Rockets

[Riley Montgomery]

I know this is a space message board but I thought you might know a little something about aviation. In my high school physics class we need to make a water rocket with a unbroken 2 liter bottle as the base. Above that 2 liter bottle can be anything else, it's just the bottom bottle needs to be perfect. Does anyone have any good suggestions on how to make this thing fly as far as possible. You can respond here or E-mail me at Orange_County1331@msn.com. Thanks!

[yale]

http://www.google.com/search?sourceid=navclient&ie=UTF-8&oe=UTF-8&q=water+rocket

[archiebald]

Ah, good old PET bottle rockets.  I haven't built any myself but I do know an awful lot about this particular type of plastic.

PET, correctly known as Polyethylene Terephthalate and used for all soft drinks nowadays.

There is a huge high school interest in these simple "toys" here in Japan due to the educational principles that can be learnt from mucking around with them with virtually 100% safety.

First things are fairly obvious.  To fly farther you need more energy and the best way to achieve that is to get more energy in the bottle at launch.  The propellent is obviously water and air.  The mass of the water provides the thrust but it is the compressed air that pushes it out the back.

I have seen and heard a lot of experimentation on the correct proportions of water and air to have in the bottle.

The starting point must be the bottle itself.  The bottle is nothing more than a fancy pressure vessel so choose a design that looks most like a pressure vessel to enable use of the highest air pressures.  It needs to be plain round (not Coca Cola contour shape).  If possible, try to find one that has been manufactured in two pieces, with the polypropylene base cup since the PET container itself has a spherical base that will generally take more pressure.  These are hard to come buy in most countries now because the industry moved to one piece bottles having 5 or 6 feet.  These footed bottles are known generically as "petalloid" design, not the best choice for a high power rocket.  Lastly, the bottle should be as heavy as possible, this may seem counter-intuitive but the additional strength in the bottle (allowing higher pressures) will more than counter-act the extra few grams of mass.

So, assuming that you have got a good strong bottle, you can then start to experiment with volume of water / air balanced against maximum pressures.  For a good bottle, you should be able to achieve a working pressure of up to 10 bar  before the bottle starts to deform (most soft drinls are about 3 to 4 bar).  A PET bottle should expand dramatically like a balloon before it bursts so you should have plenty of warning if you are getting near the limit.  A word of warning though, the only safe way to pressure test a bottle is hydraulically with 100% water and with the bottle enclosed.  1 liter of air at 10 bar will create enough of an explosion to damage your ears and project particles of PET at velocities high enough to blind you.

I have also seen a lot of work done with the nozzle design.  Don't just rely on the neck of the bottle, try to work out some different orifice sizes.

Once you are moving, don't forget aerodynamics.  The stabilizing fins should spin the bottle like a bullet and you want a good safe nosecone.

If you really want to fly, look out for the designs on 2 and 3 stage water rockets.


Try some of these links
http://dogrocket.home.mindspring.com/WaterRockets/tripreport.html
http://u.hornstein.bei.t-online.de/wr.htm
http://ourworld.compuserve.com/homepages/pagrosse/h2orfaq3.htm
(all from Google "PET bottle rocketry")

[Riley Montgomery]

I failed to mention that the air will be 80 PSI. Don't know if that means much. But I don't understand what you mean when you say bars, could you clarify that for me? Thanks for all the help.

[Thinker]

I failed to mention that the air will be 80 PSI. Don't know if that means much. But I don't understand what you mean when you say bars, could you clarify that for me? Thanks for all the help.

I suggest caution here.  Tire pressure for a typical light car or truck is usually on the order of 35 psi.  Years ago, when I was working at a gas station, the guy on the tire machine tried putting 40 psi in a older tire that had gotten a bit soft and the tire literally exploded - he got clopped good with a tire bar.

[archiebald]

Riley,
1 bar is apprximately equal to 14.5 psi.

Thinker, I work in the PET bottle industry.  PET is a fantastic material for creep resistance due to the molecular structure which is bi-axially orientated into the final bottle shape.  You'll never see a PVC, PC, PP or HDPE bottle for soft drinks because their molecules cannot strain harden as PET does.

Single trip soft drink bottles will generally take 10 bar without deformation.  A well blown bottle will only start to seriously deform around 11 to 12 bar with rupture taking place at around 14 to 15 bar.

