Arduino Uno External Power Source connection

So basically I have an arduino uno and one micro servo motor (with jumper cables). I programmed the system so that basically when I reset the board, there is a 15 second delay, then the servo motor rotates 90 degrees to let go of a rubber band. It is not a loop, it only does the program once. When the system is connected via USB to my desktop computer the program runs perfectly, but I need an external power source so that I can put it in a water rocket. I used a rectangular box 9v battery (measured at 7.4v raw) and the 9v battery connector. I soldered the copper wires in the battery connector to a metal wire that can go into the arduino ports, with electrical tape. I did a continuity test to see if the solders were good and they both turned out perfect. I put the red wire in the Vin port, and I put the black wire in the GND port, and when I connect the battery, the board turns on for a few seconds then the green light fades away and the board doesn't work. I tried again and the board kind of turned on, the green light was on, but my program didn't work at all. Any suggestions?

9v battery (measured at 7.4v raw)

The battery is dead.
A new 9V battery measures 9.6V dc MINIMUM !

Besides, a smoke alarm battery is rated for 300 to 500mAh but the internal resistance prevents it from
sourcing much current. I would consider using a 2S Lipo (8.43Vdc charged)

Wait if those rectngular 9v batteries are not good at sourcing current, do you think I should use 4 AA battery pack? Or do you think a fresh 9v battery would be a good solution?

Consider using a Lipo battery, it would have more energy and probably be lighter. You do not have much thrust. You might experiment with a dummy weight the same as your electronics and launch a few test flilghts to see what the additional load is causing you and if the rocket can even go very far.

adamby1047:
Wait if those rectngular 9v batteries are not good at sourcing current, do you think I should use 4 AA battery pack? Or do you think a fresh 9v battery would be a good solution?

If a good 9v battery works and the weight works ok with your rocket (assuming it flies on the rocket), then go to walmart and see if they have rechargeable ones in the battery section. If the battery does not fly on the rocket, then using a cell phone recharging battery to power the arduino. You might have to put a 220 ohm resistor on the uno between the 5v pin and ground to act as a dummy load to keep the battery pack from shutting down on low current. I just got one of the below for testing for $6 and delivered in the US in about 5 days. There are smaller and larger versions available.