Green Propulsion: Trends and Perspectives
Environmentally friendly alternatives could reduce the risk and cost of propulsion systems. Aerospace has been investigating possible candidates for national security space systems.
The propellants used in space programs pose environmental concerns in four main areas: ground-based impacts, atmospheric impacts, space-based impacts, and biological impacts. Ground-based impacts range from groundwater contamination to explosions caused by mishandling of propellants. Atmospheric impacts generally come from the interaction of propellant exhaust with the atmosphere. Space-based impacts generally focus on debris and effects on spacecraft. Biological impacts tend to focus on the toxicology and corrosiveness of propellants.
Space system developers have long sought to mitigate these impacts, because doing so could potentially reduce both cost and risk—especially the costs and risks associated with propellant transport and storage, cleanup of accidental releases, human exposure to toxic substances, infrastructure requirements for handling hazardous propellants, and orbital debris. The continued use of highly toxic propellants that generate environmental pollutants keeps program costs high—but the cost of developing and qualifying green alternatives also tends to be high. This has traditionally slowed development even when a green propellant provides potential performance benefits.
Also, the term “green propellant” is often confusing, as many assume a green propellant has no environmental impact. Such a propellant is generally beyond the realm of physical possibility. All propellants affect the environment in some way. For instance, all launch vehicles produce exhaust. The components of this exhaust can include soot, carbon dioxide, alumina, inorganic chlorine, water vapor, sulfates, and nitrogen oxides. All of these have an environmental impact, and may contribute to climate change, ozone destruction, or upper atmospheric contrails, depending on the atmospheric layer in which they are deposited; however, the severity and duration of the impact can vary greatly. Given this fact, a green propellant is more correctly viewed as one that seeks to minimize or eliminate a critical environmental impact in one or more of the four main areas. A green propellant is likely to have its own environmental impacts, which may be equal to the current technology in certain areas. For example, many green propellants seek to eliminate hydrazine because of its biologic impact, but they still present atmospheric or space-based effects.
The Aerospace Corporation has been investigating the potential for green propulsion systems with an eye toward helping space system designers minimize environmental impacts while improving overall efficiency and economy.
Ammonium Perchlorate Replacements
Hydrocarbon-based solid fuels have been used as rocket propellants in combination with the solid oxidizer ammonium perchlorate for several decades. Solid rocket motors are commonly used in launch-on-demand systems, boost-phase launch-assist systems, and small-lift launch vehicles. Unfortunately, much of the environmental impact (both ground and atmospheric) from the launch vehicle fleet comes from the ammonium perchlorate in solid rocket motors. Perchlorate leaching from discarded motors and from manufacturing operations can diffuse into groundwater, which can pose a health hazard for humans. The hydrogen chloride and chlorine produced during launch can destroy stratospheric ozone. Many of the exhaust components (soot, carbon dioxide, alumina, inorganic chlorine, stratospheric water vapor, and nitrogen oxide) can also contribute to climate change, either directly or through their reactions with other atmospheric species. In addition, perchlorate-based solid motors present safety challenges because they cannot be shut down once ignited.
Several green propellants are being developed as replacements for ammonium perchlorate in solid rocket motors. One potential candidate that has been gaining in popularity is ALICE (aluminum-ice). It combines the fuel, composed of nanoparticles of aluminum, with the oxidizer, oxygen, stored as water. The mixture is maintained below the water’s freezing point, so that it behaves like a solid propellant. ALICE has a higher theoretical specific impulse than conventional perchlorate-based solids. In 2009, NASA and the Air Force Office of Scientific Research launched a small suborbital demonstration vehicle powered by ALICE. The propellant has also been proposed for use on interplanetary return missions, as both water and aluminum could potentially be produced in situ on many interplanetary landing sites. ALICE produces hydrogen and alumina as its main exhaust; while these are generally billed as environmentally friendly, stratospheric alumina has the potential to reduce ozone concentrations. ALICE does present some risk of explosion (as all propellants that combine fuel and oxidizers do), but this risk has been shown to be relatively low. Alone, neither aluminum nor water is hazardous.
