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I think that is the $64 question.edit: However, I would caution that you may still need to exceed 25:1 length/diameter ratio in order get your backslide going.

nonspinning backsliders RELY on the rocket being UNSTABLE based on CardBoard cutout method. Conceptually, you freeze effective forward motion. Without forward motion, the fins are tremendously reduced in effectiveness (like trying to launch a rocket from a pad with no rail or rod for initial guidance, also why ThrustaCurve and OpenRocket either include or assume a certain length of rod or rail for valid predictions.)

i theeeeeenk the reason that BackSliders have traditionally been long rockets is because forward moving rockets are most heavily influenced from a CP aerodynamic standpoint by the fins and the nose cone, with little effect of body tube. Rockets that must acquire stability in free fall (presumed non moving or AT LEAST no aerodynamically stable effective forward movement) will NORMALLY most likely fall based on basic CG and CP based on lateral surface area, I.e. cardboard cutout. However, adding cupped or curved fins all radially in the same direction induces spin even if the rocket is not initially falling in ANY specific direction. The conservation of angular momentum resists of tumbling and ”pushes” the rocket to start to orient horizontal. This is a POSITIVE feedback loop (sort of a pleasant “vicious cycle”, if you will) as the more horizontal the rocket get perpendicular to the fall vector, the FASTER the rocket spins, the more it “wants“ to be horizontal. My theory is that THIS force MAY be able to overcome the normal nose down attitude (ballistic recovery) EVEN if by the CardBoard Cutout method the rocket’s CG is still forward of the CP. I think this is why in the Alway papers, some of the rockets initially were good backsliders while they were spinning, but went ballistic when they stopped spinning.

the problem with a fat Big Bertha type rocket is going to be the nose cone is big.

I am going to define some terms that I guess I am making up, I don’t know that they already have an established nomenclature

Coordinated Established Moving Stability: CEMS, referring to Rocket stability while it is already moving in constant forward motion along a flight path parallel with its long axis. This is the condition of a stable rocket in mid flight during ascent. HOPEFULLY it is the situation of your rocket when it reaches the end of your rod or rail. I thought of using the term Dynamic Stability (meaning Moving Stability) but this term is already use and has a different meaning. THIS is the condition ASSUMED for Barrowman Equations and I believe for OpenRocket and RocSim. A key here is a very low angle of attack Usually near 0 . Note: to have ANY angle of attack there MUST be both a coordinated forward trajectory and motion. A non-moving object has NO defined angle of attack.

FreeFall Stability: FFS, situation when the Rocket has NO coordinated forward motion. May not be the best term. The rocket is in this situation at a number of times. First, sitting on the Pad motionless. Second, at a perfect vertical apogee (rarely achieved at least with low power rockets, this would be a rocket going perfectly straight up when kinetic energy “runs out” and the rocket is aimed upward but for a moment is motionless, hanging there until it starts to fall.). Third, and most likely for our Magnus rockets and @Rktman ’s backslide, when there is an uncontrolled tumble, in our case induced by the vent port puff slinging the rocket sideways. This effectively decombobulates the rocket motion, for a moment or moments, while it may be moving, it has either no angle of attack or a variable angle of attack which when present if far from 0.

Barrowman and OpenRocket and RocSim are based on CEMS. I don‘t use these programs much (they don’t work for my square Helis and AirBrakers, which have been my passion for the last few years.). BackSliders are definitely based on FFS, which is basically the CardBoard cut out. Horizontal Spin will definitely succeed under the right FFS conditions (CG BEHIND CP, rocket wants to fly backwards.). But my theory is that it can ALSO work with CG FORWARD of CP, as long as it is not too extreme.

anyway, will be fun to play with. Key components of magnitude of Magnus Force should be related to

Length (likely directly related. longer = more)

Diameter (positively related, larger diameter = more, although I don’t know if it is a Linear or nonlinear relationship, definitely positive)

Rate of rotation (again positive, faster = more, but not sure of linear vs nonlinear.)