Very cool! Are these designed to be removable/replaceable? If so, having the Kevlar shock cord in the middle won't be a major problem as it can be replaced/inspected regularly as required.
My Removable BT-20 Baffle System
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Thanks -- I made them 18mm so should be removable but I suppose it could be glued in place.
What do you like better -- 4x 1/2 baffles rotated 90° at each step or 4 baffles alternating right / left (basically rotated 180° at each step). Or do you prefer the full baffle with holes at different orientations?
I would assume that these might eject and push the parachute out which I suppose is also a successful outcome.
I was thinking I would include some of these for free with the Coleoptere kits I am selling right now so that I could get some feedback on how they work. If you are willing to pay for shipping I am happy to send you a couple.
I don't have a big reference with baffles in terms of overall design. I focus my designs on the half-moon style because that's easy for me to make by hand, appear to be easy to maintain (shake out loose bits after a launch) and seem to work well to protect the parachute. I guess if I had to choose one design, I'd pick the 4 1/2 bffles rotated 180 degrees of each other (similar to the half-moon design I'm using).
If these get ejected with the parachute, they could be like a piston baffle hybrid, eh? Hmmm, that might be a great idea in that if they get caugh up on something and can't move like a piston should, they allow gasses to pass through and the recovery system can be ejected that way. But they might need to be modified to restrict the flow of gasses a bit more than a conventional baffle, though? Seems like an interesting idea...
I'd love to try your designs out, but I am limited in the number of engines that I have. And of the ones I do have, I have to ration them with respect to the testing and launching that I do. In other words, because I don't have enough engines to do the testing I want, I know I won't be able to properly test your designs. Thank you for the offer, though!
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An extra plus for baffles is that they can double as a shock cord attachments, with the added benefit of protecting the shock cord from the ejection blast.
I theorize that one of the major causes of MOTOR MOUNT WITHOUT BAFFLE shock cord failure (including Kevlar, which is somewhat heat resistant but definitely not flameproof) Is ejection gas burn through, as the cord attachment at the motor mount just above the motor is where the heat is the greatest.
@hcmbanjo published a relatively easy way for installing an easily examinable (and if necessary replaceable) shock cord motor mount of non-minimum diameter rockets.
https://www.apogeerockets.com/education/downloads/Newsletter338.pdf
it just hit me that the mount design cannot be used for minimum diameter rockets, but the solution there is outside the box (or at least outside the tube.). Put the Q-tip tube OUTSIDE the rocket in the fin root and run the fillet over it. Minuses are might look a but funny (although if you ar @neil_w you just put a decal over it labeling it as the “Plasma Recirculation Duct”) and adds a touch of drag, pluses are that it actually strengthens the fin root and obviously it gives you a way to regularly inspect and if necessary replace your shock cord.
I am woefully ignorant on 3D printing. I would guess though that the plastic is melted into filaments, so there must be some heat limitations of the finished product. Would it function well as a baffle? is there enough room to coat exposed parts to JB Weld or other surface protectant?and if so would that be enough?
ABS has a melting point at around 200° C and is extruded at 255° C. I have used it for numerous parts that are in line with ejection charge or next to thrust flame but have not tried to directly stop an ejection charge. If it is a removable part you could always scrap it after a few uses. I will have to test them.
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Heretic theory:
Interestingly, with original gap staging it was assumed that PARTICLES of flaming propellant were forced into the sustainer motor, whereas Emma Kristal gave a nice NARAM presentation that concluded it was the hot gases (and their accompanying infrared radiation) that went easily through the nozzle of the upper stage to light the small area of exposed propellant.
Conversely, the assumption is that BAFFLES work by preventing the hot gases from reaching the chute. But realistically, the volume of the air in most baffles is small, so the hot gases DO eventually get through . The baffle DOES serve to elevate the chute AWAY from the direct front of the motor, but I am less sure that the small volume of ambient temperature gas is sufficient to get the chute out before all the hot gases pass through.
