Optimal piston launcher volume?

Discussion in 'Competition Rocketry' started by retortec, Feb 7, 2017.

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  1. Feb 7, 2017 #1

    retortec

    retortec

    retortec

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    I have been having a lot of fun playing with piston launchers this year. Starting with the Sunward kit I rapidly moved on to building my own from scratch. Fixed head and floating head pistons with internal wiring. I even started turning my own pistons from brass and aluminum. But with all the information I found not much on piston volume is out there. I hope this question makes it to some one with a little more experience. So here is the question based on a floating head piston. Do you get more velocity off a short piston tube with the rocket popping off before the head is pulled from the center post? Logically if the head is pulled the rocket is leveraged against the mass of the tube not the launcher. The tube would be accelerated in the opposite direction absorbing some of the energy. I know a longer center post and tube provide more stability but I am just asking about motor performance.
     
    Last edited: Feb 8, 2017
  2. Feb 8, 2017 #2

    Gus

    Gus

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    It's great you are having so much fun with pistons. Unfortunately there is no simple answer to your question about piston volume.

    Nothing in model rocketry (with the possible exception of streamer performance) is more complicated than pistons. Piston performance depends on the interplay between a number of important variables.

    Here are some of the most important variables:

    Motor size and performance (widely variable even for the same motor type)
    Rocket weight (see the Alway brothers NARAM R&D report)
    Piston tube diameter
    Piston tube length
    Piston tube weight
    Piston tube material (carbon, fiberglass, paper)
    Piston cleanliness
    Floating head vs. fixed head piston
    Piston head shape
    Internal vs external ignition wires
    Tightness and nature of how motor is held in, and released from, piston
    Initial volume in piston prior to motor ignition

    The best recent investigations into piston performance, with some very surprising results, were done by Patrick Peterson and the Neutron Fusion team at NARAM 55 and 56.

    NARAM 55:

    NARAM 56: https://www.nar.org/wp-content/uploads/2014/05/Neutron-Fusion-NARAM-56-RD-FAI-pistons-1.pdf

    Patrick's work yielded the fascinating result that piston acceleration is not uniform but quickens and slows and, for a particular model weight and motor combination, actually has almost sinusoidal nodes of peak piston length, sometimes much longer than what we previously thought. Really good work.

    So, again, no simple answer to your question but huge fun to play with and explore.

    At the most recent world championships in Lviv, Ukraine my teammates Matt and Dr. B each had their own new piston designs. Each will vigorously defend his design as being the best approach. Matt finished first in the world in S5 scale altitude. Dr. B. finished first in the world in S1 altitude. :)

    So I can't tell you whose approach is best, but I can tell you that spending years flying with pistons and constantly tweaking can yield big results.

    Steve ("the depth player" whose butt was dragged onto the medal stand by fantastic teammates with better pistons than mine)
     
    Last edited: Feb 8, 2017
  3. Feb 8, 2017 #3

    retortec

    retortec

    retortec

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    Nice! The NARAM56 article has some real good nuts and bolts on calculating volume and tube length. I like the idea that the choices made in the piston launchers construction actually shapes the thrust curve. I will keep digging and experimenting. Thanks
     
  4. Feb 10, 2017 #4

    caveduck

    caveduck

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    Piston acceleration oscillations were actually the subject of a NARAM R&D report from a really long time ago, probably NARAM-34 or before, but little from that era is available online. It didn't result in anyone trying really long pistons like we're seeing now. Some of the papers may be included in old volumes of the NAR Tech Review.
     
  5. Feb 10, 2017 #5

    retortec

    retortec

    retortec

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    I started wondering after a little math if the piston acceleration oscillations could be produced in something much shorter. Overall volume and resistance of such a long bore can be matched with a larger piston with less travel. The variable of expanding gasses and how they behave in similar but different shaped volume is big. It may be a build and see kind of approach.
     
  6. Feb 13, 2017 #6

    Micromeister

    Micromeister

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    Over the years my fixed Metal head pistons and current floating head pistons have evolved from 18" piston tubes to 16" , 14, 12 and now I'm using 9" tubes for pistions for MMX, 13mm, 18mm and 24mm BP motors.
    Since 2014 I've found I get much better overall performance from these shorter launch tubes. Could be I've progressed in my use of Piston Launcher but I believe going with the shorter, Lighter slide tubes have resulted in very good increases in overall achieved altitudes.
     
