**2. Support design**

The present project, commissioned by the S3-Swiss Space System, deals with the development and qualification of three stages to orbit a composite vehicle. The first and second stages, the Airbus and the Suborbital Aircraft Reusable (SOAR) vehicles, respectively, are reusable, whereas the third stage is an expandable booster. All details of the SOAR mission are in Ref. [1] see **Figure 1**. This project requested the design of new supports for the separation wind

The S3 SOAR separation from the Airbus A300 is a critical issue during the mission. The aerodynamic coefficients are affected by the proximity of the vehicles. To correctly design the separation, the aerodynamic database must be known with sufficient accuracy. For this reason,

An earlier wind tunnel campaign had already been performed with the construction of an initial aerodynamic database [2]. To carry out further wind tunnel experiments, an improve‐ ment in the knowledge of support design is required. In fact, to design supports for the SOAR, in this configuration, is particularly complicated due to the presence of the Airbus below and behind the vehicle (the A300 empennage also impedes the use of a support from the base). Effectively, the first sting designed produced considerable interferences on the model. Therefore, the purpose of this work is to design supports for the SOAR vehicle in the following two configurations: scale 1:180 for the wind tunnel test of the SOAR in the presence of the

tunnel test in the VKI facility and the study of the respective interferences.

an important campaign of wind tunnel testing is necessary.

176 Recent Progress in Some Aircraft Technologies

Airbus and scale 1:80 for the wind tunnel test of the SOAR alone.

**Figure 1.** Illustration of the main steps of the mission.

The support in wind tunnel tests is necessary to hold up the model in the test section, but it is also an artificial device that, especially from an aerodynamic point of view, does not exist. From this definition, the following design constraints are immediately derived: to minimize the aerodynamic interferences with the flow ensuring adequate mechanical properties to sustain the model. It is also necessary to consider the wind tunnel test conditions (*M*=0.7, *α*max=15°), the model shapes, and the use of steel as material for the supports. Furthermore, the support has to allow the internal passage of cables that interface the 6 degrees of freedom internal balance with the measurement device. Finally, the test section dimensions of the VKI-S1 wind tunnel (0.4×0.36 m) and the need to adopt a new configuration with respect to the previous one are considered. This is because the use of a dorsal strut (see **Figure 2**) did not provide satisfactory results regarding the interferences with the models. Some additional constraints are present in scale 1:180 composite testing: to test the separation with the presence of the Airbus empennage and to consider the nominal relative position of the two vehicles. This is necessary to permit a relative motion of the SOAR over the Airbus.

Once requirements about flow measurement and passage of cables are acquired, the two parameters to be optimized are to minimize the aerodynamic interferences and to give adequate mechanical properties.

**Figure 2.** Picture of the old support used in the first separation wind tunnel test.

In fact, to minimize interferences, the sting has to be as long as possible and with the diameter as small as possible. These requirements are not in agreement with the structural properties for which stings should be short and have a large diameter. An important parameter in this dissertation is the "critical sting length" that is "the shortest sting length, which does not change the level of an aerodynamic measurement obtained with longer stings" [3]. The critical sting length is influenced by the Mach number, Reynolds number, boundary layer at the base of the model, sting, and model base diameter. The Reynolds number plays an especially important role. In fact, if the flow is laminar at the base of the model, *L*<sup>c</sup> is as much as 12 to 15 times the base model diameter (*D*); on the contrary, with turbulent flow, *L*<sup>c</sup> is reduced to 3–5·*D*.

It is necessary to pay attention that the diameter does not have to modify the typology of the boundary layer at the model base. The minimum diameter allowable, from load considera‐ tions, is approximately 0.25 times the model base diameter [4]. For the maximum value, it is necessary to consider that, in the transonic flow, minimum interferences exist with a sting diameter up to 0.4 times the base [5].

In this flow field, the common choice is to use a "straight sting," which is the best solution to reduce the interferences. This is a tube that enters the model at the base. With this choice, all the supporting structure is downstream of the model and it is used with an internal balance system [6].

