i think for Back Slider Recovery (bSr) and possibly for Belly Flop Recovery (bFr) BT-5 is problematic. At least for bSr, the transition from stable boost to horizontal glide requires a significant shift of the effective CG (eCG) and effective CP (eCP), essentially inverting them along the length of the rocket. For bSr, this is accomplished with an eccentric intentionally discombobulating force (tempted to abbreviate that, the EIDF
), which usually changes the angle of attack from near zero, where Barrowman calculations rule and lateral surface area of the body tube has little or no affect on CG (have I got that right, @neil_w
?), to Center of Lateral Area Calculations (CLA) (aka”Cardboard Cut Out) calculations where body tube lateral surface area is essentially an equal factor and definitely DOES a come into play. Problem with smaller body tubes, the tube mass to lateral surface area for most paper tubes is in some way proportional (not sure if directly, but somehow) to body diameters. For same tube thickness, the lateral surface area/mass is much greater than for smaller tubes of same thickness.
so for a longer BT-50, lengthening the tube definitely increases both lateral surface area and mass, but from CLA perspective, the eCP is shifted far more than the eCG. Because the BT-5 is effectively “thicker” in the paper component compared to diameter, the difference of shift with a longer (but smaller diameter) tube is LESS than with a proportional lengthening of a larger diameter tune (again assuming the same tube wall thickness and material.)
there are practical limits on this.
First is proportionality, for wider body tubes to get the same “SuperRoc” length to diameter proportions (Alway patent said at least 30 to 1, ideally 50 to 1) https://patents.google.com/patent/US6926576B1/en
the Length of the rocket gets challenging once you get beyond a BT-50. Optimal length for a BT-100 Rocket at 50 to 1 is about 15 feet.
second, the advantage of a larger diameter is dependent on the same wall thickness and material. I am sure the High Power Guys know why you use heavier (more thick walled or different structural material) for bigger rockets. Using the same wall thickness isn’t likely to be structurally sound for rockets of this size.
i am glad you brought this up, @lakeroadster
, because you are one of the biggest proponents of the Swing Test, which is was thinking was a THIRD stability test which my first rendition of Screwball failed.
i theeeeeenk Swing Tests are the Cats Pajamas for rocket that are stable based on CLA, as a rocket stable based on CLA again I theeeeeeeenk should also be stable by Barrowman. But to do a swing test for Barrowman is harder, because you have to start with and maintain a zero angle of attack, if you go significantly off that (don’t ask me what angle is “significant”, I dunno.). I think it CAN be done, but is harder, you can’t just throw the rocket out there and start swinging it.
i don’t know if @Rktman
still has his backslider, but if he was interested it would be cool to see if it fails the Swing Test. I have a tail piece from one of my back sliders, I might just play with it a bit.
this may explain why a number of people have reported rockets that failed the swing test but flew perfectly well.
and it may be that I just am crummy at doing swing tests!