Liquid Propellant

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Liquid propellant (5/21097) (John Canning; Scott Norton; Doug Gwyn)IndexHomeAboutFrom: [email protected] (John Canning)Date: Sep 8 1991

The concept of using a liquid propellant in guns is not anew one. I have worked here at the Naval Surface Warfare Centerat Dahlgren, VA - the Navy's primary gun technology laboratory -for over twenty years. (Please, no comments regarding the IOWAincident. I wasn't involved in that affair, and couldn't commenton it even if I wanted to, which I don't.) I know that researchwork on liquid gun propellants has been going on for about aslong as I've been here, maybe longer. There have been a number ofprograms, but none has panned out to date. Since hope springseternal, there is still work going on, but I won't comment onthem either for a number of reasons - not the least of which isthat I've got a wife, three kids, and a dog that look to me tokeep them clothed (The dog wears a sweater in snow.) housed, andfed, 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 regardinggun internal ballistics and how they relate to liquidpropellants, and relate to you my own personal experience withone such program from a long time ago. I can talk about this onebecause it was my project (not by choice), and because it wasnever classified.

First, the internal ballistics concepts: Normal gunpropellants for large bore guns are produced in propellantgrains. These grains are fairly good size, measuring about halfan inch in diameter and about an inch or so in length. (Hey, big

guns means big grains!) Consider a solid propellant grain and theway it burns. For all intents and purposes you can assume thatwhen a gun is fired, a particular grain will begin to burneverywhere on its exterior surface simultaneously. Overall burnrate is a function of the particular propellant, breech pressure,and the exposed surface area that is burning. As the propellantbegins to burn, the pressure behind the projectile continues torise until the projectile begins to move. When the projectilebegins to move, the volume in the breech increases. When thevolume increases, the pressure in the breech tends to drop. Well,at least it doesn't rise as fast as it did before the projectilestarted to move. Anyway, the point here is that projectile motionwill adversely affect breech pressure because breech volume keeps

increasing. This, in turn, causes a drop in burn rate, or atleast adversely affects the acceleration of burn rate.

At the same time as this is going on, the surface area ofthe propellant grain (Remember our solid propellant grain?) isalso decreasing because it is being burned away, exposing morepropellant, which also burns away, making it even smaller, ... adinfinauseum. The net result is that propellant gas is generatedat much slower rates than you'd like and the projectile doesn'tcome out as fast as you'd like. This does not mean that you can


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stand in front of a gun and thumb your nose at the gunner. Whatyou really want to have happen is to have the pressure behind theprojectile rise as continuously and as smoothly as it can.

One way to help do this is to use what is called aperforated propellant grain. These things look like strangehamburger helper noodles. Instead of a single hole, or"perforation", through the center, they have a bunch. Seven- andnine-perf propellants have been used. The idea here is that whenthey begin to burn all over, "all over" includes the insides ofthe perforations too. So, while the outside burning surface isgetting smaller, the inside burning surface is getting bigger. Ifthe propellant type, grain design, gun, and projectile are allproperly matched you get a smoothly rising pressure as theprojectile moves down the barrel, but not for the entire lengthof the barrel. ("What does all of this have to do with liquidpropellants?", you ask. Be patient, I'm getting to that.)

Now, consider the size and shape of pistol powder. A lot ofit looks like ground pepper. (An exaggeration, but bear with me.)What do you think would happen if you were to use it as thepropellant for a large bore gun? It has a LARGE total initialarea to set on fire. Remember that burn rate is a function ofthis surface area. (Click! The brain lights come on!!) The

initial burn rate would be so high it would cause the pressure tospike behind the projectile before it can move, which would raisethe burn rate, which would spike the pressure further, whichwould.....BANG!!!!

As the smoke clears, we find the remains of a gun mountscattered hither, thither, and yon around the countryside. Now,here's where the part about liquid propellants comes in. Solidpropellants are just that - solid. You can predict what thesurface area exposed to burning is as a function of time withthem. You can't do this as well with liquid propellants becausethey slosh around and change their exposed surface shapes andamounts - even while burning. The people that continue to

experiment with these propellants play statistical games in thisregard. I'm not saying that they'll never get there, but I amsaying that they've got their work cut out for them - and theycontinue to blow up gun mounts. Also, one of the biggestvariables in gun accuracy at range is muzzle velocity. They willneed a VERY predictable burning rate in order to properly controlmuzzle velocity.

