New Propellants Provide “Greener” Options for Future Space Flights
Currently, if you want to maneuver a satellite or space craft while it is in space, you use hydrazine. This highly toxic and corrosive chemical has been used for years to provide maneuverability to a majority of space vehicles. The problem with hydrazine is its corrosive and toxic nature. These properties make handling hydrazine both expensive and dangerous. Acute exposure to hydrazine can damage the liver, kidneys, and central nervous system. Scientists are currently looking into a replacement for hydrazine that would be far less toxic while still providing the same if not greater boost, cost, and ease of manufacturing. Research is currently being done into energetic ionic liquids, solid, and hybrid propulsion sources.
Hydrazine is hypergolic in liquid form. Hypergolic means that the propellant ignites when mixed with certain chemicals. This allows for a simple propulsion system that does not require an ignition source, while providing enough boost (1-30 Newtons based on the application) for maneuvers in space. Hydrazine has been around since World War II when it was used in propulsion for the first German jet powered aircraft. Since then, it has been used for propulsion in maneuvering satellites and spacecraft such as NASA’s Voyager craft. Advances in technology have found feasible replacements for hydrazine. Scientists have maximized the potential of hydrazine in terms of performance. The safety and personnel hazards posed by the propellants also incur extra costs, while posing a danger to people. In 2008, the Navy destroyed a defunct satellite that was re-entering the atmosphere due to concerns that the hydrazine aboard could harm people if it was dispersed into the atmosphere. Hydrazine has served its purpose, but scientists are looking to the future and investigating the next generation of cleaner and safer propellants.
Solid propellants have been used for years to initially launch vehicles into space. This design powers the boosters that lifts the shuttle into space. They are simple and reliable systems that provide large amounts of boost. However, the usefulness of solid motors to maneuver a spacecraft once in space is limited. Solid propellants burn until they are out of propellant and, while they can provide boost for longer time periods, once started, the reaction cannot be stopped. Another drawback to solid motors is that they are heavier and do not provide the precision accuracy of their liquid and hybrid counterparts. These qualities severely limit the uses of solid propellants in maneuvering satellites or space vehicles.
Energetic ionic liquids are hypergolic liquids that are currently being researched in both America and Europe for use as propellants. The Air Force is currently working with NASA and several civilian companies to research and test AF-M315E. This propellant is an energetic ionic liquid that provides more boost while being essentially nontoxic. While still under development, hardware is being made for AF-M315E to be used by NASA in a 2015 space flight. This flight will test the performance of the propellant under the conditions it would see in many of its proposed uses. AF-M315E is currently the best option replacing hydrazine for NASA, as well as military applications. Initial testing has shown that it does not require the use of respirators when being handled by personnel and is considered a mild irritant. Hydrazine is both a strong irritant and corrosive. Unlike hydrazine, AF-M315 is not highly flammable and does not detonate when exposed to high temperatures. It also does not respond to explosive or electric shock, which could turn a simple failure into a disaster during spaceflight.
The Swedish Space Corporation has developed an energetic ionic liquid called LMP-103S that provides almost the same power as AF-M315E. While still under US evaluation, LMP-103S promises to be comparable to AF-M315E and has already been tested in space in a thruster configuration very similar to hydrazine. Both energetic ionic liquid propellants under development provide more propulsion power than hydrazine, while at the same time being safer to handle and transport. They both provide more boost and do not need a system any larger than that currently used. This allows companies to either use less propellant, saving weight and therefore money, or have more maneuvering time and/or power, expanding mission capabilities. All of these benefits would greatly reduce the cost of space systems, while providing greater capabilities. At this time it is unclear if AF-M315E or LMP-103S would be a superior propellant, but both look to be highly viable options for replacing hydrazine. Research in energetic ionic liquids for propulsion uses has not been completed yet. There may still be other discoveries or improvements under development that will appear in the next decade.
Hybrid rocket propulsion is produced by combining a liquid with a solid propellant. It can provide the benefits of both liquid and solid motors but is more difficult to design and engineer for space maneuvering than a liquid motor. Hybrid motors allow the system to be turned on and off as needed, much like a liquid motor. Hybrid motors require around the same space as a liquid propulsion system but would be more complicated to design for a variety of spacecraft. Another complication with hybrid motors is that they have a longer ignition delay than liquid propellants, further complicating the system when immediate propulsion is needed. If a program has the time and money, a hybrid motor would be the best option for propulsion but would require extensive testing and design for each application.
There are a variety of options for replacing hydrazine in the near future. Energetic ionic liquids have the best potential to be a safer, more widely used propulsion fuel that will reduce the costs for space programs in terms of time and money in the future. Solid and hybrid systems are feasible but there use as a general purpose maneuvering propellant is not viable at this time. Energetic ionic fluid’s full potential has not been fully examined yet, but already they are meeting and exceeding almost all requirements. The next ten to twenty years will be very exciting to see what replaces hydrazine and maneuvers future space vehicles through the cosmos.
- T.W. Hawkins, A.J. Brand, M.N. McKay, M. Tinnirello, Reduced Toxicity, High Performance Monopropellant at the U.S. Air Force Research Laboratory, 2010.
- Green Propulsion Technology from ECAPS and Moog Outshines Hydrazine, 2011.
- ECAPS Capabilities.