I have noticed with wadding when I failed to use enough, and with my “tubeless” balsa helicopters, that there are two areas where burns seem to focus. One is directly in front of the motor—- makes sense, that is where heat at ejection is most concentrated. But often on parachutes and my tubeless balsa (I now add protection, either glued on aluminum foil or Mylar tape) that the burns on balsa or melted areas on chutes, were often very localized, as if they were caused by hot burning particles.
Remember that regular delay black powder motors and dedicated zero delay booster motors have a very different internal “stack”.
The zero delay motors have only two components, from back to front
clay nozzle
solid propellant slug.
The standard rocket delay motor has FIVE components
Clay nozzle
Solid propellant slug
Delay slug (generates smoke but little if any thrust)
Ejection charge (not sure if same stuff as main propellant but generates a big propellant force or “over pressure”)
Clay cap.
Surprisingly, from Ms. Kristal’s research project, the in BOOSTER motors the solid propellant slug seems to burn completely, even though it breaks through, it doesn’t generate much in the way of particles.
The STANDARD DELAY, motors, on the other hand, definitely generates particles (as a minimum, the clay cap fragments) and whether it is hot clay or I’d guess true propellant fragments THESE are one if not THE major source of chute burns.
I am cheap and fly scratch built rockets with home made plastic chutes. Maybe the more dedicated rocketeers (@Ronz Rocketz , @kuririn , etc) that fly low power with fabric chutes can chime in. When you do have chute burns, are they more commonly broad areas (which would be expected from failure to stop hot cases effectively) or small burn holes (more suggestive of impacted hot particles?,)
Relevance to baffle design is you only need to coat or otherwise protect the sides facing the motor, and maybe the lateral walls.
One of the questions that has always “baffled” me is why the standard cardboard tube in front of the motor doesn’t routinely (in fact, in my admittedly limited experience almost never) catches fire. I think the reason is exposure time, yes it gets very hot but only for an extremely short time (but apparently long enough to light a sustainer motor’s exposed propellant!). Cardboard burns easily enough but it DOES require a finite time of exposure, and an unimpeded ejection charge (even with baffle or sufficient wadding) blows through so fast it does not have time to burn the tube, even for minimum diameter. An EXCEPTION is sub minimum diameter pop pods, which I have used for rear eject recovery or pop pod gliders (tricky for gliders, but yes you CAN use a minimum diameter motor on a pop pod glider.)
https://www.rocketryforum.com/threa...w-sr-73-raven-glider-and-new-gyskelion.38883/
Anyway, if you DOWNSIZE the body tube just forward of the motor, either the hyperconcentrated heat AND/OR the small residual flame that lasts a fraction of a second after ejection (really well seen here on the second and third tests of the B motors just AFTER ejection fires)
Is sufficient to burn through after only 1 or 2 flights (which is why I put a rolled up aluminum can piece here when I used a small diameter tube in front of a larger diameter motor.)
Anyway, would love a cheap, removable baffle that would also work as a shock cord mount for low power, and would fit in a BT-20 or BT-50. What I HATE about the standard tri-fold mounts is they break up the smooth surface of the inside of the tube, and sometimes impede deployment, especially when recovery gear space is very limited.
Interestingly, with original gap staging it was assumed that PARTICLES of flaming propellant were forced into the sustainer motor, whereas Emma Kristal gave a nice NARAM presentation that concluded it was the hot gases (and their accompanying infrared radiation) that went easily through the nozzle of the upper stage to light the small area of exposed propellant.
Conversely, the assumption is that BAFFLES work by preventing the hot gases from reaching the chute. But realistically, the volume of the air in most baffles is small, so the hot gases DO eventually get through . The baffle DOES serve to elevate the chute AWAY from the direct front of the motor, but I am less sure that the small volume of ambient temperature gas is sufficient to get the chute out before all the hot gases pass through.
