Space & Astronomy/Space and Astronomy (October 1993).iso/mac/TEXT/SPACEDIG/V15_4/V15NO476.TXT

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Date: Tue,  1 Dec 92 05:02:47
From: Space Digest maintainer <[e-mail]>
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Subject: Space Digest V15 #476
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Space Digest                Tue,  1 Dec 92       Volume 15 : Issue 476

Today's Topics:
                          Another SSTO paper
                  Comparative Launcher Reliabilities
                       escape systems (2 msgs)
                hubble time, toutatis, swift-tuttle...
            hypergolics (was Re: Pumpless Liquid Rocket?)
             Kuiper belt planetesimals and Planet X claim
 Observing Toutatis (was Re: US/World News in Brief Nov 26 10 am PST)
                     Shuttle replacement (3 msgs)

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Date: 30 Nov 92 16:06:40 GMT
From: "Allen W. Sherzer" <[e-mail]>
Subject: Another SSTO paper
Newsgroups: sci.space

Here is another paper for the Freshmen Orientation written by Henry
Spencer.

   Allen
-------------------------------------


(Semi-)Technical Aspects of SSTO by Henry Spencer

This paper will try to give you some idea of why SSTO makes technical
sense and is a reasonable idea.  We'll concentrate on the overall issues,
trying to give you the right general idea without getting bogged down
in obscure detail.  Be warned that we will oversimplify a bit at times.

Why Is SSTO Challenging?

Getting a one-stage reusable rocket into orbit doesn't look impossible,
but it does look challenging.  Here's why.

The hard part of getting into orbit is not reaching orbital altitude,
but reaching orbital velocity.  Orbital velocity is about 18,000mph.
To this, you have to add something for reaching orbital altitude and for
fighting air resistance along the way, but these complications don't
actually add very much.  The total fuel requirement
is what would be needed to accelerate to 20-21,000mph.

So how much is that?  (If you don't want to know the math, skip to the
next paragraph for the results.)  The "rocket equation" is
desired_velocity = exhaust_velocity * ln(launch_weight / dry_weight),
where "ln" is the natural logarithm.  The exhaust velocity is determined
by choice of fuels and design of engines, but 7,000mph is about right
if you don't use liquid hydrogen, and 10,000mph if you do.

The bottom line is that the launch weight has to be about 20 times the
dry weight (the weight including everything except fuels) if you don't
use liquid hydrogen, and about 8 times the dry weight if you do.  This
sounds like hydrogen would be the obvious choice of fuel, but in practice,
hydrogen has two serious problems.  First, it is extremely bulky,
meaning that hydrogen tanks have to be very big; the Shuttle External
Tank is mostly hydrogen tank, with only the nose containing oxygen.
Second, some of the same properties that make hydrogen do well on the
weight ratio make it difficult to build hydrogen engines with high thrust,
and a rocket *does* need enough thrust to lift off!  Both of these
problems tend to drive up the dry weight, by requiring bigger and heavier
tanks and engines.

So how bad is this?  Well, it's not good.  Even with hydrogen, an SSTO
launcher which weighs (say) 800,000lbs at launch has to be 7/8ths fuel.
We've got 100,000lbs for tanks to hold 700,000lbs of fuel, engines to
lift an 800,000lb vehicle, a heatshield to protect the whole thing on
return, structure to hold it all together at high acceleration... and
quite incidentally, for some payload to make it all worthwhile.  Most
of the dry weight has to go for the vehicle itself; only a small part
of it can be payload.  (That is, the "payload fraction" is quite small.)
To get any payload at all, we need to work hard at making the vehicle
very lightweight.

The big problem here is:  what happens if the vehicle isn't quite as
light as the designer thought it would be?  All rockets, and most aircraft
for that matter, gain weight during development, as optimistic estimates
are replaced by real numbers.  An SSTO vehicle doesn't have much room for
such weight growth, because every extra pound of vehicle means one less
pound for that small payload fraction.  Particularly if we're trying to
build an SSTO vehicle for the first time, there's a high risk that the
actual payload will be smaller than planned.

