Author Topic: Fluorine  (Read 134701 times)

Offline DonPMitchell

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Fluorine
« on: October 13, 2010, 06:27:24 PM »
I was doing some calculations about energy per kilgoram of propellant.  It seems like oxygen is better than fluorine.  Oxygen has two valence bonds, so you need half as much of it to fully oxydize fuels.

So do people get better performance form fluorine because of other issues?  Maybe dissociation is less?  I'm only looking at heat of combustion here, and I now there is much more involved with a rocket engine.
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Offline spacecat27

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Re: Fluorine
« Reply #1 on: October 13, 2010, 11:33:24 PM »
Don, do you know this classic work?

http://forum.nasaspaceflight.com/index.php?action=dlattach;topic=20872.0;attach=207708

Been a lonnnng time since I read it, but I remember he talked about fluorine quite a bit.... with a healthy dose of shall we say.... caution.  :shock:

Offline ijuin

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Re: Fluorine
« Reply #2 on: October 14, 2010, 12:15:50 AM »
Fluorine does theoretically offer a better specific impulse than oxygen. The main obstacle is that fluorine is very nasty stuff to work with. First of all, it has a nasty tendency to react with the walls of the tank you store it in, as well as the fuel lines and pumps, and the interior of your combustion chamber, unless you coat them with the right materials (said materials adding mass to your system, as well as having less thermal and mechanical resistance than the stuff that you would use for an oxygen engine). Second, both fluorine and the exhaust from a hydrogen/fluorine engine are highly toxic--besides dissociated fluorine and hydrogen, you will get lots of hydrogen fluoride, which is essentially extremely highly concentrated hydrofluoric acid--and your engine will be spewing dozens or hundreds of tons of it if you are using it in the first or second stage of your rocket. Thus, a hydrogen/fluorine rocket will produce pollution several times worse than a comparable aluminum/ammonium perchlorate solid rocket or a hydrazine/nitrogen tetroxide rocket, much less hydrogen/oxygen.

Finally, to add to all of these issues, fluorine is quite a bit more expensive than liquid oxygen or even nitrogen tetroxide, which would further add to the cost of using such a rocket beyond the design, construction, and environmental costs.

Offline Bob B.

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Re: Fluorine
« Reply #3 on: October 14, 2010, 06:17:56 PM »
According to all my calculations, fluoride burns hotter than oxygen.  Let's consider the simple case of combustion with hydrogen.  To make things easy, we'll use gaseous reactants at 298.15 K, a stoichiometric mixture, and no dissociation.

First, oxygen and hydrogen:

O2 + 2 H2 --> 2 H2O

The enthalpy of the reactants must equal the enthalpy of the products.  The standard enthalpy of formation, i.e. Hfo at 298.15 K, of gaseous molecular oxygen and hydrogen is zero.  Therefore, the enthalpy on the right side of the equation is zero, thus that on the left side must also be zero.

At 298.15 K the enthalpy of water vapor is –241.83 kJ/mol.  As water heats up its enthalpy increases.  To make the equation balance, the temperature of the water on the right side of the equation must be that which will make its enthalpy equal to that of the reactants, i.e. zero.  Water's enthalpy is equal to zero at a temperature of about 5,000 K.

Now fluoride and hydrogen:

F2 + H2 --> 2 HF

Like oxygen and hydrogen, the standard enthalpy of formation of gaseous molecular fluoride is zero; therefore, as before, the enthalpy of the products must be zero.  At 298.15 K the enthalpy of HF is –272.55 kJ/mol.  The standard tables and equations for calculating enthalpy generally go to only 6,000 K, but by extrapolating the curve, I estimate the enthalpy of HF is zero at about 7,800 K.

In practice, the temperatures would be lower than this due to dissociation.  Furthermore, rockets don't use a stoichiometric mixture, thus further lowering the temperature.  Nonetheless, the above demonstrates that the combustion of F with H will produce higher temperatures than the combustion of O with H.

The exhaust gas velocity of a rocket engine, and thus the specific impulse, is roughly proportional to (T/M)1/2.  For the above two reactions we get:

(5000/18)1/2 = 16.67 and (7800/20)1/2 = 19.75

Remember, however, this is an over-simplified scenario.  When taking into consideration dissociation and realistic mixture ratios, fluoride will out perform oxygen by about 5% when burned with hydrogen.  When burned with hydrazine, fluoride does about 12% better than oxygen.


(edit)  Something else that further lowers the flame temperature is that the propellants in a rocket are cryogenic liquids rather than gases.  Some of the heat of combustion has to vaporize the propellants and raise their temperatures, thus resulting in a lower final temperature.
« Last Edit: October 15, 2010, 01:45:51 PM by Bob B. »

Offline Bob B.

