unisat

Mass Budget (3U Configuration)

Reference: CubeSat Design Specification Rev. 14 (Cal Poly), ECSS-E-ST-10-02C (Verification), GEVS-SE Rev. A

Profile scope: this file details the 3U CubeSat reference build. For other profiles (1U, 1.5U, 2U, 6U, 12U, CanSat minimal / standard / advanced) see the per-class BOMs under hardware/bom/by_form_factor/ and the envelopes in flight-software/core/form_factors.py.

1. Detailed Component Breakdown

1.1 Structure Subsystem

Component Part Number / Spec Qty Unit Mass (g) Total (g) MGA (%) With Margin (g)
3U Primary Structure ISIS 3U Structure Kit 1 280 280 5 294
PC/104 Spacers (M3×40mm) Custom AL6061-T6 8 3 24 5 25
Rail Feet (anodized AL) CDS Rev.14 compliant 4 12 48 5 50
Deployment Switch (kill) Endurosat KS-01 2 5 10 5 11
Solar Panel Hinges Custom spring-loaded 2 15 30 15 35
Fasteners (M3 Ti) DIN 912 Ti Grade 5 60 1.2 72 5 76
Separation Springs P-POD spec compliant 4 2 8 5 8
Subtotal Structure       472   499

1.2 Electrical Power Subsystem (EPS)

Component Part Number / Spec Qty Unit Mass (g) Total (g) MGA (%) With Margin (g)
EPS Board (PC/104) Custom PCB, 4-layer 1 85 85 10 94
MPPT Controller IC SPV1040 + passives 3 2 6 10 7
Battery Cell NCR18650B Panasonic NCR18650B 4 46.5 186 2 190
Battery Holder + Tabs Custom AL bracket 1 35 35 10 39
Battery PCM (protection) Custom PCB 1 12 12 10 13
DC-DC Converters TPS62130 (3.3V, 5V) 3 3 9 10 10
Load Switches TPS22918 (8 ch) 8 0.5 4 10 4
Solar Cells (body mount) Spectrolab UTJ 5 8 40 5 42
Solar Panel PCB (body) FR4, 1mm 5 18 90 5 95
Deployable Solar Panel 1-wing, GaAs cells 1 65 65 10 72
Subtotal EPS       532   566

1.3 On-Board Computer (OBC)

Component Part Number / Spec Qty Unit Mass (g) Total (g) MGA (%) With Margin (g)
OBC PCB (PC/104) Custom, 6-layer 1 45 45 10 50
STM32F427VIT6 MCU ARM Cortex-M4, 180MHz 1 1.5 1.5 5 2
FRAM (NV storage) FM25V20A, 256KB 2 0.5 1 5 1
NOR Flash W25Q128JV, 16MB 2 0.5 1 5 1
SD Card + Holder 32GB Industrial 1 4 4 5 4
RTC Crystal 32.768 kHz 1 0.2 0.2 5 0.2
Watchdog Timer IC MAX6369 1 0.3 0.3 5 0.3
Voltage Regulators LDO, passives - - 5 10 6
Connectors (PC/104) Samtec ESQ-120 2 6 12 5 13
Misc passives R, C, L, ESD, TVS - - 8 15 9
Subtotal OBC       78   86

1.4 Communication Subsystem

Component Part Number / Spec Qty Unit Mass (g) Total (g) MGA (%) With Margin (g)
UHF Transceiver Board Custom (CC1125 based) 1 55 55 10 61
UHF PA (1W) SKY65116-34 1 2 2 10 2
UHF LNA + SAW filter SPF5189Z + TA0968A 1 3 3 10 3
UHF Monopole Antenna Tape-spring, NiTi 1 12 12 10 13
S-band Transmitter Board Custom PCB 1 60 60 15 69
S-band PA (2W) RFMD RFPA5522 1 5 5 10 6
S-band Patch Antenna Microstrip, FR4 1 30 30 10 33
RF Cables + SMA conn RG-178 + SMA 4 5 20 10 22
Subtotal COMM       187   209

