«DESIGN OF AN INTEGRAL THERMAL PROTECTION SYSTEM FOR FUTURE SPACE VEHICLES By SATISH KUMAR BAPANAPALLI A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL ...»
In Chapter 5, the ITPS designs for corrugated-core structures were presented. The various input parameters used for these design were approximate in nature. Some of the reasons for using approximate values were the lack of availability of the exact data, difficulties in modeling and analysis, and lack of clear understanding of the expected capabilities of the ITPS. It should also be mentioned that these designs do not constitute a final design for an ITPS structure. Many other issues like joining of dissimilar materials and manner of attachment of the ITPS panels to the vehicle are beyond the scope of this research work.
In this chapter, many of these input parameters are varied and the resulting effects on the
ITPS designs are studied. The parameters that are varied are:
Heat transfer parameters
• Emissivity of top face sheet
• Heat loss from bottom face sheet
• Initial temperature of the structure (prior to beginning of reentry phase)
• Coefficient of convection on top face sheet after landing.
Loads and boundary conditions
• Pressure load on top face sheet
• In-plane load on bottom face sheet
• Boundary conditions on the ITPS panel The heat transfer parameters were mostly assumed values and, therefore, it is necessary to verify whether or not these values have a huge impact on the ITPS design. The loads and boundary conditions were studied because the ITPS design will be significantly influenced by the type of vehicle, the magnitude of the loads (which are generally different for different positions on the vehicle), and manner in which the panels are attached to the vehicle (which determine the boundary conditions on the panel).
Apart from providing an insight into the significance of the assumptions of input parameters, this study will also help formulate a set of guidelines for ITPS design. For example, the studies on the effects of material properties will help in material selection for ITPS panels.
Effect of initial temperature on structure can help determine whether it is beneficial to alter the reentry trajectory to “pre-heat” the vehicle before the actual reentry.
6.1.1 Changing the Emissivity of the Top Surface of ITPS The emissivity of the top surface determines the temperature of the top face sheet and the heat radiated out by the ITPS. Thus, it determines the amount of heat allowed into the ITPS.
While a high emissivity increases the peak temperature of the top face sheet, it also benefits in decreasing the amount of heat entering the ITPS.
The optimization procedure (Section 5.2.4) was carried out for the ITPS by reducing the emissivity from 0.8 to 0.7. The change in heat flux, entering the ITPS through the top surface, due to the two different emissivity values is illustrated in Figure 6-1. Compare this to the maximum heat flux input on the ITPS, which is 34069 W/m2. The amount of heat entering the ITPS is only a small fraction of the total heat flux input. This demonstrates the effectiveness of the top face sheet acting as a hot structure (Section 2.1). Although, the curves show a very slight difference in total integrated heat, the peak values show a difference of about 25%. The change in ITPS designs are presented in Table 6-1. The ITPS mass increases by about 5% compared to baseline designs. It can be concluded that emissivity has a marginal impact on the ITPS design and a small difference in the choice of the emissivity values does not throw off the optimized designs by much. Another important point to observe is that the change in design variables is reflected only in the bottom face sheet thickness (tB), which increases by approximately 15%.
This shows that the bottom face sheet acts as the most effective heat sink among all the sections in the ITPS.
6.1.2 Allowing Heat Loss from the Bottom Face Sheet In the baseline designs, the bottom face sheet was assumed to be perfectly insulated. This is a conservative approach as far the peak bottom face sheet temperature constraint is concerned.
In reality, the bottom face sheet loses heat to the interior of the vehicle in the form of radiation to the surroundings and conduction through the attachment points. In order to simulate the heat loss from the bottom face sheet, an emissivity of 0.2 was prescribed to the bottom surface. With this emissivity value, the peak heat flux from the bottom face sheet was equal to 418 W/m2 compared to peak heat flux value of 9,000 W/m2 (Figure 6-1) entering the top surface of the ITPS. The amount of heat loss is very small compared to the heat entering the ITPS. The heat entering top surface was same in both cases with the difference being less than 1% during the reentry phase.
Even this slight difference leads to 7–10% decrease in the optimized ITPS mass, as shown in Table 6-2. The only major change in the variables is the reduction of the bottom face sheet thickness. Thus, it can be concluded that the heat loss from the bottom face sheet plays a significant role in determining the optimized mass of the ITPS.
Figure 6-1. Heat flux entering the ITPS through the top surface for two different emissivity values imposed on Design I of Table 5-7. The dotted line shows the percentage difference between the two by comparing it with the average heat flux load. Positive value of heat flux imply heat entering the ITPS and negative values imply heat exiting the ITPS.
6.1.3 Increasing the initial temperature of the structure
baseline model was 295 K. This resulted in an increase in the ITPS mass by 17 to 27%, as shown in Table 6-3. This increase is due to the reduction in the thermal capacity of the structure as a result of increase in initial temperature.
supported (Section 5.2.2 and 5.2.3), the bottom face sheet stress constraint had a major impact on the ITPS design. The bottom face sheet stresses were largest at the reentry time (tmax∆T) when the temperature gradient in the panel was the highest (details in Section 5.2.2 and 5.2.3). In this case, an increase in the initial temperature of the structure results in a decrease in the magnitude of the temperature gradient. Thus, the stresses in the bottom face sheet decrease. As a result, the ITPS weight reduces considerably. This is an illustration of the impact of boundary conditions on the ITPS design. In ITPS designs where stress constraints are not active, the ITPS mass increases with increase in initial temperature of the structure, whereas in the case of the designs where stresses created due to the temperature gradient have an influence over the ITPS design, the ITPS mass decreases with increase in initial temperature.
