Shockie--OK, let's conduct a little experiment to verify that momentum is, in fact, imparted by the "launcher" and not by the motor. Insert a model with a dead/used motor onto a piston and light an Estes igniter. Better buy an expensive super-fast video camera to capture that instant burst of momentum and velocity you'll see as the rocket goes soaring skyward. If you prepped everything right, the rocket didn't budge, because proper use of a piston also includes friction fitting the motor into the piston tube. My high school physics would suggest that the friction imparted easily trumps the pent-up energy of the igniter gas, as is evidenced by the fact that the rocket didn't move. The NET effect of the launcher, in this case, is negative.
Now replace the dead motor with a good motor. Repeat experiment. Notice slight change in performance--this time the rocket shoots away. To what do we attribute the change in performance--a "hot" igniter or the addition of a motor? It's the MOTOR that enables the piston to transfer energy to the rocket.
If you want to limit or ban them, by the way, why arbitrarily pick on just altitude events? Pistons offer a performance advantage in practically every event, including duration and the various scale/PMC events.
--Chan Stevens
Chan:
Talking about engines:
Engines are not part of the "launcher", therefore if all you do is use a piston with an engine and a typical igniter then there is not momentum being added to the model by the launcher; its all coming from the engine itself(plus a minute amount from the igniter)
On the other hand, if you place an "additive" gaseous producing substance, whether it be BP, a custom igniter made from BP, a custom igniter using pyrogen, or any other gas producing powder in the piston launcher itself, then it becomes part of the "launcher" and confined gases under pressure will produce additional momentum and impart it to the model.
This is basic gas dynamics and physics. Did you see my earlier post on BP gas generation? Did you see the formula?
https://www.rocketryforum.com/showthread.php?t=45000&highlight=BP+gas+generation
On my other post I needed to make 2 clarifications:
1. A spider should be allowed to be used in NAR competition, as it is not typically enclosed, therefore it will not impart any additional momentum to the model.
1a. A "single-shot" type or form of "spider" built into the piston head using a custom igniter or BP should be banned from NAR competition based on 5.5
Igniters and Pistons
https://www.nakka-rocketry.net/igniter.html
Chan, what I said was that competitors would be limited to typical commercial igniter's like Estes/Quest; If you were to repeat your igniter experiment with some custom igniters, I'm sure that you would notice some movement in the piston that exceeded the friction.
And I know that if you placed as little as 0.1 g of BP or any other combustable powder in a piston you would see movement; friction normally in a well designed piston is minimal if designed correctly.
I used a piston head that had been wet sanded from 320 to 1000 grit sandpaper and it had a mirror smooth finish to it, used piston tubes that had been impregnated with epoxy resin while curing on an internal teflon rod to get a uniform internal wall.
Before use I either used a spray graphite or a spray teflon: the result was near frictionless surface, with the spray graphite/teflon acting as a sealing O ring between the piston tube wall and the piston head itself. It literally floats through the piston tube.
Chan, I also said they would be banned in those events where ALTITUDE plays a role in that specific competition event. I didn't say, just altitude events.
2. I had forgotten that the NAR will now start to allow some FAI competition events in NAR competition; I support this, so I will allow pistons with BP,etc for use in these competitions only. My reasoning here is that the NAR FAI competition events are designed to give NAR people exposure to and practice in real time FAI events; therefore thye use of BP,etc in pistons is appropriate as the rest of the world allows it .
The fact that the FAI sporting Code has the same momentum code as the pink book, but allows it, just goes to show me that in the rest of the world, at least in the world of FAI competition, basic gas dynamics and physics do not apply.
terry dean
