Level 2 cert thrust ratio requirement?

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Tonimus

Tonimus

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A question came up this weekend. Has anyone heard of a documented minimum thrust to weight ratio for a L2 certification flight with the NAR? If so, where is that rule located? It ended up not being an issue for me, but it could have. Thanks in advance.
 
NFPA specifies a safe min thrust to weight ratio of 3:1.

Most rocketeers use a min of 5:1 to be safe.
 
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5 to 1 is the lowest I go, literally hunting out motors for that ratio on low test flights. Every time I load one up I worry about it on the pad too, because it feels so heavy for the motor. Then it takes off great and all is well.
 
I have not heard of a documented minimum thrust ratio for Level 2. The various rules of thumb (3:1, 5:1) became rules of thumb for good reasons but I personally would not want to see them codified. The NAR checklist is deliberately (and rightly IMHO) vague on this:

Is (are) the motor(s) sufficient to safely fly the model? Use motor manufacturer's recommendations or recommended motor
lists for similarly sized models as a starting point (Also consider, model weight, configuration, and finish when evaluating
motor capabilities). Is (are) the either NAR, Tripoli or CAR certified? Motors must be currently certified to be used.
Low current igniter?  Yes  No
 
I searched extensively for the weight ratio (but not the NFPRs). I couldn't find it. I did find NAR guidance on minimum launch speed of four times the max cross wind.

Personally I look for my safe speed off the rail, and on larger rockets I want 500 feet of altitude. (The simulation assumes straight up, and chutes are sometimes slow to open.)
 
NFPA 1127 4.9.1: "The maximum liftoff weight of a high power rocket shall not exceed one-third (1/3) of the certified average thrust of the high power rocket motor(s) intended to be ignited at launch."

Or if you prefer the version in the NAR HPR safety code (aka condensed NFPA 1127): "My rocket will not weigh more at liftoff than one-third of the certified average thrust of the high power rocket motor(s) intended to be ignited at launch."

Note that it specifies certified average thrust. Not the initial thrust or even the imprinted average. The imprinted average is usually close to the certified value but can vary by up to 20% and still be in compliance with NFPA 1125.
 
UhClem said:
NFPA 1127 4.9.1: "The maximum liftoff weight of a high power rocket shall not exceed one-third (1/3) of the certified average thrust of the high power rocket motor(s) intended to be ignited at launch."

Or if you prefer the version in the NAR HPR safety code (aka condensed NFPA 1127): "My rocket will not weigh more at liftoff than one-third of the certified average thrust of the high power rocket motor(s) intended to be ignited at launch."

Note that it specifies certified average thrust. Not the initial thrust or even the imprinted average. The imprinted average is usually close to the certified value but can vary by up to 20% and still be in compliance with NFPA 1125.
Click to expand...

The Safety code. Duh !?! :facepalm: :blush:
 
Keep in mind that even if you are using a motor with a certified average thrust ratio of 3:1, you need to look at the thrust curve to see if it has enough speed off the rail. If the peak thrust is later, then you may be well below the amount of thrust needed to get safely off the pad.
 
Again, this wasn't an issue for me, but I could see this being an issue in the future, which is why I wanted to clear this up. Thanks guys!
 
More important than the minimum 3-1, which is for very little or NO wind, is the safety factor.
If wind is over 10-12 I want 20 to 1. More wind=more thrust.
You must also figure in diameter and length.

A 3in glass version vs a 5.5 paper,will require more thrust [5.5 ]even if weight is same.
 

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