NASA Ares V Interstage

Ares V Core Interstage

The Ares V Interstage is a 33 foot diameter, 48 foot tall cylindrical (barrel) that is axially compressed but must also withstand crushing and internal pressure causing compressive and tension hoop panel loads.

HyperSizer used on Ares V

As for all launch vehicles, it is essential that it be designed to minimum weight. To establish minimum weight, trade studies were performed with HyperSizer to determine weight trends and the most efficient combination of architectural design, panel concept, cross sectional dimensions, material system, and layup sequence. Many panel concepts were considered and each concept was optimized to find the lightest weight panel designs based on ring frame spacing.

HyperSizer used on Ares V

From the sandwich panel family, weights were reported for honeycomb sandwich, reinforced core sandwich, and integral blade sandwich. From the stiffened panel family bonded hat stiffened, integral blade stiffened, and Boeing’s PRSEUS (Pultruded Rod Stitched Efficient Unitized Structure) stiffened panels were analyzed using HyperSizer.

Weight Trends Generated with HyperSizer

By plotting the trend lines for each panel concept, the optimal solution was determined from the lightest weight combination of panel and ring frame dimensions. Once a general trend line was determined an estimated optimum solution was obtained. The panel designs were then matured by iterating HyperSizer with FEA static and buckling solutions, using “master” full scale finite element models.

HyperSizer used on Ares V

HyperSizer used on Ares V

In general, hat panels are lighter than honeycomb sandwich panels for axially loaded cylindrical structures that have ring frames for buckling support.

Hat Stiffened Panels

Hat stiffened panels have more design variables than sandwich panels. The additional design variables of a stiffened panel provide more opportunity for weight savings and if fully explored, as is done with HyperSizer, a stiffened panel with a proper combination of cross sectional dimensions and laminates can be lighter than a honeycomb sandwich.

HyperSizer used on Ares V

Additionally, the hat stiffener achieved panel buckling stability primarily by adding 0° plies in the crown and increasing hat height to obtain a high EI. For cylindrical launch structures with many ring frames, the panel aspect ratio created a direct load path which put the stiffeners into column compression so hat panels were ideal for the Ares V Interstage.

Postbuckling for Stiffened Panels

Weight savings is possible if stiffened panels are allowed to postbuckle. HyperSizer optimizations and analyses were performed for the hat panel, allowing the skin to initially buckle at limit load. The skin between the stiffeners (open span) buckles, and then the web may or may not buckle depending on the current design, but in either case, the additional load is accumulated in the corners of the cross section in the laminates effective widths.

Ring Frame Optimization

All ring frames are sized to a required EI stiffness to prevent global buckling, material strength, local buckling of each beam object, and crippling.

Figure 12

By imposing the stiffness requirement, the ring frames size adjusts to tall beams with wide flanges. By virtue, this makes the web and flanges buckling critical. To meet the local buckling requirement, more 45 degree plies are added to the web. To prevent the flanges from becoming too wide, 0 degree fibers are added to the flanges to achieve the required EI. The ability to tailor the laminates to meet the design criteria allows for weight savings in composite ring frames.

Fabrication and Testing

During fabrication, the hats were not autoclaved. Even still, the structural performance from testing proved them structurally efficient. The flange to skin co-cured bond was very strong; it never pulled off even when the skin was allowed to go far into post buckling with a large amplitude mode shape.

HyperSizer used on Ares V

Cost Reduction

Affordability is currently an important concern for NASA's heavy lift launch vehicles. With this in mind, a weight to cost metric was established. A metallic skin stringer design fabricated using friction stir welding (FSW) of the hat shaped stiffener onto the skin is relatively inexpensive. Any cross-sectional shape of aluminum can be extruded at a very low cost.

HyperSizer used on Ares V

Stiffened panels by definition of their many cross-sectional sizing variables, provide a wider spectrum of coupled weight-cost metrics. Sandwich panels provide few sizing variables and as a result, have a more narrow spectrum of coupled weight-cost metrics. Composite materials with layup tailoring provide a wider spectrum of coupled weight-cost metrics than metallics. So on the far spectrum, composite stiffened panels, particularly closed-section hat shaped, provide the widest range of coupled weight-cost performance metrics. Based on the flexibility of the hat composite stiffened panel, it can be optimized to better meet a target weight-cost metric.

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