From: glhurst@onr.com (Gerald L. Hurst)
Newsgroups: rec.pyrotechnics
Subject: Re: Zn/S mixture
Date: 2 Nov 1995 08:20:59 GMT
Organization: Consulting Chemist
Lines: 35

In article <478f1lINNant@newsman.murdoch.edu.au>, mykestan@csu.murdoch.edu.au (Myke Stanbridge) says:

>Its
>product is, of course, vaporized zinc sulphide of 
>a high molecular weight, which is not a very good 
>product for making a fast rocket. The momentum it
>transfers compared to nitrogen molecules (~28) is
>rather low. Look up a physics book for thrust and
>specific impulse for more information.

I think the problem is more one of energy per unit
weight rather than molecular weight per se. A
reaction energy of O.5 kcal/g is a piddly amount 
compared with the 1.0 or better value you might 
get from a reasonable mixture of organic fuel and 
a fair oxidizer. Nitrogen molecules won't propel 
you along any better than zinc sulfide on an equal 
energy-per-unit-weight basis.

It turns out, of course, that reactants with high
atomic weights generally yield relatively low
energy-per-weight exhaust products because the
big old Zn atom and its cousins have only a few 
valence shell electrons just like the lower atomic 
weight species but tote around a lot more mass. 
The same is true of sulfur; it accepts 2 electrons 
in its simplest reaction with metals, just as 
oxygen does, but it has about twice as much mass 
to shove around.


Jerry

From: glhurst@onr.com (Gerald L. Hurst)
Newsgroups: rec.pyrotechnics
Subject: Re: Zn/S mixture
Date: 8 Nov 1995 02:15:09 GMT
Organization: Consulting Chemist
Lines: 69

In article <47oi61INNa6@newsman.murdoch.edu.au>, mykestan@csu.murdoch.edu.au (Myke Stanbridge) says:

>Note: Zn + S is about 0.52 kcal/gram while Black powder
>is about 0.65 kcal/gram. This is not big enough to make
>the difference observed when used in similar rockets. I
>would like to know why BP rockets perform at least some
>3x better than ZnS rockets; using a propellant which is 
>afterall just a little bit less piddly than ZnS? 
>
>Recall: <Mw> for BP (75:15:10) is about 80.84 - whereas 
>        <Mw> for Zn+S is about 54.44, where <Mw> refers
>to the average formulation molecular weight. 
>
>Just luv them technical terms! 

Mike, you need to quote enough of the article to 
which you are replying to give us an idea of the 
subject matter.

The average molecular weight of the propellant 
composition has absolutely nothing to do with
the performance. For example, one could make
a rocket from a nitrated polymer with a MW 
of over 10,000. In your black powder, the MW
of the sulfur is closer to 250 than it is to 32.
Similarly, the carbon and KNO3 have "MWs" 
corresponding to the crystallite magnitude since
they are made of semi-infinite lattices and the
the  empirical formula units are discrete only in 
our minds.

Zn/S and BP rockets differ in their performance
largely for the same reason that green powder,
and fine and course BP all differ from one
another - they may have the same gross energy
output, but they burn at different rates and
sometimes in different places (e.g. outside
the rocket).

Specific impulse is a measure of what you MAY
be able to get out of a propellant if you 
formulate it and handle it properly. Energy 
gives you an idea of the potential specific 
impulse but it is no guarantee for several 
reasons. You also need to know how much of 
your energy is wasted in heating the products
- not all of it goes into thrust. Look at the
extreme case of thermite: Lots of energy but
practically none of it manifested as thrust.

You want energetic reactions that don't
drive a rocket, well, then watch a car rust
or an oil-base paint harden :)

Now, the bright side is that if you can get
a reaction to yield only simple, preferably
only monatomic or diatomic gases (for low
heat capacity) and if you can control the
reaction rate, then you can happily build
a rocket that will only be limited by the
energy release per gram no matter what
the molecular weight of the EXHAUST gases 
is.

And when you have mastered this science, NASA
will have a place waiting for you.

