Power budget

The beacon can run continuously; the transponder needs scheduling

The spacecraft is a tumbling 1U with body-mounted silicon cells on five faces, so energy is the limiting resource. The 48 Wh battery is large compared with any single eclipse. What limits operations is the orbit-average energy balance.

Orbit-avg generation

0.78 W

EOL, tumbling, β = 0°

Beacon-mode load

0.68 W

✓ Positive balance

Transponder load

2.86 W

3.84 W at max LTM current

Max transponder duty

4.8 %

β = 0° · 8.0 % at β = 45°

Battery

38.6 Wh

EOL (80 % of 48.2 Wh BOL)

Eclipse DoD

5.9 %

transponder at max load; limit 30 %

1. Solar generation

Each face carries 6 cells at about 0.20 W (AM1.5), giving 1.64 W per face at AM0. The cells are on five faces (±X, ±Y, −Z); +Z is the antenna cap. For a randomly tumbling cube each face averages |cos θ| = ¼, so the mean sunlit input is 1.25 face-equivalents = 2.05 W before losses.

Loss factor	Multiplier
Cell temperature (+50 C, -0.45 %/C)	0.89
LT3652 input regulation (8.44 V) vs 2S Vmp	0.90
Blocking Schottky (CDBA240LL)	0.96
Cover / EVA encapsulant	0.97
Radiation & UV degradation (EOL, 2 yr)	0.96
LT3652 charger efficiency	0.85
Combined	0.609

The net sunlit power into the battery is 1.25 W. At 525 km the orbit period is 95.1 min, and the worst-case eclipse (β = 0°) is 35.7 min (38 %), giving an orbit-average of 0.78 W.

Orbit-average generation vs beta angle

525 km circular orbit, tumbling, EOL. Above about 68° the orbit is in full sun.

Show data table

β (deg)	Generation (W)	Eclipse fraction
0	0.779	0.375
5	0.780	0.375
10	0.782	0.373
15	0.785	0.370
20	0.790	0.367
25	0.797	0.361
30	0.805	0.354
35	0.816	0.345
40	0.831	0.334
45	0.850	0.318
50	0.877	0.297
55	0.913	0.268
60	0.969	0.223
65	1.073	0.140
70	1.247	0.000
75	1.247	0.000
80	1.247	0.000
85	1.247	0.000
90	1.247	0.000

2. Loads by operating mode

Orbit-average load by mode and subsystem

Battery-side watts, including regulator efficiency. Orbit-average generation is 0.78 W.

Mode	FC + sensors	LTM	Magnetorquers	Heater	Total (W)	Continuous-use balance
Detumble (LEOP)	0.172	0.000	0.300	0.000	0.472	✓ +0.31 W
Safe	0.122	0.278	0.000	0.100	0.499	✓ +0.28 W
Beacon (telemetry only)	0.191	0.484	0.000	0.000	0.675	✓ +0.10 W
Transponder (typical load)	0.191	2.669	0.000	0.000	2.860	✗ -2.08 W deficit
Transponder (max load)	0.191	3.652	0.000	0.000	3.843	✗ -3.06 W deficit

Flight controller 3.3 V loads (datasheet-typical)

Load	mA @ 3.3 V	mW	Source
RP2350 MCU (150 MHz active)	30.0	99.0	DS
Flash / PSRAM / SD NAND (avg)	3.0	9.9	DS
IMU LSM6DSO + mag LIS2MDL	0.8	2.6	DS
MCP23017 GPIO expander	1.0	3.3	DS
INA219 x2 power monitors	2.0	6.6	DS
I2C muxes / buffers / watchdog / RTC	3.0	9.9	DS
Face sensors x5 (MCP9808, VEML7700, TCA4311A)	3.5	11.5	DS
Antenna-cap + battery sensors (TMP112, VEML6031)	1.0	3.3	DS
MCP25625 CAN controller (LTM on)	5.0	16.5	DS

The 3.3 V rail comes from the TPS54226 at 85 % efficiency (Eco-mode). The magnetorquers add an estimated 0.30 W during detumble only. The battery heater (1.0 W element) is budgeted at 10 % duty in safe and cold cases.

LTM supply current (public AMSAT figures, 5 V LTM_VSYS)

LTM mode	mA	mW at LTM
safe	40	200
beacon	70	348
transponder	475	2375
transponder_max	650	3250

The beacon figure is a 310 mA transmit current at about 11 % duty over a 40 mA baseline. Transponder current is 300–650 mA depending on uplink loading. The TPS54225 rail is modelled at 72 % efficiency at light load and 89 % at heavy load (forced-CCM part).

3. How much transponder time?

Maximum sustainable transponder duty vs beta angle

Fraction of time the transponder can be on, with the beacon on the rest of the time, at zero net energy per orbit

Show data table

β (deg)	Max duty (%)
0	4.7
5	4.8
10	4.9
15	5.0
20	5.2
25	5.6
30	5.9
35	6.4
40	7.1
45	8.0
50	9.2
55	10.9
60	13.4
65	18.2
70	26.2
75	26.2
80	26.2
85	26.2
90	26.2

Battery state of charge under a battery-gated transponder policy

Transponder ON above 80 %, back to beacon below 70 %. Achieved duty: 5.0 % at β = 0°, 7.9 % at β = 45°.

Result. At worst case the transponder can run about 69 minutes per day. That's enough for roughly 7 ten-minute passes over high-demand regions. Commanded or battery-gated scheduling is required. Ways to increase transponder time:

Triple-junction cells (about 29 % vs about 22 % for Si) would add about 30 %.
Higher-β orbits (dawn-dusk SSO) give up to 26 % duty.
Cells on the +Z cap.
Reducing LTM output power at low uplink load.

Assumptions to verify

The cell P_mp of the KXOB101K08TF (datasheet) and a measured face I-V curve under AM0-equivalent light.
The LT3652 operating point with a 2S face (see open items).
LTM current in each mode, measured on the FlatSat through the FC's TLM3 (LTM_VSYS) channel and INA219.
The orbit and LTAN from the Transporter-22 manifest, which set the β-angle history.

Model source: analysis/power_budget.py.