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Re: Fluorine
« Reply #4 on: October 14, 2010, 06:42:33 PM »
Thus, a hydrogen/fluorine rocket will produce pollution several times worse than a comparable aluminum/ammonium perchlorate solid rocket or a hydrazine/nitrogen tetroxide rocket, much less hydrogen/oxygen.

I often see people comment on the toxicity of the exhaust from hypergols, but I question this.  Although the reactants are extremely toxic, the products should be pretty benign.  For instance, hydrazine/nitrogen tetroxide produces water and nitrogen, along with some hydrogen if burned fuel rich.  If MMH or UDMH is the fuel, there will also be some carbon monoxide and carbon dioxide.  There will be a few other trace gases due to dissociation, but nothing toxic.

If you've ever seen the launch of a Titan missile, there is a nasty looking red vapor cloud at start-up that is quite toxic, but that is just a one-time thing resulting from the start sequence.  The nitrogen tetroxide is injected into the chamber prior to the fuel, which turns into a red vapor when is hits the air.  Once the engine reaches the correct mixture ratio, the exhaust should be non-toxic.

Is there something I do not understand?

Offline ijuin

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Re: Fluorine
« Reply #5 on: October 14, 2010, 10:06:39 PM »
In theory, yes. However, there is always some small amount of unburnt fuel or oxidizer in the exhaust, if for no other reason than because no matter how you try, you can not pump them into the combustion chamber at exactly the right proportions for complete combustion (you may be off by a couple percent). Additionally, engines are often deliberately run with a fuel-rich mixture in order to deliberately increase the proportion of free hydrogen and carbon monoxide as compared to water and carbon dioxide, since doing so will lower the mean molecular mass of the exhaust, which increases specific impulse for a given operating temperature (most rocket engines run at less than the theoretical maximum temperature). This practice also results in some unburnt fuel in the exhaust.

For an everyday-life comparison, think of an automobile engine. Most auto engines also burn a fuel-rich mixture in order to maximize the energy per combustion chamber volume (a bigger chamber meaning a heavier/bulkier engine) at the expense of energy per unit fuel. As such, a significant fraction of unburnt or under-burnt fuel remains in the exhaust.

Offline Bob B.

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Re: Fluorine
« Reply #6 on: October 14, 2010, 11:48:28 PM »
I understand what you’re saying, but my understanding of “unburnt fuel” is that there isn’t enough oxygen present to fully oxidize all the carbon and hydrogen.  That is, instead of carbon dioxide we have mostly carbon monoxide.  And instead of all the hydrogen going into water, we have some HO, H and H2.  At the high temperatures of combustion, I doubt any of the original fuel molecules will survive without breaking down into simpler forms.  The “unburnt fuel” is the carbon and hydrogen capable of further oxidation, not fuel molecules that survive in their original form.  The exhaust of a Titan missile includes carbon and hydrogen in forms capable of further combustion, but it is not spewing unreacted hydrazine.

Offline DonPMitchell

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Re: Fluorine
« Reply #7 on: October 15, 2010, 03:46:46 PM »
I figure there just aren't enough rocket lauches to worry about polution.  If airline jets burned UDMH, I'd be worried.

I've been looking at Tsander's papers on rocket propellants.  Tsiolkovsky or Oberth are trivial.  But Tsander was a real engineer, and I haven't seen an entropy-enthalphy diagram since p-chem class at U. of Minn.
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Offline Bob B.

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Re: Fluorine
« Reply #8 on: October 16, 2010, 01:15:57 PM »
I figure there just aren't enough rocket lauches to worry about polution.  If airline jets burned UDMH, I'd be worried.

UDMH is certainly more dangerous to handle and transport than jet fuel, and I sure wouldn't want to be around if there were a mishap, but in regard to the exhaust produced, UDMH is actually a more "green" fuel.  Every ton of kerosene burned produces about 3.1 tons of carbon dioxide, while one ton of UDMH produces 1.47 tons of CO2.

Because of its extreme toxicity, UDMH is highly regulated and, as such, very expensive.  It is my understanding that the over-regulation and high cost of the propellants is what priced the Titan rocket into retirement.

I've been looking at Tsander's papers on rocket propellants.  Tsiolkovsky or Oberth are trivial.  But Tsander was a real engineer, and I haven't seen an entropy-enthalphy diagram since p-chem class at U. of Minn.

Ethalpy is simply a measure of the total energy in a thermodynamic system.  In combustion, the amount of energy needed to form the products is less than the amount of energy needed to form the reactants.  Since the products have less "internal" energy, the surplus energy is transformed into kinectic energy and raises the temperature.

This is why F2/H2 burns hotter than O2/H2.  It takes less energy to form a HF molecule than it does to form a H2O molecule; therefore, more of the ethalpy goes into raising the temperature than into forming the products.

