This article is a review devoted to the possibility of manufacturing composite lattice structures for space hardware components with cost-effective approach and the feasibility of their application.

Composite materials with carbon or aramid fibers and polymeric matrices have high specific strength and stiffness and, in combination with automatic manufacturing processes, make it possible to fabricate composite structures with high level of weight and cost efficiency.

As you may know, the substitution of metal alloys by composite materials, in general, reduces structure mass by 25 −40%. Moreover, in some cases the combination of high mechanical characteristics of composites with proper structural conceptions and fabrication processes (which allow these characteristics to be realized with great efficiency) gives a more qualitative improvement of structure characteristics. One example of this design approach is lattice isogrid and anisogrid structures.

Lattice structures are usually made in the form of thin-walled cylindrical or conical shells and consist of systems of helical and circumferential ribs with one-sided sheathing, two-sided sheathing or without sheathing (see Fig.1). They are made by continuous filament winding from graphite and aramid-epoxy composites.

                                                                                                                

Figure 1 - Composite anisogrid lattice adapter (ESA)

But with all the advantages of such structures their wide application is not observed at the moment. A numbers of aerospace giants (such as AIRBUS, RUAG, CRISM, etc.), which have extensive opportunities to research and test such structures, use them in their projects.

But what about a large number of startups that have been engaged in the development and manufacture of light and ultralight launch vehicles in recent years?

One of the key criteria in the development and operation of such rockets in the growing market of private space companies is commercial efficiency to provide launch services for the Customer at the lowest price. Taking this into account, we will try to understand the appropriateness of lattice structures made of composite materials in elements of launch vehicles.

There are many gaps in designing such structures from composite materials for young startup teams at the moment. Yes, there are existing methods, and new ones are being developed. But to apply them it is necessary to perform a large amount of work, such as testing and correlation of calculations or simulations with the obtained test results. In fact, it’s the development of own estimation methodologies taking into account the influence of production technology and analysis of test results. This increases the time for designing products from scratch and confirming the obtained characteristics with the declared ones.

The fact that many startups will use composite materials in body parts indicates that they have design methods and technology for working with such materials. But the production of lattice structures has some features. Product characteristics fully depend on the correct implementation of each stage of development, namely "Design-manufacturing-material properties" chain.

Lattice structures are made by automatic winding, during which impregnated tows are placed into helical and circumferential grooves formed on the surface of the mandrel. There are several processes to fabricate lattice structures by now:

   a) Free forming of ribs involves traditional winding process, but tows are placed at a distance from each other on tops of the tows of the previous layer.

   b) The low-density foam core with grooves cut for shaping the ribs is applied to the surface of mandrel or on preformed inner sheathing.

   с) An elastic coating with grooves for shaping the ribs is laid on the mandrel surface (see figure 2).

   d) A thin metal shell with preformed grooves for ribs is fixed on the mandrel surface.

Figure  2 - Manufacturing of conical adapters

The process must be suitable for full automation especially for large scale components. This should be achieved using existing automatic composite layup equipment.

 - all structural features must be integrated during the layup phase – a one-step approval process, in order to minimize production lead time and cost;

 - the tools must be re-usable;

 - predictable and implemented finite dimensions and micro-structural quality must be guaranteed;

 - the process must be suitable for both lattice and grid-stiffened structures;

 - small local last-minute geometry changes must be included;

 - the process must result in minimal scrap rate of the material;

 - minimum post-machining/finishing is required.

An important stage of development of lattice structures is the experimental study. There are 2 approaches for the testing process:

  • Testing of full-scale components. The structure of material in components is so intricate and dependent on features a structure and corresponding manufacturing process that the determination of mechanical characteristics from specimens or scaled models is not possible.
  • A series of tests on samples and their correlation with mathematical models and non-destructive testing methods of the qualification models.

Now let's return to the question of the feasibility of lattice structures in cost-effective design of launch vehicles.

        If we conventionally divide the production process of the first sample into the development and manufacturing stages, then with the second approach to testing, the percentage of the cost of the product will look something like this (by main activities):

Development:

One-off product:

Design and Analysis

~15%

Design and Analysis

~20%

Manufacturing

~25%

Manufacturing

~33%

Testing

~19%

Materials

~9%

Non-Destructive Inspection

~12%

Machining

~16%

Others

~29%

Non-Destructive Inspection and Non-Destructive Testing

~7%

 

Others

~15%

Compared with a honeycomb structure, a similar mesh structure will have the following advantages:

- from 25% to 30% lower mass;

- up to 30% lower manufacturing cost;

- up to 20% less time for the product manufacture.

This is true for both large and small structures such as transitional compartments of small launchers, payload adapters, as well as heavily loaded planar structures.

But the given percentages are correct in case of using the existing equipment for automatic winding and possible need to modify it for a new product. If there is no equipment at all, the purchasing of it may cause the additional investment.

The expediency of investing in the specified equipment is determined by the project’s payback conditions and the list of economic indicators of the launch cost. For example, if the basic concept of a launch vehicle involves the use of aluminum structures, then it’s not appropriate to introduce lattice structures since it involves the purchase of specific equipment. Or, if the manufacturing of launch vehicles includes wound composite tanks, the modification of the used equipment for lattice structures seems appropriate. At the same time, the extent of adoption of such products into the launch vehicle plays an important role, because it’s not profitable to expand production for the sake of one payload adapter, since the gain in mass will not cover the costs of preparing production, and this fact should be taken into account.

To comprehensively disseminate the technology for the development and manufacture of products based on mesh structures, the following measures seem to be the most appropriate:

  1. Consolidation of efforts of several start-ups to develop the technology and distribute it on a large scale on mutually beneficial terms.
  2. Development of universal multifunctional structures;
  3. The high level of unification of developed products in order for mass production and cost reduction;
  4. Consideration of the use of additive technologies to reduce the share of investment in specific equipment.

After analyzing the materials given above, it can be said that the use of lattice structures in projects that assume cost-effective approach to the development of aerospace products is appropriate, subject to the recommendations on technology dissemination given above, but not limited to it.

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