SCIENCE/TECHNOLOGY

Advanced Energetic Materials Emerge For Military and Space Applications

• A new generation of propellants and explosives is being developed by teams of chemists in industrial and government labs

Stu Borman, C&EN Washington

In a research effort that is little known outside the defense commu­nity, teams of chemists, working pri­

marily in industrial and government labs, are developing a new generation of explosives and propellants for potential military and space applications.

"We've had some tremendous discov­eries, only some of which could be talked about until now because of secu­rity issues," says scientific officer Rich­ard S. Miller of the Office of Naval Re­search (ONR), Arlington, Va., a major funding source for basic research on propellants and explosives. "It's an area many people aren't familiar with, but it's fascinating chemistry."

A significant amount of work on new high-energy materials is conducted at defense companies, often with support from federal agencies like ONR and the Army's Armament Research, Develop­ment & Engineering Center (ARDEC) at Picatinny Arsenal, N.J. Considerable re­search on energetic materials is also be­ing done at the national laboratories (es­pecially Los Alamos, Lawrence Liver-more, and Sandia), at military research facilities (such as the Naval Weapons Center, China Lake, Calif., and the Na­val Surface Weapons Center, White Oak, Md.), and in academia.

Researchers at the defense firms say that sustained federal grant support has been a key element in making it possible for them to help bring new explosives and propellants from conception to reality. "The problem is that these materials don't have much [nondefense] commercial ap­plication," says chemist Thomas G. Arch­

ibald of the Aerojet Division of Gencorp, Sacramento, Calif. Without federal funding, compa­nies do not tend to pursue such research, he says, because the risk is too high and the commer­cial potential too low.

On the part of the govern­ment, this research grants pro­gram helps create the new mate rials it seeks, and also gives it some rights to those materials. Typically, a company will pro­vide a royalty-free license for government use of high-energy materials developed with federal assistance.

With such royalty-free licenses in hand, the government can get "the best price from different companies willing to manufac­ture the material," says Archibald. "So the government really makes out. I think it's the appropriate way to do research."

Ammonium dinitramide (ADN) is high on the list of promising new ener­getic materials for a variety of space and military applications. The compound, considered to be of chemical interest as a new type of oxide of nitrogen, has the formula NH4N(N02)2.

ADN can act as an explosive—that is, it can serve a dual role as oxidizer and fuel. But ADN's primary anticipated use (perhaps a decade or two from now) is as an oxidizer in solid-fuel rocket pro­pellants. Solid-fuel rockets in which it potentially could be used range from small surface-to-air missiles to large in­tercontinental ballistic missiles to the booster rockets used to put space shut­tles in orbit.

In many solid-fuel rockets, a propel-lant (consisting of oxidizer, fuel, and a rubbery matrix to hold everything to­gether) is packed around a central air channel. Upon ignition, the propellant burns at the face of the channel, which acts as a combustion chamber. The ex­haust is expelled through a nozzle at the rear, propelling the rocket forward.

Thiokol photo

18 JANUARY 17,1994 C&EN

ADN could find use as an oxidizer for propellants in future solid-fuel rockets. The current generation of such rockets includes the Patriot surface-to-air missile (top left), the Peacekeeper intercontinental ballistic missile (above), and the two booster rockets (white) on the space shuttle (bottom left).

Several years ago, senior chemist Jeffrey Bottaro of SRI International, Menlo Park, Calif., synthesized ADN and other dini­tramide salts in the belief that the notori­ously sensitive and unstable dinitramines would show increased stability in the an­ionic form. The dinitramides are more sta­ble than the dinitramines, and are also

predicted to have 5 to 15% high­er performance (lift capacity) than current propellant formula­tions. SRI was granted a U.S. patent (No. 5,254,324) on the materials and has filed addition­al patent applications covering processes for making ADN.

Last year, Bottaro and his col­leagues at SRI were shocked to learn that ADN actually had been secretly used in the former Soviet Union since the 1970s in a number of different missile sys­tems. Soviet use of ADN was a closely guarded military secret and was apparently unknown to U.S. intelligence. SRI first learned about this when a Soviet group came to the U.S. last year to mar­ket an ADN-based propellant and presented a paper on the technology at a meeting of the American Institute of Aeronau­tics & Astronautics.

