DoD Eyes Smaller Propulsion Systems
Written by Peter Buxbaum
Smaller propulsion systems' modularity and
other benefits must be complemented by
improvements in performance and other
criteria before units gain acceptance.
The Department of Defense, through programs such as Operationally Responsive Space, wants smaller satellites built, modular in design, which can be rapidly configured to meet immediate mission requirements. That program requires the development of smaller propulsion systems that can be integrated into a modular design.
To a certain extent, military space programs have a model for the miniaturization of propulsion components in ballistic defense systems. Civilian space programs are already studying ballistic defense systems in an effort to adapt components, including propulsion systems. Military space programs will likely be doing the same.
Meanwhile, federal government initiatives encourage the use of environmentally friendly, or at least friendlier, propellants. Here, research is ongoing and some progress has been made both in the United States and abroad. But U.S. military space agencies have yet to adopt any of these developments as their own, finding that they lack the performance characteristics that DoD desires. That situation may change, however, after the European Space Agency launches a satellite fueled by a new greener liquid propellant later this year.
Miniaturization, modularity and environmental friendliness all militate in favor of electric propulsion systems. Here, again, research is ongoing, but current electric systems lack the performance criteria necessary to be attractive to military space programs. On the other hand, it is possible that electric propulsion could be soon incorporated in a hybrid propulsion system, which would reduce their mass and make them greener.
“The military wants to get away from the customer approach to production, where each satellite is a one-off,” said Robert Huebner, vice president of AMPAC In-Space Propulsion, a subsidiary of American Pacific Corp. headquartered in Niagara Falls, N.Y. “With unique platforms, you can’t easily change configurations. This leads to very costly production.”
Design of future satellites will have to be modified to accommodate this modular vision. “Typically, systems, sensors and other components tend to be wrapped around the propulsion system,” Huebner explained. “To replace something in the propulsion system, you have to tear apart the vehicle. With Operationally Responsive Space, DoD seems to want to plug mission pieces into the system together and launch it.”
Modularity also implies the ability to reuse propulsion components from one platform to another, according to Dick Bregard, vice president of defense programs at Aerojet, a supplier of missile and space propulsion systems based in Sacramento, Calif. “The Air Force wants to repackage current technologies for new platforms,” he said. “They might want to take a propulsion system or components of a system from Project X and use is somewhere else.”
Another way to look at modularity is to view the military’s requirement as the desire for more capability in the same package. “The military wants to be able to reuse the same launcher and launch container,” said Bregard. “The volume is set, but when upgrades come they still want to stay within the same envelope. They don’t want to change the launcher. They want to make it more capable.” A similar approach is being taken by the Missile Defense Agency with its Standard Missile programs, Bregard noted. The MDA has begun to test a third level of standard missile and is currently planning for as many as three additional iterations.
One of the implications of the smaller size of planned satellites and the modularity of their design involves the reduction in mass and the elevation in performance of propulsion systems. “Masses of propulsion systems are being driven down,” said Huebner. “But at the same time, programs are coming up with more requirements for sensors to help warfighters do their jobs. Cutting masses means propulsion systems are being increasingly challenged.”
For Huebner, this challenge focuses on improving the efficiency of engines rather than making components lighter. “That way you don’t have to use as much propellant,” he said. “For most systems, propellant mass is a large fraction of the total.”
This also suggests, according to Huebner, a move away from chemical propulsion systems and toward electric or hybrid systems. “Current electric systems won’t do the job for small satellites,” he noted. If current research being conducted under Air Force funding bears fruit, electric propulsion systems may provide a future solution.
Military space programs could be looking to progress made in ballistic missile defense systems for indications of how they can reduce the size and mass of their propulsion systems. “The military can take advantage of the packaging and integration techniques we have incorporated in the THAAD and MKV,” said Art Weiss, director of attitude control propulsion systems at Pratt & Whitney Rocketdyne, a division of United Technologies Co. based in Canoga Park, Calif.
THAAD is the Terminal High Altitude Area Defense anti-ballistic missile, a terminal phase defense system that has gone into production. The MKV, or Multiple Kill Vehicle, was envisioned to allow more than one kill vehicle to be launched from a single booster. (In April, Secretary of Defense Robert Gates announced the termination of this program. Weiss expects MKV technologies to be repurposed elsewhere.)
