Space & Astronomy/Space and Astronomy (October 1993).iso/mac/TEXT_ZIP/spacedig/V15_2/V15NO256.ZIP/V15NO256

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Date: Mon, 28 Sep 92 05:02:39
From: Space Digest maintainer <[e-mail]>
Reply-To: [e-mail]
Subject: Space Digest V15 #256
To: Space Digest Readers
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Space Digest                Mon, 28 Sep 92       Volume 15 : Issue 256

Today's Topics:
     Redefining failure, space camps, & other changes of subject
      Safety of flyby & aerobraking for large payloads at earth
            Space Platforms (political, not physical : -)

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Date: 27 Sep 92 23:08:17 GMT
From: Nick Szabo <[e-mail]>
Subject: Redefining failure, space camps, & other changes of subject
Newsgroups: sci.space,talk.politics.space

>> Paul Dietz writes, in response to Dennis Wingo:
>>In other words, "the shuttle's not a failure, and it's not NASA's
>>fault anyway!".   Can you write two sentences without contradicting
>>yourself?
>>
>>The purpose of the shuttle was to reduce the cost of getting into
>>space.  The shuttle has been a dismal failure in meeting this,
>>its primary goal.
>
In article <[e-mail]> [e-mail] writes:

>Only half right.  The primary reason for the Shuttle is REGULAR access to
>space.

The stated goal being a flight every week, the actual result being less
than a flight per month.  Of course, we're allowed to redefine that
endlessly as well, just as we're allowed to decrease the functionality

of the space station, increase its cost from the promised $8 billion
to over $100 billion, and use accounting gimmickry and clever arguments
to say, "hey, that's what we planned all along!"  I was there when
the promise was made, so let's not dredge up the Boeing accounting -- SSF
was sold as an $8 billion project, in disregard of what Boeing
engineers actually projected as the costs, information that was
not made available to me at the time. We were shamefully involved
in promoting the wrong numbers, because we were operating by wishful
thinking; we didn't bother to think critically and ask the right
questions. Similar NASA/NSS figures on astronaut projects in the
future should be con sidered against this history of deceit.

>["space camp"]

We were talking about commerce here.  The tiny revenue earned from
space camps is lost in the NASA bottom line, and what NASA chooses
to propagandize to children in space camps hardly constitutes what
is good for industry.  It's hard to remember, since Dennis keeps
changing the subject, but this thread was about industrial
competiveness.

The camp's content doesn't reflect what children dream of, BTW. It
reflects what NASA PR wanted them to dream of when they designed the camps --
big fat NASA contracts for all the traditional goals.  In my high
school physics class the Viking project was one of the teacher's
motivational schticks; astronauts were nowhere in sight.  Polls show
that Americans support the astronaut and automated programs equally
even when not shown the relative program costs.

Furthermore, "keeping people interested in space" has no predictable
correlation to space project funding.  If anything there may be
an inverse correlation; the height of interest during Apollo
corresponded with the deepest cuts to NASA.  The most visible programs
are the most likely targets for budget cuts when Congress gets
around to trying to convince us that they're fiscally responsible.


--
[e-mail]        Tuesday, November third  ##  Libertarian  $$  vote
Tuesday  ^^ Libertarian  --  change  **  choice && November 3rd @@Libertarian

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

Date: Mon, 28 Sep 1992 06:15:17 GMT
From: Nick Szabo <[e-mail]>
Subject: Safety of flyby & aerobraking for large payloads at earth
Newsgroups: sci.space

What are the costs and benefits of using the earth
for gravity assist and aerobraking of large payloads?
What should be the policies concerning these flybies?

Here's how a gravity assist works. The planet is moving,
so there's our energy source. The slingshot can be computed
with the patched-conics approximation.  If we do a Hohmann
ellipse to the planet in the inertial frame, the trajectory
is a hyperbola in the frame of the planet.  The energy of
the vehicle is the same at symmetric points on opposite sides
of the hyperbola in the planet frame.  If we exit the rendezvous
moving in the same direction as the planet, we gain velocity
in that direction in the inertial frame.  If we exit the
rendezvous moving opposite the direction of the planet, we lose
inertial velocity.  Either gaining or losing velocity can
be useful, depending on where we're going.

Aerobraking is simpler to understand.  In layman's terms,
the air slows the spacecraft down, just like wind
resistance slows down a bicycle.  In orbital mechanics
terms, the spacecraft exchanges momentum with the particles
in the atmosphere.  An interesting variant, called cometary
aerobraking, vaporizes a piece of ice a split second before
it intercepts the spacecraft at high velocity.  The spacecraft
uses the temporary cloud of gas to aerobrake, as if it were
a planetary atmosphere.