Several energetic salts have also been proposed as replacements for ammonium perchlorate. Solid motors based on ammonium dinitramide (ADN) could potentially have a 4-percent-higher specific impulse than perchlorate-based systems without producing hydrogen chloride exhaust. However, ADN is more prone to detonation under high temperatures and shock. Also, its density is about 8 percent less than that of ammonium perchlorate, so its performance is lower than a one-to-one comparison would suggest. In the late 1990s, ATK produced a solid ADN propellant that was thermally stable—but autoignition still occurred at about 110 kelvin less than that of comparable ammonium perchlorate propellants. NASA has been developing a solid motor based on ADN and has been working with the Swedish Defense Research Agency to investigate a solid fuel that would overcome the limitations of current formulations. Hydroxylammonium nitrate (HAN) is another energetic salt that has been proposed as an ammonium perchlorate replacement. In 2007, Raytheon and Aerojet demonstrated a 150-pound thruster based on HAN.
Hybrid rockets have also been investigated as a green alternative to perchlorate-based solid rockets. Hybrid propulsion systems use a solid hydrocarbon fuel (typically a polymer) and a liquid oxidizer. They have several advantages over conventional perchlorate-based solids in that they are nontoxic and nonhazardous, they can be shipped as freight cargo, and they can be shut down in case of an on-pad anomaly. They also have better performance attributes—they can be throttled for thrust control, they can potentially be restarted on demand, and they have higher achievable specific impulse. The disadvantage of hybrid motors is that many of the oxidizers would need a propellant management system, which often adds mass and cost to the vehicle—although some proposed self-pressurizing oxidizers could eliminate
this as a liability. Hybrids produce exhaust products similar to those of conventional liquid motors, with carbon dioxide, soot, water vapor, and nitrogen oxides as potential components. Soot levels may be similar to those of conventional solids that also use solid hydrocarbons as binders. Because the solid fuel and liquid oxidizer are not mixed initially, they have a lower explosion risk than conventional solid motors. Scaled Composites has produced a hybrid launch vehicle, SpaceshipTwo. The vehicle, designed to perform only suborbital human space flights, is being tested, and commercial passenger flights are expected to start in 2011.
Hydrazine is a multipurpose propellant that can be used as a hypergolic bipropellant with nitrogen tetroxide or in a monopropellant thruster with a catalyst. Hydrazine derivatives are still used as bipropellant fuels in launch vehicles in several countries. The United States no longer flies rockets based on bipropellant hydrazine derivatives, but small hydrazine monopropellant thrusters are often used by spacecraft—and these are the applications that are typically targeted for green replacements. Hydrazine storage is a concern on the ground. Because of its toxicology, it is costly to handle. The atmospheric impact is not a large driver in hydrazine research, but the space environment is a cause for concern. Hydrazine is naturally unstable, and unvented hydrazine tanks have been known to rupture in space, posing a debris risk at the end of mission life.
The Swedish company ECAPS produced a satellite thruster based on an aqueous ADN solution that produced higher specific impulse than monopropellant hydrazine. It was used on the formation-flying Prisma satellites. Thus, ADN is a potential replacement not only for perchlorate, but for hydrazine monopropellant as well. Similarly, HAN can potentially be used in aqueous solution as an alternative to hydrazine.
