If You Can't Quantify These, Restart
The hard truth: These four budgets—power, mass, link, and timing—are the foundation of every successful hardware project. If you're estimating instead of calculating, or if your margins are wishful thinking, you don't have a design yet. These numbers tell you if your project is physically possible before you waste months building the wrong thing.
Every constraint you ignore early becomes a crisis later. Run out of power? System brownouts. Exceed mass budget? Launch delayed or cancelled. Link budget broken? Can't communicate. Timing violated? Crashes and unpredictable behavior.
Professional engineers maintain spreadsheets for these budgets from day one. Every component, every feature, every change updates the budget. Margins below 20% trigger red flags. Negative margins trigger project reviews.
Power Budget
Why it matters: Every component consumes power. Batteries are finite. Solar panels are limited. Power is often the hardest constraint in portable, embedded, or space systems. Account for every milliwatt or face brownouts, thermal issues, or mission failure.
What to Track
- Every component's power draw (nominal, peak, standby)
- Duty cycles (percentage of time in each mode)
- Average power consumption over mission profile
- Peak power events (antenna transmit, motor actuation)
- Power source capacity (battery watt-hours, solar watts)
- Conversion losses (regulators typically 80-90% efficient)
- Margin: 20-30% above calculated need
Example Power Budget
| Component | Power (W) | Duty Cycle | Average (W) |
|---|---|---|---|
| Flight Computer | 5.2 | 100% | 5.2 |
| Radio (TX) | 8.5 | 15% | 1.3 |
| Radio (RX) | 1.8 | 85% | 1.5 |
| Sensors (IMU, GPS, etc) | 2.1 | 100% | 2.1 |
| Payload Camera | 3.5 | 20% | 0.7 |
| Average Total | — | — | 10.8 W |
| Solar Available (daylight) | — | — | 15.0 W |
| Margin | — | — | 28% ✓ |
Don't forget: Eclipse periods, battery charging losses, degradation over time, thermal effects on efficiency, and fault scenarios (one solar panel fails). Model worst-case, not best-case.
Mass Budget
Why it matters: Launch costs scale with mass. Payload capacity is limited. In aerospace, every gram costs money and performance. Exceed your allocation and your project doesn't fly—literally.
Example Mass Budget
| Subsystem | Mass (kg) | Percentage |
|---|---|---|
| Structure & Chassis | 2.8 | 28% |
| Power (solar, battery, regulators) | 2.2 | 22% |
| Avionics (computer, sensors) | 1.5 | 15% |
| Communications (radio, antennas) | 1.2 | 12% |
| Payload (camera, instruments) | 1.8 | 18% |
| Subtotal (known) | 9.5 | 95% |
| Allocation (launcher limit) | 10.0 | 100% |
| Margin | 0.5 kg (5%) | ⚠ Tight |
Red Flag: 5% margin is dangerously low. Real parts always weigh more than datasheets suggest. Fasteners, cables, connectors, potting compound, thermal paste—all add up. Industry standard is 20-30% margin. At 5%, you're one heavy cable away from failure.
Link Budget
Why it matters: You can build perfect hardware, but if the signal can't reach the receiver, the mission fails. Link budgets prove your communication system works under worst-case conditions: maximum range, minimum antenna pointing, atmospheric attenuation, interference.
Link Budget Components
| Parameter | Value | Notes |
|---|---|---|
| TX Power | +30 dBm | 1 Watt output |
| TX Antenna Gain | +3 dBi | Omnidirectional dipole |
| Free Space Path Loss | -160 dB | 500 km, 435 MHz |
| Atmospheric Loss | -2 dB | Low elevation angle |
| RX Antenna Gain | +15 dBi | Ground station Yagi |
| Received Power | -114 dBm | — |
| Receiver Noise Floor | -130 dBm | kTB calculation |
| Required SNR | 10 dB | For BER < 10^-6 |
| Link Margin | 6 dB | ✓ Acceptable |
Margin interpretation: 3 dB margin = barely works. 6 dB = acceptable. 10+ dB = comfortable. Negative margin = link fails. Account for fading, multipath, Doppler shift, and pointing errors.
Timing Budget
Why it matters: Real-time systems have hard deadlines. Miss a deadline and you get crashes, instability, or mission failure. Timing budgets prove your CPU can handle all tasks within their required periods with margin for interrupts and worst-case scenarios.
Example Timing Budget (RTOS)
| Task | Period (ms) | WCET (ms) | Utilization |
|---|---|---|---|
| Attitude Control | 10 | 2.1 | 21% |
| Sensor Reading | 50 | 1.8 | 3.6% |
| Telemetry Generation | 1000 | 15 | 1.5% |
| Command Handler | 100 | 5.2 | 5.2% |
| Health Monitor | 500 | 8.0 | 1.6% |
| Total CPU Utilization | — | — | 33% |
| Target Maximum | — | — | 70% |
| Margin | — | — | 37% ✓ |
WCET = Worst-Case Execution Time: Not average, not typical—worst case. Measured under maximum load with all branches taken. Add 20-30% for interrupt handling, context switches, and cache misses. Rate Monotonic Analysis (RMA) or similar schedulability analysis is essential for hard real-time systems.
The Reality Check
If Your Margins Are Negative or You Can't Fill in Numbers
You don't have a design—you have a wishlist. Go back to requirements and make hard trade-offs. Cut features, choose lower-power components, reduce data rates, simplify algorithms. Engineering is about satisfying constraints, not wishful thinking.
How to Maintain Budgets
- Create spreadsheets on day one, before any hardware decisions
- Update budgets whenever requirements or design changes
- Review budgets in every design review meeting
- Track against allocation, not just total—subsystem budgets prevent local overruns
- Measure actual values when hardware exists, update models
- Maintain margin—never design to 100% utilization
- Document assumptions (temperature, efficiency, degradation over time)
Professional discipline: Amateur engineers skip budgets because "it'll probably be fine." Professional engineers maintain budgets religiously because they've seen projects fail from "probably fine." Which one do you want to be?