Date: Jul 15, 2010 Author: Dale McGeehon Source: MDA (
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by Dale McGeehon/techapps@nttc.edu
A new rocket-fuel formulation promises to bring greater safety to commercial space launches without sacrificing performance.
The less-toxic fuel, developed by Missile Defense Agency-funded Ogden Engineering & Associates, LLC (Tucson, AZ), could benefit NASA astronauts as well, because they would no longer have to decontaminate their spacesuits if they were exposed to leaked fuel during spacewalks. And while the fuel itself would not be used as a safer solvent for the dry-cleaning industry, the fuel is in the family of ionic liquids that are being evaluated by the industry as a nontoxic replacement for conventional solvents that, when spilled, often contaminate groundwater.
The quest for less-toxic fuels took a big leap in 1998 when the Air Force Research Laboratory started working with ionic liquids. But those liquids rarely have been used because they are slow to ignite—about 65 milliseconds. (In contrast, monomethylhydrazine (MMH), a common but highly toxic chemical used in rocket fuel, has an ignition time on the order of 1 millisecond.)
So government agencies have wanted to reduce the ignition time of the newer, safer formulations, and MDA has been a key participant. The agency awarded Ogden Engineering Phase I and II SBIR contracts in 2005 and 2006, respectively, to investigate novel additive technologies to create fuel formulations with ionic liquids that could significantly reduce ignition delay times.
With chemical engineering assistance from the University of Alabama-Huntsville, Ogden conducted more than 1,000 tests over a four-year period, experimenting with various additives and conventional oxidizers to improve physical properties and reduce overall ignition delay time.
For Ogden's hypergolic green fuel (HGF) formulation, the predicted specific impulse (a measurement of rocket engine efficiency) is comparable to that of conventional bipropellants as well, said Dr. Gregory Ogden, founder and co-owner of the company. Under similar chamber pressure and expansion ratios, HGF has a predicted specific impulse of 303 seconds—fairly close to the highly toxic MMH, which has a theoretical specific impulse of 326 seconds. Hydrazine--another highly toxic rocket fuel—has a predicted specific impulse of 329 seconds.
The density of Ogden's HGF allows it to rival the very toxic fuels when it comes to specific impulse, Dr. Ogden said. HGF has a specific gravity of 1.25, making it about 40 percent denser than MMH, which has a specific gravity of 0.88. And the HGF is 25 percent denser than hydrazine, which has a specific gravity of 1. The greater density means that fuel tanks carrying the company's HGF can be made smaller, allowing for a larger payload. The HGF would be used mostly in upper-stage rocket engines. In missiles, the HGF would be appropriate for the divert attitude and control system used to maneuver an interceptor.
Company officials claim that their HGF could be used in standard bipropellant engines without major modification. But perhaps the greatest advantage of the fuel is its lack of toxicity. "If it spills on the ground you don't need to wear a Class A hazmat suit and a SCBA breathing apparatus to go clean it up," Ogden said.
Moreover, ionic liquids, which are salts, have a low vapor pressure and are liquids at room temperature. In contrast, MMH and hydrazine have a high vapor pressure—meaning they easily turn into a vapor cloud at room temperature. MMH and hydrazine are toxic and can cause health issues if inhaled and they can contaminate anything they touch.
"Whenever there is a hydrazine-type fuel, you could replace it with something like our HGF," Ogden said. "Where you are balancing health risks with ignition speed, as we all know, speed always wins. But in other applications where fast is fast enough, then there are some real viable alternatives. I know that, in some systems, the long-term risk to operators and support personnel could outweigh the need for higher performance."
According to Ogden, the fuel could be a good replacement for the hydrazine used in the Space Shuttle's Vernier thrusters, which keep it in the right orbit. The thrusters often can leak, and when astronauts go on space walks they can get coated with hydrazine ice. They then must decontaminate themselves before returning to the shuttle. If HGF were used instead, however, that decontamination would be unnecessary because HGF is nontoxic, according to Ogden.
As for commercialization, Ogden said it makes the most sense to license the technology to existing rocket-fuel manufacturers because they already have met the regulatory requirements for propellant manufacturing. One novel application would involve a mobile plant put on the back of a flatbed truck. The nontoxic fuel would be exempt from meeting burdensome transportation guidelines, so a missile or rocket component that uses the fuel could be filled off-site and then transported to the final launch location—rather than waiting to do the fueling only at the launch site.
Ionic liquids could be formulated to replace toxic solvents that have high vapor pressure, such as benzene. They also could be formulated to replace methyl tertiary butyl ether, or MTBE, which is a gasoline additive used as an oxygenate to raise the octane number of gas. The problem with MTBE is that it often leaks from underground storage tanks, contaminating groundwater and causing serious problems where aquifers provide a significant portion of the drinking water supply.
With the right formulation, ionic liquids also could be used as a safe alternative for solvents in the dry-cleaning industry. The solvents used in dry cleaning, perchloroethylene, or PCE, and the older trichloroethylene, or TCE, both contaminate groundwater, and exposure to the latter is thought to increase one's risk of cancer.
The biggest challenge for the company is converting the military's mindset about this technology. For example, the Navy uses solid rocket motors, and is resistant to put liquid propulsion systems onboard ships, Ogden said. In November 2009, the company successfully conducted a hot-fire engine test. With additional testing, Ogden suspects that government prime contractors will be much more interested in HGF.
