Baffles VS Pistons.... What's Your Choice? And, Why?

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K'Tesh

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In a recent post about the NCR SA-14 and the Aerotech Strongarm I made the comment that if either used a piston, I'd not build the piston, in favor of installing a baffle. Later I pointed out that pistons violate the KISS principle and are an invitation to Mr. Murphy's Law. I said that I've heard too many cases of pistons jamming/failing due to residue buildup. I stand by my position. However, this has now hijacked that thread. So, seeing the interest, I'm spinning it off here.

My baffle of choice is the 1/2 moon baffle (or a variant of it, such as Back_at_it's diagram seen in the image below).

1633465414265.png

or the version used by Binder Design for the Velociraptor (the baffle is visible just above the talon fins in the image below).

1633464961849.png


The advantage of the 1/2 moon baffle is that any residue that are loose inside the area between the baffle and the back of the booster section can be easily shaken out with some careful angling of the booster. The disadvantage that I can see is the plastic cap from Aerotech motors can be a source of damage to a weakly built baffle, or getting caught inside other baffle designs. I like to make my rockets zipperless, and as the coupler that forms the joint between the booster and the recovery bay is a good place, far from the motor, to put my baffle, and also acts as a shelf (with a small grill to prevent the parachute from falling into the back of the rocket) and thus decreases the odds of the parachute "hanging up" in the body tube, resulting in a poor recovery situation.

And now that I've said my part... What's your choice? Why? What are the advantages/disadvantages of it?

Pointy Side Up!
Jim
 
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heada

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Size and purpose. On DD, I like pistons over baffles. On non-DD, baffles over pistons. That also relates to size. LPR and MPR I go with baffles and HPR I go with pistons.

That said, I don't use pistons much anymore anywhere. They're a good idea but they're prone to issues. You have to clean the BT out well between flights and swings in environmentals (temp and humidity) can cause them to stick. Instead I use flame protectors (nomex blankets) and deployment bags. They're much easier to replace and don't have all the drawbacks of a piston. Out of all the pistons I've used, the PML set up is one of the better ones.
 

T-Rex

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I have never tried a piston. Been using baffles since I became a BAR.
 

teepot

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I don't use either one. What concerns me with baffles is the possibility that it will slow and cool the expanding gas. A laundry shelf I might use if I remembered to do it. From what I've read on the forum pistons seem prone to problems. I do use a shock cord protector and a Nomex blanket.
 

Voyager1

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I’ve used pistons quite successfully in the past, but I agree with Jims’ concerns about residue buildup and the possibility of jamming. When everything works well pistons are great, due to the increased efficiency and the subsequent reduction in the ejection charge volume required.

I have not had any experience with baffles, so I can’t comment on their use.

I typically just use Nomex blankets or deployment bags these days, although for my current FrenzyXL 4” build, I will be using a redundant CO2 ejection system.
 

DigBaddy

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I've got one rocket with a baffle (90 degree like the Qualman, but homemade with basswood), and no rockets with pistons, so I'm clearly over qualified to post about things here.

Love the baffle in that one rocket. I need to do more, I'm just usually not motivated enough to make them. No nomex, no wadding. Pack up the stuff in the tube and go. Shake and turn and the crud comes out.

No complaints about the rockets with nomex either. Works great. Less work building.
 

Charles_McG

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I have one rocket with both - it comes down in two pieces.

I've been using in pistons in dual deploy situations with very short payloads. I find they take up the least volume. But I've only just started using chute protectors, so I may change my mind.
 

bad_idea

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I've been using in pistons in dual deploy situations with very short payloads. I find they take up the least volume.
How is your piston laid out in terms of aspect ratio, bulkheads, and where the chute sits?
 

Tractionengines

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Estes kits get nomex blankets, and longer kevlar shock cords.

On MPR and HPR I like pistons. YES...they have some maintenance issues. 1) Clean the tubes after each flight. We should do this anyhow. 2) Check fit of piston. I keep them a little loose. Sliding easily will still hold enough pressure to get positive displacement of chute.

I just had a parachute failure the other week... I went without the piston in a rocket that just pops the nose. With no nose weight, it was too light to pull parachute and harness out. Ejection was just after apogee and the airflow was pushing the chute back in. After the "hard landing" I picked it up and tipped it nose down and the chute / harness falls out. Built a piston for it, used 2/3 the powder, had good recovery on the next flight

Mike
 

bad_idea

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Here’s my recent Nike Nike Smoke sustainer. In this case, the piston faces down. I tend to build the powder cup into the piston. A couple of mine use 24mm motor casings as charge wells.
View attachment 484452
Thank you for the details and the photo. I've been using pistons of 1:1 or longer aspect and have lately considered testing a much shorter piston. Your success with that layout is encouraging.
 

