Between 1996 and 1998 NASA and USAF experimented a system to tow a vehicle behind a transport aircraft. The idea was to use an aerotow system to pull a winged launch vehicle to an altitude somewhere around 40,000′ at which point it would be released from the tow, fire its own rocket engines to climb to 400,000′ from where it could launch small satellites into low earth orbit.
The launch vehicle would then reenter the atmosphere where the pilot would use jet engines to fly the vehicle to a runway landing. The launch vehicle would then be serviced for another flight.
According to a NASA fact sheet, the demonstration, called the Eclipse Project, was used by NASA’s Dryden Flight Research Center to help demonstrate that a decades-old concept used to tow sailplanes into the air may help lower the cost of placing small satellites into low earth orbit.
The test and demonstration program, carried by NASA Dryden at Edwards AFB, Calif., used an Air Force C-141A Starlifter transport as the tow aircraft. A QF-106A Delta Dart was chosen as the towed aircraft to simulate a future launch vehicle because of its low-aspect ratio and relatively high wing loading. Following a period of modifications, tow system engineering and installation work, simulations, tests, and crew training, the delta-wing QF-106 was successfully towed aloft six times by the C-141 to a peak altitude of 25,000 feet to validate the aerotow concept.
The F-106s were originally all-weather interceptors dating to the mid-1950s that had been converted into unpiloted drones and used for air-to-air missile training at Tyndall AFB, Florida. One of the QF-106s was modified as the project’s primary towed aircraft and the second was a backup.
The main aspects of the project, which included modifications to the C-141A and the towed aircraft, were extensive ground testing to validate the structural integrity of the towing system, installation of a flight data instrumentation system, analytical studies to predict the towing and flight characteristics of both types of aircraft, and actual flight test operations.
The heart of the C-141A modifications was installation of an Air Force parachute extraction qualification pallet near the aft end of the aircraft’s cargo bay. On the pallet was a mandrel to which the QF-106 towline was attached. Built into the mandrel was a guillotine-like device, originally designed to cut nylon parachute straps during parachute extractions that cut the QF-106 towline once it was time to separate from the C-141A.
The clamshell cargo doors at the rear of the C-141A were removed for the project and the moveable cargo ramp was placed in a locked-up position. The pressure bulkhead door, normally used to pressurize the cargo bay during cruise flight, was also removed for the test project.
The main modification to the QF-106, besides requalifying it for piloted flight, was creating a nose-mounted tow mechanism on the aircraft that would withstand the loads and stresses of towed flight.
An attach point called a weldment and a towline release mechanism was mounted on the upper fuselage just forward of the canopy. The release system was from a standard B-52 landing drag chute system that was modified to be actuated by the push of a button on the pilot’s control stick. A manually operated backup towline release system was also added, with the handle installed in the cockpit.
The upper nose area and the sides of the fuselage were strengthened with aluminum and stainless steel skin doublers to help distribute tow loads and to bridge a major fuselage assembly point.
There were 14 flights in the program — eight to accumulate in-flight data about wake turbulence, calibrate air data instrumentation, validate simulator predictions, and determine the optimum position of the QF-106 behind the tow aircraft. The remaining six flights were the towed missions which successfully demonstrated the aerotow concept.
On the first flight in October 1996, project officials used a Dryden F-18 to investigate wake turbulence behind the C-141A to begin selecting the best and safest QF-106 tow position. Two months later, with smoke generators mounted on the C-141A’s wingtips, in-flight studies of wake turbulence and how it may impact the towed aircraft were carried out.
The first untethered pairing of the two project aircraft was in July 1997. During the flight, the QF-106 was flown about 1,000 feet behind and several hundred feet lower than the C-141A as project pilots and engineers examined potential wake turbulence and took a final look at the preferred F-106 tow location.
The next four flights with the QF-106 began in October 1997 with a functional check flight to validate simulator predictions and takeoff characteristics, and to give engineers a final instrumentation check. The pre-test series ended with a flight to calibrate air data instrumentation and a final validation of simulator predictions.
In December 1997, a successful high-speed taxi test with both aircraft linked by the towline was carried out. The test concluded with the release of the QF-106 during takeoff rotation and a final look at the takeoff characteristics of the C-141A configured as the tow aircraft.
The six towed test flights were carried out between December 1997 and February 1998.
Takeoff procedures were the same for each towed flight. After the spooled towline was attached to the QF-106 and preflight checks were completed, both aircraft were taxied to predetermined positions on the runway. The engine on the QF-106 was set at the idle power setting and remained there during the entire towed sequence to give the aircraft hydraulic and electrical power, and also thrust if an emergency release occurred. The towline was then unreeled, attached to the tow aircraft, and its brakes were locked. The C-141A was slowly moved forward to increase tension on the towline to 6,000 pounds. At that point the brakes on the C-141A were released, its engines were powered up, and brakes on the QF-106 were slowly released so that both aircraft moved in unison to keeping the towline taunt during the takeoff roll.
Takeoff speed of the C-141A was about 132 mph. The QF-106 began rotating at 138 mph but remained on the runway until it reached a speed of 189 mph. By then, the tow aircraft was several hundred feet into the air and both were accelerating in speed.
During each flight, tests were carried out to study QF-106 handling qualities, turns, climb angles, towline tensions, and wake vortices and turbulence. Data were also obtained associated with towline “bungee” oscillations as varying forces caused it to stretch and contract.
Throughout the series of flights, tests were conducted at altitudes that ranged between 10,000 feet and 24,000 feet. The top speed of the C-141A was limited to 230 mph because of the removal of the rear cargo doors.
During most tow sequences the QF-106 flew about 300 feet lower than the C-141A, though it was as much as 400 feet lower on some tests to study “bungee” and lateral control conditions. During one test the QF-106 was placed 70 feet above the C-141A to check a high-tow position. Dynamic oscillations resulted.
Once the towed portion of each flight was completed, the usual release altitude of the QF-106 for its powered flight back to Edwards AFB was about 10,000 feet. The towline release mechanism on the QF-106 performed satisfactorily on each flight.
The towline was released from the C-141A at low altitude and allowed to fall to the ground before the aircraft was landed at Edwards AFB. During the separation phase, the behavior of the plummeting towline was also studied so that abrasion and damage could be minimized in any future projects.
The joint Eclipse test project successfully demonstrated the feasibility of using the decades-old aerotow concept to launch future space vehicles.
Photo credit: U.S. Air Force and NASA
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