That is the ultimate reason why nobody has yet built an SSTO space
launcher:  its performance is hard to predict.  Megaprojects like the
Shuttle can't afford unpredictability -- they are so expensive that
they must succeed.  SSTO is better suited to an experimental vehicle,
like the historic "X-planes", to establish that the concept works and
get a good look at how well it performs... but there is no X-launcher
program.

Why Does SSTO Look Feasible Now?

The closest thing to SSTO so far is the Atlas expendable launcher.  The
Atlas, without the Centaur upper stage that is now a standard part of
it, has "1.5" stages:  it drops two of its three engines (but nothing
else) midway up.  Without an upper stage, Atlas can put modest payloads
into orbit:  John Glenn rode into orbit on an Atlas.  The first Atlas
orbital mission was flown late in 1958.  But the step from 1.5 stages
to 1 stage has eluded us since.

Actually, people have been proposing SSTO launchers for many years.
The idea has always looked like it *just might* work.  For example,
the Shuttle program looked at SSTO designs briefly.  Mostly, nobody has
tried an SSTO launcher because everybody was waiting for somebody else
to try it first.

There are a few things that are crucial to success of an SSTO
launcher.  It needs very lightweight structural materials.  It needs
very efficient engines.  It needs a very light heatshield.  And it
needs a way of landing gently that doesn't add much weight.

Materials for structure and heatshield have been improving steadily
over the years.  The NASP program in particular has helped with this.
It now looks fairly certain that an SSTO can be light enough.

Existing engines do look efficient enough for SSTO, provided they can
somehow adapt automatically to the outside air pressure.  The nozzle
of a rocket engine designed to be fired in sea-level air is subtly
different from that of an engine designed for use in space, and an
SSTO engine has to work well in both conditions.  (The technical
buzzword for what's wanted is an "altitude-compensating" nozzle.)
Solutions to this problem actually are not lacking, but nobody has
yet flown one of them.  Probably the simplest one, which has been
tentatively selected for DC-Y, is just a nozzle which telescopes,
so its length can be varied to match outside conditions.  Making
nozzles that telescope is not hard -- many existing rocket nozzles,
like those of the Trident missile, telescope for compact storage --
but nobody has yet flown one that changes length *while firing*.
However, it doesn't look difficult, and there are other approaches
if this one turns out to have problems.

We'll talk about landing methods in more detail later, but this is one
issue that will be resolved pretty soon.  The primary goal of the DC-X
experimental craft is to fly DC-Y's landing maneuvers and prove that
they will work.

So... with materials under control, engines looking feasible, and
landing about to be test-flown, we should be able to build an SSTO
prototype:  DC-Y.  The prototype's performance may not quite match
predictions, but if it works *at all*, it will make all other launchers
obsolete.

Why A Rocket?

As witness the NASP (X-30) program, air-breathing engines do look like
an attractive alternative to rockets.  Much of the weight of fuel in
a rocket is oxygen, and an air-breathing engine gets its oxygen from
the air rather than having to carry it along.  However, on a closer
look, the choice is not so clear-cut.

The biggest problem of using air-breathing engines for spaceflight is
that they simply don't work very well at really high speeds.  An
air-breathing engine tries to accelerate air by heating it.  This works
well at low speed.  Unfortunately, accelerating air that is already
moving at hypersonic speed is difficult, all the more so when it has
to be done by heating air that is already extremely hot.  The problem
only gets worse if the engine has to work over an enormous range of
speeds:  NASP's scramjet engines would start to function at perhaps
Mach 4, but orbital speeds are roughly Mach 25.  Nobody has ever built
an air-breathing engine that can do this... but rockets do it every week.

Air-breathing engines have other problems too.  For one thing, to use
them, one obviously has to fly within the atmosphere.  At truly high
speeds, this means major heating problems due to air friction.  It
also means a lot of drag due to air resistance, adding to the burden
that an air-breathing engine has to overcome.  Rocket-based launchers,
including SSTO, do most of their accelerating in vacuum, away from
these problems.

Perhaps the biggest problem of air-breathing engines for spaceflight
is that they are *heavy*.  The best military jet engines have thrust:weight
ratios of about 8:1.  (This is at low speed; hypersonic scramjets are not
nearly that good.)  The Space Shuttle Main Engine's thrust:weight ratio,
by comparison, is 70:1 (at any speed).  The oxygen in a rocket's tanks
is burned off on the way to orbit, but the engines have to be carried
all the way, and air-breathing engines weigh a lot more.

