🚀 go-pugleaf

In article <ewright.726515610@convex.convex.com> ewright@convex.com (Edward V. Wright) writes:
>The "extreme mass ratio" is an aerospace legend.  We've been
>building vehicles with similar mass ratios for the last 30
>years.  The Shuttle external tank has the right mass.  So
>did the Saturn S-IVB stage.

I would love to see some convincing evidence of this, as this is the
key risk to SSTO in my opinion -- If I get sold of this, I'll try to 
convince my Congresscritter.

Unfortunately, these are not convincing.  I agree it is not a problem
to build just any old structure with mass ratio >>12 (the required range
for H2/O2 rockets).  Aircraft exist that do this (gliders, for example).
Even reusable craft.

However, there are contributors to "dry mass" which SSTO will have which
your test cases won't.  Using OLD technology, these are very significant,
enough to kill your mass ratio.  Using NEW technology, NASP derived in
part, I certainly hope the mass ratio will climb over breakeven point, but
I still regard it as undemonstrated, nor have I seen the evidence which
shows it to be a "low risk" goal.

The contributors I see as significant:

1) A thermal protection system, capable of surviving multiple reentries
   (many) suitable for reuse.  On previous vehicles, this is ablative and
   non reusable, or (on Shuttle), heavy and only marginally reusable.
   NASP composites may combine hull structure with thermal protection, but
   they are new and carry some risk.

2) Low mass cryogenic tanks and insulation.  Yes, the SS ET has the right
   mass ratio BY ITSELF.  However, you have to carry all that tank volume
   IN ADDITION to all the other vehicle dry mass -- this looks slightly
   less attractive, to say the least.  Again, wound composite tanks are
   expected to help here -- but they are new and carry some risk.

3) All the "extra stuff" affiliated with onboard power generation, life
   support, airlocks, etc.  This seems to me to add up amazingly fast with
   a manned presence on board.  This isn't tough, and is traditionally a
   relatively small contributor, but it seems to loom large if we assume,
   in (1) and (2), that the other contributors diminish.  

Now, when I say there is risk, this does NOT mean SSTO is not possible,
nor does it mean it's not a good idea.  It just means it's not a sure
thing, nor is it trivially obvious that just "trying to do it" will 
achieve the desired result.  Hats off to the engineers if they suceed!
However, I have not seen any convincing argument, based on technology
trends, or detailed analyses of what technological advances are being
depended upon, that the risks are "tolerable" in the sense of a profit
seeking project.

Disclaimer:  All opinions expressed are my own, and do not reflect the 
     position of Intel, Portland State University, or Zippy the Pinhead.  
============================================================================
Jeff Greason                  "You lock the door ... And throw away the key.
  <greason@ptdcs2.intel.com>   There's someone in my head, but it's not me."
  <jeffg@eecs.ee.pdx.edu>				-- Pink Floyd