Plasma Treatment of Composite Adhesive Bonds
The low density of fiber-reinforced composites—along with their adjustable high stiffness and strength—makes them the material of choice for many space applications; however, these materials are susceptible to bond failures caused by deficiencies in surface-preparation techniques. The most common preparation technique relies on mechanical roughening (often sanding), which uses abrasion to remove surface contaminants and increase roughness. However, the abrasion can damage the reinforcement plies of advanced composites, reducing effective bond strength. In addition, contamination and inconsistencies in surface preparation are problematic. Therefore, the spacecraft community needs a more consistent and reliable process for creating high-performance bonds.
The Aerospace Corporation has been evaluating plasma treatment for surface preparation of composite hardware and has found that this process can address the lack of consistency and reliability in current industry practices. “The atmospheric plasma treatment is noncontacting, requires minimal operator intervention, and can be applied to complex shapes while significantly reducing the risk of physical damage to the composite since the process affects only the outer few nanometers of the treated surface,” said Rafael Zaldivar of the Materials Science Department, lead investigator for the project.
The plasma is generated by a capacitive discharge at atmospheric pressure to produce a uniform high-density mix of ions, electrons, and free radicals. These reactive species are then directed onto a surface. A number of physical processes can occur during plasma treatment: ablation (cleaning by removing low-molecular-weight organic contaminants); etching (affecting the surface morphology of the substrate); crosslinking (interconnection of long-chain molecules); and surface activation (chemical bonding of reactive molecules with the substrate).
“Our initial experimental work has shown that plasma treatment not only enhances the consistency of the mechanical performance of bonded hardware, but also increases strength. Strength increases in excess of 50 percent have been realized. In addition, the fracture toughness of these bonded joints, critical to long-term durability, has also been shown to improve by more than 100 percent compared with conventional preparation methods,” said Zaldivar. Structural-design limits are currently determined by applying statistical margin-to-test results determined from the materials and processes intended for use in flight production. Process changes that deliver a smaller variation of results or higher bond strengths will allow an increase in structural design limits and therefore increase the trade space for vehicle design (size, mass, and power).
A critical aspect of this work has been identifying the mechanism responsible for many of the improvements. Using x-ray photoelectron spectroscopy (XPS), the researchers have identified the distribution of activated groups that are formed as a function of treatment conditions and have correlated them to mechanical performance. In the case of epoxy bonded graphite/epoxy composites, an increase in the concentration of surface carboxyl species translates to an increase in adhesive bond strength.
“These dramatic improvements are a result of enhanced chemical bonding that is now possible at the interfaces between these newly formed carboxyl groups on the surface of the composite and the epoxide groups within the adhesive,” said Zaldivar. “However, not all resin systems used in composites develop the necessary type of functional groups for improvements in strength when treated with plasma. Understanding how the chemical structure of the initial composite material and the plasma treatment conditions combine to result in the necessary type of functional groups is paramount for tailoring our interfacial reactions. Tailoring of composite interfaces not only potentially increases the capabilities of current systems, but also opens a wide array of possibilities for new materials systems to be used in composite hardware.”
Many resin systems available for composite manufacture today have not been used for space applications, primarily because of drawbacks associated with their poor bonding behavior. Aerospace has recently developed a process to modify bonding surfaces of these materials to make them more susceptible to plasma treatment improvements. For example, polycyanurate-based composites, which are commonly used in national security space structures, do not show the magnitude of improvement that some of the epoxy matrices do after plasma treatment. Zaldivar said that by modifying the critical bonding interfaces, the concentration of the active species responsible for bond strength can be controlled locally and increased by more than 300 percent from that of an unmodified system.
“Plasma surface preparation techniques may be able to lower costs and improve reliability, average strength, and consistency,” said Zaldivar. “Contractors are likely to embrace the technology when it becomes widely available, but if historical precedence holds, they may do so without an understanding of the underlying chemistry and physics of the adhesive bond enhancement mechanisms. The level of understanding that will result from this work is important if the space industry is to move to any sort of qualified atmospheric plasma process.”
Preventing Radio-Frequency Breakdown in Satellite Components
Both military and commercial satellites rely on radio-frequency (RF) systems for communication and navigation payloads. The RF power demand for these systems has continued to grow with increasing user needs and higher available satellite power. Global Positioning System (GPS) III and the Mobile User Objective System (MUOS) are just two examples of satellites with unparalleled RF power requirements at multiple frequencies.
