desired velocity at launch rail departure

Forget the calculations. I want to know the bottom line here: what is the desired velocity at launch rail departure for any rocket in any given wind conditions (as long as it keeps within the NAR guidelines for launch/wind)? Is 50 feet/sec fast enough or do I need to be going faster for a safe launch? Or, would something slower be ok? I rely on Rocksim for my calculations.

gary7 said:

Forget the calculations. I want to know the bottom line here: what is the desired velocity at launch rail departure for any rocket in any given wind conditions (as long as it keeps within the NAR guidelines for launch/wind)? Is 50 feet/sec fast enough or do I need to be going faster for a safe launch? Or, would something slower be ok? I rely on Rocksim for my calculations.

Click to expand...

You ask a simple question, and expect a simple answer but there is none.

From https://www.nar.org/NARhpsc.html and https://www.nar.org/cabinet/HPRSafetyCode.pdf

Launcher. I will launch my rocket from a stable device that provides rigid guidance until the rocket has attained a speed that ensures a stable flight, and that is pointed to within 20 degrees of vertical. If the wind speed exceeds 5 miles per hour I will use a launcher length that permits the rocket to attain a safe velocity before separation from the launcher.

a speed that ensures a stable flight means the speed at which the fins exert aerodynamic control of the vehicle. The minimum rod speed depends on your rocket design and is generally between 30 fps (20 mph) to 45 fps (30 mph).

a safe velocity means a speed where fins can maintain aerodynamic control without stalling due to excessive angle of attack. The higher the rocket velocity, the lower apparent wind angle, and therefore the angle of attack of the fins. https://www.nar.org/pdf/launchsafe.pdf

Assuming you are using the same length rod for all wind conditions, and you reach the stable speed with that rod length in winds up to 5 mph, then you need to increase the thrust to weight ratio from the standard 5:1 to a higher value. The recommended T/W ratio for a rocket on a normal length rod is T/W = wind speed in mph:1 where 5:1 is the minimum and 20:1 is the minimum in a 20 mph wind. You may use a longer launch rod and use a lower T/W ratio, but the concept is to have the separation velocity to be 4 time the cross wind speed with a minimum rod speed of 20 mph (30 fps) to 80 mph (120 fps) in a 20 mph wind.

Bob

Bob, for there to be no simple answer, you certainly gave me one that I can be happy with as I (learn to) pick the appropriate motor for whatever condition. Thanks, Bob.

Gotta love this forum. I know we don't always agree, but there is such a wealth of knowledge here and in my local NAR group (I just forgot to ask them this question).

i agree that was a simple enough answer that would help anyone ....or you could do like i do and fly Vmax on windy days:horse:

I have been told (and use) 30mph is a safe speed off the rail/rod. I know it is just a rule of thumb type number.