First HPR Build - Wildman Punisher 3

[cautery]

So, it is 3 days short of a year since I ordered the main materials for my Level I/II Cert. airframe. Time, money, et al. prevented me from getting started with the ACTUAL build, but I have spent many hours thinking through the project.... over, and over, and over.

Not much to report right now, but I wanted to get an image posted, reserve some posts right after this OP, and maybe ask a few questions that I need answered before they obstruct the crit. path.

I'll edit for complete specifications later, but here's a brief run-down:

Wildman Punisher 3 (no change... 54mm motor mount, HEDD)
Aeropack retainer (considered doing the 75TAC, decided to wait 'til a later build)
AT 54mm Hardware
AT I229T-14A for Level I
AT J135W-14A for Level II (if I do my part right!)
Drogue & main chutes, Kevlar cord, et al from Rocketman. Thanks Buddy! You WILL get a report after I fly!
Electronics: TeleMega v5.0 main computer, EasyMini v2.0 for backup, TeleBT v4.0 Ground Station, etc.

Here is the obligatory image to christen the build:



Should be a lot of fun!

Be well!

 

[cautery]

11222022 - Booster Free Volume Calculator (DRAFT) created. Tool to assist in determining the FREE/Open Volume in your booster for use in calculating an estimated Ejection Charge Weight and Vent Port Sizing.
12052022 - Centering Rings - Vents and Notches - #1 (Marking and Setup)
12052022 - Centering Rings - Vents and Notches - #2 (Drilling & Completion)

 

[cautery]

Tools, Techniques, et al. that I MAY use and want to keep notes on so I won't forget. Generally, it's a reminder list.

1) Cut sheer pin heads off flush.

2) Vent as much as possible aft through holes in the centering rings. O-rings glued over holes to reduce adhesive migration (rfjustin ?). Altimeter Vent Port Sizing says the 3 port diameter is 0.1794". Shear Pin Calculator spreadsheet from Speedmotion Rockets says 0.2226". Doable.... Try to increase sample size from other calculators, and if possible, review the background math.

3) 0.86 grams of BP recommended for booster/drogue. Start minimum 10% below and work up (0.774 g start). Record the ground testing.

HERE is where the tool creation starts below...

 

[cautery]

Reserved #3

 

[cautery]

Appears that I am accumulating a list of questions for which I need answers/decisions in order to progress.
Any opinions/recommendations/advice from y'all more experienced builders are greatly appreciated:

Questions:

1) Vent Band? Yea or Nay? I'm generally aware of the use(s) of the vent band... But I am not convinced that I want to use one. WHY would I? Why shouldn't I? I am leaning toward deleting it from the build to eliminate a body seam, reduce weight and a few tasks.

2) Vents: (x3 places?): Cert flights aren't going above 8k ft or so, but I still need to address vents as to purpose, number, and location/characterization. Ideally, I would have ZERO penetrations into the airframe that will "see" the air flow from flight. So, I at least want to minimize the sizes, et al. After cert, this WM Punisher is going to go really fast and pretty frick'n high.

a) Avionics Bay - Two baro. altimeters, so have to have vents in avbay. # and size? Location? I am leaning toward 3 vents for the avbay.... The calculator here: RocketryCalculator.com gives the following answer for a 2.875" ID on the AvBay, 5.8785" inside length of avbay, and 3 ports (ostensibly 120 separation at 180 degree phase shift with respect to "Front Fin line" (chosen as longitudinal datum).

3 ports? - 3/32" is slightly oversized.
1 port? - 5/32" almost exactly what the calc called out.... but I think it's big enough to get cooked....
Ideally, the avbay vents will serve double duty at the access hole(s) to the Schurter switch(es). And I am leaning toward 3 vents 180 degrees out from "front" datum line, mirroring the nosecone sheer pin locations radially about the airframe.... (neater lines; dressed)

b) Nosecone vent: Gonna have plenty of room for the main chute in the nosecone (HEDD), and I am using 3 x 2-56 sheer rivets to hold it on. I REALLY don't want to drill a hole in the straight section of the nosecone for a vent. Can y'all think of an option? Not sure how much internal pressure differential could be generated in that small volume (reduced by chute and cord) by altitude induced external pressure drop. Is there any trick for avoiding the nosecone vent? Could I vent through a tube through the forward bulkhead to the interior of the avbay (tube is to direct potential hot gases to a "safe place" inside the avbay (most of the sensitive bits will be inside a triangular sled. The nosecone vent (sized to actual volume with "payload" compressed to yield max volume would be a good deal less net flow area and thus could vent to avbay and save a hole in the nosecone.