“DoD is downsizing their tactical satellites so that they can quickly integrate together mission components and get them launched,” Weiss continued. “The launch configuration is driven by the immediate needs of warfighters. Military space programs have started looking at the size of propulsion systems already incorporated in missile defense systems in order to size up their payload.”
The missile defense program is already where it needs to be as far as the size of its propulsion systems, according to Weiss. “There is no current push to reduce sizes anymore,” he said. “Military space programs could take advantage of what we have already done and figure out how best to integrate that in what they want to do. At least some of the smaller satellites will have even less volume than we do in our ballistic defense kill vehicles.”
The National Aeronautics and Space Administration has already expressed interest in incorporating some missile defense technologies in their designs, an indication that DoD may also be following suit.
Propellants are also an evolving aspect of space and missile propulsion systems. The military is interested in utilizing substances that are safer to the humans who handle them as well as to the environment, but to date, no suitable greener propellants have been deployed. The greening of satellite propulsion systems could also run counter to DoD’s efforts at cost containment through modularization.
“Some of the ways propulsion systems are changing is in the military need for more insensitive munitions,” said Aerojet’s Bregard. “Insensitivity” generally refers to the safety and transportability of the material in question, as far as their imperviousness to shock and vibration.
There is some crossover in this concept to the area of propellants in that the military is striving to employ more stable and less toxic substances to fuel satellite and missile launch and throttling functions.
“The military is trying to use fuels that are less hazardous to humans and to the environment in how they are mixed and used,” said Bregard. “They used to rely on acid-based materials that were very hazardous to the individuals handling and loading them. They are trying to get away from that and to materials less hazardous to individuals and that don’t leave a toxic cloud.”
This trend militates in favor of solid as opposed to liquid propellants, according to Bregard. The Navy also limits the types of fuels it is willing to carry on ships. This has also sparked investigation into the more stable solid fuels.
Solid fuels are likely to make headway in the case of the smaller propulsion devices used to adjust the trajectory and attitude of launch vehicles in the case of spacecraft or kill vehicles in the case of ballistic defense systems. “There are some breakthroughs in controllable solids for these types of systems,” said Bregard. “The Navy also prefers solid fuels and is in the forefront of this trend.”
There are problems with solid fuels, however. Ammonium perchlorate, a major ingredient in solid propellant, is toxic to humans once ignited. Alternatives have been explored, but have not yielded adequate performance results.
“There is a desire to go away from ammonium perchlorate,” said Bregard, “but there is not yet a good solid energy source that can readily replace it. This involves how the armed services tailor their performance considerations versus the environmental impacts. Military laboratories and private industry contractors are looking into it, but at this point there is no suitable trade they can come up with that applies a significantly greener propellant that does not increase risks elsewhere.”
In the case of liquid propellants, efforts are being made to replace the high performance but toxic hydrazine and nitrogen tetroxide that are often used as propellants. AMPAC is working on potential solutions, but operational systems still need development.
In addition, the experimental fuels may be greener, but are not entirely environmentally friendly. “There is still toxicity associated with the propellants under investigation,” said Huebner.
Military space and missile defense agencies, and their industry partners, are also working to make propellants cheaper, according to Bregard. “Work is going on behind the scenes to make products cheaper, more flexible and more capable,” he said. “Sometimes there is a tendency to lose sight of the cost factors of the newer types of fuels as opposed to their older petroleum-based counterparts.”
Part of cost reduction involved with green propellants is that you don’t have to have people in SCAPE suits deal with the propellants and you don’t have to have specialized facilities to store them.
“There is also a cost aspect of going to greener propellants,” Huebner added. “The fact that the infrastructure for handling toxic propellants has been in place for so many years has slowed the adoption of greener or safer propellants.”
Some greener fuels also compromise performance, he added. “The Air Force Research Laboratory is doing a lot of good work in the area of greener propellants,” said Huebner. “Some of these have been demonstrated on a small scale, but they require performance improvements before they are going to be of interest to military space programs.”
Research conducted in Sweden has yielded a greener liquid propellant that will soon be used to launch a satellite by the European Space Agency. Weiss expects missile defense officials in the United States to be looking at this new liquid propellant for missile defense applications if the European launch proves successful.
Other possibilities for the application of greener liquid propellants include the use of methane- and ethanol-based fuels, according to Bregard. Methane in particular is increasingly being considered as a greener alternative to current liquid propellants. ♦