All these maneuvers allow us to tap into the energy
already stored in the orbits of the planets and minor
planets.  They can greatly reduce the mass of propellant
and tank needed for a mission; in the case of Galileo
the Venus-Earth-Earth-Jupiter trajectory saved it from
being cancelled when it had to substitute a smaller
upper stage for the powerful Centaur.

Very large payloads can benefit from these trajectories
for the same reason, especially at earth, which raises
the important question of safety.  We cannot tolerate
bringing a dinosaur-killer sized asteroid anywhere near
earth, or coming towards earth near an intercept trajectory.
Reentry of c. 100 ton Shuttles is safely performed and
tolerated towards inhabited areas, and natural fireballs
and meteorites massing several tons each hit the earth
harmlessly every year.  Somewhere between these two extremes,
we need to figure out the margins of safety and enforce them.

There are several techniques for using earth to change the orbital
trajectory of objects:

* fast aerobraking (eg Shuttle, Apollo)
* slow aerobraking (eg Hiten)
* gravity assist (eg Galileo)

All three of these can play important roles in reducing the costs of
capturing space materials from comet fragments and asteroids into
various earth orbits.  The delta-v savings are roughly up to an order
of magnitude for gravity flyby, and up to two orders of magnitude for
aerobraking.

For gravity assist, the following need to be considered:

* what is the margin of error due to fringe atmospheric density,
  gravity anomaly and trajectory measurement error?
* how quickly and precisely can the on-board engines compensate for
  trajectory errors directly before and during the flyby?
* can the operation be timed so that a worst-case error will cause
  reentry over an uninhabited area (eg the ocean)?
* how much material on board is strongly toxic or radioactive?
* what is the worst-case scenario wrt the mass, composition, and
  worst-case error trajectory of the payload?

For slow aerobraking, we must also consider the above points, paying
close attention to the fringe atmospheric density, since that is what
we are using to change the trajectory.

For fast aerobraking, we need to pay very close attention to upper
atmospheric density at all levels.  The error margins are much less.
Unless the worst-case scenario is trivial or the spacecraft is
well-controlled aerodynamically, fast aerobraking is much more
dangerous than slow aerobraking or gravity flyby.

Given these data points, we must then determine whether the project
is ethically and politically tolerable, and whether it can be insured.

For the sake of discussion, I make the following initial proposed rules
of thumb:

gravity assist & slow aerobrake
    * <.0001% chance of trajectory error sufficiently large for reentry
    * if reentry, >98% chance it will occur over & towards
      uninhabited area
    * payload mass limits:
          solid metallic materials: 5,000 tons (no piece > 1 ton)
        stony materials: 10,000 tons (no piece > 5 tons)
        carbonaceous materials or loose regolith: 30,000 tons
        volatile ices w/pores: 50,000 tons
        strongly toxic or radioactive material:
            (varies by material; 1 ton typical)

The shape and attitude of the container also play a major role.
For example a long, thin cylinder will be dispersed more widely
than a sphere if it hits the atmosphere sideways instead of
headlong.

fast aerobrake:
    * aerobrake must contain control surfaces sufficient to
      give <.001% chance of reentry due to error
    * if reentry, >98% chance it will occur over & towards
      uninhabited area
    * no strongly toxic or nuclear materials on board
    * mass limits:
solid metallic materials: 200 tons
all other materials: 400 tons (note: ceramic heat shield will
        be significant % of mass for any payload)

Significant amounts of simulation, study of real-life artificial
and natural reentries, and benefit/risk analysis need to go into
determining the actual safety margins.  Calculations like those
done by Zdenek Sekanina, to predict the ability of comet material
to penetrate the earth's atmosphere, need to be perfected.

Privately financed insurance with unlimited liability
should be required for all such payloads.  If no insurance
company is willing to underwrite the risk it is a good sign
for the public that the maneuver is too risky and should be
not be allowed.  On the other hand if the insurance industry
volunteers to take on the risk, this is a good sign that the
risks, financial and physical, are minimal, and that the payoff
directly to the companies, and indirectly to mankind as a whole,
are well worth it.



--
[e-mail]        Tuesday, November third  ##  Libertarian  $$  vote
Tuesday  ^^ Libertarian  --  change  **  choice && November 3rd @@Libertarian

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

Date: 28 Sep 92 07:43:29 GMT
From: Tim Starr <[e-mail]>
Subject: Space Platforms (political, not physical : -)
Newsgroups: sci.space,talk.politics.space,alt.politics.marrou,alt.politics.libertarian

In article <[e-mail]> [e-mail] (ken bell) writes:
}
}Does the libertarian party support US withdrawl from the UN?
}//////////////////////////////////////
}/* Kenny  *   Welcome to Mind Wars! */
}//////////////////////////////////////

Yep.


Tim Starr - Renaissance Now! - Think Universally, Act Selfishly
[e-mail]

"True greatness consists in the use of a powerful understanding to enlighten
oneself and others." - Voltaire

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End of Space Digest Volume 15 : Issue 256
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