NASA Langley Research Center
Hampton, Virginia

Ionizing Radiation Spacecraft Design Considerations

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In order to develop efficient and reliable spacecraft, scientists need to eliminate those uncertainties in the space environment definition which adversely affect spacecraft operations and cost. In addition, existing radiation models should incorporate new flight data and new models should be generated as needed for advanced spacecraft design.

New data from science missions exist that increase the understanding of the space environment and it should be used to update and expand existing models and create new models. Models need to accurately reflect: the effects of transient belts due to solar storms, atmosphere modulation, and trapped heavy ions; the neutron environment induced in spacecraft; and the LDEF data on the iron component of solar energetic particles. In addition, new data on proton flux and directionality from LDEF indicates that the present AP8 model is inadequate.

In conjuction with a requirement for models, there exists a need for other analysis tools. There is an increased reliance on radiation analyses in spacecraft design in order to assess both risk and cost. Designers needs to be able to address a variety of important radiation effects: dose (which can range from 20 to 30 rad/year), single-event effects (which affect microelectronics), displacement damage and sensor noise. Overall, the analysis determines the amount of redundancy, and hence cost, of the spacecraft. For example, single-event upsets in microelectronics are a serious concern and it is important to determine the amount of shielding necessary to protect components. If a component is used in a data collecting processing unit with large memory, redundancy might not be a large consideration because the data will indicate if a single-event upset occurs. In contrast, the flight controls require more redundacy in order to keep the spacecraft operating properly.

A low level, standardized tool needs to be developed in order to get quick solutions to radiation problems. Such a tool not only will save cost but could also be available to students which will help train the next generation of spacecraft engineers. For a more thorough examination and analysis, high fidelity tools need to be developed to perform a variety of radiation analyses: radiation effects predictions and design requirements, design trades and verification, functional impacts, system reliability assessments, and radiation testing requirements. Radiation effects prediction methods, computer codes and tools to assess SEUs in microelectronics are needed along with the incorporation of the latest methods for LET spectra predictions.

Communication cycle time for vital information is another area for potential increased efficiency. Generally, radiation specialists have rapid access to the latest radiation environment/effects data, models and analysis codes. The expertise of these specialists should be made readily available to spacecraft designers.

In addition to the needs for design and support tools, physical parts for the manufacture of spacecraft are less available due to the decrease in DoD development of radiation hardened parts and greater reliance on unhardened commercial parts, and some offshore fabrication, for space applications. Also, ground-based radiation test facilities are becoming oversubscribed. Designers need component parts with low radiation susceptibility and ground test facilities to verify radiation tolerance. The following committtees are addressing issues relating to parts selection, test procedures and tests facilities: NASA Integrating Radiation Hardness Design Assurance Working Group, NASA Parts Steering Committee, and NASA/DoD Facilities Committee for Radiation Testing.

RECOMMENDATIONS

Below are some recommendations to lead to more reliable and cost-effective spacecraft programs:

Models

1. Include data from non-U.S. missions: Russian, ESA, and NASDA.
2. Utilize existing non-U.S. models to augment present models.
3. Archive flight data which is relevant to design applications for both short and long term usage.
4. Assure timely release of models to the users - spacecraft designers and engineers.

1. Develop radiation effects prediction methods including LET spectra prediction in order to expand knowledge of the SEU problem.
2. Develop computer codes suitable for radiation assessments.

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