Vortex Generator-Scratch Ex. Rocket

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Well-Known Member
Apr 29, 2009
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I have been interested in the possibilities of rocket designs using ducted air for some time and have followed the discussions on thrust augmentation with interest. This spring I finished converting a Dynastar Sky Torpedo to the Skyscoop by adding airscoops to direct air around the mmt to reduce base drag. It turned out to be a good flyer but I didn't observe any real gains in performance.
So in this design to reduce the drag from scoops I'm routing air from the nosecone through the center of the rocket and out the middle of a triple 24mm cluster. The largest diameter tube that fits in a BT-80 with 3x24mm is 14mm.
Firstly I laid out the rings on 1/8 birch plywood using the Estes tube marking guide and a centering ring.

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The mmt holes were cut with a 1" hole saw hammered and filed to closer to the motor tube size. The major challenge with these rings is to get them to hold together during fabrication as the motor tubes reach the outer edge of the ring. My solution was to leave some of the plywood waste on the rings when cutting them out with my bandsaw. After the mmt is glued together the excess material will be sanded off the rings. The clear tube with the reducer mounts in the center of the rings and joins to a BT-50 coming down from the nosecone. Ted

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The motor mount parts were test fitted and then motor hooks and engine stops were glued on. After curing the entire mount was glued together with 20 minute epoxy(used throughout) in one step. I used 2 inch balsa spacers between the rings and lots of tape. It can be difficult to get the rings level and all the tubes parallel but it turned out close to perfect. I then cut strips of bt-20 and glued them onto the central plastic tube between the motor tubes. This was done to protect the plastic from any hot gases and to provide attachment points for the through the wall fins. The plastic reducer at the top of the mount was covered by a slice of 1" I.D. tube to protect it from ejection gas. The extra 1/8 ply on the rings was removed with a dremel sanding wheel and hand work. until the mount fit smoothly in the BT-80 airframe.

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Well I didn't mean to abandon this build thread but my desktop is down. It needed it's annual wipe. My kids have been playing games on several kids sites and picked up some kind of malware that corrupts the OS. The rocket is nearly finished but the details will have to wait until I can access my pics. Ted
My desktop is back at last! I got it to the doc just in time as the hard drive was barely working and had to be replaced.
The V.G. just needs paint and vinyl on the fins. I drilled some more holes in the 1/8 plywood fin core and laminated 1/4 inch thick balsa onto them and sanded it to an airfoil. The fins have two tabs, one to fit between the motor rings and another at the top glued to the central vent tube.
The motor mount was beefed up with pieces of bt-50 tube glued onto the clear vent tube and a shroud put around the bt-50to14mm reducer to protect it from ejection gases. Epoxy glue was liberally added to all the motor mount joints. It may seem like over-building but the reason will become apparent later.

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The nosecone is designed to admit air into the bt to pass through and out the motor mount. I thought about several different shapes and their tendency to act as a ringfin, an effect I wanted to minimize. The central passage through the bt is 24mm and simply cutting a hole that size in the end of the nc seemed pretty draggy so I found a conical plastic shot glass for the interior passage that was cut to mate with the internal tube. The bottom of the nc was cutoff leaving an inch or so of shoulder on it. I made a bulkhead from 1/8 plywood with a 1and3/16s hole through it to accommodate a loose tube joining the shotglass in the nc to the internal tube. The joiner tube will be taped in place for flight allowing the cavity in the nc to house a small altimeter and adjustable noseweight. Three 1/4 inch eyebolts were glued on the bulkhead and it was epoxied into the nc. Ted

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I pondered for some time about the dynamic stability of this rocket.At angles of attack just what effect would the nosecone cause? Would it cause the rocket to veer off course or dampen movement off the axis? I decided eventually that the best thing to do was spin stabilize the rocket using airfoils on the fins. I will pay the price in drag but can guarantee? a stable flight if it's statically stable. The fins will be unpainted on the curved side and if it's stable I can reduce the airfoil by sanding the fins.
I'm planning to use this rocket for altitude testing of the internally vented design versus a conventional rocket. I've saved the piece of nosecone cut off the tip and will tape it in place for comparison testing. Ted

