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Aerotech SU 29mm Motor Confusion

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qquake2k

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Okay, I'm trying to wrap my feeble mind around all the different Aerotech single use 29mm motors. So far, the only 29mm rocket I have is the Hijacker. It's 3" diameter and 45" long. Weighs in at 16oz (according to the manufacturer). The recommended SU motors are F20 and G80. But when I look at the motors available, I find F20W, F23FJ, F25W, F26FJ, F27R, F32T, F42T, F50T, G38FJ, G40W, G77R, G78G, G79W, and G80T. I've tried researching the different motors, but all I can find are small graphs with thrust curves. The graphs show me that some of the motors have high initial thrust and short burn time, or low initial thrust and longer burn time. I get that. But some are so close, they're almost identical. Some of the motors aren't shown in the graphs. They're in the charts, but the charts only show the motors' physical size, total NS, propellant weight, motor weight, and delay times. They don't list maximum thrust, burn time, maximum liftoff weight, etc.

So is the major difference really the propellant type? I read about them in the Aerotech catalog. White Lightning, Black Jack, Redline, etc. Would I be safe in assuming that if the Hijacker will fly fine on a G80T, then it will also fly fine on a G77R or G79W? They all have somewhat similar thrust curves. Or is it mostly trial and error? Is there any more comprehensive information available somewhere for the Aerotech motors?
 

Handeman

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http://www.thrustcurve.org/ is your friend. Check the data and thrust curves for any motor.

The general rule is 5 to 1 thrust to weight, based on average thrust. You can use the thrust curves to determine that a motor with 3 to 1 might be safe if the initial thrust was 5 to 1 or high and will get the rocket stable.

The manufacture graphs are OK, but thurstcurve.org has actual file formats (most you can look at with Notebook or other text editors) and give you a little more info. It isn't cluttered with other motors either.
 

tomar

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I hear you.

Here's what I would do. If you have RockSim or WRASP, simulate the flight with each motor. You'll find differences in performance (altitude, acceleration, etc.). A lot of times, the differences are simply the difference in propellant types. Some of the propellant fomulations generate better altitude while others are for visual effects.

What a position to be in, so many motors to choose from. Wow! What a difference from just 10 years ago.
 

MarkH

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The F20 and G80 are at the low and high ends of the avg thrust and total impulse levels for the motors you listed. If it flies on those two it will fly on everything else in between. The G80 has the highest total impulse so it should give it the most altitude.
 

cjl

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Agreed. If it will fly on an F20, and a G80, then you should be good to go on any motor in between, including all that you mentioned in your post.
 

MattieShoes

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I disagree. The two most important numbers would be total impulse to make sure it gets high enough and impulse over the first fraction of a second to make sure it leaves the rod/rail with enough speed for the fins to render the rocket stable. Ideally, we'd want a motor to at least exceed the lowest recommended engine in both areas.

The F20 has 60.5 Ns of total impulse. The F23, F27, and F42 all have less total impulse.

The F20 has about ~50 Newtons of impulse over the first fraction of a second. The F23, F27, F26, F10, and G25 all have less impulse in that first fraction of a second.

So the avoid list includes: F10, F23, F26, F27, F42, and G25.

Of those, I'd still feel fine with the F23 and F42, they'd just be low-altitude. The others probably would work but I'd run some sims before using them to make sure it's leaving the rod/rail with enough speed, or at least launch it on a safer motor first just to see how fast it's leaving the pad.
 

MattieShoes

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Exactly. ;)

Mark K.
I was just going off all the AT SU 29mm engines from thrustcurve :)

I imagine it'd probably fly fine on any of them, but I'd err on the side of safety at least until I flew it a few times and had a feel for how fast it leaves the pad. :)
 

MarkH

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The F20, F27, and F42 all have about the same total impulse, 50-55 Nsec. Yes the F23 is a little weaker at 42Nsec, and the initial spike is weaker too. The F42 has a much higher initial spike and will take off much faster than the F20, F27 or F23. I've flown all but the F23 in my AT Initiator and the F42 definitely will make it lift off faster. To me, the F27 seems a little quicker off the pad than the F20.

