Spool rocket flatulence?

[LithosphereRocketry]

So at my local club launch recently I saw a few rockets fly that basically were high-power spools. Every time the motor would make a noise almost like the video I've seen of hybrid motors and very much like a Skidmark- a loud buzz-ish part to the usual roar.

Has anyone else seen this with very draggy rockets that rely on a plate for stability? Does anyone know why this happens? If I had to make a guess I would guess some instability with how the smoke clings to the aft ring. I have no scientific basis for that though.

 

[mpitfield]

If I had to make a guess I would guess some instability with how the smoke clings to the aft ring. I have no scientific basis for that though.

You mean base drag? I have watched spool design rockets launch, and for the most part they are great flights and fun to watch. Every time I see one I always think that I should build one. I have never noticed the phenomenon that you are describing but I will pay closer attention next time. I can imagine some acoustical effect bouncing off the base but that is just a guess and above my knowledge. I will be interested to read any informed responses to your thread.

 

[rstaff3]

You mean base drag? I have watched spool design rockets launch, and for the most part they are great flights and fun to watch. Every time I see one I always think that I should build one. I have never noticed the phenomenon that you are describing but I will pay closer attention next time. I can imagine some acoustical effect bouncing off the base but that is just a guess and above my knowledge. I will be interested to read any informed responses to your thread.

I'm with Michael. I have one I've flown on H's and have seen some other HPR versions. I don't remember the motors on those but also don't remember any funky sounds.

 

[KennB]

It would seem the buzzing sound most likely generated by the disks of the spool vibrating from the air flow over and around them. If the disks had a very heavy cross-section, it would take a lot of energy to vibrate them; if they are thin, it would take less energy. It would be interesting to put the same spool up on a mid-power motor to see if it would be quieter.

 

[Daddyisabar]

Since you are impeding your flight with excessive drag the rocket is moving slower and the motor is working hard to lift the oddroc. So it is a bit lower and closer to the ground for you to hear as opposed to a beautiful, sleek 3-4FNC flying high nice and quick out of ear shot. The colder and denser the air the more the effect you get on motor noise. Also oddrocs tend to have bits sticking out and lots of disrupted airflow to make strange noises and whistles. Different motors have different sounds. There are nice sharp quick burners and then there are those with lots of gunk coming out the back, clogging things up and making lots of noise. Some motors might have tiny voids or uneven mix causing pops, maybe the igniter gets stuck in the nozzle. Maybe when someone is flying a weird one you are paying more attention, a little more scared than usual and you hear things?

 

[dhbarr]

Without knowing which motor(s), it's hard to say.

 

[LithosphereRocketry]

Without knowing which motor(s), it's hard to say.

The ones I heard were an I200W and a H-something Redline in (I think) the same case.

Sent from my LGL44VL using Rocketry Forum mobile app

 

[Threemorewishes]

I had a twelve inch diameter spool that I often flew on 38mm motors. My favourite motor was the CTI I223 Skidmark as it had a pronounced crackle in the exhaust note. I flew it on many different types of motors and I can't say I heard any noise like you describe that were similar to hybrids.

 

[BABAR]

Are you SURE it was the rocket? Did the RSO leave the microphone on?

 

[LithosphereRocketry]

Are you SURE it was the rocket? Did the RSO leave the microphone on?

There was no microphone present...

I recently heard a similar effect on a small (9") saucer on a B motor, so not limited to spools or HPR. Not much overall noise but definitely a WUHvwuhvwuhvwuh note to it.