And what's the payoff?  The X-30, if it is built, and if it works
perfectly, will just be able to get into orbit with a small payload.
This is about the same as SSTO, at ten times the cost.  Where is the
gain from air-breathing engines?

The fact is, rockets are perfectly good engines for a space launcher.
Rockets are light, powerful, well understood, and work fine at any
speed without needing air.  Oxygen may be heavy, but it is cheap (about
five cents a pound) and compact.  Finally, rocket engines are available
off the shelf, while hypersonic air-breathing engines are still research
projects.  Practical space launchers should use rockets, so SSTO does.

Why No Wings?

With light, powerful engines like rockets, there is no need to land
or take off horizontally on a runway, and no particular reason to.
Runway takeoffs and landing are touchy procedures with little room
for error, which is why a student pilot spends much of his time on
them.  Given adequate power, vertical takeoffs and landings are easier.
In particular, a vertical landing is much more tolerant of error than
a horizontal one, because the pilot can always stop, straighten out
a mistake, and then continue.  Harrier pilots confirm this:  their
comment is "it's easier to stop and then land, than to land and then
try to stop".

What if you don't have adequate power?  Then you are in deep trouble
even if your craft takes off and lands horizontally.  As witness the
El Al crash in Amsterdam recently, even airliners often don't survive
major loss of power at low altitude.  To make a safe horizontal landing,
especially in less-than-ideal weather conditions, you *must* have enough
power to abandon a bad landing approach and try again.  Shuttle-style
gliding landings are dangerous, and airline crews go to great lengths
to avoid them; the Shuttle program, with the nation's best test pilots
doing the flying and no effort spared to help them, has already had
one near-crash in its first fifty flights.  Routine access to space
requires powered landings.

If we are going to rely on powered landings, we must make sure that power
will be available.  Airliners do this by having more than one engine,
and being able to fly with one engine out.  SSTO is designed to survive
a single engine failure at the moment of liftoff, and a second failure
later.  Since (at least) 7/8ths of the takeoff weight of SSTO is fuel,
it will be much lighter at landing than at takeoff.  Given good design,
it will have enough power for landing even if several engines fail.
If SSTO has an engine failure soon after liftoff, it will follow much
the same procedure as an airliner:  it will hover to burn off most of
its fuel (this is about as quick as an airliner's fuel dumping), and
then land, with tanks nearly empty to minimize weight and fire hazard.

Note that in an emergency, vertical landing has one major advantage
over horizontal landing:  horizontal landing requires a runway, preferably
a long one with a favorable wind, while a vertical landing just requires
a small flat spot with no combustible materials nearby.  A few years ago,
a Royal Navy Harrier pilot had a major electronics failure and was unable
to return to his carrier.  He made an emergency landing on the deck of a
Spanish container ship.  The Harrier suffered minor damage; any other
aircraft would have been lost, and the pilot would have had to risk
ejection and recovery from the sea.

Given vertical landing and takeoff, is there any other use for wings?
One:  crossrange capability, the ability to steer to one side during
reentry, so as to land at a point that is not below the orbit track.
The Shuttle has quite a large crossrange capability, 1500 miles.
However, if we examine the history of the Shuttle, we find
that this was a requirement imposed by the
military, to make the Shuttle capable of flying some demanding USAF missions.
A civilian space launcher needs a crossrange capability of, at most, a
few hundred miles, to let it make precision landings at convenient times.
This is easily achieved with a wingless craft:  the Apollo spacecraft
could do it.

Finally, wings are a liability in several important ways.  They are heavy.
They are difficult to protect against reentry heat.  And they make the
vehicle much more susceptible to wind gusts during landing and takeoff
(this is a significant limitation on shuttle launches).

SSTO does not need wings, would suffer by carrying them, and hence does
not have them.

Why Will It Be Cheap And Reliable?

This is a good question.  The Shuttle was supposed to be cheap and
reliable, and is neither.  However, there is reason for hope for SSTO.

The Shuttle's costs come mainly from the tremendous army of people
needed to inspect and refurbish it after each flight.  SSTO should get
by with many fewer.

