A.5.2.2.2 Preliminary Design1
We base the preliminary design of the skirt on the simple geometry of a truncated cone. The bottom of the cone connects to the stage below it and the top of the cone connects to the stage above it. We assume that the skirt is made of a thin wall without any support from stringers or support rings. In Figure A.5.2.2.2.1, the basic geometry is presented in the sketch.
Fig. A.5.2.2.2.1. Geometry for the preliminary design of the skirt.
(Jessica Schoenbauer)
In the above figure, we illustrate that the cone angle, α, is the taper angle from the vertical axis, l is the axial length of the cone,r1 is the radius of the small end of the cone, and r2 is the radius of the large end of the cone.
Applying our assumptions and geometric constraints, we vary the quantities of the cone angle, the wall thickness, and the material to design a skirt. The skirt design intends to support the maximum applied axial load while minimizing the cost associated to the material and manufacturing. In addition to the constraints mentioned above, the skirt’s cone angle is prescribed to be between ten and sixty degrees1, as suggested in a paper in NASA’s design specifications, and the skirt length must be equal to or greater than the length of the nozzle it houses.
For certain launch vehicle configurations, we know that using the ten degree minimum taper angle for the skirt will not provide a skirt that is long enough to house the nozzle. For these situations, we add a cylinder to the bottom of the cone that interfaces with the stage below it so that the nozzle is housed by the skirt. Although placing the cylinder on the top of cone to interface with the stage above it would reduce the mass, and hence the cost, we cannot place it in this configuration because it may not be large enough in diameter to accommodate the nozzle. We illustrate the two possible geometries for the skirt and where they are in association with the launch vehicle in the figure below.
Fig. A.5.2.2.2.2. Skirt configurations and there position in the launch vehicle.
(Jessica Schoenbauer)
The figure shows the two skirts on a three stage launch vehicle. Skirt 1 connects the first stage to the second stage. This skirt could not house the nozzle by using the minimum cone angle and therefore had the cylinder added to the bottom of it to allow for housing of the nozzle. Skirt 2 follows the other configuration for the skirt connecting the second stage to the third stage. The bottom of the cone connects to the second stage while the top of the cone connects to the third stage.
References
1 “NASA Space Vehicle Design Criteria (structures): Buckling of Thin-walled Truncated Cones,” NASA SP-8019, September 1968.
Authors: Jessica Schoenbauer