by RP Mueller · Cited by 13 — A lightweight bulldozer blade prototype has been designed and built to be used as an excavation implement in conjunction with the NASA Chariot lunar
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Lightweight Bulldozer Attachment for Construction andExcavation on the Lunar Surface Robert P. Mueller1National Aeronautics & Space Administration (NASA) Kennedy Space Center, KSC, Florida 32899, USA R. Allen Wilkinson2 and Christopher A. Gallo3 National Aeronautics & Space Administration (NASA) Glenn Research Center, Cleveland, Ohio, 44130, USA Andrew J. Nick4 and Jason M. Schuler5Arctic Slope Regional Corporation (ASRC) Aerospace, Kennedy Space Center, KSC Florida 32899, USA and Robert H. King6Engineering Division, Colorado School of Mines, Golden, CO 80401, USA A lightweight bulldozer blade prototype has been designed and built to be used as an excavation implement in conjunction with the NASA Chariot lunar mobility platform prototype. The combined system was then used in a variety of field tests in order to characterize structural loads, excavation performance and learn about the operational behavior of lunar excavation in geotechnical lunar simulants. The purpose of this effort was to evaluate the feasibility of lunar excavation for site preparation at a planned NASA lunar outpost. Once the feasibility has been determined then the technology will become available as a candidate element in the NASA Lunar Surface Systems Architecture. In addition to NASA experimental testing of the LANCE blade, NASA engineers completed analytical work on the expected draft forces using classical soil mechanics methods. The Colorado School of Mines (CSM) team utilized finite element analysis (FEA) to study the interaction between the cutting edge of the LANCE blade and the surface of soil. FEA was also used to examine various load cases and their effect on the lightweight structure of the LANCE blade. Overall it has been determined that a lunar bulldozer blade is a viable technology for lunar outpost site preparation, but further work is required to characterize the behavior in 116th C and actual lunar regolith in a vacuum lunar environment. ‘Chief, Surface Systems Office, NE-S, KSC, FL 32899, AIAA Senior Member. 2 Research Scientist, Space Processes and Experiments Division, Mail Stop 110-3, NASA Glenn Research Center, 21000 Brookpark Road, Cleveland, OH 44135 Systems Engineer, Systems Engineering Division, Mail Stop 86-1, NASA Glenn Research Center, 21000 Brookpark Road, Cleveland, OH 44135 “Mechanical Engineer, Advanced Systems Division, Mail Stop ASRC-l5, ASRC Aerospace, Kennedy Space Center, FL 32899 Mechanical Engineer, Advanced Systems Division, Mail Stop ASRC-15, ASRC Aerospace, Kennedy Space Center, FL 32899 6 Professor of Engineering, Engineering Division, Colorado School of Mines, Golden, CO 80401 American Institute of Aeronautics and Astronautics
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Nomenclature w= blade width l= blade length blade height d= vertical cut depth = rake angle S= external friction angle g= gravitational constant y= material density c= cohesion4)=internal friction angle q=surcharge mass eb=blunt edge thickness =blunt edge angle s=side plate thickness v=vehicle velocity r=blade radius h=soil prism heightI. Background – Why is Lunar Excavation Useful? AMONG the numerous operational and technical challenges of establishing a lunar outpost is the development of construction and excavation technologies. With the exception of Apollo 12, all of the Apollo landers touched down in an empty landscape devoid of space hardware. Apollo 12 landed 180 meters away from the Surveyor 111 lander and its engine exhaust ejected regolith off the surface, effectively sandblasting the nearby Surveyor III. Multiple landings at a lunar outpost will eject the loosely packed top levels of regolith at the outpost hardware. One method of mitigation is the construction of landing pads and protective berms. It is proposed that landing pads can be constructed by excavating to the densely compacted regolith 30 cm below the surface and using lunar regolith to build berms as blast barriers to surround the excavated area. The NASA Lunar Architecture Team (LAT) proposed an architecture in 2006 to establish a lunar outpost that includes in-situ resource utilization (ISRU). The architecture is planned to be implemented in a series of missions to establish a lunar outpost that begin in 2019 and continue at 6 month intervals into 2027 (Cook7). By 2023, the outpost might be configured as shown in Fig. I (Mueller and King8). This architecture is just one example of many architecture scenarios being studied by NASA and all data is representative only and not definitive. In addition to building landing pads and blast protection berms, lunar regolith excavation is also useful for a variety of other civil engineering and construction type of tasks which are listed in table I below. American Institute of Aeronautics and Astronautics
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50 m Landing Pad-Y-Surface Mobility 200 m PUBcavator B SPUD. Storage50 m Trench 19 Lander MPU C-‘ _____IK-ISRU Module MPU Habitata1–ExcavatorA2020-A I MPULander iMPU BO2 Mining Area SPUCCmm 101Dm2Dm H20 Mining Area Figure-I. Example 2023-B Typical Lunar Outpost Site Plan Outpost development requires excavation for landing and launch sites, roads, trenches, foundations, radiation and thermal shielding, etc; furthermore, ISRU requires excavation as feedstock for water processing and oxygen production plants. Several alternative scenarios are under consideration at NASA that include either carbothermal or hydrogen reduction process to extract Oxygen from regolith and using special materials to avoid covering habitat modules. Table I presents the mass of excavated regolith required by task for each mission for the Hydrogen reduction processing and habitat structures shielded by regolith (H2 Reduction 02/Hab Shield) alternative. 20192020a2020b2021a2021b2022a2022b2023a2023b Cable Trenches37,64456,46509,41109,411000 Roads150,000150,000112,50037,500037,50037,50000 Landing Pad588,750588,7500000000 Berms282,726282,7260000000 Foundations01,2004,5001,2002,2506004,2001,3500 Hab/Shield Trench00357,6680178,8340178,83400 Hab/Shield Roof00066,88066,88066,88066,88000 02 ISRU0000000250,000250,000 Ice ISRU000000050,00050,000 Total Regolith (MT)1,0591,079475115248114287251250 TotalIce00000005050 Regolith (MI)Table I: Excavation requirements (Kg) for a Representative Lunar Outpost Mission 3American Institute of Aeronautics and Astronautics
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To construct these surface features excavation devices similar to earth-moving equipment will be needed. Engineers at the Kennedy Space Center have developed the first prototype of a lunar excavation blade. The Lunar Attachment Node for Construction and Excavation (LANCE) blade attaches to the prototype lunar truck Chariot, built by the Johnson Space Center. The LANCE blade is a lightweight implement which, in combination with the Chariot platform, can perform lunar site preparation activities such as area clearing of rocks, leveling, dozing, grading, and berm construction. The blade is made from an aluminum support structure with a unique composite moldboard. The composite moldboard is constructed from alternating layers of carbon fiber and proprietary toughening polymer fibers in an epoxy resin matrix, with a polyurethane topcoat. The toughening polymer allows the composite to have an increased elastic deformation zone, similar to metals but with the decreased mass of a carbon fiber composite. Polyurethane on the outermost surface provides a tough wear resistant finish. The 4.1 meter long blade can be independently adjusted using an electromechanical actuator to actively set digging depth. The lightweight construction of the entire system amounts to a mass savings of over 70% compared to a similarly sized commercial dozer blade.II. Methods of Plume Mitigation Lander rocket engine plume interaction with the lunar regolith causes ejecta of the regolith particles at speeds of 1,000 rn/s to 2,000 mIs according to research done by Metzger’°. These particles are typically sized from 20 microns to 100 microns in diameter, but nevertheless can cause damage to emplaced lunar assets. Analysis of the Apollo landing plumes has established that the plume ejecta travels in a wide low swath that extends up to 3 degrees from the horizontal surface (Metzger’°). Therefore, one method of plume mitigation is to erect a barrier at the perimeter of the landing area to block the path of these ejected particles. Since transported mass from the Earth is extremely expensive, it is more cost effective to use in-situ regolith to build a local barrier such as a regolith berm. There are other methods of plume mitigation as well, such as creating a hardened landing pad by sintering, polymer deposition or other methods, In addition the barrier could be made of non-lunar materials that have been brought or re-cycled. For the purposes of this paper, only in-situ regolith berms will be considered as a blast barrier. III. Lunar Attachment Node for Construction & Excavation (LANCE) A lunar bulldozer blade was developed in a custom design to interface and operate with the Chariot mobility platform also known as a “Lunar Truck”. The Chariot can use a variety of implements and the bulldozer blade known as Lunar Attachment Node for Construction & Excavation (LANCE) is one instance of such implements. The intended use of the LANCE blade is as a prototype “proof of concept” demonstration device for lunar site preparation. It is made of an aluminum frame with a composite carbon fiber /epoxy mold board. Figure 2 – LANCE Blade Isometric View American Institute of Aeronautics and Astronautics
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A. LANCE Configuration The LANCE bulldozer blade was mounted onto the Chariot mobility platform via a structural interface consisting of five structural hard points that connected directly to the chrome-molybdenum Chariot steel frame. The blade was mounted in a static fashion, with future enhancements expected which will make give it a motion controlled vertical degree of freedom in order to vary the cut depth as desired. Another future enhancement that is expected is a quick attach mechanism which will allow rapid assembly to the Chariot. The blade was mounted as close as possible to the Chariot frame in order to reduce cantilever moment loads and deflections. The result is an extremely rigid assembly which provides superior control due to the close coupling of the chariot and blade structures. By avoiding a long cantilever arm, as is typical in commercial applications, several issues are eliminated such as wash-boarding of the excavation surface and other delayed control loop effects. Another important aspect of the blade design is the fact that it covers the port to starboard span of the rover wheels so that the bulldozed and graded surface provides a smooth path for the wheels to drive on, thereby enhancing control and allowing slot-dozing techniques to be used. Figure 3 – LANCE Blade Grading a Simulated Landing Site in Moses Lake, Washington State, USA B. Lightweight Blade Construction Techniques Typically excavation tools are made with heavy welded steel construction. This type of construction is very efficient for a terrestrial application, but when designing for aerospace application the goal is typically to maintain strength and durability while minimizing weight. To achieve this goal the design team looked at many different construction techniques. For example, welded high strength steel with smaller wall thicknesses, welded aluminum, composites, and hybrid aluminum-composite structures. Mass trades were performed and the high strength steel and welded aluminum frames were comparable while the composite structure approximately weighed 2 times less. The high strength steel with smaller wall thicknesses presented difficult challenges during the manufacturing process specifically during welding. The composite structure was very promising but required complex finite element analysis and very specific manufacturing processes. The welded aluminum frame requires special weld techniques due to the fact that aluminum anneals around the weld affected areas. The annealed aluminum can have up to 70% loss in material strength when compared to a heat treated aluminum stock. Two techniques were considered for avoiding this loss in material strength, removing the load paths from the weld affected areas and post-weld heat treatment. Removing the load paths from the weld affected areas generally meant adding more material and was found to be not optimized for light weight design. The post-weld heat treatment required certain alloys of aluminum stock. Aluminum 6061is typically used for post-weld heat treatment, while also being readily available and easy to machine and weld. Post-weld heat treatment was found to increase the annealed aluminum’s material strength to within 85% of 606 1-T6 material strength6. After considering the required manufacturing processes for the high strength steel, welded aluminum, and composite structure the welded aluminum frame using post weld heat treatment was selected. The hybrid aluminum composite frame was reinforced by making the front moldboard of American Institute of Aeronautics and Astronautics