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The next stage was to make the internal tube and supports. I drilled several holes around two 50/80 centering rings. These holes will allow passage of the ejection gases around the internal tube, form a parachute compartment in the forward 1/4 of the airframe and function as a baffle to protect the twin 18 inch chutes. I used a full 34 inch length of bt-80 for the airframe and cut three pairs of slots in it for the fin tabs. The mmt was recessed two inches into the airframe and glued in place with plenty of epoxy.
After the mmt was dry I mounted the cr's on the central tube and attached a length of kevlar below the upper ring for a shock cord. The central tube was attached to the mmt and glued in place in the bt.
The next step is decorating it. I plan to use a vinyl film over the flat side of the fins to cover the holes. Two of the fins will have black and one will have a mirror surface, hopefully producing a visible flash as it rotates. I'd like to use a pattern on the airframe that will show some visible effect during rotation. Can anyone suggest something? Ted

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Here are some pics of the built rocket. The fins will be skinned after painting. If you noticed their is no visible launch lug. I plan to use the internal passage and a sabot? coupled to the airframe below the mmt. The air passage starts out 43mm in diameter,decreases to 23mm through the nosecone, increase to 24mm for most of the length then decreases again to 14mm for about three inches through the mmt. Accordingly I'll use a six foot length of 1/2 inch O.D. steel tubing as a launch rod up the center of the rocket. I'll also be making a custom launcher for this bird consisting of 3/4 inch plywood discs on a flat X base stacked with the rod through the center. A six foot length of sonotube with three supporting legs will be placed onto the launcher with the rocket in place to form a tube launcher. The aforementioned sabot is a cone shaped device attached by a coupler below the mmt to confine the motor exhaust and help propel the rocket. It will fit loosely enough to drag separate after exiting the tube. I expect it to sound like a cannon at launch:D
The overall length of the rocket is 37 and 3/8s inches. I think I'll use the cardboard cutout method to determine the center of pressure as this generally gives a conservative estimate. The nosecone is obviously draggy pushing the C.P. forward but so are the fins bringing the C.P. rearward so hopefully they will cancel each other out. The C.G. will be set at one caliber from the C.P. for the first flight and possibly reduced later if advisable.
I'm very interested to hear your comments or criticism particularly regarding paint schemes. Ted

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Really cool work, I'll be interested in the results. I don't know how much you'll gain. You will be sending a lot of air flow down a rather long and narrow tube. Even if the ID is super smooth, friction may get you and reduce the airflow down the center to nothing.

A very interesting next step would be to make a rocket with replaceable / interchangeable tubes so you can test different lengths to see what effects you can archive.
Thanks for your interest. I agree that the outflow might not be much but I'm unsure as this is all blue sky stuff to me. There are a few bumps inside the tube at the joints and the restriction near the end will raise the pressure too. I recall reading on another topic describing airflow through aircraft coolers, increasing the pressure of restricted air before exit raised it's velocity.
I may shorten the airframe later after some flights and observe the effects. I plan to fly it on D-12's to start and I expect altitudes in excess of 800 feet on those motors and maybe some E-9's if all goes well. I ordered a Quest How High altimeter from my LHS yesterday so it's time to get busy on the launcher. Ted
I'm still hoping for some paint scheme suggestions...So far I've thought about a barber pole stripe. When rotated it will appear to climb the airframe and it will be easy to do using the tube spirals. I would guess at high rotations this effect wouldn't be seen as it would appear as a blur. By using two primary colors such as yellow and red a color change to orange should become apparent when it's spinning rapidly.
Another idea is to paint it all black with the aforementioned reflective surface on the rear of one fin with a vertical reflective stripe on the airframe to flash with the fin.
What I'm hoping to achieve is a visibly changing appearance at differing rates of rotation. Ted