I have noticed with wadding when I failed to use enough, and with my “tubeless” balsa helicopters, that there are two areas where burns seem to focus. One is directly in front of the motor—- makes sense, that is where heat at ejection is most concentrated. But often on parachutes and my tubeless balsa (I now add protection, either glued on aluminum foil or Mylar tape) that the burns on balsa or melted areas on chutes, were often very localized, as if they were caused by hot burning particles.
Remember that regular delay black powder motors and dedicated zero delay booster motors have a very different internal “stack”.
The zero delay motors have only two components, from back to front
clay nozzle
solid propellant slug.
The standard rocket delay motor has FIVE components
Clay nozzle
Solid propellant slug
Delay slug (generates smoke but little if any thrust)
Ejection charge (not sure if same stuff as main propellant but generates a big propellant force or “over pressure”)
Clay cap.
Surprisingly, from Ms. Kristal’s research project, the in BOOSTER motors the solid propellant slug seems to burn completely, even though it breaks through, it doesn’t generate much in the way of particles.
The STANDARD DELAY, motors, on the other hand, definitely generates particles (as a minimum, the clay cap fragments) and whether it is hot clay or I’d guess true propellant fragments THESE are one if not THE major source of chute burns.
I am cheap and fly scratch built rockets with home made plastic chutes. Maybe the more dedicated rocketeers (@Ronz Rocketz , @kuririn , etc) that fly low power with fabric chutes can chime in. When you do have chute burns, are they more commonly broad areas (which would be expected from failure to stop hot cases effectively) or small burn holes (more suggestive of impacted hot particles?,)
Relevance to baffle design is you only need to coat or otherwise protect the sides facing the motor, and maybe the lateral walls.
One of the questions that has always “baffled” me is why the standard cardboard tube in front of the motor doesn’t routinely (in fact, in my admittedly limited experience almost never) catches fire. I think the reason is exposure time, yes it gets very hot but only for an extremely short time (but apparently long enough to light a sustainer motor’s exposed propellant!). Cardboard burns easily enough but it DOES require a finite time of exposure, and an unimpeded ejection charge (even with baffle or sufficient wadding) blows through so fast it does not have time to burn the tube, even for minimum diameter. An EXCEPTION is sub minimum diameter pop pods, which I have used for rear eject recovery or pop pod gliders (tricky for gliders, but yes you CAN use a minimum diameter motor on a pop pod glider.)
https://www.rocketryforum.com/threa...w-sr-73-raven-glider-and-new-gyskelion.38883/
Anyway, if you DOWNSIZE the body tube just forward of the motor, either the hyperconcentrated heat AND/OR the small residual flame that lasts a fraction of a second after ejection (really well seen here on the second and third tests of the B motors just AFTER ejection fires)
Is sufficient to burn through after only 1 or 2 flights (which is why I put a rolled up aluminum can piece here when I used a small diameter tube in front of a larger diameter motor.)
Anyway, would love a cheap, removable baffle that would also work as a shock cord mount for low power, and would fit in a BT-20 or BT-50. What I HATE about the standard tri-fold mounts is they break up the smooth surface of the inside of the tube, and sometimes impede deployment, especially when recovery gear space is very limited.
I have thought about this very thing, and here's my discussion addressing it:
https://www.rocketryforum.com/threads/baffling-observations-about-size-and-girth.174573/
TL;DR: "I think what's happening is that the baffle is providing a small delay in the time it takes for the damaging hot gasses and bits to hit the recovery system...But the baffle can only do so much for so long. If the recovery system hasn't left the main body tube "on schedule" what's getting spewed by the baffle will be just as hot and damaging to a recovery system as if there was no baffle to begin with."
I have experienced two types of parachute burns. First, there's the type where tiny holes get melted into the parachute, resumably due to hot bits from the ejection charge. But I have also experienced a melted parachute even though the disposable wadding prevented ALL hot bits from making direct contact with parachute. In other words, it was the heat that melted the parachute, not the burning bits. And when I examined the disposable wadding, the part that was in direct contact with the ejection gasses and bits was completely black, like I sprayed it with black spray paint. But the sheet of the disposable wadding kept its integrity and did not break, tear, split or otherwise have an opening. It's this second type of parachute "burn" that I discuss in the above link.
