jlabrasca
Well-Known Member
- Joined
- Oct 31, 2016
- Messages
- 1,429
- Reaction score
- 476
I haven't put a baffle in a rocket above a 24mm MMT, but I do like them. The ones I have handled the small red cap on AT Hobbyline motors quite well. Have you considered how the baffle will handle the larger caps used on the AT HPR motors with ~1.5g of BP? I've never had an issue with them, but I suspect they come off the motor quite energetically.
Of course that is assuming you will even be flying AT motors with motor eject.
No, you got it. I am planning to send it up on a 38mm AT DMS J motor, and to bring it down with a JLCR.
I have considered that the baffle is going to take some punishment, which is why I cut the top and bottom plates from 3/16" aircraft plywood and will epoxy the baffle to the airframe. There's weight penalty, but this design isn't intended to set an altitude record.
If I got the geometry right, the width of holes at the bottom end of the baffle will be just about 80% of the diameter of the ejection cap. I am going to face the inside and outside of that hexagonal cone with aluminum tape.
I've never flown a DMS motor, but I know the RMS have a large plastic cap for the forward closure that holds the powder in. I imagine they could do more damage to a baffle then the hot ejection gases. I was just wondering if you were going to fly the AT RMS and had considered those caps.
I understood your question -- I just did a poor job of answering it.
I think the cap on the DMS motor is the same as the cap on the RMS.
It has an ejection cap, anyway, and I have considered the impact of that cap on the baffle. I am overbuilding the baffle (and the rest of the rocket) by a wide margin (I hope).
View attachment 341964
The DMS cap is an “innie”, a small red rubbery plug, that inserts into the ejection charge well after drilling the delay, inserting the washer, and pouring in the black powder. It’s about 3/8 inch diameter. ]The RMS cap is an “outie”, red plastic, sized to fit on the outside of the forward closure, so they are much larger. There wouldn’t be room inside the RMS ejection charge well for the powder and some kind of plug.
I wouldn’t worry about baffles being struck by a cap so much as being plugged by the cap, preventing ejection gases from escaping. Your tapered design looks like it could be susceptible to trapping the cap, but your design does not look like it could be plugged that way. If you’re flying an RMS motor you could just forego using the cap and wrap electrical tape around the forward closure to hold the powder in place. That has worked for many years and I’ve never had problems with it plugging baffles.
Edit: I do wonder about that 1.5 mm ply though; it’s pretty thin.
I would do the perforated disk out of 1/4 inch plywood and use it to anchor my shock cord as well.
Very nice build! I like the charcoal avoidance shielding on your ejection baffle.
I’ve found cutting ogive nose cones to be a bit difficult. The only way I’ve managed to get good results is to make a hole the desired diameter in a piece of scrap wood. Then I use it as a template, with a framing square, to mark the cut line on the nose cone. It looks like you’ve got the saw to do the job hanging on the wall in your picture. (6 ½” Shark Saw)
I shot two coats of Rustoleum 2X high gloss black. After that dries and cures, I'll try to top it with Spastix black and Spastix color changing paint -- to show off the bevels.
The problem with having a robot that can do precise and repeatable cuts, is that EVERYTHING starts to look like a job for the precise and repeatable robot.
View attachment 342412View attachment 342413View attachment 342414
Also finished the fillets on the motor mount and got some paint on my paint-test fin (weather was really nice here today).
I'm looking forward to having my robot to do my bidding. Soon, very soon. Nice cutting jig. Looking forward to seeing how it comes out.
From my experience, I found it easier and better looking to shoot Rustoleum Colour shift over rustoleum flat black.
It makes the contrast better if you have colour shift sections next to black: I did colour shift roll patterns over gloss black, and in the sun, the contrast between the gloss black and colour shift isn’t great in the shine.
Since then I tested the flat black and it worked better.
It’s also easier to see how much you’re laying down over flat black than it is on gloss, so I was less likely to put down too much in each pass. (Which I did the first time... ☹️
Loving this thread and all the different techniques.. really looking forward to possibly getting to see and fondle this beauty at one of our launches
Looks like it's comming along great, and a nice solid fin can. Are the socket head screws shown going through the tail cone for motor retention?
Paul B.
Enter your email address to join: