Correspondence

Who could have thought that anyone outside our immediate group would correspond with us ? Admittedly there's the Oxford contingent, but they have been seeded by lights from the one true flame. Nevertheless the letters have started, and still they come.

Rockets

Dr Bruce K Brownstein writes on 15 April 1996:

My name is Kenneth I am currently working on a rocket of my own. The only difference between yours and mine is the fact that mine is powered by air and water pressure. To be honest with you this is for a contest the contest is to keep a two liter bottle air-born for as long as possible using air and water pressure. I am curious if that in fact you in your travels have come across any type of design for a rocket(s) of this sort, or if you have any of your own designs that you could send me. Thanks for your time and it would be apreciated if you could write back to me as soon as possible. I am in urgent need of a design, for the competition is in a week. Thanks again.

Martin replies:

Thanks for the email. I have posted it to our web page in the hope of stimulating further comments.

I'm afraid that I have little practical experience with air/water rockets but I think that many of our designs would be amenable to such a device. In your position I'd go for a single bottle design, inverting the bottle so that the screw on top faces the ground. I'd discard the top and insert a bung (perhaps with a mallet) into which a tube had been attached to allow the inward passage of air. Pressure testing suggests that the bottles can withstand a pressure of about 90psi so you want the cork to eject a little below this figure. You then have to optimize the quantity of water in the rocket, but I can offer no other advice than experimentation here.

Have fun building your rocket!

Conor replies:

With regard to your correspondence with the rocket home page about pressurized air powered rockets, I am afraid we havn't bothered with pressurized air powered rockets, it seemed to be a solved problem. Rarely a day goes by when the evening doesn't find some man or some woman in a pub telling me how their friends or their friends friend or more exceptionally they themselves have flown such a rocket. This very evening someone, in, as you could guess, a pub, was telling me that very tale. It is this that inspired me write, my mate Clive, a caver, claims some mates of his made a water and pressurized air powered rocket and it flew so high it became a dot in the sky and then disappeared and never returned or at least was never found. This is the sort of height to aim for, or to use the sort of English to which Churchill so famously objected, this is the sort of height for which to aim.

Anyway, I am sorry to say we have never tried making pressurized air powered rockets. I would suggest aladiting (do you have araldite in america, you might call it 'cold weld' or 'liquid metal') to a coke bottle ('soda' bottle) top a piece of plastic tubing, aralditing similar to a pump and connecting them with some tight fitting pvc tubing pushing enough pvc tubing over the piping that it takes 80psi to pop. This is only a guess.

The only piece of practical advice I can offer is to attach the fins ('vlad stuff') below the water's release point ('thrust nozzle'). This, obviously, is vital for stability. The easiest way we have found to do this is to make a skirt. Get a second coke bottle, cut of the top just below the point it begins to curve and the bottom just above. Stick the rocket nozzle down into the cylinder thus produced and gaffer tape it in place. Stick the vlad stuff to the skirt. If you don't have gaffer tape in america there is just no point.

Obviously the best way to win a time-of-flight contest is to add a parachute. I have all sorts of ideas on how to do this, but none that have been tested.

If you have any success or more typically a litany of failure please send an account to the rocket home page. Pictures are also welcome.

Water rocket radiation leaks

Mark Knoepfle writes on 15 April 1996:

Sir,
Imagine my delight, after searching the net for someone that had done some sort of work with water rockets, in finding your rocket page.

I am a senior in aerospace and mechanical engineering double major at the U of Arizona. Like the fool I am, I though it would be fun to try to build a water rocket. Just a diet coke (20 oz.) bottle, some water, and a tire valve. Pump it up until it pops and flys. Simple eh? It seemed like a weekend kick until I decided to pull out the (very dusty) Mechanics and Thermodynamics of Propulsion from a class last year. The damn thing. I found myself thinking about how to calculate things like the exit velocity etc. The max height equation danced just out of reach of completion. The question, simple in nature, of how much water and how high a pressure still remain unanswered. I suppose the engineering approach would be to build and test. For some ungodly reason I've got to do some paper and pencil work before I lay a saw to pvc pipe.

I don't even think a nozzle is even necessary as water is pratically incompressible. If the gushing liquid has air in it a nozzle might help, but it couldn't be much.

Have any of you done any calculations as to the exhaust velocity? Any guess? Could I interest you in some eqs? Might spark your desire to go back to diff eq.s. Not even p d eq.s ( no Green's fcn.s here ).

Martin replies:

Thanks for the email, it's nice to see other people are reading the page besides our immediate collaborators. I don't think any of us have done any calculations beyond trivial back of envelope affairs. In some ways this is a deliberate choice as many of us are theoreticians and to try and model the rockets seems a little like cheating; in some sense it's more the challenge and enjoyment of building rockets without careful design that drives us.

