cycle are higher power and small component size, because two-phase-flow and heat-transfer coefficients are much larger than those for gas in non-two-phase systems. Unfortunately, little is known at present about the behavior of two-phase flows and associated heat transfer in a reduced-gravity environment. The possibility of developing a multipurpose, multiuser facility for multiphase flow research on the ISS should therefore be considered. Such a facility would also act as a catalyst for bringing together national and international researchers to address the problem in a cost-effective and comprehensive manner. Significant insights into the static and dynamic behavior of granular materials and dusts could be gained through experiments on the ISS. Such an understanding would be of value for applications on Earth and for human and robotic exploration of the Moon and Mars. (It is noteworthy that the Mars rover Spirit has been stuck in the martian soil, a granular material, since May 2009.)
Advances in propulsion performance (specific impulse, efficiency, thrust to weight, propellant bulk density), reliability, thermal management, power generation and handling, propellant storage and handling, and strategies for refueling on orbit are all key drivers for dramatically reducing mass, cost, and mission risk. The ISS provides unique opportunities for advances in a number of these areas through research on processes such as cryogenic two-phase fluid management, propellant transfer, engine starts, flame stability, active thermal control of injectors and combustors, and cryogenic fluid management.
In summary, the ISS platform is an essential and integral component of any implementation of the physical sciences research outlined in this decadal survey.
The decadal survey committee strongly recommends that NASA intensify the utilization of the ISS as a world-class research laboratory engaged in both basic and applied research that enables space exploration and is enabled by the microgravity environment of the ISS. The goal should be to maximize the utilization of existing facilities and to engage world-class scientists and engineers to carry out research that leads to the development of space-related technologies. Ground-based experimental and theoretical work should form a significant component of the overall activity.
Cross-disciplinary research should be emphasized, and a research portfolio with prioritization should be developed and shared with the technical community.10 To develop a vibrant research community that is committed to space-related research, NASA should have a firm plan for sustaining the research by providing adequate resources. Aside from benefiting directly from the research, NASA would be contributing to the creation of the workforce for the future. The process, from the acceptance of a proposal to preparation of the flight experiment to conduct of experiments on the ISS, should be streamlined, with a reduction in time from start to finish. This is essential to keep graduate students and other researchers engaged in the research activity. Some of the experimental rigs that have already been flown on the ISS can serve as facilities for future research investigations in the physical sciences. NASA should reconsider the placement of a centrifuge on the ISS so that long-duration partial-gravity experiments can be conducted. NASA should also strengthen and expand its collaborations with international partners. This would allow access to the facilities of the partner countries, avoid a duplication of research, and allow U.S. researchers to accomplish much more than they could otherwise.
Discussed throughout this report are various topics within each of the areas described above. The committee reiterates, however, that although the ISS is a key component of the research infrastructure to be utilized by a biological and physical sciences research program, it is only one component of a healthy program. Other platforms will play an important role and, in particular, research on the ISS will have to be supported by other platforms, including a parallel ground-based program, to be scientifically credible.