L2-PWR-RTG CRITICAL POWER ENERGY Level 2 · hardware

Radioisotope Thermoelectric Generators

RTG Emergency Backup Power System

The RTG emergency backup power system is a passive, solid-state generator that converts the decay heat of plutonium-238 into electricity via thermoelectric couples. Functioning as the tertiary power tier in an N+2 settlement architecture, each 90-kilogram unit produces 300 to 500 watts of electrical power to sustain critical keep-alive loads, including cryo-coolers, command and data handling cores, and emergency communications. During the 14-day lunar night when secondary solar arrays cannot generate power, and in lunar vacuum conditions requiring reliable passive heat rejection without moving parts, the system maintains 0.9999 availability, degrading from 500 watts at beginning-of-life to 150 watts across its 100-year design life.

Emergency backup power using radioisotope decay heat with thermoelectric conversion providing continuous low-level power

Purpose

Provide ultra-reliable always-on emergency power for critical keep-alive functions (cryo-coolers, CDH core, emergency communications) if both nuclear reactor and solar arrays fail. RTGs have no moving parts and degrade predictably, making them ideal for deep backup.

Context

Emergency/tertiary power source in the N+2 architecture. Lowest power output (~300-500 We per unit) but highest reliability (no moving parts, passive operation). Complements L2-PWR-NUC (primary) and L2-PWR-SOL (secondary). Multiple units may be deployed for adequate keep-alive power.

Principles

▸Radioactive decay of Pu-238 produces continuous thermal energy (~0.57 W/g)
▸Thermoelectric couples (Seebeck effect) convert temperature differential to electricity
▸No moving parts: extremely reliable with predictable degradation
▸Power output decreases with Pu-238 half-life (87.7 years) and thermoelectric degradation
▸At 100 years, Pu-238 retains ~45% of initial thermal output
▸Thermocouple degradation (sublimation, dopant diffusion) further reduces electrical output

Typical implementations

▸MMRTG (Multi-Mission RTG): ~110 We BOL, ~2 We/kg, used on Curiosity/Perseverance
▸GPHS-RTG: ~300 We BOL, used on Cassini, New Horizons, Ulysses
▸eMMRTG (Enhanced MMRTG): ~145 We BOL using SKD couples, in development
▸GPHS module: standard 250 Wt heat source building block
▸Voyager RTGs: still operating after 47+ years (significantly degraded)

Lunar considerations

▸Lunar surface provides better heat rejection than deep space (radiator to ground + sky)
▸Must be shielded or separated from biological preservation areas
▸Thermal output useful as supplemental heating for critical systems
▸Predictable degradation allows mission planning for power budget evolution
▸No refueling possible: initial Pu-238 load must last mission duration
▸Multiple GPHS modules provide graceful degradation (lose one, others continue)

Specifications

Functional

primary function	Convert radioisotope decay heat to DC electrical power for emergency keep-alive systems
inputs	Pu-238 fuel (pre-loaded, self-sustaining)
outputs	DC electrical power: ~300-500 We total (from multiple units at BOL), Waste heat: ~2-4 kWt (usable for heating), Telemetry: temperatures, power output, voltage
bol power we	500
eol power we 100yr	150
availability	0.9999
design life years	100
no moving parts	True
autonomous operation	True

Physical

mass kg	90
dimensions	Multiple units, each ~65 cm diameter x 66 cm length (MMRTG form factor)
materials	Pu-238 oxide fuel pellets, Iridium cladding (GPHS module), Fine Weave Pierced Fabric (FWPF) carbon-carbon aeroshell, PbTe/TAGS thermoelectric couples, Aluminum housing fins
operating temp c	Hot junction ~500°C, cold junction ~200°C
radiation field	Alpha/neutron emission from Pu-238 (lower than fission reactor)
thermal	Self-heating, always warm

Operational

power consumption w	0
thermal range c	200, 500
lifetime years	100
mtbf hours	1000000

Interfaces

Provides

→Power Conversion & Conditioningpowercritical
DC electrical output ~300-500 We total (decreasing over mission life)
→Active Thermal Control Systemthermalsupporting
Continuous waste heat ~2-4 kWt, usable for heating critical components
→Power Management & Controldataessential
RTG status: temperatures, power output, degradation tracking

Requires

←Active Thermal Control Systemthermalessential
Heat rejection path for RTG fins (passive, but fin orientation matters)
←Primary Structuremechanicalcritical
Mounting structure, ~90 kg per unit pair
←Radiation Shieldingenvironmentalsupporting
Separation distance or minor shielding from sensitive payloads