NASA Dryden Flight Research Center is supporting a Phase II Small Business Innovation Research (SBIR) contract between the U.S. Air Force Research Labs and Kelly Space & Technology (KST). KST's innovation is to use an aerotowed reusable launch vehicle to put small satellites into low orbits around the Earth. In support of this idea, the SBIR is to demonstrate aerotow with representative aircraft; namely, a C-141A as the towing airplane and a QF-106A (a modified F-106) as the towed airplane. NASA Dryden has developed a computational simulation of the dynamics of the tow rope and towed airplane, conducted dynamic-stability studies, developed test plans, and completed successful ground, taxi, and flight tests.

Figure 1. Eigenvalues of Oscillations of the Tow System in short-period, phugoid, and "bungee" (simple longitudinal) modes were identified, and a flight envelope for stable towing of the QF-106A behind the C-141A was predicted on the basis of computational simulations of the dynamics.

Towing a launch vehicle to altitude should make it possible increase the payload and decrease the cost of the launch. No previous aerotow experiments produced tow-dynamics data of any consequence. Dryden's expertise in conducting unusual flight tests was needed to perform simulations of the dynamics (see Figure 1), determine a safe flight-test approach, and conduct the aerotow flight tests.

Dryden provided support for the modifications that were made in converting the F-106 into the towed experimental airplane, and for its operation and maintenance. Dryden also provided research instrumentation, the test range, flight-safety, operations, research pilots, and research engineering analysis of the aerotow system. The Air Force Flight Test Center also supported this project by providing the C-141A airplane and flight crew.

Figure 2. The C-141A Airplane Towed the QF-106A Airplane in flight tests. The airplanes were connected by a tow rope 1,000 ft (305 m) long.

Tests of the entire tow system (see Figure 2) have been completed. These included high-speed taxi (through rotation of the QF-106A airplane) and successful flight tests.

The results of the tests showed that, among other things, the tow rope is not a straight line as previously assumed. The rope exhibits considerable sail from the airflow. Observed stability boundaries do not match those predicted from the simulations; with a differential altitude of approximately 200 ft (60 m) between the aircraft, the tow-rope tensions were stable. The rope sail also alters the trim of the aircraft relative to the predictions from the simulations; the angle at the point where the rope meets the QF-106A was found to be more acute than it was predicted to be, making it necessary to use more elevon deflection. Analysis was continuing at the time of reporting the information for this article.

This work was done by Al Bowers and Jim Murray of Dryden Flight Research Center.