Advanced Hydropneumatic Rocketry

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Stage Two Rockets

To get more power from the simple design as outlined in the previous page, there are many approaches that can be made.

Increase the size. There is, it is claimed, no substitute for size, through whether this size in measured in cubic inches, litres, millimetres or whatever depends on the subject in question. Simply put, a larger rocket will generally reach further than a small one, as the old cube:square relationship holds. Or at least it holds for bottles of the same sort. This does not hold for bottles of different types as they have different wall thicknesses. Indeed finding different bottles with a higher volume to weight ratio will also result in better performance.

The cube:square relationship says that as the dimensions of the rocket are increased, the surface area of the shape (and hence the weight, for the softdrink bottles are made from a thin sheet of constant thickness) will go up by the square of the increase, while the volume of water (and hence the impulse of the rocket) will go up by the cube of the increase. Hence an increase in dimensions of 2 will give a rocket that is 4 times as heavy but 8 times as powerful, so it will have twice the range.

Increase the pressure. The rocket is propelled by the air pressure within, which is equal to the pressure required to push the cork out of the neck. Increasing this pressure will result in a more powerful rocket.

The pressure required to push the cork from the bottle can be increased by choosing a tighter fitting cork, allowing the water to swell the cork, using a glue to fix< the cork in place, or using a fixed lid with a valve to control flow.
One way to increase the maximum pressure achievable in a cork stoppered bottle would be to put the screw top on over the cork. To do this a hole must be put in the screwtop first. They air injection tube can then be pushed through the hole and into the cork so the air can be injected. When the correct air pressure is reached, aim the rocket and unscrew the lid. The cork will be pushed out and the rocket will launch.

Increase the exhaust velocity. The impulse provided to a rocket is equal to the reaction mass (in this case the water) multiplied by the exhaust velocity. One way of increasing the velocity is to increase the gas pressure as above, but other techniques involve optimising the nozzle shape and providing a constant pressure.

Constant gas pressure will give a higher average flow with the same peak pressure. This requires the generation of gas as in Stage Three. Advanced nozzle designs can be obtained from any good rocketry reference.

Stage Three

Fermentation Powered Rockets

The simple compressed air rockets have a few drawbacks.

You need a source of compressed air. This can be difficult to find and it takes time to pump it up.
As the water escapes, the air pressure, and hence the exhaust velocity, and hence thrust, drops.

Are there ways around this? You Bet! The Easiest is the Fermentation Powered Rocket.

Fermentation Powered Rockets

As always, the rocket starts with a plastic softdrink bottle. This is filled with water. But this time you add sugar.

This sugar can be of any form, whatever is the cheapest and most convenient. Raw sucrose (table sugar) is my choice, but you can use honey, fruitjuice, malted grain, potato peelings, whatever. Remember however that you are probably going to get this stuff all over you so don't be too adventurous.

When using sucrose I find that a 1:10 ratio is good. ie. 1 litre of water takes 100g of sugar.

Then you add some brewers yeast. This is available in supermarkets but if not, try adding some yeast containing product such as wine.

Now you leave the bottles (with the lid on TIGHT) in a cool dark place. By cool I mean 20-30 degrees celsius, people living in colder places might regard that as a warm dark place. It is also wise to choose a place where a leak of fermenting sugar water won't cause any damage. So not in your clothes cupboard. (I'm not making THAT mistake again:)

Anyway, after a month or so, the sugar should be nicely fermented. I mean the plastic bottle should feel like glass because they are SO HARD from the internal pressure.

Now you take the bottle to your launching site, give it a good shake, faceit in the direction of launch and sunscrew the cap. There will be a loud Woosh, the rocket will have disappeared into the distance, and you will be soaked in fermented sugar water (so wear old clothes.)

Advantages

No need for an airpump.
Many rockets can be prepared ahead of time and let off quickly.
Because the CO2 is disolved in the water and coming out of solution, pressure is maintained during the thrust process.
Because the gas is disolved in the water, more water can be used. The bottle can be nearly full. This means that the thrust can last for longer.
Because you use the screwtop rather than a cork, higher pressures can be used.This can be adopted for the pump rockets
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Increase the size. There is, it is claimed, no substitute for size, through whether this size in measured in cubic inches, litres, millimetres or whatever depends on the subject in question. Simply put, a larger rocket will generally reach further than a small one, as the old cube:square relationship holds. Or at least it holds for bottles of the same sort. This does not hold for bottles of different types as they have different wall thicknesses. Indeed finding different bottles with a higher volume to weight ratio will also result in better performance.	The cube:square relationship says that as the dimensions of the rocket are increased, the surface area of the shape (and hence the weight, for the softdrink bottles are made from a thin sheet of constant thickness) will go up by the square of the increase, while the volume of water (and hence the impulse of the rocket) will go up by the cube of the increase. Hence an increase in dimensions of 2 will give a rocket that is 4 times as heavy but 8 times as powerful, so it will have twice the range.
Increase the pressure. The rocket is propelled by the air pressure within, which is equal to the pressure required to push the cork out of the neck. Increasing this pressure will result in a more powerful rocket.	The pressure required to push the cork from the bottle can be increased by choosing a tighter fitting cork, allowing the water to swell the cork, using a glue to fix< the cork in place, or using a fixed lid with a valve to control flow. One way to increase the maximum pressure achievable in a cork stoppered bottle would be to put the screw top on over the cork. To do this a hole must be put in the screwtop first. They air injection tube can then be pushed through the hole and into the cork so the air can be injected. When the correct air pressure is reached, aim the rocket and unscrew the lid. The cork will be pushed out and the rocket will launch.
Increase the exhaust velocity. The impulse provided to a rocket is equal to the reaction mass (in this case the water) multiplied by the exhaust velocity. One way of increasing the velocity is to increase the gas pressure as above, but other techniques involve optimising the nozzle shape and providing a constant pressure.	Constant gas pressure will give a higher average flow with the same peak pressure. This requires the generation of gas as in Stage Three. Advanced nozzle designs can be obtained from any good rocketry reference.