But if s not clear that lack of awareness of the Soviet work cost SRI unnecessary R&D ex­pense. "We would have ended up putting the same amount of effort into an ADN program here—in fact, probably more—if we'd known that the Soviets had it," says Robert J. Schmitt, direc­tor of the organic process and environmental chemistry de­partment at SRI.

The earlier Soviet discovery also apparently poses no threat

to SRI's patent position because the Sovi­ets used ADN secretly, never filed for pat­ents on the material, and hadn't published or publicly divulged anything about it un­til last year.

ADN is primarily envisioned as a re­placement for ammonium perchlorate (AP), a widely used oxidizer for pro­

pellants and explosives. AP is current­ly the predominant oxidizer used in space propulsion systems and a variety of military rockets and missiles.

One potential application of ADN would be in space shuttle booster rock­ets. The propellant currently used in the space shuttle contains AP (oxidiz­er), powdered aluminum (fuel), and a hydrocarbon binder.

Unlike AP, ADN is chlorine-free. The smoky white contrail (or "signature") formed by chlorinated oxidants like AP can allow missile launches to be detected more easily, making the launches less se­cure militarily. The chlorine in AP also has potential environmental implica­tions. SRI points out that an ADN oxi­dant potentially could be used in re­duced-signature missiles and would also be more environmentally benign.

Another plus for ADN is its signifi­cantly improved lift capacity relative to AP, possibly enabling larger payloads to be put into orbit. "Calculations indi­cate that on the space shuttle you could put at least 8% more mass into orbit per shot," says Schmitt. "That's quite a bit of additional poundage."

In practice, the 8% figure could turn out to be significantly lower. Nevertheless, the National Aeronautics & Space Adminis­tration has been briefed about the new oxidizer, and it recently participated with ONR in some modest funding to help move along the ADN scaleup process.

Miller is optimistic about ADN, but also cautious. "I'm not saying ADN is going to revolutionize the booster busi­ness in the near future, because it's a long, long process," he says. "You're talking 10 to 20 years before ADN would ever replace AP."

A second promising dinitramide salt formulation is KDN-AN, a cocrystal-lized form of potassium dinitramide (KDN) and ammonium nitrate (AN).

CL-20 andTNAZ represent new generation of explosives New explosives Current explosives

OoNN .NOo

° 2 N - ^ < A N / N 0 2

OoN \ N CH2

OPN

1ST A^. N \

HoC- ~NOo

N02 NOo

CL-20 TNAZ

0 2 N -

N0 2

1 /—K

k N-

1 N02

HMX

0 2 N \ k , ^ / K N

^j -NOo N 1 N02

RDX

,N02

02N^

CH3 1 ..~

^ \ / N ° 2 n s T N02

TNT

JANUARY 17,1994 C&EN 19

SCIENCE/TECHNOLOGY

^ i

New explosives like CL-20 and TNAZ could one day be used in "smart" weapons like this cruise missile, shown destroying a reinforced concrete target after having been launched from a submerged submarine 400 miles away.

Thiokol Corp., Brigham City, Utah, has patented the material and is develop­ing it as an oxidizer for space launch motors and other applications.

AN has long been considered a high­ly desirable oxidizer for solid-fuel rocket

Miller: tremendous discoveries

propellants because of its extremely low cost, low sensitivity (low probability of unplanned ignition), low signature, and absence of halogens. "We've always wanted to use ammonium nitrate as a very low cost and environmentally con­scientious oxidizer, even though if s not as energetic as AP," says Miller.

However, AN has a crystalline phase stability problem that causes unpredict­able ballistic performance in some cases and catastrophic rocket-motor failure in others. This has limited the use of AN without phase stabilizers to systems that aren't exposed to temperature fluctua­tions or humidity and that contain only small amounts of AN in the formulation. Unfortunately, AN with phase stabiliz­ers exhibits dramatically reduced energy content, negating any advantage of us­ing an AN-based oxidant.

But Thiokol chemists Thomas K. High-smith and Robert B. Wardle recently found that small amounts of KDN have a dramatic phase-stabilizing effect on AN without the energy penalty. In fact, KDN-AN's energy content is almost equivalent to that of AN with no stabilizer. KDN-

AN also has im­proved ballistic performance—in­creased thrust per unit time, reduced

Research team working on ADN at SRI International includes (left to right) chemists Alan Dodge, Paul Penwell, and Robert Schmitt, chemical engineer David Bomberger, and chemists Bock Loo and Jeffrey Bottaro.

variability, and better predictability— compared with AN propellants without KDN.