c) Booster vent: Have not calculated the booster free volume yet, but clearly, this portion of the airframe generally has the most vacant volume. I am considering trying to figure out a way to vent the booster through the front centering ring and thence through the rear centering ring (a la flyfalcons). I think I can drill the front CR before I glue it in and stop the epoxy runover with rfjustin's o-ring trick. Aft ring drilled prior to gluing in the retainer. No one said a vent HAD to be on the SIDE of the booster.d) Now..... some day this thing will go well in excess of MACH 1, so I have seen what happens around the edges of vent holes.... they burn/soften/deform the edges of the hole..... a hole drilled in a 90 degrees.

What if the vent hole is drilled in at an angle (tilted down from 90 degrees perpendicular to BT toward the AFT end of the rocket.... 45-60 degrees? This reduces the effective opening area and provides a little "hood" to help stop the air from grabbing at the holes. Slightly less visible in the sunshine as well. Mostly I want to have an effective vent strategy with minimal penetration area through front/side profiles.

3) Rail Buttons: I have fly-away rails, but I am going to install rail buttons as backup (and just keep the PEM nuts filled with set screws when rail buttons are not used. The rear button will be located 0.5" in front of the aft centering ring and will be placed PRIOR to installing the motor tube. The FWD rail button will be at STA 30.4005 (from tip) or 18.3995" from end of booster tube. This works out to be a) about 6 calibers in front of rear rail button, and 1.5 calibers forward of the worst case CP. This is 17.9" between buttons... some bit more than the 75mm flyaway guides. Should I leave it as is or spread 'em out even further? And model for a 60-72" or better rail?

Bunch more decisions to make, but at least I am moving forward now!

 

[boatgeek]

Appears that I am accumulating a list of questions for which I need answers/decisions in order to progress.
Any opinions/recommendations/advice from y'all more experienced builders are greatly appreciated:

Questions:

1) Vent Band? Yea or Nay? I'm generally aware of the use(s) of the vent band... But I am not convinced that I want to use one. WHY would I? Why shouldn't I? I am leaning toward deleting it from the build to eliminate a body seam, reduce weight and a few tasks.

If you use a switch band, you don't need a specific alignment between your fin can and the AV bay. That's one less place to have to do a precision alignment. The fin can to AV bay doesn't need shear pins; just do a friction fit.

2) Vents: (x3 places?): Cert flights aren't going above 8k ft or so, but I still need to address vents as to purpose, number, and location/characterization. Ideally, I would have ZERO penetrations into the airframe that will "see" the air flow from flight. So, I at least want to minimize the sizes, et al. After cert, this WM Punisher is going to go really fast and pretty frick'n high.

See below.

a) Avionics Bay - Two baro. altimeters, so have to have vents in avbay. # and size? Location? I am leaning toward 3 vents for the avbay.... The calculator here: RocketryCalculator.com gives the following answer for a 2.875" ID on the AvBay, 5.8785" inside length of avbay, and 3 ports (ostensibly 120 separation at 180 degree phase shift with respect to "Front Fin line" (chosen as longitudinal datum).

3 ports? - 3/32" is slightly oversized.
1 port? - 5/32" almost exactly what the calc called out.... but I think it's big enough to get cooked....
Ideally, the avbay vents will serve double duty at the access hole(s) to the Schurter switch(es). And I am leaning toward 3 vents 180 degrees out from "front" datum line, mirroring the nosecone sheer pin locations radially about the airframe.... (neater lines; dressed)

A hair oversized is fine; and I think 3 is usually better than any other number. 1 can cause weird problems. I'd make these 1/8", 1/4" to 1/2" if you're accessing switches. I would strongly recommend against Schurter switches in favor of screw switches or pull pins. My experience with Schurter switches has not been good for long term use.

b) Nosecone vent: Gonna have plenty of room for the main chute in the nosecone (HEDD), and I am using 3 x 2-56 sheer rivets to hold it on. I REALLY don't want to drill a hole in the straight section of the nosecone for a vent. Can y'all think of an option? Not sure how much internal pressure differential could be generated in that small volume (reduced by chute and cord) by altitude induced external pressure drop. Is there any trick for avoiding the nosecone vent? Could I vent through a tube through the forward bulkhead to the interior of the avbay (tube is to direct potential hot gases to a "safe place" inside the avbay (most of the sensitive bits will be inside a triangular sled. The nosecone vent (sized to actual volume with "payload" compressed to yield max volume would be a good deal less net flow area and thus could vent to avbay and save a hole in the nosecone.

