brainwagon

A recent issue of Make magazine had an article about launching water rockets via hydrogen/oxygen combustion: basically an electric current is used to break water into its constituent hydrogen and oxygen, which then bubbles up in the rocket, forcing out some additional water. To launch, this hydrogen is ignited, and recombines quickly into water vapor, but also generates a huge burst of pressure and launches the rocket.

And here was my question: how big of a burst of pressure does it really generate? Is it safe? 2 liter bottles have a burst pressure of around 120psi or so. That’s actually reachable with ordinary mechanical means, and it is pretty easy to understand and monitor: the pressure increases slowly, and can be monitored if necessary by an ordinary pressure gauge.

The chemical reaction isn’t so easy to wrap one’s head around. You actually need to know some physics. Sadly, I’m mostly self taught, so I have to work through these things slowly. A discussion with Michael and Tom over lunch the other day reminded me of the ideal gas law. One mole of hydrogen and one half mole of oxygen combine to create one mole of water vapor. But I must admit: I know very little about the realities of combustion physics.

So, I did what I always did: I googled. And I found this interesting page. It estimates the pressure to peak at around 160psi, well beyond the burst pressure of the common 2L bottle. To keep the rocket from bursting, it is vital then to dilute the reaction by the introduction of ordinary air. The nitrogen won’t combust, and should limit the overpressure.

I’ll have to muddle over the details more sometime, but it’s good reading.

Powering Water rockets with hydrogen combustion