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The "Eclipse" Project

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The Eclipse Tow Launch Project
Developing an Alternative Space Launch System

NASA Technical Briefings

Dryden Flight Research Center, Edwards, California

 

Project Description:

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The project acquired the nickname "Dope on a rope".  Mark wasn't very fond of it, but after completion of the project, he said he preferred "dope on a rope" to what the NASA folks at Edwards were calling him--"The Drag Queen!"

Pilot Mark Stucky

In 1997 and 1998, the Dryden Flight Research Center at Edwards, California, supported and hosted a Kelly Space & Technology, Inc. project called Eclipse, which sought to demonstrate the feasibility of a reusable tow-launch vehicle concept. The project goal was to successfully tow, in-flight, a modified QF-106 delta-wing aircraft with an Air Force C-141A transport aircraft. This would demonstrate the possibility of towing and launching an actual launch vehicle from behind a tow plane.

Dryden was the responsible test organization and had flight safety responsibility for the Eclipse project. Dryden provided engineering, instrumentation, simulation, modification, maintenance, range support, and research pilots for the test program.

The Air Force Flight Test Center (AFFTC), Edwards, California, supplied the C-141A transport aircraft and crew and configured the aircraft as needed for the tests. The AFFTC also provided the concept and detail design and analysis as well as hardware for the tow system and QF-106 modifications. Dryden performed the modifications to convert the QF-106 drone into the piloted EXD-01 (Eclipse eXperimental Demonstrator–01) experimental aircraft. Kelly Space & Technology hoped to use the results gleaned from the tow test in developing a series of low-cost, reusable launch vehicles. These tests demonstrated the validity of towing a delta-wing aircraft having high wing loading, validated the tow simulation model, and demonstrated various operational procedures, such as ground processing of in-flight maneuvers and emergency abort scenarios.

 

Eclipse Videos

 

 

The Eclipse Tow Launch Demonstration Project

Dryden Flight Research Center, Edwards, California

 

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NASA Dryden Flight Research Center supported a Kelly Space and Technology, Inc. (KST)/U.S. Air Force project, known as Eclipse, which demonstrated a reusable tow launch vehicle concept.

The purpose of the project was to demonstrate a reusable tow launch vehicle concept conceived and patented by KST. Kelly Space obtained a contract with the USAF Research Laboratory for the tow launch demonstration project under the Small Business Innovation Research (SBIR) program. The USAF SBIR contract included the modifications to turn the QF-106 into the Experimental Demonstrator #1 (EXD-01), and C-141A aircraft to incorporate the tow provisions to link the two aircraft, as well as conducting flight tests. The demonstration consisted of ground and flight tests.

These included a Combined Systems Test of both airplanes joined by a tow rope, a towed taxi test, and six towed flights. The project's primary goal of demonstrating the tow phase of the Eclipse concept using a scaled-down tow aircraft (C-141A) and a representative aerodynamically-shaped aircraft (QF-106A) as a launch vehicle was successfully accomplished.

On Feb. 6, 1998, the Eclipse project made its sixth and final towed flight, bringing the project to a successful completion. Preliminary flight results determined that the handling qualities of the QF-106 on tow were very stable; actual flight measured values of tow rope tension were well within predictions by the simulation, aerodynamic characteristics and elastic properties of the tow rope were a signficant component of the towing system; and Dryden's high-fidelity simulation provided a representative model of the performance of the QF-106 and C-141A airplanes in tow configuration. Total time on tow for the entire project was five hours, 34 minutes, and 29 seconds. All six flights were highly productive, and all project objectives were achieved.

 

Project Goals

All three of the project's objectives were successfully accomplished. The objectives were: demonstration of towed take-off, climb-out, and separation of the EXD-01 from the towing aircraft; validation of simulation models of the towed aircraft systems; and development of ground and flight procedures for towing and launching a delta-winged airplane configuration safely behind a transport-type aircraft.

 

Participant's Roles

NASA Dryden served as the responsible test organization and had flight safety responsibility for the Eclipse project. Dryden also supplied engineering, simulation, instrumentation, range support, research pilots, and chase aircraft for the test series. Dryden personnel performed the modifications to convert the QF-106 into the piloted EXD-01 aircraft. During the early flight phase of the project, Tracor, Inc., provided maintenance and ground support for the two QF-106 airplanes.

The Air Force Flight Test Center (AFFTC) provided the C-141A transport aircraft for the project, its flight and engineering support, and the aircrew.

