Future space exploration, such as the Artemis program, journeys to Mars, and future lander missions will require thermal control systems (TCSs) with the ability to adapt to a wide range of thermal loads due to vastly fluctuating external temperatures. Current TCSs employ radiators that can achieve a turndown ratio (defined as the ratio of the maximum to minimum heat rejection rates) of 12:1 by utilizing regenerative heat exchangers and a two-fluid-loop system, both of which are heavy and complex. However, future missions will demand radiators that can provide turndown ratios of 12:1 while remaining light, functionally passive, and simply designed. Previous work has investigated using shape memory alloy (SMA) components in single phase radiator prototypes to achieve efficient heat rejection. Preliminary analysis shows that SMA-based radiators can enable turndown ratios as high as 37:1. In this paper, the design, fabrication, and testing of an SMA torque tube driven radiator prototype is discussed. The SMA torque tube is attached to a heat rejecting panel that resembles flat radiator panels currently installed on the International Space Station. As the temperature of the working fluid in the TCS increases, the SMA torque tube actuates and rotates the panel, allowing for more radiative heat rejection to occur. This new design matures the concept past a previous prototype that merely demonstrated actuation under single-phase (e.g., liquid water) flow. The current radiator prototype has been designed to function not only with closed-loop, single-phase fluid flow, but also in conjunction with a two-phase TCS and even as a heat pipe. Both approaches take advantage of phase transformation of the working fluid to improve overall TCS efficiency and decrease complexity. During testing, a heated two-phase working fluid was circulated through the system, resulting in a maximum angular actuation of 67 degrees, thus demonstrating two-phase operation for the first time. These results give confidence that an SMA torque tube-driven radiator can outperform current radiators as development continues.