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  #11  
Old 03-13-2016, 12:00 AM
wjwj wjwj is offline
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Really, epoxy? Isn't epoxy flame-resistant?

Your post got me thinking, so I did some theoretical physics calculations. Here is what I know:

The rocket plane is expected to weigh no more than 300g. According to the aforementioned site, the 7sec sucrose rocket produces an average thrush of 2lbf, or 9N.

If the plane weighs 300g, its weight is 3N. Therefore, the net force of the rocket is 6N upwards.

The acceleration at that weight and thrust is 20m/s^2, or 2G. Plenty for a rocket plane, since a rocket plane, by definition, is controllable!

This calculation reassures me that if I can mimic the end-burner sugar rocket, my rocket plane has a shot at working.

(P.S. - if the rocket plane accelerates for 7sec, it would reach speeds of >300mph without air resistance. Of course, air resistance will greatly limit my speed, but still cool!)
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  #12  
Old 03-13-2016, 12:37 AM
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That is why I suggested an endburner.
I have zero experience with rc planes boosted with rockets, but had remembered that Aerotech have 24mm RC hardware available with end-burning reloads so I figured it might work for you.
7sec with 9N average is similarish to an aerotech E6(7.1s, 5.3N)
http://www.thrustcurve.org/motorsearch.jsp?id=38
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  #13  
Old 03-13-2016, 12:49 AM
wjwj wjwj is offline
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You were right! That's why I asked. Thanks for the suggestion.

I was actually going to use either E6's or F10's originally. However, I want to do many launches, and each motor costs over $20 USD. That's as much as it costs me to fill up my car!
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  #14  
Old 03-13-2016, 05:41 AM
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You can end--burn APCP (Ammonium Perchlorate Composite Propellant) and get low-thrust rocket motors.
You can only core-burn KNO3/Sugar or KNO3/Epoxy motors and expect any sort of aprreciable thrust. The amount of thrust and burn time can be adjusted by core geometry and nozzle size.
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  #15  
Old 03-13-2016, 06:15 AM
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Quote:
Originally Posted by wjwj
You were right! That's why I asked. Thanks for the suggestion.

I was actually going to use either E6's or F10's originally. However, I want to do many launches, and each motor costs over $20 USD. That's as much as it costs me to fill up my car!

You can do whatever you want. So do it. Ignore the nay sayer. (I do, as do others around here)
Go with James (Recrystallized Rocketry) semi-positive experience with endburning and build on it.
Make some motors and do the tests, make more motors do more tests and if your results match or exceed those of James' tests, go fly.
Keep in mind that the E6 has an initial kick of ~12N (Red trace), which I suspect is done with a short core before transitioning to end burning.
An example of this short core is shown at the nozzle end in the second pic of a BlackPowder motor (eg, Estes, Quest).
The length of the core will affect the initial thrust and also the length of burn.
You will need to test this, along with nozzle diameter, to tweak your thrust curve.

You may also want to have a look at simulation programs like BurnSim. Will save you lots of time.
I've never actually tried to sim an endburner in BurnSim, but I'm sure there is a way to sim a short core transitioning to end burn. You might have to cheat, with a short Bates grain and a solid grain. Have a play

You've got me intrigued now, I'm going to fire up BurnSim
Edit: If you want the vital statistix for KNSU in Burnsim(they're not included) send me a PM.
More Edit: More reading: http://www.thefintels.com/aer/exper...ropellants2.htm
MORE edit... BurnSim has a Cored End-Burner option, make life simpler
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Last edited by /// : 03-13-2016 at 06:52 AM.
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  #16  
Old 03-13-2016, 08:46 AM
astronwolf astronwolf is offline
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Quote:
Originally Posted by wjwj
... I want to do many launches, and each motor costs over $20 USD. That's as much as it costs me to fill up my car!

The quest for cheap motors isn't cheap. In the end, you might develop the capability to make inexpensive motors, but the process of getting there carries some expense.
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  #17  
Old 03-13-2016, 03:08 PM
wjwj wjwj is offline
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I'll just try both! That's the beauty of experimental motors. I'll first try the sugar rockets as James' tests looked promising. If that doesn't work, I'll try dabbling with something else. As I mentioned before, though, I am more of an RC enthusiast and not so much a rocket enthusiast, so I probably won't do too much experimenting.

Also, do you think it would help if I increased the diameter of the motors? This would cause more propellant to burn and might increase the thrust a bit.

The other idea is to drill into the propellant a little, creating an initial core-burner for a powerful takeoff, then a long end-burner would follow to sustain the flight and add altitude.
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  #18  
Old 03-13-2016, 03:21 PM
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"The other idea is to drill into the propellant a little, creating an initial core-burner for a powerful takeoff, then a long end-burner would follow to sustain the flight and add altitude."

In a home made motor you can do this, but limit the depth and the width. In commercially made motors, I wouldn't do it. At all.
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  #19  
Old 03-13-2016, 03:25 PM
wjwj wjwj is offline
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Quote:
Originally Posted by Rocketflyer
In a home made motor you can do this, but limit the depth and the width. In commercially made motors, I wouldn't do it. At all.


Interesting. Can I ask why? I never planned on modifying commercial motors.
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  #20  
Old 03-13-2016, 03:35 PM
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Quote:
Originally Posted by wjwj
Interesting. Can I ask why? I never planned on modifying commercial motors.


By drilling out part of an end burner, you make it into a 'mini port burner', which burns faster. This will give you the kick to get off the ground. To deep, too wide, causes the grain to burn quite fast, sometimes too fast, with negative results. Altering a ready made motor increases the likely hood of having that motor blowing out the nozzle, the top cap, or both. At worst, splitting the sides open. Not cool.

From and old micro-grain enthusiast.
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