Kelly Space and Technology provided the modification design and fabrication of the hardware that was installed on the EXD-01 aircraft.

Kelly Space and Technology hopes to use the data gleaned from the tow tests to develop a series of low-cost reusable launch vehicles, in particular to gain experience towing delta-wing aircraft having high wing loading, and to demonstrate various operational procedures such as ground processing and abort scenarios.

 

Background

The first successful towed flight occurred on Dec. 20, 1997. Prior to the first tow test flight, the C-141A and EXD-01 were used to conduct a series of tethered taxi tests to validate the tow procedures. Previously, a successful joint flight test was conducted in late October 1996, by Dryden, AFFTC, and KST, in which one of Dryden's F-18 chase aircraft flew at various ranges and locations behind the C-141A to define the wake turbulence and wingtip vortex environment. This flight test was replicated in July 1997, with an unmodified QF-106 flight proficiency aircraft.

 

Simulation

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To enhance flight safety and reduce the number of unknowns during flight tests, the Eclipse project relied on a high-fidelity flight simulator. Full nonlinear mathematical models of the EXD-01 aircraft, C-141A aircraft, and the tow rope were modeled in the Eclipse simulator, which had a piloted and an off-line, batch version.

The former was used to train pilots for normal and emergency operations, while the latter was used for dynamic analyses and for validation of major design decisions. As the flight project progressed the simulation validated the flight data. An additional benefit of the fully validated simulation is the ability to extrapolate the Eclipse tow dynamics to larger, future tow launch concepts.

 

Aircraft Modifications

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Modifications to the QF-106 included shortening the nose pitot boom, and the addition of a tow cable attachment and release mechanism. In addition, cockpit modifications such as the addition of a tow rope tension display were made.

Modifications to the C-141A were minimal, with all of its towing and tow rope jettison equipment placed on a standard cargo pallet secured in the rear of the aircraft. During earlier tests, smoke-generating devices were placed on the C-141's wingtips to enhance the visualization of the aircraft's wake vortices as the EXD-01 flew behind it.

The QF-106 was selected by KST because the aircraft has a delta wing planform representative of the Astroliner spacecraft that the company plans to build. The QF-106 is a rugged, reliable aircraft, which was available from the Air Force's drone target aircraft inventory. The C-141A was chosen because it can be configured as a tow aircraft with no modification to the airframe.

The 1000 foot-long tow "train" used to tow the EXD-01 aircraft consisted of the tow rope, made of a synthetic fiber normally used in marine applications; a breakable link designed for safety and to break if too much stress was placed on the tow train. Nylon straps were used on the first three flights in the middle of the tow train for damping to help any spring tendencies in the tow train. The straps were not needed for the last three flights

 

Funding

Kelly Space and Technology obtained a contract with the USAF Research Lab for the tow launch demonstration project under the Small Business Innovation Research (SBIR) program. The USAF SBIR contract includes the modifications to the QF-106 and C-141A aircraft to incorporate the tow provisions to link the two aircraft.

 

Technology Commercialization

Kelly Space and Technology's Eclipse Astroliner concept, which the current Eclipse project is demonstrating, will use a Boeing 747 to tow the planned Astroliner vehicle from a conventional runway to a launch altitude of about 45,000 feet. At that altitude the tow line would be released, and the Eclipse Astroliner's rocket engine would be ignited. Releasing a vehicle at that altitude results in greater fuel economy for the launch vehicle than would a typical ground-launching. The Eclipse Astroliner would then climb to its designated payload separation altitude of about 400,000 feet. Following ejection from the Eclipse Astroliner, the payload's upper stages would be ignited to deliver the payload to its specified orbital destination. The Eclipse Astroliner vehicle then would descend as a glider, much like the Space Shuttle, to land using either remote piloting capability or an automatic landing system.

 

 

 



 

NASA Technical Briefings

Dryden Flight Research Center, Edwards, California

 

The Eclipse project was comprised of three phases: wake turbulence assessment, airdata calibration, and tethered operations. The first two phases have been completed successfully. The tethered operations were initiated in late 1997.

