By Joseph MendiolaDecember 20, 2019
REDSTONE ARSENAL, Ala. (Dec. 19, 2019) - Spectrometer technology could give Soldiers the ability to check the status of tactical missile while in the field.
The concept is featured in a recent technical paper written in partnership with engineers from Alabama A&M University and the U.S. Army Combat Capabilities Development Command Aviation & Missile Center Weapons Development and Integration Directorate.
Chemists are using Raman spectroscopy to detect changes in chemical compounds inside a tactical missile's propellant, which fuels the missile when fired. Raman spectroscopy is the method of shining a monochromatic light or laser on a chemical sample and then measuring the frequency and intensity of the scattered light that results.
"If you shine light that has a specific frequency on to a material, most of the light is reflected, but then we have a small amount that is scattered," said Dr. Paul Ruffin, senior researcher and adjunct professor at Alabama A&M University. "What happens is that ... some of the energy from the light goes into that material (and) the frequency of the (resulting) light is reduced that's called a vibrational frequency. If anything changes in that material, we can see a change in that frequency of that light."
That change in vibrational frequency is measured. Different molecules have different vibrations - what chemists call a fingerprint - and the changes in the scattered light's frequencies and intensities help identify the types of molecules in the sample, as well as the amount of each chemical in the sample respectively.
Propellant in certain tactical missiles contains a stabilizer called methyl-nitroaniline or MNA. Normally, AvMC chemists remove the propellant from the missile, sample it, do a chemical extraction, pull the MNA out of propellant and then use a gas chromatograph or liquid chromatograph to quantify it. This gives a true indication of the status, but also makes the missile unusable, a process called de-milling. With the Raman spectroscopy method, the remaining MNA will be measured by using a Raman laser probe analyzing the propellant surface. Currently, tests have been on propellants removed from a rocket motor.
Tactical missile propellant is typically made up of nitrate esters like nitroglycerin and nitrocellulose. The propellant includes stabilizers like MNA because, without it, the esters can degrade when they interact with heat, water or moisture. Over time, the chemicals naturally break down and release gases like nitrogen dioxide. If the NO2 gas interacts with moisture, it creates nitrous or nitric acid which, when accumulated considerably in one spot, can generate enough heat to auto-ignite the propellant. Instead, the MNA stabilizer interacts with the NO2 gas and negates it, creating a stable form.
The MNA is what needs to be measured accurately, said Chris Marshall, AvMC WDI missile sustainment supervisory chemist. The next step in the project after fine-tuning the MNA analysis is integrating a system into the missile to allow for screening. "The issue they're going to have to find down the road is finding a way to get their probe into the missile," Marshall said. "They're talking about maybe fiber optics, having a port or slot to insert the probe they're using. Once we get past the MNA quantification, can we devise a way to get the probe into a missile non-destructively?"
The AAMU researchers are developing a prototype version of a portable unit that works by laser. "It's going to be a fieldable system," said Ruffin. "We want to make this Soldier-friendly."
The current process requires missiles in the field be shipped back to Army facilities in the U.S. to do the de-milling. The Raman spectroscopy technique could decrease some of the hassle.
Marshall said the Raman spectroscopic technique won't completely eliminate the previous method of testing, but it will provide Soldiers in the field an accurate way to screen the amount of MNA in the missile while in the field.
"It's going to be a lengthy process, and it's very new and early on," said Marshall. "There's a big push, though, to find a nondestructive technique - either through sensors or things like this - that someone could put a probe in and check the health of a motor without having to break it down."
This all goes back to the stockpile reliability plan, which determines the performance and safety of tactical missile inventories in the field through the missile's shelf life. SRP is the mechanism for extending shelf life and allowing continuing use. AvMC is recognized worldwide for SRP testing and analyses approaches, and has partnerships with the U.S. Navy, U.S. Air Force and 14 foreign military agencies.
"Say you have a certain lot of missiles you don't know any information about. Now you could have a way to check them right there on site. That's the biggest thing," said Marshall. "If the Soldier says, 'I don't know the state of these, are they bad or good?', this can give a good idea if you still could be using these missiles or you have to send them back (to the U.S.) to get tested."
The CCDC Aviation & Missile Center, formerly known as the Aviation & Missile Research, Development and Engineering Center (AMRDEC), is part of the U.S. Army Combat Capabilities Development Command, which conducts responsive research, development and life cycle engineering to deliver the aviation and missile capabilities the Army depends on to ensure victory on the battlefield today and tomorrow. Through collaboration across the command's core technical competencies, CCDC leads in the discovery, development and delivery of the technology-based capabilities required to make Soldiers more lethal to win our Nation's wars and come home safely. CCDC is a major subordinate command of the U.S. Army Futures Command.