The basic SSTO concept opens major possibilities for simple, quick
refurbishment.  With no discarded parts, nothing needs to be replaced.
With no separating parts, there is no need to re-assemble anything.
In principle, an SSTO vehicle should be able to "turn around" like
an airliner, with little more than refuelling.

Of course, this is easier said than done.  But there is no real reason
why SSTO should need much more.  Its electronics experience stresses
not much worse than those of an airliner -- certainly no worse than
those of a jet fighter.  Its structure and heatshield, designed to fly
many times, will have sufficient margins that they will not need
inspection and repair after every flight.  Most space-vehicle components
don't inherently need any more attention than airliner components.

The one obvious exception is the engines, which do indeed run at much
higher power levels than airliner engines.  But even here, airliner
principles can be applied:  the way to make engines last a long time
is to run them at less than 100% power.  SSTO engines have it easy in
one respect:  they only have to run for about ten minutes at the start
of the flight and two or three minutes at the end.

Still, the Shuttle engines certainly are not a shining example of low
maintenance and durability.  However, it's important to realize that
the Shuttle engines are not the only reusable rocket engines.  Most
liquid-fuel engines could be re-used, were it not that the launchers
carrying them are thrown away after every flight.  And the durability
record of these other engines -- although limited to test stands -- is
*much* better.  The RL-10 engine, which will be used in DC-X, is rated
to fire for over an hour, in one continuous burn or with up to ten

restarts, with *no* maintenance.  Several other engines have comparable
records.  Conservatively-designed engines are nowhere near as flakey
and troublesome as the Shuttle engines.

Here again, DC-X should soon supply some solid evidence.  Although its
engines and other systems are not the same ones that DC-Y would use,
they should be representative enough to demonstrate rapid, low-effort
refurbishment, and the DC-X program will try to do so.

Airliners typically operate at about three times fuel costs.  The fuel
cost for an SSTO vehicle would be a few dollars per pound of payload.
It may be a bit optimistic to try to apply airline experience to the
first version of a radically new vehicle.  However, even advanced
aircraft typically cost no more than ten times fuel cost.  Even if
SSTO comes nowhere near these predictions, it should still have no
trouble beating existing launchers, which cost several thousand dollars
per pound of payload.

We can look at this another way:  head counts.  Airlines typically have
about 150 people per aircraft, and most of those sell tickets or look
after passengers' needs.  Perhaps a better example is the SR-71, which
is like SSTO in that it was an advanced craft, pushing the frontiers
of technology, operated in quite small numbers.  Although it is hard
to get exact numbers because of secrecy, it appears that USAF SR-71
operations averaged perhaps one flight per day, using perhaps eight
flight-ready aircraft, with a total staff of about 400 people.  That's
50 per aircraft.  If SSTO can operate at such levels -- and there is
every reason to think it can -- it should have no trouble beating
existing launchers, which typically have several thousand people
involved in preparations for each and every launch.  (NASA's Shuttle
ground crew is variously estimated at 6,000-10,000 for a fleet of
four orbiters flying about eight flights a year.)

As for reliability, the crucial reason for thinking that SSTO will do
a lot better than existing launchers is simple:  testing.  It should
be feasible and affordable to test an SSTO launcher as thoroughly as
an aircraft.  This is *vastly* more thorough than any launcher.  The
F-15 fighter flew over 1,500 test flights before it was released for
military service.  No space launcher on Earth has flown that many
times, and the only one that even comes close is an old Soviet design.
It is no wonder that the Shuttle is somewhat unreliable, when it was
declared "operational" after a grand total of four test flights.
By aircraft standards, the Shuttle is still in early testing.  Some
expendable launchers have been declared operational after *two* tests.

Each and every SSTO vehicle can be tested many times before it carries
real payloads.  Moreover, since SSTO can survive most single failures,
it can be tested under extremes of flight conditions, like an aircraft.
For example, unlike Challenger, an SSTO vehicle would launch with
passengers and cargo in freezing temperatures only after multiple
test flights in such conditions.  There will always be surprises when a new
craft is flown in new conditions, but SSTO should encounter -- and
survive -- most of them in test flights.