The wake turbulence produced by the C-141A and the handling qualities of the QF-106 in that wake were evaluated and assessed in several flights. From the results of these flights it was determined that proper positioning of the QF-106 behind the C-141A provided stable, controllable flight conditions. For one flight test, smoke-generating devices were placed under the C-141A's wings to enable visualization of the aircraft's wake vortices. Also, one of Dryden's F-18 chase aircraft flew at various distances and lateral positions behind the C-141A to probe the wake in an effort to define the wake turbulence environment. Finally, this probe test was replicated by the unmodified QF-106. It was found that both aircraft, particularly the QF-106, were very controllable even in wake turbulence. Testing also confirmed viability of the chosen tow rope length and a low tow position.

The modified EXD-01 was flown in the fall of 1997 to obtain the airdata calibration with the modified nose-boom, which has been shortened to prevent interference with the tow rope. The tethered flights began in late 1997.

 

Test Aircraft

The QF-106 was selected by KST because the aircraft has a delta wing planform representative of the Astroliner spacecraft that the company plans to build. The QF-106 is a rugged, reliable aircraft which was available from the Air Force's drone target aircraft inventory. The C-141A was chosen because it can be configured as a tow aircraft with no modification to the airframe.

Dryden added a research instrumentation system to obtain airspeed, aircraft motion, tow rope tension, and tow rope angle measurements. Modifications to the QF-106 included shortening the nose pitot boom and addition of a tow rope attachment and release mechanism. The cockpit was modified to provide the pilot with a tow rope tension display and also the two (an electrical and a mechanical) tow rope releases. A video camera was installed near the aircraft's nose to provide the control room with a view of the tow rope during the flight. These aircraft modifications were performed by Dryden personnel. No sigificant modifications were made to the C-141A. All of the towing and tow rope jettison equipment will be placed on a standard cargo pallet secured in the rear of the aircraft. A video display of the tow rope and EXD-01 was installed, as well as a flight test instrumentation pallet to obtain C-141A aircraft parameters. Differential GPS will be used to determine the separation distance between the two aircraft.

 

Simulation

To enhance flight safety and reduce the number of unknowns during flight tests, the Eclipse project used a high fidelity simulator. Full nonlinear mathematical models of the EXD-01 aircraft, C-141A aircraft, and the tow rope were modeled in the Eclipse simulator which had both a piloted and an off-line, batch version. The former was used for pilot training for normal and emergency operations, while the latter was used for dynamic analyses and for validation of major design decisions.

As the flight project progressed the simulation was be validated with flight data. An additional benefit of the fully validated simulation was the ability to extrapolate the Eclipse tow dynamics to larger, future tow launch concepts.

 

Flight Test Approach

A build up approach was used in the tow demonstration flight phase. That meant each test mission build upon the knowledge and experience gained from the prior test. The first steps were to validate all test and flight procedures. Initial missions would also validate predicted performance of both aircraft, particularly during takeoff and climb out. For subsequent missions, the EXD-01's performance and handling qualities were evaluated at various flight configurations. At first, the EXD-01 was flown in a high drag configuration with landing gear and speed brakes extended, whereas final flights were conducted in a 'clean' or "landing gear up" configuration.

Two tow rope configurations were used. The first flight tests employed a tow rope that consisted of three primary elements: a 1000 foot Vectran rope that is bisected by a 50 foot section of 8-ply nylon strap. Damping characteristics of the tow rope were significantly improved by the nylon segment. Then flight testing used a tow rope that was made of a single 1,000 foot Vectran element. The two airplanes were staged on the runway, during which the hookup to the tow rope was made. The C-141A then added tension to the tow rope by taxiing forward slowly, then it accelerated, taking off at 120 knots airspeed. In tow, with engines at idle, EXD-01 rotated at 130 knots and lifted off at 165 knots. The EXD-01 pilot then positioned the airplane in a 'low tow' position at a -20 degrees elevation angle throughout the tow. The EXD-01's engine were at idle power throughout the towed portion of the mission, to enable it to 'power up' rapidly after release for a conventional landing. All towed flights concluded with the release of the EXD-01 from tow at the target altitude.

 

 



 

 

NASA Technical Briefings

Dryden Flight Research Center, Edwards, California

Flight tests have been successful.

 

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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. 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.

 

 

 

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

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 aero-towed reusable launch vehicle to put small satellites into low orbits around the Earth. In support of this idea, the SBIR is to demonstrate aero-tow 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.

Towing a launch vehicle to altitude should make it possible increase the payload and decrease the cost of the launch. No previous aero-tow 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 aero-tow 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 aero-tow system. The Air Force Flight Test Center also supported this project by providing the C-141A airplane and flight crew.

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.


