Launch Tubes

[Brian]

Is it better to have a hollow launch tube or a solid one, excusing the fact that air needs to pass through it?
A hollow launch tube would hold more pressurized air unlike a solid one, but a solid launch tube would need less water.

[longpaddle]

Is it better to have a hollow launch tube or a solid one, excusing the fact that air needs to pass through it?
A hollow launch tube would hold more pressurized air unlike a solid one, but a solid launch tube would need less water.

My two cents says that the amount of water should not change. The amount of water is calculated on the size of the motor..eg 2liters, 1/3 water = 650ml.. less water means less burn time, less height.
So I don't believe the soild rod will affect the amount of water needed for a full burn.
However it will likely effect the volume of air. The compression will be the same but the amount of compressed air would be a little less.
That said, none of it is likley big enough to make a measurable difference.
Anyone else?

[Brian]

I think it's USWR X-12 which uses no water but instead a launch tube. A launch tube relpaces some water because it is also one of the rockets reaction masses and is more efficient because the rocket does not have to carry it, unlike water.
Above the waterline a hollow launch tube can hold more presurized air but when launched water could fill the launch tube, a waste of water and valuable space. It could be that the water wasted is equivalent to the air gain in a hollow launch tube so there may not be a measurable difference. A solid launch tube will have more area to be reacted on.
Only experimentation will show.

[longpaddle]

That is one I would like to test and get the data on.
The reaction mass of the launch tube compared to the extra water it replaces. Which is more effcient?
More water, longer burn, more weight
Soild tube, shorter burn, less weight.
Let me know what you find out.

[U.S. Water Rockets]

I think it's USWR X-12 which uses no water but instead a launch tube. A launch tube relpaces some water because it is also one of the rockets reaction masses and is more efficient because the rocket does not have to carry it, unlike water.
Above the waterline a hollow launch tube can hold more presurized air but when launched water could fill the launch tube, a waste of water and valuable space. It could be that the water wasted is equivalent to the air gain in a hollow launch tube so there may not be a measurable difference. A solid launch tube will have more area to be reacted on.
Only experimentation will show.

You must be thinking of another rocket.

Our team name is U.S. Water Rockets for a reason: all of our rockets use plain water as the reaction mass and air pressure as the energy storage. If we used anything else, then we would have to change our name.

Frankly, U.S. Stomp Rockets is a really crappy sounding name.

Plus, waterless water rockets are not legal in record competition and we've been defending our records since 2004 against various efforts to cheat the established rules. Perhaps one of these unsavory sore losers is trying to obscure the truth with wild stories like this. If you didn't hear it from us, then it's probably a wild guess.

To your original topic: the purpose of our launch tube is to act as an internal guide rail. This is important because with a rocket that goes so high we need a very accurate launch vector to be sure the rocket does not go off course. An internal guide rail is also an inherently safer design because external guides are much more likely to become horizontal projectiles in the event of an explosion.

The launch tube also protects our compressor from water draining into the cylinder via the air hose because the tube keeps the air inlet above the water line. It eliminates the need for traps and drains in the air plumbing.

Without getting into a bunch of math that 3 people in the world would ever read through, you can run the simulations yourself and see the piston action of the launch tube does not work as efficiently as the water reaction mass (hint: the launch tube can only be expelled at the same speed as the rocket is moving, and the rocket is not moving really fast for the first few feet. The water mass can exit much faster - because it is not being held back by the ground - and impart more energy to forward momentum).

Try some simulations and see the velocity curves for yourself.

[Brian]

Sorry for the misunderstanding!
The launch tube does increase power shown by a picture analysis I took http://www.wra2.org/forum/viewtopic.php?f=3&t=1432 Between frame 3 and 4 the acceleration decreases after clearing the launch tube.

(hint: the launch tube can only be expelled at the same speed as the rocket is moving, and the rocket is not moving really fast for the first few feet. The water mass can exit much faster - because it is not being held back by the ground - and impart more energy to forward momentum).

The launch tube, a slower reaction mass, under the same forces, a rocket will need to move faster, because the energy wants to take the path of least resistance. A launch tube attached to a planet has more resistance force than water, therefore the rocket, supplying less inertial resistance will move. If less energy is used accelerating the water, more energy is used accelerating the rocket. A slower reaction mass has more time to imparted a resistance force against the rocket.