Jerry

From: glhurst@onr.com (Gerald L. Hurst)
Newsgroups: rec.pyrotechnics
Subject: Re: Zn/S mixture
Date: 8 Nov 1995 23:05:52 GMT
Organization: Consulting Chemist
Lines: 98

In article <47prq7$2n2@ixnews5.ix.netcom.com>, silent1@ix.netcom.com (The Silent Observer) says:

>In article <47oi61INNa6@newsman.murdoch.edu.au>, Myke Stanbridge 
>(mykestan@csu.murdoch.edu.au) says...
>
>>Note: Zn + S is about 0.52 kcal/gram while Black powder
>>is about 0.65 kcal/gram. This is not big enough to make
>>the difference observed when used in similar rockets. I
>>would like to know why BP rockets perform at least some
>>3x better than ZnS rockets; using a propellant which is 
>>afterall just a little bit less piddly than ZnS? 
>>
>>Recall: <Mw> for BP (75:15:10) is about 80.84 - whereas 
>>        <Mw> for Zn+S is about 54.44, where <Mw> refers
>>to the average formulation molecular weight. 
>
>
>Well, actually, BP has only about twice the specific impulse of Zn/S 
>micrograin -- micrograin runs about 80, BP is approximately 130, if I 
>remember aright.
>
>The difference isn't in the average molecular weight (AMW) of the 
>propellant, though -- it's the in the AMW of the reaction products.  The 
>reaction product of a Zn/S propellant is mainly zinc sulfide, previously 
>quoted here as MW 97+.  The products of BP combustion are CO, CO2, N2, 
>KOH, SO2, and possibly a little CS2 -- the exact proportions, and 
>presence of other impurities, varies in a regular witch's brew, but 
>those are the mainstays.  Of them, CS2 is the heaviest, at MW about 84, 
>but it's a >very< minor component -- CO2 and CO are the largest part of 
>the product, with SO2 the heaviest component present in significant 
>quantity.  The AMW of the resultant is most likely in the range of 44, 
>give or take a bit -- less than >half< that of zinc sulfide.
>
>Why is this an advantage?  A given amount of energy, measured in 
>kcal/mole, will raise the temperature of a "light" gas (one of low AMW) 
>more than a heavier one, and the resulting higher gas temperature can do 
>more work; this gives a higher exhaust velocity, which means more thrust 
>for a given mass flow rate -- and thus, more impulse for a given 
>propellant mass.  In addition, a lighter gas is less prone to become a 
>liquid (or even a solid) either under pressure (as ZnS may do in some 
>motor chambers) or when the temperature drops during the passage through 
>the nozzle (as certainly happens with ZnS exhaust -- that's why it's so 
>smokey).  The only BP components that become liquid or solid in the 
>exhaust are the KOH and CS2 -- neither of which is a huge fraction fo 
>the total exhaust mass.

Silent, I guess you don't agree with my postings on this matter, but
let me say again that I do not believe that the MW of the exhaust
gases is of direct consequence in determining specific impulse.

It is true that high molecular weight generally results in low energy
per molecule, but given an equal energy per mole basis the temperature
which is achieved does NOT depend on molecular weight, but on
molecular complexity because of the associated degrees of freedom
which result in the partitioning of energy among translational,
vibrational and rotational modes. In short, a given amount of heat 
will raise the temperature of a mole of nitrogen (MW = 28) higher
than it will a mole of water (MW = 18) because the nitrogen is 
merely diatomic whereas the water is triatomic. If the structure
of gaseous ZnS is diatomic (which it may or may not be) then
its molar heat capacity is probably comparable to that of Nitrogen,
carbon monoxide or AlO (not Al2O3) which are all similar. It
so happens that CO2 has an exceptionally high heat capacity
compared with other common exhaust products, including water.

I would look elsewhere for an explanation of the relatively
low specific impulse observed with Zn/S propellants. There are
plenty of other places to look :)

Jerry

Newsgroups: rec.pyrotechnics
From: nelson@cs.rochester.edu (Randal Nelson)
Subject: Re: Zn/S mixture
Organization: University of Rochester Computer Science Department
Date: Fri, 10 Nov 1995 02:23:34 GMT
Lines: 86

> Lots of discussion about importance of molecular weight, or lack thereof...

What no one has mentioned so far is, as far as rockets are concerned,
the critical factor is mainly the energy per gram of fuel rather than the
energy per mole.

In an ideal engine, the thermal energy of the propellant is
completely converted into kinetic energy with a velocity vector directed
away from the nozzle.

