Brief:
This is my entry for the EMRR “Retro Spaceship” rocket design. When this contest was first introduced I was very excited that I finally had a good excuse to build a vacuum former so I picked “Cool Rockets Space Bucket”. My daughter who helped throughout build said it looks like a rocket that Jimmy Neutron would build so we have named it “The Neutron.” Neutron is designed to use a D/E motor and is rear engine ejection with a 18-24 inch Nylon parachute.

Construction:
Component Description:

• .030” Thick Polystyrene Sheet
• 3/32” Balsa Sheet
• 1/4” Balsa Sheet
• 3/16 Wood Dowel
• BT55 – Body Tube
• BT50 – Motor Tube - 2.75” Long
• BT20 – Motor Mount Tube – 6.25” Long
• 2 - 1/8” Launch lugs
• Centering Ring (T20 to T55)
• Centering Ring (T50 to T55)
• Centering Ring (T20 to T50)
• 36” - 300 # Kevlar String
• 12” - .75” elastic shock cord
• Nose weight
• 24” Nylon parachute

Plans/Vacuum Forming:

The first step was to layout the design and determine its size. Once the overall layout was complete I started to build the vacuum former; the design I used is available in the featured tips section of EMRR. I experimented with different shapes and master material. I settled on the pink insulation foam for the master material. It lasts for about 5 cycles and then starts to degrade from the heat. The other tip I discovered was to use baby powder as the release agent. It protected the foam and allowed for much easier removal of the master.

Construction of the body and cockpit

As I mentioned above the body and cockpit are made by vacuum forming polystyrene sheet into the appropriate shape. I have included template (template A) showing the outside size of the body master and cockpit. In addition pictures of the foam masters are included. The body halves are made from the same master to help ensure a good fit when assembled together. The cockpit section is made from a separate master. In this master I added the windshield and porthole recesses. The masters are both made from pink insulation foam by sanding them into each shape.

Each of the vacuum formed shapes were trimmed after forming and test fit. The cockpit section was then trimmed to fit the top body section. After trimming to a satisfactory fit, the cockpit was secured to the top body section using 5 minute epoxy.

Construction of body tube

Start with the 12” BT55 tube and cut two disks out of the ¼” balsa to fit the inside diameter of the BT55 tube. These will be used to plug the end to the tube. Wrap the Kevlar shock cord around one disk and glue the second disk to the first sandwiching the Kevlar cord between them. Insert the disk into the body tube so the Kevlar is feed down the tube. The Kevlar cord will be attached to the motor tube in a later step. Glue the disk (and Kevlar) assembly roughly 1 inch from the end of the BT55 tube. Fill this space with nose weight. I used BB’s (about 4 oz). Now cut a third disk out to plug the end of the tube and glue it in place.

Next, cut the 3/32” balsa per the templates: two of template B (side pieces) and 2 of template C (top and bottom pieces). Mark the tube as if you were mounting 4 fins 90 degrees apart. Mount the 2 side pieces (template B) to the tube 180 degrees apart. Mount the top and bottom pieces (template C & D) on the body tube 90° from the 2 side pieces. Be careful to make sure you mount in the same orientation – front should be toward the end of the tube with the plug that was just inserted. Lastly, on one side of the tube mount the launch lugs at the very front and rear of the body tube along the joint between the side piece and tube. Now add a generous glue fillet to each of the 4 balsa pieces. Be careful on the launch lug joint – make sure you leave clearance for the launch rod.

Construction of motor tube and shock cord

Start with the BT50 tube 2.75” long. Glue a centering ring into the end of the motor tube so that a “D” engine sticks out 3/8”. Now take the BT20 tube 6.25” long and glue it into the centering ring mounted in the motor tube (see picture). Next, cut a small groove on the inside of the T20 to T55 centering ring. Glue this .375” from the end of the tube opposite the motor tube. Slide the Kevlar cord (attached to the body tube) through the groove on the centering ring and tie it around the tube. Now pull the Kevlar up to the centering ring and apply a bead of epoxy to the Kevlar, centering ring. and tube joint (see picture). Next, epoxy the .75” shock cord to the BT20 tube (see picture). Finally, mount the last centering ring (T50 to T55) on the motor tube 1.5-2.0” from the end of the motor tube. Once this assembly is dry, test fit it in the body tube and make sure the motor tube slides in and out very easily. Leave it motor sub-assembly in the body tube for now.

Final assembly of the body and body tube

This was the most challenging step. First, I had to make clearance cuts (about .5”) from the rear of the top and bottom body halves for clearance of the body tube (see picture). Once there was clearance for the top and bottom sections, I taped the top half of the body to the body tube sub-assembly and poked straight pins through the balsa wings that overhang the body. The pins were placed about 1 inch apart. The purpose of the pins was to ensure the top and bottom halves were aligned well. The top and bottom halves were mounted to the side wings using 5 minute epoxy and the pins were removed. After the epoxy was set I cut the excess balsa as close to the body sections as possible leaving a slight overhang. I then sanded the wings to a smooth transition between the top body section, bottom body section, and wings. There were a few areas I was not satisfied with this interface so I filled these with thinned Emery’s wood putty and then sanded smooth. I repeated this pr ocess a number of times until I was happy with the transition.

