MadSci Network: Engineering

Re: Bottle rocket design features

Date: Wed Nov 10 08:33:17 1999
Posted By: Artem Evdokimov, Postdoc
Area of science: Engineering
ID: 942000991.Eg

Dear User,

I am afraid that your question somewhat exceeds the time I can afford to 
spend on engineering a "super" bottle rocket, so forgive me if I would not 
go into too much detail. I will try to outline some general ideas which may 
be of use.

First of all, I will assume that the total weight of the rocket with the 
fuel (water) should be under 300g. This places a cap on how much total 
thrust can you expect in an ideal case. I suspect that the first place to 
invest time in optimizing would be the engine - namely the size of the 
nozzle opening and the pressure of air above the liquid. Also, if you 
maximize the pressure you might want to use concentrated saline solution 
instead of water, especially if you are *not* limited by fuel weight, but 
even if you are, expelling more mass per second creates larger thrust.

Two bottles may be utilized in sequence i.e.  glue them together with  
epoxy (i advise to be very generous with epoxy and to use some kind of 
cloth tape soaked in epoxy rather than just plain glue to ensure that the 
thing won't blow up in your face). You then get a total volume of 3 L which 
means that in addition to water (saline) you can pack in extra compressed 
air which can also create thrust. I think coke bottles are good to 3 atm at 
least, be careful not to exceed the allowed presure.

Filling the smaller bottle with heium won't do much good - simply because 
the volume is not enough to create sufficient lift.

Parachute - by all means ! Just make sure that the surface area is enough. 
I leave it to you to design a good parachute opening system - there are 
several ways to do this. Adding a parachute is a very good idea from the 
point of view of safety - less chance that the rocket will injure someone.

Generally, streamlining your design would help, but make sure that you do 
not add too much extra weight. A simple nose cone, glued and sanded 
(polished) to the main body should reduce the drag.  Also make sure that 
your fins are sturdy and are glued well, because a good bottle rocket 
develops enough thrust to rip poorly glued fins off.

As always, be accurate and align your rocket accurately. If you have the 
option of testing the rocket's stability in a "wind tunnel" i.e. for 
example by  hanging it in a strong draft, or out of the car window (be 
careful) it may help to improve performance. 

Hope it helps.


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