I also think that the area of the main body tube just above the engine (where the ejection charge occurs) survives for as long as it does is primarily because the ejection gasses and hot bits move parallel to the main body tube walls. Contrast this with a recovery system, disposable wadding or baffle which will come into perpendicular contact with the hot gasses and bits. This is just my take and opinion, I very well could be wrong...
Mostly quarter size holes. I had one with burn marks that look like an art exhibit but it's in the Bertha that I lost Yesterday. I rarely get holes from a baffle. The only problem with them is that it takes away 5 to 6 inches of the BT for laundry. In my Super Mars Snooper, I have to put the altimeter into the NC which so far has been working fine.
My understanding is that most baffles create a convection which traps the burnt particles in the centrifugal vortex but allows the pressure wave through. The offset half-circle baffle creates the vortex but not sure about offset holes and if that creates much of a vortex inside. The multiple chamber baffles like Qualman are not so much vortex but more like entrapment within a maze. I really like the Qualman baffles because they're sturdy and cheap. They do add some weight to the back of the rocket. They're easy to attach the shock cord to using an eyelet screw. I still attach a Kevlar cord because then I don't have to worry about threading a replacement elastic through the eyelet.
In the past, I sprayed the nylon chutes with fire retardant but not sure where I put the bottle lately.
https://www.amazon.com/gp/product/B003VASNKU/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1
This is not quite what you are talking about (motor tube burn through), but I found it interesting nonetheless:
https://www.apogeerockets.com/education/downloads/Newsletter418.pdf
I've started using either the Apogee outer reinforcement, or a thicker motor tube after reading this.
Hans.
Very interesting read, thank you.
I would imagine the removable baffle with the addition of the dead space in front as a built-in feature should hopefully provide a similar benefit as the outer sleeve of the stuffer tube. I hope to test this in the near future. One idea I have is to build a largely stock Estes Patriot, but replace the stock MMT with a longer MMT that can hold the thrust ring, removable baffle (70mm long, not 53mm long) and standard 18mm engine. I'll probably use an Estes 18mm retainer ring in the back, too.
hcmbanjo
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Here's some "Baffle Autopsies" showing very little damage after a few flights.
These are the Centuri style disk baffles.
https://modelrocketbuilding.blogspot.com/search?q=autopsy
Thanks for sharing. Those definitely look to be in much better shape than my removable baffle, although I wonder how much of that is due to the use of a BT-60 (or similar) rocket instead of a BT-20.
How far away was that Centurion baffle from the top of the engine?
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I also found that interesting reading.
one of the mysteries is
same size motor, same size tube, why the burn through when used as a long motor mount but not for a minimum diameter rocket?
Kind of blows my theory of the “persistent flame” for a few microseconds post ejection out of the water.
I like the idea that the Bernoulli effect “sucks”* the motor tube INWARD and causes the ejection charge to either concentrate within or possible even directly (or partially directly) focus on the tubing, particularly the “concentrate” part as it would correlate with what I am seeing when I downsize the chimney.
*”sucks”: okay, no such force as “suction”, technically creates a situation where the pressure inaide the tube is less than outside the tube, so the greater force is OUTSIDE pushing the tube inward. In a flying minimum diameter rocket there is airflow OUTSIDE the rocket, not sure what velocity is INSIDE compared to outside, thus no easy way to know the pressure difference easily, but evidentially insufficient to cause significant inward deformation of the tube since the effect does not occur in minimum diameter rockets.
back to baffles.
the fact that 1/2 moon baffles work has “baffled” me a bit, I think it supports a linear particle theory.
