A10 Thrust Curve

[PaulK]

Hi Doc,

I recently bought some A10-0T motors with the COBRA labels, and noticed the thrust curve looks different from the current A10 packs. I'm familiar with the current A10 thrust curve, which has a short peak of ~12N, then drops down to ~1N for a full burn time of about 0.8 sec. The cobra labeled A10 motors include a thrust curve that drops to 0 at 0.25 sec. Has the curve on these motors actually changed over the years, or was that older thrust curve just wishful thinking?

Thanks

[shockwaveriderz]

wishful thinking

terry dean

[Carl@Semroc]

Since the 2004 Estes catalog and the latest (1995) curves from S&T both show the long tail and a 2.38N average thrust instead of 10N average thrust. I would think that someone must have put at least a few on a test stand and got those results, then just mislabeled the engines as A10 instead of A2.

The earliest ones must have been about a .25 second burn to have given them the higher average of 10N. I wish I had been watching all the engine designations versus actual performance more closely over the years. It is hard to separate reality and wishful thinking.

[billspad]

Since the 2004 Estes catalog and the latest (1995) curves from S&T both show the long tail and a 2.38N average thrust instead of 10N average thrust. I would think that someone must have put at least a few on a test stand and got those results, then just mislabeled the engines as A10 instead of A2.

The earliest ones must have been about a .25 second burn to have given them the higher average of 10N. I wish I had been watching all the engine designations versus actual performance more closely over the years. It is hard to separate reality and wishful thinking.

For model rocket motors the average thrust labeled on the motor is a combination of wishful thinking and marketing. The wording of NFPA 1125 allows the manufacturer to use pretty much any number he wants. I'm told the problem goes back many years and is unfixable because the NFPA will not allow any wording that allows grandfathering existing products and there's no way anybody is going to convince the manufacturers to voluntarily rename a lot of their motors.

The A10 is probably the only example where the mislabeling is actually a good thing. As you can see from the thrust time curve the 10 is caused by the large initial spike and the average is brought down by the long tail. I believe the long tail is there so the booster version of the motor won't blow through too soon. For the A10-3 you'll notice that the measured delay was 2.35 seconds. If you take the tail off of about .6 seconds and add it to the delay you get an honest A10-3.

[PaulK]

...The A10 is probably the only example where the mislabeling is actually a good thing. As you can see from the thrust time curve the 10 is caused by the large initial spike and the average is brought down by the long tail. I believe the long tail is there so the booster version of the motor won't blow through too soon...Funny, I was thinking that the booster would be better off without the tail; we want the sustainer to light at max velocity, which would be right at the end of the initial peak. Wouldn't the rocket be slowing down during that long tail?

[STRMan]

Funny, I was thinking that the booster would be better off without the tail; we want the sustainer to light at max velocity, which would be right at the end of the initial peak. Wouldn't the rocket be slowing down during that long tail?

F=MA

As long as the force of the thrust is = to the force of gravity + the force of friction, the speed will remain constant during the long tail. If the model is too heavy or draggy, it will decelerate. A lightweight slippery sustainer would not, and might even continue to accelerate.

[billspad]

Funny, I was thinking that the booster would be better off without the tail; we want the sustainer to light at max velocity, which would be right at the end of the initial peak. Wouldn't the rocket be slowing down during that long tail?

Going by what I've seen when A10's cato by blowing through too soon, I think that without that tail you'd be staging somewhere around the top of the rod.

The rocket would be slowing down but you don't always want to stage at maximum velocity since drag increases with velocity. That's assuming you're going for maximum altitude. A lot of things come into play but I think the A10 is the way it is because it's the only way it can be made.