In-Space Propulsion Technology Research Articles

Assessment of near-term fuel screening and qualification needs for nuclear thermal propulsion systems

Nuclear thermal propulsion (NTP) is an in-space propulsion technology capable of both high specific impulse (850–1000 s) and thrust (44–1112 kN), which can help reduce trip times for crewed missions beyond low Earth orbit. NTP technology has been demonstrated during historic programs. Over 20 ground test reactor experiments were performed, which demonstrated the prototypic reactor operations, during the Nuclear Engine for Rocket Vehicle Application (NERVA)/Rover program (1955–1972). Although historical programs have shown that NTP is a viable in-space propulsion technology, developing NTP in modern programs is contingent on the development and qualification of ultrahigh-temperature nuclear fuel technologies that can withstand engine operating conditions. In historical NTP development programs such as NERVA/Rover, prototypic reactor/engine schemes were ground tested to assess the overall system feasibility and to qualify the reactor fuel forms for eventual flight systems. Although this approach is effective to verify fuel performance under prototypic conditions, relying solely on full-scale NTP reactor tests as the pathway for verifying or qualifying fuel is inefficient and cost prohibitive today. Additionally, modern nuclear licensing requirements state that before test reactor approval, reactor components and fuel elements should be qualified via non-nuclear (out-of-pile) and nuclear (in-pile) testing under representative operating conditions. Using this methodology, fuel matures as production scale fabrication methods are established, and as produced fuel performance is demonstrated. This paper provides an overview of historical approaches to NTP fuel performance maturation, including fuel screening and qualification needs, and provides insight for establishing an efficient testing paradigm that can be implemented to rapidly and affordably develop NTP fuel forms for eventual qualification.

Light-induced propulsion of graphene-on-grid sails in microgravity

Light sailing is the only existing in-space propulsion technology that could allow us to visit other star systems in a human lifespan. In order to best harness radiation pressure, light sails need to be highly reflective, lightweight and mechanically robust. This is traditionally achieved by the use of nanometer-thin reflective layers supported by a micrometer-thick substrate that endows them with the necessary sturdiness. This combination usually results in a sail mass that is too high to be efficiently used for extrasolar exploration. Here, we propose a potentially scalable sail design that combines a hollow substrate with an atomically-thin 2D material which, thanks to its ultimately low surface density, allows reducing the mass contribution of the substrate. To demonstrate the potential of such sails, we have studied the laser-induced displacement of graphene-on-copper sails in vacuum and in microgravity. In these conditions, 0.25 mg samples are accelerated by using lasers of different wavelengths (450 and 655 nm) and power (0.1–1 W). The measured thrust is one order of magnitude larger than the theoretical calculations for radiation pressure alone. This calls for further theoretical studies and attracts interest on graphene as light-sail material.

Deep space transportation enhanced by 20 kW-Class Hall Thrusters

Deep space is the new frontier for human exploration, with Moon and Mars identified as fundamental targets. Improving in-space transportation capabilities has been recognized as one of the critical enabler for sustainable and affordable space programs in Earth proximity and beyond. Envisioning the presence of future deep space infrastructures, cargo transferring becomes a major issue that can benefit from improvements in in-space propulsion technology. Electric propulsion could represent the turning point, thanks to the combination of new system architectures and technology advancements, e.g. cluster architecture and magnetic shielding, and improved capability of on-board power generation. High-power Hall Thrusters are considered the most promising solution for future space exploration, thanks to a favourable thrust to power ratio, higher than Gridded Ion Engines. Reusable platforms, based on Hall Thrusters, could represent a valid alternative to chemical-propelled spacecraft. These systems could be exploited to support human presence in deep space, delivering life support items and providing on-orbit servicing capabilities. In this paper, the typical mission analysis tools have been exploited to analyse the selected scenarios. The analysis highlights possible advantages achievable adopting high-power Hall Thrusters on board reusable platforms. Since the design of these spacecraft envisions the adoption of a 20 kW-class Hall Thruster string, the mass and power budgets are obtained for those subsystems that are most affected by this critical technology. Then, the feasibility of each scenario is assessed considering the needs defined not only by the traffic plan, in terms of loading/unloading cargo and transfer duration, but also by the peculiar mission and physical constraints. Last, the different platform design solutions are compared with respect to their electric propulsion configurations, in order to identify the possible commonalities in terms of architecture and technology, in line with the current trend of modularity and affordability.

