Water Rockets

[PakledHostage]

I bought my son a water rocket for his birthday, then I found this site: Water rocket equations

I think I am going to have to build a test stand...

[PakledHostage]

The one I got him means business. I might not have bought it if I knew to what extent it does (he's only 5). But with adult supervision, I guess it is OK. He thinks it is pretty awesome and it gives me an interesting new project.

I think I will do some simulations in MathCAD to find the theoretical optimal starting water to air ratio vs. launch air pressure. It might be fun to then build a test stand to try to refine the model and optimize the launches a bit more.

[sevenperforce]

13 hours ago, PakledHostage said:

The one I got him means business. I might not have bought it if I knew to what extent it does (he's only 5). But with adult supervision, I guess it is OK. He thinks it is pretty awesome and it gives me an interesting new project.

I think I will do some simulations in MathCAD to find the theoretical optimal starting water to air ratio vs. launch air pressure. It might be fun to then build a test stand to try to refine the model and optimize the launches a bit more.

I remember getting my kids one for Christmas 2021. We had a lot of fun with it. We figured out pretty quickly that more water meant a longer thrust duration while less water meant greater liftoff thrust AND greater liftoff specific impulse. Optimized at around 2/5ths full I think. 

[RKunze]

 

17 hours ago, PakledHostage said:

The one I got him means business. I might not have bought it if I knew to what extent it does (he's only 5).

I built some myself a couple of years ago when my kids where around that age. And yes, those things pack quite a punch at launch - I wouldn't let a five year old play with something like that unsupervised (besides, daddy wants to have fun with rockets as well ).

It's actually pretty easy, basically just take an empty soda bottle and hot-glue some cardboard fins at the end for the rocket, and stick a small tube through a cork that fits snugly into the bottles' opening for the launcher. For launch, fill bottle with water (1/3 of the volume works pretty good), jam cork into bottle, connect open end of the tube to a pump (airtight), point to the sky (and more important, away from anything that could be damaged) and pump until liftoff.

For a more elaborate, reusable, but still simple version, hot-glue half a tennis ball to the front of the rocket (protects the rocket a bit on impact, and gets the dry COM into the first third of the vehicle which gives quite a bit more stability in flight), use plastic for the fins (mainly because cardboard and water are not a durable combination), and use a garden hose quick connect system both as the nozzle and a launch clamp (which means you can pump to a predefined pressure and then launch at will by releasing the connection - preferably through some kind of remote release mechanism at a few meters distance, as that is a) much safer and b) will keep you mostly dry at launch).

And if you really want to make a hobby out of it, build something like this  (I never got that far, I didn't even build a single-stage one with a parachute for retrieval).

 

Edited June 12, 2023 by RKunze
Fixed the messed up quote...

[PakledHostage]

So I created a MathCAD spreadsheet to compute the water rocket's performance and it reveals a few interesting things:

1. The density of the propellant does not significantly affect performance. Sugar brine has a density of about 1300 kg per cubic metre and using that density in the model only yeilds about 1-2% more altitude. The reason being that the Ve is that much lower because the same internal rocket pressure can't accelerate the heavier fluid as much as it does the lighter fluid.

2. The boost phase of the flight is over in about 1/3 of a second, and the rocket accelerates to about 95 kph in that time. It then flies ballisticly.

3. Performance isn't that sensitive to the size of the nozzle. At least not with variation of only a couple millimetres around the existing nozzle radius.

4. The biggest determinant of performance (perhaps not surprisingly) is internal rocket pressure and the amount of water in the chamber. In my son's rocket, launching slightly less than 1/2 full with water seems to yeild the best performance, but  that would change depending on the rocket geometry.

My model accurately predicts the flight duration and I am working on figuring out how to verify other parameters. I haven't taken into account head losses through the nozzle or back pressure in the propellant due to acceleration, but that later effect seems to be relatively small compared to the other sources of error in the model.