 

 



 

NASA Technical Briefings

Dryden Flight Research Center, Edwards, California

 

Dryden was the Responsible Test Organization and had flight safety responsibility for the Eclipse project. Dryden was also supplying engineering, instrumentation, simulation, modification , maintenance, range support, research pilots, and chase aircraft for the test series.

The Air Force Flight Test Center (AFFTC) supplied the C-141A transport aircraft and crew and configuring the aircraft as required for the tests. They also provided general engineering support and assuming flight safety responsibilities for the C-141A.

KST provided concept and detail design and analysis as well as hardware for the tow system and QF-106 modifications. Dryden performed the modifications to convert the QF-106 drone into the piloted EXD-01 (Eclipse eXperimental Demonstrator -01) experimental airplane.

KST will use the results gleaned from the tow tests in developing a series of low-cost, reusable launch vehicles. These tests demonstrated the viability of towing a delta-wing aircraft having high wing loading, validate the tow simulation model, and demonstrate various operational procedures, such as ground processing, in-flight maneuvers, and emergency abort scenarios.

 

 



 

NASA Technical Briefings

Dryden Flight Research Center, Edwards, California

Past Project - Eclipse Tow Launch Demonstration

 

On February 6, 1998 the Eclipse project accomplished its sixth and final towed flight, bringing the project to a successful completion. Preliminary flight results determined that the handling qualities of the QF-106 on tow were very stable, actual flight measured values of tow rope tension were well within predictions by the simulation, aerodynamic characteristics and elastic properties of the tow rope are a significant component of the towing system, and Dryden's high fidelity simulation provided a representative model of the performance of both the QF-106 and C-141A airplanes in tow configuration. All six flights were highly productive and all project objectives were achieved.

 

Project Summary

NASA Dryden Flight Research Center supported and hosted a Kelly Space and Technology, Inc., (KST)/USAF project, known as Eclipse, which demonstrated a reusable tow launch vehicle concept. The goal of the project was to successfully tow in flight a modified QF-106 delta-wing aircraft with an Air Force C-141A transport-type aircraft. Dryden was the Responsible Test Organization and had flight safety responsibility for the Eclipse project. Dryden also supplied engineering, instrumentation, simulation, modification, maintenance, range support, research pilots, and chase aircraft for the test series.

The Air Force Flight Test Center (AFFTC) supplied the C-141A transport aircraft and crew and configuring the aircraft as required for the tests. They also provided general engineering support and assumed flight safety responsibilities for the C-141A. KST provided concept and detail design and analysis as well as hardware for the tow system and QF-106 modifications. Dryden performed the modifications to convert the QF-106 drone into the piloted EXD-01 (Eclipse eXperimental Demonstrator -01) experimental airplane. KST planned to use the results gleaned from the tow tests in developing a series of low-cost, reusable launch vehicles.

These tests were to demonstrate the viability of towing a delta-wing aircraft having high wing loading, validate the tow simulation model, and demonstrate various operational procedures, such as ground processing, in-flight maneuvers, and emergency abort scenarios.

 

 



 

 

NASA Technical Briefings

Dryden Flight Research Center, Edwards, California

 

Simulations In Support Of Towed Flight Demonstration

Two independent simulations run simultaneously to study towed-aircraft response before flight.

 

The Eclipse flight project was established to demonstrate a reusable-launch-vehicle concept developed by Kelly Space and Technology, Inc. An F-106 delta-wing aircraft was chosen as the towed vehicle, and a C-141A transport-type airplane was selected for the towing vehicle. These airplanes are shown in Figure 1. Dryden Flight Research Center was the test organization with responsibility for safety of flight on the Eclipse project.

To enhance safety of flight, simulations of the two airplanes were implemented along with a simple mathematical model of a tow rope. A computational simulation of an F-106 airplane had been implemented at Langley Research Center to support some vortex-flow flight experiments, and this simulation was revived at Dryden. The C-141 simulation was adapted from an existing B-720 simulation at Dryden by replacing the mathematical model of the aerodynamics of the B-720 airplane with linear aerodynamic coefficients based on the performance of the C-141 airplane. The mathematical model of the B-720 engine was modified with a thrust multiplier to match the C-141 static sea-level thrust. In addition, the simulation was updated with C-141 weight, inertia, and center-of-gravity data. Existing simulation cockpits were used without modification.