Conclusion

Although there is reason for some uncertainty about the exact performance
of the first SSTO spacecraft, the basic approach being taken is sensible
and reasonable.  It should work.  The imminent test flights of the DC-X
test craft should resolve most remaining technical concerns.  Nobody can
be sure about costs and reliability until DC-Y is flying, but there is
reason to believe that SSTO should be much better than current launchers.
If the program is carried through to a flying DC-Y prototype in a timely
way, it really could revolutionize spaceflight.

--
+---------------------------------------------------------------------------+
| Allen W. Sherzer | "A great man is one who does nothing but leaves        |
|  [e-mail]     |  nothing undone"                                       |
+----------------------145 DAYS TO FIRST FLIGHT OF DCX----------------------+

------------------------------

Date: 30 Nov 92 16:56:31 GMT
From: Claudio Egalon <[e-mail]>
Subject: Comparative Launcher Reliabilities
Newsgroups: sci.space

I am wondering if it makes sense at all to compare reliability of a
manned rate spacecraft with unmanned rate spacecraft. Of course, a
manned rate spacecraft is supposed to be more reliable...

Claudio O. Egalon

------------------------------

Date: 30 Nov 92 14:48:52 GMT
From: Chris Jones <[e-mail]>
Subject: escape systems
Newsgroups: sci.space

In article <[e-mail]>, BrianT@cup (Brian Stuart Thorn) writes:
>         when Gemini 6 misfired at T+1 second in 1965, Wally Schirra
>   opted to stay on top of the Titan rather than use the ejection seats.
>   This despite the fact that Titan was fully fueled, a few inches off
>   the launch cradle, and the engines had conked-out. That's not exactly
>   a testament to the Gemini escape system.

It's probably a testament to Schirra's innate self-confidence in his ability to
detect that the stack had NOT lifted off, despite what his instruments were
telling him.  Although the engines had ignited, there was no lift off.  An
electrical plug had shaken loose, which was supposed to be a fail-safe
indicator of liftoff.  Schirra has said that he didn't think they had moved, so
he held off ejecting.  It was the correct decision.

I don't know how many other times aborting has been considered.  I know that
when Apollo 12 was struck by lightning, the thought crossed Conrad's mind.  In
_Carrying the Fire_, Mike Collins relates how he pointed out to Neil Armstrong
that he had a piece of his suit hooked around the abort handle, and the
reaction he imagined an accidental abort would get in the papers.
--
Chris Jones    [e-mail]

------------------------------

Date: 30 Nov 92 18:52:46 GMT
From: Pat <[e-mail]>
Subject: escape systems
Newsgroups: sci.space

In article <[e-mail]> [e-mail] (Brian Stuart Thorn) writes:
>
>   Well just a nitpick here, but Gemini and Space Shuttle both used
>   Ejection Seats and survival in either system was consider very low.
>   In fact, when Gemini 6 misfired at T+1 second in 1965, Wally Schirra
>   opted to stay on top of the Titan rather than use the ejection seats.
>   This despite the fact that Titan was fully fueled, a few inches off
>   the launch cradle, and the engines had conked-out. That's not exactly
>   a testament to the Gemini escape system.
>
>   -Brian
What should be remembered is that ejection seats are the worst way to
egress an aircraft. i think around 1/4 of all seats usage seriously
injures the occupant.   that high G launch and the odds of impacting
debris make it risky.  i think schirra knew this and trusted his guts
over the risk of the seats.

------------------------------

Date: Mon, 30 Nov 92 11:52:06 -0600
From: [e-mail] (Phil G. Fraering)
Subject: hubble time, toutatis, swift-tuttle...

\Ben Zellner has also been trying to get Hubble to observe Toutatis;
/according to Dave Tholen and J.R. Spencer, the asteroid should be
\"barely resolvable" (i.e., it will appear as bigger than one pixel) if
/its size is as big as their estimate of about 2.7 km-- it should
\subtend about 0.15 arcsec.  Dave had planned to observe Toutatis at
/opposition this summer, so maybe he has better estimates now.

Hmm.... to tie this in with the Swift-Tuttle thread of several
hundred years ago (or so it seems), has anyone gotten some time
on Hubble for observing Swift-Tuttle as it's leaving the inner
solar system?