[Ultimate Steve]

On 6/11/2023 at 8:49 PM, PakledHostage said:

The one I got him means business. I might not have bought it if I knew to what extent it does (he's only 5). But with adult supervision, I guess it is OK. He thinks it is pretty awesome and it gives me an interesting new project.

Ah, I built a lot of those out of 2L bottles back in the middle school and high school days! They are fun and it is a great way to get into rocketry, either instead of or in addition to Estes/similar model rockets. If you and your son are interested in building your own at some point I will detail my adventures with water rockets, I'll ping @RKunze here because he (?) seems like he would be interested, and it seems we both know of and possibly were inspired by Air Command Rockets.

 

Kinda long so I'll put the whole thing in a spoiler.

Spoiler

First off, for anything more advanced than single bottle based rockets, you are going to want to check out Air Command Rockets' website: http://www.aircommandrockets.com/

They were what got me inspired to build mine.

 

There were four prototypes, I believe. A family friend had donated me a PVC pipe based air rocket launcher designed to hook into an air compressor. The rockets would be built out of duct tape and paper, and the pressure would be built up and released, sending the rockets a couple hundred feet. I made an extension for it that would allow it to hold standard drinking bottles down and pressurize them. However it was very janky and unreliable, and required me sitting right next to it to set it off, thus, fearing for my life, I never took it up to the pressures those bottles were capable of, and also because there was no seal, it leaked too much to reach those pressures without prematurely spraying all of the water out! Also I got soaked every launch, but it was part of the fun!

I don't remember precisely what mine were called, but they were part of my X program, encompassing pyro, water, and air rockets. I believe I've reached around X-12, but there were a few repeats because I couldn't keep track of the names myself!

 

The X-6 would have been my first self built water rocket, a simple 2 liter bottle with cardboard fins duct taped to it. Probably got 30-50 feet into the air by my judgement. It rapidly deteriorated however as the cardboard fins did not take kindly to the water.

 

I might be mixing up the X-7 and the X-8, but the X-7 was probably another 2 liter bottle based rocket, this time with another 2 liter bottle serving as a nose cone, and fins with layered cardboard, construction paper, and packing tape, as an attempt at waterproofing.

It performed by far the best out of any of my water rockets, probably reaching above 60 feet, but this is just by estimation and was never scientifically measured. The Estes altimeters don't take well to water, hard impacts, and low altitudes, and I wouldn't have gotten one for a few years at that point. I hadn't really learned trigonometry at that point, so strapping some rods to a protractor and standing a known distance away would also come later.

 

The X-8 was a rocket designed to test splicing multiple bottles together using tape and whatever glues I had on hand. I believe I attempted to seal 2 or 3 (probably 2) 1.25L bottles together (we were out of 2L bottles at the time I believe), combined with another bottle as a nose cone, much larger fins designed like the X-7's fins, and a metal pipe as a counterweight because it was tail heavy from the fins.

It failed horrendously, my pathetic attempt at a seal probably couldn't hold water at ambient pressure, let alone the tens of PSI I was targeting! Even during attempted pressurizations and launches, it probably needed help getting off of the launch pad, and never flew more than a foot or two under its own power, although it was fun to throw around (probably not a good idea because of the metal pipe, but it did make it very aerodynamically stable). The large fins also needed a better attachment mechanism than tape as they proved to be quite floppy.

 

The pinnacle of my water rocket journey was the X-9. I decided to do it like the pros. I saved up a LOT of 2L bottles, actually bought stuff instead of relying on my family's grand collection of basement junk, and bothered measuring things to ensure they would go on correctly.

I believe the volume of the fuel tank ended up being around 8-12 liters, with either 3 or 4 spliced pairs of 2 liter bottles and one normal 2 liter bottle connected by tornado tubes, all glued together by Air Command's recommended glue.

It had its own section for a parachute, built from a broken umbrella, which, due to my lack of electronics knowledge, was actively deployed using remote control from an R/C car I had disassembled for the purpose, which never worked more than 1/4 of the time in testing.