I've put a copy of your message on the web page; I hope that's all right.

Rocketry Society from Udine

Anto Maria Prati writes on 15 July 1996:

Model Rocketry

John writes on 23 August 1996:

dear fellow rocketeer,

I, being in the U.S. am sadly disappointed that the government over there regulates fun with such alacrity as to prevent thrill seekers from buying an using solid composite motor devices. This business with the liquid nitrogen and water sounds very clever and must take tremendous mental effort to see reproducible results. I heartily commend your efforts! As my wife is of Longford - in the midlands as she says - I look for interesting things to do when we visit the folks. I feel certain that there is no conventional rocketry as I have come to know here in the states. Alas, England has a small group who may hopefully spread the tales of safety and allow the hobby to grow.

As a suggestion, and I do not claim responsibility for any forthcoming damages, you may want to investigate the feasibility of using your coke bottles with a chlorine bleach/aluminum foil reaction. I remember clearly the expolsions my cousin could produce with a bit of aluminum foil rolled and stuffed into a two liter bottle followed by a small amount of clorox and tightly capped. Since rocket motors are little more than properly vented bombs, this information I hope is useful to you. If not, at least you can make really loud bangs!

going in September

PS - if you can find a way to narrow the neck into a better nozzle, you can bettter maximize velocity.

More Rockets

Dennon Shockey writes on 16 December 1996:

Dear Martin,

You Don't know me , but I got your E-mail address from the Extragroup correspondence page. Anyway I'm 16 year old student in a grade 11 physics course and have a chance to get bonus marks if we create a bottle rocket out of a 2 litre pop bottle and has a parachute for comming down. That's it there is now other restrictions. Could you please send me if you could, just a basic design for this if you have one. It would be much appreciated:-)

Thanx

Conor, lettersmith, replies:

I regret the delay in replying to your query, it is now probably too late for you to gain bonus marks in physics. The delay was because I in my own modest way was also trying to get bonus marks in physics, though in my case I was trying to construct a new gravitational instanton using the hyperkähler quotient. I have named this gravitational instanton \X.

I am afraid that the easiest way to fire a coke bottle rocket is to use compressed air, something I have never tried. If you look on the extragroup correspondence page, you will find the address of people who have. If you think compressed air an unworthy way to gain bonus marks, try making a hs rocket, full instructions are given in the rocket home page, if you have any queries, email me.

We have never succeeded in making a parachute, perhaps, if you do, you will email us with an account of how you did it.

Conor.

More Rockets

Steve Slaven writes on 16 January 1997:

Martin,

I teach eighth grade science in Winamac, Indiana, USA. I also coach a Science Olympiad team. One of the events in Science Olympiad is to build rockets made of 2-L bottles. The object of the game is to keep the rocket aloft longer than the competition.

Instead of using liquid nitrogen we use compressed air with a maximum pressure of 75 psi. My students and I have had rockets composed of three 2-L bottles achieve altitudes of 90-95 meters. We are not satisfied that we have reached maximum altitude yet. We tend to have stability problems.

Have you any suggestions on how to build a stable bottle rocket?

Conor replies:

Steve,

Many thanks to your letter to the rocket home page. Our attitude to compressed air rockets has always been that they are the best way to acheive maximum height but somehow have never bothered making any. I am impressed and incredulous to hear your rockets reached almost a hundred metres, are those by the eye estimates or did you use triangulation. We have never used triangulation since triangulation involves maths and most of us do maths for a living.

Now stability is the cutting edge problem. My answer is that the difference between the aerodynamic centre and inertial centre has to be as large as possible. This means you should try and get your weight up top, that is add small fins as far down the rocket as possible and add some weight to the top. A good way of adding fins is described in my description of hairspray rockets, which can be found in the {sl Latest News} section of the page. The Oxford group added weight to the top of their rocket by making a papermache cone. The other important thing is for the rocket to take off fast, that is why I have never had much problem with stability, the hs rocket tends to take off very fast, but become ballistic too soon into flight. Bodil and the Beast had terrible stability problems with their julia clip rockets because they never took off fast enough. You can adjust take off speed by widdening your nozzle.

Please keep in touch, or even better send us an account, design and a couple of pictures for the page.

C.

then added:

Maybe I ought also mention that we launched all our rockets along a six foot shower rail. That helped the rockets reach stable speeds before free flight. The launch pole was bent and destroyed when the last bertha exploded.

C.

This is http://rocket.tc
Last modified: