MOTIVATION

With recent advancements in space exploration technology, a complete new generation of reusable launch vehicles (RLVs) designs has been proposed. These reusable launch vehicles have a unique aerodynamic design. For example, the proposed X-33 has a truncated base, as shown in Fig.1, in order to accommodate an aero-spike engine. The large base area of these RLVs causes a large base drag, which adversely affects the flight characteristics (i.e. the cross range, the down range, the glide slope angle, and the descent velocity) during an un-powered re-entry into the earth's atmosphere. As the drag increases, the glide slope angle  becomes steeper,  thereby causing the vehicles to have poor range characteristics. With limited range, landing options are severely restricted for such vehicles.

Figure 1 -  X-33 flight vehicle. Figure is courtesy of NASA.

POTENTIAL SOLUTION

Earlier research work has shown that there exists a potential way of reducing the base drag without altering the design of these RLVs. Hoerner[1] showed that there was a possible means of reducing the total drag on the bodies with a truncated base. He correlated a large set of drag data for bluff bodies and showed that an empirical relationship between the fore-body viscous drag coefficient (CD_fb) and the base drag coefficient (CD_b) existed. From the relationship provided by Hoerner shown in Fig. 2, it can be concluded that, the total drag can be substantially reduced by increasing the fore-body drag. The location where total drag reaches its minima is called the "drag bucket." RLVs operating in the drag bucket region will have optimal drag. As shown in Fig. 2, most of the old generation of RLVs are on the right side of the drag bucket, whereas the new generation of RLVs are on the extreme left section of the curve. Hence, for the new generation of RLVs, an increase in the viscous fore-body drag can reduce the base/total drag.

Figure 2 -  Hoerner's relationship between CD_fb and CD_b.

OBJECTIVES

1)  To verify the base drag reduction phenomena at the higher Reynolds number.

2) Understand the mechanism behind the base drag reduction phenomena.

3) Efficient active/passive control of base drag. 

MODEL DESIGN

To investigate the effect of fore-body drag / boundary layer state on base drag, a model with the following features was needed: (i) the ability to easily manipulate boundary layer thickness, (ii) a low base-area-to-fore-body-area ratio, and (iii) the flexibility to perform different measurements. In order to incorporate these features, the modular design concept shown in Fig.3  was used. The model had an half angle of 2.5 degree ,which kept the base-area-to-fore-body-area ratio low, and the modular design provided the ability to vary the base-area-to-fore-body area ratio. Interchangeable plates allowed for changing the surface roughness on the model and for using different plates to perform various measurements. The model had three important sections: (i) the leading edge, (ii) the ramp area, and (iii) the base area as shown in Fig. 3. The leading edge was elliptical to provide a smooth transition between the ellipse and ramp sections. For this study, all tests were made at zero degree angle of attack. 

Fig. 3 Schematic of the models used in the base drag experiment

WIND TUNNEL

The experiments discussed here were conducted in the University of Wyoming Aeronautical Laboratories (UWAL) 2' x2' wind tunnel. The wind tunnel is a fan-driven, open-return design with a 0.61 x 0.61 x 1.219 m test section. Using a variable-speed motor, free-stream velocities of 10-50 m/s are possible at Reynolds's number up to 2.5x10^6. The inlet section of the tunnel has a honeycomb insert and three screens located just upstream of a contraction section with a 12:1 ratio. The measured free-stream turbulence above the model was 0.3%.

TEST CASES

LEGEND

A :Pressure measurements on fore-body 

B: Pressure measurements on fore-body 

C: skin friction measurements on the fore-body

D: Hot-wire survey before the separation

E: Hot-wire measurements in the wake of the model