Rational: 1/2 moon baffles really shouldn’t slow down the ejection gas flow that much, if anything they may accelerate the velocity a bit as they restrict the area.
ejected PARTICLES can take at least two paths,
“Go with the flow”: the particles deviate and scoot around the baffles with the gas flow
or
”Linear”: the particles travel at least INITIALLY in a straight line. With three 1/2 moon baffles they MUST impact at least one of the baffle plates, and maybe only after bouncing pack and cooling a tad they follow the airflow ooit and scoot around the baffle plates
if they “go with the flow” I would think that 1/2 moon baffles wouldn’t work well, but we know they do.
an interesting static test would be to set up a gap booster rocket on a test stand with a three half-moon baffle in place. It really shouldn’t effect the heat of the gas that much (I have gapped over 50 inches successfully.). Would be interesting to see if the sustainer lights.
sounds like I have a project to try!
I'm following this thread closely, as I put baffles in everything. I detest wadding. Half the time it doesn't even work for me, as I have had many instances of "wadding blow by" where the chute still got singed.
Anyway, I'm interested in the longer term durability of baffles - clogging, burning, or?? Good to see decent experimentation to find out. And the idea of a removable baffle is one of those simple ideas that no one has thought of before. My current thinking after reading this is to use a long "stuffer" tube with the motor and baffle stacked (with a spacer). Remove both after use. Easy.
Hans.
Anyway, I'm interested in the longer term durability of baffles - clogging, burning, or?? Good to see decent experimentation to find out. And the idea of a removable baffle is one of those simple ideas that no one has thought of before. My current thinking after reading this is to use a long "stuffer" tube with the motor and baffle stacked (with a spacer). Remove both after use. Easy.
Hans.
Go for it! I plan on doing this set up too (the only difference being that I think my "spacer" will be built into the baffle), although I more than encourage someone else to beat me to it and to report their results. Then I will have the pleasure of confirming their findings/observations.
Baffle 1.1 - Testing
I conducted two ground tests with Baffle 1.1 and my Estes Wizard (that's now painted and 100% finished). The first test was with a C6-7 engine and it worked perfectly. The ony issue I noticed was some bubbling of the paint around the engine area of the rocket, presumably due to the heat. I assume the paint was fully dry, as I finished painting it about a week ago and even used a dehumidifier to help it dry.
The second test was with a C6-5 engine and the test was perfect, except the engine shot out the back of the rocket. However, the nose cone, disposable wadding and parachute ejected just fine.
Here are a few pictures of Baffle 1.1 after 10 total tests (8 with an A8-3 engine and 2 with C6-X engines):
As you can see, the baffle seems to look about the same. The only difference was that the center support rod made out of balsa had burned away near the top of the baffle (furthest away from the ejection charge). It may have also burned away near the bottom of the baffle, but I can't see inside that area of the baffle.
Preliminary Conclusions About Baffle 1.1
When making another baffle of this size (53 mm long and for a BT-20 rocket), I will coat the inside with epoxy or glue. I also believe I can get 6-8 launches out of this disposable/removable baffle, even when using a C BP engine. If using an A engine, I have no doubt I can get 10-12 launches. If using a B engine, I'd estimate I could get about 8-10 launches.
Next up, I will test Baffle 2.0, that's 70mm long and has JB Weld epoxy coating all parts of the interior of the baffle. It was also made out of a spent engine casing. These tests will be primarily (if not exclusively) with C BP engines.
I conducted two ground tests with Baffle 1.1 and my Estes Wizard (that's now painted and 100% finished). The first test was with a C6-7 engine and it worked perfectly. The ony issue I noticed was some bubbling of the paint around the engine area of the rocket, presumably due to the heat. I assume the paint was fully dry, as I finished painting it about a week ago and even used a dehumidifier to help it dry.
The second test was with a C6-5 engine and the test was perfect, except the engine shot out the back of the rocket. However, the nose cone, disposable wadding and parachute ejected just fine.
Here are a few pictures of Baffle 1.1 after 10 total tests (8 with an A8-3 engine and 2 with C6-X engines):
As you can see, the baffle seems to look about the same. The only difference was that the center support rod made out of balsa had burned away near the top of the baffle (furthest away from the ejection charge). It may have also burned away near the bottom of the baffle, but I can't see inside that area of the baffle.