Practically speaking, you probably aren't going to pressure separate at reasonable altitudes, especially since a typical fit will leak some air anyway. It's worth checking to see what the total pressure force will be at the highest altitude you can get with the largest motor you can fit in the fin can. If that's a problem, drill a 1/8" hole in the side of the nose cone. I would just do that anyway, just above the top of the AV bay.

c) Booster vent: Have not calculated the booster free volume yet, but clearly, this portion of the airframe generally has the most vacant volume. I am considering trying to figure out a way to vent the booster through the front centering ring and thence through the rear centering ring (a la flyfalcons). I think I can drill the front CR before I glue it in and stop the epoxy runover with rfjustin's o-ring trick. Aft ring drilled prior to gluing in the retainer. No one said a vent HAD to be on the SIDE of the booster.d) Now..... some day this thing will go well in excess of MACH 1, so I have seen what happens around the edges of vent holes.... they burn/soften/deform the edges of the hole..... a hole drilled in a 90 degrees.

What if the vent hole is drilled in at an angle (tilted down from 90 degrees perpendicular to BT toward the AFT end of the rocket.... 45-60 degrees? This reduces the effective opening area and provides a little "hood" to help stop the air from grabbing at the holes. Slightly less visible in the sunshine as well. Mostly I want to have an effective vent strategy with minimal penetration area through front/side profiles.

I think you're overthinking this. Drill a 1/8" hole in the side of your fin can somewhere between the bottom of your AV bay and your forward centering ring. This is the least of the aerodynamic sins you're going to commit with this rocket. If you're not going to CNC mill the fins and fillets to a truly optimum profile, you're losing more than you would from the hole. You also don't really need to worry about aerodynamic heating until you get well over Mach 2. If you can't do that with the largest motor you can fit, don't worry about your fin can vent.

3) Rail Buttons: I have fly-away rails, but I am going to install rail buttons as backup (and just keep the PEM nuts filled with set screws when rail buttons are not used. The rear button will be located 0.5" in front of the aft centering ring and will be placed PRIOR to installing the motor tube. The FWD rail button will be at STA 30.4005 (from tip) or 18.3995" from end of booster tube. This works out to be a) about 6 calibers in front of rear rail button, and 1.5 calibers forward of the worst case CP. This is 17.9" between buttons... some bit more than the 75mm flyaway guides. Should I leave it as is or spread 'em out even further? And model for a 60-72" or better rail?

Bunch more decisions to make, but at least I am moving forward now!

I like my rail buttons a little closer together, but I'm sometimes in the minority on this. You lose control authority as soon as the top rail button leaves the rail, so it should be no higher than the CP to get the rocket going as fast as possible before it leaves the rail.

 

[cautery]

Appreciate the response!!

If you use a switch band, you don't need a specific alignment between your fin can and the AV bay. That's one less place to have to do a precision alignment. The fin can to AV bay doesn't need shear pins; just do a friction fit.

I am constitutionally incapable of NOT creating a "specific alignment" twixt the avbay and the nosecone/airframe. The sled will be registered as well. ALL of the avbay mass is arrayed about the centerline as closely as possible.... switch will require reaching through a hole OR pulling a pin (if I can find a way to mount it "square" considering the triangular sled inside.
I am also constitutionally incapable of using a "friction fit". There will be sheer pin (or 3). I want to KNOW that it is not coming apart (absent catastrophic failure) until the charge(s) are fired. Friction fit on fiberglass? I just can't!

A hair oversized is fine; and I think 3 is usually better than any other number. 1 can cause weird problems. I'd make these 1/8", 1/4" to 1/2" if you're accessing switches. I would strongly recommend against Schurter switches in favor of screw switches or pull pins. My experience with Schurter switches has not been good for long term use.

Gotcha.... I have a number of different switches (Schurter, screw x 3, mag, "Remove Before Flight" pin (micro-switch), et al. I will play around with these as I build the avbay and create the wiring harness, et al.

Practically speaking, you probably aren't going to pressure separate at reasonable altitudes, especially since a typical fit will leak some air anyway. It's worth checking to see what the total pressure force will be at the highest altitude you can get with the largest motor you can fit in the fin can. If that's a problem, drill a 1/8" hole in the side of the nose cone. I would just do that anyway, just above the top of the AV bay.

