A single crewed coastal patrol boat costs €200,000–500,000 per year to operate. A single helicopter hour costs €1,000–3,000. Our first 3-drone MVP patrol fleet — hardware only, no software — cost around €1,780. That's not a typo. The maths of autonomous SAR only works because PX4 has commoditised industrial drone hardware for a decade. Here's exactly what we bought and why.
The spec
| Item | Qty | Unit | Total |
|---|---|---|---|
| Holybro X500 V2 Development Kit | 3 | €350 | €1,050 |
| Zeee 4S 5200 mAh 120C LiPo | 8 | €45 | €360 |
| ESP32 WiFi telemetry module | 3 | €15 | €45 |
| ISDT D2 Mark 2 charger | 2 | €100 | €200 |
| RadioMaster Pocket transmitter | 1 | €80 | €80 |
| Spare props, cable ties, velcro | 1 | €45 | €45 |
| Total | ~€1,780 | ||
The airframe — Holybro X500 V2
X500 V2 is the reference airframe for PX4 development. Folding arms, 500 mm wheelbase, ~1.8 kg MTOW. It ships as a "development kit" — Pixhawk 6C flight controller, M9N multi-constellation GPS, 2216 KV920 motors, BLHeli_S 4-in-1 ESCs, 10-inch propellers, power distribution board. Solderless assembly — motors connect via bullet connectors, ESCs are pre-soldered to the PDB. Plug, calibrate, fly.
Why it's the right pick for our MVP: it's the airframe PX4 community effectively standardises on, which means every SITL test, every online tutorial, every troubleshooting forum post references it. Running hardware that matches the simulator reduces "works in SITL, fails on real drone" debugging to close to zero. 20-minute flight time on a 4S 5200 mAh battery is enough for realistic coastal patrol segments. 10 m/s sustained wind capability covers most operational conditions.
What it's not: weatherproof, long-endurance, or payload-flexible. It's a minimal platform. Production hardware with rain tolerance, 45+ minute endurance, or thermal payload capacity will be a different airframe — tracked in the backlog. For MVP, this is the right baseline.
The battery strategy — 8 cells for 3 drones
Eight 4S 5200 mAh LiPo packs across three drones is the minimum for continuous 24/7 patrol. The arithmetic:
- Each sortie is ~18 minutes of flight (20-minute spec minus safety margin).
- Battery cool-down + swap + recharge = ~60 minutes at 1C charge rate on the ISDT D2 Mark 2.
- At any moment: 3 batteries in drones (one flying, one standby, one transit/charging on the drone we just landed), 4 batteries on chargers (2 per charger × 2 chargers), 1 reserve battery.
- Rotation: one sortie every ~45–70 minutes. Charge cycle matches sortie cycle. Patrol never has to wait for a charger.
With fewer batteries the rotation breaks — you get patrol gaps when every pack is either flying or charging. With more batteries you're carrying dead weight in the field kit. Eight is the pragmatic minimum; we'll add as we find we need them.
Zeee 4S 5200 mAh packs at 120C are overkill on discharge rating (X500 peak current is well under 60 A; 120C of 5.2 Ah is 624 A) but the headroom extends cycle life. Expect 300+ cycles at the conservative 1C charge rate, less if we fast-charge at 2C.
The telemetry — ESP32 over WiFi, not SiK radio
X500 kits ship with SiK telemetry radios by default — the 433 MHz pair that most people use for QGroundControl. We swap them for ESP32 WiFi modules (mRo or equivalent). Why:
- One laptop is the ground station. No second radio plugged in, no special-purpose hardware. The MacBook's built-in WiFi hotspot is the ground-station network.
- All three drones to one port. Every ESP32 forwards MAVLink to port 14550 on the MacBook. The Overwatch MAVLink bridge multiplexes by system ID — three drones, one UDP socket, three fleet cards in the UI.
- Higher bandwidth than SiK. 57600 or 921600 baud over WiFi; fast enough for 50 Hz attitude streams from every drone without saturating.
- SiK still as a fallback. If the hotspot is jammed or out of range, plug the ground-side SiK radio back in via USB and set
MAVLINK_PORTto the serial device. Overwatch bridge handles both.
The chargers — ISDT D2 Mark 2
Two AC dual-channel 200 W chargers cover four batteries simultaneously (two per channel × two channels × two chargers — wait, that's four; "two per charger × two chargers" is the correct framing). Key features we actually use:
- 1C balance charge at 5.2 A for our 5200 mAh packs — about 60 minutes for a full recharge. Sustainable charge rate for cycle life.
- Storage mode — discharges packs to 3.8 V/cell for overnight storage between sessions. Important for battery longevity when we're not flying every day.
- Built-in cell balancing. Detects imbalanced packs before they become unreliable.
The D2 Mark 2 isn't the cheapest charger on the market but it's the simplest, most reliable dual-channel option at the AC-input, 200 W-per-channel spec. Customer teams running continuous operations will eventually want Pro-grade gear; for MVP this is the right pragmatic choice.
The RC transmitter — RadioMaster Pocket
One RadioMaster Pocket on 2.4 GHz ELRS, bound to all three drones. This is Layer 3 of the three-layer safety architecture — the hardware kill switch that works regardless of the ground station or the autopilot's state. Every operator carries it during flight. Every operator drills the kill-switch throw until it's muscle memory.
The Pocket is compact, light, and affordable (~€80). It covers us for manual override in every conceivable emergency and costs less than a single fleet battery.
What this fleet does and doesn't do
Does: Continuous 24/7 patrol of a coastal zone within ~100–300 m of the operator's MacBook hotspot. Live MAVLink telemetry to the ground station. Fleet Advisor optimisation of relay parameters in real time. Emergency RTH / kill switch from the ground station UI. Three-layer safety: GS button + PX4 onboard failsafes + RC transmitter kill. Weather-triggered abort. Altitude-separated relay handoffs. Support for up to 20 drones with one additional cost-scaling factor (more batteries, more chargers).
Doesn't: Live video (requires Pi Zero 2 W + Camera Module 3 addition at ~€50/drone for Phase 2). On-drone detection (same Pi upgrade). Extended range beyond ~500 m (requires cellular modem per drone or long-range digital radio like Herelink). Thermal imaging, zoom gimbal, or other mission-specific sensors (hardware upgrade path tracked in HARDWARE_UPGRADES.md).
MVP is literal. Everything listed under "doesn't" is a known Phase 2+ upgrade with a bill of materials. Phase 2 adds live video and detection to all three drones for an additional ~€150 in hardware. Phase 3 is "production-ready fleet" at a still-small number.
Why the price matters for the pitch
Coastguard procurement officers have a reference number in their head: what does a crewed patrol boat cost per year? What does a manned helicopter hour cost? €200k, €500k, €1k, €3k — the numbers are in the high hundreds of thousands or thousands. A 24/7 capable drone patrol fleet for under €2k of hardware (plus the operator's MacBook, plus Overwatch) sounds impossible until they see it working.
The demo is what closes it. On demo.adrone.company we run a 24-hour Curacao coastal patrol simulation compressed to 120 seconds of real time. Three drones rotating through relay handoffs, Fleet Advisor tuning parameters, alerts firing on detections, all at the scale of the real deployment. The spec sheet and the price follow from there.
If you want to put this fleet in front of your procurement committee, get in touch. We'll walk you through the hardware list, the upgrade path, and the total cost of ownership against whatever crewed alternative you currently fund.