Products from NASA's in-space propulsion technology program applicable to low-cost planetary missions

Since September 2001, NASA's In-Space Propulsion Technology (ISPT) program has been developing technologies for lowering the cost of planetary science missions. Recently completed is the high-temperature Advanced Material Bipropellant Rocket (AMBR) engine providing higher performance for lower cost. Two other cost saving technologies nearing completion are the NEXT ion thruster and the Aerocapture technology project. Under development are several technologies for low-cost sample return missions. These include a low-cost Hall-effect thruster (HIVHAC) which will be completed in 2011, light-weight propellant tanks, and a Multi-Mission Earth Entry Vehicle (MMEEV). This paper will discuss the status of the technology development, the cost savings or performance benefits, and applicability of these in-space propulsion technologies to NASA's future Discovery, and New Frontiers missions, as well as their relevance for sample return missions.

Development priorities for in-space propulsion technologies

During the summer of 2010, NASA's Office of Chief Technologist assembled 15 civil service teams to support the creation of a NASA integrated technology roadmap. The Aero-Space Technology Area Roadmap is an integrated set of technology area roadmaps recommending the overall technology investment strategy and prioritization for NASA's technology programs. The integrated set of roadmaps will provide technology paths needed to meet NASA's strategic goals. The roadmaps have been reviewed by senior NASA management and the National Research Council.With the exception of electric propulsion systems used for commercial communications satellite station-keeping and a handful of deep space science missions, almost all of the rocket engines in use today are chemical rockets; that is, they obtain the energy needed to generate thrust by combining reactive chemicals to create a hot gas that is expanded to produce thrust. A significant limitation of chemical propulsion is that it has a relatively low specific impulse. Numerous concepts for advanced propulsion technologies with significantly higher values of specific impulse have been developed over the past 50 years.Advanced in-space propulsion technologies will enable much more effective exploration of our solar system, near and far, and will permit mission designers to plan missions to “fly anytime, anywhere, and complete a host of science objectives at the destinations” with greater reliability and safety. With a wide range of possible missions and candidate propulsion technologies with very diverse characteristics, the question of which technologies are ‘best’ for future missions is a difficult one. A portfolio of technologies to allow optimum propulsion solutions for a diverse set of missions and destinations are described in the roadmap and herein.

Shock Radiation Measurements for Mars Aerocapture Radiative Heating Analysis

NASA's In-Space Propulsion Technology program is supporting the development of shock radiation transport models for aerocapture missions to Mars and Venus. Phenomenological models of nonequilibrium shock radiation will be incorporated into high-fidelity flowfield computations used to predict the aerothermal environments for a Mars or Venus aerocapture entry vehicle. These models are validated with shock radiance measurements obtained at flight-relevant conditions. A comprehensive test series in the NASA Ames Electric Arc Shock Tube facility at a representative freestream condition was recently completed. The facility's optical instrumentation enabled spectral measurements of shocked gas radiation from the vacuum ultraviolet to the near infrared. The instrumentation captured the nonequilibrium postshock excitation and relaxation dynamics of dispersed spectral features. A description of the shock tube facility, optical instrumentation, and examples of the test data are presented.

Recent advances in solar sail propulsion systems at NASA

Supporting NASA's Science Mission Directorate, the In-Space Propulsion Technology (ISPT) Program is developing solar sail propulsion (SSP) for use in robotic science and exploration of the solar system. SSP will provide longer on-station operation, increased scientific payload mass fraction, and access to previously inaccessible orbits for multiple potential science missions. Two different 20-m solar sail systems were produced and successfully completed functional vacuum testing last year in NASA Glenn's Space Power Facility at Plum Brook Station, Ohio. The sails were designed and developed by ATK Space Systems and L’Garde, respectively. These sail systems consist of a central structure with four deployable booms that support the sails. These sail designs are robust enough for deployments in a one atmosphere, one gravity environment, and are scalable to much larger solar sails—perhaps as much as 150 m on a side. In addition, computation modeling and analytical simulations have been performed to assess the scalability of the technology to the large sizes ( > 150 m) required for first generation solar sail missions. Life and space environmental effects testing of sail and component materials are also nearly complete. This paper will summarize recent technology advancements in solar sails and their successful ambient and vacuum testing.