I mean, from what I've heard, if you want some time to do it in
1996 or 1998, now's the time to start filling out forms, right?

\"Do you know the asteroids, Mr.Kemp?...    Bill Higgins
/Hundreds of thousands of them.  All
\wandering around the Sun in strange        Fermilab
/orbits.  Some never named, never
\charted.  The orphans of the Solar         [e-mail]
/System, Mr. Kemp."
>                                           [e-mail]
\"And you want to become a father."
/  --*Moon Zero Two*                        SPAN/Hepnet: 43011::HIGGINS

One of these days I've got to ask where that's from.

BTW, Bill, you think you could drop me a line with a couple
pro-SSC arguments? All the good ones I've seen recently have
been anti... what did you think about Dyson's statement in
Physics Today from a couple years back, about new electron
accelerators?

Finally, could an updated Fermilab do everything cheaper and
better?

--
Phil Fraering
"...drag them, kicking and screaming, into the Century of the Fruitbat."
<<- Terry Pratchett, _Reaper Man_
PGP key available if and when I ever get around to compiling PGP...

------------------------------

Date: Mon, 30 Nov 92 23:46:59 GMT
From: [e-mail]
Subject: hypergolics (was Re: Pumpless Liquid Rocket?)

> So we're talking about a few grams of thermite, an aluminum frying pan
and
> a bottle of Perrier?  The last two are easy enough to obtain, but the
first
> one is a bit tougher...
>


Nah. Thermite's easy. Nothing but very finely powdered Aluminum and Iron.
A very fine file, some aluminum pans and an old iron skillet and a lot of
time might do the trick.

Thermite is not all that easy to ignite though. I used to use Potassium
Permanganate and Glycerine to start it off.

--
=======================================================================
Give generously to the Betty Ford   Dale M. Amon, Libertarian Anarchist
Home for the Politically Correct            [e-mail]
=======================================================================

------------------------------

Date: 30 Nov 92 17:13:48 GMT
From: Anita Cochran <[e-mail]>
Subject: Kuiper belt planetesimals and Planet X claim
Newsgroups: sci.astro,sci.space

In article <[e-mail]>, [e-mail] (Tom Van Flandern) writes:

> [e-mail] (Andrew C Goldish) writes:

> > Now that objects have been sighted that could possibly prove the existence
> > of the Kuiper belt, ...

>      The one such object recently sighted apparently is not a Kuiper belt
> comet, but a possible member of the Saturn family of asteroids or comets. All
> other searches for Kuiper belt objects have so far proved fruitless.  And the
> reasons for expecting a Kuiper belt at all have now been called into
> question.  The whole concept is close to being ready to file away next to
> "cold fusion."

Gee, I was not aware that 1992QB1 was NOT a Kuiper object.  Actually,
as I understand it (unless there is a new orbit in the last week) there
are two possible solutions.  One is a circular orbit with the distance
putting it as a Kuiper Belt distance and the other is the Saturn
family object that Tom mentions.  But, as of a little over a week ago,
the orbit was still indeterminate.  That is why Brian Marsden has asked
that I try to get a new position in late December or January.  Jan Luu
(one of the discovers of QB1) was supposed to observe it Thanksgiving
but I am not sure if she got anything.  Our run was pretty well weathered
out.  Thus, there is still a possibility of 1 object in an orbit which
matches the predicted Kuiper belt.


--
 Anita Cochran  uucp:  !utastro!anita
                arpa:  [e-mail]
                snail: Astronomy Dept., The Univ. of Texas, Austin, TX, 78712
                at&t:  [phone]

------------------------------

Date: 30 Nov 92 04:37:21 GMT
From: Bill Higgins-- Beam Jockey <[e-mail]>
Subject: Observing Toutatis (was Re: US/World News in Brief Nov 26 10 am PST)
Newsgroups: sci.space

In article <[e-mail]>, [e-mail] (Mitchell E. Gold) writes:
> In article <[e-mail]> [e-mail] (UPI-Radio) writes:
>        A University of Florida astronomer says a two-mile wide asteroid is
>    approaching Earth and will give astronomers a rare close-up view
>    December 8th. Dan Dyurda says the asteroid will not pose a threat when
>    it crosses the Earth's orbit... coming within 2-point-2 Million miles of
>    the planet.
> Can anyone venture a guess as to the possibility of realigning Galileo to
> get a shot of this? Any more information on how close it'll be?