"The discovery that you can cocrystal-lize a few percent of potassium dinitra-mide with ammonium nitrate is poten­tially a very important technological advance/' says Miller. "It is really inter­esting and fascinating chemistry. The cocrystallized material eliminates many of the problems of ammonium nitrate. It gets rid of the phase stability problem, it increases the burning rate, and it may increase the efficiency of aluminum combustion."

Another new energetic material has the imposing chemical name hexanitro-hexaazaisowurtzitane, but is better known as CL-20. CL-20 is a very high-energy crystalline compound whose method of synthesis and detailed performance data are still classified.

"It's the highest energy molecular ex­plosive known to man," says Miller. "It's a symmetric polycyclic nitramine— absolutely fascinating chemistry and just a beautiful molecule."

Potential military applications for CL-20 include minimum-signature tac­tical propulsion, boost propulsion for strategic missiles or space launches, and special warheads for "smart" or light weapons. Potential nonmilitary

applications include charges and ex­plosives for construction and struc­ture demolition.

CL-20 was first synthesized in 1987 by chemist Arnold T. Nielsen, now retired, at the Naval Weapons Center, China Lake. Two synthetic routes to the compound from a pre­cursor are currently used. One is a modification of the Nielsen synthe­sis in which more economical re­agents are used, and the second is a

20 JANUARY 17,1994 C&EN

synthesis developed by Thiokol chem­ists Wardle and Jerald C. Hinshaw.

Both syntheses were designed to in­troduce the compound's high-energy nitramine functional groups at the last step in the process, a nitration. Because of safety concerns, says Wardle, "we try very hard to leave as much of the ener­gy out until the end as we can."

CL-20 is currently in the pilot-plant stage both at Thiokol and Aerojet. Appli­cations for the high-energy compound are under development, and several commercial and military products based on CL-20 are planned.

Another high-energy material of in­terest to the military is the explosive 1,3,3-trinitroazetidine (TNAZ). Chemist Kurt Baum of Fluorochem, Azusa, Calif., and Archibald (then at Fluorochem) developed TNAZ in the early 1980s as part of the same high-energy nitramine program that engendered ADN and CL-20.

The x-ray crystal structure of TNAZ was first obtained by x-ray crystallog-rapher Richard D. Gilardi and cowork­ers of the Laboratory for the Structure of Matter, Naval Research Laboratory, Washington, D.C. The group has also determined the structure of CL-20 and other new high-energy materials devel­oped with ONR funding.

TNAZ is more powerful than two

Thiokol chemists who have helped develop CL-20 and other high-energy materials

include (left to right) Thomas Highsmith, Lou Cannizzo (seated), Jerald Hinshaw,

Wayne Edwards, Bob Hajik, Robert Wardle, Reed Blau, and Scott Hamilton. Thiokol

facility where they work in Brigham City, Utah, has a "rocket garden" that includes a

space shuttle booster rocket.

widely used explosives of British origin—HMX (Her Majesty's Explo­sive or High-Melting Explosive, tetranitrotet-raazacyclooctane) and RDX (Research Depart­ment Explosive or Roy­al Demolition Explo­sive, trinitrohexahydro-triazine). HMX and RDX represent the cur­rent generation of mili­tary explosives. TNT (trinitrotoluene) is bet­ter known to the pub­lic. Although TNT is still widely used militarily, there are environmental prob­lems in its manufacture and its explosive power is lower than that of HMX and RDX.

An advantage of TNAZ over HMX and RDX, in addition to its higher en­ergy content, is that it is melt-castable, meaning that "you can make a big ket­tle of it and pour it," explains Archi­bald. "If you had a shape charge that you wanted to make, you could pour TNAZ in and it would conform to the shape. That's a real valuable property." Shape charges are a class of weapons used to attack heavily armored targets

Aerojet chemists (left to right) Thomas Archibald, William Harvey, and Roland Carlson filter a TNAZ intermediate at Aerojet's Advanced Oxidizer Pilot Plant, Sacramento, Calif.

like tanks, where the explosive force must be focused in a particular direction to penetrate and defeat the armor.

The major problem with TNAZ right now is that it is very expensive. "With bombs you're talking about very large quantities of material," says Rao Sura-paneni, chief of the warheads group at ARDEC. "Right now we're still in the initial stages of producing TNAZ and we haven't got the cost down yet, so it's not that practical at this point."