With increasing power levels comes increasing risk for RF breakdown within high-power components. RF breakdown is an electrical discharge—such as a plasma or multipactor discharge—that can degrade high-fidelity communication signals and cause physical damage to susceptible components. These discharges can lead to complete loss of essential communication or navigation signals and prevent proper satellite operation. As such, preventing RF breakdown is essential.
In response to this growing risk, The Aerospace Corporation is leading new research into plasma and multipactor breakdown. This program, led by Timothy Graves, Space Materials Laboratory, is pursuing basic research into the underlying phenomenology while helping contractors develop better hardware and testing requirements.
“Aerospace has a unique window into the real-world issues experienced by RF component manufacturers. This allows us to tailor our research programs to solve problems of today and tomorrow through a physics-based understanding of these concerns,” said Graves. “Our goal is to decrease risk through an improved understanding throughout the satellite process. From component design, through extensive ground testing, to on-orbit operation, we depend on the success of these RF components. Our research is providing new ways to improve success in each of these areas.”
Multipactor breakdown is one of the highest concerns for high-power RF component designers today. Also referred to as multipaction, this discharge type can occur when electrons impact material surfaces in resonance with the RF electric field. This resonance depends primarily on three components: the RF voltage (how fast the electron is accelerated), the RF frequency (how long before the electric field changes direction), and the geometry (how far the electron travels before hitting a surface).
As electrons resonantly impact electrode surfaces, the electron density grows through secondary electron emission. The secondary electron yield, defined as the number of emitted electrons per incoming electron, is a fourth parameter for multipactor breakdown, such that the secondary electron yield is greater than 1 to develop the discharge. When these conditions are met, the formation of a large electron density can perturb the RF system and substantially increase the risk of plasma breakdown and component damage.
Detecting multipaction in complex devices can be difficult, yet early detection in product development is critical for satellite cost and schedule. In some cases, devices with undetected multipaction in unit-level tests may experience catastrophic failures after integration into the satellite system. To prevent this, Aerospace has characterized various breakdown signatures and developed new diagnostics for improved detection sensitivity.
New software-based phase-nulling diagnostics for multipactor detection have been recently developed at Aerospace using fast analog-to-digital processing to analyze the relationship between forward and reflected power signals. With software, the system monitors for any complex impedance change caused by multipactor or plasma formation. These software-based systems have significant advantages over manually controlled analog devices, including higher stability, improved interpretation, and greater sensitivity.
Multipaction depends on the material surface, which can vary strongly with contamination. These discharges also dynamically change as multipacting electrons impact surfaces, desorb contaminants, and/or form new surface thin films, a process known as multipactor conditioning. Aerospace has performed extensive research into multipactor contamination and multipactor conditioning on various materials, specifically characterizing a new multipactor mode referred to as transient-mode multipactor discharge.
“The transient-mode multipactor discharge forms similarly to a conventional discharge, yet as the electrons remove contaminants and change the secondary electron yield, the multipactor is extinguished,” Graves said. “This transient process can repeat indefinitely under continued contamination until the device is damaged.” Several Aerospace surface science studies are investigating dynamic surface changes with multipactor exposure. Initial results have shown the formation of thin films that can initially improve the voltage threshold for multipaction. Further studies are planned with potential application to surface science and nanotechnology research areas.
Graves credits the success of this program to the diverse scientific backgrounds available at Aerospace. “Our multidisciplinary team includes researchers in RF engineering, plasma physics, materials science, and systems engineering. With experts in each of these areas, we have made strong and unique contributions toward mitigation of RF breakdown.” The program’s team includes participants across many departments, including William Cox, Tom Curtiss, Rostislav Spektor, and Jason Young, all of Electric Propulsion and Plasma Science; Gouri Radhakrishnan and David Witkin, of Materials Science; Jerry Michaelson, of Communication Systems Implementation; and Frank Villegas, of Antenna Systems.
The program has also had strong participation from students at UCLA, Loyola Marymount University, Embry-Riddle Aeronautical University, Purdue University, and the University of Maryland. Graves also cites an “excellent collaboration with many government contractors to work together toward the common goal of better device performance and reliability.”