I wish them well because there are some significantlogistics benefits that could be had from such a system, but I'mnot holding my breath.

Now, I promised to tell you about a liquid propellant

program that I was personally involved in. A looooong time ago wehad a member of "Upper Management" who isn't here anymore.(Matter of fact, I'm not sure he's anywhere anymore - he may havedied by this time.) This individual thought it would be a goodidea if Navy ships wouldn't have to load propellant charges whenthey re-stocked, but could just strike down projectiles into themagazines. His idea was that we ought to be able to manufactureour own propellant from the sea. After all, he reasoned thatwater 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


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benefit.

Thus was born the Hydrogen-Oxygen Gun Program, or the HOGProgram as we called it. (I swear that I'm not making this up. Istill have the design folder with all my material in it.) Ithought it was a dumb idea from the beginning, but I was the dutystuckee. Well, you can imagine the ribbing I took at the hands ofmy co-workers. I think I heard every "pig" joke that was everdreamed up. Anyway, I ran into a number of design problems thatsingly and collectively killed the program. Thank God! I wasbeginning 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 therequired hydrogen and oxygen and then to liquefy it to get thenecessary energy density. Wrong! You get hydrogen and chlorinewhen you electrolyze seawater. A nasty combination in anybody'sbook.

"Well, how about we only keep the hydrogen and get the oxygenfrom liquefying air? After all, we do this all the time onboardcarriers to supply pilots while they're flying." I had to hand itto this guy, he got an "A" for inventiveness. I began to look atthis 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 gotto research liquid propellants. One BIG difference in this setupover everyone else - we were working at cryogenic temperatures.

I ran into the "exploding gun" phenomenon during some earlytesting that we did using just high pressure gaseous hydrogen andoxygen in a closed bomb that we were using to try to establishburning rates vs pressure curves. Fortunately, it was a small,thick walled bomb (for safety reasons) and all we did was destroythe pressure gage that was screwed into the side. The downsidewas that it went off before we were ready for it. The ordnancemen had just finished hooking everything up and were exiting thetest 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 gonehaywire and asked us to check it. We bled the pressure off thebomb and backed the pressure gage out. It fell apart in our handsand looked like someone had hit it with a sledge hammer! Theclick had been the test mixture detonating. We found out laterduring our investigation that one of the ordnance men had used arag with some oil or grease on it to clean out the bomb betweentest 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 outthat when petroleum products get chilled to cryogenictemperatures, they become shock sensitive. Fire departments getreal nervous when they hear they've have a truck accidentinvolving liquid oxygen on paved roads. Don't even think ofthrowing 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 safetyregs state that this material is dangerous if left in a confined


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space - and they defined a confined space as "three" walls.Remember the Hindenburg.

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

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

The research funding ran out and the results looked sonegative that it was decided not to renew the effort. As much ofa pain it had been, it had been a real learning experience for ayoung engineer. I look back on this experience with mixedemotions.

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

know.

John S. Canning

Combat Systems EngineerNaval Surface Warfare CenterDahlgren, VA

Subject: Re: Liquid Propellant?From: [email protected] (Scott Norton)Date: Jan 05 1996Newsgroups: sci.military.moderated

In our discussion of Liquid Propellant (LP) for artillery, there areboth general discussions (using a variety of liquid propellants) anddiscussion 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 standsfor) in water. It is nonflammable, and passes the Army's insensitive

munitions (IM) safety criteria. It will only react when underpressure. It is about as acid as lemon juice, and as poisonous asasprin. Its biggest problems are that it gets more sensitive ifcontaminated 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 fueltanker or "water buffalo" type of container, but one designed toprevent build-up of high pressure.

The big advantages of LP are in the logistics area:


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

- It is truly incremental. While current field artillery firesdifferent increments, unused bags are not used again. If you fireonly a single increment, the remaining charges are just tossed out.

