Space Forum / Space Flight / November 2003

http://www.astronautix.com/lvfam/orion.htm

http://216.239.41.104/search?q=cache:r5AyDJX2ao0J:www.andrews-space.com/en/news/
MMO%2520JPL%2520Talk.pdf+micro+fission&hl=en&ie=UTF-8

http://ffden-2.phys.uaf.edu/213.web.stuff/Scott%20Kircher/fissionfusion.html

The URLs above describe a sort of ship that's possible to build using
nuclear fuel.  In the 1940s and 1940s nuclear pulse units - miniature
a-bombs - were proposed as a means to propel spacecraft.  This
resulted in Project Orion, which was cancelled with the signing of the
Nuclear NonProliferation Treaty in 1963.

Since that time the same technologies that were explored to create
inertial confinement fusion were also explored to create very small
inertial confinement fission - so called, micronukes.  Micronukes -
nuclear hand grenades, can be used directly for propulsion, or
indirectly as triggers for relatively clean mini-H-bombs.  In either
case, total energy yeilds are such that total containment of the blast
is feasible, and we end up with spaceships the size of ocean liners to
supertankers - capable of flying across the solar system with ease.

Check it out;

http://www.niac.usra.edu/files/studies/final_report/pdf/76McNutt.pdf
http://fusionenergy.lanl.gov/Documents/MTF/Mtfrefs8-99.PDF
http://128.97.43.7/bapsf/papers/Gekelman-laserJGR.pdf

http://hypertextbook.com/physics/modern/fusion/index.shtml
http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/fission.html

Lithium-6 Deuteride produces 10 kiloton TNT equivalent explosion when
0.156 kg of it are detonated.  At 0.82 gram per cc, this means that
190 cc of the stuff are needed for each blast. A sphere 7.1 cm across.

A 2 ton TNT equivalent fission trigger consisting of 100 mg of
Plutonium is made from wire about the size of a paperclip.  If made
from the world's existing stockpile of nuclear weapons;

http://www.nrdc.org/nuclear/nudb/datab19.asp

There would be plenty to go around.  Also, Deuterium is abundantly
available in the world's water supplies.  And, Lithium-6 consists of
7.4% of the world's supply of Lithium.  The US imported 3,000,000 kg
last year

http://minerals.usgs.gov/minerals/pubs/commodity/lithium/450301.pdf

A minimum traditional weapon (not the advanced type supposed here)
contains about 5 kg of Plutonium.  So, we have about 50,000 kg
available from current weapons stockpile.  So;

  50,000,000 grams Pu - 0.1 gram --> 500 million triggers
3,000,000,000 grams Li-6/yr - 156 grams --> 19.2 million units/year

Deuterium is relatively unlimited - since its abundantly available in
the world's water supply.

So, we have enough materials to last us 25 years with 20 million
blasts per year.

156 grams expanding with 10 kiloton 41.84e15 joules of energy - has an
average velocity of;

  E = 1/2 * m * V^2 --> V = SQRT(2*E/m)
                          = SQRT(2*41.84E15/0.156)
                          = 23,160,532 m/sec

So, if our weapon's experts can design a miniature nuclear explosion
that efficiently deposits the bulk of its energy into the reacting
medium, we can obtain exhaust velocities exceeding 20,000 km/sec!

Compare this with the Space Shuttle's 4.5 km/sec exhaust speed !!!

Okay, with this kind of performance its easy to see that we can do
amazing things.

For example, to move 20 million kilometers (2e10 meters) at 1/10th gee
constant (after escaping Earth) - accelerating half the time and
slowing the other half - to land softly on Mars (assuming its 20
million km away at the time) requires

  D = 1/2 * a * t^2  and V = a * t --> t = V/a --> D = 1/2 * a *
V^2/a^2

  D=V^2/(2a) --> V = SQRT(D*2*a)
                   = SQRT(2e10*2*0.982)
                   = 198,191 m/sec
                   = 198.2 km/sec

To get to the half way point, and the same amount to slow - with
slight variations due to the relative speeds of the planets which
amount to a few 10s of kilometers per second.

