Talk:Water rocket

center of gravity, fins section

in the fins section it states that on launching a rocket's cg will move backward just after launching, this is incorrect as on launching the g forces push the water (or whatever) toward the back (and gravity before launching) and it is then pushed out REDUCING the mass at the back of the rocket i can understand the cg of the propellant moving aft but the cg of the whole rocket will move forward because the mass of propellent is dissapearing. —Preceding unsigned comment added by 203.109.165.215 (talk) 01:51, 28 January 2010 (UTC)

Nah. As the propellant is used up, the water flows down to make up for it.- Wolfkeeper 02:02, 28 January 2010 (UTC)

New additions: Fins and Predicting Height

We have expanded the section on fins to discuss the physics behind the stabilizing torque, with a reference to a NASA page. We have added a section with a derived/reduced equation for the maximum height and have provided our (numerous) simplifying assumptions. We have done this following a project researching water rockets in a mechanical engineering class at the University of Michigan. See the reference to the lecture material from William W. Schultz. Laura Rose P (talk) 17:29, 16 April 2012 (UTC)

@Laura Rose P: I'm not buying it.

The equation to predict maximum height doesn't account for thrust, which is related to the pressure in the bottle and the diameter of the nozzle. The formula

$h=\left({M_{i} \over M_{R}}\right)^{2}\left({P_{i} \over \rho g}\right)$

doesn't account for thrust or nozzle diameter. For a given pressure, if the nozzle diameter is small enough, the rocket won't rise at all, and if it's too large, the rocket will eject all of its thrust mass at once, in which case the maximum height is determined solely by the momentum imparted to the empty mass of the rocket. I would think there's probably some optimal diameter that maximizes the height.

Also the paragraph that follows is dubious: An independent variable that influences peak height is weight/mass. Depending on the thrust of the rocket propulsion system, a rocket requires a minimum mass to overcome the deleterious effects of drag. For example, the greater the thrust/the less the original weight of the rocket, the more weight or mass must be added to the rocket to insure maximum apogee. No, that's all wrong, and contradictory. If you launch a high mass and a low mass with the same velocity, the high mass will go higher due to its higher momentum to overcome air drag... BUT given the same initial amount of thrust mass and pressure in a water rocket, the low-mass rocket will reach a higher terminal velocity. Depending on the initial masses, the low-mass rocket could achieve a higher apogee than the high mass rocket — the opposite of what the paragraph claims.

Accordingly, I have marked these as dubious in the article. ~Anachronist (talk) 21:57, 19 April 2017 (UTC)

After working through the problem using actual physics in a numerical simulation, I found that the assumptions made in this section don't give a useful answer for apogee height. There are far too many dynamically changing variables for a steady-state formula like this to be useful. Flow through the nozzle is not uniform, particularly at lower pressures, air pressure changes rather significantly before the water runs out, the mass changes of a lot, and the acceleration is far from constant. The final burst of air after the water is gone also imparts a significant increase to velocity (like 30% or more). And drag is a big deal at peak velocity.

The paragraph about adding ballast turns out to be partially valid, but misses the point. There is a sweet spot of ballast mass that results in maximum apogee, but too much or too little ballast won't reach the maximum altitude.

I am removing this section. If anyone wants, I can share the numerical simulation. ~Anachronist (talk) 21:57, 1 May 2017 (UTC)

Commercial products

This article focuses almost(?) entirely on home-built water rockets. Shouldn't there be some coverage of the toys of this type that have been sold over the years?

I had a couple of rockets that were about seven inches long. The body was shaped like a narrower-than-usual football, maybe two inches diameter at the thickest point. Two fins at the tail, angled to produce spin, for stabilization. They came with a hand pump, also about seven inches long, which included a clamp mechanism to hold the rocket on the pump output and then release it when desired. The rockets were made of clear or colored-transparent plastic so you could see to put the right amount of water in, and then while pumping you could see the air bubbles rising through the water.

One day the little tube on the pump that the rocket sat on while being pumped broke off. And of course that meant you couldn't pump up the rocket any more. I was very sad.

That would be solidly at the "kid's toy" end of the scale. At the other end, somebody - may have been Mattel - made a multi-stage water rocket that was styled as a close match to the Saturn 5 moon rocket stack. It was two or three feet tall. A long string was attached to the base and to a clamp that was supposed to pull out when the rocket reached a certain height; pulling the clamp out released the second stage. There might have been a third stage. A neighbor kid had one. I never saw it work but it looked very impressive.

There were many more, especially during the height of enthusiasm for the space program in the 60s. Jeh (talk) 20:45, 14 March 2019 (UTC)

@Jeh: See Wikipedia:Be bold. You are welcome to add more material.

I have fond memories of those water rocket toys myself. A couple years ago when I visited NASA at Cape Canaveral, the gift shop had those mini-narrow-football-shaped water rockets. I was happy to see they were still being made! So I bought one for my child but I was disappointed at the poor quality of the hand pump (it leaked pressure and felt flimsy) compared to what I remembered as a child.

I also remember seeing (and wanting) that multi-stage version in stores when I was a kid, but I never saw one actually working. ~Anachronist (talk) 18:08, 8 April 2019 (UTC)

I, too fondly remember the small water-rocket toys. So that makes at least three of us with personal recollections. Finding reliable secondary sources of information about them, is however, a different matter.... Gambaguru (talk) 21:02, 10 July 2020 (UTC)

I am owning two types of commercial rockets. One is a nice kit that has to be completed by a PET bottle to make a rocket. It is call "Rokit". I used it to create the high speed video (weblinks) and image sequence you can see in the article. The other one is less like a rocket but more like an "air gun". It is sold from different manufacturers under names like "jump rocket" or "stomp rocket". I am not providing links here on wikipedia, but you will find it quickly with the search engine of your choice. Complete water powered model rocktes are unknown to me. -- Dr. George (T) 21:50, 10 July 2020 (UTC)

Edit request

Please add a hatnote:

{{confused|nuclear salt water rocket|steam rocket}}

Not to be confused with nuclear salt water rocket or steam rocket.

-- 64.229.88.43 (talk) 03:52, 25 July 2022 (UTC)

 Done Handmeanotherbagofthemchips (talk) 17:53, 1 August 2022 (UTC)