Viewing your launches, your launch tube seems very flexible?

[U.S. Water Rockets]

Sorry for the misunderstanding!
The launch tube does increase power shown by a picture analysis I took http://www.wra2.org/forum/viewtopic.php?f=3&t=1432 Between frame 3 and 4 the acceleration decreases after clearing the launch tube.

(hint: the launch tube can only be expelled at the same speed as the rocket is moving, and the rocket is not moving really fast for the first few feet. The water mass can exit much faster - because it is not being held back by the ground - and impart more energy to forward momentum).

The launch tube, a slower reaction mass, under the same forces, a rocket will need to move faster, because the energy wants to take the path of least resistance. A launch tube attached to a planet has more resistance force than water, therefore the rocket, supplying less inertial resistance will move. If less energy is used accelerating the water, more energy is used accelerating the rocket. A slower reaction mass has more time to imparted a resistance force against the rocket.

Viewing your launches, your launch tube seems very flexible?

Hi Brian,

Our launch tube is actually quite stiff. It looks flexible because it is long relative to the diameter. X-10 and X-12 were built with a receptacle in the top of the pressure vessel to center the launch tube, so the tube has to be almost as long as the rocket to mate with the centering receptacle. We didn't anticipate the length of the tube being an issue at the time so the length to diameter ratio is not optimal and the stiff launch tube rebounds like a spring.

Regarding your video analysis:

We can do a little though experiment here to examine what is going on. keep in mind that we are going to make the assumption that this is a typical launch tube and a typical rocket. Assume a 22mm bottle and a hollow launch tube with a reasonably thin wall and a rocket volume proportional to the nozzle.

Think of it this way: the launch tube is connected to the earth, so it is not contributing any mass to the rocket. The launch tube is also displacing very little volume inside the rocket. You end up with a single force acting on the rocket, which is constant through the entire duration of the launch tube phase. The weight of the rocket is fixed and the pressure inside is fixed, until the launch tube end is reached. Ignoring friction and air resistance, the acceleration of the rocket is easy to calculate. You can also determine the highest speed possible given an infinite launch tube length. You also can calculate how long the launch tube would need to be to reach the theoretical maximum launch tube velocity.

The reason stomp rockets work as well as they do is because they type of rocket uses no water, so the mass is low and the maximum launch tube speed is very high. As soon as you add water, the mass limits the speed you can reach on a launch tube that will actually fit inside the rocket.

In the case of the water rocket you will see that the launch tube contributes a certain amount of velocity to the rocket and the majority of the acceleration of the rocket happens as the water is removed because the mass of the rocket goes down dramatically. The mass of the rocket is the largest restriction on the speed, so as it goes down, the rocket becomes easier and easier to accelerate.

You can see this effect by analyzing our 300FPS launch video from 2008:

[youtube][/youtube]

Keep in mind we are talking about typical water rockets here and you could find ways to modify a rocket to create corner cases to support nay theory. For example, a rocket with a 1mm nozzle and 1mm launch tube will behave vastly differently than a 22mm/22mm setup.

[Brian]

There is a lot of force on your launch tube so I wouldn't blame it.
The water in the place of a launch tube would also contribute no mass to the rocket, so the power increase from launch tubes is probably negligible. I guess there only water filters for now.

[WRA2]

This is a very interesting discussion.

Has anyone considered that a solid launch tube will affect smaller volume rockets more than a large one. The percentage of space occupied by a solid tube will be greater in a smaller rocket than a large one so a solid vs. hollow will have a greater difference when used in a low volume rocket. The solid tube will occupy a larger portion of the available volume in a small volume rocket. This may also affect the "ideal fill" ratio as well as the total volume of the rocket will decrease by the volume of the solid launch tube.

[Brian]

For experimentation, this would be the way to go.

[U.S. Water Rockets1]

For experimentation, this would be the way to go.

Sometimes, you have to pick the design that is best for one purpose that is a compromise in some other area. For example, an FTC rocket is great for aerodynamics, and the polycarbonate is one of the strongest plastics available, but to get those benefits, the rocket is going to be very low volume, so it needs to be build as lightweight as possible. Launch tubes are no different. Everything is a compromise!

Keep experimenting!

[Nick B]

My vote goes for the longest possible hollow launch tube and air supply line, no check valves.