Friday, October 10, 2008

Custom kit

I recently was lucky enough to get a bunch of rockets and motors. An educator had an Estes bulk pack that he didnt want anymore. My plan is to build three rockets from the set: a 2 x 18mm cluster model, a stretched rocket for staging, and this 3 x 18mm cluster. The bag of motors is just a bunch of A8-3s... so that explains why the cluster designs, I need to find a way to burn those up in bulk.

This rocket has four body tubes, three nose cones, and six fins. To ensure stability, I added a few grams of white glue to the nose cone. The last step will be adding a launch lug; probably a 3/16ths or if not, a 1/4th. CA glue was used for reinforcements, and white glue for the initial assembly. The central tube will eject a small parachute, the outer two (nacelles) will eject a streamer each. Perhaps a short delay on the outer two then a longer delay on the center would be a nice dual deploy of sorts.

7 comments:

DTH Rocket said...

Cool. I don't often build kits, but when I do I like to modify it. Otherwise I feel like I'm building what everybody else has already built.

Nice design!

thimscool said...

How do you keep something like this flying straight without active control surfaces?

It seems that the most trifling imperfection would lead to a spin, at best...

R2K said...

Thimscool...

These rockets use a very old technology that we also see in arrows and weather vanes: aerodynamic stability. The center of balance for the rocket (where it spins, center of gravity) is well ahead of the center of pressure. That means that more air pushes against the back than the front. So it if tries to fly sideways, the back will get pushed harder than the front. That pushes the back back. Above 50 mph in a well designed rocket, this will make it very stable indeed. You can throw a dart or an arrow (or a rocket) backwards and it will tumble in mid air and fly straight. Its simple but reliable.

Active guidance is very hard and expensive. One other method is spin stabilization, like spinning a football.

DTH did I explain that well enough? :)

thimscool said...

What about spin along the axis of symmetry?

R2K said...

The fins will fight spin (unless they are "canted" or angled and then will produce spin) to some extent. But often rockets will spin, some very quickly indeed (to 300 rpm) depending on flaws or fin angles etc...

Spin isnt a bad thing per se; it can make a rocket more stable, and looks really cool.

I am most impressed by the power of rockets when I see them spin. Recently I saw a K rocket fly with just two fins. It spun up to make it stable, and got up to 100+ rpm. Can you imagine the power involved in that?

rocketry said...

Nice design. I really like the looks of this one. I enjoy doing clusters from time to time. Do you use a clip whip?

R2K said...

Nope! I cant claim to be great at clustering as I have done only a few so far. I did a 5 D flight that shockingly worked well (lucky). Then one or two AP clusters with three motors with recovery. I flew my 4 motor cluster once and only two motors lit. That is the limit of my experience. I know the theory, but when it comes to doing it I have been lax at best. I will do better, however.

Rather than a clip whip, for three motors I will use two lengths of braided copper to make rails. Then the clip goes to each rail.

Next post will show my funny little 2 motor rocket made from the same parts as this one.