Author Topic: Fluorine  (Read 134702 times)

Offline Bob B.

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Re: Fluorine
« Reply #15 on: October 18, 2010, 09:08:22 AM »
How fast does the carbon monoxide react with atmospheric oxygen anyway (or disperse for that matter)?

I have no idea, though I suspect dispersion will depend a lot on the wind.  CO has the same molecular weight as nitrogen, so I don't think we'd have to worry about the gas sinking or rising in elevation.

Fast enough that most people standing outside of the safety perimeter around the launch pad are safe from CO inhalation risk during/after the launch?

Good question.  From what I've read, CO becomes dangerous to humans at a concentration of about 100 ppm.  In just ten seconds of operation, a Soyuz releases enough CO that it must disperse into a volume of about 46 million cubic meters for the concentration to fall below 100 ppm.  That doesn't seem to me to be a problem if there isn't much wind (causing dispersal in all directions) or if the spectators are up wind.  However, if a strong breeze is blowing in the direction of the spectators, it might be possible for a cloud of CO to reach the perimeter before it has dispersed to safe levels.  But even in that case, I doubt the levels would stay high for very long before the cloud moved on and/or dispersed.

I really don't have answers to your questions; I'm just trying to make some intelligent guesses.  What are your toughts?

Offline DonPMitchell

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Re: Fluorine
« Reply #16 on: October 18, 2010, 10:32:28 AM »
So you'd have to be standing for a while within 300 meters of the Soyuz launch and assume the gas doesn't disperse or blow away.  I think getting your eyebrows burned off would be a bigger concern than CO poisoning.

Just to put things in perspective, compare the rate at which rockets are launched (dozens per year?) to the 87,000 airplanes that take off every day in the USA.  That's why I'm not very concerned about polution from rockets.  And I bet automobiles are a vastly bigger problem than airplanes.
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Offline Satanic Mechanic

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Re: Fluorine
« Reply #17 on: October 18, 2010, 10:37:20 AM »

I really don't have answers to your questions; I'm just trying to make some intelligent guesses.  What are your toughts?
The CO will want to bond to the oxygen to create CO2 and ozone.  How fast it will want to convert over depends on a few factors.  This is a large volume of CO going out in a short time.  I would have to crack open the chem books to figure out the rate.
I have had CO poisoning before when I was at a dragrace track years ago and it is not fun.

I have been reading your guys post about the Flourine on the side.  I know its use in the enrichment process of uranium, separating out the U-235 from the U-238.  I know that Flourine is corrosive as hell.  Great topic none the less.

SM

Offline DonPMitchell

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Re: Fluorine
« Reply #18 on: October 18, 2010, 01:50:01 PM »
Isaac Asimov, in one of his nonfiction science books, called Fluorine the "pale yellow assasin".  A lot of chemists died studying its properties.
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Offline Bob B.

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Re: Fluorine
« Reply #19 on: October 18, 2010, 08:49:46 PM »
Here is a snippet about fluorine from John D. Clark's book Ignition! An Informal History of Liquid Rocket Propellants (1972).

Quote
Fluorine is not only extremely toxic; it is a super-oxidizer, and reacts, under the proper conditions with almost everything but nitrogen, the lighter of the noble gases, and things that have already been fluorinated to the limit.  And the reaction is usually violent.

It can be contained in several of the structural metals – steel, copper, aluminum, etc. – because it forms, immediately, a thin, inert coating of metal fluoride which prevents further attack.  But if that inert layer is scrubbed off, or melted, the results can be spectacular.  For instance, if the gas is allowed to flow rapidly out of an orifice or a valve, or if it touches a spot of grease or something like that, the metal is just as likely as not to ignite – and a fluorine-aluminum fire is something to see.  From a distance.