“Our research will continue to adapt to meet our customer needs. We hope to pave the way for improved computational prediction capability in complex structures, improved device testing with enhanced diagnostics, and, lastly, improved understanding of breakdown phenomenology—toward our main goal of ensuring space mission success.”
Beyond-Next-Generation Access to Space
What will launch systems look like beyond the next generation of spacelift systems? What technology is needed to enable such systems? What are the risks, and how will they be met? What missions will these vehicles perform, and at what operating costs?
Prompted by such questions, The Aerospace Corporation in 2009 began a research program to identify possibilities for the generation of launch system architectures beyond those currently planned or under development, and to identify the technologies that would enable such systems. Options for these beyond-next-generation spacelift systems are being examined for satellite and human spaceflight applications.
The Air Force Spacelift Development Plan provides the architectural blueprint for launch following the EELV program. This plan recommends development of a reusable
booster with expendable upper stages to significantly lower the cost of next generation launch vehicles. Aerospace has helped the Air Force develop a detailed technology road map for operational deployment of this concept. The beyond-next-generation effort builds upon the investment in reusability and operable designs from the Air Force’s plan to evaluate fully reusable two-stage-to-orbit systems and single-stage-to-orbit systems. Today’s technological advances offer the potential to expand the missions and markets for beyond-next-generation systems while reducing the cost per flight and improving turnaround time between flights. The vision for future missions includes routine, highly responsive space access (also referred to as satellite launch on demand), space tourism, and point-to-point passenger/cargo delivery between major cities (e.g., Los Angeles to Tokyo in less than 2 hours).
“The initial phase of the study found that the success of future systems is closely tied to achieving operational efficiencies more characteristic of aircraft than of today’s rockets—for example, turn time, maintenance effort/hours, and mission abort options. Success was not so dependent on dramatic improvements in performance,” said Jay Penn of the Launch Systems Division and principal investigator of the study. Aerospace co-investigators include Greg Richardson, Greg Meholic, Bob Hickman, Joe Tomei, Glenn Law, and Fred Peinemann.
“Success will likely be driven by the flight rate of reliable, reusable launch vehicles that meet the demands and price markets of future missions. Two-stage-to-orbit vehicle architectures and modern engines could satisfy future performance needs, but launch vehicles based on today’s technology would become extremely cost prohibitive in new markets,” said Penn. The costs of maintaining today’s technology—rather than investing in future technologies—would be prohibitive because of low flight rates, major refurbishment needs between flights, and significant failure costs. Most critical to the future is investment in technologies focused on operability that would dramatically alter launch vehicle design approaches and yield fully reusable, low-cost, highly operable space-access platforms. Investments in novel air-breathing propulsion concepts and supporting propulsion technologies offer opportunities to increase system robustness and performance. However, these concepts introduce a new set of design challenges because of their highly integrated engine cycles.
Research has concentrated on three primary areas: the relationship between future spacelift markets and missions, advanced launch-vehicle architectures and performance and operability metrics outside of conventional approaches, and game-changing and emerging technologies. The range of emerging technologies that have been explored includes lightweight structures that use emerging advances in carbon nanotube-based materials, space elevators (a concept in which payloads are lofted to orbit on a carriage that ascends a long, very exotic tether having greatly varying properties along its length), nuclear thermal propulsion, and constant volume combustion devices/pulse-detonation engines that rely on an inherently simpler and more thermodynamically efficient cycle than those used on existing engines.
Ten single-stage-to-orbit vehicle designs were modeled; these encompassed vertical and horizontal launch options using various propellant combinations and diverse rocket and air-breathing propulsion systems. The long list of technologies required for these designs includes passive and active highly operable high-temperature and lightweight thermal protection systems; high-temperature seals and actuators; highly integrated propulsion systems, aerodynamics, and control; advanced vehicle health monitoring; and a range of technologies that foster rapid vehicle turnaround for the subsequent flight. Even with innovative propulsion and material technologies, the single-stage-to-orbit designs either did not meet the performance criteria or resulted in vehicles with large gross liftoff weights and large dry weights.