Tractionengines

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I use short pistons. I have also used centered bulkhead, as well as bulkheads high or low. Depends what other hardware is around, and which side needs more volume.

Here is the one in a LOC Cyclotron I just built this summer. 3" Dia. X 2" long, bulkhead centered.
20210922_212820.jpg 20210922_212942.jpg
The main body tube is removable from motor mount / fin can. Makes wiping it out very easy. As well as cleaning, checking, and replacing the kevlar harness.

Mike

( The whole thing of an article from years ago about binding, and which end is up etc. has some serious flaws in the analysis. )
 

Sooner Boomer

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The only time I feel like I need to use a piston is with a large diameter short rocket. Estes Fat Boy and Big Daddy are two mod rock examples. A LOC Warloc is an example going to the other extreme.

As much as possible, I build (or re-build) using 3/4 D baffles in a coupler. For extra strength, you can build them along a center "spine" rather than just gluing the baffle disks to the body tube.
 

beeblebrox

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I definitely prefer a baffle. My 3d printed ones work perfectly. the hole thru the center is for passing thru a kevlar line. Place a knot at the aft end and seal it with RTV blob. the separate disc is bonded on last. This is meant to built inside a standard tube coupler. Fillet the three fins inside the coupler before gluing on the top plate. The one below is for a standard, LOC/AeroTech 2.6" body tube. Pistons can jam, sometimes even having the shock line get pinched between the piston and the airframe. Also the line at the motor side of the piston can get burned anyway.

ALSO: Before anyone decides to troll me on this... These are made from PLA+ - more heat resistant than regular PLA. These have held up fine for many flights. An ejection charge does not supply enough heat for long enough to melt this design. The support is over a much longer length too (5.7"), compared to AeroTech baffles, which have been known to melt sometimes.

If anyone wants the STL files, PM me. This design could also be fabricated with plywood as well but would be more of a pain to build...
I have made these from BT-55 size thru 5.56" LOC tube.

1633536063298.png
 

icyclops

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I definitely prefer a baffle. My 3d printed ones work perfectly. the hole thru the center is for passing thru a kevlar line. Place a knot at the aft end and seal it with RTV blob. the separate disc is bonded on last. This is meant to built inside a standard tube coupler. Fillet the three fins inside the coupler before gluing on the top plate. The one below is for a standard, LOC/AeroTech 2.6" body tube. Pistons can jam, sometimes even having the shock line get pinched between the piston and the airframe. Also the line at the motor side of the piston can get burned anyway.

ALSO: Before anyone decides to troll me on this... These are made from PLA+ - more heat resistant than regular PLA. These have held up fine for many flights. An ejection charge does not supply enough heat for long enough to melt this design. The support is over a much longer length too (5.7"), compared to AeroTech baffles, which have been known to melt sometimes.

If anyone wants the STL files, PM me. This design could also be fabricated with plywood as well but would be more of a pain to build...
I have made these from BT-55 size thru 5.56" LOC tube.

View attachment 484543
Interesting design. I would think anytime the gas has to go up and then back down, then up and out you would lose some pressure…not so? Or not enough to really matter? Most of the baffle designs i have seen the gas moves in an upward or sideways motion as its just meant to control hot particles and gases….glad this baffle works good for you.
 

beeblebrox

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Interesting design. I would think anytime the gas has to go up and then back down, then up and out you would lose some pressure…not so? Or not enough to really matter? Most of the baffle designs i have seen the gas moves in an upward or sideways motion as its just meant to control hot particles and gases….glad this baffle works good for you.
Not so much the gas as the whole system is closed anyways, the big thing is catching burning chunks, powder grains don't burn instantly, they will bounce off the first wall they hit, rather than make the turn... This design is loosely based on the baffles sold by Semroc, which also work well. That one has tubes extending past each other.

1633622239390.png
 

MoeFaux

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Interesting design. I would think anytime the gas has to go up and then back down, then up and out you would lose some pressure…not so? Or not enough to really matter? Most of the baffle designs i have seen the gas moves in an upward or sideways motion as its just meant to control hot particles and gases….glad this baffle works good for you.
I think not so.

