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The gravity tractor concept is a proposed method to deflect an imminent asteroid impact through gravitational tugging over a time scale of years. In this study, we present mission scenarios and operational considerations for asteroid mitigation efforts involving multiple gravity tractors. We quantify the deflection performance improvement provided by a multiple gravity tractor campaign and assess its sensitivity to staggered launches. We next explore several proximity operation strategies to accommodate multiple gravity tractors at a single asteroid including formation-flying and mechanically-docked configurations. Finally, we utilize 99942 Apophis as an illustrative example to assess the performance of a multiple gravity tractor campaign.
Tests were made in the 20-foot propeller-research tunnel to investigate the possibility of obtaining increased net efficiencies of propeller-nacelle units by enclosing the engines in the wings and by using extension shafts. A wing of 5-foot chord was fitted with a propeller drive assembly providing for several axial locations of tractor propellers and pusher propellers. A three-blade 4-foot propeller and a three-blade 3 1/2-foot propeller of special design were tested in this wing with spinners and fairings ranging in diameter from 6 to 16 inches. A 16-inch NACA cowling was tested for comparative purposes. Two types of cuffs were also employed. It was found that the net efficiency of a conventional round-shank propeller mounted on an extension shaft in front of or behind a wing increased with an increase in the diameter of the spinner and the shaft housing within the scope of the tests. The largest spinner used had a diameter that might favorably compare with that of a radial engine cowling. The efficiencies for the pusher position appeared to be more critically affected by spinner size than those for the tractor position. The spinners with large diameters for the pusher position resulted in a higher efficiency than those for the corresponding tractor arrangements; the reverse was true for the small spinners. The use of propeller cuffs in combination with a spinner of small diameter generally resulted in net efficiencies that were comparable with those found for the large-spinner combinations.
Given sufficient warning time, Earth-impacting asteroids and comets can be deflected with a variety of different "slow push/pull" techniques. The gravity tractor is one technique that uses the gravitational attraction of a rendezvous spacecraft to the impactor and a low-thrust, high-efficiency propulsion system to provide a gradual velocity change and alter its trajectory. An innovation to this technique, known as the Enhanced Gravity Tractor (EGT), uses mass collected in-situ to augment the mass of the spacecraft, thereby greatly increasing the gravitational force between the objects. The collected material can be a single boulder, multiple boulders, regolith or a combination of different sources. The collected mass would likely range from tens to hundreds of metric tons depending on the size of the impactor and warning time available. Depending on the propulsion system's capability and the mass collected, the EGT approach can reduce the deflection times by a factor of 10 to 50 or more, thus reducing the deflection times of several decades to years or less and overcoming the main criticism of the traditional gravity tractor approach. Additionally, multiple spacecraft can orbit the target in formation to provide the necessary velocity change and further reduce the time needed by the EGT technique to divert hazardous asteroids and comets. The robotic segment of NASA's Asteroid Redirect Mission (ARM) will collect a multi-ton boulder from the surface of a large Near-Earth Asteroid (NEA) and will provide the first ever demonstration of the EGT technique and validate one method of collecting in-situ mass on an asteroid of hazardous size.
The Asteroid Redirect Mission (ARM) concept would robotically visit a hazardous-size near-Earth asteroid (NEA) with a rendezvous spacecraft, collect a multi-ton boulder and regolith samples from its surface, demonstrate an innovative planetary defense technique known as the Enhanced Gravity Tractor (EGT), and return the asteroidal material to a stable orbit around the Moon, allowing astronauts to explore the returned material in the mid-2020s. Launch of the robotic vehicle to rendezvous with the ARM reference target, NEA (341843) 2008 EV5, would occur in late 2021 [1,2]. The robotic segment of the ARM concept uses a 40 kW Solar Electric Propulsion (SEP) system with a specific impulse (Isp) of 2600 s, and would provide the first ever demonstration of the EGT technique on a hazardous-size asteroid and validate one method of collecting mass in-situ. The power, propellant, and thrust capability of the ARM robotic spacecraft can be scaled from a 40 kW system to 150 kW and 300 kW, which represent a likely future power level progression. The gravity tractor technique uses the gravitational attraction of a station-keeping spacecraft with the asteroid to provide a velocity change and gradually alter the trajectory of the asteroid. EGT utilizes a spacecraft with a high-efficiency propulsion system, such as Solar Electric Propulsion (SEP), along with mass collected in-situ to augment the mass of the spacecraft, thereby increasing the gravitational force between the objects [3]. As long as the spacecraft has sufficient thrust and propellant capability, the EGT force is only limited by the amount of in-situ mass collected and can be increased several orders of magnitude compared to the traditional gravity tractor technique in which only the spacecraft mass is used to generate the gravitational attraction force. This increase in available force greatly reduces the required deflection time. The collected material can be a single boulder, multiple boulders, regolith, or a combination of different material types using a variety of collection techniques. The EGT concept assumes that the ability to efficiently collect asteroid mass in-situ from a wide variety of asteroid types and environments is a future capability that will be developed and perfected in the future by the asteroid mining community. Additionally, it is anticipated that the mass collection would likely be performed by a single or multiple separable spacecraft to allow the SEP spacecraft to operate at safe distance from the asteroid.