Offline ijuin

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Re: Fluorine
« Reply #9 on: October 17, 2010, 12:04:42 AM »
UDMH is certainly more dangerous to handle and transport than jet fuel, and I sure wouldn't want to be around if there were a mishap, but in regard to the exhaust produced, UDMH is actually a more "green" fuel.  Every ton of kerosene burned produces about 3.1 tons of carbon dioxide, while one ton of UDMH produces 1.47 tons of CO2.

True, but CO2 is the least toxic pollutant produced. How much CO (deadly to mammals at like 100 parts per million) or nitrogen compounds gets produced per ton of UDMH/N2O4, compared to hydrocarbon fuels? I'd rather have more CO2 and less toxins than less CO2 and more toxins, thank you.

Offline DonPMitchell

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Re: Fluorine
« Reply #10 on: October 17, 2010, 12:27:58 AM »
I wouldn't worry about these simple modecules like CO.  THey oxydize in the atmosphere, and automobiles probably generate a million times more CO than rockets.  UMDH is probably much worse than its combustion products.

H2O4 is no picnic either, you don't want to inhale that orange cloud that comes out of the exhaust of one of these rockets.  But again, once it's released into nature, nitrogen oxides combine with water to form acid and then chemically combine with minerals in soil to form nitrates...at that point its harmless plant food.
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Offline DonPMitchell

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Re: Fluorine
« Reply #11 on: October 17, 2010, 01:53:27 AM »
WRT thermochemistry of rocket engines, I need to read Sutton's chapter 4 slowly, and hopefully then I will grok this.
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Offline Bob B.

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Re: Fluorine
« Reply #12 on: October 17, 2010, 10:48:24 AM »
WRT thermochemistry of rocket engines, I need to read Sutton's chapter 4 slowly, and hopefully then I will grok this.

I'm not familiar with Sutton's book, but any thermodynamics book ought to have a chapter on chemical reactions that will explain all this.  I'm certainly no expert but I've studied the subject enough to understand the basic concepts.

Offline Bob B.

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Re: Fluorine
« Reply #13 on: October 17, 2010, 03:48:27 PM »
UDMH is certainly more dangerous to handle and transport than jet fuel, and I sure wouldn't want to be around if there were a mishap, but in regard to the exhaust produced, UDMH is actually a more "green" fuel.  Every ton of kerosene burned produces about 3.1 tons of carbon dioxide, while one ton of UDMH produces 1.47 tons of CO2.

True, but CO2 is the least toxic pollutant produced. How much CO (deadly to mammals at like 100 parts per million) or nitrogen compounds gets produced per ton of UDMH/N2O4, compared to hydrocarbon fuels? I'd rather have more CO2 and less toxins than less CO2 and more toxins, thank you.

Just to be clear, I was replying to Don's facetious comment about airline jets burning UDMH.  It is my understanding that jet engines operate at very lean mixtures, thus CO2 would dominate over CO. (Beside, CO would eventually oxidize to CO2 in the atmosphere through natural processes.)  Furthermore, there is plenty of nitrogen in the air for the formation of nitrogen compounds – UDMH is worse in this regard than kerosene, but not significantly so.  I don't consider CO2 a toxin, but to the global warming crowd (and the EPA), it is a pollutant.  In this regard, kerosene is twice as bad as UDMH.

Of course UDMH will never be used as an jet fuel, so let me get back to rockets…

You are correct that there are no nitrogen compounds with Kerosene/LOX, however there is about twice the amount of carbon compounds produced versus UDMH/NTO.  I ran some calculations for the engines of Soyuz (Kerosene/LOX) and Long March CZ-2E (UDMH/NTO).  I figured the exhaust expanding to one atmosphere pressure.  Here is the composition of the exhaust by mass:

Soyuz / Engines: RD-107 (strap-ons)
O/F=2.48, Pc=60 atm, Pe=1 atm

CO    35.23%
CO2   33.74%
H2O   29.90%
H2     1.07%
HO    <0.06%
H     <0.01%
O2    trace
O     trace
C     trace

Long March CZ-2E / Engines: YF-20B
O/F=2.12, Pc=71 atm, Pe=1 atm

N2    35.63%
H2O   26.14%
CO2   19.21%
CO    17.65%
H2     1.37%
H     trace
HO    trace
NO    trace
O2    trace
O     trace
N     trace
NO2   trace
C     trace
Oxides of nitrogen make up 73 parts per billion.

Calculations were performed using the computer program STANJAN.  For kerosene, C12H26 was used.


Offline ijuin

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Re: Fluorine
« Reply #14 on: October 17, 2010, 10:54:03 PM »
How fast does the carbon monoxide react with atmospheric oxygen anyway (or disperse for that matter)? 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?