«DESIGN OF AN INTEGRAL THERMAL PROTECTION SYSTEM FOR FUTURE SPACE VEHICLES By SATISH KUMAR BAPANAPALLI A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL ...»
11. The trusses closer to the panel edges experienced a disproportionately high amount of load when compared to the trusses close to the center of the panel. Other truss-core geometries (arrangement of trusses with respect to the face sheet) need to be explored so as to distribute the loads more evenly among the trusses.
Some material trade-off studies for corrugated-core ITPS were carried out during the preliminary stages of the ITPS design process. The results were not presented in the dissertation because the input parameters for the designs were altered significantly as the design process evolved. In the light of these different input parameters the results of older designs would not appear coherent. A companion study  used the code developed for this research work to carry out a more meaningful material trade-off study. However, this study did not take the deflection and stress constraints into consideration for the optimization process. Some of the conclusions that can be drawn from these combined studies are presented below.
12. Beryllium alloys are superior to Aluminum alloys in all aspects. Use of Beryllium alloys also facilitates in designing a much lighter ITPS. Therefore, in spite of the toxicity concerns of Beryllium, it is worthwhile to use Beryllium alloys in the ITPS designs.
13. Titanium alloys have excellent high temperature properties and are suitable for top face sheet and web material. However, the service temperature of these alloys limits their use to low heat flux regions.
14. Inconel alloys, with a higher service temperature than titanium alloys, allow ITPS design for higher heat flux. However, high density of inconel leads to prohibitively high ITPS weight in corrugated-core structures.
15. Ceramic matrix composites, for example Nextel-718/Alumina Silicate, have many superior properties like low density, high service temperature, stiffness comparable to aluminum, low thermal conductivity and low coefficient of thermal expansion. However, very low tensile strength of these materials is their “Achilles’ heel”. Otherwise, these materials are the most suitable for ITPS designs for top face sheet and web.
Now that a general framework for the ITPS design has been established with this research work, new structure geometries need to be explored. Corrugated-core structure design provides a lot of insight into the factors affecting typical ITPS designs. It is necessary to build on these to improve the structure geometry for a lighter ITPS. A drastic change like a transition to truss-core structures should be avoided. Instead, the active constraints in the corrugated-core designs should be studied and step-wise changes in geometry should be implemented.
The excessive thermal deflection of the webs, due to thermal gradients, causes large stresses in the webs and bottom face sheet. A classic technique to relieve thermal stresses is to provide local provisions for expansion of the structure so as not to accumulate high stresses. One of the changes that need to be immediately explored is to divide each web in the length-direction.
If the web in Figure 5-6 were to be split into two parts along the centerline (dotted line in Figure 5-6), and a small gap provided between the two partitions, then the thermal deflection can be cut into half. This will reduce the stresses considerably while not adversely affecting the load carrying capacity of the panel.
Buckling of the edges is another major factor influencing the design. For example, buckling of the web-edges close to the panel edges is one of the first buckling modes. By tactically placing stiffeners in the webs and face sheets, such buckling modes can be prevented.
Stiffeners do not add to the weight or heat conduction path of the ITPS significantly, while drastically improving the buckling resistance of the structure. Therefore, this line of design improvement has a potential for reducing the ITPS weight considerably.
Although truss-core geometries considered for this research did not yield a feasible structure, other geometries need to be explored that can distribute the thermal stresses more evenly over all the trusses. In the truss-cores used for this research, the trusses close to the panel edges experienced large loads due to the top face sheet expansion, while the trusses close to the center of the panel experienced very small loads. One of the suggestions to improve this design is to arrange all the trusses so that they tilt towards the center point of the panel. This will impart more loads to the interior trusses. Stress concentration in the top face sheet will still be a problem in these structures. There is a need to device a method to overcome the stress concentrations that arise as a result of FE modeling. In practice, these stresses will be much lower due to the manner in which the trusses are attached to the face sheets. A method to overcome the stress singularities in the beam-plate models for truss-core structures can help reduce the cost of FE calculations to a large extent.
In this research work, heat flux loads for a Space Shuttle-like vehicle were considered.
Most future space vehicles are likely to be space capsules like Apollo. The heat loads for space capsules are many times higher than the loads on the Shuttle. As of now there are no materials that have service temperature to withstand these loads. Therefore, ablative materials have to be used to provide thermal protection. It would be very interesting to combine the ablative materials with ITPS structures so that the ablative material thickness controls the amount of heat flowing into the structure.
The number of FE experiments carried out to obtain the response surface approximations was manageable for the corrugated-core design. However, as the design gets complicated, the number of design variable will increase. Therefore, methods to reduce the number of FE analyses need to be explored in future. One of the suggestions is to couple approximate analytical formulas with the accurate FE analysis to carry out a multi-fidelity optimization procedure.
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