Thermal energy/gm -> kintic energy/gm = v^2/2

Thus the ultimate theoretical performance of a fuel is
characterized by a specific velocity. For historical reasons, this
velocity is generally divided by the acceleraation of gravity, g to
yield a specific impulse in seconds - basically, the time a given unit
of fuel could support its own weight (if it wasn't changing)
against gravity. Since g is about 10m/sec, this means that the
specific velocity of a fuel with Isp 200 is about 10*200 = 2000m/sec.

So the ultimate theoretical performance of a fuel depends only
on the available thermal energy per gram, not on the molecular
weight of the components, the products, or the degrees of freedom
in the atoms

As a practical matter, however, the molecular weight tends to be highly
correlated with the Isp for several reasons.

First, the energies of chemical reactions are typically more or less
proportional to the number of atoms involved, not their weight.
In fact, reactions of heavier atoms are generally a bit less energetic
per atom due to the closer spacing of typical electronic energy levels.
Thus low-atomic weight (correlates with low molecular weight for simple
molecules) generally have proportionately more energy available per
gram, thus higher Isp.

Second, if we assume that the products behave as an ideal gas,
(a bad assumption, but it's the best possible situation), then
theoretically, all the thermal energy can be converted - by an
infinitely long nozzle. However, as several people have
pointed out, the temperature of a high molecular weight ideal gas with a given
thermal energy per gram will be proportionally higher than that
of a low molecular weight gas with the same energy per gram.
This is bad news for nozzle and chamber components in the presence
of the HMW gas.

Third, high molecular weights are also associated with complex molecules
and lots of internal degrees of freedom.
Although this energy is theoretically convertible with a nozzle,
the process is slower and less efficient (i.e. you need to make your nozzle
longer and longer)  since the vibrational and
rotational energies must be redistributed into the euclidean degrees of
freedom through collisions before it can be extracted by a nozzle).

Fourth, high molecular weight exhaust products tend to be correlated
with high solidification temperatures. This is bad news, since as
soon as a product solidifies, most of its energy get dumped into
huge numbers of internal degrees of freedom, and the nozzle extraction
efficiency through redistribution into the 3 euclidean DOFs is
essentially zero. If there is no other transfer mechanism, this
energy tends to be lost through radiation.
One solution that is used, e.g. in propellants using aluminum as
a fuel because of its extremely high energy content per gram
in combination with an oxygen source, is to provide a source of
low molecular weight gas that can extract the energy from the
solid particles via conductive cooling; this heated gas can then be
accelerated by the nozzle.

Computation of the actual Isp for a nozzle is a horribly complicated
business. The relevant parameter is the expansion ratio, (and secondarily the
length, because of the effect of reaction rates) but how much of th
available chemical energy can be converted to
kinetic energy with a specific configuration is a horribly complicated
function of reaction kinetics (for a given chemical system, only
some fraction of the chemical energy is available at any given
temperature) reaction rates, thermal transfer coefficients, etc. etc.
Even NASAs best models contain significant approximations, and
must be verified experimentally.

RN

-- 
  Randal Nelson			 716-275-8488	University of Rochester
		      nelson@cs.rochester.edu	Computer Science Department
..!{allegra,decvax,rutgers}!rochester!nelson	Rochester, New York,  14627

From: glhurst@onr.com (Gerald L. Hurst)
Newsgroups: rec.pyrotechnics
Subject: Re: Zn/S mixture
Date: 10 Nov 1995 08:26:44 GMT
Organization: Consulting Chemist
Lines: 37

In article <1995Nov10.022334.29828@cs.rochester.edu>, nelson@cs.rochester.edu (Randal Nelson) says:

>> Lots of discussion about importance of molecular weight, or lack thereof...
>
>What no one has mentioned so far is, as far as rockets are concerned,
>the critical factor is mainly the energy per gram of fuel rather than the
>energy per mole.

No, it merely looks that way because the thread has drifted to 
other related topics. The original question can be paraphrased
as "Howcum ZnS has roughly the same energy per gram as BP, but
doesn't deliver nearly as much SI."

And at least one answer thread said essentially: "'cause there are
a number of ways energy makes its way out of the rocket other than
by spurting out a stream of vibratationless, rotationless particles
all traveling at 180 deg with respect to the direction of the rocket."

