LEGO RC Submarine 3.0: Balloon Ballast + On-Board Compressor

How a simple balloon-and-compressor trick delivers real underwater depth control — and what you can build with it

If you’ve ever wondered whether a LEGO model can truly “swim” under its own power, the Brick Experiment Channel’s “Building a Lego-powered Submarine 3.0 – balloon and compressor” shows a clever, repeatable way to do it. The core idea is brilliantly simple: instead of trying to waterproof LEGO motors or poke unreliable shafts through the hull, you keep all the electronics dry inside a sealed container and control buoyancy with a small air balloon that’s inflated by a LEGO compressor. Inflate the balloon and the sub displaces more water, becomes more buoyant, and rises; vent some air and it sinks. Steering is achieved by changing propeller direction via a linear magnetic coupling, so you transmit torque through the hull wall without holes or leaky seals. Pool and open-water tests at the end of the video make it clear this isn’t just a tabletop trick—it actually cruises. 

Why does the balloon method work so well for LEGO? First, it avoids the hardest engineering problem: dynamic seals. Any rotating shaft through a wall wants to leak under pressure. By using magnets on either side of the hull to couple rotation, the prop direction can be reversed and steered with no penetrations. Second, LEGO’s compact pneumatic parts can act as an ad-hoc compressor, which means you can actively trim buoyancy while the sub is in the water. In the video, depth control is explicitly handled by the balloon + compressor, while heading changes come from flipping the prop’s thrust direction. It’s a tidy system that plays to Technic’s strengths. 

There are trade-offs, and the creator discusses them in later write-ups: air compresses as you go deeper, so the same amount of air inside the balloon provides less lift at depth. That means the sub may feel twitchy around neutral buoyancy, and you’ll likely need either careful weighting or sensor-assisted control if you want ultra-stable cruising at, say, pool-bottom depth. (As an example, a ~60 ml air volume can shrink noticeably by 1.5 m depth, nudging the craft negative.) Still, for a LEGO-first approach without custom hull machining, it’s a remarkably effective path—and the 3.0 build became one of the channel’s most-liked projects since its 2021 release. 

Below, you’ll find practical tips for adapting the technique and a bunch of MOC ideas you can chase—whether you want a screen-accurate sub, a whimsical aquarium rover, or a sci-fi exploration craft.

Tips: How to use the balloon-and-compressor submarine technique

• Start with a robust clear hull (a gasketed food/storage container or tube) so you can see internals and spot leaks quickly. Keep all electronics dry inside.

• Drive the prop with a linear magnetic coupling: stack strong magnets on an interior motor hub and mirror that stack on the exterior prop hub. No holes, far fewer leaks.

• Trim for near-neutral buoyancy on the surface: add/removes weights until the craft barely sinks when the balloon is empty and rises when it’s slightly inflated. This gives the compressor “headroom.” 

• Expect depth-dependent behavior: air compresses with pressure, so lift decreases as you dive. Plan to add a little extra inflation the deeper you go. 

• Short, direct air lines reduce loss; keep your balloon near the center to avoid nose- or tail-heavy trim when it expands.

• Use grippy tires or printed hubs on the external prop if your coupling slips; smooth plastics in water can shear under load.

• Protect the balloon from sharp Technic edges; a bit of soft sleeve or smooth tiles nearby helps.

• Fail-safe vent: add a quick vent (manual valve) you can pop open if the sub gets “too floaty.”

• Range-test your radio at water level before deeper trials; water attenuates signals fast.

• Pool first, creek later: run initial tests in a clean pool, then try calm natural water once you trust the seal.

Ideas: MOCs to build with this technique

• Research ROV for aquarium inspection with a camera “viewport.”

• Classic yellow “explorer sub” with bubble canopy and robotic arms.

• Steampunk mini-sub with exposed gears and copper-colored accents.

• Rescue minisub (inspired by real DSRV craft) with detachable ballast pods.

• Sci-fi scout pod that hovers above the bottom using gentle, automated trims.

• Under-ice probe diorama (use white plates/tiles as “ice” and film underneath).

• Treasure-hunter sub with a claw for “sunken brick” retrieval games.

• Marine biology rover with LED “lamp” and sampling basket (LEGO nets).

• Harbor tug tender—tiny workboat that can dive just below the surface and pop up.

• Educational buoyancy lab where students tweak weights and balloon fill to hit neutral buoyancy lines.

The balloon-and-compressor approach is one of the cleanest ways to get real, controllable underwater motion from LEGO without resorting to complicated seals or non-LEGO housings. It’s hands-on physics—buoyancy, pressure, lift—wrapped in a fun Technic challenge. Watch the video, study the coupling and ballast choices, and then experiment: start in a tub, graduate to the pool, and refine until your sub glides exactly where you want it. When you’ve got a design you’re proud of, share it—there’s a whole community of builders learning from each iteration.

Disclaimer: This article was created with the assistance of AI. While efforts have been made to ensure accuracy and originality, the content may include automatically generated text and should be considered as informational only.