It's all a work in progress with port placement. Looking back, my most reliable designs have been the BT-5 and BT-20 diameters, particularly the X-8, with dozens of flights and zero failures. It's very light with disproportionately large fins which have luckily never broken. These two models have a forward port just behind the nosecone. I'm now building in larger sizes to make more visible and carry an altimeter.
We have determined that four 3/16" diameter ports spaced circularly at the same station line on a BT-50 confer a definite weakness in that section of the tube. We won't be doing that again.
The transitions serve a dual, even triple purpose to us. Firstly, to replace a major length of BT-60 tube with a transition and a BT-50 or BT-20 tube of the same length results in a measurably lighter model. Secondly, they form a bulkhead which can protect both the ports behind or any cargo such an altimeter in front of it. Nextly, and most importantly, we feel the Magnus effect is enhanced resulting in the wondrous spiraling effect on the descent, keeping the landing nearer to the pad. Also, we are very concerned with the strength and reliability of the ejection event, so wish to reduce internal volume as well as use 24mm motors in lieu of the 18mm.
Your astute questions make you a highly desirable correspondent!
The ratios you speak of are of considerable interest to me. Each of my designs considers this issue both in practical and philosophical ways. There are trade-offs. My next build will incorporate the Golden Ratio in the fin design, believe it or not.
Instability with my models does not seem to be an issue at all. Today we had steady winds 7 mph gusting to over 10. The only rocket which appeared to head into the wind (weathercock) was the one of post #582. The model of post #584 went up straight as a laser. We made many other flights this morning in addition to those pertaining to the subject models. We are exploring length to diameter ratios between 40 and 50 to one for HSR. Some of our models with larger fins areas spin to to 420 rpm. Those with smaller fin areas don't spin as rapidly. The faster the spin, the greater the Magnus effect, we think.
So far, the simplest and most reliable HSR I know of is the BT-20 Magnus X-8. Launched on C6-5, you get 4 beautiful spirals and land close to the pad. But also limited visibility and no altimeter.
Regarding
“Instability with my models does not seem to be an issue at all. Today we had steady winds 7 mph gusting to over 10. The only rocket which appeared to head into the wind (weathercock) was the one of post #582.”
If I read that right, you haven’t had any UNSTABLE rockets, and the worst you have had is OVERSTABLE. So from a BOOST stability standpoint, you haven’t reached the lower limit of effective size of fins.
From HSR standpoint, regarding your ballistic or other failures, have any been secondary to insufficient fin size?
So the Transitions are used to
1. Reduce mass
2. Reduce forward diameter, which reduces Magnus forward, so the rear Magnus overwhelms forward Magnus, and induces rotation. And
3. Gives you a bulkhead to protect the altimeter. Altimeters are a problem, on a standard rocket putting their mass as forward as possible is win-win, but for HSR that same forward weight favors ballistic over HSR.
FlightSketch Mini is
- 3.16g (0.11oz) ready to fly with CR1225 battery (included)
But currently and commonly out of stock.
It does fit inside a BT-20 coupler.
PNut is 0.26 oz. including battery (7.37 grams), also fits inside a BT-20. And is in stock.
$65.
http://www.perfectflite.com/pnut.html
Apogee has a vac-formed lightweight BT20 nose cone that weighs 1gram.
Compares with standard BT20 plastic nose cone
Weight: 5.9 g (0.21 oz)
Best of all worlds? Pass through transition To allow ejection charge to go through, small forward BT-20 payload bay with altimeter, lightweight cone, puff port just behind payload.
Of course, if it goes ballistic you lose at least the nose cone, maybe altimeter.