3FNC motor confusion

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Klatuso

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I cannot run any rocket simulation software because I have a Chromebook. It doesn't run software. So I'm building a 3FNC from Pemberton Technologies. I have two 29mm motors, an Aerotech Econojet F20-7 and G79-14. My BAR novice intuition says that these both should fly the rocket well. The F20 is weak but should get it up and parachute deployed adequately. The G79 will send it high, over 2,000 feet and the 14 second delay will not deploy the parachute at apogee but on the way down. With a padded shock cord a zipper should be prevented and letting it drop a bit could prevent it from drifting too far.

I emailed Pemberton and Apogee rockets about these motors but received rather different responses. Perhaps motor selection is more of an art than a science.

Apogee recommended against both motors. Tim Van Milligan responded personally and how impressed I was by that! I felt honored! He wrote that the F20 hasn't enough thrust and too long a delay. That concerns me. He wrote the G79 motor "would be good off the pad. But the delay is too long." But when I visit their recommended motor page for this rocket it has an Aerotech G54-14 motor. Same 14 second delay.

The Pemberton response from someone named Layne (?) was more enthusiastic. The G79 would "rock the 3FNC's world." asked for videos of the flight. "Now, the F20 may just be sufficient." and did say that he/she would need to run the simulation but was on the road and would need to run the simulation software.

Bottom line, I am pleased by the personal response to a question from a BAR who can't run the simulation software. One is conservative and the other liberal. Given that neither are completely wrong I'm still a bit confused. Obviously, what I should do is purchase an Aerotech F37W-M as recommended on the Apogee website, I don't feel like spending the $18.00. I am inclined to go with the G79, I have 80 acres and more open land around me.

I'm not starting a debate between Pemberton and Apogee, just making an observation about the art of motor selection and wondering if there are other opinions.
 
For quick and dirty sims based on rocket weights and a few other criteria use Thrustcurve.org no special software needed other than a internet browser.
 
Here you go. The biggest problem I see with the G79 is it needs a delay between 7 and 10 seconds.

OR.jpg
 
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Would the G79 give me enough altitude for it to fall for 7 seconds more? In your opinion?
 
Lane is The Pemberton.
Tim Van Milligan responded personally and how impressed I was by that! I felt honored!
I don't want to rain on your parade, but Tim answers everyone personally. I imagine Lane does as well. These are small companies and don't have much of a customer service staff. If it's not much of an honor, it's still a sign of good service, and the reason I buy from Apogee despite the lower prices elsewhere.
I cannot run any rocket simulation software because I have a Chromebook.
[Misconstraunce danger = Might sound snarky but isn't]I'd cultivate a relationship with someone who has a computer to run OR or RockSim on now and then. Family member, club member, other rocketry buddy, whatever. It's super useful.[/Misconstruance danger]

Apogee recommended against both motors. He wrote that the F20 hasn't enough thrust and too long a delay. That concerns me. He wrote the G79 motor "would be good off the pad. But the delay is too long." But when I visit their recommended motor page for this rocket it has an Aerotech G54-14 motor. Same 14 second delay.
OK, so Tim, being the more conservative, points out the risks of each motor. The G79-14 (if that's really what you've got) would give too great a velocity at deployment. You believe you're protected against zippering, but if the speed is great enough there can be other hazards, most notably shredding the chute. The chute shreds act like streamers and you come in fast. Do you think the rocket can take a hard landing? Are you sure enough about the anti-zippering?

The F20 will be slow off the pad, so highly susceptible to weather cocking. Can you fly on a windless day? Do you have enough field to recover anyway?

How disappointed would you be if any of these bad things happen?

After considering these things, especially the last one, only you can make the decision as to whether the reward is worth the risk, and which risk you're more comfortable taking.

The Pemberton response from someone named Layne (?) was more enthusiastic. The G79 would "rock the 3FNC's world." asked for videos of the flight. "Now, the F20 may just be sufficient." and did say that he/she would need to run the simulation but was on the road and would need to run the simulation software.
So, the more optimistic response says that the F20 is risky. Personally, I'd stay away from it, but see above.

From what others have said about the G79-14 not being available in single use, you've either got a single use motor with a shorter delay or an RMS motor with an adjustable delay, so maybe you're OK. Tim's response would have been based on a G79-14 as you stated, and Lane's on the G79s that are available. If that's the case, i.e. if you can pick your delay (I assume you read the -14 from something on the motor, so I think it's lots more likely to be the RMS) then forget everything else I've typed and use that.

Take a look at these flight results from other flyers: https://www.rocketreviews.com/3fnc---pemberton-technologies.html

The picture Qquake provided shows a nice, long motor mount tube. Perhaps after a successful flight on a G79- you could stick in a longer Aerotech or Ceseroni case and do your L1 flight.

Also, last thing, would the G44 that Pemberton's web site recommends really break the bank?
 
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I have an RMS motor and the G79-14 reload. I do not have a delay tool. Perhaps I will let that motor and reload sit on the shelf a bit.

I don't want to rain on your parade, but Tim answers everyone personally. I imagine Lane does as well. These are small companies and don't have much of a customer service staff. If it's not much of an honor, it's still a sign of good service, and the reason I buy from Apogee despite the lower prices elsewhere.[Misconstraunce danger = Might sound snarky but isn't]I'd cultivate a relationship with someone who has a computer to run OR or RockSim on now and then. Family member, club member, other rocketry buddy, whatever. It's super useful.[/Misconstruance danger]


OK, so Tim, being the more conservative, points out the risks of each motor. The G79-14 (if that's really what you've got) would give too great a velocity at deployment. You believe you're protected against zippering, but if the speed is great enough there can be other hazards, most notably shredding the chute. The chute shreds act like streamers and you come in fast. Do you think the rocket can take a hard landing? Are you sure enough about the anti-zippering?

The F20 will be slow off the pad, so highly susceptible to weather cocking. Can you fly on a windless day? Do you have enough field to recover anyway?

How disappointed would you be if any of these bad things happen?

After considering these things, especially the last one, only you can make the decision as to whether the reward is worth the risk, and which risk you're more comfortable taking.


So, the more optimistic response says that the F20 is risky. Personally, I'd stay away from it, but see above.

From what others have said about the G79-14 not being available in single use, you've either got a single use motor with a shorter delay or an RMS motor with an adjustable delay, so maybe you're OK. Tim's response would have been based on a G79-14 as you stated, and Lane's on the G79s that are available. If that's the case, i.e. if you can pick your delay (I assume you read the -14 from something on the motor, so I think it's lots more likely to be the RMS) then forget everything else I've typed and use that.

Take a look at these flight results from other flyers: https://www.rocketreviews.com/3fnc---pemberton-technologies.html

The picture Qquake provided shows a nice, long motor mount tube. Perhaps after a successful flight on a G79- you could stick in a longer Aerotech or Ceseroni case and do your L1 flight.

Also, last thing, would the G44 that Pemberton's web site recommends really break the bank?
 
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Got any drill bits?

every 1/32nd = 1 second of delay removed. I use a 5/32 bit.
Measure needed delay to remove on bit, wrap tape around that line for a "stop" so bit cannot dig deeper...or just use a bit stop if you have one.
 
Good advice. I just joined the Albuquerque Rocketry Society, and those folks launch really big rockets!
 
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