Electric propulsion offers another potential green alternative. Electric thrusters encompass a wide range of designs, including arcjets, resistojets, ion thrusters, and Hall thrusters. They use a magnetic field to trap injected electrons that are used to ionize an injected gas—usually xenon. Electric thrusters have the advantage of high specific impulse (compared to chemical thrusters), and they can potentially use much less propellant than a hydrazine vehicle to achieve the same maneuver. The main disadvantages are that they require an electric power source and generally offer only low thrust, which means they take longer to deliver a satellite to orbit. Many satellites have appropriate power generators onboard for other applications, so electrical thrusters need not add mass—but the longer delivery time can make them unattractive for certain missions. Thus, many satellites that use electric thrusters still must have hydrazine onboard for certain maneuvers. The xenon released from electric propulsion is generally not an environmental hazard, although the ionic plume can affect the space environment. Electric propulsion has been used by Russian satellites for a long time and is gaining acceptance in the United States; the technology is flying or will fly in many major U.S. Air Force programs and has helped lower the amount of hydrazine needed for these missions.
Several potential bipropellant formulations that use liquid oxygen (LOX) are being produced as potential hydrazine replacements. In general, these propellants are much less toxic than hydrazine derivatives; however, the cryogenic (< 91 kelvin) nature of liquid oxygen makes it hazardous to produce and difficult to store for long periods. Some of the first launch vehicles ever developed used LOX formulations, and kerosene/LOX and hydrogen/LOX engines are still in use today. LOX-based formulations are less popular for spacecraft propulsion because they require bulky tanks and feed lines and need large amounts of energy for refrigeration. Still, LOX/hydrocarbon fuels offer higher specific impulse than hydrazine monopropellant. Specific impulse is a way to describe the efficiency of rocket and jet engines. It is the ratio of the thrust produced by an engine to the rate of fuel consumption: it has units of time and is the length of time that one unit weight of propellant would last if used to produce one unit of thrust continuously. Thus an engine with a higher impulse would be more efficient because it would produce more thrust for the same amount of fuel used. Several different designs of LOX/hydrocarbon fuels have been demonstrated recently. For example, an 870-pound LOX/ethanol reaction-control thruster was developed and test-fired by Northrop Grumman in 2003, and Aerojet has also tested a LOX/ethanol reaction-control thruster. In 2007, XCOR and ATK test-fired a 7500-pound motor based on LOX and methane; the pressure-fed engine was sponsored in part by NASA’s Advanced Development LOX/Methane Engine program. In 2008, the Pratt & Whitney Rocketdyne RS-18 monopropellant thruster was modified to use LOX/methane and was tested by NASA at White Sands, New Mexico. Methane engines have performance close to that of traditional LOX/kerosene engines, but generally have lower fuel density. An advantage of methane over kerosene is the possibility to use a fuel-rich gas generator without soot formation; it also features a high cooling efficiency. Methane is injected in a gaseous state, thus lowering the risk of combustion instabilities. One reason for the renewed interest in LOX for spacecraft is the potential for in situ resource use during planetary missions.
High-test hydrogen peroxide has always been attractive as a monopropellant, as its decomposition products are water and oxygen. It can also be used as part of a bipropellant. In the past, hydrogen peroxide was used for satellite propulsion, but fell out of use as good catalysts for hydrazine thrusters became available. One major drawback of hydrogen peroxide is that storage becomes more difficult as the purity increases. A purity of at least 67 percent is needed to generate sufficient energy from a thruster. Several sub- marine accidents have resulted from unintended explosions of hydrogen peroxide propellants used in torpedoes. Still, hydrogen peroxide has been safely used by Russian Soyuz launch vehicles (82 percent purity) for more than 40 years to drive the main turbine pump in the gas generator and in the reaction-control-system thrusters used for the descent phase. Hydrogen peroxide was also used as the oxidizer in the British Black Arrow launch vehicle. As a monopropellant, hydrogen peroxide has a performance about 20 percent lower than hydrazine. The volume specific impulse achievable with 90 percent hydrogen peroxide is higher than for most other green propellants because of its high density. The most significant technological challenge for creating hydrogen peroxide monopropellant thrusters has always been the development of effective, reliable, long-lived catalytic beds. Also, alternative decomposition techniques are still needed to fully exploit the higher performance offered by 98 percent hydrogen peroxide. Current research is generally focused on microthrusters for small satellites. Lawrence Livermore National Laboratory developed a microthruster that uses 85 percent hydrogen peroxide; it flew on a 25-kilogram satellite. For the last decade, General Kinetics has offered 3-, 6-, and 25-pound force monopropellant and bipropellant thrusters and gas generators based on hydrogen peroxide.