But, as is usually the case, the stuff can be handled if you go about it sensibly, and if you want to fire it in a rocket, Allied Chemical Co. will be glad to ship you a trailer truck full of liquid fluorine.  That trailer is a rather remarkable device in itself.  The inner fluorine tank is surrounded by a jacket of liquid nitrogen, to prevent the evaporation and escape of any fluorine into the atmosphere.  All sorts of precautions – pilot trucks, police escorts, and what not – are employed when one of those trucks travels on a public road, but sometimes I’ve wondered what it would be like if a fluorine tank truck collided with one carrying, say, liquid propane or butane.

The development of large fluorine motors was a slow process, and sometimes a spectacular one.  I saw one movie of a run made by Bell Aerosystems, during which a fluorine seal failed and the metal ignited.  It looked as though the motor had two nozzles at right angles, with as much flame coming from the leak as from the nozzle.  The motor was destroyed and the whole test cell burned out before the operators could shut it down.
« Last Edit: October 18, 2010, 11:15:13 PM by Bob B. »

Offline ijuin

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Re: Fluorine
« Reply #20 on: October 19, 2010, 03:01:19 AM »
"The lighter of the noble gases"? That implies that Fluorine reacts with the heavier noble gases such as Krypton and Xenon. . .

Offline Bob B.

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Re: Fluorine
« Reply #21 on: October 19, 2010, 08:43:54 AM »
"The lighter of the noble gases"? That implies that Fluorine reacts with the heavier noble gases such as Krypton and Xenon. . .

The National Institute of Standards and Technology Chemistry Database lists following compounds:

Krypton difluoride (KrF2)
Xenon monofluoride (XeF)
Xenon difluoride (XeF2)
Xenon tetrafluoride (XeF4)
Xenon hexafluoride (XeF6)


Offline DonPMitchell

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Re: Fluorine
« Reply #22 on: October 19, 2010, 08:48:10 PM »
And it does react with nitrogen to form NF3.  Xenon oxide also exists, and is an explosive.
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Offline Bob B.

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Re: Fluorine
« Reply #23 on: October 20, 2010, 04:37:40 PM »
And it does react with nitrogen to form NF3.  Xenon oxide also exists, and is an explosive.

A search of NIST shows several fluorine-nitrogen compounds:

http://webbook.nist.gov/cgi/cbook.cgi?Formula=FN&AllowExtra=on&NoIon=on&Units=SI

Offline DonPMitchell

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Re: Fluorine
« Reply #24 on: October 22, 2010, 12:04:30 AM »
That paper "Ignition!" was really a fun read.  As of the 1970s, when he wrote it, the highest specific impulse ever achieved in an engine was 542 sec.  Wow!  That was using liquid lithium + fluorine + hydrogen. 

Beryllium has higher energy than lithium, but all attempts to utilize it have failed due to practical aspects of the substance, the difficulty of getting it to fully burn.
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Offline Bob B.

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Re: Fluorine
« Reply #25 on: October 22, 2010, 09:09:05 AM »
That paper "Ignition!" was really a fun read.

Yes, I enjoyed it.  I guess being thrilled by a book about the history of rocket propellants just proves my nerdiness.

As of the 1970s, when he wrote it, the highest specific impulse ever achieved in an engine was 542 sec.  Wow!  That was using liquid lithium + fluorine + hydrogen.

I tried to computationally verify that result but couldn't get it to work.  I could never find a mixture that would outperform straight fluorine + hydrogen.  I'd like to know the mixture ratio and operating conditions of that engine.

Beryllium has higher energy than lithium, but all attempts to utilize it have failed due to practical aspects of the substance, the difficulty of getting it to fully burn.

Didn't he also talk about boron as a rocket propellant?  I found it interesting that chemists have tried so many different compounds and combinations.  As I recall, Clark's conclusion about "exotic" propellants was that they are just too much trouble to be worthwhile.  The only prediction I remember him making that turned out not to be true was about the imminent use of fluorine compounds as oxidizers.  The one he seemed pretty high on was chlorine pentafluoride (ClF5).