6.2.1 Effect of Boundary Conditions The effect of boundary conditions can be understood by comparing the ITPS designs in Tables 5-6 and 5-7. For designs in Table 5-6, simply supported boundary conditions were imposed on the bottom face sheet edges on all four sides of the panel. For designs in Table 5-7, the boundary conditions were relaxed by removing the simply supported boundary conditions on the edges that are parallel to the webs. The reduction in ITPS weight due to relaxation of the boundary conditions was of the order of 25–48%. The major reason for this reduction in weight is due to the decrease in stresses in the bottom face sheet. In the relaxed boundary conditions designs, stress constraints were not active. Therefore, the bottom face sheet thickness was reduced significantly. Other advantages of relaxing the boundary conditions are that the length of the panels is significantly higher and the panel height is also reduced considerably. Relaxation of constraints on the bottom face sheet leads to reduction in stresses in all sections of the ITPS panel. Thus, it can be concluded that the boundary conditions play a major role in influencing the ITPS design.
6.2.2 Increasing the Pressure Load on the Top Surface The pressure load on the top face sheet was doubled to 2 atmospheres (202,650 Pa). The changes in ITPS designs are presented in Table 6-4. There was a marginal increase of 1 to 5% in the ITPS mass. Higher pressure may cause greater increase in ITPS mass. However, for the spacecraft surfaces that are not stagnant regions like nose cone or wing leading edges on the Space Shuttle, the pressures experienced on the top surface are typically less than 1 atmosphere.
Therefore, the corrugated-core ITPS structures can withstand these loads with a sufficient margin.
6.2.3 Increasing the In-Plane Load The in-plane load was increased from 30,000 N/m in the baseline models to 150,000 N/m.
While the baseline load is a typical load for the backshell of a space capsule, 150,000 N/m is a typical load experienced by the mid-section of the Space Shuttle. The increase in the ITPS mass is negligible. A large portion of the in-plane load is borne by the bottom face sheet. Even the bottom face sheet is a long plate; the webs attached to it act as stiffeners and prevent it from buckling. Thus, the corrugated-core ITPS structure is able to withstand significantly large inplane loads without any increase in the ITPS mass.
The weight of optimized ITPS designs (presented in Chapter 6) ranged from 3.5 to 5 lb/ft2.
Typical weights of ARMOR TPS (developed for VentureStar) ranged between 2 to 3 lb/ft2.
Taking into account the capability of the ITPS to withstand substantially higher loads as compared to ARMOR TPS, the ITPS weight can be considered to be reasonably good.
As mentioned earlier, the aim of this design is not to design the ITPS for any particular vehicle, but to study the feasibility of the ITPS concept. Therefore, the characteristics that influence ITPS design are of major interest and the conclusions drawn from the results presented in Chapters 5 and 6 are outlined here. Some general conclusions drawn from this research work are listed below.
1. Designs with a small number of strong supports (corrugated-cores) are more suitable for ITPS applications when compared to designs with large number of weak supports (trusscores).
2. The most severe load influencing the ITPS design is the large thermal gradient through the thickness of the ITPS panel. Any future designs should focus on geometries that take this thermal gradient into account before considering other loads.
3. Emissivity of the top face sheet has a moderate impact on the ITPS weight. Considering that emissivity of a surface falls under the purview of manufacturing, not much can be done to improve the emissivity value through the design process. However, keeping the value as high as possible always helps in reducing the heat coming in and reduces the ITPS weight.
4. Bottom face sheet is the most efficient heat sink of all the sections in the ITPS panel. The optimizer tends to increase the heat capacity of the structure by first increasing the thickness of the bottom face sheet. If this thickness reaches the upper bound, then the optimizer increases the thickness of the ITPS panel, which increases the insulation capacity of the panel along with the heat capacity. The bottom face sheet is usually made of low density, high heat capacity materials like Aluminum and Beryllium and that is one of the major reasons why the optimizer prefers to increase the bottom face sheet thickness first before increasing the height of the panel.
5. Since bottom face sheet is typically the thickest section of the ITPS, a designer should take care so that majority of the loads are borne by the bottom face sheet. One of the examples for this research work is the manner in which in-plane loads were imposed only on the bottom face sheet.
6. Even a relatively small heat loss from the bottom face sheet can lead to a significant weight reduction in the ITPS. While assumption of perfectly insulated boundary condition leads to a conservative design, one must guard against excess conservatism. It is difficult to estimate the exact amount of heat lost from the bottom face sheet as it depends on the vehicle architecture and could be different at different regions even on the same vehicle.
Although it is preferred to be conservative in the generic designs of an ITPS, final designs should definitely take the heat loss into consideration as it could lead to considerable savings in launch weight.
7. Boundary conditions exert the most significant influence on the ITPS design. Tighter constraints can render the panel too heavy or in some cases the design may not be feasible.
Boundary conditions are dependent on the manner in which the ITPS panels are attached to the frame of the vehicle. During the preliminary design process and when the exact vehicle configuration is not available, it is difficult to obtain the correct boundary conditions.
Therefore, the design process should be carried out with different boundary conditions to obtain a set of generic designs that can later be narrowed down when the vehicle specifications become clearer.
8. Increase in pressure load by 2 times and in-plane load by 5 times lead to a marginal increase in ITPS mass. The corrugated-core structures possess a sound inherent design to withstand these loads.
Conclusions for truss-core structures for ITPS applications:
9. Truss-cores produce large stress concentrations at the junction points between the trusses and face sheets. Stress concentrations in the truss-core design could be a result of modeling error. In reality the area of contact between the truss and face sheets is much larger and this would lead to lower stresses. It is necessary to explore methods to reduce the magnitude of the stresses so as to eliminate the modeling error.
10. The trusses in truss-core designs considered in this research were too weak to withstand the forces produced as a result of large thermal gradients in the structure. If the trusses were to be made stiffer, then the stress concentrations at the truss-plate junctions increase significantly.