DESICOS

New Robust DESIgn Guideline for Imperfection Sensitive COmposite Launcher Structures

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 Obiettivo del progetto (Objective)

'European Space industry demand for lighter and cheaper launcher transport systems. The proposed project DESICOS contributes to these aims by a new design approach for imperfection sensitive composite launcher structures, exploiting the worst imperfection idea efficiently: the Single Perturbation Load Approach. Currently, imperfection sensitive shell structures prone to buckling are designed according the NASA SP 8007 guideline using the conservative lower bound curve. The guideline dates from 1968, and the structural behaviour of composite material is not considered appropriately, in particular since the imperfection sensitivity of shells made from such materials depends on the lay-up design. The buckling loads of CFRP structures may vary by a factor of about 3 just by changing the lay-up. This is not considered in the NASA SP 8007, which allows designing only so called 'black metal' structures. Here is a high need for a new precise and fast design approach for imperfection sensitive composite structures which allows significant reduction of structural weight and design cost. For most relevant architectures of cylindrical and conical launcher structures (monolithic, sandwich, isogrid - without and with holes) DESICOS will investigate a combined methodology from the Single Perturbation Load approach and a specific stochastic approach which guarantees an effective and robust design. A recent investigation demonstrated, that an axially loaded unstiffened cylinder, which is disturbed by a large enough single perturbation load, is leading directly to the design buckling load 45% higher compared with the respective NASA SP 8007 design. All results will be summarized in a handbook for the design of imperfection sensitive composite structures. The potential will be demonstrated within different industrially driven use cases.'

Introduzione (Teaser)

NASA guidelines for the design of thin-walled structures, such as fuel tanks of launchers date back to the late 1960s, but are still in use. To support the aerospace industry in the design of new launchers, an EU-funded project set out to develop an alternative methodology.

Descrizione progetto (Article)

The existing guidelines for the design of cylindrical structures prone to buckling had been developed without considering new exciting developments in composite materials. Today's space industry's demand for lightweight launchers of lower cost or the ever increasing payload requirements were not addressed either.

More recently, different approaches have been adopted to predict the maximum load for launcher structures. Using non-linear finite element models, good agreement has been achieved with experimental results. However, the computational cost of these theoretical models used in the pre-design phase is too high when faster tools are needed.

The 'New robust design guideline for imperfection sensitive composite launcher structures' (http://www.desicos.eu/ (DESICOS)) project proposds a deterministic methodology. The so-called 'Single perturbation load approach' (SPLA) uses the radial profile of a load applied as an indication of the structure's sensitivity to imperfections.

Despite the advantages of this methodology, a reasonable computational effort still needs to be dedicated to the calculation of the knock-down factor. In engineering composite structures, the knock-down factor is used to account for the fact that imperfections reduce the stress that these can withstand to a small fraction of the stress at which a perfect structure buckles.

Within the DESICOS project, the new methodology was further developed and a way to combine the SPLA concept with an analytical or semi-analytical approach was investigated. With a view to applying SPLA in the preliminary design of launcher structures, it is validated against statistical approaches based on extensive testing of cylindrical structures made of composite materials.

All the results are summarised in a handbook which will be dedicated to the design of composite launcher structures. The potential of the new methodology to reduce the development timescales and manufacturing costs, while increasing the payload capacity, will be demonstrated within case studies to be carried out during the second phase of the DESICOS project.