From: jcannin@nswc-wo.navy.mil (John Canning)
Date: Sep 8 1991

     The  concept of using a liquid propellant in guns is  not  a 
new  one. I have worked here at the Naval Surface Warfare  Center 
at Dahlgren, VA - the Navy's primary gun technology laboratory  - 
for  over twenty years. (Please, no comments regarding  the  IOWA 
incident. I wasn't involved in that affair, and couldn't  comment 
on  it even if I wanted to, which I don't.) I know that  research 
work  on  liquid gun propellants has been going on for  about  as 
long as I've been here, maybe longer. There have been a number of 
programs,  but  none has panned out to date. Since  hope  springs 
eternal,  there  is still work going on, but I won't  comment  on 
them  either for a number of reasons - not the least of which  is 
that  I've got a wife, three kids, and a dog that look to  me  to 
keep them clothed (The dog wears a sweater in snow.) housed,  and 
fed, and the government has a lien on certain sensitive  portions 
of my anatomy regarding breaches in security. 

     I  WOULD like to comment on some general concepts  regarding 
gun   internal   ballistics  and  how  they  relate   to   liquid 
propellants,  and relate to you my own personal  experience  with 
one such program from a long time ago. I can talk about this  one 
because  it  was my project (not by choice), and because  it  was 
never classified.

     First,   the  internal  ballistics  concepts:   Normal   gun 
propellants  for  large  bore guns  are  produced  in  propellant 
grains.  These grains are fairly good size, measuring about  half 
an inch in diameter and about an inch or so in length. (Hey,  big 
guns means big grains!) Consider a solid propellant grain and the 
way  it burns. For all intents and purposes you can  assume  that 
when  a  gun  is fired, a particular grain  will  begin  to  burn 
everywhere  on its exterior surface simultaneously. Overall  burn 
rate is a function of the particular propellant, breech pressure, 
and  the exposed surface area that is burning. As the  propellant 
begins  to burn, the pressure behind the projectile continues  to 
rise  until  the projectile begins to move. When  the  projectile 
begins  to  move, the volume in the breech  increases.  When  the 
volume increases, the pressure in the breech tends to drop. Well, 
at least it doesn't rise as fast as it did before the  projectile 
started to move. Anyway, the point here is that projectile motion 
will adversely affect breech pressure because breech volume keeps 
increasing.  This,  in turn, causes a drop in burn  rate,  or  at 
least adversely affects the acceleration of burn rate.

     At  the same time as this is going on, the surface  area  of 
the  propellant grain (Remember our solid propellant  grain?)  is 
also  decreasing because it is being burned away,  exposing  more 
propellant, which also burns away, making it even smaller, ... ad 
infinauseum.  The net result is that propellant gas is  generated 
at  much slower rates than you'd like and the projectile  doesn't 
come  out as fast as you'd like. This does not mean that you  can 
stand  in front of a gun and thumb your nose at the gunner.  What 
you really want to have happen is to have the pressure behind the 
projectile rise as continuously and as smoothly as it can.
     
     One  way  to  help  do  this is to  use  what  is  called  a 
perforated  propellant  grain.  These things  look  like  strange 
hamburger   helper  noodles.  Instead  of  a  single   hole,   or 
"perforation", through the center, they have a bunch. Seven-  and 
nine-perf propellants have been used. The idea here is that  when 
they  begin to burn all over, "all over" includes the insides  of 
the  perforations too. So, while the outside burning  surface  is 
getting smaller, the inside burning surface is getting bigger. If 
the  propellant type, grain design, gun, and projectile  are  all 
properly  matched  you  get a smoothly  rising  pressure  as  the 
projectile  moves down the barrel, but not for the entire  length 
of  the  barrel. ("What does all of this have to do  with  liquid 
propellants?", you ask. Be patient, I'm getting to that.)

     Now, consider the size and shape of pistol powder. A lot  of 
it looks like ground pepper. (An exaggeration, but bear with me.) 
What  do  you  think would happen if you were to use  it  as  the 
propellant  for  a large bore gun? It has a LARGE  total  initial 
area  to  set on fire. Remember that burn rate is a  function  of 
this  surface  area.  (Click! The brain  lights  come  on!!)  The 
initial burn rate would be so high it would cause the pressure to 
spike behind the projectile before it can move, which would raise 
the  burn  rate, which would spike the  pressure  further,  which 
would.....BANG!!!! 

