Ok, here's the first installment of the fun. I need this stuff for another project, so it's a win-win to need it for this forum.
Let's start with piston launchers...
Floating head pistons have been around for years in competitive rocketry. They are virtually isolated to that discipline, and half the rocketry community doesn't even know about them. Half the rocketry community are scared to death of building a glider, too, but that's it's own sad story for another day. Hopefully we can take away some of the voodoo and make it more accessible to the masses. I've pulled in some of the still shots that FT captured for reference here since they did things with the camera that I'm not equipped to do. So let's get started.
Why would you want to use a piston? After all, it's complex, the alligator clips have to just barely be attached to the ignitor so they'll slip free with the slightest tug, and the tube is constantly filling up with soot!
Anyone who has launched model rockets is familiar with the hiss prior to the rocket whoshing away. That hiss is hot gas firing out of the nozzle, but at low enough speeds that there is no useful thrust. Propellant is being burned, but until the combustion chamber fully pressurizes, there is no useful thrust. A piston launcher confines that gas in a closed chamber, just like an engine cylinder, and puts its expansion to work. The tube pressurizes, and pushes down on the piston. Since the piston rests atop a stake that you've punched into the ground, it can't move.
Instead, the cylinder has to move upward to relieve the pressure, carrying the rocket with it. When the piston reaches the end of the tube, it hits a collar which prevents its escape. At this point, the tube fully pressurizes, and would rupture but for one thing: it's friction fitted to the base of the engine casing. The pressure separates the tube from the casing, propelling the casing backward and shoving the rocket forward.
Physics now comes into play. The pressure of the tube being shoved upward as it pressurizes develops significant speed--so much so that the entire rocket-piston assembly is catapulted into the air. When the tube fully pressurizes and separates, it is thrown backwards. Newton's third law now kicks it--the rocket is throw upwards--all of this having happened before the engine has produced any useful thrust! Now when the thrust kicks in, the rocket is already moving very, very fast, and in the case of a little A engine propelling a heavy glider, your altitude potential nearly doubles!
The result is displayed to an absolutely stunning extent by this sequence:
Notice that in the brief time that after separation, literally a fraction of a second, with the piston still flying through the air, the rocket has travelled a tremendous distance, now going well over 100 mph on its brief 0.3 second burn (yes, you read that correctly--the thrust lasts for less than 1/3 of a second!).
The acceleration is so much faster than a traditional rocket launch that you don't even need a launch rod like you used with your Estes rockets. The minimal guidance needed is provided by the stake that the piston rides on. Basically, wherever that rocket is pointed, that's where it's going--period. I've launched this model in winds exceeding 25 mph--more than double the strength at which most glider fliers call it quits, and it still tracks dead straight--no weathervaning into the wind.
In the next post, I'll show the guts of the piston system.