bobkrech
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The 5:1 thrust to weight ratio recommendation is a rule of thumb, not a regulation. An average rocket on an average length rod/rail will accelerate to a minimum launch velocity (where the control surfaces (fins) generate enough corrective force to insure the rocket will travel straight in the upward direction on a calm to light air day if the thrust to weight ratio is 5:1.
The generally accepted minimum stable velocity for a model rocket is ~ 20 mph (30 fps), and 30 mph (45 fps) for a high power rocket (The relative fin area on a model rocket is usually larger than that on a high power rocket which accounts for the difference.). The fins are an aerodynamic lift device. If the angle of attack of the fin is too large the fins will stall the rocket will tumble. The angle of attack where this happen is profile dependent and could be as low as 15 degrees for an square edged flat fin. If the ratio of the rocket velocity divided by the cross-wind velocity is > 4, then the angle of attack is < 15 degrees. For a high power rocket at a minimum rod velocity of 30 mph, if the cross-wind is 7.5 mph or lower, the angle of attack on the fins is < 15 degrees so the rocket will not stall and tumble leaving the rod/rail.
So with a 6' rod/rail, an average high power rocket will leave the rod/rail at 30 mph. The fins will not be stalled and provide corrective actions if the wind is 7.5 mph or lower. This is the basis of the 5:1 lift-off thrust to weight ratio. If the launch rail is longer, a lower T/W ratio is fine as long a the rocket leaves the rail at 30 mph or faster and the wind is 7.5 mph or lower.
If your rocket is stable at a lower velocity, then a lower leaving velocity is ok, so a lower T/W would be ok. If your rocket/motor combination does not conform to this norm, bring a sim to validate your motor selection.
Regardless, the RSO has the final say on the subject, whether or not there is a "club" rule.
Bob
The generally accepted minimum stable velocity for a model rocket is ~ 20 mph (30 fps), and 30 mph (45 fps) for a high power rocket (The relative fin area on a model rocket is usually larger than that on a high power rocket which accounts for the difference.). The fins are an aerodynamic lift device. If the angle of attack of the fin is too large the fins will stall the rocket will tumble. The angle of attack where this happen is profile dependent and could be as low as 15 degrees for an square edged flat fin. If the ratio of the rocket velocity divided by the cross-wind velocity is > 4, then the angle of attack is < 15 degrees. For a high power rocket at a minimum rod velocity of 30 mph, if the cross-wind is 7.5 mph or lower, the angle of attack on the fins is < 15 degrees so the rocket will not stall and tumble leaving the rod/rail.
So with a 6' rod/rail, an average high power rocket will leave the rod/rail at 30 mph. The fins will not be stalled and provide corrective actions if the wind is 7.5 mph or lower. This is the basis of the 5:1 lift-off thrust to weight ratio. If the launch rail is longer, a lower T/W ratio is fine as long a the rocket leaves the rail at 30 mph or faster and the wind is 7.5 mph or lower.
If your rocket is stable at a lower velocity, then a lower leaving velocity is ok, so a lower T/W would be ok. If your rocket/motor combination does not conform to this norm, bring a sim to validate your motor selection.
Regardless, the RSO has the final say on the subject, whether or not there is a "club" rule.
Bob
