Overview of free-piston Stirling engine technology for space power application

An overview is presented of free-piston Stirling engine activities, directed toward space power applications. One of the major elements of the program is the development of advanced power conversion. Under this program the status of the 25 kWe opposed-piston Space Power Demonstrator Engine (SPDE) is presented. Initial differences between predicted and experimental power outputs and power output influenced by variations in regenerators are discussed. Technology work was conducted on heat-exchanger concepts to minimize the number of joints as well as to enhance the heat transfer in the heater. Design parameters and conceptual design features are also presented for a 25 kWe, single-cylinder free-piston Stirling space power converter. Projections are made for future space power requirements over the next few decades along with a recommendation to consider the use of dynamic power conversion systems, either solar or nuclear. A cursory comparison is presented showing the mass benefits of a Stirling system over a Brayton system for the same peak temperature and output power. A description of a study to investigate the feasibility of scaling a single-cylinder free-piston Stirling space power module to the 150 kWe power range is presented

Outdoor test stand performance of a convertible engine with variable inlet guide vanes for advanced rotorcraft propulsion

A variable inlet guide van (VIGV) type convertible engine that could be used to power future high-speed rotorcraft was tested on an outdoor stand. The engine ran stably and smoothly in the turbofan, turboshaft, and dual (combined fan and shaft) power modes. In the turbofan mode with the VIGV open fuel consumption was comparable to that of a conventional turbofan engine. In the turboshaft mode with the VIGV closed fuel consumption was higher than that of present turboshaft engines because power was wasted in churning fan-tip airflow. In dynamic performance tests with a specially built digital engine control and using a waterbrake dynamometer for shaft load, the engine responded effectively to large steps in thrust command and shaft torque. Previous mission analyses of a conceptual X-wing rotorcraft capable of 400-knot cruise speed were revised to account for more fan-tip churning power loss than was originally estimated. The new calculations confirm that using convertible engines rather than separate lift and cruise engines would result in a smaller, lighter craft with lower fuel use and direct operating cost

The 1988 overview of free-piston Stirling technology for space power at the NASA Lewis Research Center

The completion of the Space Power Demonstrator Engine (SPDE) testing is discussed, terminating with the generation of 25 kW of engine power from a dynamically-balanced opposed-piston Stirling engine at a temperature ratio of 2.0. Engine efficiency was greater than 22 percent. The SPDE recently was divided into 2 separate single cylinder engines, Space Power Research Engine (SPRE), that serves as test beds for the evaluation of key technology disciplines, which include hydrodynamic gas bearings, high efficiency linear alternators, space qualified heat pipe heat exchangers, oscillating flow code validation, and engine loss understanding. The success of the SPDE at 650 K has resulted in a more ambitious Stirling endeavor, the design, fabrication, test, and evaluation of a designed-for-space 25 kW per cylinder Stirling Space Engine (SSE) to operate at a hot metal temperature of 1050 K using superalloy materials. This design is a low temperature confirmation of the 1300 K design. It is the 1300 K free-piston Stirling power conversion system that is the ultimate goal. The first two phases of this program, the 650 K SPDE and the 1050 K SSE are emphasized

Free-piston Stirling technology for space power

An overview is presented of the NASA Lewis Research Center free-piston Stirling engine activities directed toward space power. This work is being carried out under NASA's new Civil Space Technology Initiative (CSTI). The overall goal of CSTI's High Capacity Power element is to develop the technology base needed to meet the long duration, high capacity power requirements for future NASA space missions. The Stirling cycle offers an attractive power conversion concept for space power needs. Discussed here is the completion of the Space Power Demonstrator Engine (SPDE) testing-culminating in the generation of 25 kW of engine power from a dynamically-balanced opposed-piston Stirling engine at a temperature ratio of 2.0. Engine efficiency was approximately 22 percent. The SPDE recently has been divided into two separate single-cylinder engines, called Space Power Research Engine (SPRE), that now serve as test beds for the evaluation of key technology disciplines. These disciplines include hydrodynamic gas bearings, high-efficiency linear alternators, space qualified heat pipe heat exchangers, oscillating flow code validation, and engine loss understanding

