Rocket interstages

Interstages are primary structures intended to withstand the huge axial compressive loads and vibrations of the launch as well as to allocate the engines and equipment of the next stage.

The fabrication of heavy lift rocket interstages based on anisogrid lattice designs using the Latticestruct system may result faster, simpler, and more cost-effective than the state of the art filament winding and fibre placement fabrication methods.

Lattice cylindrical shells. A proof-of-concept anisogrid cylinder as those used in modern rocket interstages is shown on the left. A close-up view of the node part is provided. Two example models (scale 1:200) of anisogrid interstages that can be assembled using the offered nodal construction system are shown in white, one following a triangular pattern and the other hexagonal. Two finite element models of anisogrid cylinders are illustrated on the right.

The optimal masses of a medium (example 1) and a heavy-lift (example 2) aluminium rocket interstages* have been calculated and compared for five cylindrical shell designs to illustrate the technical performance of the technology in Space applications.

Mass savings for rocket interstages. The optimal masses for a 2.8 m diameter and 1.2 m high interstage for a medium-lift rocket that withstands 650 t of axial load (example 1) and for a 10 m diameter and 15 m high interstage for a heavy-lift rocket that resists 3800 t (example 2) are shown. In both cases, the anisogrid design with hollow circular ribs is by far the lightest.

The anisogrid design with hollow circular ribs, which only can be cost-effectively fabricated with the offered nodal system, outperforms all others under the same specifications. It is more than 2 times lighter than isogrids with or without skin, more than 3 times than anisogrids with solid rectangular ribs, and more than 4 or 5 times than thin-walled monocoques.

Furthermore, if the examples were fabricated using hollow unidirectional carbon fibre ribs instead of aluminium, an exclusive advantage of the offered technology, the performance would be doubled again in comparison with competing technologies (metallic machining for isogrids and filament winding for anisogrids). This means an unprecedented 4-fold weight reduction with respect to the best state-of-the-art technologies.

Despite nodes mass contribution has not been directly included in the comparisons, its weight usually ranges the 10-30% so the results would remain very favourable even in the worst case

 * The material properties, dimensions, and design loads for these interstage examples were R= 1.4 m, L= 1.2 mm, E= 73.1 GPa, F= 3872 kN and M= 1864 kNm (1.5 safety factor included) in example 1, and R= 5.03 m, L= 14.86 m, E= 71.7 GPa, F= 38.29 MN (1.4 safety factor included) in example 2. The material was aluminium of σ= 414 MPa, v=0.3, and ρ=2800 Kg/m3. Example 1 was taken from "Design and Analysis of Vented Interstage in a Typical Launch Vehicle, Krishnan and Saladavi 2015", and example 2 from "Buckling imperfection sensitivity of axially compressed orthotropic cylinders. Shultz and Nemeth 2010".