Hmmmm, let me see if I can figure out that math... I'll bleed excess pressure from nosecone to exterior through the avbay before I drill a hole in the nosecone. I can use a tube to completely bypass the NC outflow (if any) thru the interior of the avbay and into the booster section and thnce to the airframe tail... thus not figure into the data logging of the av-bay environment.

I think you're overthinking this. Drill a 1/8" hole in the side of your fin can somewhere between the bottom of your AV bay and your forward centering ring. This is the least of the aerodynamic sins you're going to commit with this rocket. If you're not going to CNC mill the fins and fillets to a truly optimum profile, you're losing more than you would from the hole. You also don't really need to worry about aerodynamic heating until you get well over Mach 2. If you can't do that with the largest motor you can fit, don't worry about your fin can vent.

Heh... Is it possible to over-think? It likely won't see more than mach 1.66 and 15k feet, but it's not just about THIS rocket or the certs.... It's about acquiring new skills and refining the ones I have..... It all build toward the LVLIII and altitude stuff I want to do

I like my rail buttons a little closer together, but I'm sometimes in the minority on this. You lose control authority as soon as the top rail button leaves the rail, so it should be no higher than the CP to get the rocket going as fast as possible before it leaves the rail.

The main tradeoff is get a little faster speed off a shorter rail before front button departs rail VS. GIANT reduction in trajectory excursions.... I'd rather use a longer rail to get the speed but KNOW that it came off the rail nuch closer to the chosen elevation (nominally straight up with ZERO lateral vector component.


Thanks again for the detailed response! Be well!

 

[crossfire]

The Punisher 3 is my favorite kit. You can see mine in the picture on the left of my post

 

[cautery]

Free Volume Calculator for Ejection Charge Calculations - Booster (v0.003)

This is a tool to assist you in determining the FREE/Open Volume in your booster for use in calculating an estimated Ejection Charge Weight (or Vent Port Sizing. And I added the math to give you the "Effective Body Tube Length" in Version 0.003, so you can go straight to the BP Estimator (or other) to get a hopefully more accurate starting place for Ejection Charge determination.

 

[UPscaler]

Welcome! Excellent choice. The Punisher is my favorite kit.

Braden

 

[cautery]

Welcome! Excellent choice. The Punisher is my favorite kit.

Braden

Your videos had a lot to do with my choice.

 

[StreuB1]

Excellent choice on rocket and great motor selections!!! That I229T is going to rock in that bird!!!

The J180T might also be a nice choice for L2 motor but you can't go wrong with the J135W either.

I like switch bands and I always use (3) vent holes with one used for electronics activation. It can be done so many different ways, all are good. Up to you to decide what way you like best. Do your investigations and thinking and then go for it. It won't be the end of the world either way.

Looking forward to see what you come up with!

 

[cautery]

Thought for sure I would get a "lot" done today.... Well.... I did get a bunch done; just doesn't look like it.

I completed much of the layout work on the booster tube, av-bay, and motor mount tube. When fitting the dual booster harnesses, I realized that I needed to drill the vent ports in the CRs....

But what size? Need a free volume for that calculation. Also need the free volume to calculate ejection charge weight(s) for the booster.
I never want to have to do all this stuff manually again unless absolutely necessary, thus, my descent into the black hole of Excel spreadsheets for the rest of the day (posted above).

So, no actual building today beyond some layout, notching the front CR, determining start point for charge weight workup and the vent port number/sizes to drill in the centering rings.

I am however just about ready to sand and make the first bond though! Woohoo!

 

[thzero]

I'm not sure I follow why you think removing the switch/vent band does much for you (a half ounces at most?), Trying to route the pressure through the centering rings, etc. just seems overly complicated. Doubt you'll see much reduction in performance for the extra seam due to the switch band either.

 

[boatgeek]

Thought for sure I would get a "lot" done today.... Well.... I did get a bunch done; just doesn't look like it.

I completed much of the layout work on the booster tube, av-bay, and motor mount tube. When fitting the dual booster harnesses, I realized that I needed to drill the vent ports in the CRs....

But what size? Need a free volume for that calculation. Also need the free volume to calculate ejection charge weight(s) for the booster.
I never want to have to do all this stuff manually again unless absolutely necessary, thus, my descent into the black hole of Excel spreadsheets for the rest of the day (posted above).