The convertible engine: A dual-mode propulsion system

A variable inlet guide vane (VIGV) convertible engine that could be used to power future high-speed rotorcraft was tested on an outdoor stand. The engine ran stably and smoothly in the turbofan, turboshaft, and dual (combined fan and shaft) power modes. In the turbofan mode with the VIGV open, fuel consumption was comparable to that of a conventional turbofan engine. In the turboshaft mode with the VIGV closed, fuel consumption was higher than that of present turboshaft engines because power was wasted in churning fan-tip air flow. In dynamic performance tests with a specially built digital engine control and using a waterbrake dynamometer for shaft load, the engine responded effectively to large steps in thrust command and shaft torque. Previous mission analyses of a conceptual X-wing rotorcraft capable of 400-knot cruise speed were revised to account for more fan-tip churning power loss that was originally estimated. The calculations confirm that using convertible engines rather than separate life and cruise engines would result in a smaller, lighter craft with lower fuel use and direct operating cost

Test stand performance of a convertible engine for advanced V/STOL and rotorcraft propulsion

A variable inlet guide vane (VIGV) convertible engine that could be used to power future high-speed V/STOL and rotorcraft was tested on an outdoor stand. The engine ran stably and smoothly in the turbofan, turboshaft, and dual (combined fan and shaft) power modes. In the turbofan mode with the VIGV open, fuel consumption was comparable to that of a conventional turbofan engine. In the turboshaft mode with the VIGV closed, fuel consumption was higher than that of present turboshaft engines because power was wasted in churning fan-tip air flow. In dynamic performance tests with a specially built digital engine control and using a waterbrake dynamometer for shaft load, the engine responded effectively to large steps in thrust command and shaft torque

The effects of urbanization on floods in the Austin metropolitan area, Texas

This report looks at compiled data from 1956-1980 to "provide a technique for estimating the magnitude and frequency of flood-peak discharges at ungaged sites and to estimate the effects of changes in urbanization on flood peaks."The effects of urbanization on flood peaks in streams in the Austin metropolitan area were studied in two separate analyses. In the first analysis, annual peak discharge records at 13 streamflow-gaging sites were used to compute a recorded flood frequency relation for each site. Rainfall and streamflow data for 10 to 20 storms for each of these sites were used to calibrate a rainfall-runoff model in which a 55-year rainfall record was used to simulate 55 annual peak discharges. These simulated discharges also were used to develop a flood-frequency relation at each site. The flood-frequency relations from recorded and generated data were then combined by weighting the recorded flood frequency by the years of record at each site to produce a combined (or weighted) flood frequency at each site. Flood frequencies for all 13 sites were subsequently regressed against basin characteristics at each site to determine possible effects of urbanization. The regression analysis of the combined flood-frequency data for the 13 sites yielded an equation for estimating floods of a given recurrence interval at ungaged sites in the Austin area as a function of the contributing drainage area, the total impervious area percentage, and basin shape. The regression equation estimates that a near fully developed hypothetical drainage basin (impervious area percentage, 45) would have discharges for the 2- and 100-year recurrence interval that are 99 percent and 73 percent greater, respectively, than discharges for those frequencies from a rural drainage basin (impervious percentage, 0). In the second analysis, records at one streamflow-gaging site on Waller Creek were analyzed for changes in rainfall-runoff and flood-frequency relations due to urbanization. Annual peak discharges from 1956 to 1980 and data from a total of 80 storms at the Waller Creek site were analyzed. Both analyses showed increases comparable to those predicted using the equations developed from the 13-station analysis. The last 14 years of record (the near fully developed land-use stage for the Waller Creek analysis) at the two sites on Waller Creek were part of the 13-station analysis. The U.S. Geological Survey, in cooperation with the Texas Department of Water Resources began limited investigations of urban watersheds in Austin in 1954, with the installation of two streamflow-gaging stations and three recording rain gages in the Waller Creek watershed. In 1963, a streamflow gage and three recording rain gages were installed at Wilbarger Creek watershed, a rural area just north of Austin. In cooperation with the City of Austin, the urban study was expanded in 1975 to include additional streamflow and rainfall gaging stations and the collection of surface water-quality data. The number of streamflow-gaging stations increased from 2 to 25 and the number of recording rain gages increased from 3 to 31.Waller Creek Working Grou