I don't know if this has been suggested. It trades power for control. You may not get the bottle to fly 50 ft, but maybe 15 feet straight up reliably is a nice trade-off.
Summary:
(a) suspend 5 mentos inside the bottle above the soda using a paper clip threaded through holes in the mentos. See here:
http://www.youtube.com/watch?v=GAHGz...0coke%20mentos
(b) Use a cork to hold the pressure until it builds.
(c) Construct a rudimentary launch platform from another 2 liter bottle so it launches straight up.
Details:
(1) Buy a cork that will fit firmly into the mouth of a 2 liter diet coke bottle. They have these at Michaels for <$2 for 4 of them.
(2) Create a launcher. Take an empty 2 liter bottle. Remove the label and soda. Use a box cutter or exacto knife to cut the bottle in half with a clean straight cut. 1/2 inch above the halfway point is actually perfect (yes above). Throw out the top of the bottle. The bottom will be your launcher.
(You will put the corked full bottle inside it and turn the whole assembly upside down to launch.)
(3) Prepare mentos. Take four mentos (5 is slightly better but dips into the soda and causes permature mixing thus ruining the launch.) Poke a hole through the centers of the mentos. I use a large thick paper clip or very thin nail for this because a normal clip just bends. String the four mentos through a small paper clip and then press the end of the clip into the bottom (inside) of the cork. The mentos need to be flush against the cork or they will mix with the soda too early.
(4) Cork the rocket. Open a new Diet Coke 2 liter bottle (Actual Diet Coke Brand is reported as the best, adn Dieat Coke beats non-diet becuase it's less sticky to clean up). Push the cork in as tightly as possible being careful not to crush the bottle and make sure NOT to let the mentos hit the soda!! Take the launcher and place it over the top of the rocket. You can get a little airtight seal here but don't worry if not - the main importance of the launcher is to hold the bottle upright.
(5) Turn the whole thing over (do not shake), and place upside down on the pavement and immediately back away. Don't hesitate because it can go off pretty fast.
The rocket should shoot upward maybe 15 feet or so. The trick is getting the cork as tight as possible. A rubber stopper might be better than a cork but I didn't try that. Then again if too tight you're back to the problem with the cap (not coming off until you destroy it). A 20 oz bottle might work even better, less weight, but I didn't try that. Also try a few more mentos and dump a little soda out first - more air space, more released gas should cause a faster and more explosive release???
It seems fairly safe, especially compared to slamming the capped bottle down, but use caution, consider eye protection, and of course avoid young kids around. It can still go at odd angles. Have fun.