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-   -   Floating Head Piston Questions (http://www.oldrocketforum.com/showthread.php?t=20415)

shockwaveriderz 04-04-2022 11:52 AM

Floating Head Piston Questions
 
Would it be safe and correct to say that the height that a traditional floating head will fly once it leaves the piston rod, with the rocket attached, would be determined by the engine used?

If this is so, what kind of heights are we looking at ? 5ft ? 10ft? More?

I'm specifically talking about 1/4A3, 1/2A3 and A3 13mm motors.

Different motors with different thru-time curves are going to pressurize the piston differently?

it also seems to me that wind speed would have a detrimental effect on the rocket/piston tube assembly. Anybody ever seen such a combination hit with a strong wing and noticed how much it tilts before separation of the two?

astronwolf 04-04-2022 12:26 PM

Quote:
Originally Posted by shockwaveriderz
Would it be safe and correct to say that the height that a traditional floating head will fly once it leaves the piston rod, with the rocket attached, would be determined by the engine used?


Sounds like a nice NARAM R&D project for you. Even though floating head pistons are kind of passé these days, a decent report might still get you more than flight points.

tbzep 04-04-2022 12:38 PM

Quote:
Originally Posted by astronwolf
Even though floating head pistons are kind of passé these days
What is the current weapon of choice in national/international competition?

astronwolf 04-04-2022 03:39 PM

Quote:
Originally Posted by tbzep
What is the current weapon of choice in national/international competition?

Floating head pistons have been banned from international competition. It's fixed pistons only. I think you can still use the floating head variant in US competition, but I haven't checked lately.

tbzep 04-04-2022 03:46 PM

Quote:
Originally Posted by astronwolf
Floating head pistons have been banned from international competition. It's fixed pistons only. I think you can still use the floating head variant in US competition, but I haven't checked lately.

Are they using the typical zero volume piston that's been around since the 70's, or have they come up with something different in the fixed piston relm?

shockwaveriderz 04-04-2022 04:41 PM

Quote:
Originally Posted by tbzep
Are they using the typical zero volume piston that's been around since the 70's, or have they come up with something different in the fixed piston relm?


it's basically the same design, Except now they have design upgrades like grooved teflon piston heads, some had pull pins to release a piston collar clamp, although that seems to have petered out. I never used a grooved teflon piston head but I may have been the 1st person to use a teflon piston head back at the Flyoffs at NARAM-47. Some use springs to absorb the piston head stop shock. A new feature is to use 10mm diameter piston tubes with a 13mm on top to fit the engine. Apogee Newsletter #428 has a good look at some Bulgarian and Ukrainian pistons.

shockwaveriderz 04-04-2022 04:45 PM

Quote:
Originally Posted by astronwolf
Floating head pistons have been banned from international competition. It's fixed pistons only. I think you can still use the floating head variant in US competition, but I haven't checked lately.

wolf, your correct on both counts. I'm not 100% sure but I think they were banned for safety reasons and perhaps they might have just been a little better than the USA's competitors.

the safety reason being, that the piston tube which might be made out of fiberglass, kevalr or carbon fiber that's 2-3 ft onlg might bounch off somebodies head.

georgegassaway 04-04-2022 05:41 PM

Floating Head pistons are still fine for NAR contest flying.

Gus 04-04-2022 11:56 PM

Would it be safe and correct to say that the height that a traditional floating head will fly once it leaves the piston rod, with the rocket attached, would be determined by the engine used?

Motor plays a part. Strength of fit between motor and piston tube is very important. Piston tube length and weight are very important. Piston head plays a role. Rocket weight is an important part.

If this is so, what kind of heights are we looking at ? 5ft ? 10ft? More?


For both floating and fixed head pistons the rocket generally leaves the piston tube a fraction of a second after the aft end of the piston tube hits the piston head. Almost no difference between fixed and floating heads. The old theory was that floating head pistons sailed some ways above the rod before the rocket leaves the piston. Not actually so.

I'm specifically talking about 1/4A3, 1/2A3 and A3 13mm motors.


These 3 particular motors behave very similarly on a piston. A10s are a bit different with their much quicker burn.

Different motors with different thru-time curves are going to pressurize the piston differently?


Correct

it also seems to me that wind speed would have a detrimental effect on the rocket/piston tube assembly. Anybody ever seen such a combination hit with a strong wing and noticed how much it tilts before separation of the two?


Most U.S. fliers fly a piston in a tower. Wind is rarely a problem. Many Europeans tend to fly naked, piston without a tower. Wind doesn't affect their pistons but will affect the rocket itself once it leaves the piston.

Gus 04-05-2022 12:02 AM

Quote:
Originally Posted by tbzep
Are they using the typical zero volume piston that's been around since the 70's, or have they come up with something different in the fixed piston relm?


Many of us now use a short tube (4"-6") on top of our piston tubes creating an air gap between the rocket motor and the piston head. With black powder motors this is less for performance than it is for ease of use with fixed head pistons. Added benefit is that most of the soot from the motor ends up in the buffer tubes making repeat use of the longer piston tubes more possible.

You can piston launch composite motors, like the Quest Q-Jets , or other Aerotech single use motors if you use a buffer tube to prevent chuffing. Without such a buffer tube composites have difficulty igniting in a zero volume piston.


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