Preliminary Conclusions About Baffle 1.1
When making another baffle of this size (53 mm long and for a BT-20 rocket), I will coat the inside with epoxy or glue. I also believe I can get 6-8 launches out of this disposable/removable baffle, even when using a C BP engine. If using an A engine, I have no doubt I can get 10-12 launches. If using a B engine, I'd estimate I could get about 8-10 launches.
Next up, I will test Baffle 2.0, that's 70mm long and has JB Weld epoxy coating all parts of the interior of the baffle. It was also made out of a spent engine casing. These tests will be primarily (if not exclusively) with C BP engines.
Looks like the outside is charred. Correct? If so, there may be some "blow by" going past the outside of the baffle.
Hans.
Hans.
Possibly, but I doubt it. Because I added the index card on one side of the baffle, it's a much tigher fit and no longer slides out with a few shakes. So I doubt any "blow by" would be greater now than in the initial 8 tests with the A8-3 engines. I'm pretty sure much of the "dirtiness" of Baffle 1.1 during the 2 C engine tests is the result of residue from the inside of the rocket's main body tube rubbing off.
Hmm.... Hadn't thought about that.
So the removable baffle is both a baffle and a tube scrubber?
Hans.
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So you think you got a bit of both, particle burns and hot gas melting?
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Opens up another rabbit hole. May be something @BEC , @ksaves2 , or @prfesser can answer.
i figured all Estes 18mm motors, while having different propellant loads, had the SAME ejection charge, if you are getting different (lower) burn throughs from As the Cs, either the previous statement is NOT true, or the longer unused forward motor casing in the A motor (I think it is only half full, where a C is packed near to the tip.) provides enough space to significant cool the gases before they hit the baffle. Kind of doubt that, as the length is not that much, but I don’t have any other ideas. I guess the immediate “pre-ejection” empty chamber is larger for the Cs, maybe that’s a factor?
Given that the gas and particle effects are so difficult to separate, it seems the most definitive way to test this experimentally is to eject a packed chute with equivalent hot gas independent of any motor action, possibly from a cylinder or pump paired with an exceptionally powerful heating element, and measuring the effects of this versus packed chutes exposed to hot particles propelled mechanically, perhaps by gravity.
Temperature would be challenging to control, equipment design/acquisition would be tricky at best, and the generated data would be crude, but it should at least give a general idea what kind of damage both of these combustion products inflict, under which conditions they’re inflicted, and what the damage pattern looks like for each.
I’d expect melting, fusing, and ash staining from gas and burning/holes from particulates.
We know a well-designed baffle will protect from particulate effects by simply blocking a straight-line path from the forward end of the motor, but how they protect from gas damage, we have only a vague idea. Perhaps it may be worth looking into the design of things like firearm suppressors to see which considerations are important in gas cooling, then combining that with the maze or serpentine arrangements that trap particles.
Temperature would be challenging to control, equipment design/acquisition would be tricky at best, and the generated data would be crude, but it should at least give a general idea what kind of damage both of these combustion products inflict, under which conditions they’re inflicted, and what the damage pattern looks like for each.
I’d expect melting, fusing, and ash staining from gas and burning/holes from particulates.
We know a well-designed baffle will protect from particulate effects by simply blocking a straight-line path from the forward end of the motor, but how they protect from gas damage, we have only a vague idea. Perhaps it may be worth looking into the design of things like firearm suppressors to see which considerations are important in gas cooling, then combining that with the maze or serpentine arrangements that trap particles.
I disagree that the differences in the location of the ejection charge in the motor casing are negligible.
When looking at a C engine, the clay cap is just a few mm below the end of the casing. But with the A engine, the clay cap is roughly halfway down the casing. That's potentially a 30mm+ difference. And remember, the baffle location installtion rule of thumb is at least 1 main body tube width above the end of the engine. In a BT-20 or BT-50 rocket, this amount to more than a 1 body tube differene even before we get to the end of the motor's casing.