Nitrous oxide is similar to hydrogen peroxide in its usability as an oxidizer in a bipropellant or as a monopropellant. Nitrous oxide offers potentially 80 percent of the specific impulse of hydrazine. Unlike other nitrogen oxides, it is nontoxic, noncorrosive, and stable under ordinary conditions; however, like most monopropellants, it can explode under certain conditions, so handling and shipping are a concern. Nitrous oxide is easily liquefied under pressure and is often stored as a liquid. High vapor pressure (50 atmospheres at room temperature) enables self-pressurization of the propellant tank, which can save weight. The reaction products—nitrogen and oxygen—are not hazardous. Catalysts are generally used to accelerate the decomposition of nitrous oxide because the decomposition temperature tends to be high. Developing space-qualified monopropellant catalysts are an issue with nitrous oxide, although some progress has been made on small resistojet thrusters for microsatellites. A monopropellant based on nitrous oxide is being developed by Firestar Engineering with support from NASA and DARPA (Aerospace has been involved in reviewing the safety testing program). It has higher performance than hydrazine, and may soon fly on the International Space Station. Much of the current investigation has focused on the use of nitrous oxide as the oxidizer in hybrid bipropellants—although in this application, ozone destruction from the nitrogen oxides in the exhaust would have to be considered.
Another hydrazine alternative, cold-gas thrusters, has been used on satellites for many years, both for maneuvering and for attitude control. They are suitable for applications that require very low total impulse. Cold-gas thrusters offer a wide range of chemical propellants because the gas need not be a combustible to provide thrust. Nitrogen has been used in many designs and has a specific impulse of 68 seconds. The main disadvantage of cold-gas thrusters is their low performance; however, for nanosatellites (where space is limited), their simplicity is an important advantage. Low-thrust propulsion engines using cold-gas thrusters are commercially available from various sources. (see sidebar, The National Environmental Policy Act).
Examples of Aerospace Research
Aerospace has a long history of investigating electric propulsion systems. Early programs focused on hydrazine arcjets, ion engines, and Hall-efffect thrusters. More recent work has complemented the corporation’s expertise in small satellites.
For example, a recent study verified the feasibility of a two-stage air-breathing Hall thruster. The system would be used to compensate for the increased orbital drag acting on a satellite at lower altitudes. A small, responsive satellite flying at lower altitudes could achieve better optical performance or present simpler design constraints. A key aspect of the thruster design was an ionization stage based on electron cyclotron resonance.
For many years, Aerospace scientists have been working with cold-gas microthrusters manufactured from photo-sensitive glass-ceramic materials. The technology would be suitable for a miniature spacecraft. The material has also been investigated for use in arrays of one-shot microthrusters that could be individually activated on orbit. More recent work geared toward the propulsion needs of CubeSats has focused on the use of a UV laser to activate a solid polymer and produce usable exhaust; the use of a laser to permit the combustion of solid propellants at pressures that would otherwise be too low; the use of ADN monopropellant; and electrolysis of water—essentially a fuel cell—to generate hydrogen gas as propellant.
Another intriguing study examined the introduction of a liquid hydrocarbon (n-heptane) and a gaseous oxidizer in a modified shock tube to create a pure gas-phase fuel and oxidizer mixture. Also, plans for new research facilities at Aerospace include the capability of studying liquid-oxygen/liquid-hydrogen thrusters.
Aerospace has developed a hybrid motor sizing code to answer fundamental questions such as: What is the performance gain or loss that will result from using hybrids instead of conventional perchlorate solids or liquids? What materials and design techniques could be used to increase payload-to-orbit capability? What is the estimated cost, and how does that compare with a conventional launch vehicle?