     As  the  smoke clears, we find the remains of  a  gun  mount 
scattered  hither, thither, and yon around the countryside.  Now, 
here's  where the part about liquid propellants comes  in.  Solid 
propellants  are  just  that - solid. You can  predict  what  the 
surface  area  exposed to burning is as a function of  time  with 
them.  You can't do this as well with liquid propellants  because 
they  slosh  around and change their exposed surface  shapes  and 
amounts  -  even  while  burning. The  people  that  continue  to 
experiment with these propellants play statistical games in  this 
regard.  I'm  not saying that they'll never get there, but  I  am 
saying  that they've got their work cut out for them -  and  they 
continue  to  blow  up  gun mounts.  Also,  one  of  the  biggest 
variables in gun accuracy at range is muzzle velocity. They  will 
need a VERY predictable burning rate in order to properly control 
muzzle velocity.

     I  wish  them  well  because  there  are  some   significant 
logistics benefits that could be had from such a system, but  I'm 
not holding my breath.

     Now,  I  promised  to tell you  about  a  liquid  propellant 
program that I was personally involved in. A looooong time ago we 
had  a  member  of "Upper Management"  who  isn't  here  anymore. 
(Matter of fact, I'm not sure he's anywhere anymore - he may have 
died  by this time.) This individual thought it would be  a  good 
idea if Navy ships wouldn't have to load propellant charges  when 
they re-stocked, but could just strike down projectiles into  the 
magazines.  His idea was that we ought to be able to  manufacture 
our  own  propellant from the sea. After all,  he  reasoned  that 
water  is made from hydrogen and oxygen, and you can burn  those, 
so  we  ought to be able to figure out a way to use this  to  our 
benefit. 

     Thus  was born the Hydrogen-Oxygen Gun Program, or  the  HOG 
Program as we called it. (I swear that I'm not making this up.  I 
still  have  the  design folder with all my material  in  it.)  I 
thought it was a dumb idea from the beginning, but I was the duty 
stuckee. Well, you can imagine the ribbing I took at the hands of 
my  co-workers.  I think I heard every "pig" joke that  was  ever 
dreamed  up. Anyway, I ran into a number of design problems  that 
singly  and  collectively killed the program. Thank  God!  I  was 
beginning to hate the sight of bacon.

     For  starters, seawater has salt as one of  its  components. 
This  guy's  idea  was to electrolyze  seawater  to  produce  the 
required  hydrogen and oxygen and then to liquefy it to  get  the 
necessary  energy density. Wrong! You get hydrogen  and  chlorine 
when  you electrolyze seawater. A nasty combination in  anybody's 
book.

     "Well, how about we only keep the hydrogen and get the oxygen 
from  liquefying air? After all, we do this all the time  onboard 
carriers to supply pilots while they're flying." I had to hand it 
to this guy, he got an "A" for inventiveness. I began to look  at 
this avenue. It began to look like we could actually do it, so  I 
started to look at gun designs to handle it. This was where I got 
to research liquid propellants. One BIG difference in this  setup 
over everyone else - we were working at cryogenic temperatures. 

     I ran into the "exploding gun" phenomenon during some  early 
testing that we did using just high pressure gaseous hydrogen and 
oxygen  in a closed bomb that we were using to try  to  establish 
burning  rates vs pressure curves. Fortunately, it was  a  small, 
thick walled bomb (for safety reasons) and all we did was destroy 
the  pressure gage that was screwed into the side.  The  downside 
was  that it went off before we were ready for it.  The  ordnance 
men had just finished hooking everything up and were exiting  the 
test  chamber. I was right behind them when we all heard  a  very 
loud  click. We thought something had dropped off the  test  rig. 
Our instrumentation man reported that the pressure gage had  gone 
haywire  and asked us to check it. We bled the pressure  off  the 
bomb and backed the pressure gage out. It fell apart in our hands 
and  looked  like someone had hit it with a  sledge  hammer!  The 
click  had been the test mixture detonating. We found  out  later 
during our investigation that one of the ordnance men had used  a 
rag  with some oil or grease on it to clean out the bomb  between 
test runs. Petroleum products and high pressure oxygen don't mix! 
This  presents  a  serious  problem  for  shipboard   operations, 
particularly for a space like a gun mount.

