Radiation hardening balance vs mission risk and cost

Background

Orbital Tugs represent the primary logistics backbone for Phase 1 Initial Swarm Deployment, responsible for transporting 2,000-8,000 kg payloads across cislunar and heliocentric space over 7-15 year operational lifetimes. The consensus document specifies dual-string avionics with redundant flight computers, dual star trackers, and redundant IMUs to achieve fault tolerance during long-duration missions. However, significant divergence exists among technical models regarding the appropriate level of radiation hardening: Claude specifies RAD750 processors with full radiation-hardened avionics, Gemini proposes automotive-grade COTS electronics with spot shielding to achieve 10× cost reduction, and GPT recommends radiation-tolerant (not fully hardened) dual-string computers with environment-specific assessment.

This question emerges directly from the tension between unit cost targets—ranging from $1.15M (Gemini) to $60-120M (GPT) per vehicle—and the requirement for reliable 25,000+ hour thruster operation in radiation environments spanning 0.3-1.5 AU from the Sun.

Why This Matters

The electronics hardening strategy fundamentally determines both fleet economics and mission reliability. At production scales of 800+ units, the difference between RAD750-class processors ($200,000-300,000 per unit) and automotive-grade COTS alternatives ($500-5,000 per unit) compounds to hundreds of millions of dollars in program cost. However, a single-event upset (SEU) or total ionizing dose (TID) failure during proximity operations could result in collision with swarm elements, loss of payload, or cascading debris events.

Operating distance creates critical dependencies: tugs operating at Gemini's proposed 0.3 AU (Mercury-adjacent) experience approximately 10× the solar particle flux compared to 1 AU operations, dramatically increasing both SEU rates and cumulative TID. The consensus recommendation for depot-based operations means tugs will accumulate radiation exposure over multiple mission cycles rather than single-use profiles, making lifetime dose calculations essential.

The autonomy architecture compounds this risk—collision avoidance requires <1 second response times, and thruster fault safing demands <100ms reaction. If radiation-induced processor errors occur during these critical windows, autonomous systems cannot rely on ground intervention. Fleet-wide adoption of inadequate hardening could result in systematic failures as vehicles age, potentially grounding the entire logistics capability during critical deployment phases.

Key Considerations

Radiation Environment Characterization: Solar proton events (SPEs) dominate at inner heliocentric distances, while galactic cosmic rays (GCRs) present steady-state background. The 0.3-1.5 AU operating envelope spans environments with order-of-magnitude differences in flux intensity. Trapped radiation in cislunar space (NRHO/HEO staging) presents different spectral characteristics than heliocentric transit.

Component Susceptibility Thresholds: Automotive-grade electronics typically tolerate 10-50 krad TID; radiation-tolerant components handle 100-300 krad; fully hardened parts survive 300 krad to 1 Mrad. A 7-15 year mission at 1 AU accumulates approximately 30-100 krad behind 100 mils aluminum equivalent shielding, depending on solar cycle phase.

Shielding Mass Penalties: Spot shielding critical components adds mass that reduces payload capacity or requires larger solar arrays. Each kilogram of shielding mass trades against the 2,000-8,000 kg payload budget and affects the 180-210 m² solar array sizing.

Failure Mode Criticality: Not all electronics require equal protection. Star trackers and IMUs feeding autonomous collision avoidance demand higher reliability than housekeeping telemetry processors. Selective hardening strategies must map component criticality to protection levels.

Qualification and Testing Costs: Radiation testing campaigns for COTS components require extensive lot sampling and may reveal part-to-part variation that complicates fleet-wide reliability predictions. Testing costs partially offset procurement savings.

Research Directions

1. Develop Mission-Specific Radiation Environment Models: Generate Monte Carlo simulations of cumulative TID and SEU rates for representative mission profiles at 0.3, 0.7, 1.0, and 1.5 AU, incorporating solar cycle variability and depot dwell times. Quantify the 90th-percentile worst-case exposure for 15-year operational life.

2. Conduct COTS Component Characterization Study: Identify candidate automotive-grade processors, memory, and power management ICs; compile existing radiation test data from literature; estimate lot-to-lot variability and failure distribution curves against mission dose requirements.

3. Perform Shielding Mass-Performance Trade Study: Model spot shielding configurations for critical avionics boxes, quantifying mass penalty versus TID reduction. Determine break-even points where shielding mass exceeds cost-equivalent radiation-hardened component procurement.

4. Analyze Failure Consequence Propagation: Map electronics failure modes to operational consequences using fault tree analysis. Identify which subsystems require full hardening versus radiation-tolerant versus COTS-with-shielding based on criticality to autonomous collision avoidance and fleet safety.

5. Benchmark Operational Heritage: Survey radiation performance data from existing long-duration SEP missions (Dawn, Psyche, DART) and commercial GEO satellite fleets to establish empirical failure rates for various hardening approaches in comparable environments.