1.5 ADCS (Attitude Determination and Control)

Component Part Number / Spec Qty Unit Mass (g) Total (g) MGA (%) With Margin (g)
ADCS Controller PCB Custom, PC/104 1 40 40 10 44
Magnetorquer (X, Y rods) Custom air-core, 0.2 Am² 2 30 60 10 66
Magnetorquer (Z coil) PCB-embedded, 0.1 Am² 1 15 15 10 17
Reaction Wheel Assembly CubeWheel Small (3-axis) 3 60 180 5 189
RW Driver Electronics Custom H-bridge 3 8 24 10 26
Magnetometer HMC5883L (3-axis) 1 2 2 5 2
Sun Sensors (coarse) Photodiode arrays 6 3 18 10 20
Gyroscope BMI088 (3-axis) 1 1 1 5 1
Subtotal ADCS       340   365

1.6 GNSS Subsystem

Component Part Number / Spec Qty Unit Mass (g) Total (g) MGA (%) With Margin (g)
GNSS Receiver u-blox MAX-M10S 1 2 2 5 2
GNSS Patch Antenna Taoglas CGGP.25.4.A.02 1 8 8 10 9
LNA + SAW filter - 1 3 3 10 3
Coax cable RG-178 1 5 5 10 6
Subtotal GNSS       18   20

1.7 Payload Subsystem

Component Part Number / Spec Qty Unit Mass (g) Total (g) MGA (%) With Margin (g)
Camera Module OV5647 + optics (f=3.6mm) 1 35 35 10 39
Camera Lens Assembly Custom 30m GSD @ 550km 1 180 180 15 207
Camera Interface Board Custom MIPI-CSI adapter 1 20 20 10 22
Radiation Sensor RADFET + dosimeter PCB 1 45 45 10 50
Radiation Shielding AL 2mm local shield 1 80 80 10 88
IoT Relay Payload LoRa SX1276 + ant 1 25 25 15 29
Subtotal Payload       385   435

1.8 Thermal Control

Component Part Number / Spec Qty Unit Mass (g) Total (g) MGA (%) With Margin (g)
MLI Blanket (5-layer) Kapton + Mylar 2 15 30 10 33
Kapton Film Heater (1W) Minco HK5578 2 5 10 10 11
Thermistors (10kOhm NTC) Vishay NTCS0402E3 12 0.1 1.2 10 1.3
Thermal Interface Material Bergquist GP5000S35 - - 8 10 9
Thermal Washers + Isolators Ultem spacers 8 1 8 10 9
Subtotal Thermal       57   63

1.9 Wire Harness

Component Part Number / Spec Qty Unit Mass (g) Total (g) MGA (%) With Margin (g)
PC/104 Stack Connectors Samtec ESQ-series 6 6 36 5 38
Internal cables (AWG28) Teflon insulated 15 3 45 15 52
Antenna deploy mechanism Burn wire + dyneema 2 8 16 10 18
Debug/JTAG connector 10-pin Cortex 1 2 2 5 2
Misc (tie wraps, tape) Kapton tape, lacing cord - - 12 20 14
Subtotal Harness       111   124

2. Mass Budget Summary

Subsystem CBE Mass (g) MGA Average (%) MEV Mass (g)
Structure 472 5.7% 499
EPS (incl. battery + solar) 532 6.4% 566
OBC 78 10.3% 86
COMM 187 11.8% 209
ADCS 340 7.4% 365
GNSS 18 11.1% 20
Payload 385 13.0% 435
Thermal 57 10.5% 63
Harness 111 11.7% 124
Total 2,180 8.5% avg 2,367 
CBE  = Current Best Estimate (measured or datasheet values)
MGA  = Mass Growth Allowance (per AIAA S-120A-2015)
MEV  = Maximum Expected Value = CBE * (1 + MGA)
Parameter Value
CBE Dry Mass 2,180 g
MEV Dry Mass (CBE + MGA) 2,367 g
System Margin (20% on MEV) 473 g
Total Allocation 2,840 g
CubeSat 3U Mass Limit 4,000 g
Remaining Unallocated 1,160 g (29.0%)

2.1 Mass Growth Allowance Tracking

Per AIAA S-120A-2015, MGA depends on design maturity:

Design Phase MGA Guideline Project Status
Conceptual (SRR) 25-35% Complete
Preliminary (PDR) 15-25% Complete
Detailed (CDR) 5-15% Current
As-Built (FRR) 2-5% Upcoming
Flight (measured) 0% -

Current MGA average of 8.5% is consistent with CDR-level maturity. Components with > 10% MGA are those still in prototype phase (camera lens, S-band, IoT payload).