     If high pressure oxygen and petroleum products are a  safety 
problem,  then cryogenic oxygen presents a nightmare!  Turns  out 
that   when   petroleum  products  get   chilled   to   cryogenic 
temperatures,  they become shock sensitive. Fire departments  get 
real  nervous  when  they  hear they've  have  a  truck  accident 
involving  liquid  oxygen  on paved roads. Don't  even  think  of 
throwing  a rock toward the wreck, let alone walking over to  it! 
(Not sure who'd be that stupid, but you never know!)

     Cryogenic  hydrogen  is no better. As I recall,  DoT  safety 
regs state that this material is dangerous if left in a  confined 
space  -  and  they defined a confined space  as  "three"  walls. 
Remember the Hindenburg.

     In  order  to  avoid instant detonation  of  the  propelling 
charge,  it was obvious that we couldn't have the  entire  charge 
inside the breech at the same time. We began to investigate  ways 
of continuously injecting the charge for the length of the  burn. 
Sliding  seals at these temperatures were a problem, as  was  the 
overall injection mechanism design.

     Lastly, we found that the steel that guns are normally  made 
of becomes brittle at cryogenic temperatures, not well suited  to 
the kinds of shock loading you would expect from gun firings.  So 
we also found material problems.

     The  research  funding  ran out and the  results  looked  so 
negative that it was decided not to renew the effort. As much  of 
a pain it had been, it had been a real learning experience for  a 
young  engineer.  I  look  back on  this  experience  with  mixed 
emotions.

     I  don't work guns anymore, haven't for over ten  years.  My 
comments here are based on my past experience, and don't  reflect 
any  official  opinion of the Center. Just thought you  ought  to 
know.

                                John S. Canning

                                Combat Systems Engineer
                                Naval Surface Warfare Center
                                Dahlgren, VA

Subject: Re: Liquid Propellant?
From: norton@ACM.ORG (Scott Norton) 
Date: Jan 05 1996
Newsgroups: sci.military.moderated

In our discussion of Liquid Propellant (LP) for artillery, there are
both general discussions (using a variety of liquid propellants) and
discussion specific to the US Army's Crusader (formerly AFAS) gun.

Crusader's LP is a monopropellant, premixed.  It contains HAN
(hydroxyl ammonium nitrate) and HES (can't recall what that stands
for) in water.  It is nonflammable, and passes the Army's insensitive
munitions (IM) safety criteria.  It will only react when under
pressure.  It is about as acid as lemon juice, and as poisonous as
asprin.  Its biggest problems are that it gets more sensitive if
contaminated by iron ions, and for it to pass the all the IM tests,
the tank geometry is constrained.  So it can't just be kept in a fuel
tanker or "water buffalo" type of container, but one designed to
prevent build-up of high pressure. 

The big advantages of LP are in the logistics area: 

- It can be replenished by pumping, so the resupply operation goes
quicker. 

- It is truely incremental.  While current field artillery fires
different increments, unused bags are not used again.  If you fire
only a single increment, the remaining charges are just tossed out.

A disadvantage of the LP design used in the Crusader is its
regeneratively pumped piston approach.  To get this incremental
capability, and to avoid high peak pressures, the fuel is metered into
the combustion chamber, pumped by the chamber pressure acting on a
piston at the back of the chamber.  If things go correctly, the piston
pumps LP into the combustion chamber faster than it combusts.  But if
the piston hangs up or if the seals leak, the combustion can proceed
into the fuel reservoir, causing a "pressure reversal."  This can
destroy the gun.  (Note: the combustion goes back into the
as-yet-uninjected fuel, not into the storage tank.)  This problem is a
design problem--once solved, it should not be a problem in fielded
systems. 

There are a few ways in which the theoretical advantages of LP get
diluted by practical considerations.  For example, because the HAN-HES
LP contains a substatial percentage of water, its volumetric energy
density is less than a good solid propellant.  Although the
tank-stored LP packs more efficiently than granular solids in
increments, the solid is more energy-dense.

Also, the piston is a big mechanism, and its size and weight may be
too great a price to pay.

Still, the Army is spending big bucks on LP.  Some of the difficulties
are fixable in design, and the advantages remain even if they are
diluted.  And the equivilent solid-propellent technology is no cake
walk, either.  New systems are held to very high standards for IM,
safety, enviromental cleanliness, and performance.  Current
solvent-based granular propelling charges produce a big waste stream
when manufactured and when demilitarized, have trouble meeting IM
requirements, and require lots of work by the gunners to use.