Offline ijuin

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Re: Fluorine
« Reply #26 on: October 22, 2010, 10:46:08 PM »
That paper "Ignition!" was really a fun read.  As of the 1970s, when he wrote it, the highest specific impulse ever achieved in an engine was 542 sec.  Wow!  That was using liquid lithium + fluorine + hydrogen. 

I assume you mean the highest for chemical reactions. NERVA engines had already been tested at 800+ seconds by that time IIRC.

Offline Bob B.

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Re: Fluorine
« Reply #27 on: October 23, 2010, 09:24:38 AM »
That paper "Ignition!" was really a fun read.  As of the 1970s, when he wrote it, the highest specific impulse ever achieved in an engine was 542 sec.  Wow!  That was using liquid lithium + fluorine + hydrogen.
I assume you mean the highest for chemical reactions. NERVA engines had already been tested at 800+ seconds by that time IIRC.

Yes, in fact, here is Clark's exact quote:

Quote
Using lithium and fluorine alone (no hydrogen) their maximum specific impulse was 458 seconds.  But when they proportioned the lithium and fluorine to burn stoichiometrically to LiF, and injected hydrogen to make up 30 percent of the mass flow, they measured 542 seconds - probably the highest measured specific impulse ever attained by anything except a nuclear motor.

Elsewhere in the book he mentions that the Phoebus-1 nuclear motor had achieved a specific impulse of 760 and that impulses above 850 are expected soon (the book's copyright is 1972).  I'm pretty certain the latter NERVA motors did reach around 850.

In my last post I complained that I'd like to know the exact fluorine-lithium-hydrogen mixture, yet Clark gives it to me in the above quote.  I didn't remember him being so specific about it.  I'll have to try to run those calculations again because I don't remember being able to get anywhere close to 542 seconds when I did it a few years ago.
« Last Edit: October 23, 2010, 12:50:48 PM by Bob B. »

Offline DonPMitchell

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Re: Fluorine
« Reply #28 on: October 23, 2010, 01:34:24 PM »
Which is a better propellant for nuclear engines, hydrogen H2 or Helium?  Helium has higher molecular weight, but its monatomic so almost no Cp from internal vibrational states.  I don't know how those factors trade off.

Of course if you can get the hydrogen hot enough to dissociate, then its the clear winner.
« Last Edit: October 23, 2010, 01:37:13 PM by DonPMitchell »
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Offline Bob B.

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Re: Fluorine
« Reply #29 on: October 23, 2010, 05:42:49 PM »
Which is a better propellant for nuclear engines, hydrogen H2 or Helium?  Helium has higher molecular weight, but its monatomic so almost no Cp from internal vibrational states.  I don't know how those factors trade off.

Of course if you can get the hydrogen hot enough to dissociate, then its the clear winner.

There is very little hydrogen dissociation until the temperature gets up around 3,000 K.  I researched NERVA a couple years ago and I don't remember all my sources, but I'm pretty sure those engines operated at less than 3,000 K.  We're therefore dealing predominately with molecular hydrogen.

It takes far more energy to heat hydrogen to a given temperature than it does to heat an equal mass of helium to the same temperature, by a factor of about 3 to 1.  However, the materials of construction and the ability to cool the engine will limit the temperature at which the engine can operate.  Let's say we run the engine at the highest temperature the materials will allow.  This means that for a given reactor, we can put three times as much helium through the engine as hydrogen.  Alternatively, we can heat the same mass of helium with a reactor 1/3 the size.  However, at a given temperature hydrogen will have a much better specific impulse because its molecular weight is half that of helium.

So which is better, hydrogen or helium?  The answer is, it depends.  Hydrogen is a more efficient fuel that produces a higher specific impulse.  On the other hand, helium makes more efficient use of the power plant.  I would say that in most practical applications hydrogen is the better choice because you require less propellant, which is usually the most massive part of the system.  If you had a situation where not much propellant was needed, helium might be better because you could downsize the reactor and reduce the dry mass of the system.  However, in the latter case it's probably simpler to use chemical propulsion.