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  #11  
Old February 5th 05, 08:13 PM
Allen Thomson
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Probably such things have been hashed out before,


I just noticed that such hashery has been going on in
a thread called "The Tortoise and the Hare" in
rec.arts.sf.science . Worth a look.

  #12  
Old February 6th 05, 06:51 PM
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Len Lekx wrote:

= This is rather old, and I doubt much research has
= gone into it... but if it could be developed, it
= would be an *immense* improvement in rocket
= technology...

= From an article by Jerry Pournelle...

= "Take Boron-11 (11B5). Bombard with protons. The
= result is a complex reaction that ends with helium
= and no nuclear particles. It could be a direct
= spacedrive. For those interested, the basic
= equation is

= 11B5 + p = 3(4He2) + 16MeV

= and 16 million electron volts gives pretty
= energetic helium. The exhaust velocity is better
= than 10,000 kilometers/second, giving a
= theoretical specific impulse of something over a
= million."

Just guessing, but I suspect the "complex reaction"
is hiding a requirement that this process take place
at pressures typical only of stellar interiors.

One wonders a bit, if the reaction is so productive,
that it is not used in weapons or tokomak projects
compared to deuterium or tritium, since 11B5 seems to
be the _majority_ isotope, and thus abundant and
presumably cheap:

Boron 5B Ar(B) = 10.811(7)
Isotope Atomic mass Mole fraction in NIST SRM 951 boric acid [2,32]
10B5 10.012 9371(3) u 0.198 27(13)
11B5 11.009 3055(4) u 0.801 73(13)


http://www.iupac.org/publications/pa.../7410x1987.pdf

That would make it, one would guess, a good power
source for "big push" ion propulsion systems.

FWIW

xanthian.

  #13  
Old February 6th 05, 10:33 PM
Paul F. Dietz
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wrote:
Len Lekx wrote:


= "Take Boron-11 (11B5). Bombard with protons. The
= result is a complex reaction that ends with helium
= and no nuclear particles. It could be a direct
= spacedrive. For those interested, the basic
= equation is

= 11B5 + p = 3(4He2) + 16MeV

= and 16 million electron volts gives pretty
= energetic helium. The exhaust velocity is better
= than 10,000 kilometers/second, giving a
= theoretical specific impulse of something over a
= million."

Just guessing, but I suspect the "complex reaction"
is hiding a requirement that this process take place
at pressures typical only of stellar interiors.

One wonders a bit, if the reaction is so productive,
that it is not used in weapons or tokomak projects
compared to deuterium or tritium, since 11B5 seems to
be the _majority_ isotope, and thus abundant and
presumably cheap:



It doesn't work in magnetic fusion reactors because
the bremsstrahlung losses exceed the fusion output
(this is true of anything beyond DT, D3He, or perhaps
DD).

In a bomb, the compressed fuel is enclosed in a capsule
that retards the escape of radiation. It comes into
thermodynamic equilibrium with the photons, which
assume a blackbody spectrum at the temperature of the
fuel. The energy per volume of this radiation goes
as T^4, so the hotter the fuel, the more energy bleeds off into
this radiation bath, and the more the fuel has to be
compressed to compensate. High Z materials are also
harder to compress.

Anyway, deuterium is also pretty cheap, as nuclear weapons
systems go.

BTW, 11B+p is not aneutronic, since there are neutron-producing
side reactions like 4He + 11B -- 14N + n, as well as
p + 11B -- 12C + gamma that produces a very hard photon.

Paul
  #15  
Old February 7th 05, 05:43 PM
Len Lekx
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On Sun, 06 Feb 2005 16:33:26 -0600, "Paul F. Dietz"
wrote:

It doesn't work in magnetic fusion reactors because
the bremsstrahlung losses exceed the fusion output
(this is true of anything beyond DT, D3He, or perhaps
DD).


How about electrostatic containment...? If the reaction yields
only charged particles, a series of oppositely-charged plates should
keep the reaction contained.

Anyway, deuterium is also pretty cheap, as nuclear weapons
systems go.


Who said anything about weapons? The idea is to channel the
reaction into a high-velocity jet for a reaction drive. :-)

BTW, 11B+p is not aneutronic, since there are neutron-producing
side reactions like 4He + 11B -- 14N + n, as well as
p + 11B -- 12C + gamma that produces a very hard photon.


OUCH! :-O Guess we'd need shielding then...

  #16  
Old February 9th 05, 02:40 AM
Paul F. Dietz
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Len Lekx wrote:

How about electrostatic containment...? If the reaction yields
only charged particles, a series of oppositely-charged plates should
keep the reaction contained.


Ditto. Bremsstrahlung comes from the fact you have to have electrons
around to neutralize the plasma. and those electrons will get
hot, and radiate (if the electrons are kept cold by some mechanism
then the power loss from ions-electrons will be even larger.)

Paul
  #17  
Old February 9th 05, 06:22 AM
Len Lekx
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On Tue, 08 Feb 2005 20:40:35 -0600, "Paul F. Dietz"
wrote:

How about electrostatic containment...? If the reaction yields

Ditto. Bremsstrahlung comes from the fact you have to have electrons
around to neutralize the plasma. and those electrons will get
hot, and radiate (if the electrons are kept cold by some mechanism
then the power loss from ions-electrons will be even larger.)