Wind-tunnel tests were performed on a scale model of a cab-over-engine tractor-trailer vehicle and several modifications of the model. Results from two of the model configurations were compared with full-scale drag data obtained from similar configurations during coast-down tests. Reductions in fuel consumption derived from these tests are presented in terms of fuel quantity and dollar savings per vehicle year, based on an annual driving distance of 160,900 km (100,000 mi.). The projected savings varied from 13,001 (3435) to 25,848 (6829) liters (gallons) per year which translated to economic savings from $3435 to about $6829 per vehicle year for an operating speed of 88.5 km/h (55 mph) and wind speeds near the national average of 15.3 km/h (9.5 mph). The estimated cumulative fuel savings for the entire U.S. fleet of cab-over-engine tractor, van-type trailer combinations ranged from 4.18 million kl (26.3 million bbl) per year for a low-drag configuration to approximately twice that amount for a more advanced configuration.
In the tractor-rocket seated-extraction candidate system for Space Shuttle Orbiter crew escape, the crewmember is pulled from his seat and away from the Orbiter via an elastic pendant, using a system of rails to guide the extraction trajectory through an opening on the window frame for flight deck crew and through the side hatch for the middeck crew. A three-body simulation has been developed to model the flight-mechanics aspects of the concept, where the three bodies are the astronaut (six DOF), the tractor rocket (six DOF), and the Shuttle Orbiter (three DOF); attention is given to crewmembers' clearance of the Orbiter structure and engine plumes.
This report is the third of a series giving the results obtained in the 20-foot wind tunnel on the interference drag, and propulsive efficiency of nacelle-propeller-wing combinations. The first report gave the results of the tests of an NACA cowled air-cooled engine nacelle with tractor propeller located in 21 positions with reference to a thick wing. The second report gave the results for several engine cowlings and nacelles with tractor propeller located in four positions with reference to same wing. The present report gives results of tests of the same nacelles and cowlings in the same positions with reference to a smaller wing of Clark y section. The lift, drag, and propulsive efficiency were determined at several angles of attack for each cowling and in each nacelle location.
Experimental measurements of a generic tractor-trailer were obtained in two wind tunnels at Ames Research Center. After a preliminary study at atmospheric conditions in the 7- by 10-Foot Wind Tunnel, additional testing was conducted at Reynolds numbers corresponding to full-scale highway speeds in the 12-Foot Pressure Wind Tunnel. To facilitate computational modeling, the 1:8-scale geometry, designated the Generic Conventional Model, included a simplified underbody and omitted many small-scale details. The measurements included overall and component forces and moments, static and dynamic surface pressures, and three-component particle image velocimetry. This summary report highlights the effects of numerous drag reduction concepts and provides details of the model installation in both wind tunnels. To provide a basis for comparison, the wind-averaged drag coefficient was tabulated for all configurations tested. Relative to the baseline configuration representative of a modern class-8 tractor-trailer, the most effective concepts were the trailer base flaps and trailer belly box providing a drag-coefficient reduction of 0.0855 and 0.0494, respectively. Trailer side skirts were less effective yielding a drag reduction of 0.0260. The database of this experimental effort is publicly available for further analysis.
Material collected from an asteroid's surface can be used to increase gravitational attraction between the asteroid and a Gravity Tractor (GT); the spacecraft therefore operates more effectively and is referred to as an Enhanced Gravity Tractor (EGT). The use of tethers and solar sails to further improve effectiveness and simplify operations is investigated. By employing a tether, the asteroidal material can be placed close to the asteroid while the spacecraft is stationed farther away, resulting in a better safety margin and improved thruster efficiency. A solar sail on a spacecraft can naturally provide radial offset and inter-spacecraft separation required for multiple EGTs.