The tow-rope model assumes that the tow rope lies on straight line between the two airplanes. On the basis of results from laboratory tests, the rope tension was modeled as quadratic in elongation and linear in elongation rate. This tow-rope model was verified initially by implementing it in a glider simulation and having a glider pilot subjectively evaluate the performance.

Initial studies were performed with the F-106 simulation alone. In these studies, it was assumed that the C-141 airplane was a point mass that would be unaffected by the forces on the tow rope. C-141 takeoff trajectories were generated and recorded in the C-141 simulation. These trajectories were played back in the F-106 simulation to study the takeoff performance of the towed F-106. This first cut showed some interesting results. The F-106 performance on tow was quite different from that of a sailplane. There appeared to be a lower and an upper bound on the tow angle between the two airplanes. Flight beyond these bounds would cause divergent pitch and sometimes roll oscillations. Fortunately, the oscillation amplitude would increase slowly enough that the pilot was able to recognize the problem and correct for it by flying back within the bounds. The simulation was already providing important information to the flight-test team.

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Figure 1. These Aircraft Were Used To Demonstrate the concept of towing a reusable aerospace vehicle to launch altitude.

Figure 2. Simulations of the Towing and Towed Airplanes were synchronized to obtain consistent results.

 This work was done by Ken Norlin and Jim Murray of Dryden Flight Research Center and Joe Gera of Analytical Services and Materials, Inc. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com under the Machinery/Automation category. DRC-98-33

To make the simulation study more realistic, it was decided that simulations of both airplanes should be performed simultaneously. To do this, it was necessary to link two independent six-degree-of-freedom (6-DOF) simulations — essentially creating a 12-DOF simulation. Although this seemed challenging at first, it turned out to be quite simple. The two simulation computers were linked with a fiber-optic reflective memory interface; this linkage enabled the sharing of airplane positions, velocities, and tow-rope forces between the two simulations.

To obtain consistent results, it was decided to synchronize the two simulations. The frame rates of both simulations were increased to 100 Hz, and flags in shared memory were created to enable the simulations to synchronize by polling. The interrupt driver in the F-106 simulation was used to generate the 100-Hz frame pulse, and the C-141 simulation simply waited for the F-106 simulation to indicate that a new frame should be started. The synchronization scheme is shown in Figure 2.

The results of the linked simulations confirmed the results of the F-106 simulation. The assumption that the C-141 airplane could be treated as a point mass turned out to be a good one. The C-141 pilot could not feel the effects of the F-106 doing normal small-amplitude maneuvers on tow.

The availability of two independent simulations also afforded a capability to achieve quicker, more productive, simulation sessions. Instead of generating a C-141 trajectory and then preparing and transferring the resulting data for playback in the F-106 simulation, the C-141 pilot could simply hit a "simulation reset" button and immediately try a different takeoff profile. This enabled the F-106 test pilot to quickly get the feel of the towed operation, and soon this pilot's task became easy. This setup also proved valuable for evaluating various failure scenarios during full mission simulation with the control room being fed by a stream of data generated by the simulator and transmitted by pulse-code modulation.

Six towed flights were performed in a demonstration program that was completed on February 6, 1998. Extensive instrumentation was used so that flight results could be compared with simulation results. It turns out that the simulation tow model was good at predicting rope tension, but a little conservative in predicting stability. The F-106 pilot was able to fly to more extreme tow angles before encountering the divergent oscillations. Part of this difference between the simulation and the flight tests may be due to the assumption of a straight tow rope in the simulation. During the flight tests, the tow rope would "sail" and develop significant curvature. In later flights, the tow rope was marked at regular intervals and video images were recorded so that this phenomenon could be studied in more detail. With the flight-instrumentation data and video images, it should be possible to develop a more realistic tow-rope model that can be incorporated into the simulation.


NASA photo by Tom Tschida

 

 

 

 

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The f-106 engine was turning at idle to provide electric power, and hydralic pressure for flight controls, during take offs and advanced to full power for landing, when the tow rope was released.

Close Up Of The Tow Cable Lock & Release Mechanism.

Close Up Of The Tow Cable Lock & Release Mechanism.

 

 

Cockpit Of The F-106

 

 

Dryden 1997 Research Aircraft Fleet on Ramp - X-31, F-15 ACTIVE, SR-71, F-106, F-16XL Ship #2, X-38, and X-36

 

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Photographs and Text Courtesy of NASA and Dryden Flight Research Center

 

Last Updated

02/10/2014

 

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