Fair question, Mike, but why use Galileo's dinky cameras when far
larger telescopes are available?

Think of the Earth as a big spacecraft, loaded with state-of-the-art
instruments, which is on course for a flyby of the asteroid 4179
Toutatis on 8 December.  This is closer than any known asteroid will
pass to our planet before 2000.  It is a cosmic coincidence that
Galileo is coming within 300 km of the Earth on exactly the same day.
(It *would* be interesting to see a plot showing the orbits of both,
though.)

Probably the most interesting observation, if it works, will be Steve
Ostro's gang using the Goldstone radar. As he wrote for the Division
of Planetary Sciences meeting in October:

  "Expectations for the echo strength, for the accuracy of the
  delay/doppler prediction ephemerides, and for the delay/doppler
  dispersions of echo power suggest that several minutes of
  observation can yield a delay-doppler image possessing good noise
  statistics and placing ~10,000 resolution cells on the target. A
  Goldstone track lasting ~5 hours should be able to produce an
  imaging sequence with ~100 frames, each having a fractional
  spatial resolution comparable to that of Galileo's first images
  of Gaspra. Radar observations are planned for Goldstone from late
  November through mid December and for Arecibo during mid
  December."

Ben Zellner has also been trying to get Hubble to observe Toutatis;
according to Dave Tholen and J.R. Spencer, the asteroid should be
"barely resolvable" (i.e., it will appear as bigger than one pixel) if
its size is as big as their estimate of about 2.7 km-- it should
subtend about 0.15 arcsec.  Dave had planned to observe Toutatis at
opposition this summer, so maybe he has better estimates now.

"Do you know the asteroids, Mr.Kemp?...    Bill Higgins
Hundreds of thousands of them.  All
wandering around the Sun in strange        Fermilab
orbits.  Some never named, never
charted.  The orphans of the Solar         [e-mail]
System, Mr. Kemp."
                                           [e-mail]
"And you want to become a father."
  --*Moon Zero Two*                        SPAN/Hepnet: 43011::HIGGINS

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Date: 30 Nov 92 10:31:54
From: Steinn Sigurdsson <[e-mail]>
Subject: Shuttle replacement
Newsgroups: sci.space

In article <[e-mail]> [e-mail] (Pat) writes:

   In article <[e-mail]> [e-mail] (Steinn Sigurdsson) writes:
   >
   >Ooh, I love analogies: so, Allen, would you argue that an Aircraft
   >carrier is best left anchored in mid-ocean and the planes flown in?
   >After all, it seems silly to sail the thing back and forth all the
   >time when what you really want is a launch platform for aircraft
   >(somewhere) in the ocean, no?
   >

   Actually that is how carriers operate.  the air wings fly off as the carrier
   approaches home port so they can under go depot level service.  also
   that way they are available for other ops while the carrier refits.

   the carrier then steams off shore and the wings are flown on.

I've forgotten now what Allen's original point was, but
the point of the extended analogy is that carriers _do_ go
to port for servicing (unless they're a big island in the
Atlantic :-( ), not whether they kept the planes on board during
servicing...

------------------------------

Date: 30 Nov 92 16:05:58 GMT
From: Gary Coffman <ke4zv!gary>
Subject: Shuttle replacement
Newsgroups: sci.space

In article <[e-mail]> [e-mail] (Henry Spencer) writes:
>In article <[e-mail]> [e-mail] (Gary Coffman) writes:
>
>>... airliners receiving that grade of service have engines *fall
>>off* in flight. Suppose a fuel feed line fatigues from multiple flights.
>>It wasn't X-rayed before flight because this is airliner grade servicing.
>>So the thing lets go as they pass through 10,000 feet on their way to
>>a landing at O'Hare. A couple of tons of rocket fuel starts streaming
>>down among the firing engines as they pass over the Loop. What's their
>>abort mode? ...
>
>Or suppose that #3 engine on a 747 somehow fails messily, and wipes out
>#4 while it's at it, and this happens with a heavy fuel load over Amsterdam.
>What's their abort mode?  Answer:  they die and so do a lot of others.
>The question is not "can unsurvivable failures occur?" but "how likely is
>such a failure?".  Something meant to be certified as an airliner, e.g.
>DC-1, will have to be built to keep the probability of such failures low.
>That means, as with airliners, careful analysis of fatigue lives of parts
>and inspection/replacement schedules set up to avoid problems.  (It also
>means, as with airliners, that there will probably be an occasional crash
>due to unanticipated problems.)

