From: henry@spsystems.net (Henry Spencer)
Newsgroups: sci.space.tech
Subject: Re: The cost (in weight) for Reusable SSTO
Date: Sun, 28 Mar 1999 22:37:10 GMT

In article <kemJ2.876$Vc2.18603@news-west.eli.net>,
Larry Gales <larryg@u.washington.edu> wrote:
>An SSTO with a useful payload using Kero/LOX is easy to do -- provided that
>it is *expendable*.  All of the difficulty lies in making it reusable...

There are people who are sufficiently anti-SSTO that they will dispute the
feasibility of even expendable SSTOs (apparently not having read the specs
for the Titan II first stage carefully).

>   (1) De-orbit fuel: I understand that it takes about 100 m/s to de-orbit.

That's roughly right.  Of course, in favorable circumstances you could play
tricks like using a tether to simultaneously boost a payload higher and
de-orbit your vehicle.  (As NASA's Ivan Bekey pointed out, this is one case
where the extra dry mass of a reusable vehicle is an *advantage*, because
the heavier the vehicle, the greater the boost given to the payload.)

>   (2) TPS (heat shield): the figures I hear for this are around 15% of the
>orbital mass

Could be... but one should be very suspicious of this sort of parametric
estimate.  It's often possible to beat such numbers, often by quite a large
margin, by being clever and exploiting favorable conditions.  Any single
number for TPS in particular has a *lot* of assumptions in it.

>   (4) Landing gear: about 3%

Gary Hudson pointed out a couple of years ago that, while 3% is common
wisdom, the B-58 landing gear was 1.5%... and that was a very tall and
mechanically complex gear designed in the 1950s.  See comment above
about cleverness.

I would be very suspicious of any parametric number for landing gear which
doesn't at least distinguish between vertical and horizontal landing.

>   (5) Additional structure to meet loads from differnet directions (e.g.,
>vertical
>        takeoff, semi-horizontal re-enttry, horizontal landing).  This is
>purely
>        guesswork on my part, but I assume about 8%

Of course, here the assumptions are up front:  you're assuming a flight
profile that many of us would say is simply inferior -- overly complex,
difficult to test incrementally, and hard on the structure.

>I would appreciate it if anyone could supply more accurate figures.

More accurate figures either have to be for a specific vehicle design,
or are so hedged about with assumptions that they are nearly meaningless.
--
The good old days                   |  Henry Spencer   henry@spsystems.net
weren't.                            |      (aka henry@zoo.toronto.edu)

From: henry@spsystems.net (Henry Spencer)
Newsgroups: sci.space.tech
Subject: Re: The cost (in weight) for Reusable SSTO
Date: Tue, 30 Mar 1999 04:15:51 GMT

In article <36FFC7D3.A89DB1E4@et.byu.edu>,
Jonathan A Goff  <jongoff@et.byu.edu> wrote:
>> ...one should be very suspicious of this sort of parametric
>> estimate.  It's often possible to beat such numbers, often by quite a large
>> margin, by being clever and exploiting favorable conditions.  Any single
>> number for TPS in particular has a *lot* of assumptions in it.
>
>Of course, if you cut the mass very far, you usually will
>rapidly hit diminishing returns and drive costs up pretty
>high.

Yes and no.  It depends on whether you are cutting the mass by shaving
grams off an existing design -- in which case that's true -- or by finding
a new way of doing things which is inherently lighter.
--
The good old days                   |  Henry Spencer   henry@spsystems.net
weren't.                            |      (aka henry@zoo.toronto.edu)

From: henry@spsystems.net (Henry Spencer)
Newsgroups: sci.space.tech
Subject: Re: The cost (in weight) for Reusable SSTO
Date: Wed, 31 Mar 1999 23:57:27 GMT

In article <37026475.7A250E61@et.byu.edu>,
Jonathan A Goff  <jongoff@et.byu.edu> wrote:
>> Yes and no.  It depends on whether you are cutting the mass by shaving
>> grams off an existing design -- in which case that's true -- or by finding
>> a new way of doing things which is inherently lighter.
>
>Shaving grams off an existing design is always expensive,
>however redesigning usually also is pretty expensive.

If you do it early enough, it's not bad.  One of the major errors in the
shuttle design process was the unwillingness to tear up the drawings and
redesign, even when the constraints had changed enough that the chosen
design was clearly not working out.  There were at least two occasions
when that *should* have been done.  See N.S. Norway's comments about the
R-100 vs. R-101.

