Backup build for Havoc - 38mm MD

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Legendontour
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Backup build for Havoc - 38mm MD

Postby Legendontour » Sun Dec 08, 2019 9:22 pm

As xmas approaches, I fear my promised PML 1/4 patriot will not materialise, so I need to construct something to fly at Havoc to use the motors I have ordered. It seems a few 38mm MD rockets have been built over the last few years with some expertise showing through on the forum. I could splash out and order a kit from the US, but have decided to challenge myself to scratch build something with what I have available - namely some 38mm PML Phenolic and coupler, and the matching Urethane nose cone. These were some of the "in stock" items in the shop. I figure it will also be fairly easy to pack in my backpack come February.
Loosely based on the old PML Cirrus Dart, but with DD I'm juggling lengths and numbers to find the optimal cut point for my single length of tube. At the moment I've settled on 55/35cm with a 1cm switch band on a 6 inch coupler.
I splashed out on the eggtimer sale after having fun building my first set and Ive got a TRS that I'd like to put in the AV bay and mount the antenna on the underside with the battery. Backup option is a quantum in the AV bay and a mini tied to the shock cord somewhere. The nose cone is solid, so no room in there!
My question is around fins. How have people come up with the optimal fin size/shape. Playing around with OR it seems that a clipped delta profile with a 15cm root and a 4cm span is the optimal shape for altitude (8330ft) But 4cm just seems a little skinny for me. 5cm gets be over 8000 feet still. Ive discovered that fin sim is only available to US residents via a bank cheque, so looks like I'm unable to utilise that. Ive ordered some 1.5mm g10 glass sheet and plan to to tip to tip on top to build them up the 3mm.

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SpaceManMat
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Re: Backup build for Havoc - 38mm MD

Postby SpaceManMat » Tue Dec 10, 2019 6:39 pm

My preference is for the clipped delta, the reason is to get enough fin out in the clean air. You want to avoid turbulent airflow that is close to the body. If you make your span to small then I think OR will actually give you a warning as the calculations assume the span is larger than the body tube diameter, again I believe the issue is the airflow close to the body. Not sure how fast you are going but there is a design rule that can be used to optimise the slope of the leading edge to reduce the shock wave.
QRS: 124
AMRS: 32 L2 RSO
Highest Altitude: 13,647 feet
Fastest Flight: Mach 1.55
Largest Motor: CTI 1115J530 IM
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