Analyzing fundamental problem of designing incinerated payload fairings made from composite polymer materials

The article considers fundamental engineering problem of designing the payload fairing (PLF) for a launch vehicle (LV) made from polymer composite material (PCM) with high temperature resistance and minimum mass, with the PLF being incinerated after separation from the launch vehicle on the atmospheric section of the descent path. The energetic material (EM) is introduced into the PCM, which provides heating of the PCM structure to the ignition temperature and subsequent PLF combustion under the conditions of the incoming air flow. A comparative analysis of the problems arising at different life cycle stages in creating and operating traditional (PLFT) and incinerated (PLFI) payload fairings is carried out. Example options for energetic materials and possible designs of the individual PLFI elements are given.


Introduction
The environmental effect of launch vehicles (LV) in the impact zones of spent stages boosters (SS) is one of the main problematic issues in the LV operation. It should be noted that for each launch path, the largest area is formed by the impact zones of payload fairing (PLF), interstages and aft-interstages. So, for the "Soyuz" type LV, the total area of impact zones for exhaust boosters (side blocks and central block) is ~ 35% and impact zones for PLF and AIS are ~ 65 %; for "Zenit" LV, impact zones area for the 1st stage booster is ~ 19 -20% and the impact zones for PLF are ~ 80 -81 %; the "proton-M" LV impact zones area for the 1st stage booster is ~ 28 %, and for PLF it is ~ 23 % (if special impact zone is allocated), the impact zones area for the 2st stage booster is ~ 49 % [1,2]. After each LV launch in the impact zones, work is carried out to find the separated stages, the payload fairing, to cut and remove them to storage sites, with subsequent recycling, etc. Besides damage to the environment and economic activity of local business, this results in additional social tension.
Currently, the problems associated with the entry of space objects into the atmosphere with hypersonic speeds and degradation of these object structure materials are thoroughly considered [3]. There are known technical solutions providing almost complete absence of impact zones for the spent stages [4]. For the most part, such studies relate to the SS, while "dry" compartments, such as PLF halves, separate at such motion parameters that heating does not reach sufficient temperatures to ensure their combustion process in the dense layers of the atmosphere [5].
In works [6,7] a method is proposed as one of the possible solutions to reduce the impact zones. This method is based on the introduction of an energetic material (EM) into the existing PLF structure. The material, when burned, will heat the PLF structure material to the ignition temperature when moving along the atmospheric section of the descent path. These studies have shown the possibility of the PLF destruction, however, to bring the structure to a finely dispersed state requires a significant increase in the EM mass and, accordingly, all PLF because of the currently used PCM. Modern PLF represent a two-leafed three-layer structure (figure 1), the outer layers of which are made from PCM based on carbon fibers and aluminum honeycomb filler [8]. The widespread use of PCM in rocket and space technology products is due to the possibility to create structures with a number of unique mechanical and physical properties, such as high heat resistance, low coefficient of friction and thermal expansion, high resistance to atmospheric influences and chemical reagents, etc. [9], which hinders the implementation of the proposed method of the PLF combustion. Based on the above said, it is proposed to replace the currently used PCM, honeycomb core with the materials and structures capable of providing the traditional requirements for operation on all life cycle stages of the launch vehicle (manufacturing, storage, operation on the launch and technical complexes, on the active sector of the ascent trajectory) [10].
When an incinerated payload fairing (PLF I ) is designed, fundamental problems arise in the field of engineering, in particular: a) when creating a PCM with new thermal, physical and chemical properties; b) the choice of possible energetic materials that are part of the PCM; c) the choice of PCM structures providing durability, the ability to provide combustion conditions, etc.

Problem statement
Based on the analysis of the problem state for the design and construction of the incinerated PLF I structures, the problem statement is formulated as follows: to determine requirements to design parameters of the PLF I made from PCM with introduced EM, providing the required thermal power characteristics of the PLF I in the boost phase of the LV launch with further combustion on the atmospheric part of the ascent trajectory after separation from the LV.
Solutions to the formulated problem of the PLF I design and construction are proposed as based on the existing scientific and methodological framework for designing traditional PLF T from PCM [11] and provide for the solution of the following task sequence: -comparative analysis of the PLF T and PLF I operation stages; -comparative analysis of the PLF T and PLF I design parameters; -comparative analysis of the PLF T and PLF I design stages. Illustrated by selecting the filler structure for the PLF I , the replacement of aluminum honeycomb filler with the filler made as a corrugated structure from EM is to be considered.