I think one way to do a test is to drill out a small hole in the clay cap and test the engine. If done correctly (or luckily), there will be a signficant reduction in the amount of ejected particles in relation to the hot gases. One could also put a fine metal screen as a barrier b/w the engine and the recovery system to further catch some burning bits (but still let the hot gasses through). I think doing this test in a BT-50, 55 or 60 rocket will be best, to provide a wider diameter for the gasses/bits to flow and compensate for the reduced gas/bits flow caused by the fine metal screen.
Maybe we can arrange some sort of group test where 1 person does the testing with...50 engines or w/e, but many of us chip in a few bucks to help fund the research.
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You have a point, I may be comparing apples to kumquats. My experience is with long gap staging, which is a wholly different animal, I got a kick out of an Apogee article by Tim Van Milligan (who is obviously very intelligent and experienced, not many rocket companies have survived to compete with Estes!) that said you could gap stage up to around 11 inches, I have routinely done 36 or more for years. So from that perspective the difference in empty space at the forward end of the motor is negligible. But pretty much for baffle usages, you are using either a black powder motor with an EJECTION CHARGE and a CLAY cap, or a composite motor. I don’t use many composites, but I know they have and ejection charge and I think some plastic cap or something . I am curious if the composite burn through pattern (when you don’t use enough wadding or have a baffle) is DIFFERENT between black powder and composite, maybe the CLAY fragments get hot?
anyway, I squat corrected, the length of the empty space between an A And a C may make a significant difference in burn through.
I also SUSPECT that simply moving the baffle further from the forward end of the motor SIGNIFICANTLY improves the life expectency of the baffle.
That’s a plus, another plus is the more forward the baffle, the better the effect on CG.
another plus is it provides a “shelf” for your recovery gear, keeps it from slide backward from acceleration at launch, so you recovery gear both stays closer to the nose AND also positively impact CG.
The minus (or at least one minus) is the further forward you move the baffle, for a rocket of given length, the less space you have between the baffle and the nose cone for your recovery gear (shock cord and chute or streamer.). So long as you aren’t building scale or a longer rocket doesn’t bother your aesthetic sense, this is pretty easily achieved by just lengthening the body tube.
in practice, that was a BIG difference between Estes Cosmic Cobra and the Helicat. They are ALMOST the SAME rocket, the only differences that I can see are the Cobra is 19.5” long and since you have to pack the BLADES folded inside the rocket, it is a nightmare to get the blades, the chute, and the wadding in there and still have it “loose” enough for ejection. The Helicat 30.25” long and that extra 10.25 inches make it a dream to pack. The other difference I think is mainly cosmetic, both have plastic fin cans (I think these are essentially ready to fly or nearly ready to fly), the Cobra fins are angle forward and the Helicat backward. As for the unreliability of the Cobra due to tight packing leading to either failed deployment or a melted chute, just do a search on this forum for Cosmic Cobra and you will see others have had same problem.
back to baffles, the closer you put the baffle to the nozzle (the further back you put it), the “tougher” you need it to be and the sooner it is going to have to be replaced. Since baffles have more mass than wadding, which is close to negligible, this may be a factor both in the amount of materials (and thus $$cost) of the baffle and mass of the rocket. Food for thought.
My understanding is that many (most?) composite engines use blackpowder as their ejection charge.
I agree that you want the baffle as far away from the ejection charge as possible to improve baffle longevity (along with other benefits you mentioned). But I also think there's a "law of diminishing returns" going on here. For example (hypothetically), going from 10 to 20mm of space between the baffle and the end/top of the engine will make little difference. But going from 40 to 50mm might make a far bigger difference, as that might mean the difference between avoiding an ejection charge flame altogether or having direct exposure to that flame.
As an FYI, an Estes Wizard should be able to handle a 70mm long baffle and still have sufficient room above it for a 7 inch parachute (made out of thin HDPE from grocery bags), Kevlar and elastic shock cord of sufficient length and a Jolly Logic AltimeterTwo that's partially slid into the nose cone. (after removing the altimeter's outer casing). I think the Yankee can also make this happen, but the nose cone's shape is different, so the altimeter slides into it a bit less.