Crusader is looking to be more like a naval gun, with a smaller crew,
an autoloader, and a high rate of fire.  LP would be a big help.

For comparison, not that Otto fuel, used in torpedoes, is also a
liquid monopropellant.  While the Army LP is somewhat difficult, by
comparison to Otto Fuel, Army LP is grape juice.  Otto fuel is
downright evil.

--  
Scott Norton
Norton@ACM.org
Defense Technology, Inc.
2361 Jefferson Davis Hwy, Suite 500
Arlington VA  22202-3876
(703) 415-0200, fax: (703) 415-0206

Subject: Re: Liquid Propellant?
From: norton@ACM.ORG (Scott Norton) 
Date: Jan 23 1996
Newsgroups: sci.military.moderated

In my previous post concerning Liquid Propellant, I said that the fuel
was HES.  It is, in fact, TEAN.  Again, I'm posting away from my
sources, so I can't recall what TEAN stands for exactly.  The T is
either tri or tetra; the E is either ethyl or ethanol; the AN is
ammonium nitrate.

-- 
Scott Norton
Norton@ACM.org
Defense Technology, Inc.
2361 Jefferson Davis Hwy, Suite 500
Arlington VA  22202-3876
(703) 415-0200, fax: (703) 415-0206

Subject: Re: AFAS - liquid vs solid propellant
From: norton@ACM.ORG (Scott Norton) 
Date: Mar 24 1996
Newsgroups: sci.military.moderated

In article <DMq62B.GC1@ranger.daytonoh.attgis.com> "LEYTE C" <A7QA@unb.ca> writes:

> A recent article in Jane's Defence Weekly (24 Jan 1996) brought up 
> an issue that is dividing the US Army in relation to AFAS artillery 
> system development. While the project has been thus far concentrating 
> on the development of liquid propellant, there may be a decision to 
> revert to the less risky solid propellant technology. Proponents of 
> the solid propellant system cite its proven technology and increased 
> work for Army depots, while the liquid propellant camp says the new 
> technology is vital to gaining the advantages in range and firepower 
> sought by AFAS. I would tend to agree with the latter argument. Isn't 
> the whole goal of the AFAS/Crusader programme to develop a tube 
> artillery system that can outrange the enemy (with efficiency) ? My 
> personal opinion about these types of decisions is that if you are 
> going to spend all that money on a new weapon intended to serve for 
> quite some time to come, you might as well go with the latest 
> feasible technology. Besides, if a decision is made to revert to 
> solid propellants now, it will be necessary to redesign many key 
> components of the AFAS/Crusader system. Any opinions or thoughts on 
> the issue of solid vs liquid propellant for AFAS ? 


Liquid propellent would be really good, but the current design is not
without its own tradeoffs.

First, there have been some problems in development of the gun's
regenerative pumping system.  It is critically important that the
seals not leak, or the supply part of the system will ignite and blow
up.  

Second, the regenerative piston is a big hunk of metal.  About as big
as a 16 in gun's breech plug.

Finally, the bulk storage advantages of the current HAN-TEAN
monopropellant are diluted somewhat if you want to meet insensitive
munitions criteria.  You can't have too large a volume, or you'll get
a high-order reaction to some of the tests, like shaped charge impact.
Instead of one big tank, you have to go to a series of pipes, like a
car's radiator.

Some general explaination for those who don't know what AFAS is, or
why liquid propellant is being discussed.

AFAS, now called Crusader, is the US Army's Advanced Field Artillery
System, an autoloading, self propelled 155 mm howitzer.  It will have
a matched support vehicle for rapid rearming as well.  One of its
goals is to get a simultaeous time-on-target by shooting three rounds
on three different trajectories.  The first gets a small charge and a
high trajectory, and the following rounds get large charges and
flatter trajectories.  They all hit at once, increasing the lethality
since the troops don't have time to fall prone or crawl into holes.

