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A water rocket is a type of model rocket using water as its reaction mass. The pressure vessel (the engine of the rocket) is constructed from thin plastic or other non metallic materials (usually a used plastic soft drink bottle) weighing 1,500 grams or less. The water is forced out by compressed air. It is an example of Newton's third law of motion.

I want to destroy my competition

Discussions about water rocket powered cars, sleds or boats.
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JR Inventor
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I want to destroy my competition

Post by JR Inventor » Wed Nov 29, 2017 2:57 am

Hi everyone! I'm new and I've never made a water rocket before. I'm here because I'm making a strange boat from things I found around the house and I think it would be cool to add a water rocket.


(Skip this if you just want to talk about the rockets)
Now, here's a little bit about my boat so far without the rockets.
It's made from an old wooden door with the entire center cut out so only the edges of the door are left. On top of the remaining door there is a much lighter plywood board that serves as the deck, and it is supported over the door by 4 stronger pieces of wood. All of this is glued together with wood glue by the way. On top of the plywood deck I have my chair which is a large comfortable but lightweight car-seat. This car-seat slides forwards and backwards on top of pvc "rails." Also, I took my old shoes, glued them to the deck, and cut out the heel part so I could keep my feet planted as I slide back and forth.

You're probably wondering how this boat floats. Well, underneath each side of the door I have 2 large pontoons made of long plastic tubes made from plastic bottles. There are ten total tubes which is more than enough to keep me and and boat out of the water. Anyway, to row this boat I'm using my rowing mechanism invention.


Now, I have some questions for how I should add this water rocket propulsion.

-I want there to be a lot of force and I read that you should only pressurize a 2 liter bottle to 50 psi and that doesn't seem like a lot to me. My question is, can I make a water rocket out of a pvc pipe and attach that to my boat? I'm assuming that a pvc pipe can safely hold a much higher psi.

-If I can make a high psi water rocket out of a pvc pipe, would help push the boat at all?

-Is it possible to make a launch lever on this theoretical pvc pipe rocket.

Any thoughts or advice? So far, I hardly understand how water rockets work so I'd appreciate very simplified answers. Thanks!



skysaber89
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Re: I want to destroy my competition

Post by skysaber89 » Wed Nov 29, 2017 3:58 pm

Basically, to answer your questions, yes. I will begin with the 2 Liter bottle.

Let me preface this post by saying air based systems are extremely dangerous even at lower pressures, as air is a compressible fluid and stores a substantially higher amount of energy than non-compressible fluids, such as water, at the same pressures. I wouldn't advise using anything that uses high air pressures when the pressure vessel will be in the remote vicinity of any fleshy bits, such as your head. ALSO the explosions are a lot louder and could result in permanent hearing loss because of these high energy. We blew up a bottle and it was heard at least 1/8 mile from testing site, if not more. Some higher pressure explosions could be compared in noise to a shotgun.

Many of the groups on this forum, including myself, have conducted various pressure tests of 2 liter bottles. They all tend to fail between 150 to 200 psi. My specific tests have failed around 180. The reason not to put more than 50 PSI in a 20 liter is the risk of permanent deformation, material degradation, and drastically shortened lifespan due to these factors. As you fill the bottle with air, it expands. The bottle has a certain level of elasticity to it, which is why you can bend the bottle, and it will USUALLY go back into its original shape, but perhaps with some stress lines in the material, dependent upon how much you deformed it.
However, past a certain threshold the bottle can no longer return to its original size, thus resulting in permanent damage to the bottle. Because you filled it so much, the plastic has stretched, and its wall thickness has thinned. This results in the walls being weaker. My particular tests tended to fail at the neck and cap area. so I suspect the bottles could actually hold slightly more than 180 psi (if i were to devise a better testing rig. However my team has no real needs to do this as we primarily work with fully custom air frames.).


Okay now that I'm done blathering on about why its bad to over-pressurize bottles, onto answering your questions.