This assumes you're ok with sticking the engine a full 1/2 inch (instead of the 1/4 inch) out the back of the rocket. But that extra 1/4 inch shouldn't be a problem because the extra 10 to 15 grams (from the baffle and altimeter) are still located WELL ABOVE the Wizard or Yankee's Center of Pressure. I also prefer this extra 1/4 inch of rocket engine sticking out the back as it makes removing a spent engine easier.
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I don’t know of any hard or fast rules, but having the baffle 20 mm or less from the motor casing seems waaaaay too close. I am completely guessing, but I would expect that about 5 cm would be the minimum that I would want to have the baffle from the motor casing and also would further take a guess that having it much more than seven or 8 cm probably doesn’t gain you very much for low power. The problem is, for relatively small and particularly short rockets, this can potentially be an issue. A large number of my scratch built mainly BT-20 and BT-50 rockets are 18” long not including nose cone and fins, predominately because that’s the size the tubes come in. So LENGTHWISE room for a baffle isn’t really an issue. I do a lot of asymmetric fin rockets, and the extra length really helps the stability.
I also agree with you in that I like to have a little more engine casing sticking out if I can handle it. Part of that is that I will usually have about a quarter inch of tubing extending below the most caudal aspect of the fins that I put a mylar piece of tape around hot protect the paint from my tape wrap) and I will use an external tape wrap for motor retention which for low power has worked for me pretty well, in fact I’ve had engine hooks fail when a tape wrap doesn’t fail, and a further fact is that now even if I HAVE an engine hook I usually put a piece of tape around that to go with the belt and suspenders approacu (a term I think appropriated from @lakeroadster .)
It probably is, especially if you glue in your baffle and hope it'll last as long as your rocket does. But if you have a removable/disposable baffle made out of parts you'd be throwing away, it's not a big deal. And remember, the baffle I tested with eight A8-3 engines and two C6-X (Baffle 1.0/1.1) had only 20mm of space b/w the baffle and top of the engine.
The only question becomes how short can the life of the baffle be before replacing it becomes a burden. I'm guessing that for most LPR fliers, getting at least 8 launches (and that's being conservative) is sufficient, as long as the baffle's only cost is time. Personally, I'm aiming for 10, and I think that's why I'm probably going to make a 70mm long baffle (as opposed to a 53mm long baffle) my primary baffle. I could go with the 53mm (or even 50mm) baffle to save weight, but if I'm going to be that concerned with weight, I'll just skip the baffle and use wadding. I may eventualy try out a 50mm baffle for the heck of it, though.
Baffle 2.0
The next version of my removable baffle is complete. It weighs 6.0 grams with the stainless steel eyelet and the half-moon plates are made from card stock while the center support shaft is made from a wooden toothpick. All inner surfaces of Baffle 2.0 are coated with JB Weld. Here are a few pictures:
I will do my initial tests in a modified Estes Alpha III. The The main body tube is from an A***ee Apprentice and the fin can and nose cones are from the Alpha III; more pics of the finished rocket later. The MMT is basically a stock MMT that's longer to accommodate the removable baffle. I also installed an Estes 18mm retainer ring on it. Here's how it looks on the Alpha III so far:
I hope to do some testing tomorrow or this weekend. Will report back when I do.
The next version of my removable baffle is complete. It weighs 6.0 grams with the stainless steel eyelet and the half-moon plates are made from card stock while the center support shaft is made from a wooden toothpick. All inner surfaces of Baffle 2.0 are coated with JB Weld. Here are a few pictures:
I will do my initial tests in a modified Estes Alpha III. The The main body tube is from an A***ee Apprentice and the fin can and nose cones are from the Alpha III; more pics of the finished rocket later. The MMT is basically a stock MMT that's longer to accommodate the removable baffle. I also installed an Estes 18mm retainer ring on it. Here's how it looks on the Alpha III so far:
I hope to do some testing tomorrow or this weekend. Will report back when I do.
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