Next, I cut clearance for the launch rod in the rear portion of the body. Using a straighten coat hanger I made a deformation mark on the front of the body by pushing the hanger through the launch lugs from the rear. With an Exacto knife I made a clearance hole in the front of the body section for the launch rod (see picture); now double check that the launch rod will slide smoothly through both launch lugs and body.

Construction of top and bottom wing detail

The top wing was cut from 1/4” balsa sheet using the attached templates (template E). The two pieces were glued together and top edge rounded. A 3/16” dowel was sanded to half the thickness to make the rib detail on the wing. Each dowel was tacked down at one end and formed into the curved shape. Each rib was then clamped and glued into final configuration. Final sanding and fitting to body was done prior to gluing to the body.

The bottom wing was cut from ¼” balsa sheet using the attached template (template F). The wing was cut out and sanded smooth rounding all the edges. Holes were drilled and sanded to the correct size shown in the template. The two dishes were also cut from ¼” balsa sheet (template G & H). Each dish was sanded to create the curved back portion. A hole was then drilled through each dish and enlarged by sanding to fit in the appropriate location on the wing. The dishes were then glued in place. Final sanding and fitting to body was done prior to gluing.

Each wing was filled using Elmer’s wood putty thinned with water and sanded smooth.

Final Assembly of the body and wings

The top wing was test fit to the body sanding where needed to improve fit. The wing is then mounted to the body using 5 minute epoxy. The same technique was used for mounting the bottom wing. Large fillets of epoxy were applied to the joints between the body and top wing. This was repeated for the bottom wing.

Plexiglas side wings

After testing flying the prototype rocket (see below) I found I had to add wings on the side of the rocket to make the launch more stable. After I completely the rocket I added 2 .060” Plexiglas wings (template I). I attached each to the rocket using 5 minute epoxy.

Finishing and painting

After all the balsa wings were filled with Elmer’s wood putty watered to a thin paint consistency. The entire rocket was primed using gray primer. The base cost was then air brushed tan (Model Maker – British Gulf Armor Light - 4813). Next the main body was masked and the cock-pit was painted blue (Testors – Gloss Dark Blue -1211). I waited about 24 hours for the cock-pit to dry and then masked the main body and cock-pit; then air brushed the top and bottom wing with dark red (Model Maker – Rust - 4675). Next the center line was painted dark red. Next came the windshield with light blue (Model Maker – Hellblau RLM - 4788). Next, the seams and rivets were added by hand using a brown colored pencil. Finally, the trim and communication disks were hand painted with silver (Model Maker – Silver – 4678).

Flight:

I built two of these rockets, one served as the flight tester (prototype) and the second as the final painted model. The prototype took a beating with 7 flights and 5 failures most of these failures caused by parachute separation resulting in a lawn dart. One advantage to living in Northern Wisconsin; there was 13-20 inches of snow on the ground to cushioning the rocket so lawn darts became snow darts

On the final flight day (12-15-07) it was a cool -1F. The plan was to launch the prototype and 3 launches of the final painted model.

First flight – Prototype – successfully launched with an E9-4 motor – nice, straight, with only a few twists as it gained altitude. The model arched over at apogee and as I prayed for the ejection – it finally occurred – “will the shock cord hold?”. “Yes” - the parachute opened, not completely, but enough. The rocket suffered significant damage to the nose and lost most of its nose weight. I think I will finally retire this one. Successfully? – I will leave that up to the EMRR judges – they will have to make this call.

Second flight – Final painted model – with a record of 5 failures out of 7 flights, I was extremely nervous. The temperature and a polystyrene model likely meant failure would result in destruction. Count down 5…4…3…2…1 – launch (E9-4 motor) – straight off the launch rod, 2-3 twists as it climbed. It reach apogee and started descending fast – “ejection, ejection, where are you” – finally ejection, parachute out, no shock cord failure – “YES!!!!”. Upon recovery I found that one for the side Plexiglas fins separated at the glue joint and the Kevlar shock cord was severely burned and likely would not last another flight. I decided after inspection that I would retire this model after only one flight – maybe in the future I will attempt a larger motor, but I need to i mprove the shock cord/rear engine ejection design.

I will leave the successful flight call to the EMRR judges. I had 1 with the final painted model and 2 with the prototype. Each of these occurred using an E9-4 motor. In all the flights the models twisted slightly under boost, arched over and ejection was very late.

Summary:
If I rebuild this rocket I will shift from the rear engine ejection system to a nose cone ejection design. The rear engine ejection is too unreliable, it stresses all the components especially the shock cord. In addition, when using a T55 body tube and T50/T20 combo motor tube there is very little space for a parachute.

Thanks EMRR for inspiring me with this contest – keep up the great work!!

Also, thanks to my recovery crew Eric and Abby without you I would have had to walk through knee deep snow.

Editor's Note: Ray also did this nice write up in his club's newsletter