Re; Response from CoB of Boeing on SSTO ...
  
   Boeing has been looking at several TSTO vehicles and has carried
out extensive conceptual studies of advanced launch systems for some
time.  A good reference on this might be: "Comparison of Propulsion
Options for Advanced Earth-To-Orbit (ETO) Applications (IAF-92-
0639)." by V.A. Weldon and L.E. Fink from Boeing.   The paper
describes a propane-fueled TSTO launch system claimed to achieve
aircraft-like operational efficiencies without the problems
associated with liquid hydrogen fuel.  Basically, it's a high-speed
airplane launching a Hermes-type spaceplane
   The design (the concept is also called "Beta") as laid out in the
paper can launch at least 10,000 pounds into polar orbit, or 20,000
pounds to space station orbit including a crew of eight persons and
life support.  System design reliability is .9995.
   Beta is a 360-foot-long first stage powered by two large ramjets
and 12 high- speed civil transport (HSCT) turbofans.  A 108-foot-
long reusable orbiter is trapeze-mounted in the belly of the first-
stage aircraft, which also could accommodate a longer and heavy
payload on an expendable second stage.
   To launch the orbital vehicle, the first stage takes off like a
normal HSCT and accelerates to Mach 3.  At that point the turbofans,
modified to burn catalyzed JP-7, would shut off and the ramjets,
would take over.  At Mach 5.5 the orbiter or the ELV would swing
out, ignite and proceed to orbit. Both vehicles would land like
aircraft at the conclusion of their respective missions.
   Estimated total weight of the combined configuration at takeoff
is about 1.5 M lbs, roughly equivalanet to a fully loaded An-225.
The orbiter stages weighs about 400,Klbs including 335 Klbs
of LOX and subcooled propane to power two 250 Klbs vacuum thrust
rocket engines. Propellants would be stored at 91 degrees Kelvin,
with the propane in a spherical tank mounted forward of the 15-by-
25-foot cargo bay and the two-seat orbiter crew station. LOX would
be stored aft.  Weldon and Fink claim the key to this design's
success is the structurally efficient airframe and the compact
tankage allowed by the high-density supercooled hydrocarbon fuel.
     The paper compares TSTO design to SSTO design.  They conclude
while a SSTO has a slightly lower recurring cost, a TSTO is easier,
cheaper, and less risky to develop, simpler to build, has greater
safety and mission versatility and doesn't carry the hard-to-handle
and bulky hydrogen fuel. The conlcude "In conjunction with its major
use of airplane type engines and fuel, as well as its inherent self-
ferry capability, it is probably the system most likely to provide
as close to airline-like operations as possible with a practical
configuration, until a single stage airbreather/rocket concept can
be shown to be operationally viable."
  
   Weldon and others at Boeing have been working on TSTO designs for
some time.  I expect this, or a similar concept (perhaps the HTHL
SSTO they proposed for the SDIO SSTO first phase) is being re-
examined as a basis for a bid on the first phase of SpaceLifter.
   Does it threaten DC-???.  Possibly -- There is a set of on-going
studies trying straighten out the government's future space
transportation strategy.  MDC and Boeing (as well as other firms)
are providing data to a joint study team back in DC.  There are
various factions and options vying for attention -- including
shuttle upgrades, shuttle replacement (what was called the "4-2-3"
architecture), SpaceLifter, ELV upgrades, and various advanced
vehicles (ALES, Beta, DC-??, NASP, FSTS, SSTOs of several types,
etc.)  NASA/DOD/DOT are trying to put together a coherent strategy
for future US gov't space transportation systems, and trying to
juggle near-term launch needs (like for DoD and NASA) against
medium-term needs (including commercial considerations), and against
the investment and risk of going to "leap frog" new technologies
like SDIO/SSTO and NASP and Beta.
   It's a heck of a problem.  The worst part of the problem isn't
that there aren't promising ideas and concepts -- there are dozens
of them -- but how they balance cost and risk versus real needs in
the near term.  They should have a draft report in mid-June, with a
final report coming by the end of the fiscal year.
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 Wales Larrison                           Space Technology Investor
  