Excuse Me

Excuse Me 3D frame

Andrew MacMillen's Level 3 Rocket



  • Excuse Me is designed and simulated using Rocksim 7. The design file and the Ratt hybrid motor file are available for review.
  • System 3 General Purpose Epoxy is used throughout with slow hardener for structural joints or long working time, and medium or fast hardener for non-structural purposes. Structural joints are initially cured at garage temperature (60-65 deg. F) for maximum penetration.
  • JB Weld is used for critical high stress fillets, such as nose cone bulkhead and motor mount adapter thrust rings
  • all sub-assemblies & the final rocket pieces are post-cured at 110 deg. F to ensure maximum epoxy cross-linking.
  • all bulkheads, centering rings and fins are birch aircraft plywood, 5, 7 or 9 ply depending on part thickness
  • all hardware is stainless steel (if possible) where exposed to motor exhaust or deployment charges
  • the entire airframe is laminated with Aerosleeves 6" biaxial fiberglass sleeving, and glassine coating is removed from all tubing before construction
  • the nosecone to upper airframe & upper to lower airframe joints utilize 2-56 x 1" nylon shear screws
Excuse Me 2D
Click to enlarge

Nose Cone:

Nose Cone


  • Nose cone: Performance Rocketry fiberglass 5:1 conical, 27.70" x 5.54"
  • Tracker Bay: 20" x 1.25" PVC & end caps
  • Nosecone centering ring: 5.25"/1.96" x 0.50"
  • Nosecone reinforcement: 6 ea 6-32 x 1" flat head wood screws
  • Harness point: 3" x 3/8" stainless steel U-bolt

The tracker bay is constructed from a 20" x 1.25" piece of PVC pipe, with an end cap glued onto the tip, and a male threaded adapter on the base. A female threaded cap will close the bay for flight. A 1/8" hole is drilled on the side of the bay for pressure relief. The forward section of the PVC pipe & cap are roughened with 100 grit sandpaper to enhance the foam bond below.

The tracker bay sub-assembly is glued with JB Weld thru the nosecone centering ring with the faceplate of the male threaded adapter on the outside of the centering ring.

Next the 3" x 3/8" stainless steel U-bolt is drilled & fastened just off the centerline of the nosecone centering ring (to allow the female cap to close the tracker bay).

The completed nosecone centering ring and tracker bay sub-assembly is set into the nosecone, with 2 part foam securing the forward end of the tracker bay in the nosecone, and JB Weld securing the centering ring to the nosecone.

A fillet of JB Weld is applied around the nosecone/bulkhead joint for added strength.

Lastly, a 1/8" hole is drilled through the nose cone centering ring for pressure relief.

Upper Airframe:

Upper Airframe


  • Body tube: LOC BT-5.38 x ~36" (length TBD by parachute packing)
  • Aerosleeves 6" biaxial fiberglass sleeving
  • Ebay stops: 4 ea 3/8" x 1" dowels

The upper airframe is a plain tube laminated with fiberglass/epoxy, with 3/16"" pressure relief holes in the main & drogue bays. 4 sections of dowel are glued equidistantly at the appropriate point serving as stops for inserting the electronics bay from the nosecone end. 2 of the dowels are glued 3/8" apart to align the electronics bay dowel.

After glassing and finishing the electronics bay stops, a 1/4" bead of CA is soaked into both inside ends of the airframe to stiffen & protect the edges, then sanded to fit the nosecone & lower airframe coupler.




  • Main chute: SkyAngle CERT-3 XLarge
  • Main chute protector: 24" x 24" Nomex
  • Main harness: 15' & 25' 1/4" tubular Kevlar
  • Drogue chute: SkyAngle Classic II 32"
  • Drogue chute protector: 15" x 15" Nomex
  • Drogue harness: 15' & 25' 1/4" tubular Kevlar
  • Harness connectors: 6 ea 1/4" stainless steel Quick Links

Recovery uses the standard dual deployment setup, with the drogue attached between the booster & upper airframe, and the main between the upper airframe and the nosecone.

The parachutes are sized to achieve ~75 fps descent on drogue from apogee, and ~15 fps on main.

Deployments occur via the primary and secondary deployment charges on either end of the electronics bay.

5/16" QuickLinks are used for each of the 3 attachment points in both the main & drogue bays, and are attached to 1/2" tubular Kevlar.