You most certainly can use PVC. However, there are a couple of caveats. The higher diameter pipes, such as 2 inch, are not rated for pressure and could explode. Check the pressure rating of the pipes before you start filling them with air. Also there is a lot involved with water rockets, including total volume and nozzle diameter. Plus, I am guessing that this rocket will be horizontal(?), so there could be some problems with that, but I have never tested for thrust while the vessels are on their sides. If someone else who has done the water rocket car challenge or something of the sort could chime in, that would be awesome. Also, how high pressure are you talking here? 200 psi or 2000?

Yes, it will TECHNICALLY push the boat. How much of a reaction it creates is dependent on the factors I mentioned earlier (vessel volume, fill pressure, nozzle diameter) as well as others such as water fill percentage and the weight of the overall system. Generally speaking, using a high volume vessel with a relatively high pressure would result in a stronger, longer thrust than a small vessel with the same pressure, assuming the nozzle diameter to be equivalent across the two vessels.

By launch lever, do you mean a pressure release lever which initiates the rocket's thrusting? If so, yes, but the implementation of such a device at the moment I am unsure of. Again, ask the water rocket car guys.

This probably isn't the simplified answer you wanted, but the information is very important nonetheless, especially for a novice.

if you have any more questions, you can reply back.

(This wiki article explains the basics of how water rockets work: https://en.wikipedia.org/wiki/Water_rocket )



JR Inventor
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Re: I want to destroy my competition

Post by JR Inventor » Wed Nov 29, 2017 8:26 pm

skysaber89 wrote:
Wed Nov 29, 2017 3:58 pm
Basically, to answer your questions, yes. I will begin with the 2 Liter bottle.

Let me preface this post by saying air based systems are extremely dangerous even at lower pressures, as air is a compressible fluid and stores a substantially higher amount of energy than non-compressible fluids, such as water, at the same pressures. I wouldn't advise using anything that uses high air pressures when the pressure vessel will be in the remote vicinity of any fleshy bits, such as your head. ALSO the explosions are a lot louder and could result in permanent hearing loss because of these high energy. We blew up a bottle and it was heard at least 1/8 mile from testing site, if not more. Some higher pressure explosions could be compared in noise to a shotgun.

Many of the groups on this forum, including myself, have conducted various pressure tests of 2 liter bottles. They all tend to fail between 150 to 200 psi. My specific tests have failed around 180. The reason not to put more than 50 PSI in a 20 liter is the risk of permanent deformation, material degradation, and drastically shortened lifespan due to these factors. As you fill the bottle with air, it expands. The bottle has a certain level of elasticity to it, which is why you can bend the bottle, and it will USUALLY go back into its original shape, but perhaps with some stress lines in the material, dependent upon how much you deformed it.
However, past a certain threshold the bottle can no longer return to its original size, thus resulting in permanent damage to the bottle. Because you filled it so much, the plastic has stretched, and its wall thickness has thinned. This results in the walls being weaker. My particular tests tended to fail at the neck and cap area. so I suspect the bottles could actually hold slightly more than 180 psi (if i were to devise a better testing rig. However my team has no real needs to do this as we primarily work with fully custom air frames.).


Okay now that I'm done blathering on about why its bad to over-pressurize bottles, onto answering your questions.

You most certainly can use PVC. However, there are a couple of caveats. The higher diameter pipes, such as 2 inch, are not rated for pressure and could explode. Check the pressure rating of the pipes before you start filling them with air. Also there is a lot involved with water rockets, including total volume and nozzle diameter. Plus, I am guessing that this rocket will be horizontal(?), so there could be some problems with that, but I have never tested for thrust while the vessels are on their sides. If someone else who has done the water rocket car challenge or something of the sort could chime in, that would be awesome. Also, how high pressure are you talking here? 200 psi or 2000?