3. Center of Mass Calculation

Coordinate system: X = along long axis (from -Z face to +Z face), Y = lateral, Z = normal to largest face. Origin at geometric center of the 3U envelope (50mm x 50mm x 170mm from P-POD rails).

3.1 Component Positions and CG Contributions

Subsystem Mass (g) X_cg (mm) Y_cg (mm) Z_cg (mm)
Structure 472 0.0 0.0 0.0
EPS Board 346 -50.0 0.0 0.0
Battery Pack 186 -50.0 0.0 -15.0
OBC 78 -20.0 0.0 0.0
COMM 187 10.0 0.0 5.0
ADCS 340 50.0 0.0 0.0
GNSS 18 40.0 15.0 20.0
Payload (Camera) 235 -60.0 0.0 -20.0
Payload (Rad+IoT) 150 20.0 0.0 10.0
Thermal 57 0.0 0.0 0.0
Harness 111 0.0 0.0 0.0

3.2 Aggregate Center of Mass

X_cg = SUM(m_i * x_i) / SUM(m_i)
     = (472*0 + 346*(-50) + 186*(-50) + 78*(-20) + 187*10 + 340*50
        + 18*40 + 235*(-60) + 150*20 + 57*0 + 111*0) / 2180
     = (-17300 - 9300 - 1560 + 1870 + 17000 + 720 - 14100 + 3000) / 2180
     = -19670 / 2180
     = -9.0 mm

Y_cg = (18*15) / 2180 = 0.12 mm  (essentially centered)

Z_cg = (186*(-15) + 187*5 + 18*20 + 235*(-20) + 150*10) / 2180
     = (-2790 + 935 + 360 - 4700 + 1500) / 2180
     = -4695 / 2180
     = -2.2 mm

CG Location: (-9.0, 0.1, -2.2) mm from geometric center

CDS Rev. 14 Requirement: CG must be within 2 cm of geometric center in X and 1 cm in Y, Z. Status: COMPLIANT (9.0 mm < 20 mm in X, 0.1 mm < 10 mm in Y, 2.2 mm < 10 mm in Z)

4. Moments of Inertia Estimate

Using parallel axis theorem for each component modeled as a rectangular prism:

I = I_cm + m * d²

Where I_cm is the moment about the component’s own center and d is the distance to the satellite CG.

4.1 Principal Moments of Inertia

Axis Moment of Inertia (kg*m²) Ratio
I_xx (roll, along long axis) 0.0027 1.00
I_yy (pitch) 0.0089 3.30
I_zz (yaw) 0.0092 3.41

Products of inertia are small (< 5e-5 kg*m²) due to near-symmetric layout.

The near-equality of I_yy and I_zz with I_xx being smallest confirms the 3U elongated shape. This is favorable for gravity-gradient stabilization as a backup to active ADCS.

4.2 Inertia Requirements for ADCS Sizing

The magnetorquer authority must satisfy:

tau_max = M_dipole * B_earth  >  3 * (I_max / T_orbit) * omega_max

Where:
  M_dipole = 0.2 Am² (per axis)
  B_earth  = 30 uT (at 550 km, minimum)
  tau_max  = 0.2 * 30e-6 = 6.0e-6 Nm

  Required: 3 * (0.0092 / 5742) * 0.1 rad/s = 4.8e-7 Nm

  Margin: 6.0e-6 / 4.8e-7 = 12.5x   --> ADEQUATE

5. Mass Contingency and Risk

Risk Impact (g) Mitigation
Camera lens heavier than estimated +50 Lighter optics material (Ultem vs. glass)
Additional radiation shielding needed +100 Spot shielding only, accept higher dose
S-band antenna redesign (higher gain) +30 Trade study: gain vs. mass
Harness routing longer than planned +30 Strict harness routing plan
Thermal design margin +20 Optimize MLI coverage
Total contingency +230 Within unallocated margin (1,160 g)

6. References

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