One proposed method to deflect a potential Earth impacting asteroid is via the “gravity tractor” method. Here, a spacecraft, hovering close to an asteroid using ion engines, uses its gravitational pull to change the asteroid?s orbit away from an impacting path. The proposed Asteroid Redirect Robotic Mission was slated to demonstrate the feasibility of this technique on the asteroid 2008EV5, and measure the amount of deflection. In this paper, the questions of how long the tractoring needs to be to cause a measurable deflection, and how the spacecraft can be used to measure it were examined.
Aerodynamic drag tests were performed on a conventional cab-over-engine tractor with a 45-foot trailer and five commercially available or potentially available add-on devices using the coast-down method. The tests ranged in velocity from approximately 30 miles per hour to 65 miles per hour and included some flow visualization. A smooth, level runway at Edwards Air Force Base was used for the tests, and deceleration measurements were taken with both accelerometers and stopwatches. An evaluation of the drag reduction results obtained with each of the five add-on devices is presented.
Force and moment data were obtained from a one-twenty-fifth scale wind tunnel model of a cab-over-engine tractor trailer combination. The tests define the aerodynamic characteristics of the baseline (unmodified) vehicle and several modified configurations. The primary modifications consist of: (1) greatly increased forebody corner radii, (2) a smooth fairing over the cab-to-trailer gap, (3) a smoothed underbody, and (4) rear streamlining (boattailing)of the trailer. Tests were conducted for yaw angles from 0 deg to 30 deg. The reduction in drag, relative to the baseline, obtained by combining the modifications are compared for the zero yaw condition with full scale coast down drag results for similar configurations. The drag reductions obtained from the model and full scale tests are in good agreement.
Built-in safety mechanism for tractor-trailer "fifth-wheel" coupling keeps rig together in case kingpin failure. Modified coupling utilizes all standard components, such as two wear plates, kingpin, and kingpin latch. It is modified by adding semicircular lip to top wear plate, matching semicircular slot to bottom wear plate, and two latching stop mechanisms.
A wind tunnel investigation was conducted to determine the influence of several physical variables on the aerodynamic drag of a trailer model. The physical variables included: a cab mounted wind deflector, boattail on trailer, flow vanes on trailer front, forced transition on trailer, and decreased gap between tractor and trailer. Tests were conducted at yaw angles (relative wind angles) of 0, 5, 10, 20, and 30 degrees and Reynolds numbers of 3.58 x 10 to the 5th power 6.12 x 10 to the 5th power based upon the equivalent diameter of the vehicles. The wind deflector on top of the cab produced a calculated reduction in fuel consumption of about 5 percent of the aerodynamic portion of the fuel budget for a wind speed of 15.3 km/hr (9.5 mph) over a wind angle range of 0 deg to 180 deg and for a vehicle speed of 88.5 km/hr (55 mph). The boattail produced a calculated 7 percent to 8 percent reduction in fuel consumption under the same conditions. The decrease in gap reduced the calculated fuel consumption by about 5 percent of the aerodynamic portion of the fuel budget.
This paper reports wind-tunnel tests giving the lift coefficients of large-scale wing-nacelle combinations both with and without the propeller. The tests were made to show the effect of nacelles, and idling and stopped propellers on the landing speeds of tractor monoplanes. Four types of nacelles with various cowlings were used in numerous positions with respect to both a Clark Y and a thick airfoil. The effect of both the idling and stopped propeller on lift, and consequently on landing speed, was negligible. A nacelle with exposed engine cylinders when placed directly in front of an airfoil caused a slight reduction in lift, consequently an increase in landing speed, over the condition with the wing alone. With this exception no appreciable effect on landing speed was indicated for any of the other combinations.
This report is the fifth of a series giving the results obtained from wind tunnel tests on the interference drag and propulsive efficiency of nacelle-propeller-wing combinations. This report gives results of tests of an NACA cowled air-cooled engine nacelle with tractor propeller located in 12 positions with reference to a Clark Y biplane cellule.
The effects of propeller operation on the static longitudinal stability of single-engine tractor monoplanes are analyzed, and a simple method is presented for computing power-on pitching-moment curves for flap-retracted flight conditions. The methods evolved are based on the results of powered-model wind-tunnel investigations of 28 model configurations. Correlation curves are presented from which the effects of power on the downwash over the tail and the stabilizer effectiveness can be rapidly predicted. The procedures developed enable prediction of power-on longitudinal stability characteristics that are generally in very good agreement with experiment.