**Figure 3.** The SOAR (light blue) with the new support (orange) over the Airbus (dark green) in the nominal position; scale 1:180.

The use of a straight sting is possible only for the scale 1:80 model, see **Figure 3** whereas the presence of the Airbus, with its empennage, in the composite configuration (scale 1:180) prevents its usage. see **Figure 4** The only remaining possibility, in this case, is to use a straight sting that enters in the SOAR base (rear surface) but at the other extremity is connected with an inclined bar before the Airbus empennage. The presence of the cables inside the support

**Figure 4.** The SOAR with the new circular sting; scale 1:80.

In fact, to minimize interferences, the sting has to be as long as possible and with the diameter as small as possible. These requirements are not in agreement with the structural properties for which stings should be short and have a large diameter. An important parameter in this dissertation is the "critical sting length" that is "the shortest sting length, which does not change the level of an aerodynamic measurement obtained with longer stings" [3]. The critical sting length is influenced by the Mach number, Reynolds number, boundary layer at the base of the model, sting, and model base diameter. The Reynolds number plays an especially important role. In fact, if the flow is laminar at the base of the model, *L*<sup>c</sup> is as much as 12 to 15 times the base model diameter (*D*); on the contrary, with turbulent flow, *L*<sup>c</sup> is reduced to 3–5·*D*.

It is necessary to pay attention that the diameter does not have to modify the typology of the boundary layer at the model base. The minimum diameter allowable, from load considera‐ tions, is approximately 0.25 times the model base diameter [4]. For the maximum value, it is necessary to consider that, in the transonic flow, minimum interferences exist with a sting

In this flow field, the common choice is to use a "straight sting," which is the best solution to reduce the interferences. This is a tube that enters the model at the base. With this choice, all the supporting structure is downstream of the model and it is used with an internal balance

**Figure 3.** The SOAR (light blue) with the new support (orange) over the Airbus (dark green) in the nominal position;

The use of a straight sting is possible only for the scale 1:80 model, see **Figure 3** whereas the presence of the Airbus, with its empennage, in the composite configuration (scale 1:180) prevents its usage. see **Figure 4** The only remaining possibility, in this case, is to use a straight sting that enters in the SOAR base (rear surface) but at the other extremity is connected with an inclined bar before the Airbus empennage. The presence of the cables inside the support

diameter up to 0.4 times the base [5].

178 Recent Progress in Some Aircraft Technologies

system [6].

scale 1:180.

needs a cavity that decreases the stiffness, with the necessity to pay particular attention to the thickness of the walls. From the definition of the critical Reynolds number, it is possible to see that, for the scale 1:80, the flow at the base is definitely turbulent, whereas, in scale 1:180, the transition is around the base. In any case to design a sting 12 to 15 times the length of *D* is not possible due to the constraint of the Airbus empennage, and the longest one possible was designed.

Also, for the diameter, in scale 1:180, it was not possible to respect the rules explained for the presence of the cable inside, and again the smallest one possible was chosen.

The inclined part of the support is clearly a critical point for the interferences and it dramati‐ cally breaks the flow in the proximity of the rear portion of the SOAR. For this reason, it made it as less as possible inclined (*Β* angle small) with respect to the constraint of the empennage angle (otherwise the support would approach the tail of the Airbus too closely). Another solution, adopted to try to have small interferences, is to use an airfoil shape as section of the inclined bar. In fact, in the transonic flow, a thin airfoil allows for a dramatic reduction of the drag [7]. The aim of this is to choose the thinnest airfoil possible, which is compatible with the structural constraints and the presence of cables inside. The choice was a NACA 0016 cut in the rear part.

Two different shapes of sting sections are evaluated in this work: circular and elliptic. The circular one is the most common and there are many references in the literature [3, 4, 6]. On the contrary, an elliptic sting should reduce the interferences and the drag especially at a high angle of attack [8]. These configurations will be evaluated at every step of the design process, first from a structural point of view and then with a flow analysis using a CFD software. Because there is a lack of literature on the use of the elliptic sting, its use and the possible advantages of this choice are investigated in depth. All support dimensions are in Ref. [9].
