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making 12 volts from 2- 9 volt batteries

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(unknown):
I have a hobby flying model rockets. There are different categorys of rockets the small rockets, that will only need 9 volts to fire there engines and mid to high power rockets that use 12 volts to fire there motors. I am trying to reduce the amount of equipment needed to carry out into the field to light up a few rockets of different sizes. If I take two 9 volt batteries and put them together to get 18 volts how  or what is the best way to reduce the voltage to 12 volts. I plan on puting a switch to go from 9 volts to 12 volts for the 2 different size igniters. The components I have now like LEDS are 12 volt.

dinges:
You need a voltage regulator. Google for '7812' and 'voltage regulator'. A manufacturers datasheet will give you all the info you need.
If you need the 12V ONLY for the LEDs (no other electronics that needs exactly 12V), you could do without a voltage regulator. LEDs will work on any voltage (DC). In series with the LED is usually a resistor (I think, in your case, about 470-1000 ohm). All you need to do is increase the size of this resistor by 18/12.
But, if there's ANY electronics there that needs 12V, go for the voltage regulator solution. It's not that hard to build, as far as electronics goes. I suppose you know how to hold a soldering iron, if not, find someone who can (HAM, local TV-repairman, etc.)
I don't know how much current these igniters use, but the 7812 can supply max. 1A (some special versions can go up to 1.5 and 3A)
My advice remains: find someone local who can help you out. If you ply him correctly, he might even build the complete thing for you.
Given more details, a better answer could be given.
Peter.

Opera House:
The rule of thumb is to go with the highest voltage battery it is convienent to carry with you.  Use a switching regulator to reduce the voltage to whatever you want.  When I was searching around a while ago I saw some projects that did almost exactly this.  The projects were for the model airplane guys.

Opera House:
Just curious.........Why does it matter if you put 12 or more volts on a 9V igniter? Aren't these one time use.

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