So, no actual building today beyond some layout, notching the front CR, determining start point for charge weight workup and the vent port number/sizes to drill in the centering rings.

OK, now you're definitely overthinking this. You don't need to vent the annular space between the motor mount and the body tube. At best, it's a little extra work for no tangible benefit. At worst, you'll get crud in there that you can't get out.

You might also want to think about the precision of the BP calculation before you go whole hog on things like CR thickness and charge wells. If your calc is +/- 10% (at best!), then worrying about details that change the volume by 1% or less doesn't make a lot of sense. Also, variations in how you build charges (eg ematch on top of BP or underneath) have a bigger impact than a couple of percent change in volume. That said, there's probably no need to take those things out of the spreadsheet already made.

 

[cautery]

I'm not sure I follow why you think removing the switch/vent band does much for you (a half ounces at most?), Trying to route the pressure through the centering rings, etc. just seems overly complicated. Doubt you'll see much reduction in performance for the extra seam due to the switch band either.

Thanks a bunch for the response! Even though you disagree with some of my plan, I consider it a useful exercise to think through and defend WHY I want to use a particular technique.

Switch/Vent Band

Mostly based on my personal preference I am sure, but in favor of NOT using a vent band:

PRO:
1) DOWN 1 pieces to mess with
2) One less bond to layout, sand, bond, et al. (And the VB to coupler/av-bay tube is very slightly sloppy, so for someone who tends toward the OCD side of the room, it's less aggravation for me trying to figure out hour to get it bonded perfectly concentric so that when cured I cannot feel the interfaces to booster and NC.)
3) UP 33 feet, UP 2 mph, DOWN 17.4 grams (0.613 oz ) actual, more than a half ounce
4) Down 1 seam... (no double cuts in any decoration spanning the seams, et al.)
5) Relieves me of the 1" limitation of where to put vent port(s), switch port(s). I can put them where I want.
6) ...and more.

CON:
1)???

Frankly, as I originally posted, I have yet to be convinced of its benefit(s). Would you list the top three? Please.
As I said, I am LEANING toward its deletion... not quite decided yet.

PRO:
Venting aft:
1) Less holes in the airframe that "see" the flowing fluid.
2) Less holes (even a single larger one) to SEE.
3) It's a cool technique to learn.... which is the Secondary Mission of this entire build.

CON:
1) A little more effort and planning required.
2) ?

 

[cautery]

OK, now you're definitely overthinking this. You don't need to vent the annular space between the motor mount and the body tube. At best, it's a little extra work for no tangible benefit. At worst, you'll get crud in there that you can't get out.

You might also want to think about the precision of the BP calculation before you go whole hog on things like CR thickness and charge wells. If your calc is +/- 10% (at best!), then worrying about details that change the volume by 1% or less doesn't make a lot of sense. Also, variations in how you build charges (eg ematch on top of BP or underneath) have a bigger impact than a couple of percent change in volume. That said, there's probably no need to take those things out of the spreadsheet already made.

LOL! Overthinking? Think so? Is that actually possible? Maybe. Does it matter? Likely as not, no.

As long as I...
1) Certify successfully, and
2) Learn a bunch for next time,
then I have accomplished the missions.

Venting Annular Volume is actually a function of needing to use those pathways aft to vent the forward booster tube volumes aft. Not only will they be drilled, I will also break/chamfer/radius the edges to potentially improve the flow from a minimum sized port. Crud? Not likely from me... maybe the odd piece from a field somewhere, but pump some 40 psi air through each area and not much is going to stay in there.

BP Charge Weight Precision - I am absolutely NOT relying on the "precision" of my calculations. I am ESPECIALLY not relying on the precision OR accuracy of the calculations of others. I am building and testing TOOLS that, if successful, will allow me to increase the confidence level of the calculators/sims for future projects.
While THIS project does not necessarily require most any of the "little extras" or more uncommon techniques, future projects.... "More, Better, Faster....". Every little detail will become increasingly important. That's my thought process anyway. (e.g. BP Charge Weight Calculations - Some day, I hope to have tools that I can trust to be accurate with decent precision to reduce ground testing time, increase peace of mind, etc. Some day I hope to learn enough to be able to push HPR forward.... at least in some small way.)

Really appreciate you taking the time to respond! Be well, sir!

