The austrian company PeakTechnolgy sponsored us with the opportunity and the materials to use their autoclave and pre-pregs. We also had the fortune of being supported by another company KerblModellbau, who did the milling for us and also provided us with the mould-material.
The fancy shape of our STR-03-Fincan is therefore rather a „because we can“ than the result of careful simulation and wind-tunnel testing 😉 We wanted to integrate features that we read about, in papers about fins in particular and aerodynamics in general. After some discussion in the team we agreed upon a concorde-delta-wing-like planform:
The fins profile is defined by a series of LD-Haack graphs (blue curves in the picture) starting from the fins edge to a specific point at the centerline (the line of highest swelling; red curve in the picture). The centerline itself is the projection of the weighted mean between planforms front-line and the back-line onto another parable defining the fins thickness from about 10mm at the root to a sharp end at the tip.
After smoothing of the edges and rounding the passage between fin and shaft the a third of the outer composite-layer of our Fincan looks like this:
The composite-part should be built up like this:
- Surface Layer of thin Carbon-Fiber-Cloth 45°/45° (respective to rocket axis).
- Two Layers of unidirectional Carbon-Fiber-Fabrics 90° (respective to rocket axis).
- Another Layer of thin Carbon-Fiber-Cloth 45°/45° to maintain a symmetric laminate.
- Projected cured laminate thickness of outer shell is 0,5mm.
- Fin-Core-Elements milled from PMI-Foam (Rohacell 71).
- A Layer of Carbon-Fiber-Cloth 45°/45° inside the tube-segment (after the three-part-mould is closed).
- A Layer of unidirectional Carbon-Fiber-Fabric at 0° inside the tube-segment.
- A Layer of Carbon-Fiber-Cloth 45°/45° inside the tube-segment for a symmetric laminate again.
- Projected cured laminate thickness of in-tube-laminate is also 0,5mm
The negative-mould therefore is made of three parts. They are joined by nine aligning pins and are made from LAB973.
When we got the milled moulds, we spent about a day and a halve sanding, sealing, polishing and at last applying the release-agent:
With the moulds ready for lay-up, we started with the first layer of 45°/45°-carbon-cloth-prepreg; compacting it into the mould with a vacuun-bag and trimming the ends with a scalpel.
The surface-layer was followed by two layers of unidirectional pre-preg at 90° and to maintain a symmetrical laminate, finished with another layer 45°/45°.
Each layer compacted into the mould and carefully trimmed afterwards.
Next we prepared the fin-cores. We milled them out of a special PMI-foam(trade-name: Rohacell) at a CNC-mill@Happylab. The fin-cores got covered in a special adhesive-film and also being vacuum-compacted afterwards.
Each mould got a fin-core attached on the left side. The black adhesive-film is very sticky;) open spots were also covered with snippets of the adhesive-film-material.
All three moulds ready to be put together:
Next we added another three layers of caron-cloth in the shaft. Sort of just rolled them in. A ring of aluminium (already covered in red plastic-foil in the picture) serves as structure to form a section with smaller diameter as a joint for the rocket-tube. All holes and gaps are masked with tape.
Package sealed and vacuumized:
After 5 hours at 120°C and at a pressure of 3 bar in the autoclave:
And there is the polished result: