Recommendations for bottom of rocket?

What are some recommendations to protect the bottom airframe, motor and motor tube at the bottom of a rocket?

I am guessing the easy thing to do is put in extra center rings until flush at bottom? Of coarse this idea will effect motor tube location.

Any other ideas? We are talking Level 2 5.45" rocket out of loc tubing, no glassing.

Thanks!!!

I always use a 1/2" birch plywood ring at the back that has a step. One diameter matches the airframe, and one slides inside. This also transfers the thrust from the motor to the centering ring to the airframe. All mechanical, no epoxy or glue joints under stress.

Edward

This is the ideal application for thrust plates, which I first saw on Apogee's site last year, but i think others are making/selling them too. Basically what Alphahybrids just described, but made from anodized aluminum for durability and coolness.

It looks like they may not stock the exact size you're looking for, but there might be someone doing custom work around here... otherwise you might stick with a plywood version that you could make at home.

I have been pleased with the products from https://scpconcepts.com/. He also does custom work.

I tend to stick with wood because it is easy to fabricate and weighs less than aluminum. I don't like adding weight to the wrong end of a rocket.

Edward

It would help to see a picture... If the aft is flush then the previous posts of wood or aluminum plates apply. But I often recess the aft CR so the motor retention is hidden. In the latter case I have added couplers and even "stiffy" couplers along with an internal Kevlar laminate to provide a tough rim that will take extreme abuse.

I'm working with larger motors, M to O, but my plate is a combination centering ring plus removable full diameter plate. Both are plywood. The combination of the two, bolted together, comprise the thrust ring. I paint the removable part with high temp engine enamel (black) and a little clear coat each burn. This protects the wood. Retention is bolts and modified washers capturing the thrust ring of the motor tube.

I have adaptors for motors from 114mm (half N to baby O), 88mm (lower N), and 75mm (M range). For 114mm this is just a ring. For the others, it is a ring with a smaller hole, with attached allthread rods extending into the rocket, holding small rings. These rings match alignment with internal rings, providing centering for the smaller diameter motors.

It is quick and easy to swap between motor sizes this way. Plus if I burn up an outer thrust plate it is easy to fabricate a replacement. I've burnt one up that was from inferior plywood - peeled the outer layer like an onion with a 7600 motor! Better plywood has worked flawlessly with the larger 88s. I just wipe it down and add another layer of paint each flight to replace what burns off.

With the size rocket you are working with there are many motor arrangements you could use. The limitation of course is the tubing taking the thrust. It probably wouldn't survive larger clusters for instance. But if you think the rocket is beefy enough, some sort of swappable core arrangement like I used may be of benefit. For instance you could swap between a 38mm, a pair of 38mm, and a 54mm motor. Otherwise it just adds complexity. The removable outer thrust ring isn't needed at all for more modest motors as there isn't enough heat transferred to do much if any damage. Large motors are another story.

An old way to protect the outer surface of the ring is to coat it thickly with epoxy. When that burns down, sand a bit and re-coat. Though I'm liking the high temp engine paint instead... Both methods work, and are a lot lighter and cheaper than an aluminum ring.

Gerald

I assumed the question related to physical strength, if it is heat/flame resistance, then yes, as Gerald said, I also use 1200 degree BBQ paint on the aft ends of all my rockets.

+1 for this.

SCP's thrust plates make for a clean installation and protect the bottom of the rocket.

Worsaer said:

I have been pleased with the products from https://scpconcepts.com/. He also does custom work.

Click to expand...

If you use wood for the aft CR, paint it with thin epoxy to seal it, sand it smooth, and give it a good coating of paint. I prefer black gloss, because it's easy to clean off the soot with a baby wipe.

the only thing protecting my 5.5 inch loc tube rocket is an aeropack. Excellent motor retention, plus because my aft CR is flush, it takes the hit so my CR doesn't....(it's got a thin coat of epoxy on it so it would seal & paint pretty)

Up to L2 as far as thrust distribution is concerned, if you have a strong internal structure to your fins such as a good bond at the root, internal fillets, or fiberglass strips that connect the finroots to the air frame, I think a thrust plate is a little overkill. That is a matter of personal preference, though. On some of my more recent original designs however, I have started to add another centering ring forward of the fins which adds additional structural support to the airframe and also reduces the amount of BP needed for recovery. For my L3 project I am going to have a total of 4 centering rings.