Electronics bay:

electronics bay


  • Harness point: 3" x 3/8" stainless steel U-bolt
  • Outer ebay bulkhead: 5.38" x 0.25"
  • Inner ebay bulkhead: 5.29" x 0.25"
  • Electronics bay: LOC TC-5.38 x 10.00"
  • Ebay hardware:
    • Ebay closure: 2 ea 11" x 3/8" stainless steel All-thread + nuts & washers
    • Charge connectors: 8 ea brass binding posts & nuts
    • Charge holders: 4 ea 1.25" PVC end caps
    • Charge holder screws: 4 ea 8-32 x 3/4" truss head screws & nuts
    • Ebay retention: 6 ea 8-32 T-nuts, 8-32 x 3/4" screws, & 1" x 1" x 1/4" plywood blocks
    • Electronics sled: 9.00" x 5.00" x 0.093" G10 fiberglass
    • Sled/Allthread guides: 2 ea 8.00" x 7/16" aluminum tube
    • Sled/ebay guides: 4 ea 8.00" x 3/8" x 3/8" basswood strips
    • Harness load distribution: 4 ea 5" x 1" x 1/8" aluminum bar
  • Inner ebay bulkhead: 5.29" x 0.25"
  • Outer ebay bulkhead: 5.38" x 0.25"
  • Harness point: 3" x 3/8" stainless steel U-bolt

2 bulkhead sets are created by centering and face gluing the inner & outer bulkheads together. Both sets of glued bulkhead are centered and 3/8" holes co-drilled for the All-thread, then 3/8" holes for the U-bolts. The aluminum load bars are also drilled with matching holes for the All-thread & U-bolts. The bulkheads are also drilled for the charge holder screws & brass binding posts, and the altimeter wires. All components are mounted and epoxied in place, and wiring is connected to the bindings posts thru the bulkhead and the holes epoxied shut. The lower/drogue bulkhead also has a 3/8" x 1" dowel screwed & glued on the outer face as an alignment pin for the electronic bay stop inside the upper airframe. Lastly the edges and 1/4" of the inside of the ends of the coupler are stiffened by soaking in a bit of CA, then sanded smooth to fit the inner bulkhead.

Once the electronics bay assembly is complete, measure the distance from the outer face of the aft/main bulkhead to the center line of the arming screw switches. Insert the electronics bay into the upper airframe and drill 4 equidistant 1/4" altimeter vent holes (2 are also altimer arming ports) thru the airframe and electronics bay. Drill 3 ea equidistant 8-32 holes on both ends of the bay and through the airframe, then epoxy 1" x 1" x 1/4" plywood blocks with embedded 8-32 T-nuts inside the bay. These capture the electronics bay inside the upper airframe. Use CA to seal all holes edges and sand smooth. Mark which 2 are arming ports on both the electronics bay and the upper airframe.

The electronics sled is a 9.5" x 5.25" piece of .093" G10 fiberglass, with 2 ea 7/16" brass tubes glued to alternate sides with JB Weld to hold it on the All-thread. The 2 batteries, 2 arming switches, and 2 altimeters are mounted on the electronics sled. The sled is sized to just fit between the upper & lower inner bulkheads, and across the diameter of the electronics bay.

The electronics bay (coupler) has 4 ea 8" x 3/8" x 3/8" basswood strips epoxied to the inner surface as sled guides.




  • Altimeter: G-Wiz MC
  • Altimeter: Missileworks RRC2X-40K
  • Arming switches: 2 ea Newton's 3rd Rocketry screw activated SPST
  • Battery holders: 2 ea 5-sided high retention

The electronics are two fully independent & redundant systems of battery, holder, switch and altimeter. All onboard wiring is 24 ga stranded, and the altimeter to deployment charge binding post wires are 24 ga hi-flex stranded.

Per previous flights using these same altimeters, the accelerometer based G-Wiz MC will fire the primary drogue charge at or just before apogee, and the barometric Missleworks RRC2X-40K will fire the backup drogue charge just after apogee. The RRC2X will fire the primary main charge at 1000' and the G-Wiz will fire the backup main charge at 800'.

Lower Airframe:

Lower Airframe
  • Lower airframe:
    • Harness point: 3" x 3/8" stainless steel U-bolt
    • Harness load distribution: 5" x 1" x 1/8" aluminum bar
    • Upper Booster: LOC BT-5.38" x 33.5"
    • Booster bulkhead: 5.38" x 0.50"
    • Airframe coupler: LOC TC-5.38 x 10.00"
    • 64mm motor centering ring: 5.38"/2.51" x 0.25"
    • 98mm motor centering ring: 5.38"/3.95" x 0.25"
    • Booster coupler: LOC TC-5.38 x 10.00"
    • Rail buttons: 3 ea Giant Leap 1010 rail buttons & 8-32 x 3/4" pan head screws
    • Rail button blocks: 3 ea 8-32 T-nuts & 1" x 1" x 1/4" plywood blocks
    • Lower Booster: LOC BT-5.38" x 40.00"
    • 98mm MMT centering ring: 5.38"/4.08" x 0.25"
    • Fin pocket rails: 6 ea 18.00" x 0.75" x 0.375" balsa strips
    • Upper Fin set: 3 ea 11.00" root, 10.00" sweep, 2.75" tip, 4.50" span, 0.75" TTW, 0.375" thick
    • 98mm motor mount: LOC BT-3.9 x 34.00"
    • Lower Fin set: 3 ea 5.50" root, 1.50" sweep, 2.75" tip, 4.50" span, 0.75" TTW, 0.375" thick
    • 98mm MMT centering ring: 5.38"/4.08" x 0.25"
    • Thrust rods: 6 ea 2.25" x 0.25" carbon fiber arrow rods
    • Motor retainer: 3 ea 10-24 x 1.00" thread couplers & fender washers
    • 98mm MMT centering ring: 5.38"/4.08" x 0.25"
    • aft stiffener: LOC TC-5.38 x 10.00"
    • aft flame protector: 16.90" x 2.75" x .09" aluminum roof flashing
  • 98mm/64mm motor adapter:
    • 64 motor mount: LOC BT-2.56 x 30.00"
    • 98mm/64mm centering ring: 3.90"/2.28" x 0.19"
    • 98mm/64mm centering ring: 3.90"/2.28" x 0.19"
    • 98mm/64mm thrust ring: 4.15"/2.68" x 0.75"
  • Motor Retention:
    • Retention plate: 5.38"/2.20" x 1/16" stainless steel
    • Screws: 3 ea 10-24 x 2.50" screws