 

[Crazyrocket]

The reason I use a vent band on the punisher and any other rockets that use HEDD is that it provides a stop to prevent the avbay from going too far into the nose cone and getting potentially wedged during assembly or during flight. I do have other rockets that are not HEDD where I do not use a vent band and actually put the avbay vent holes right on the seam.

I agree with other that the losses for having extra seams and weight is negligible given that the motor thrust curve could vary significantly more than the weight/drag that you are concerned about.

Just my two cents. Good luck with the build and your certification attempts!

 

[thzero]

Thanks a bunch for the response! Even though you disagree with some of my plan, I consider it a useful exercise to think through and defend WHY I want to use a particular technique.

Switch/Vent Band

Mostly based on my personal preference I am sure, but in favor of NOT using a vent band:

PRO:
1) DOWN 1 pieces to mess with
2) One less bond to layout, sand, bond, et al. (And the VB to coupler/av-bay tube is very slightly sloppy, so for someone who tends toward the OCD side of the room, it's less aggravation for me trying to figure out hour to get it bonded perfectly concentric so that when cured I cannot feel the interfaces to booster and NC.)
3) UP 33 feet, UP 2 mph, DOWN 17.4 grams (0.613 oz ) actual, more than a half ounce
4) Down 1 seam... (no double cuts in any decoration spanning the seams, et al.)
5) Relieves me of the 1" limitation of where to put vent port(s), switch port(s). I can put them where I want.
6) ...and more.

CON:
1)???

Seems questionable at best, but hey, its your rocket... as long as it has enough boost, its CG is greater than 1 caliber ahead of CP, and it deploys safely...

 

[boatgeek]

LOL! Overthinking? Think so? Is that actually possible? Maybe. Does it matter? Likely as not, no.

As long as I...
1) Certify successfully, and
2) Learn a bunch for next time,
then I have accomplished the missions.

Venting Annular Volume is actually a function of needing to use those pathways aft to vent the forward booster tube volumes aft. Not only will they be drilled, I will also break/chamfer/radius the edges to potentially improve the flow from a minimum sized port. Crud? Not likely from me... maybe the odd piece from a field somewhere, but pump some 40 psi air through each area and not much is going to stay in there.

BP Charge Weight Precision - I am absolutely NOT relying on the "precision" of my calculations. I am ESPECIALLY not relying on the precision OR accuracy of the calculations of others. I am building and testing TOOLS that, if successful, will allow me to increase the confidence level of the calculators/sims for future projects.
While THIS project does not necessarily require most any of the "little extras" or more uncommon techniques, future projects.... "More, Better, Faster....". Every little detail will become increasingly important. That's my thought process anyway. (e.g. BP Charge Weight Calculations - Some day, I hope to have tools that I can trust to be accurate with decent precision to reduce ground testing time, increase peace of mind, etc. Some day I hope to learn enough to be able to push HPR forward.... at least in some small way.)

Really appreciate you taking the time to respond! Be well, sir!

OK, I'll stop trying to talk you out of doing it the hard way.

Best of luck, and I'll leave you with a quote from my materials prof in college: "Engineering is the science of good enough."

 

[cautery]

The reason I use a vent band on the punisher and any other rockets that use HEDD is that it provides a stop to prevent the avbay from going too far into the nose cone and getting potentially wedged during assembly or during flight. I do have other rockets that are not HEDD where I do not use a vent band and actually put the avbay vent holes right on the seam.

I agree with other that the losses for having extra seams and weight is negligible given that the motor thrust curve could vary significantly more than the weight/drag that you are concerned about.

Just my two cents. Good luck with the build and your certification attempts!

OK.... now THAT is a valid reason to use the switch/vent band. With 3 rivets top and bottom, I don't think that's going to happen. Perhaps, as an alternative to the band, positive stops could be put inside the airframe and nosecone just below the desired position to remove the potential for assembly issues.

Why did you choose to put the vents ON the seam? Seam is an interruption already so put all the disturbances in one place? Or the holes are staggered to only match up one way? Just curious. I still want to test vents that are angled back towards the tail.... not on this rocket, but soon.
I can't let motor thrust variability be a reason NOT to gather up all the "tiny" bits to forge them into a significant plus. Clearly makes little difference here, but I am somewhat confident that "the little things" will become important as things scale up. I want to have the skills, tools, and knowledge to take advantage when the time comes.

Thank you for the response... Your switch band point made points! Be well! Happy Thanksgiving!

 

[cautery]

OK, I'll stop trying to talk you out of doing it the hard way.

