Beacon Independent Power Source
GPHS-RTG (Pu-238) Beacon Power Supply with Supercapacitor Buffer
Self-contained radioisotope thermoelectric generator (RTG) with supercapacitor buffer providing ~1 We continuous power for the beacon for 100+ years, completely independent of the main Ark power system.
Purpose
Guarantee beacon operation through any scenario that disables the main power bus — reactor shutdown, solar array damage, cabling severed, distribution faults — and outlast any chemical battery technology over the 100-year mission.
Context
Provides DC power to L3-COM-BEAC-XMIT and L3-COM-BEAC-ENC; uses ~250 mW thermal-to-electric conversion of Pu-238 alpha decay. Supercapacitor buffers the burst current of the transmitter (5 W for 0.5 s) from the steady-state RTG output.
Principles
- ▸Pu-238 has 87.7-year half-life — natural fit for a 100-year mission with predictable, modest power decay (~0.79%/year)
- ▸GPHS (General Purpose Heat Source) modules: 250 W thermal each, with PbTe/TAGS thermoelectrics ~6.5% efficient → ~16 We per module; a small fraction is sufficient for beacon
- ▸Supercapacitors (electric double-layer capacitors) store burst energy without battery aging — 100 F at 5 V stores ~1.3 kJ, enough for thousands of bursts
- ▸Light-Weight Radioisotope Heater Units (LWRHU): 1 W thermal each, used on Galileo, Cassini, Mars rovers — pair with thermoelectric to make milli-watt RTGs
- ▸Self-test of thermoelectric output verifies RTG health; degradation is gradual and predictable
- ▸Hermetic radioisotope encapsulation per NASA Pu-238 safety standards survives any credible Ark accident
Typical implementations
- ▸Multi-Mission RTG (MMRTG) — used on Curiosity, Perseverance: 110 We BOL, 14 years
- ▸GPHS-RTG — used on Galileo, Cassini, New Horizons
- ▸Light-Weight Radioisotope Heater Unit (LWRHU) — paired with thermoelectric for milliwatt sources
- ▸Soviet Strontium-90 RTGs (lighthouses) — decades of unattended operation
- ▸ESA-developed Americium-241 RTGs — alternative isotope with longer half-life (432 years)
Lunar considerations
- ▸Pu-238 RTG must meet international safety standards for launch and lunar disposal — sealed iridium-clad pellet inside graphite reentry/impact body
- ▸Lunar surface thermal environment helps RTG cold-junction (vacuum allows radiative rejection)
- ▸Long mission life makes Pu-238 (87.7 y) preferable to shorter-lived isotopes like Cm-244 (18 y)
- ▸Am-241 (432 y half-life) is an emerging alternative — ESA's main RTG program — gives flatter power curve over 100 years
- ▸Beacon RTG is small enough to be transported as part of Ark cargo without requiring dedicated launch licensing beyond standard mission
Specifications
Functional
| primary function | Provide continuous, independent electrical power to beacon over 100+ years |
| inputs | None — radioisotope source is sealed and self-contained |
| outputs | Regulated DC power: 3.3 V at ~250 mW continuous, Burst power via supercapacitor: 5 W for 0.5 s every 1 hour, Voltage/current telemetry to L3-COM-BEAC-ENC (for health-flag inclusion) |
| isotope | Pu-238 (or Am-241 future) |
| thermal power w bol | 4.0 |
| electrical power w bol | 0.25 |
| electrical power w eol 100yr | 0.13 |
| supercapacitor capacity f | 100 |
| supercapacitor max voltage v | 5.0 |
| burst capability w s | 2.5 |
| lifetime years | 100 |
Physical
| mass kg | 1.5 |
| dimensions | 120 × 80 × 80 mm hermetic isotope module + capacitor pack |
| materials | Iridium-clad Pu-238 fuel pellet, Graphite Impact Shell (GIS), Carbon-bonded carbon fiber (CBCF) thermal insulation, PbTe (lead telluride) thermoelectric couples, Tantalum / electrolytic supercapacitor cells, Stainless steel outer enclosure |
| operating temperature c | -180, 150 |
| radiation self | Alpha emitter (sealed); external gamma <1 mrad/hr at 1 m |
| ultimate load g | 50 |
Operational
| power output w bol | 0.25 |
| power output w eol 100yr | 0.13 |
| thermal range c | -180, 150 |
| lifetime years | 100 |
| mtbf hours | passive — limited by isotope decay |
Interfaces
Provides
- Regulated DC with burst capability
- Always-on 3.3 V supply
Requires
- Shielding compliance to keep external gamma exposure within safe limits
- Mounting location with thermal isolation from surrounding sensitive electronics
Cite this entry
Lunar Ark Codex. "Beacon Independent Power Source" (L3-COM-BEAC-PWR). Retrieved 10 September 2026, from https://lunarark.com/entry/L3-COM-BEAC-PWR
Licensed CC-BY-SA 4.0. You may reuse and adapt this entry with attribution, under the same licence.