AVAILABLE TODAY FOR TOMORROW'S NEEDS

• HIGH EFFICIENCY REDUCES ISOTOPE INVENTORY
• DEMONSTRATED LONG LIFE

Benefits

NASA Glenn Research Center and the Department of Energy (DOE) are developing a Stirling convertor for an advanced radioisotope power system to provide spacecraft on-board electric power for NASA deep space missions. Stirling is being evaluated as an alternative to replace Radioisotope Thermoelectric Generators (RTGs) with a high-efficiency power source. The efficiency of the Stirling system, in excess of 20%, will reduce the necessary isotope inventory by a factor of at least 3 compared to RTGs. Stirling is the most developed convertor option of the advanced power concepts under consideration [1,2].

Background

NASA Glenn has been investigating Stirling radioisotope power systems for deep space missions since about 1990. This work grew out of earlier Stirling efforts conducted for DOE for a Stirling automotive engine and for the NASA Civil Space Technology Initiative (CSTI) to develop Stirling for a nuclear power system to provide electrical power for a lunar or Mars base. NASA Glenn also provided technical management for DOE for the Advanced Stirling Conversion System (ASCS) terrestrial dish Stirling project. Overall, NASA Glenn has been developing Stirling technologies since the mid-1970's.

Systems using Stirling convectors are also being analyzed by NASA Glenn for other space applications including solar dynamic power systems for space-based radar [3] and as a deep space alternative to the radioisotope system [4,5], a combined electrical power and cooling system for a Venus lander, and lunar/Mars bases and rovers.

Status

DOE is developing the Stirling convertor for the radioisotope power system under contract with Stirling Technology Company (STC) of Kennewick, WA [6,7]. Two 55-We convertors operating in a dynamically-balanced opposed arrangement are now being tested by STC, as shown in figure 1. Both design convertor power and efficiency have been demonstrated. NASA Glenn is providing technical consulting for this effort under an Interagency Agreement with DOE.

Fig. 1 - Two opposed 55 We Stirling Convertors (Courtesy DoE/STC)

NASA Glenn is evaluating key technology issues for the radioisotope Stirling development through the use of two NASA Phase II Small Business Innovation Research (SBIR) contracts with STC. Under the first SBIR, STC demonstrated a synchronous connection of two thermodynamically independent Stirling convertors and a 40 to 50 fold reduction in vibrations compared to an unbalanced convertor [8]. Synchronization was achieved over a wide operating range. System operation was also demonstrated with the synchronized convertors feeding a battery charger load. This connection method is now being used to connect the DOE/STC 55-We convertors. The second SBIR contract is for the development of an Adaptive Vibration Reduction System that will essentially eliminate vibrations over a mission lifetime, even in the unlikely event of a failed convertor.

NASA Glenn has also initiated an in-house project to provide supporting technology for the overall Stirling radioisotope power system development [9]. Tasks for this new effort include convertor performance verification, controls technology development, materials and joining evaluations, heater head structural life assessment, permanent magnet characterization and thermal aging tests, finite element analysis (FEA) of a lightweight alternator concept, convertor electromagnetic interference (EMI) characterization, demonstration of convertor operation under launch and orbit transfer load conditions, and a lightweight radiator conceptual design. The vibration test facility in the Structural Dynamics Laboratory at NASA Glenn is shown in figure 2. This facility will be used for the testing under launch and orbit transfer loads. These project tasks build on NASA Glenn expertise developed as part of previous Stirling research, especially for the Stirling space power development during the NASA CSTI project [10-12].

Fig. 2 - Vibration Test facility at NASA Glenn

In support of DOE, Orbital Sciences Corporation [13,14] and Lockheed Martin have completed system studies for a Stirling radioisotope power system. These systems were based on a STC Stirling convertor design. Power system layouts using either two or four convertors have been studied. The choice of two or four convertors per power system is dependent, in part, on system redundancy requirements.

Demonstrated Long Life Capability

The design of the 55-We Stirling convertor for the radioisotope power system is based on previous successful STC development efforts, particularly those for the 10-We radioisotope terrestrial convertor, RG-10, and the 350-We RG-350 aimed at commercial cogeneration and remote power [15]. One RG-10 has now been on life test for over 50,000 hours (5.7 years) with no convertor maintenance and no degradation in performance. Another RG-10 convertor has recently been fueled with isotope, and field testing is beginning. Multiple units of the RG-350 and their companion cryocoolers have accumulated over 80,000 total hours of operation, much of this at independent third-party test sites. STC has also completed extensive component life testing and, in particular, has over 1000 years of total test time (230 flexures) on the critical flexural bearings. These numerous test hours on various systems and on key components provide a high confidence that the 55-We convertor will meet its life and reliability goals.

The 55-We convertor uses the following proven STC long life design approaches: non-contacting moving parts with flexural bearings and clearance seals, no lubrication, integral alternator inside the pressure vessel, hermetic sealing, and Inconel 718 for the hot-end material. The use of Inconel 718 allows the necessary long lifetimes at a hot-end temperature of 650°C. Increased efficiency and/or decreased radiator size can be achieved by using advanced hot-end materials operating at a higher temperature.

Demonstrated Dynamic Balance

The Stirling convertors for the radioisotope power system will be operated in synchronized opposed pairs to minimize vibrations under normal operating conditions (see figure 3). With two RG-350 convertors operating in this configuration, STC has achieved a 40-50 fold reduction in vibrations compared to an unbalanced convertor, a value that appears to be well below pixel smear limits for deep space sensing. The convertor pairs are connected electrically in parallel and mechanically through external attachments on the cold-end pressure vessels.

Fig. 3 - Synchronized Opposed Convertors

The Adaptive Vibration Reduction System (AVRS) will further reduce vibration levels by a factor of 10 or more under normal operating conditions. It will achieve this with an active balance system with feedback from a vibration signal and will cancel the fundamental vibration and up to 10 harmonics. Even more importantly, the AVRS will be adaptive and will add the ability to adjust to any changing convertor conditions over the course of a mission. Thus, it should allow successful dynamic balancing over a mission lifetime and will be able to demonstrate its adaptive ability through up-front testing. The AVRS is now being developed on two RG-350 convertors and will also be demonstrated on the DOE/STC 55-We convertors.

Stirling cryocoolers are currently used to cool vibration-sensitive sensors in space applications. STC has demonstrated a cryocooler vibration level of only 0.007 g's using similar technology to the AVRS. This technique has been shown to be effective with reasonable power and mass budgets. One key difference for balancing power convertors is that the frequency is not fixed as it is in coolers. Thus, the frequency must be measured on a continual basis and factored into the control algorithm.

Initial tests of the AVRS on the RG-350 have shown a 500 fold reduction in unbalanced vibrations under normal operating conditions with two synchronized convertors in an opposed configuration. This compares to the 40 to 50 fold reduction with just the synchronized convertors and was accomplished with only 2 W of power dissipation. Testing was also done with a simulated failed convertor, and a 50 fold vibration reduction was obtained with only 7 W of power dissipation; this power dissipation scales to less than 2-3 W for a 55-We convertor. These initial tests only balanced the fundamental of the vibration signal (no harmonics) and, thus, vibration levels should be even further decreased as development of the full control algorithm is completed.

References

Last Revision: January 4, 2000

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