Yes, it will TECHNICALLY push the boat. How much of a reaction it creates is dependent on the factors I mentioned earlier (vessel volume, fill pressure, nozzle diameter) as well as others such as water fill percentage and the weight of the overall system. Generally speaking, using a high volume vessel with a relatively high pressure would result in a stronger, longer thrust than a small vessel with the same pressure, assuming the nozzle diameter to be equivalent across the two vessels.

By launch lever, do you mean a pressure release lever which initiates the rocket's thrusting? If so, yes, but the implementation of such a device at the moment I am unsure of. Again, ask the water rocket car guys.

This probably isn't the simplified answer you wanted, but the information is very important nonetheless, especially for a novice.

if you have any more questions, you can reply back.

(This wiki article explains the basics of how water rockets work: https://en.wikipedia.org/wiki/Water_rocket )

Thanks for replying!

By the way, the rocket would be horizontal, but possibly angled toward the water a bit.

Also, I should probably explain how I plan on using this rocket. There is going to be a boat race across a 25 yard pool. I would like to edge the rockets at the start of the race. This would hopefully help accelerate the boat to give a head start before the rowing begins.

I want the rockets to be safe of course and I'm worried about a few things.

-You brought up the deafening sound. I didn't even know about that but is it possible to avoid having such a loud explosion sound? Or is that sound always going to happen? What if the water rocket is weak and just for show?

-Obviously, if I do use a pvc pipe I don't want it to explode. Can I guarantee that the pipe won't be in any danger of exploding if I pump it to a psi that is much lower than its burst psi. Example: it has a burst psi of 800 and I fill it to 200. Also, to further guarantee that there is no danger, would wrapping the pipe in many layers of gorilla tape could help minimize damage at all if there is somehow a pvc burst?

-Can the water ejected from the pvc pipe rocket be dangerous? Should I angle the pipe towards the water slightly to so that it hits the water and has no chance of injuring someone? Can it injure someone?

-Others scenarios that I thought could be dangerous would be if the rocket broke free from the boat and hit someone. I would want to guarantee that doesn't happen and my idea to prevent that would be to heavily gorilla tape the entire length of the pipe to the wooden wall by wrapping it around and around both the pipe and wood. Also, I would use fiber fix as an additional guarantee.

Also, what about the cap to the pvc pipe? I imagine there would be a permanent cap on the front of the pvc pipe (front of the boat) which would be locked in place with a pvc cement. This cap is probably not likely to get blown from the pipe like a bullet I hope. What about the cap on the back where the water is expelled when I initiate the rocket's thrusting. That cap can't be cemented in place. Will it get blown out like a bullet before the thrust? What about when I do initiate the thrust? How do I make sure the cap doesn't go flying?

Can these safety measures be guaranteed? If not, then could they be guaranteed with a low psi rocket made from 2 liter bottles? If the rocket can't help push the boat at all, I would still like a water rocket just for show.

Thanks again for all of the insight. Sorry if I have no idea what I'm talking about. I appreciate your helpfulness!



skysaber89
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Re: I want to destroy my competition

Post by skysaber89 » Fri Dec 01, 2017 5:58 pm

Those are all great questions, and it's good that you've considered them and their implications. Let me progress through them and answer each individually.

Also, henceforth PV shall be used in place of pressure vessel as I am a lazy human.

As far as explosion sound. the loudness is correlated to how much air volume is in the PV and to what pressure it burst at. The boom happens because the rapid expansion of air pushes the atmospheric air out of the way so fast. How you would avoid this is essentially killing the performance of the rocket. To contain the violence of the event, most people test their PVs underwater, while also being filled completely with water. This minimizes the amount of pressurized air in the system, and what is left gets slowed down substantially by the submersion. So instead of a boom you would hear a kind of loud pop, depending on how much water the PV was in. However, let me discuss this further in conjunction with your next couple of questions.