So, a spacecraft that could achieve a 500 km/sec final velocity would
be able to execute a constant 1/10th gee flight to Mars and back, when
it was near Earth.

This trip would take;  t = 198,191 /0.982 = 201,823 seconds = 56 hours

to each half way point.  A round trip wold take 224 hours - LESS THAN
10 days!

The amount of propellant needed to carry on board would be given by;

  Vf = Ve * LN(1/(1-u)) ---> u = 1 - 1/EXP(Vf/Ve)
                               = 1 - 1/EXP(500/20,000)
                               = 0.0247

Less than 2.5% of the spacecraft mass is needed to be the pulse units
described above.

Okay, so 20 million blasts per year of 0.156 kg pellets translate to
3,000 tons again - divide this by 2.5% - obtains 124,800 tons per year
carried to and from mars in this way.

Of course, this is very inefficient.  The most efficient way to carry
stuff by rocket is to have the exhaust speed equal the final speed.
So, if we carry sufficient propellant to energize it to match the
final speed - and pack it around the pellets - then, we can compute;

   u = 1 - 1/EXP(Vf/Ve)
     = 1 - 1/EXP(1)
     = 0.6321

But, this 63.21% is energized to 500 km/sec.  That's 125 GJ per kg of
propellant.  20 million pellets, each producing 41.83e15 joules of
energy, yeilds 836.6e21 joules per year.  This gives 6.7 trillion kg
of propellant.  Divide this by 0.6321 and we obtain 10.6 trillion kg
of rockets.  Multiply by 0.3679 to obtain 3.9 trillion kg of payload.

So, an energy efficient rocket fleet would have enough fuel to carry
nearly four billion tons of payload to and from mars each year - with
flight times meaasured in Weeks - and do this for 25 years.  That's
100 billion tons.  Or 15 tons for every man woman and child on the
Earth!

Clearly, we have the capacity to set up the sort of interplanetary
trading between Earth and mars that we now enjoy throughout the
world's oceans.

That wasn't venom.  When I get venomous, it's lots worse than that.

My point is that it's unwise to use a web page, written by someone who's
neither an optics guy nor an astronomer and clearly doesn't know much
about telescope engineering, as a reference for how to build huge space
telescopes.  (As another example, no astronomer would put transparent
optical elements in front of the mirror, because you can't make such
elements transparent to a wide enough range of wavelengths -- one of the
big assets of a reflecting telescope in space is being able to work well
into the UV and IR, and that requires that all optical elements be
mirrors, not lenses.)

A space mirror still faces differential thermal stresses (heating will
seldom be exactly even over a large structure -- this is a major issue
for the design of things as small as spacecraft antennas) and transient
distortions from pointing accelerations.

On the contrary, segmentation generally drives cost down, because segments
are easier to make and easier to handle for maintenance.  The only
question is whether you can combine the segments into a mirror of high
optical quality, and the answer to that is now unquestionably yes.

If you can get the plastic flow to go the right way, which is by no means
self-evident.  Besides, there's no reason to bother.  A big thin mirror is
going to be flexible even at room temperature, no matter what you do.

Just an aside:

There is creativity and open-mindedness here. That's why
your suggestions are being examined and discussed, and why
my wild postings get answers, rather than being dismissed
out of hand.

However, sometimes new ideas are incorrect, or are based
on incorrect facts. Do not mistake disagreement or
criticism of your ideas for lack of open mindedness or
suppression of creativity. It's just debate, and people
are just as allowed to disagree with you as you are
allowed to post new ideas.

In this particular case, you used a reference written by
someone Mr. Spencer apparently thought was ill-informed
about telescopes. Mr. Spencer did not say, "He's a moron,"
or, "You're a moron for using that website," which would've
been a crude dismissal. Instead, he highlighted (highlit?)
what he thought to be the errors in the site's logic. By
listing his objections (as opposed to making an unsupported
dismissal of the website), he made clear his position. You
were free to agree or disagree with his logic in further
debate.

In summary: don't mistake detailed but constructive
criticism for suppression of creativity. It's just debate,
and people come to this newsgroup for debate.

Mike Miller, Materials Engineer