The easiest thing to do would be to use the recommended engines for the AT Initiator as a guide. Since the Initiator and Hijacker both weigh about the same 14.oz and 16 oz. per manufacturer specs, they should both lift off at about the same speed for a given motor, (with the Hijacker being slightly slower off the pad because it weighs a little more). The Hijacker would get a little less altitude than the Initiator since it is a little heavier and is 3" dia vs 2.6" for the Initiator.

I disagree. The two most important numbers would be total impulse to make sure it gets high enough and impulse over the first fraction of a second to make sure it leaves the rod/rail with enough speed for the fins to render the rocket stable. Ideally, we'd want a motor to at least exceed the lowest recommended engine in both areas.

The F20 has 60.5 Ns of total impulse. The F23, F27, and F42 all have less total impulse.

The F20 has about ~50 Newtons of impulse over the first fraction of a second. The F23, F27, F26, F10, and G25 all have less impulse in that first fraction of a second.

So the avoid list includes: F10, F23, F26, F27, F42, and G25.

Of those, I'd still feel fine with the F23 and F42, they'd just be low-altitude. The others probably would work but I'd run some sims before using them to make sure it's leaving the rod/rail with enough speed, or at least launch it on a safer motor first just to see how fast it's leaving the pad.
 
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hardinlw

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One thing I have not seen mentioned is the delay. A low impulse motor will need a shorter delay than a high impulse motor. The F20W is in the lower end of the F impulse range, while the F50T is in the high end. You'd need a longer delay with the F50 than with the F20. Aerotech publishes a "Motor Matrix" which should be on their web site. It lists all the engine options for their kits. If you can find one that is similar in size and weight to your rocket and calls for the same delay on the F20 and G80 as your kit instructions, then all the other options for that Aerotech kit should be OK for your rocket.
 

dpower

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Okay, I'm trying to wrap my feeble mind around all the different Aerotech single use 29mm motors. So far, the only 29mm rocket I have is the Hijacker. It's 3" diameter and 45" long. Weighs in at 16oz (according to the manufacturer). The recommended SU motors are F20 and G80. But when I look at the motors available, I find F20W, F23FJ, F25W, F26FJ, F27R, F32T, F42T, F50T, G38FJ, G40W, G77R, G78G, G79W, and G80T. I've tried researching the different motors, but all I can find are small graphs with thrust curves. The graphs show me that some of the motors have high initial thrust and short burn time, or low initial thrust and longer burn time. I get that. But some are so close, they're almost identical. Some of the motors aren't shown in the graphs. They're in the charts, but the charts only show the motors' physical size, total NS, propellant weight, motor weight, and delay times. They don't list maximum thrust, burn time, maximum liftoff weight, etc.

So is the major difference really the propellant type? I read about them in the Aerotech catalog. White Lightning, Black Jack, Redline, etc. Would I be safe in assuming that if the Hijacker will fly fine on a G80T, then it will also fly fine on a G77R or G79W? They all have somewhat similar thrust curves. Or is it mostly trial and error? Is there any more comprehensive information available somewhere for the Aerotech motors?
Some of this just comes with reading through all the documentation and understanding the terms Average Thrust, Total Impulse, and reading the charts. The Aerotech catalog really tells most of what you need to know, sit back and study it for while. If you ask nicely, they may even send you a printed catalog.

Some other tips:

There are three different lengths of AT SU motors- Econojet F, Regular F, and G motors. The Econojets (F20/F23/F27/F42) are in the shortest case, and have quite a bit less total impulse than the full F25 & F50, though are a less expensive. (Note that the F32 is a 24mm motor) The G motors are the longest, and vary widely in total impulse, based primarily on their propellant, from the G38 at 87N-s to the G80 at 137N-s. The higher average thrust number generally provides a higher lift off weight (assuming the appropriate delay is chosen). Some of the motors are very similar, and simply have different color/sound effects. The catalog explains the propellants nicely. Once you do this for a while, you'll appreciate the wide variety of motors Aetotech makes. It really adds spice to the hobby.
 

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