Session 3: Power System Engineering
The purpose of this session was to obtain insights into how solar-based power systems work as a whole and the performance improvements that could arise from advanced technology.
Robert Francis (The Aerospace Corporation) began the session by discussing some of the general issues related to improving solar-based power systems. He noted that the DOD has a technology roadmap with specific performance numbers in terms of energy density and volumetric efficiency for solar arrays. This DOD roadmap projects improvements that are about half the level of improvement proposed in the draft NASA roadmap for TA03. Francis thinks that most of the mass savings in future solar power systems will come from improvements to solar array supporting structures— not from improvements in the solar cells mounted on the arrays. He noted some of the challenges in moving to very-high-power arrays include the need for higher voltages for the spacecraft electrical bus and difficulties in ground testing.
Azam Arastu (Boeing) focused most of his talk on the two advanced, lightweight, and compact, solar arrays currently under development at Boeing with the support of the Defense Advanced Research Projects Agency (DARPA) and the U.S. Air Force Research Laboratory. Both the Fast Access Testbed Spacecraft (FAST) and the Integrated Blanket/Interconnect System (IBIS) arrays are capable of generating very high power and employ highly efficient IMM solar cell technology and advanced deployment structural concepts to provide significant improvements in the specific power over the current state of the art. The FAST array uses linear solar concentrators and is less expensive as it uses fewer solar cells. It also is more tolerant to radiation and more effective in the low-light environments of deep space, but it has more precise pointing requirements. The IBIS array, which is constructed using thin, flexible, and planar solar modules, packs more compactly than FAST and has less precise pointing requirements. Both array systems are designed to be highly modular and scalable so they can eventually be assembled into systems providing solar arrays capable of generating upwards of 300 kW of power.
The group discussion spent some time on the issue of modularity with both speakers saying that the slight increase in weight caused by modular systems is outweighed by the savings due to simplicity and the ability to mass produce components. When asked why NASA needs to invest in arrays when there is already a robust technology development, the speakers suggested that NASA can focus its investment on spacecraft integration issues and the unique environmental requirements for operation in space. One of the speakers also said that even non-financial support from NASA could help ensure technological progress continues. Finally there was some discussion on bus voltage: 200 V DC was identified as a near term possibility; 300 V DC possible over the long term; and improving performance beyond that achievable with 300 V DC may require shifting from a DC power system to an AC power system.
Session 4: Nuclear Systems
Joseph Nainiger (Alphaport) began the session on nuclear systems by discussing the history of RPSs and fission power systems. He noted that while the United States has extensive flight experience with RPSs, it has only flown one fission power system (in 1965) and has not manufactured and ground tested a fission nuclear thermal rocket since 1972. Over the near term, RPSs will continue to fill the need for a reliable source of electrical power in environments that cannot use solar power (even with advances in solar power technology). Fission systems are enabling for missions with high power demands independent of sun proximity or illumination. Nainiger praised the current effort at NASA to develop and test the non-nuclear components of a fission power system, while adding that a system capable of producing at least 1 MW of electrical power will only be possible with a sustained and aggressive technology development effort. He noted that fission power systems will require a significant investment in infrastructure, and he suggested that efforts should be taken to capture technology developed in the past. Naininger also asserted that restarting the production of Pu-238 is a critical national need.
Gary Bennett (formerly with NASA) concurred that NASA has a critical need for Pu-238. He postulated that more than half of the missions proposed in the recent planetary decadal survey (NRC, 2011) would benefit from an RPS, if enough Pu-238 were available. He discussed the new RPSs under development and urged NASA to focus on the Stirling engine power converter that is at the heart of the Advanced Stirling Radioisotope Generator