Granted that the chamber would need some kind of shielding - I
never disputed that. The best theoretical fission, fusion, or
antimatter drives would also need to be shielded... so I just took
that as a given. :-)

  #18  
Old February 10th 05, 12:13 AM
Paul F. Dietz
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Len Lekx wrote:

Ditto. Bremsstrahlung comes from the fact you have to have electrons
around to neutralize the plasma. and those electrons will get
hot, and radiate (if the electrons are kept cold by some mechanism
then the power loss from ions-electrons will be even larger.)



Granted that the chamber would need some kind of shielding - I
never disputed that. The best theoretical fission, fusion, or
antimatter drives would also need to be shielded... so I just took
that as a given. :-)


Shielding wasn't the problem. The problem is the electrons
are sucking energy out of the ions faster than fusion can replace
it.

Paul
  #19  
Old February 11th 05, 05:32 PM
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Henry Spencer wrote:

Current technology can stack up an array of ion thrusters, but

powering
them is problematic. That is much too big for current-technology

solar;
the problem is not the solar cells but the structural dynamics of

enormous
lightweight solar arrays. 100MW space reactors are not off-the-shelf
items either. Solving either in the next 20 years is conceivable,

but not
a small project, and probably will not happen without specific need.


But if you have lots of ion thruster, you can have lots of solar
arrays, each flying independently, powered by four ion thrusters.

Actually, 100MW solar is only 1km2. A sinlge sheet inside a square
lattice, with a steerable ion thruster every 200m along the lattice (20
in total), connected to the main unit by 100MV 1Amp cable.

Since the acceleration is so low, even a light weight latice could take
the strain.

  #20  
Old February 12th 05, 06:21 AM
John Schilling
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"Allen Thomson" writes:


John Schilling wrote:
"Allen Thomson" writes:


continuous thrust = 1 kN, ISP = 10,000 sec
(= 100 km/sec Ve) and a total run time = 1 megasecond
or greater, preferably getting toward 10 Ms.


they can make you up a cluster of high-power ion thrusters
that deliver the sort of performance you are looking for,
without even stretching the state of the art.


Could they really? That's excellent news.


And yes, the ion thruster array will be heavy (kiloton)


Apparently you have a specific design, or design in mind.
What, in general, are you thinking about?


Wasn't thinking anything specific, but since you ask:

Start with the NSTAR ion thruster developed for NASA's Deep Space 1
mission. Not that it's the best around or the most suited for your
hypothetical requirements, but it's unambiguously real and I can
talk about it without getting into anything proprietary or classified.

Weighs 8.3 kilograms, turns 3 mg/sec of Xenon and 2.4 Amps at 640 Volts
into 92 mN of thrust at a specific impulse of 3120 seconds. Lasts at
least 12,800 hours, or 4.6 megaseconds. And requires a 15 kilogram
power processing unit to turn 2.3 kilowatts of 28 VDC bus power into
the aforementioned 2.4A/640V drive power (and other important stuff
besides; it's not just a step-up transformer). Marginal cost of one
thruster/PPU combo would be on the order of a million dollars if you
were buying it commercially.

The specific impulse is less than a third of your requirement, but that
is not a fundamental limitation of the technology or even the specific
hardware, just a mission optimization - power scales as thrust times
Isp, 92 mN at ten thousand seconds would have required more power than
Deep Space One had available, and a deliverable 29 mN at ten thousand
seconds would not have resulted in enough acceleration to get DS-1 to
its destination before the ops budget ran out. In lab demonstrations
unconstrained by the requirements of particular missions, ion thrusters
using the same basic technology have delivered Isp values of 8,000
seconds, and ten thousand seconds would be no great problem.

In particular, if you run a stock NSTAR thruster on Argon rather than
Xenon, you should get 51 mN at 5,650 seconds Isp. If we increase the
drive voltage to 2kV and decrease the current to 0.75A or so, that
gets us 29 mN at 10,000 seconds Isp. We'll need to modify the
thruster to do this, increasing grid spacing, internal high-voltage
standoff, insulation thickness, etc. But as we've reduced the current,
we can reduce conductor size as well, and all of these are minor. As
the gross power has remained constant, mass will also remain constant
to the first order.

So, 29 mN thrust, 10000 seconds Isp, and well over a megasecond of life.
Two of your three requirements met.

All we have to do is gang thirty-five thousand of them in parallel, and
you've got your kilonewton of thrust as well. Eight hundred fifteen
tons of hardware, plus some integration overhead, rounds to a kiloton.

In practice, there are economies of scale to be had in using smaller
numbers of larger thrusters. Mostly in cost, but some of the NSTAR
components are running up against minimum-gauge limits in mass as
well. OTOH, there are severe difficulties if your individual thrusters
wind up larger than can be realistically tested in the largest ground
facilities[1]. So something like 800 thrusters at 100 kW each might
be appropriate. And eight power processing units with a hundred output
channels each.

And the eighty-megawatt power supply that we have agreed not to talk
about for now...


[1] Unless you can arrange for an advanced propulsion R&D team to take
up residence on ISS or one of its successors, of course. If you *can*
arrange for this, may I send you my resume?


--
*John Schilling * "Anything worth doing, *
*Member:AIAA,NRA,ACLU,SAS,LP * is worth doing for money" *
*Chief Scientist & General Partner * -13th Rule of Acquisition *
*White Elephant Research, LLC * "There is no substitute *
* for success" *
*661-718-0955 or 661-275-6795 * -58th Rule of Acquisition *

 




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