    Last edited: Feb 14, 2017
  7. Feb 23, 2017 #7

    retortec

    retortec

    retortec

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    I have been experimenting with lighter, shorter pistons. Many of my designs have been based on articles and post from you. Just saying thanks. I have built a tower with a piston base. The base has spacers and shrouds so I can experiment with many different configurations. Currently I have pistons made with CA and stainless coated craft paper, turned polished brass on fixed and floating mounts. Short and light definitely yields results but the long large volume pistons are intriguing. So I have been playing with a short large volume design. The center 13mm tube will be mounted to a light, low drag piston with the outer tube fixed. Volume and drag parameters I am compiling from a long piston model. While dragging different pistons though the mock up I can measure the drag and get a rough idea of what it will look like. Looks like the best approach is to keep experimenting on both fronts. Science is fun.
     
  8. Jun 26, 2017 #8

    TangoJuliet

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    I've been thinking a lot lately about pistons and why they work. The early 80's was my first introduction to competition model rocketry and piston launchers. I never used one but I got see other modelers use them and developed an understanding of how they worked. At ECRM I got to see it all again, with a few new (to me) variations. What I still have a hard time wrapping my head around is WHY they work.

    Let me explain... I understand that the exhaust gasses of the motor pressurize the volume of the cylinder over/around the internal shaft with either a floating or fixed piston head on it, and that "pressure" gets imparted to the rocket as the piston reaches the end of its travel providing that added "kick" to the boost phase. But wouldn't a pressure vessel/cylinder with NO piston do the same thing? And doesn't the expanding internal volume of the piston negatively effect the amount of pressure that can build up before the rocket departs from it?

    I keep thinking of the little water rocket my best friend had when we were kids playing in the yard. A plastic "rocket" roughly the shape of a V-2 was filled with water then attached/locked onto what was essentially a mini bicycle pump. He'd pump that thing several times then he'd unlock the rocket from the pump and off it would go! No moving parts.

    So... If I had a suitably strong pressure vessel that the rocket motor fit into and had more friction fit than is typical of a piston launcher, could I not get the internal pressure to build up enough to add that "kick" to the rocket and thus eliminate some of the variable? You'd have a known, constant volume. The only variable would be the amount of friction necessary to keep the model attached until sufficient pressure had built up.
     
  9. Jun 26, 2017 #9

    Nytrunner

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    I'm no expert, but I think the linear travel of the piston helps guide the rocket (like a launch rod) until it pops off.

    If you use a static container, you better hope like all getout that it detaches uniformly and enough pressure is built up to make it stable immediately.
     
  10. Jun 26, 2017 #10

    TangoJuliet

    TangoJuliet

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    In some cases that's true. Some others still use launch guidance, like a tower, or even a rod. The purpose of the piston is more for the added punch from the pressure build-up in the piston which translates to added altitude. It just seems to me that there's some inefficiency in the moving piston itself. Maybe there's some other physics I'm not understanding about it.
     
  11. Jun 26, 2017 #11

    retortec

    retortec

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    I was thinking the same thing until I sat down and really thought about the lectures posted earlier in this thread. The moving piston does help with rocket alignment and stability but it provides a lot more. As the chamber expands the pressure builds in spikes. The spikes build until the seal with the rocket is broken. The spikes far exceed the pressures found in a static chamber. I was thinking a larger piston could preform in a similar fashion but the number of pressure spikes falls off and performance diminishes. I am currently playing with 20" to 30" prisons and getting some real performance boosts.
     
  12. Jun 27, 2017 #12

    TangoJuliet

    TangoJuliet

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    I wish I could see this visually. I can understand the pressure spikes I think. But the way I'm visualizing it, the highs are a function of the friction of the piston head inside the cylinder and the lows are due to the expanding piston chamber, so I still fail to see how the pressure could be greater than a fixed pressure vessel. At ECRM a week and a half ago, one of the other competitors also mentioned using longer 30-36" pistons, while John (Micromeister) claims to have success with much shorter pistons. I suppose it's worth some experimentation to better understand.
     

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