Its fins were actual corrugated posterboard, gigantic sheets secured with diagonal braces.

It even had a payload bay at the top where I planned to seat lego astronauts and possibly a camera if it proved reliable enough to justify risking one.

Unfortunately, well... It leaked. Very badly. From what I can remember, the splices were done pretty well, but the tornado tubes were not designed to hold pressure. If I do this again I will order from multiple manufacturers and test before committing to one. Some of the might have even cracked, I don't remember.

 

So, I took it all apart and rebuilt it, adding lots more glue. This rebuild was so significant that I actually renamed it to X-10. Unfortunately, the launch pad would not seal with it, causing obtaining pressures beyond a certain point to be impossible. The pros use an O ring based system but I never got that far, I had always intended to build a new launch pad capable of supporting a rocket that huge, with a proper seal, but I just haven't yet, and the rocket is currently sitting untested in my parents' basement, as it has been for the past several years.

 

Your post has kind of made me want to get back into this. Unfortunately I don't permanently live with my parents any more so acquiring the resources for restarting NOSPWRAD (my water rocket agency) will be problematic, but I might be able to figure something out.

 

If you are curious, a summary of the full X program:

 

X-1, a clay rocket I built as an art project designed to accept Estes motors, flown with much fear and trepidation. If I remember right it burned a hole through the launch pad during attempted liftoff and never went anywhere. If it did go anywhere it was a couple feet up and promptly sideways.

X-2, clone of the X-1 built with the extra clay as we were allowed a certain amount and I was trying to make the rocket as light as possible so didn't use much.

X-3, a very tiny clay "rocket" built with the leftover clay "designed" to accept mini motors. So dangerous even I never flew it. So much so that I even forgot I had assigned it an X designation until many years later when I, slightly more professionally, revived the X program.

X-3 Red Goblin, a cardboard and wood and styrofoam rocket designed to test construction techniques and the ignition of two Estes motors simultaneously. Flew at least four times, and if I remember correctly, there were only one or two separate dual ignitions that actually succeeded, the majority of the flights ended in it veering off one way or another with only one of the engines having lit.

X-4 Red (I forgot the second half of the name), a cardboard and wood two stage rocket designed to carry a payload and test manufacturing techniques, probably the most successful of the entire X series. Obtained altitudes of a few hundred feet (altimeter and trig verified) and with successful second stage ignition and payload return. The brown first stage tube never got painted, though, and I never found it in the field.

X-5, a massive K'nex rocket with a 2 liter bottle in the base I launched on the air pressure launcher. Made it perhaps 5-10 feet laterally, but beyond a certain pressure the bottle shot through the rocket, destroying it and causing me to have to pick up a lot of K'nex pieces from the yard. Flew before X-3 or X-4 because they took me so long to finish and fly.

X-6, discussed above.

X-7, discussed above.

X-8, discussed above.

X-8 again, a rocket made entirely out of paper and tape and toothpicks. A very bad idea, but a very fun idea. Flew once, burned its fins off, and landed within a few feet of me before the ejection charge had gone off, giving me quite the scare.

X-9, discussed above.

X-10, discussed above.

X-11 Memories, a high school art project, seeing if I could make ceramic rockets succeed with my better knowledge of rocketry and better sculpting ability. The result was a lot cleaner and a lot lighter than the X-1 or X-2 but only marginally more successful. It got off the launch pad and about 10 feet up.

 

Please update us with your continued progress, and keep a mission log so you can look back on it years later! That's something I wish I had done, recorded everything in a notebook somewhere. I tried to do it but I couldn't keep up and it is something I regret not sticking with.

 

[PakledHostage]

21 hours ago, Ultimate Steve said:

Ah, I built a lot of those out of 2L bottles back in the middle school and high school days!

  Hide contents

 

 

 

Fun story. Thanks! I will have a look at that website.