RW Fire Flyer and Mach 2

I was looking at RW's stock and trying to figure out what I might buy next. I noticed the 2.6" Fire Flyer. It has a 54mm MMT! That leaves about 1/4" between the MMT and the airframe.

I simmed it on Thrust Curve, and looks like even the smallest I motors will go near mach, with all of the Js, Ks, and Ls taking it way over mach and some past Mach 2.

If I go this route and put something big in it, to the tune of 2000+ fps, what do I need to know about building it that I don't already know? How do you ensure the fins stay on with that small a fin can?

Can I build this like I would build any HPR rocket and fly it at near Mach 2?

Minimum diameter rocket fins are epoxied right to the airframe and those fins don't fall off - as long as you build it right.

The Fire Flyer has TTW fin slots and thus is not a true minD kit.

any concerns on fin flutter with the split fins?

A very good question. Anybody got one these? I don't want to spend $170 on one and then watch it explode on its maiden flight.

I dont have one, but using strong epoxy and making internal and external fillets, along with foaming it on the inside. I doubt those fins will go anywhere.

I have one, and I am at the point in the build where I need to install the motor mount and fins. Foaming the 1/4" air-gap between the motor mount and fins would be messy, probably not worth the effort. Probably fair better mixing up a load of epoxy, heavily mixed with micro balloons, and injecting it into the cavity. Not sure if that would work at all, as I do not see people using micro balloons much anymore.

Here is an image of the fin



...and a pic of the discussed area showing the centering rings for reference...

Looks like a challenge!

SinfulDarkLord said:

I dont have one, but using strong epoxy and making internal and external fillets, along with foaming it on the inside. I doubt those fins will go anywhere.

Click to expand...

I am less concerned with the fins tearing off due to attachment than the fins themselves tearing apart due to fin flutter

See post 9 in this thread
https://www.rocketryforum.com/showthread.php?121796-3-quot-fiberglass-build-need-some-fin-help&highlight=flutter

les said:

I am less concerned with the fins tearing off due to attachment than the fins themselves tearing apart due to fin flutter

See post 9 in this thread
https://www.rocketryforum.com/showthread.php?121796-3-quot-fiberglass-build-need-some-fin-help&highlight=flutter

Click to expand...

That video just demonstrates yet another example that the fin attachment is not just your only weak spot. If I am not mistaken it has an outer and inner tube. If the fins were attached directly to the outer tube with no slots or internal tabs from the fins. Chances are tube failure occured from the bending loads of the fins while fluttering.

You can see the fins were holding on pretty good to the rocket, that is until it ripped the tubing apart.

I see minimal chance of fins ripping off due to fin flutter with the method I just described.

les said:

I am less concerned with the fins tearing off due to attachment than the fins themselves tearing apart due to fin flutter

Click to expand...

I agree, the 3/32" G10 split fins may not be able to withstand very high velocities without fluttering, particularly the forward fins based on their shape.

I was concerned enough to model the fins in AeroFinSim 4.5. I've not used this software enough in extreme flights to develop a high degree of certainty of the accuracy and reliability of the predictions. However, when I am running it with the more conservative CN-alpha options, fin flutter is predicted before Mach 2.

I'm planning on keeping mine at and below 1200 ft/sec, it's not very heavy and should perform well on CTI K260's and the like.

@Bat-mite - So order one already... Too bad you did not take advantage of this kit when it was the Hump Day Happy Hour special two weeks ago... I couldn't pass it up.

Order the kit and for only $9.99 right now you can customize the fins design.

Thank You
Sales Dept