So PVC has 2 pressure ratings. The bigger one is the burst pressure, and the smaller is the working pressure, think of air hose. It's not necessarily wise to use the burst pressure as the max pressure of the tube, as it can fluctuate from pipe to pipe, as well as poor seals on the caps or joints or whatnot. So look at the working pressure of the pipe you're wanting to use. If it is still rated at like 600, and you use 200, that will be plenty fine. Just remember that the larger diameter pipes, its harder to find pressure rated stuff, since most of it is just used for flushing your poo.

If you are dead set on reinforcing it, carbon fiber is the way to go. However it is super expensive and kind of difficult to work with, especially if you haven't used it before. The next best would be fiberglass or kevlar. Gorilla tape may provide some reinforcement, but I wouldn't count on it being the saving grace if you are dangerously close to the pressure rating of the pipe. Again, probably wouldn't be an issue if the hypothetical scenario I provided were the case.

As for securing it to your raft, I would suggest using stainless pipe securing bands and screw those through the raft, and put a box in front of the PV to hold the tip in place and make sure she don't go sailing into bystanders.

Thrust safety: this is a tough one. It is, again, dependent on the pressure you use. It could potentially be harmful if you use like 800 psi, but there are many other issues that present themselves at those levels. Aiming it down at the water would definitely negate the issue of it blasting some poor soul behind the boat, but then the securing it down becomes a bit more difficult. Not impossible, just a bit harder. Plus once you angle the center of thrust you lose efficiency as it is not thrusting through the center of mass (or close to it at least) and therefore wasting energy by providing thrust in a vector which doesnt need it. Think of it like a right triangle. You have a horizontal and vertical component, and the larger the vertical component, the smaller the horizontal and therefore less forward momentum you gain.

The thrusting mechanism I'm not sure about, as I've never done something like this before. I suspect you could adapt the PV down to a smaller pipe diameter and then have a seal (which I will pursue further in a sec) that gets released. You can secure this using a rope or something and make sure its not very long to minimize the possibility that she smacks into someone (why is everything i talk about a she?).

As for how to implement this seal, IDK right now. I can help you come up with a concept later, or depending on the timing of this contest you can PM me for a quicker response.

As with anything, there is no 100% guarantee that it will be reliable and safe. We can do all we can to minimize the possibility of a catastrophic event, but just remember prepare for the worst hope for the best.

I am not saying to drop the idea however. You can definitely make it pretty fricken safe and reliable, it's just a matter of how much time and effort you are willing to put in.

Honestly, you could probably use 150 in a long pipe and get a decent amount of thrust, depending on how you tuned the nozzle.

I'm probably forgetting things, as I tend to think faster than I can type and forget where I was going with a certain thought.

TL;DR: You could use the pvc, but just be careful with its implementation. Make sure its tied down tight and don't overpressurize the tube and all should be pretty much dandy.



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anachronist
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Re: I want to destroy my competition

Post by anachronist » Sat Dec 02, 2017 6:49 pm

Some food for thought.

Water filled to about 1/3 full in a 2-liter soda bottle at 100 psi will leave the bottle in about 1/10 of a second. That is to say, you won't experience a continuous sustained thrust, you will experience a brief kick. There will be some additional thrust from air pressure remaining in the bottle after the water is gone, but that won't affect you much given the large mass of you and your boat (the residual pressure impulse adds about 30% to the vertical speed of a water rocket). A water rocket is comparable to a mortar: an explosive charge at the beginning and the rest is coasting.

I don't know the rules of your competition, but if I wanted to harness the energy in pressurized air for a boat race, I'd make a sort of engine to drive a propeller. That would make far better use of the available energy. The term for this is "pneumatic motor" or "air pressure engine" and there are a large variety of designs possible. I remember one that drove a hand-held propeller to pull a scuba diver, powered by the diver's air tank pressure. Something like that, to use the pressure efficiently over a sustained period, would be more advantageous for a boat.



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