“The single-stage-to-orbit system performance design was approximately twice the dry weight and gross liftoff weight of a similar capability two-stage-to-orbit design. Thus, even if a single-stage-to-orbit vehicle is successful, it will not yield a better beyond-next-generation solution than a two-stage-to-orbit design using far less aggressive technologies. Based on these findings, our efforts turned to two-stage-to-orbit design solutions,” Penn said.
As with the single stage, the nine two-stage-to-orbit designs studied were sized to deliver 25,000-pound payload lift to low Earth orbit. The designs and systems evaluated included rocket and turbine-based combined-cycle boosters, pulse-detonation engines, an air collection and enrichment system, and magneto-hydrodynamic-augmented propulsion, which allows more total heat to be added to the flow and increases propulsion cycle and power efficiency. Key assessments of the two-stage designs included defining performance parameters and vehicle characteristics for each concept for dry weight, gross liftoff weight, length, ground-to-orbit equivalent specific impulse, and propellant density. Also studied were spacelift and high-speed global-reach needs such as payload/dry weight, orbiter wetted area, orbit flexibility, manufacturing complexity, volumetric sensitivity, and basing flexibility.
“No attempt was made to apply individual weightings because these are highly dependent upon specific mission applications and objectives,” Penn said. “When customer and stakeholder preferences are known, weighting can be easily applied and cumulative scores determined for each concept. Assuming there is a development program for innovative propulsion and materials technologies, all two-stage-to-orbit concepts achieved reasonable gross liftoff weights and sizes. The relative merits of each concept are mission and application dependent,” Penn said.
For example, if flexible access to low Earth orbit is determined to be the most critical future need, then vertical-takeoff, horizontal-landing two-stage-to-orbit solutions appear best. In these designs, the booster stage could be based on the reusable booster system design, and the orbiter stage could be based on either a fully reusable rocket or a higher-performing but more technically advanced rocket-based, combined-cycle powered stage. If hypersonic cruise or dual use as an atmospheric transport or bomber becomes most important, then the horizontal-takeoff combined-cycle and pulse-detonation engine solutions seem most promising. If integration to traditional airport runway operations and air-traffic control is needed, then concepts employing air collection and enrichment systems are most attractive because they have acceptable payload-to-gross-liftoff-weight ratios and can accommodate existing runway limits. These air-collection designs would not have quantity/distance issues because they take off and land with no onboard oxidizer (a feature that the other concepts cannot avoid). In an air collection and enrichment system, the oxidizer is extracted from the air, so there is no large quantity of stored liquid oxygen at takeoff, and therefore no explosive hazard. If a nearer-term air-breathing solution is most appealing, then air collection and enrichment systems also show merit.
The study found that two technologies in particular combined into a vehicle concept showed the most promise when compared with the rocket-based and turbine-based combined-cycle concepts. The first is an air collection and enrichment system that uses a refrigeration-based cycle to extract the oxidizer from the atmosphere during subsonic flight for later use in the trajectory, and the second is a pulse-detonation engine that has a higher cycle efficiency than existing engines and is expected to yield improved installed thrust to weight. The pulse-detonation engine is at a lower state of development than the combined-cycle engines, but is believed to be feasible because of its inherently simpler and more efficient design. Pulse-detonation engines have been developed by hobbyists and are routinely run in academia. Penn believes that this type of vehicle concept—combining the air collection and enrichment technologies with pulse-detonation engines—merits further evaluation.
The study results allow The Aerospace Corporation to offer its customers feedback as they determine future development investments. Penn’s team is working to improve the modeling of pulse-detonation engine systems for spacelift missions and volumetrically efficient “wave-rider” hypersonic aircraft for point-to-point transport. The team will also design reference missions for study—for example, transporting 50 passengers from Los Angeles to Tokyo in less than two hours, or launching a replacement spacecraft to orbit in less than one week after an on-orbit failure. The team will evaluate concepts and system architectures against the requirements of these types of missions. The researchers will also study the environmental effects of nuclear thermal rockets, the near- and far-term benefits of carbon nanotube materials applied to launch vehicles for their unique electrical and structural characteristics, and the use of orbital propellant depots for refueling.
Aerospace Report No. ATR-2010(8161)-1, “Beyond Next Generation (BNG) Access to Space” (The Aerospace Corporation, El Segundo, CA, 2010).