The ejection charge shouldn't be thought of as a puff of air that must travel a distance and can be dissipated by a bit of resistance, but rather as a building of pressure within a closed system. Black powder burns, which turns solids into gasses, which have a significantly larger volume than the solids, which means pressure is quickly raises. In other words, an explosion.

Remember also that, within a closed system, pressure anywhere is pressure everywhere. Thus, that expansion and building of pressure on the aft side of the baffle will be transferred to your recovery bay with great efficiency regardless of how many twists and turns exist between.
 

wrad

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I mostly just fly with blanket protectors only. I am yet to fly any rockets with baffles but plan to soon for low power anyway. As for pistons, I have used in the past with great success the pml system in my io and liked it, however it’s become redundant now as I extended the rocket for dual deploy, and just use blankets. Maintaining a good smooth fit is a small annoyance with pistons, however I have seen one get stuck resulting in quantum tube rocket using the fragmentation type recovery technique
 

Sooner Boomer

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Maintaining a good smooth fit is a small annoyance with pistons, however I have seen one get stuck resulting in quantum tube rocket using the fragmentation type recovery technique
This can definitely be a problem with pistons. My personal opinion is that you should design the rocket *from the beginning* to be used with/without a baffle, a piston, etc. If you didn't design the rocket, but a modifying an existing design, you have to think all the way through the operation of all the parts and pieces (esp. the ones out of sight on the inside!). The placement of the shock cord is *very* important! If you use a conventional tri-fold method of attaching the shock cord, the piston can (and with my luck WILL) hang up on it. The shock cord has to be attached (to the body tube) below the piston.

Piston design is important, too. I make mine styled somewhat like a shotgun wad. There is a central disk just slightly smaller than the ID of the body tube. Attached below this is a short conical skirt. There are a couple of ways of doing this (ie. pleats) so that the piston slides in easily, but will "inflate" (expand at the base) slightly to seal the body tube. The top portion can be a short cylinder, or can be more complicated utilizing "petals" that wrap around the chute, but will be peeled away in the air stream (or just fall off because of loose fit. The exact design will depend on body tube diameter and size of parachute.

One thing to keet in mind! (and this is true for ALL of your builds!) - consider how the force of thrust is going to cause everything to shift. Will it cause the piston to tilt to the side? Will that tilt cause it to bind? Will the piston be forced back into the body tube? And be in a bind there?

Pistons have thrir place. Maybe not in EVERY rocket, but once you get a good design, you can use it in many flavors of rockets
 

Back_at_it

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I'm solidly on the baffle side. As K'Tesh pointed out, I too prefer the halfmoon style baffle systems using either three or four plates. I generally use them in BT55 up to BT80 rockets but I have recently done one in a 3in project. I like this design for a couple of reasons. The first is it's simplicity.

While I would love to find someone that could cut me the halfmoons to fit inside various couplers, It's simple enough to cut what I need from hobby grade ply with a scroll saw. After all they don't have to be precise as the adhesive will fill / cover any gaps. The other reason I prefer them is the ease of removing any debris from the ejection. Simply shake the rocket side to side and the junk falls out.

When building these style baffles I tend to leave about one inch between the plates. I'm not sure if placing them any closer together would hurt performance or cause issues with the ejection but so far I have flown these in rockets up to a 29mm F67 without a single failure. Just build them from plywood for strength and be sure to place them at or forward of the CG whenever possible.

The other style I'm fond of is the shown in post #16. What is commonly referred the Semroc or LOC style baffle. I do make some changes from what is pictured. First is that I build it inside a coupler. The second change is that I don't overlap the tubes as much. As long as the ends of the tubes pass each other so that the burning embers can't jump from one tube to the next, there isn't a need for them to be any longer. Finally, I spread the tubes so that the tubes are against the inside wall of the coupler. This gives support of the tube at the opening and along the entire length of baffle tube.

This style has some advantages in that it can be made very short for use in short stubby rockets. A while back I used this style in a BT80 FatBoy Restoration. Since the body is short I couldn't fit a standard 3 or 4 plate half moon style as the baffle wouldn't leave room for the laundry. I built this style baffle using a BT80 Coupler that was 1.75in long. Each baffle tube was 1.0in long. They overlapped by 1/4in and the remainder stuck out the top and bottom of the upper and lower plate. That rocket was eventually upgraded to 24mm and has flown a several times without issue. I'll admit i was hesitant to build the baffle as short as I did as I thought it might hurt flow but no issues.

Here is a my hacked up version of the pic above that represents how I build them along with a couple pics from a recent build.

