Crazy Idea

ThreeJsDad

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I am not sure where to post this so it you see it in another forum please understand I am simply spreading some seeds in an effort to get started in the right direction.

I build RC Sailplanes or a living and specifically models designed for F5J style flying. To this end they are very light and very clean. They are sent to a starting altitude of 75-150m via a small electric motors.

I have two models a 3M and a 3.5M, the smaller one is normally 680g with motor, battery and all the goodies. The 3.5 should come in just under 1Kg. Obviously these numbers go down with the removal of the motor, ESC, battery and prop system.

I would like to build a couple into rocket boost gliders but I have no idea what motor would be a good option. Obviously we need enough total power but I would prefer the wings stay attached.

All help is welcome and greatly appreciated.
 

Ez2cDave

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Aerodynamic drag during Boost, due to high airspeed, will be your biggest obstacle . . . Wing may flutter and / or fail structurally.

The second obstacle is the high possibility of your having to pilot the sailplane, to make course corrections, while under rocket power, at speed.

Dave F.
 

ThreeJsDad

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3.5m I am working on and the 3m we have been flying for a while.
 

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ThreeJsDad

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I need to make the pod mold for the 3.5m. If, with your help, we can sort out a rocket motor option I may design a dedicated boost pod and make a nice mold.
 

ThreeJsDad

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What you want are the low thrust long burn motors purpose made by Aerotech for RC. Aerotech E6, for example. Frank Burke’s https://dynasoarrocketry.com/ has more info.
If we could launch them on something as small as an E that would be awesome !!! The wing alone on the 3.5m is 550g. It has a root chord of 205mm

I just looked at his models, they are cool but most are in the 250g range when ready to fly. My 3m Katana needs to be around 700g to have the right wing loading and the 3.5m Vanquish should be close to 1Kg
 
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Alan15578

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If we could launch them on something as small as an E that would be awesome !!! The wing alone on the 3.5m is 550g. It has a root chord of 205mm

I just looked at his models, they are cool but most are in the 250g range when ready to fly. My 3m Katana needs to be around 700g to have the right wing loading and the 3.5m Vanquish should be close to 1Kg
As a rule of thumb you want a near steady state climb at thrust greater or equal to twice weight, after you accelerate as quickly as possible to that speed, and then coast to apogee. So you are probably looking at the Estes F15, G12, or even HPR motors, along with as many A10Ps as you need to get you up to speed. You will also need serious R/C skills to keep the boost wing loads low, as well as a stiff enough wing structure to keep the wings from twisting or fluttering.
 

burkefj

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I went through the likely motor options already, are you still intending on sticking with the 75-150 meter target as this is under the general 400' FAA limit for class G airspace? Or are you looking to go higher? In any case, the motors I listed in the other thread are the most likely candidates. I know the E-6 will handle vertical boost up to around 15 ounces, but with the max 45 degree launch angle allowed by AMA/NAR/Tripoli rules I don't know if it would get your larger gliders up to 75-150 meters, maybe. There is the F-12 that has less total impulse but more thrust that might work. You'd have to work out clearance for your tail surfaces and thrust line for your pod.
 
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ThreeJsDad

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I went through the likely motor options already, are you still intending on sticking with the 75-150 meter target as this is under the general 400' FAA limit for class G airspace? Or are you looking to go higher? In any case, the motors I listed in the other thread are the most likely candidates. I know the E-6 will handle vertical boost up to around 15 ounces, but with the max 45 degree launch angle allowed by AMA/NAR/Tripoli rules I don't know if it would get your larger gliders up to 75-150 meters, maybe. There is the F-12 that has less total impulse but more thrust that might work. You'd have to work out clearance for your tail surfaces and thrust line for your pod.
Thanks for all the effort. The reason for the low launch is the fun of hunting thermals. In F5J the goal is to find a thermal at a very low altitude and then ride it for 10 minutes. I realize most rocket gliders are geared towards what we call sled rides but these are quite different in their design and performance.

Keeping the tails clear of the thrust line should interesting. The fuselages are over 5' so that would help.

Thanks again.
 

burkefj

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Thanks for all the effort. The reason for the low launch is the fun of hunting thermals. In F5J the goal is to find a thermal at a very low altitude and then ride it for 10 minutes. I realize most rocket gliders are geared towards what we call sled rides but these are quite different in their design and performance.

Keeping the tails clear of the thrust line should interesting. The fuselages are over 5' so that would help.

Thanks again.
If you're building these for a living or doing competition with them you probably have a thrust stand or some way to measure the thrust of the current electric motor setup or somebody's already done that for specific props most likely, Id just use your throttle settings and thrust/duration and compare that with the rocket motor thrust curves to pick something that's similar.
 

Alan15578

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Thanks for all the effort. The reason for the low launch is the fun of hunting thermals. In F5J the goal is to find a thermal at a very low altitude and then ride it for 10 minutes. I realize most rocket gliders are geared towards what we call sled rides but these are quite different in their design and performance.

Keeping the tails clear of the thrust line should interesting. The fuselages are over 5' so that would help.

Thanks again.
Ah, I thought your altitude was simply what you were achieving with electric power, and that you wanted to try for maximum altitude as a boost glider. Please disregard response 9, which is about achieving maximum altitude with minimal fuel or impulse.

Thermals,
Alan
 

ThreeJsDad

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Ah, I thought your altitude was simply what you were achieving with electric power, and that you wanted to try for maximum altitude as a boost glider. Please disregard response 9, which is about achieving maximum altitude with minimal fuel or impulse.

Thermals,
Alan
No Issues Alan, we are allowed a 30sec motor run and I am normally at my "hunting" altitude in under 10sec. In competitions we get penalized severely for shutting off over 150m and score a lot more points if we can shut off at 10-20m. I have done a full flight shutting off at 30m but 20 and below is tough.

There is a young man from Europe who can, with some regularity, hand toss his model, hook a thermal and ride said thermal for 10 the whole 10 minutes.

I have had a 40min flight with my 2m after shutting off just below tree top height.
 

ThreeJsDad

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If you're building these for a living or doing competition with them you probably have a thrust stand or some way to measure the thrust of the current electric motor setup or somebody's already done that for specific props most likely, Id just use your throttle settings and thrust/duration and compare that with the rocket motor thrust curves to pick something that's similar.
Thanks, We have done that but I was not sure how laterally the information would compare. That makes life pretty easy. I will post some numbers and see what you glider guru's think.
 

Ez2cDave

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Keeping the tails clear of the thrust line should interesting. The fuselages are over 5' so that would help.
Another thing to consider, aside from thermal damage to the Stabilizer & Rudder, is the situation of those control surfaces being subjected to higher than Boost airspeed velocities, if they are in the exhaust plume of the rocket motor.

In addition to being under higher aerodynamic loads and stress, I suspect that they will become much more sensitive to control inputs, while under thrust.

Thoughts ?

Dave F.

zitnan rcrg boost2.jpg 1d.jpg 1Winningmodel.JPG IMG_6145.jpg nigellaunching2.jpg nigellaunching3.jpg Partsreduced.jpg
 
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