For those who don't recognize that reference...  They were the last major
airships built in Britain.  The R-100, built by private industry, was
badly underfunded but the designers were free to change their minds
repeatedly, and did.  The R-101, built by a government lab, had lavish
funding, but once the designers had made up their minds about something,
they were stuck with it.  The R-100 made a successful, although not
entirely problem-free, maiden voyage to Canada.  The R-101 had trouble
making it into the air at all, overran its schedule and budget badly, and
on its maiden voyage to India, crashed in France killing all aboard.
--
The good old days                   |  Henry Spencer   henry@spsystems.net
weren't.                            |      (aka henry@zoo.toronto.edu)

From: gchudson@aol.com (GCHudson)
Newsgroups: sci.space.tech
Subject: Re: The cost (in weight) for Reusable SSTO
Date: 31 Mar 1999 22:22:44 GMT

The rule of thumb for gear is usually 4%.  I used to do contract work for
Boeing. ;)

The B-58 gear were used a lot more than once.  A few hundred uses is a big
number for a resuable SSTO HTOL.

Gary C. Hudson

From: henry@spsystems.net (Henry Spencer)
Newsgroups: sci.space.tech
Subject: Re: The cost (in weight) for Reusable SSTO
Date: Wed, 31 Mar 1999 23:42:42 GMT

In article <37017430.50DF@erols.com>,
Brian Pickrell  <emphyrio@erols.com> wrote:
>> Gary Hudson pointed out a couple of years ago that, while 3% is common
>> wisdom, the B-58 landing gear was 1.5%...
>
>Although it's possible to be TOO clever...
>"...Enormous amounts of energy, up to ten million foot-pounds per
>brake, may be absorbed in a normal landing. Even higher energies may
>heat the brakes to
>such a degree that tires or hydraulic fluid may ignite..."

Problems with this sort of thing were not unknown even in airliners at the
time.  One of the mandatory flight tests for a new airliner, these days,
is to load it up to maximum takeoff weight, wind it up to maximum
rejected-takeoff speed on the runway, and slam on the brakes.  And then,
after the aircraft shudders to a halt, you sit there for ten minutes,
motionless, to minimize cooling airflow over the brakes and wheels.  If
any of the tires blow in that waiting period, you flunk the test and it's
back to the drawing board for some design changes.

Aviation Week published a closeup photo of the 747-400 prototype at the
beginning of the waiting period in that test.  The brakes were glowing
orange-hot.

>I don't have any special knowledge beyond what I read on this site, but
>it sounds like the Convair designers achieved this low weight by cutting
>safety margins to a knife edge...

Not really.  While their margins unquestionably were lower than those
deemed appropriate in other areas of aviation -- the pilot did have to
know what he was doing, and watch his step in certain respects -- aircraft
brakes, tires, and landing gear in general lead much harsher lives than
people tend to think.  See above.

>I think that any design that Gary Hudson is interested in should be
>designed for operational flexibility and multiple reuse; so shaving
>landing gear weight at the expense of reusability, safety, and runway
>length requirements should be looked at vary suspiciously.

Indeed so (although runway length is irrelevant for a vertical lander, of
course).  But as I've commented before, there is a difference between
shaving weight by reducing margins -- somewhat of a NASA specialty -- and
reducing weight by clever design which retains good margins.  Just because
it's always been done as X, does not mean X is optimum.  In rocketry in
particular, a lot of the common wisdom is *not* the result of exhaustive
investigation of alternatives, but rather a case of "well, we did it that
way once and it worked, so we stuck with it".

>Basically,
>the more mass there is in the landing gear, the more energy it can
>dissipate.

Other things being equal.  But other things are *not* always equal.
--
The good old days                   |  Henry Spencer   henry@spsystems.net
weren't.                            |      (aka henry@zoo.toronto.edu)

From: gherbert@crl3.crl.com (George Herbert)
Newsgroups: sci.space.tech
Subject: Re: The cost (in weight) for Reusable SSTO
Date: 31 Mar 1999 18:17:11 -0800

Henry Spencer <henry@spsystems.net> wrote:
>>Although it's possible to be TOO clever...
>>"...Enormous amounts of energy, up to ten million foot-pounds per
>>brake, may be absorbed in a normal landing. Even higher energies may heat
>>the brakes to such a degree that tires or hydraulic fluid may ignite..."
>
>Problems with this sort of thing were not unknown even in airliners at the
>time.  One of the mandatory flight tests for a new airliner, these days,
>is to load it up to maximum takeoff weight, wind it up to maximum
>rejected-takeoff speed on the runway, and slam on the brakes.  And then,
>after the aircraft shudders to a halt, you sit there for ten minutes,
>motionless, to minimize cooling airflow over the brakes and wheels.  If
>any of the tires blow in that waiting period, you flunk the test and it's
>back to the drawing board for some design changes.

I think you got the test details wrong; you have to stop the aircraft
completely, then taxi it a certain distance (to get it off the runway,
clear the airport for other operations), tire blowouts are permissible
(and expected, in the later stages, and fires aren't unknown) but the
aircraft still has to be able to taxi the required distance after the
complete stop regardless of blowouts or fires.  They usually blow
*all* the tires (the plugs in the tires, at least), sometimes have
fires on one or more gear, and usually have to replace brake parts,
hubs, etc as well as the tires.


-george william herbert
Retro Aerospace
gherbert@retro.com   gherbert@crl.com