Naturally the conversation took a number of turns regarding the 
nature of the waste heat mechanism. One school held that massive
molecules waft away more more heat because they have greater
heat capacity than their lighter brothers. An opposing school
pointed out that the heat capacities of gases are a function
of degrees of freedom, i.e. complexity of a molecule rather than
the mass, although "complex" often has connotations of mass.

At about this point, you strolled in and opined that energy per
gram is what counts, IN THEORY but ......

Anyway, the balance of your article is a fine piece of work 
which deserves to be read by all budding rocket scientists.
Who knows, one of them may be inspired to develop a coal-
burning rocket with a nozzle so efficient it spurts dry ice
at 4,000 m/s.

Jerry

From: glhurst@onr.com (Gerald L. Hurst)
Newsgroups: rec.pyrotechnics
Subject: Re: Zn/S mixture
Date: 10 Nov 1995 08:55:33 GMT
Organization: Consulting Chemist
Lines: 26

In article <47v2c4$6i7@geraldo.cc.utexas.edu>, glhurst@onr.com (Gerald L. Hurst) says:

>In article <1995Nov10.022334.29828@cs.rochester.edu>, nelson@cs.rochester.edu (Randal Nelson) says:
>
>>> Lots of discussion about importance of molecular weight, or lack thereof...
>>
>>
>>What no one has mentioned so far is, as far as rockets are concerned,
>>the critical factor is mainly the energy per gram of fuel rather than the
>>energy per mole.
>>
>No, it merely looks that way because the thread has drifted [Snip]

Sorry to quote myself. but I just noticed that part of this
thread is missing from my newsreader so that what is there
gives a somewhat misleading view of prior posts. In one of
my posts I called the energy per gram of BP and ZnS "piddly."
Mike standbridge semi quoted me in a reply on 11/7 without an 
introductory quote, but my post seems to be gone. This would
account for Randle Nelson's observation that the importance of
energy per gram had not been discussed.

Or maybe I've slipped a neuron. Sigh.


Jerry

From: glhurst@onr.com (Gerald L. Hurst)
Newsgroups: rec.pyrotechnics
Subject: Re: Zn/S mixture
Date: 11 Nov 1995 00:06:59 GMT
Organization: Consulting Chemist
Lines: 43

In article <BILLW.95Nov10021002@puli.cisco.com>, billw@puli.cisco.com (William ) says:

>Hmm.  Is exhaust velocity solely due to temperature, or is it dominated by
>fluidics?  I think what you're saying is that the molecular effects come into
>play and cause the gasseous products to behave in a non-ideal fashion (ideal
>gasses would have exhaust velocity directly related to internal temp and
>pressure, and temp would be directly proportional to energy added, right?)

If you are talking about velocity in the sense of the component
of molecular velocity directed opposite the direction of motion
of the rocket, then the temperature inside the rocket and the
heat capacity of the gases determine the limit of this velocity 
but fluidics determines how close you you can get to that limit.

Internal temperature is roughly proportional to the total energy
but deviates from this value if there are polyatomic gases
involved because their molar heat capacities tend to increase
with temperature as new degrees of freedom kick in.

Ideally, the exhaust gas temperatures would be very low because 
of the molecules having lost much of their random molecular motion 
(heat) by conversion into directed kinetic energy, in the form of 
"cold" molecules moving together at great velocity opposite the 
rocket's direction of acceleration.

>Are heat capacities constant with temperature?  Intuitively, ZnS, which
>some have theorized exists temporarilly in liquid form inside a Zn/S
>motor, is much further from ideal gas behavior than N2, CO2, and H20,
>all of which have triple points that are much lower (and so can only
>exist as gasses).  But that's just intuition - I don't actually remember
>anything I ever learned that said that gasses behave more ideally far
>above their triple points...

As was mentioned above, the heat capacities of molecules having more 
than one atom tend to increase with temperature. Gasses generally 
behave more ideally well above their critical temperatures, at low 
pressures. The critical temperature is the highest temperature at 
which a gas can be condensed. The triple point, per se, has little 
relevance to discussions of ideality except insofar as materials 
with low triple points generally also have low critical temperatures.

Jerry