Don't know if anyone here remembers them but a few years ago in the UK, the SodaStream system was popular (you bought the syrup and carbonated your own water at home).  Those bottles were a thicker version of the soft drink bottles in the shops and the minimum spec for deformation was 20 bar or 300 psi.

Just for interest, does anyone know the more common name for PET?  I'll give you a clue, you're probably wearing some right now.

[Thinker]

Thanxs Archibald...

I just read that 100 psi bit and had visions of these kids working with what amounted to bombs (that exploding tire all those years ago leaped to mind - the guy who got hit said it was like somebody had used the tire bar as a club on him).  Sounds like these PET bottles can handle 150 psi or so easy then, maybe twice that if they latch onto one of the really good ones.

[remcook]

polyester?...but we call them PET bottles anyway..

[archiebald]

Perfectly correct Remcook!!  most PET bottles get recycled into shirts and carpets.

Thinker, although a PET bottle is very strong, safety with pressurized gases is very important.  If the bottle should fail for some reason at 100 psi, there is certainly enough energy to injure somebody.

For example, if you look at the thread of a soft drink bottle you will find vent slots that allow the gas pressure to release while the cap is still retained by the thread.  The very first ones didn't do this and there were a few cases of the cap coming off the end of the thread with full pressure behind it, there were a number of near miss injuries (just missing the eyes) before the vented thread design was implemented.

By the way, while reading one of those links I saw that the height record for a PET bottle rocket was 1,060 feet using a two stage design with Carbon Dioxide as the pressurized gas....cool!!!.

[LunarOrbit]

I took a quick look at the websites you listed Archie, but can you give me a brief explanation how the second stage works? How is air added to the second stage inflight in order to cause it to "ignite"?

I don't need to know too much detail because I'm not planning on building one... I'm just curious because I had never heard to multiple stage bottle rockets before.

[archiebald]

Like I said, I haven't built one myself but my understanding is that a valve is used on the base of the second stage that "senses" when the pressure in the first stage has dropped off, i.e. when the propellent has been used up.  As the second stage comes online, it blows the first stage away.

Possibly with some more searching, my guess might be proved wrong.

[archiebald]

http://www.orgsites.com/va/bristolwaterrockets/00water.JPG

A photo of a heavy lift PET bottle rocket, and the best part is it is fully re-usable for the next launch in a matter of minutes.  Could be an idea for the Crew Exploration Vehicle.

Also take a look at this video clip of a 2 stage rocket launch.  2MB wmv format.

http://antigravityresearch.com/HTMLobj-497/2-Stage_Short.wmv

[Thinker]

Dang, some of those things are fairly elaborate!

I can almost envision somebody at NASA looking these over with a gleam in his eye, going `THIS is what we need...'

[archiebald]

Just found a nice description of how the stage separation system works and some piccies.

http://www.geocities.com/wrgarage/stage_da.jpg

http://www.geocities.com/wrgarage/newstg.gif

How it Works
During pressurization, pressure is trying to blow the second stage out of the top of the first stage fitting. This force is proportional to the cross sectional area of the nozzle tube. The second stage is retained by friction between the nozzle tube and the sheath, since air between the tube and sheath can escape to the atmosphere. The retaining force is proprotional to the surface area in contact times a friction coefficient). The weight of the sustainer (mass x 1g) also helps keep it together. Since the nozzle is long relative to its diameter, the blowoff force is smaller than the retaining forces, so no separation occurs. Pressure builds equally in both the first and second stage, since the check valve allows upward flow with no resistance. At launch, the pressure in the first stage declines, and the check valve closes. The sheath begins to expand as differential pressure builds, but the retaining force is aided by the inertia forces (second stage mass x 10g or more acceleration).
When the last of the water leaves the first stage, the pressure in the first stage abruptly drops, inflating the sheath. The retaining inertia loads are also disabled by the end of the first stage thrust, and the stages separate.

Simple, ingenious....cool.

http://www.geocities.com/wrgarage/stage.htm

[xir]

I’m really impressed i never thought that so much thought had been put into them, but i suppose that the’re far simpler to make than the ones which use ignited fuel to launch, as most peeps don’t have the know how to make one for themselves.