The upper/lower airframe coupler is constructed from a standard coupler with 2 layers of 4 oz carbon fiber cloth laminated on the inside for increased strenght & rigidity. The booster bulkhead is constructed the same as the nosecone & electronics bay bulkheads, with a 3" x 3/8" U-bolt and an aluminum backer bar. The bulkhead is epoxied 1/2" down from the top of the coupler, and a 1/2" slice of additional body tube is slit and epoxied inside the coupler for additional load resistance, and a fillet of JB Weld is applied for added strength.

After the airframe coupler is epoxied into the lower airframe, the coupler is painted with a 50% epoxy, 50% alcohol solution a couple of times to provide a smooth mating surface with the upper airframe, and sanded to fit cleanly.

The upper section of the lower airframe is a 33" section of 5.5" BT, with a special 2.6" centering ring just under the upper coupler to capture a 64mm motor (Ratt M900).

The lower section of the lower airframe is a 40" section of 5.5" BT, and the entire lower airframe section will be laminated with fiberglass/epoxy.

There is a vent hole ~1.5" below the upper end of the airframe to vent the M900, and 3 ea 1" x 1" x 1/4" plywood blocks with embedded 8-32 T-nuts glued inside the airframe for the rail guides. One T-nut block will be inside the mid-frame coupler 40" from the base, another just above the lower fin set at 5.5" from the base, and the last just below the upper fin centering ring.

Motor Mount & Fins:

Motor Mount

The upper end of the 98mm motor mount is captured by a 5.5"/3.9" centering ring just below the mid-airframe coupler. Another 5.5"/3.9" centering ring is just above the upper fin set, another just below the lower fin set, and the final 5.5"/3.9" centering ring is 1/2" above the bottom of the 98mm motor mount. This aft centering ring will have 3 holes through it backed by 10-24 3/4" washers and 1" 10-24 couplers. It also has a fillet of JB Weld for added strength. The bottom of the motor mount is 2.25" up from the base of the rocket, to allow for both the 3/4" thickness of the 98mm/64mm motor adapter thrust ring, and the 1.4" aft closure of the M900 motor.

6 ea 18.00" x 0.75" x 0.375" bals strips are epoxied 3/8" apart equidistant radially, and between the upper & lower fin centering rings. The outer lower side of each strip is reinforced with an epoxy & chopped glass fillet. This provides a pocket for each fin root.

The fins are cut from 3/8" aircraft plywood stock using a full-sized template, then the leading & trailing edges of all 6 fins are beveled at 30 degrees. The fins are epoxied into the airframe slots & fin root pockets using a custom fin alignment jig. The outer fins fillets are an epoxy, silica & phenolic microballon paste.

6 ea 2.25" sections of 1/4" carbon fiber arrow stock are epoxied inside the airframe between the lower 98mm centering ring and the aft 98mm centering ring to more effectively transfer large thrust loads to the stronger fin can section.

The inside of the airframe aft of the aft centering is reinforced by a 2.25 section of body tube slit to fit & epoxied, and protected from motor damage by a 16.9" x 2.25 of aluminum roof flashing epoxied in place.

The 98mm/64mm motor mount adapter is a single length of BT-2.6, with 2 ea 98mm/64mm centering rings along it's length, and an 3/4" thrust ring on the aft end. The thrust ring is glued in place using slow epoxy to maximize wood & cardboard penetration, and the BT-2.6 has small holes drilled to form 'epoxy rivets'. The thrust ring has a JB Weld fillet for additional strength.


Motor retention is a custom 1/6" thick stainless steel plate, secured by 10-24 x 2.5" screws into the 10-24 couplers behind the aft centering ring.

Shear pins are 2 sets of 3 ea 2-56 nylon screws, one set for the nose cone to upper airframe junction, and one set for the upper & lower airframe junction.