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!