Hubert's water rocket page

What is the maximum altitude you can get out of a single stage? I have the feeling that the 1000-foot level is a worthy target. Here are three ideas I developed to advance towards the goal in large steps:

  1. How to tailor the nozzle opening as a function of time.
  2. Given equal nozzle size and pressure, how to get the water to flow out twice as fast.
  3. Given a certain nozzle and water flow rate, how to get twice as much thrust out of the water.


A bit more down to earth: my new launcher

How to tailor the nozzle opening as a function of time.

Clearly the total height achieved can be optimized if one had the ability to vary the thrust of the engine as weight is reduced and velocity increases.
Here is how:
Suppose you have a styrofoam disk in the rocket, which fits loosely inside. It would float on the water in the bottle, and descend as the water is expended. Attach a rod to the disk, which fits through the nozzle opening. The effective nozzle area is the area of the bottle opening minus the area of the rod. The rod does not need to be straight, but can be tapered, or shaped otherwise to optimize the flow rate as a function of water level.

Construction: the disk is cut from 3/4" blue or gray styrofoam. Keep 1/4" clearance from the inside bottle wall. The rocket of course is built up out of (at least) 2 sections, and the disk is dropped in before the sections are glued together.

Find a small plastic bottle (such as a trial size shampoo bottle) with a screwtop cap. Make a hole in the center of the disk and glue the cap into the hole, female thread side down. Use the male thread on the top of the little bottle to glue to the top of your rod. You can now squeeze the bottle to hold the disk, and screw in the rod. This allows for replacement of the rod at any time for tuning, exchange etc.

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How to get the water out faster

If you could lower the viscosity of the water, it would flow out of a given nozzle, under a given pressure, faster. This means that the momentum transfer would be correspondingly higher, ergo more thrust.

Magic you say? Not really. See fast water. I also seem to remember that some fire departments use this same trick to get more water out of a hose.

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How to get more thrust out of the same water volume flow rate and velocity.

This is clearly in the 'don't try this at home' category: Suppose the water that flows from your nozzle is at 150C rather than at ambient temperature. As soon as it exits the small nozzle opening, it will expand into steam. Not only do you get thrust from the the momentum transfer of the ejected mass of water, but also from the water-steam phase transition. Steam explosions are poweful! To get the most out of this scheme, you need a shaped (parabolic) expansion nozzle, just like a real rocket engine.

Like I said, don't try this at home! I warned you.

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Last update 6 Apr 2002
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