Yes, I agree with this. My question is whether any space launch system
can accumulate enough flight hours to make any of this analysis meaningful.
It seems to me that the launch requirement for something of the DC class
is small enough that there will only be a few built, and those flown
fairly infrequently. At least they will fly at nowhere near the schedule
rates of airliners. What I'm questioning here is whether airliner grade
ground servicing can work with such a system. It would take many years
of flight experience to feel confident that all the catastropic failure
modes were sufficiently under control for airline grade servicing. It
would seem that the cost of the flights, still orders of magnitude
greater than airline costs, the costs of the payloads, again orders
of magnitude greater than most airline cargos, and the relatively
small number of flights would all conspire to demand zero defects
levels of servicing similar to what is now given Shuttle. That pushes
processing costs right back up into the stratosphere.

>>Spacecraft stresses are much higher...
>
>Spacecraft stresses are *zero* for most of the flight.  The high stresses
>last a few minutes per flight.  Hardware with a fatigue life of (say)
>1000 hours wouldn't even be legal on a 747, but should last a DC-1 its
>entire operating life.

Fatigue life of components is proportional to the stress under which
they operate. For a given part, a 10X greater stress will shorten it's
operational lifetime more than 10X greater number of flight hours.
The usual engineering response is to either make the part much stronger,
and heavier, or to use non-destructive testing after each use looking for
tiny stress cracks and the like. Now my point is that the proposed DC is
a thin margin system, mass is at a premium. How much of that margin can be
traded for reduced levels of servicing? I don't think there's much room in
the design for this. Look to auto racing for examples of this. F1 cars
work on tiny margins, and fail often. They are essentially rebuilt after
each race. Meanwhile club racers may campaign an entire season on the
same engine and chassis with only minimal servicing. The stress levels
are grossly different.

Somebody asked, "What's my solution?" to this problem. I mentioned it
earlier, a much bigger SSTO. Because of cube/square relations, a really
large SSTO could be built with ordinary structural steels and ordinary
shipwright techniques. Taking off and landing from the ocean, it poses
minimal risk to populated areas. Instant heavy lift, and low cost per
pound to orbit. Whether there's enough demand for such a system to be
economical is an unanswered question. Certainly most payloads today are
too small to justify flying such a system unless they could be combined
on a single launch. Whether payloads would start to be designed to take
advantage of the system is an unknown, but many people here could probably
think of several uses for a cheap per pound space freighter.

I think DC is an attempt to build an exotic race car when a tramp
steamer is more in the realm of the possible.

Gary

------------------------------

Date: 30 Nov 92 16:19:13 GMT
From: Gary Coffman <ke4zv!gary>
Subject: Shuttle replacement
Newsgroups: sci.space

In article <[e-mail]> [e-mail] (Brian Stuart Thorn) writes:
>
>
>   Tread very carefully here, Gary. I'm as big a supporter of the Space
>   Shuttle as anyone, but I do remember a Space Shuttle 'malfunction'
>   a few years back which screwed the heck out of three deep space
>   mission launch windows. One mission was delayed for three years (Galileo),
>   one for four years (Ulysses), and a third had to fly a longer trajectory
>   to get there after leaving ahead of schedule (Magellan).

Well that's true, but that three year delay wasn't the fault of Shuttle,
it was the fault of the managers who refused to listen to MT engineers
who said it was too cold to launch. And it was the fault of a bureaucracy
who stopped the world instead of putting strip heaters on the joints, or
saying "Gee I guess we *really* shouldn't launch when it's that cold" and
continuing to launch. It was the fault of a risk averse America.

Gary

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End of Space Digest Volume 15 : Issue 476
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