Best of luck, and I'll leave you with a quote from my materials prof in college: "Engineering is the science of good enough."

Oh heck no! I appreciate the running "sanity check". Keep it up!

Re: Quote..... Perhaps it is a good thing that I moved away from engineering in college. A lifetime saddled with "good enough" when my DNA screams "More, Better, Faster..." pretty much 24/7 would have been miserable. As a non-PE, I am not saddled with any of the constraints.

 

[boatgeek]

Oh heck no! I appreciate the running "sanity check". Keep it up!

Re: Quote..... Perhaps it is a good thing that I moved away from engineering in college. A lifetime saddled with "good enough" when my DNA screams "More, Better, Faster..." pretty much 24/7 would have been miserable. As a non-PE, I am not saddled with any of the constraints.

Perhaps it's a side effect of watching my professional work like a hawk for unnecessary work (you can run up a couple of person-years of labor pretty quickly in my industry!), but I have a hard time not being frustrated by over-complication, particularly when it seems to be complication for the sake of complication*. If you're concerned about airflow through a vent hole, make it a hair bigger instead of chamfering/rounding the inside. Weirdly, I would have an easier time of accepting the extra work if you were doing for looks rather than airflow efficiency.

This is a me issue, and mainly to explain why I'm probably going to be commenting less or not at all on this unless I see something that's a safety concern. There's only so many times that I can hear that you don't want to do things an easier way before I have to stop suggesting them.

The prof's main point was that at some point, you're more interested in whether a design meets objectives than whether it's truly ideal. At some point, you have to stop designing and start building. Also, there's little point in trying to determine if a 14mm bolt will meet requirements if you know that they'll buy imperial bolts and a 1/2" bolt isn't good enough. That absolutely does not mean that engineers aren't on a path of trying to improve designs throughout their career. There are also industries (and sectors in my industry) where you get to/have to optimize more.

* That might not be your motive, but it's hard to set that aside when you reject ideas of simplification on the basis of nebulous future projects. Again, a me issue.

 

[cautery]

Perhaps it's a side effect of watching my professional work like a hawk for unnecessary work (you can run up a couple of person-years of labor pretty quickly in my industry!), but I have a hard time not being frustrated by over-complication, particularly when it seems to be complication for the sake of complication*. If you're concerned about airflow through a vent hole, make it a hair bigger instead of chamfering/rounding the inside. Weirdly, I would have an easier time of accepting the extra work if you were doing for looks rather than airflow efficiency.

This is a me issue, and mainly to explain why I'm probably going to be commenting less or not at all on this unless I see something that's a safety concern. There's only so many times that I can hear that you don't want to do things an easier way before I have to stop suggesting them.

The prof's main point was that at some point, you're more interested in whether a design meets objectives than whether it's truly ideal. At some point, you have to stop designing and start building. Also, there's little point in trying to determine if a 14mm bolt will meet requirements if you know that they'll buy imperial bolts and a 1/2" bolt isn't good enough. That absolutely does not mean that engineers aren't on a path of trying to improve designs throughout their career. There are also industries (and sectors in my industry) where you get to/have to optimize more.

* That might not be your motive, but it's hard to set that aside when you reject ideas of simplification on the basis of nebulous future projects. Again, a me issue.

I get where you are coming from. We all have our "things". Some of us (raises hand) have a very long list of them.
Yes, I get it. Mission first. Motive is not complication for complication's sake, nor is it to choose the "easy" way because it is easy.
If my ramblings aggravate you, by all means scroll by without comment. This is supposed to be fun. I don't want to be involved in lessening folks joy.

 

[kramer714]

The reason I use a vent band on the punisher and any other rockets that use HEDD is that it provides a stop to prevent the avbay from going too far into the nose cone and getting potentially wedged during assembly or during flight. I do have other rockets that are not HEDD where I do not use a vent band and actually put the avbay vent holes right on the seam.

I agree with other that the losses for having extra seams and weight is negligible given that the motor thrust curve could vary significantly more than the weight/drag that you are concerned about.

Just my two cents. Good luck with the build and your certification attempts!

Yeah, you want the switchband for HEDD. That way your shear pins are not loaded at all during liftoff, the switchband takes up all the load.

Side note for HEDD, if you use a compact avbay, the length of the coupler that goes up into the nosecone can be used for the parachute. I only use 1.5 inches of the coupler for the avbay (rrc-3 or PerfectFlight) the remainder is open. Basically the avbay lives in the length of a 1" switchband and eliminates an eye bolt giving lots of room in the nose plus letting you put a long motor in and still having room for a shock cord / chute.

