Slotted Cardboard Airframes: Buying and Fin Slot Prep Guide
By High Power Model Rocketry | Published: 2026-11-03
Category: Buying Guides
Learn how to choose slotted cardboard airframes by diameter, slot count, and length, then prep fin slots cleanly for a scratch-built rocket that flies true.
Quick answer
Buy the slotted airframe whose inside diameter matches your motor mount and whose slot count matches your fin count, then dry-fit every fin before gluing. For most scratch builds, a 24 mm tube with 3 or 4 slots is the simplest starting point; step up to 38 mm only when your motor and fin span call for it.
Why slot count and slot length matter more than tube length
A slotted body tube saves you from cutting fin slots by hand, but only if the slots line up with your fin stock and fin placement. The supplied slotted airframes list a 1.5-inch slot length, so fins with a root chord longer than that will need trimming or a different tube. Slot count should equal fin count; a 3-slot tube suits a three-fin rocket, while a 4-slot tube suits a four-fin design. Tube length affects how much body you have for recovery and payload, not how well the fins fit.
Cardboard versus other airframe materials
Cardboard airframes are light, easy to sand, and inexpensive, which makes them a good match for scratch-built sport rockets. They are less stiff than composite tubes and can soften if moisture gets into exposed edges, so seal the slot edges and inside of the tube after assembly. If you plan to fly high-power motors or want a tube that shrugs off handling, a composite airframe is the better choice; for low- and mid-power builds, cardboard keeps weight and cost down.
The trade-off: convenience versus commit-first sizing
Pre-slotted tubes speed up assembly, but the slot positions are fixed. Measure your fin root chord, fin thickness, and desired fin span before ordering. If your fin stock is thicker than the slot width, you will need to open the slots slightly; if it is much thinner, add a thin shim or fillet to avoid a loose fit. Also check that the tube diameter matches your nose cone and centering rings, since a slotted tube is harder to adapt than a plain tube.
Decision rules for choosing and prepping slotted airframes
- Match diameter to motor mount: Measure the outside diameter of your motor mount tube or the inside diameter of your centering rings. Choose a 24 mm airframe for 24 mm mounts and a 38 mm airframe for 38 mm mounts. Do not guess from the rocket's overall size. Confirm the fit with a caliper before ordering.
- Match slot count to fin count: Count your fins. A three-fin rocket needs a 3-slot tube; a four-fin rocket needs a 4-slot tube. If you plan to add a fifth fin or use a different fin arrangement, buy a plain tube and cut slots yourself. Slot count is fixed, so decide your fin layout first.
- Check slot length against fin root chord: The 24 mm slotted tubes list a 1.5-inch slot length. If your fin root chord is longer than the slot, either shorten the root or choose a tube with longer slots. A fin that extends past the slot will not seat flush. Dry-fit a fin in the slot before any glue.
- Prep slots for a clean bond: Lightly sand the inside of each slot to remove loose fibers, then test-fit the fin. If the slot is tight, widen it with a sanding stick; if it is loose, add a thin shim or plan for a fillet. Tack the fin with a small amount of glue, check alignment, then apply full fillets. Seal exposed cardboard edges with thin CA or epoxy after assembly.
Product facts
- Slotted Cardboard Airframes: Slotted cardboard airframes come in 1-inch (24 mm) and 1.6-inch (38 mm) diameters, with 3 or 4 slots and 1.5-inch slot length on the 24 mm tubes. This range lets you match slot count to fin count and pick a diameter that fits common motor mounts without cutting your own slots.
Slotted Cardboard Airframes
Who should buy slotted cardboard airframes
Best for
- Scratch builders using 24 mm or 38 mm motor mounts who want to skip cutting fin slots
- Low- and mid-power sport rockets where light weight and low cost matter
- Builders with standard three- or four-fin layouts and fin root chords under 1.5 inches
Not for
- High-power builds that need a stiffer, moisture-resistant airframe
- Rockets with fin root chords longer than 1.5 inches or non-standard fin counts
- Builders who want to adjust slot position or angle after purchase