For both logistic and performance reasons, the Army would like to use
a liquid propellant for Crusader, rather than the granualar, bagged
powder they use in other 155 mm guns.  Liquid propellent is easier to
resupply (you pump it like diesel fuel), it packs more efficiently
into tanks and drums, it fills the chamber with no void spaces giving
greater energy density, and it can be used incrementally, with only as
much propellent as needed for the range.  (When a howitzer fires less
than a full charge, it doesn't save the powder for the next mission.
The bags have different grain sizes, and you can't throw the tail ends
into another shot.  Instead the unused propellent is discarded,
burned, or reclaimed at a recycling plant.)

Liquid propellent can also be injected in a precisely metered rate, to
help make the pressure profile less spiky and more flat-topped.  This
gives more muzzle velocity for a specific pressure limit.  In solids,
this behavior is controlled by the grain geometry.

The specific propellent developed for Crusader is a mixture of HAN
(hydroxyl ammonium nitrate) and TEAN (tri-ethanol ammonium nitrate) in
water.  HAN is the oxidizer, and TEAN is the fuel.  The water makes it
very insensitive to fast and slow cookoff, bullet impact, and
fragments.  It reacts only when under pressure.

The problem with just filling up the breech with liquid is that the
pressure shoots up to a peak exponentially.  To produce a nice, long,
flat-topped pressure profile, the propellent must be metered into the
chamber through the ballistic cycle.  Crusader has developed a
regenerative injector, where chamber pressure pushes a piston, which
forces more propellent into the chamber. 

-- 
Scott Norton
Norton@ACM.org
Defense Technology, Inc.
2920 South Glebe Road
Arlington VA  22206
+1-703-299-1656, fax: +1-703-706-0476

From: gwyn@arl.mil (Doug Gwyn )
Newsgroups: rec.guns
Subject: Re: [TECHNICAL] Binary Liquid Propellants
Date: 31 Jan 1997 18:45:03 -0500

In article <Pine.SUN.3.95.970128075224.17760A-100000@Isis.MsState.Edu> James H Galt-brown <jhg3@Ra.MsState.Edu> writes:
#	The U.S. military has been working on a binary liquid propellant
#system for some years now.

Indeed, two APG/Edgewood scientists received awards for the practical
development of binary liquid propellant systems.  Later on they were
sacrificed by their own management for local PR purposes, for the
environmental "crime" of flushing acid down laboratory drains, which
of course was standard chemical lab practice at the time they did it.

Newsgroups: sci.military
From: ahahma@utu.fi (Arno Hahma)
Subject: Re: Liquip Propellent Gun
Message-ID: <C0G17H.16t@law7.DaytonOH.NCR.COM>
Date: Wed, 6 Jan 1993 17:42:53 GMT

From ahahma@utu.fi (Arno Hahma)

In article <C0E5K8.6rK@law7.DaytonOH.NCR.COM> Fengxi Zhou <fengxi@prancer.eche.ualberta.ca> writes:

>gun? Almost all guns are fired by solid explosives. I understand GE or FMC is
>studying the possibility of liquid fired guns. Reportedly, it has higher

That is right, many companies study such guns. GE already has working
models, if not already in production/field use. Rheinmetall is
planning to get one into production soon.

>Questions:
>1. Why higher muzzle velocity than solid gun?

Because you can adjust the pressure-time history of the firing. Just
adjust the propellant feed and try to maintain a constant pressure in
the tube. That is _very_ hard to do with solid propellants - the
Germans tried it during the WW2 with little success (V-3
"Hochdruckspumpe", planning to shoot some 200 km from France to
London).

In addition, the liquid propellant has a higher specific impulse than
the solid propellants. The liquid propellant is much less erosive than
high energy powders (lower combustion temperature). All of these give
rise to a better performance.

>2. How about rapid firing? (2 step loading!)

No problem, as the propellant is fed with pumps. Load the ammo, close
the breech and fire, just like before. Pumping the propellant is done
very fast, there is a small initial volume and additional propellant
is fed during combustion, like in a diesel engine. This is usually
done using the gas pressure in the tube.

>3. Any disadvantages compared to solid propellent guns?

Yes: much more expensive. Both the gun and the propellant. It has been
estimated, that the liquid propellant costs about 10 times more than
the present solid propellants. Still, the liquid propellant gives so
many advantages, that it is considered cheaper (easy transport, storage
and waste disposal, no cartridges, no factories for loading the
cartridges, far safer and less vulnerable, etc).

>Fengxi Zhou
>fengxi@prancer.eche.ualberta.ca

ArNO
    2