Scratch - HazMat {Scratch} Original Design / Scratch Built

Contributed by Moira Jean Whitlock

Manufacturer: Scratch
(Contributed - by Moira Jean Whitlock)

Brief:
Single-staged 2 or 4 x 38mm cluster rocket. Designed for the Aerotech J350 and 570, and also suitable for the upcoming Redline equivalents.

The rocket can recover perfectly using long motor delays or with an altimeter in the payload bay. Dual deployment is also possible.

This rocket is inspired by the Quadra G, by AAA Model Aviation Fuels and is somewhat like a modified upscale of that great kit.

Construction:
The nose cone is a 4 inch by 15 inch Ace plastic cone with 8 ounces of ballast foamed into the tip with 2-part polyurethane foam. The nose cone is screwed in place with 4 wood screws and is thus removable in case electronics are to be placed in the payload section or even a second parachute is used and deployed from this section.

The body tubes are 2 48 inch lengths of Maximum Thrust Rocketry 4 inch motor tubes. They are paper and ultra thick and strong. They are further reinforced internally with Red Arrow Hobbies 4 inch phenolic couplers from the motor tubes all the way up to the middle of the payload bay. One of the couplers contains a 1/4 inch thick birch bulkhead for drogue/main attachment (if just one chute is used) and allows the rocket to separate in the middle. These couplers are affixed in place by generous amounts of West Systems slow epoxy.

There are 4 fins, fashioned from 2 pieces of .125 inch thick G10 from Scott Eakins in the ROL auctions. These pieces are 1 by 2 foot rectangles with a sliver cut off at the tip edges to make the tips 6 inches long. The trailing edges will be 12 inches each once the two pieces are slotted down the middle and interlocked together at right angles. The assembly gets a thorough epoxy fillet in the middle. Glass it with a strip of 4 oz weight glass cloth if you plan to use really hot motors with high average thrust.

The 4 38mm motor tubes are 36 inch lengths of Red Arrow Hobbies phenolic tubing and are a snug fit into the main body tube...Rocksim 4.0 insists that they don't fit, but my experience says that they do indeed...and the tubes are pushed in 12 to 18 inches and epoxied well and the gaps foamed with polyurethane foam. the remaining length of 18 to 24 inches extends outward from the main body. Be sure that the tubes are flush at the ends, that is, equal in length. The interlocking fin assembly then slides among the 4 motor tubes so that one motor tube is on each fin "corner." The assembly is epoxied into place. All the motor tubes are glassed together with wraps of 4 oz. weight fiberglass cloth. I also glassed the launch lugs onto the airframe. the airframe can be glassed as well, but I only did the tops of both and the bottom where the motor tubes start, for stress reinforcement and zipper prevention. I also glassed the fins to the motor tubes. I did this with strips of glass cloth. the entire fin face can be glassed, but I did not choose to do so. You might if you really push this rocket with mighty motors.

1/4 inch birch centering rings provide more internal support and are between the first 2 6 inch lengths of phenolic internal couplers. The second ring contains the u-bolt that connects the 6 feet of tubular Kevlar® plus 10 feet of 1/4 inch thick wire wrapped bungee. The shock cord is connected to the u-bolt with a quick link and the two materials in the shock cord are connected with one as well. I used square knots, not granny knots. The Kevlar® came from Rocket Silo and the bungee came from American Science and Surplus.

The second length of body tubing contains the coupler and 1/4 inch birch bulkhead assembly that allows middle separation and also attaches a u-bolt for the forward connection of the shock cord and drogue or main chute. This assemble is screwed into place with 6 wood screws, not epoxy, because this allows removal and replacement of electronics and bays from a point in the rocket not too high to arm on the pad. I say this because my level 3 project needed to be armed from a standing position atop my husband's pickup truck cab!!

The chute is a Sky Angle chute rated for 10 pound rockets.

A 6 inch heavy duty eyebolt is epoxied and foamed into the nose cone to form an anchor for 5 feet of wire wrapped bungee shock cord and a second chute, this one a 60 inch military surplus cargo chute from Everything Surplus, a ROL online auction vendor. The shock cord attached aftward at a 1/4 inch birch centering ring halfway down the payload tube. I'd attach it lower, but I need space for the protective bay needed for the altimeters below.

The only trick in building this rocket was the G10 cutting. Do use respiratory gear and blades meant to cut this stuff. Wood blades dull and stress on this and saws heat up as a result.

The rocket is sturdy and strong. I was pleased when the guys picked it up and marveled at its strength and weight.

I decorated it in purple and orange, with metallic accents from self adhesive mylar from local hobby shops. This also coordinated with the two chutes, which are purple and orange.

LaunchFlight:
I recommend long delays for the J350 and 570 if 4 are used, medium delay if 2 are used. I used 4 J350's. One of them was prepped with a long delay and the other 3 were not given charges, just the tracking delay grains, and their delay wells were plugged with grease and sealed with duct tape. There were aft loops of piano wire embedded in the fillets used to affix the fins, and around the necks of all 4 motors was tied a loop of 1/8 inch thick Kevlar® rope that was tied up onto the u-bolt where the recovery line was attached. I didn't want any of the motors to be ejected. I anticipated a fast and high flight.

Nomex®/Kevlar® chute protectors were employed with redundant worm bed wadding in the prep.

A Skyward Electronics altimeter was placed in the payload bay and all was a go until I accidentally set off the altimeter with a BANG. That scared me! But I was determined to let my bird fly, so I screwed on the nose and sealed the payload bay and flew the bird with the motor ejection. I really had wanted to stress test the altimeter for my upcoming level 3 flight, but that was not to be. I mainly wanted to test my building techniques to see if I could hold this bird together with a predicted mach 1.3 and altitude near 5,000 feet. I used 4 low current large tungsten bridge igniters from Fire in the Hole, and a large marine 12 volt deep cycle battery. The launch rod is a lightning rod 3/4 inch thick by 7 feet long.

I was nervous as I readied myself for this flight. I momentarily questioned my sanity. When the flight was announced, dead silence ensued as opposed to the usual guffaws when I do a funny theme rocket. I think everyone was aghast. Well, the rocket ROARED. I mean, roared straight up. The flame trail looked easily as long as the rocket. I heard a deep fawumpf sound that others confirmed was probably mach. At apogee, the chute deployed and everyone cheered then, breaking the silence. The RSO said, "I don't know about you, but I think that was awesome."

I made a fast chase a mile downrange. It ended up in a field of grazing cattle, and I was a bit scared of them because I'm an avid beefeater... Mind you, I had set up with 2 chutes and the one that finally did the job was really underrated, so I was worried. But the rocket came down flyable again and ready for the Redlines!

Summary:
If there's any CON, clusters do cost more to fly. But there's nothing like the roar you get from multiple motors. I was high on the adrenaline for weeks. the pros include ease of building and great fun for the flyer and spectators.

Other:
Always ohm out your igniters to be sure they match, or get professionally made ones from Fire in the Hole. That will ensure that all the motors light.

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