A disadvantage of the LP design used in the Crusader is itsregeneratively pumped piston approach. To get this incrementalcapability, and to avoid high peak pressures, the fuel is metered intothe combustion chamber, pumped by the chamber pressure acting on apiston at the back of the chamber. If things go correctly, the pistonpumps LP into the combustion chamber faster than it combusts. But ifthe piston hangs up or if the seals leak, the combustion can proceedinto the fuel reservoir, causing a "pressure reversal." This candestroy the gun. (Note: the combustion goes back into theas-yet-uninjected fuel, not into the storage tank.) This problem is adesign problem--once solved, it should not be a problem in fieldedsystems.

There are a few ways in which the theoretical advantages of LP getdiluted by practical considerations. For example, because the HAN-HESLP contains a substatial percentage of water, its volumetric energydensity is less than a good solid propellant. Although the

tank-stored LP packs more efficiently than granular solids inincrements, the solid is more energy-dense.

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

Still, the Army is spending big bucks on LP. Some of the difficultiesare fixable in design, and the advantages remain even if they arediluted. And the equivilent solid-propellent technology is no cakewalk, either. New systems are held to very high standards for IM,safety, enviromental cleanliness, and performance. Currentsolvent-based granular propelling charges produce a big waste streamwhen 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 aliquid monopropellant. While the Army LP is somewhat difficult, bycomparison to Otto Fuel, Army LP is grape juice. Otto fuel isdownright evil.

--Scott [email protected]

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


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Subject: Re: Liquid Propellant?From: [email protected] (Scott Norton)Date: Jan 23 1996Newsgroups: sci.military.moderated

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

--Scott [email protected] Technology, Inc.2361 Jefferson Davis Hwy, Suite 500Arlington VA 22202-3876(703) 415-0200, fax: (703) 415-0206

Subject: Re: AFAS - liquid vs solid propellantFrom: [email protected] (Scott Norton)Date: Mar 24 1996Newsgroups: sci.military.moderated

In article "LEYTE C" 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 notwithout its own tradeoffs.

First, there have been some problems in development of the gun'sregenerative pumping system. It is critically important that theseals not leak, or the supply part of the system will ignite and blowup.


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

Finally, the bulk storage advantages of the current HAN-TEANmonopropellant are diluted somewhat if you want to meet insensitivemunitions criteria. You can't have too large a volume, or you'll geta 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 acar's radiator.

Some general explanation for those who don't know what AFAS is, orwhy liquid propellant is being discussed.

AFAS, now called Crusader, is the US Army's Advanced Field ArtillerySystem, an autoloading, self propelled 155 mm howitzer. It will havea matched support vehicle for rapid rearming as well. One of itsgoals is to get a simultaeous time-on-target by shooting three roundson three different trajectories. The first gets a small charge and ahigh trajectory, and the following rounds get large charges andflatter trajectories. They all hit at once, increasing the lethalitysince 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, baggedpowder they use in other 155 mm guns. Liquid propellent is easier toresupply (you pump it like diesel fuel), it packs more efficientlyinto tanks and drums, it fills the chamber with no void spaces givinggreater energy density, and it can be used incrementally, with only asmuch propellent as needed for the range. (When a howitzer fires lessthan 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 endsinto 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, tohelp 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) inwater. HAN is the oxidizer, and TEAN is the fuel. The water makes itvery insensitive to fast and slow cookoff, bullet impact, andfragments. It reacts only when under pressure.

The problem with just filling up the breech with liquid is that thepressure shoots up to a peak exponentially. To produce a nice, long,flat-topped pressure profile, the propellent must be metered into thechamber through the ballistic cycle. Crusader has developed a

regenerative injector, where chamber pressure pushes a piston, whichforces more propellent into the chamber.

--Scott [email protected] Technology, Inc.2920 South Glebe RoadArlington VA 22206+1-703-299-1656, fax: +1-703-706-0476


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From: [email protected] (Doug Gwyn )Newsgroups: rec.gunsSubject: Re: [TECHNICAL] Binary Liquid PropellantsDate: 31 Jan 1997 18:45:03 -0500

In article James H Galt-brown 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 practicaldevelopment of binary liquid propellant systems. Later on they weresacrificed by their own management for local PR purposes, for theenvironmental "crime" of flushing acid down laboratory drains, whichof course was standard chemical lab practice at the time they did it.

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