Improving tube launch rocket performance without simply bolting on a bigger motor is harder than it sounds, and Con Hathy’s methodical experiments with a 3D-printed model rocket show exactly why the physics can bite back before they help you.
The starting point was the Arcas sounding rocket, a real-world design that used a gas-fed launch tube to reach an altitude of 100 km. Even versions without a gas generator apparently benefited from the tube arrangement, and that detail caught Hathy’s attention. The standard options for improving a model rocket’s altitude all carry costs: a larger motor raises safety concerns, and trimming weight limits what the rocket can carry. A launch tube, in principle, sidesteps both problems by using the engine’s own exhaust to push the rocket out of the tube with extra force.
Why the First Test Went the Wrong Way
To put the idea to the test, Hathy 3D printed a rocket and flew it from both a conventional launch rail and a tube, using a printed sabot fitted around the rocket to seal the tube. The expectation was straightforward: pressurised gas behind the rocket, extra boost, higher altitude. The actual result was the opposite. The tube-launched rocket performed substantially worse than the rail-launched version.
A simulation Hathy built afterwards explained why. As the rocket accelerates down the tube, the volume of empty tube behind it grows faster than the engine can fill it with exhaust gas. Rather than building up pressure behind the rocket, the arrangement was pulling a slight vacuum, actively slowing the vehicle down. It is the kind of result that looks embarrassing on paper but is genuinely useful: the simulation gave a clear target for what needed fixing.
Tube Launch Rocket Performance Recovered by Redesign
The fix centred on reducing the tube’s diameter. A narrower bore means the volume that needs back-filling grows more slowly, keeping the pressure balance in the rocket’s favour. Getting the rocket into a tighter tube required its own solution: Hathy redesigned the stabiliser fins to pop out after leaving the tube, wrapping flush against the body while the rocket is still inside. The sabot was also slimmed down and had foam added to sharpen the seal between it and the tube wall.
For the second round of flights, Hathy also attached a pressure sensor to the base of the tube, giving him real data on what the gas column was actually doing during the launch rather than relying on simulation alone. The combination of hardware changes and instrumentation paid off. According to the altimeter, the redesigned tube-launched rocket flew 72% higher than the baseline. The pressure sensor data suggested a longer tube could have extracted still more performance, which points toward future testing rather than a finished design.
What the project demonstrates clearly is that tube launching is not a free lunch. The geometry has to be right: too large a bore and the rocket fights a self-generated partial vacuum on the way out. Get the diameter matched to the engine’s output and the principle works, meaningfully so at 72% extra altitude on a small model. The pop-out fin mechanism is a neat mechanical detail in its own right, solving the packaging problem without adding significant complexity to the rocket’s structure.
For anyone working on high-performance model rocketry, the sabot-and-pressure-sensor approach Hathy used gives a replicable test methodology, not just a one-off result. The pressure sensor in particular turns what would otherwise be an altimeter-only verdict into something you can diagnose and iterate on, tube length by tube length.
