What thrust rating to use?

[stantonjtroy]

Ok, I know the rule of thumb is a 5:1 thrust to weight ratio for a good, safe boost off the pad. Rocket weight is easy, just weigh the thing. Power is easy, get it off the thrust curve. My question is Which power figure do I use for THIS PARTICULAR rule of thumb (5:1). Do I use peak thrust, average thrust or total impulse? I would assume average but I don't like to assume. Any help would be appriciated.

 

[als57]

Most folks I know use the average number to be safe.

Al

 

[darkhelmet]

Some motors, like the Estes D12, have a designed-in spike in the thrust curve at the beginning. Some HPR motors are "regressive" with a higher thrust at the beginning and a gradual taper as it burns. It's ok to use the beginning peak to get the 5:1. Look at https://www.thrustcurve.org for the plotted curves.

Use >5:1 and a longer rod/rail in stronger winds.

 

[cjl]

Ok, I know the rule of thumb is a 5:1 thrust to weight ratio for a good, safe boost off the pad. Rocket weight is easy, just weigh the thing. Power is easy, get it off the thrust curve. My question is Which power figure do I use for THIS PARTICULAR rule of thumb (5:1). Do I use peak thrust, average thrust or total impulse? I would assume average but I don't like to assume. Any help would be appriciated.

Use initial thrust. In some motors, this will be lower than average thrust. In some, it will be higher. In some, it will be the same as average thrust. In all cases, initial thrust is what will be propelling it off the rail, and therefore, it is the important parameter for stability.

 

[spacecadet]

Although that formula would tell you you can fly a 280g. model on a C6 with a peak thrust of 14N, which is pushing one's luck rather. I wouldn't go over about 150g. by design, but wouldn't be too worried at 200. It just wouldn't go very high.
Perhaps I am overly conservative.

 

[Bill P]

Higher thrust is needed for higher drag rockets and rockets with small fins.

 

[Handeman]

The whole point of thrust ratio etc is to provide enough speed for the fins to stablize the rocket by the time it leaves the guide of the rod or rail.

Speed is the value you're looking for. What speed do you need? that depends. Mostly it is a factor of fin size, calibers of stability, and wind speed. Small fins, 2+ calibers of stability, and high winds will require more speed. Large fins, 1 caliber of stability and no wind would need very little speed.

Use simulators like wrasp, rocksim, etc to determine what the speed will be at the end of the rod. Remember that you really should use the distance from the top rail button/launch lug and the top of the rail/rod as the height of the rod when doing calculations.

 

[ben_ullman]

The whole point of thrust ratio etc is to provide enough speed for the fins to stablize the rocket by the time it leaves the guide of the rod or rail.

Speed is the value you're looking for. What speed do you need? that depends. Mostly it is a factor of fin size, calibers of stability, and wind speed. Small fins, 2+ calibers of stability, and high winds will require more speed. Large fins, 1 caliber of stability and no wind would need very little speed.

Use simulators like wrasp, rocksim, etc to determine what the speed will be at the end of the rod. Remember that you really should use the distance from the top rail button/launch lug and the top of the rail/rod as the height of the rod when doing calculations.

or........

(average / 4.45)/5 = lbs at 5:1 T:W

haha you always have the explanation and I always think "Does he REALLY do that" /sarcasm

Ben

 

[GregGleason]

Like cjl said, it is the initial thrust that matters. As I have studied the issue, what I look for is what is going on in the first 0.12 to 0.25 seconds of motor burn. That initial thrust is going to help you get an idea of performance beyond the rod/rail as the fins are becoming effective for stabilizing the rocket. What helps me is taking the wRasp data and bringing it into Excel to give me a better understanding of the limits of the motor. Once I understand the max initial thrust, I can then estimate more precisely the max lift off weight. Other things to consider that the spreadsheet will not show you, is performance in moderate winds, peak altitude, etc., but it is at least a starting point to make a determination if a motor should be considered or not. An example using the the AeroTech G71R-RMS follows.

Greg

 

[Handeman]

or........

(average / 4.45)/5 = lbs at 5:1 T:W

haha you always have the explanation and I always think "Does he REALLY do that" /sarcasm

Ben

When you ask yourself that question, always answer YES! Then ask the important question, HOW?

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