Murphy's Law: Scary Lasers


June 1, 2012: Editor Note: The U.S. military is backing off from efforts to deploy combat lasers because none of the development efforts has produced a practical weapon. The following is a personal account of how the search for a combat laser began half a century ago because of an unexpected early success, followed by bizarre complications.


This is how the U.S. Army developed the first combat laser in the 1960s and 70s, but refused to deploy it because it was considered too cruel. It all started with the Advanced Propulsion Technology Branch of the Propulsion Directorate in the Army Missile Command at Redstone Arsenal in the 1960's. Their mission was the development of advanced propulsion concepts such as liquid monopropellants, bipropellants, hybrids, air breathing, etc. for application to Army missiles.

One morning, after a staff meeting with the army generals Dr. Walter Wharton, our supervisor, announced, "Men I've got a new and unusual project for you. It's not propulsion but needs all the technology and skills of propulsion. Here's the pitch: Over the past two years one of our army contractors has failed to demonstrate chemical lasing in a hardware device. The general asked us to take over the effort. And, if we achieved a satisfactory demonstration of the chemical laser, we would be the nucleus of a new laboratory and weapons effort. I told the general the problem was a natural fit to our skills. With the chemists, physicists, engineers, and technicians on our staff and our background in hardware development and testing we could do the job expeditiously."

The basic problem with using a laser as a weapon is power. A laser is focused light energy, being sent from the laser to the target in a short burst. Using batteries or generators and capacitors are too heavy for this to be practical. But the right combination of chemicals can provide the needed energy, at least in theory. The solution is a bit more complex.

Dr. Wharton led us in the analysis and evaluation of the contractor's effort and data. Their device used gaseous hydrogen and gaseous fluorine. The attempt at lasing was through the kinetic reaction states in the laser cavity to form the end product HF. Dr. Wharton, a skilled chemist, immediately determined the critical issue. To allow the intermediate activated molecules the time and space to lase to ground states in the laser cavity, would require supersonic injection by the mixing nozzles and very low cavity pressure. That environment would slow down the kinetics and stretch out the reaction zone allowing the species to lase.

Wharton designated me (Joe Connaughton, a chemical engineer) as team leader for chemist Tony Duncan, laser device operator, physicist Bill Friday, cavity optics and power, and mechanical engineer, Ben Wilson, facility design and development. We had top priority in obtaining hardware, shop, and other support services. In a matter of weeks we had the device set up and ready for operation. The big day came when we were ready to test. Dr. Wharton said, "Get that machine cranked up and don't stop till you get it to lasing. I'll be in the office, so call me if you have any problems or when it starts lasing."

We spent most of the day adjusting the flow of the gases and setting our liquid nitrogen trap and pumping speed. But near the end of the day, Bill Friday held a piece of strip recorder paper three feet from the cavity optics and yelled, "Hey! Look guys at me burn holes in this paper by that invisible laser beam!" We probably didn't project more than a hundred watts of power but it worked. We had an operating HF chemical laser, and we were in business. The next day was show time, which included all day demonstrations to various levels of management including the Commanding General.

We were off and running to build a ten kilowatt HF laser that would define the operating parameters for scale up to weapon grade hardware. It was a large modular boilerplate device designed for research studies. Calorimetric cavity mirrors for precise power measurements and ports for optical flow field visualization were included. The modular design allowed the evaluation and development of laser components to advance the technology of high-energy lasers.

The group quickly expanded to include PhD level scientists, who began to study all aspects of the chemical laser and extrapolate data to weapon system needs. Dr. Barry Allen, with contractor support, researched solid sources for the reactants. He found hydrides and fluorides that had reactant densities greater than the cryogenics were appropriate. He also worked on the successful development of chemical pumps that would replace the huge vacuum blow down system required to pump the boilerplate laser.

Drs. Kerry Patterson and Miles Holloman were analysts who developed computer models that helped guide the experiments. They also performed significant optical flow visualization measurements that showed the deleterious effects of large boundary layers in mixing nozzles and shock structures in the laser cavity (refs. 2 & 3).

Within the next decade we had fully characterized and optimized the chemical laser for scale up to an army mobile weapon system. One major problem we were unable to solve was getting lethal power onto a mobile vehicle. We were, however, able to propose a mobile broad beam laser battlefield weapon that could destroy battlefield and missile sensors. It would also burn out the eyeballs of enemy foot soldiers.

Dr. Wharton presented our mobile battlefield laser concept (including supporting design data) to several layers of army brass. He was surprised to learn that it was turned down. The weapon was quite feasible, with a solid development schedule. Amazingly, the generals turned it down on the basis that it was too cruel to use in battle, because it would burn out foot soldiers eyes. Apparently, they considered it to be in the same weapon class as poison gas.

The upshot of this initial decade or more of chemical laser effort by the Army is that the program was cancelled and the unique laser laboratory and test facility dismantled. The chemical laser group of specialists was disseminated into other missile research and development units. At the time of the termination of the chemical laser program, an Army Missile Command reduction in force also took place. The Army downgraded many chemical laser specialists because they were no longer in the mainstream of the propulsion effort.

Although this early Army chemical lasers effort died at the time, the results were a significant contribution to the Air Forces' airborne laser and fixed installation lasers later developed by the armed services. -- Joseph Connaughton





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