concept
using
NK-33s and a wet wing.

http://www.google.com/groups?hl=en&lr=&ie=UTF-8&oe=UTF-8&selm=5d2ar5%24fs5%40new
s2.delphi.com&rnum=1

I hope pasting this will work. If not, then I will paste in the entire
article. Thanks,

BobDL