Theoretical studies
The purpose of the PLF is to protect the payload from the incoming air flow, radiation, temperature changes, from mechanical effects on the atmospheric section of the LV ascent trajectory during the passage through dense layers of the atmosphere.
On the active part of the ascent trajectory, PLF T and PLF I should provide the same operational conditions. Moreover the PLF T after fulfilling its mission and separation from the LV moves in freeair conditions, and its life cycle ends with the landing in a predetermined impact zone, where it is evacuated and then disposed of. The PLF I recycling stage is carried out on the atmospheric section of the descent trajectory and requires no impact zones, as well as the expenditures for searching, removal and subsequent recycling. Table 1 shows the compared positions of PLF T and PLF I at the PLF operation stages on the active and passive parts of the trajectory. -the fundamental difference between PLF T and PLF I is the absence of impact zones, whose area are several times larger those of impact zones for spent LV first stages; -an additional operation stage is introduced for the PLF I compared to the PLF T , where the combustion process is carried out; -one of the PLF I design objectives is the choice of the descent trajectory interval, on which the combustion occurs. The thickness of the heat-shielding coatings It is determined from the conditions of structure thermal loading It is determined from the conditions of structure thermal loading + the condition of combustion on the atmospheric section of the descent trajectory 5 The parameters of the ribs crosssection It is determined from the conditions of PLF power loading At the initial stage, it is assumed to be identical to the regular PLF T The following main conclusions can be drawn from the results given in table 2: -the list and the number of the PLF T and PLF I design parameters for the design stage under consideration are assumed as the same; -for PLF T , the movement along the descent path does not affect the choice of design parameters, and the life cycle ends with recycling after falling to the ground; -the PLF I recycling stage sets new requirements to the design parameters and involves ensuring the PLF I structure combustion conditions on the atmospheric part of the descent trajectory; -the coordinates of the vector for design parameters of the traditional PLF T Х Т (х 1 , х 2 , х 3 , х 4 ,х 5 ) differ significantly from the coordinates of the vector for design parameters of the incinerated PLF I Х I (х 1 , х 2 , х 3 , х 4 ,х 5 ) due to the additional requirements caused by the changes in the PLF operation stages.
Consider the main positions of the stages to solve the PLF design problem. Table 3 presents a comparative analysis of the PLF T and PLF I design stages.

Evaluating the replacement of design and filler material
Using the choice of the filler structure as an example, a consideration should be made of the honeycomb aluminum filler replacement with the filler made as a corrugated structure, according to the method [12]. The new filler must: a) provide similar strength, operational, and technological characteristics; b) allocate a sufficient amount of heat during the combustion to heat the carbon-fiber material to the ignition temperature. It is proposed to consider PCM based on ABS structure [13] with the EM introduction as one of the possible materials. The filler is proposed to be made in the form of a corrugation, and the corresponding parameters are to be determined from the strength ensurance condition (condition a). When the chosen filler structure burns, the necessary quantity of heat (condition b) shall release. If this condition is not met, the filler mass shouls be increased in accordance with the proposed method [15]. Figure 2 shows the design diagram to the filler structure parameters on the example of cellular construction and corrugations.  As follows from the results given in table 4, there is a fundamental possibility to choose the design and material of the filler providing both strength conditions and the release of the required heat amount.

Experimental studies results
As a possible example, the filler structure in the form of an aluminum honeycomb with the parameters given in table 4 and the energetic material in the form of a mechanically activated pyrotechnic composition B 4 C + Ti, as well as with other energetic compositions [6,14], was considered. Figure 3 shows the result of the maximum structural element destruction. Experiments with pyrotechnic compositions showed the possibility of the structural element dispersion, however, it was not possible to achieve a complete mass loss for a number of reasons: -aluminum honeycomb filler is not combustible, because aluminum burning is possible only in finely dispersed form; -combustion of pyrotechnic composition is accompanied by gas formation, which leads to, besides heat loss, an increased pressure in the filler layer; -combustion of mechanically activated pyrotechnic composition provides a gas-free combustion mode, but during the chemical reaction, the combustion products evole into the condensed phase; -the introduction of pyrotechnic composition in an amount sufficient for burning the structure leads to an additional increase in the PLF mass.

Discussion
The experiments have shown the feasibility of changing not only the structure and the filler material, but also the structure and material of carbon fiber, in particular, the EM introduction in its composition.
A comparative analysis of PLF T and PLF I showed that the main difference lies at the PLF operation stage after its mission being fulfilled, namely the PLF I combustion in the atmospheric section of the descent trajectory. To ensure the combustion of the payload fairing structure, additional stages are introduced when solving the problem of PLF I design. Based on the combustion conditions, new requirements to the PLF I design parameters are formed.
As an example, the replacement of a honeycomb aluminum filler with a similar and corrugated filler made from another material was considered. Possible variants of the filler structure are shown in figure 4. а) b) c) d) Figure 4. Types of the filler structure: a) corrugated; b) honeycomb with rectangular cells; C) frame (pyramidal); d) frame (tetrahedral).