Build Your Own Two-Stage Water Rockets

Water rockets are one of the most fun and exciting science-adjacent activities one can take part in during the summer, and are popular with children and adults alike. Designs range from a bike pump with a cork in a bottle, up to significantly more advanced hardware. [Air.command]’s two-stage water rocket definitely fits into the latter category.

The build is initially somewhat confronting in its complexity, but after a thorough read-through the operating principles become clear. It’s an all-mechanical setup which relies on the weight of the upper stage and the initial acceleration of the rocket to keep the two stages coupled. It’s only when the first stage stops delivering thrust that a spring forces the two stages apart, and the upper stage rockets ever higher.

Parts-wise, everything is fairly accessible – with pieces cribbed from garden hose fittings, retractable pens and other household ephemera. It’s not the easiest thing to put together, but with perseverance and some tweaking and tuning, it’s definitely achievable for the home gamer, with no advanced tools or techniques required.

Now that you’ve got a two-stage rocket under construction, you might want to consider upgrading your launchpad. Video after the break.

 

15 thoughts on “Build Your Own Two-Stage Water Rockets

  1. that was really great fun to watch. congratulations.
    i guess you need to step back so you have some time to react, but that pretty much determines in which direction it’s going to go wrong.
    learned that as a kid with Estes engines.

      1. Ah you have a source on that? Pretty sure CASA rules don’t specify propulsion method nor care long as they’re less than 1.5kg total, motors less than 62.5g or 160N thrust and you keep it below the start of controlled airspace.

  2. As a college student we discovered with a 2L bottle, plenty of weight in the nose and about 120 psi we could get a water bottle rocket pointed straight up out of sight with a single stage. The problem is that with all that weight it landed with a terrible wallop.

    1. Well, that’s a good question. The specific impulse of a pressurized fluid rocket motor depends mainly on the velocity at which the fluid leaves the nozzle, and this is mainly determined by the pressure. Starting with pop bottles limits this, although you can just keep increasing pressure until you start getting failures, then back off a bit. But you can also do things like starting with a pop bottle and wrapping that with fiber to increase the pressure it will tolerate, keeping in mind that you’re adding weight when you do this.

      The thing to remember is that the first stage has to lift everything above it, so it always looks like the first stage is underperforming. The way to look at this is to start with the uppermost stage. Once you’ve got that stage getting the payload as high and as fast as you can, that’s when you add stages under it. Each of these gets a LOT bigger than the stages above it, because they carry their own weight plus everything above. Which is the basic rocket problem. If you ever saw a Saturn V launch, you know that these left the pad mostly on the thoughts and prayers of everybody watching. Well, not really, but these were loaded with more fuel and oxidizer than they could lift, so for the first few seconds, they were doing pretty much nothing but burning off enough expendables to get the weight down below the thrust.

      1. Not entirely true. Fully loaded the Saturn 5 weighed in at a tad over 6.2 million pounds. Thrust at takeoff was roughly 7.5 to 7.8 million pounds. On that kind of mass though, acceleration was excruciatingly slow. It took 11 seconds for the rocket to clear the tower (380 foot). That said, the venerable STS (space shuttle) was even slower. The Saturn 5 accelerated at 3.4 m/s^2. The shuttle at 1.4 m/s^2 (Source for these numbers: https://hypertextbook.com/facts/2000/JeffreyAnthony.shtml)

  3. I don’t think that in this usage, “sustainer” means what they think it means. “Upper stage” or “second stage” would be a better description. Still, good job.

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