Hence, the Army wants to use TNAZ primarily for weapons in which the per­formance of an explosive is more impor­tant than its price per pound. For exam-

JANUARY 17, 1994 C&EN 21

SCIENCE/TECHNOLOGY

NASA hails Hubble repair success, plans eight shuttle flights in 1994

pie, if TNAZ could be used in an elec­tronically guided weapon to defeat important targets with high efficiency, the cost of the energetic material per se would not be that important, says Sura-paneni, especially relative to the cost of the electronics used in such smart weap­ons. Nevertheless, materials like HMX and RDX should continue to be main­stay explosives in Army munitions for some time, he says.

In an effort to address the cost prob­lem, researchers at Aerojet and at other industrial, government, and academic labs are trying to streamline the synthe­sis of TNAZ. For example, key improve­ments in TNAZ synthesis were recently made at Los Alamos National Laborato­ry by chemists Michael Hiskey and Michael Coburn. "It takes a number of years for a new material to make it into real systems," says Archibald. "With TNAZ, we're on the beginning steps of that process/'

Other high-energy materials in the development pipeline include energet­ic polymers. In solid explosives and rocket propellants, the rubbery matri­ces used to bind the oxidizer-fuel mix­tures together are sometimes inert— that is, they don't have any energetic function. An energetic polymer can in­crease the amount of energy in such a formulation.

"The energetic polymer that's most far along is called GAP, for glycidyl azide polymer, which is being devel­oped in the U.S. by 3M Co.," says Mil­ler. "However, energetic polymers based on oxetane chemistry are not far behind. The oxetanes are under development at Thiokol and Aerojet."

For development of materials like ADN, KDN-AN, CL-20, TNAZ, and energetic polymers, federal support for collaborative research efforts has been critical. In the case of CL-20, for exam­ple, federally funded groups at several companies have made fundamental contributions to the synthesis.

And this kind of success doesn't hap­pen overnight. "It takes a while doing research for good things to come out of it," says Archibald. Some people have the idea, he says, that "you give some money to a lab and in three months a group will come up with something. That's not the case, especially in an area like explosives, where so much work has gone on over so many years. It takes a very long and very concerted, very di­rected program." •

Richard J. Seltzer, C&EN Washington

Completing a mission that riveted attention around the world, space

shuttle Endeavour landed at Kennedy Space Center in Florida in mid-Decem­ber after 11 days in Earth orbit—five of them filled with spectacular space walks by astronauts to repair and service the Hubble Space Telescope.

This dramatic mission put a successful cap on the National Aeronautics & Space Administration's program of sev­en shuttle flights in 1993, several of which were troubled by technical glitch­es and delays. And the success laid a sol­id foundation for an ambitious slate of eight shuttle flights that NASA plans this year.

"We're absolutely elated" about En­deavour's flight, says NASA associate administrator Wesley T. Huntress Jr., who oversees space science programs. "It was a mission where everything that the agency does came together."

Launched April 24, 1990, Hubble is designed to be the most powerful astro­nomical observatory ever built, far sur­passing ground-based observa­tories and helping to answer key questions in astronomy, as-

Preparingfor space shuttle flight early next month, Russian

cosmonaut Krikalev (below) receives bailout training, and

workers test Wake Shield Facility.

o •a

trophysics, and cosmology (C&EN, April 9,1990, page 4). As big as a railroad tank car and weighing 25,000 lb, Hubble is ex­pected by astronomers to "open a new window on the universe" and "revolu­tionize the study of astronomy." From orbit 370 miles above Earth, unham­pered by atmospheric distortion, it will look "back toward creation of the uni­verse," Huntress notes.

When launched, its instruments in­cluded a high-quality reflecting optical telescope, a wide-field/planetary cam­era, a faint-object camera, a faint-object spectrograph, a high-resolution spectro­graph, and a high-speed photometer. They enable Hubble to study not only visible light rays, but also ultraviolet and near infrared, and to determine the visual appearance, internal structure, size, brightness, chemical composition, age, and distance from Earth of stars, planets, galaxies, and other objects in space.

However, during tests two months af­ter Hubble was launched, NASA discov­ered that Hubble's 94-inch, painstaking­ly ground primary telescope mirror was

22 JANUARY 17,1994 C&EN