My First Design questions

[nukemmcssret]

In my previous post I asked about Basswood or plywood now I am asking a new question. I have a manufactured motor mount that I put together. Here are two pictures of the bottom of the rocket. Here is my question. Do you have the engines sticking out or flush with the body tube? Or does it matter?

 

[shreadvector]

1) You will want to be able to flex the motor hooks so that you can install and remove the motors, so that is why they are often sticking out a bit from the back end.

2) If stability is an issue, you will want the motors as far forward as possible. If the model is very stable, then they can stick out the back a bit. With a cluster rocket, you will want it to be extra stable (in case one motor does not ignite), so make sure you use big fins that extend as far back as practical.

 

[Pantherjon]

Shouldn't really matter..Aesthetically, it looks better in picture 1..But #2 would be fine too depending on the length of the rocket, it could shift the CG back far enough to upset the stability..Personally I like it recessed so the hooks are flush with the end of the tube that way the rocket will 'sit' for display if the fins don't extend past the end of the tube..

 

[bob jablonski]

That would be more of a "what you think looks good" unless the stability is close then I would move the MMT into the BT a bit more. Being that it is a cluster I would make it a bit over stable in case one of the motors dosn't light.
Mr. Bob
Starlight Dude
www.starlightrocketry.com

 

[nukemmcssret]

Hey you all, thanks for the inputs. Personally I like the engines flush it makes the rocket look sleaker. The rocket is 33" long, 2.6" O.D., Wing Diameter of 9", and the margin is 3.35 overstable. Mass 12 oz. without engines, and a margin of 1.7 with engines. Looks like I need to add some nose weight. Also here is the rocksim drawing. Thanks nukemmsccret I would like to keep this thread open for future questions.

 

[shreadvector]

Can you flex the motor hooks if you glue the mount in flush?

 

[powderburner]

1) You will want to be able to flex the motor hooks so that you can install and remove the motors, so that is why they are often sticking out a bit from the back end.

Or, leave the MMTs inside the back of the main BT and relocate the motor clips----rotate them around each individual MMT tube (about 60 degrees) so the clip is pointing into the empty space between two MMT tubes and you will be able to push the motor clips back just fine

View attachment MMT design.bmp

 

[shreadvector]

Or, leave the MMTs inside the back of the main BT and relocate the motor clips----rotate them around each individual MMT tube (about 60 degrees) so the clip is pointing into the empty space between two MMT tubes and you will be able to push the motor clips back just fine

Yes, but the photos posted seem to indicate that they are already glued as a sub-assy with the hooks on the outboard side of each motor tube. Flexus-interruptus.

 

[rokitflite]

I like the motor tubes protruding... It gives a more detailed appearance.

 

[Micromeister]

I like the motor tubes protruding... It gives a more detailed appearance.

I agree with rokitflite, letting the motor tubes protrude gives a better appearance and also give the modeler a way to add some tape to the motors/tube joint which adds a bit to ensure the motor casings Stay in the motor tubes at ejection.

While it's two late to do on this cluster, I personally like to add 2- 1/8" wide x 2.25" long strips of File folder (cardstock) to each motor tube at the mating points leaving the rear 1/2" open and clear to allow masking tape to be installed all the way around each motor tube in the cluster. Since starting this practice in the mid 90's I haven't kicked a single motor casing regardless of retainer system used.
Hope this helps.

 

[JRThro]

The rocket is 33" long, 2.6" O.D., Wing Diameter of 9", and the margin is 3.35 overstable. Mass 12 oz. without engines, and a margin of 1.7 with engines. Looks like I need to add some nose weight.

Why would you add nose weight if the stability margin is 1.7 calibers with engines installed? Anything greater than 1.0 is considered stable.

 

[nukemmcssret]

Yes you are correct. I will not have to add any weight. Thanks

 

[powderburner]

Personally I like it recessed so the hooks are flush with the end of the tube that way the rocket will 'sit' for display if the fins don't extend past the end of the tube..

I agree with Pantherjon, I like the "hidden" look and I like being able to stand a rocket on its tail without having to use a special stand. I don't like having an "exposed" motor mount unless it is a major design feature of the kit design. I guess hidden-versus-exposed is mostly a matter of personal choice.

One advantage I can think of for a protruding mount (and motor) when using Estes-type motors is that if the rocket is descending tail-first, the motor case takes any initial impact damage (and you are going to throw them away anyway). Unless you use motor hooks....

As far as stability impact, most rockets using a triple cluster are probably going to be fairly long (as low-power rockets go) and moving the motors an inch this way or that is not likely to cause a giant impact on longitudinal balance and stability should not change much if the fins are moved along with the MMT. You still need to check, though, and be ready to add nose weight if the cluster is hanging back too far. It's all part of the fun of making decisions and choosing styles when you design your own rockets.

 

[Peartree]

Test fit it with expended motors.

How hard is it to pull out used motors after flight if the mount is recessed?

I have one kit (that I really like) where the mount slid forward a bit during construction and ended up recessed much as you've described. I don't fly it nearly as often as I would have, simply because it is a pain in the heinie to get motors in/out.

 

[powderburner]

... it is a pain in the heinie to get motors in/out.

That's why you have a two-foot length of good, sturdy 1/2inch diam dowel in your range kit. Insert from the front end and push the motor from the inside.

And if the nozzle crumbles, that's why you have a good, solid mother-of-all-tools pliers in your range kit.