Baffle.jpg
9.jpg
12.jpg
16.jpg
 
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Cape Byron

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Baffles. We use them in every kit we can cram them into and we source them from Qualman. Great products and service.

Not a big fan of pistons. Poorly maintained pistons are a recipe for disaster. One piece of ejection 'grit' can ruin your day.
 

K'Tesh

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Not so much the gas as the whole system is closed anyways, the big thing is catching burning chunks, powder grains don't burn instantly, they will bounce off the first wall they hit, rather than make the turn... This design is loosely based on the baffles sold by Semroc, which also work well. That one has tubes extending past each other.

1634262167208.png
This is basically the same design as found in the Binder Design kits.
 

Mike Haberer

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Baffles. Pistons can bind. Lost a PML BullPup to a binding piston.

I prefer Qualmann baffles if I have the space to include them in the airframe. Nice design, easy to build, sturdy, not expensive. I build custom for anything great then BT-80.
 
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BEC

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I generally use baffles in anything BT-55 (1.3 inches) and bigger. I was a big fan of the Semroc cup-style baffle (for example https://www.erockets.biz/semroc-ejection-baffle-kit-conical-fiber-bt-60-sem-eb-60/) but I've sometimes had them blown out. That said, they are probably still the most common type in my fleet.

I've also used the two-offset-tube variety shown above in relatively large models (Estes PSII builders' kits for example) and have had a couple of those blown out/blown apart. I have also used the two-perforated-disc type that appears to trace back to some Centuri models and I've also recently tried a couple of David Qualman's units. The main driver for me is little to no wadding required, plus consistent placement of the recovery stuff in a long model.

I've not used pistons for recovery systems except for a couple of ejectable foam plugs in contest models. I've used piston launchers, though, so I have encountered the maintenance they entail.

I like as low maintenance as possible solutions....I'm lazy that way. :)
 

Jim Hinton

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I've used both, I find pistons to be superior at getting the laundry out of the airframe. There are two caveats. 1. maintenance matters. The piston and airframe should be cleaned after every flight. It doesn't need a lot of cleaning, remove loose abrasive debris. 2. The warning on AT reloads about the red plastic cap is true. If you are flying 29mm reloads in a 29mm mount, the red plastic ejection charge cap can hang up in the mount and create issues. Using tape and wadding to cover the ejection charge solves this issue. This can also be a big issue with baffles. Be sure to allow plenty of distance between the end of the MMT and the upstream end of the baffle. If this area is too restricted, the majority of the ejection gas will discharge through the nozzle and will frequently cause total ejection failure. Just a few lessons learned the hard way.

Jim
 

bad_idea

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The warning on AT reloads about the red plastic cap is true. If you are flying 29mm reloads in a 29mm mount, the red plastic ejection charge cap can hang up in the mount and create issues. Using tape and wadding to cover the ejection charge solves this issue. This can also be a big issue with baffles. Be sure to allow plenty of distance between the end of the MMT and the upstream end of the baffle. If this area is too restricted, the majority of the ejection gas will discharge through the nozzle and will frequently cause total ejection failure. Just a few lessons learned the hard way.
Thank you for this timely warning. Am working on a 29mm design now mostly for Aerotech mid-power reloads and had I not seen this might not have left enough room ahead of the motor. About how much space volume and/or length do you typically leave?
 

Jim Hinton

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Thank you for this timely warning. Am working on a 29mm design now mostly for Aerotech mid-power reloads and had I not seen this might not have left enough room ahead of the motor. About how much space volume and/or length do you typically leave?
That's a bit of a tough call, a wrong choice gets pretty ugly. In theory, a larger airframe needs less room. Overall, I would try to leave at least six inches between the motor or MMT (which one extends further forward) and the rear of the baffle. The baffle should allow fairly free flow of air. When using baffles with 29-40/120 reloads, you need to make sure the little red plastic cap (or other charge cover) doesn't get stuck in the baffle.

Jim
 

bad_idea

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Thanks. Had planned to put a baffle in the zipperless coupler of the fin can in a dual deploy design, less because it was needed than for expeience building it. Thinking now I'll leave the baffle out and just put my faith in a Nomex blanket for the backup motor eject for the drogue. Since I was going to use a blanket to protect from the apogee-fired charge anyway, the baffle was sort of illogical and just there to be there in this design, but I tend to try out a lot of things on the same rocket - not always necessary or optimal - just to gain experience. Will skip it this time around.
 
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