D engine mounts

[Joshua]

Hello, I'm Joshua. I just joined the forum today, after stumbling onto it some time back. I really haven't flown rockets any for several years because of the lack of a decent field to launch from. From there I discovered freeflight model planes and the fact that I could safely launch them at a local airport. The advent of micromax motors has lured me back in, and I plan to start using them sometime soon.

Anyhow, and old question of mine: I designed and built my only rocket with a 24mm mount about 5 yrs ago, one of the last rockets I built. It flew twice on D12-3's, absolutely beautiful both times, but after only the first launch, the motor mount, made from a BT50 section, was charred on the edges, the adhesive had blushed, and some of the edges of the spiral winding had started to unravel from the heat. It was like this by the time it had landed (yes, the 18" shute is a bit big, even for a BT60 rocket with a payload section). Is this a normal thing? The rocket is still airworthy, but if it were a minimum diameter rocket, it wouldn't be. Do I just have to live with this?

Thanks!
Joshua Finn

 

[jflis]

WHat you experienced is actually rare (in my experience). One thing that could cause such a problem is if you made the mount such that the end of the motor is flush with the end of the tube. If that is the case, you could prevent this by having the motor stick out about 1/4" (which is typical)

I have models with dozens, if not hundreds of flights on them without ever seeing such a thing happen.

jim
PS: Welcome to the forum! You're going to like it here

 

[powderburner]

Can you maybe tell us what the size of the main airframe tube is, and whether the motor mount is recessed inside?
The reason I ask is this: if you have a wide base area surrounding your motor it is easily possible to get some exhaust recirculation going. A trapped vortex can keep part of the hot exhaust gas circulating against the rear of the rocket, and this can indeed cause some scorching.
BTW, welcome to TRF!

 

[illini]

Another possibility is that you launched the rocket from a position too close to the blast deflector...and the deflected blast is what roasted your motor mount. Do you recall whether you had propped the model away from the blast deflector...maybe using a clothespin or a spent engine casing?

 

[Joshua]

I'll try to answer the questions below. As for the being a rare occurrence, I'm sure glad of that. I pretty much put any more large engine projects on hold indefinitely after that.

The motor mount is more or less standard. I believe it sticks out about 1/4" behind the body tube. The body is BT-60, so there's a big diameter difference. The lower centering ring is about 1/2" from the end of the motor tube. I didn't use any special stuff to hold the rocket above the blast deflector. The bottom of the body tube rested on the standoff (standard Estes launch pad). Is the clearance provided by the standoff not enough? I've never had this problem from 18mm rockets. Will a minimum diameter rocket have any cooling troubles like this? I've got a minimum diameter 24mm glider pod at home, but no glider for it (yet).

Good flying,
Joshua Finn

 

[illini]

We're all obviously grasping at straws on this one. Could be any one of the suggested causes. Bottom line: What you experienced is rare. Try using a clothespin to prop your next model 4-6" above the blast deflector and see if that works for you.

 

[jflis]

Yea, I'd be willing to bet that the problem was the model being very close to the blast deflector (I hadn't considered that earlier). The blow-back from the motor ignition could cause this very problem.

What a lot of folks do is use a clothspin to hold the model up, or (my preference) is to take a spent motor casing with the nozzle removed (just squeeze it with pliers to remove) and slide that over the launch rod. This will hold your model 2.75" above the blast deflector.

hope all this helps!

 

[GL-P]

Well, I keep my lower centering ring flush with the BT to prevent MMT damage.

That could be your problem