 

[bobtheshortstop]

Yeah, you want the switchband for HEDD. That way your shear pins are not loaded at all during liftoff, the switchband takes up all the load.

Side note for HEDD, if you use a compact avbay, the length of the coupler that goes up into the nosecone can be used for the parachute. I only use 1.5 inches of the coupler for the avbay (rrc-3 or PerfectFlight) the remainder is open. Basically the avbay lives in the length of a 1" switchband and eliminates an eye bolt giving lots of room in the nose plus letting you put a long motor in and still having room for a shock cord / chute.

View attachment 547366View attachment 547367

Dude that's pretty clever AV bay!! Super compact! Did you 3d print that yourself??

 

[kramer714]

Designed and printed. The red spacer gets bonded into the coupler, that is what the bulkhead bear against. It uses a microswtich and a screw (through a molded in insert). A hex coupler fits in the hex molded in, screws attach to the hex coupler from either end through the bulkheads (shows button head but i countersunk them). a 2s lipo fits in the pocket molded in place and is retained by the blue-green cover screwed in place. The two blue pieces go over the ends of the shock cord and then get screwed in. I didn't want to lose the volume from an Eye bolt but you could use that in the center in place of the 'blue pieces'. The short end of the coupler goes into the nosecone, the longer end goes into the booster. I have a perfectflite and a RRC2 version.

 

[cautery]

Yeah, you want the switchband for HEDD. That way your shear pins are not loaded at all during liftoff, the switchband takes up all the load.

Side note for HEDD, if you use a compact avbay, the length of the coupler that goes up into the nosecone can be used for the parachute. I only use 1.5 inches of the coupler for the avbay (rrc-3 or PerfectFlight) the remainder is open. Basically the avbay lives in the length of a 1" switchband and eliminates an eye bolt giving lots of room in the nose plus letting you put a long motor in and still having room for a shock cord / chute.

NICE! Thanks for the visuals. What a cool idea! I have a 6" coupler/avbay that I really don't need but less than half that without breaking a sweat. never thought of putting an internal bulkhead in the coupler to increase the main deployment volume. I don't need the room, but it's a great technique to have in your tool box!

And finally someone has mentioned a vent/switch band "feature" that gives me pause as to deleting it from my bird. Embarrassing that I didn't "catch" it. I wonder what the shear force would be? Need to go hunt up the math to compute it for my case. I won't fly without the band unless I can know for certain that 3 pins top and bottom will hold.... Don't want to bump up the pin size as that ups the BP charge weight which leads to other concerns.

Thank you again for the response! be well and Happy Thanksgiving!!

 

[Neutronium95]

You look to be on a good course for the build. I would suggest not getting too bogged down in making everything too precise or optimal. Like with ejection charges, the scale I bring to the field is only good to 0.1g of precision. If it's a windy day, then my measurements might be even less precise. I tend to use charges a bit on the larger side than are strictly necessary to separate the parts. Moreso after I had my Mach 2 come in ballistic at BALLS this year, despite having a seemingly adequate ground test, and seeing that at least one of the charges fired when examining the wreckage. Unless you use a grossly oversized charge, there isn't much risk of damaging a fiberglass rocket, so I'd probably start at around 1.4-1.5g for the drogue, and 0.5 for the main. Always make sure to ground test, and make sure that the parts separate well, and all of the laundry gets out of the tube.

 

[sharkbait]

NICE! Thanks for the visuals. What a cool idea! I have a 6" coupler/avbay that I really don't need but less than half that without breaking a sweat. never thought of putting an internal bulkhead in the coupler to increase the main deployment volume. I don't need the room, but it's a great technique to have in your tool box!

And finally someone has mentioned a vent/switch band "feature" that gives me pause as to deleting it from my bird. Embarrassing that I didn't "catch" it. I wonder what the shear force would be? Need to go hunt up the math to compute it for my case. I won't fly without the band unless I can know for certain that 3 pins top and bottom will hold.... Don't want to bump up the pin size as that ups the BP charge weight which leads to other concerns.

Thank you again for the response! be well and Happy Thanksgiving!!

The switch band not only transfers the the thrust load at lift off, it also transfers the drogue charge separation pressure away from your upper shear pins.

With HED deploy the switch is the way to go, convention DD with a riveted or bolted upper then it is not needed