The sizing code shows that as payload weight increases, hybrids become more competitive with conventional solids. The LOX and helium tanks (used as a pressurant) are the primary reason that hybrids have a larger inert mass than solid rockets. Future systems that use self-pressurizing oxidizers or composite oxidizer tanks would significantly improve hybrid system performance.
Enhancing the burn rate of the solid propellant would also boost hybrid motor performance. With this in mind, Aerospace has been examining the use of fuel additives to create novel fuel grains. These additives may increase burn rates and overall performance and could also enable a restart capability. Currently, hybrid rocket motors are ignited by explosive squibs or gas flames. The squib systems can only be restarted if the squib is replaced, which is not possible for upper stages already in flight. External flame sources are possible on upper stages, but these add weight. Hypergolic fuel grains, which ignite spontaneously upon contact with the oxidizer, provide the simplest and most reliable form to start and restart a motor. Studies at Aerospace have demonstrated that the addition of lithium aluminum hydride (LiAlH4) to paraffin wax produces a fuel grain that ignites upon contact with several different chemicals. A 50-pound-thrust paraffin wax/gaseous oxygen motor was constructed and tested at Aerospace, demonstrating the feasibility of hybrid motors that use LiAlH4 doped paraffin wax and nitric acid as a hypergolic ignition system.
Hypergolic propellants are extremely useful on orbit, where conventional solids are rarely used. Aerospace has been researching a novel approach to fabricating fuel grains that could be used to create hybrid motors for CubeSats. In this approach, a fuel grain is built up using a form of rapid prototyping called stereolithography, in which patterns in successive layers of a liquid photopolymer are cured to produce a three-dimensional structure. Complete control of the three-dimensional grain shape allows the design of a hybrid motor that is highly filled and where the ports are not limited to straight, axially constant shapes, as is the case for motors produced by typical casting methods. Aerospace has fabricated and tested several fuel grains using gaseous oxygen. A novel deposition apparatus has been set up that will allow printing of paraffin wax motors as well. Positive results have been shown, and the technology is at Technology Readiness Level 3.
Some proposed technologies would eliminate propellants entirely. These tend to be far-reaching and have generally been demonstrated only in small-scale ground tests, if at all. They include concepts such as space elevators, laser propulsion, nuclear propulsion, and kinetic rail guns. Aerospace scientists have even proposed the ejection of microminiature spacecraft, instead of chemical exhaust products, to provide thrust; the satellites could even be programmed to return to the host craft, enabling long-term reuse. Though intriguing, these proposals do not appear likely to affect the space industry in the near future.
One exception is the solar sail. Originally proposed almost 50 years ago, solar sails use pressure from the solar wind and solar radiation to propel the spacecraft. Current technology uses ultrathin mirrors, and future crafts could potentially use a sail that acts both as a solar panel and as a propulsion device. Solar sails are generally not practical for Earth orbits, where atmospheric drag would overcome the forces produced, although solar sails that rotate with the craft could possibly make them suitable for some orbital use. The Japanese IKAROS satellite launched in 2010 uses a solar sail as its primary mode of propulsion; it will travel to Venus and then to the far side of the sun. Several other solar sail projects are currently in the works and may soon fly.
Ultimately, the fate of green propulsion will depend on its ability to satisfy the two main drivers for its development—higher performance and lower costs. U.S. space agencies have already begun the move toward green propellants with the acceptance of LOX/hydrogen and LOX/kerosene launch vehicles and greater use of electric propulsion for spacecraft. Currently, the atmospheric impact from launch vehicles remains low, but only because launch rates remain low. Limiting environmental impacts is a key part of achieving the high launch rates that would be needed to pursue ambitious space architectures, as doing so would help achieve the cost efficiency needed to make rapid launches financially possible. Green technology could also make interplanetary missions more efficient and sample-return missions from distant bodies more feasible. Green technologies aren’t just a future possibility—they are already a part of the space architecture, and further growth seems highly likely.
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