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luminum mount plate Composite moldboard Hardened steel cutting edge and backer plate Aluminum support framethe frame out of carbon fiber epoxy, but the majority of the load bearing frame was made from the post-weld heat treated frame. The hybrid design presented less initial risk then the complete composite structure but still allowed for more weight savings while pushing the limits of typical excavation tools. The blade is several meters long to ensure that the LER’s wheels are covered during grading to cover the wheel tracks. The structure consists of vertical ribs along the length of the blade with horizontal stiffeners connecting the ribs together (Figure 4). The ribs and horizontal stiffeners fit together like ajig helping to keep the frame straight during welding (Figure 5). The blade was separated into three 2 meter sections to make manufacturing and heat treatment easier. The composite moldboard is made from 0 degree and +-45 degree layers of carbon fiber with a toughing polymer fiber in between each layer of carbon fiber. This polymer fiber helps dampen the loads from grading by distributing the loads across the moldboard. The front of the moldboard is coated with a thin layer of urethane much like truck bed liner. This urethane layer protects against abrasion from the soil during grading. The assumption is that the urethane coating helps absorb the impacts from the soil effectively creating an abrasive boundary layer. The carbon fiber moldboard is bolted to the front of the aluminum structure using the ribs (Figure 6).Figure 4: Rib and horizontal stiffener constructionFigure-5: Jig Construction The cutting edge is bolted on and made from hardened steel (Figure 6). During grading operations forward grading was found to be effective during initial leveling operations, but for final grading a back drag operation creates a very smooth surface. Opposite the cutting edge a hardened plate was bolted on to protect during the back drag operations (Figure 6). The aluminum frame is bolted onto an aluminum plate which serves as a mounting plate to the back of the rover (Figure 6).Figure 6: Lance blade diagram mounted on Chariot American Institute of Aeronautics and Astronautics
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The analysis predicted blade forces using the Balovnev equations4. Two separate equations were used to predict the blade forces. One was derived for a bucket and the other for a blade. Both were coded to compare the two different theoretical methods to the data. The test data was obtained for a wide bulldozer blade but the Balovnev blade equations did not necessarily result in the best correlation with the data. I) Balovnev Bucket Force Equations Horizontal Blade Force = wd A1 (I + cot 13 tan 6) [dgy/2 + c cot 4) + gq + BURIED * (d – I sin ) gy (1-sin 4))/(l+sin 4))] + web A2 (1 + tan 6 cot ab)(ebgy/2 ±c cot 4) ±gq + dgy(1-sin 4))/(l+sin 4)))+ d A3 (2s + 4l tan 6) [dg’)’/2 + c cot 4) + gq + BURIED * (d – l sin 3) gy (I-sin 4))/( 1 +sin 4))](3) where BURIED = I if entire bucket is below the soil otherwise BURIED = 0 A1=A(13) A2 = A(ab) A3 = A(it!2) Replace x’ in the following equations with f3 for A1, ab for A2 or it/2 for A3 if x [0.5 [sin’ (sin 6/sin 4)) – 6] A(x) = (I- sifl 4) cos 2x)/(l-sin 4)) ifx > 0.5 [sin’ (sin 6/sin 4)) – 6] A(x) = [cos 6 (cos 6 + (sin2 4) – sin2 6)2)/(l-sin 4))] exp[(2x – it + 6 + sin1 (sin 6/sin 4))) tan 4)] Vertical Blade Force = Horizontal Blade Force * cos (13 +- 6)! sin(13 + 6)(4) 2) Balovnev Blade Force Equations lf(13 <= (0.5 sin'(sinö/sin4)) - 6/2)) Al = (l-sin4) cos(213)) / (l-sin4)) lf(13 > (0.5 sin'(sinö/sin4)) – 6/2)) Al = (cosö (cosö + (sin24)-sin2ö)°5)) / (l-sin4)) Exp((213 – it + 6 + sint(sinö/sin4))) tan4)) A2 = 0.8 gyw (tan6+tan4)) cos24) A3 = sin(l!(2r/h)) A4 = A2 rh / (2tanö) A5 = A2 cos(A3) r2 / (1+tan26) A6 = A2 sin(A3) r2 tan6 /(1+tan26) A7 = Exp(2A3 tan6) A8=(A4+A5)(A7-l)-A6(A7+l) B I = Al wgyd2 sin(f3+6) / sin13 / cosö / 2 B2 = Al dwc sin(f3+6) / sinf3 / cos6 / tan(4)) B3 = Al A8 sin(13+6) / cosö / ((tan13+Tan(ic/4-4)/2)) / (tan13 Tan(ir/4-4)/2))) B4 = Al 0.8 gydwh sin(13+6) / sinf3 / cosö B5 = 0.8 gywh2 cos24) / 2American Institute of Aeronautics and Astronautics
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Horizontal Blade Force = BI + B2 + B3 + B4 + B5(5) 3) Luth-Wismer Blade Force Equations A third set of equations was used to predict the forces on the blade to provide an additional comparison to the measured forces. This third set is designated as the Luth-Wismer equations5. The Luth-Wismer equations predict a blade force higher than the Balovnev blade equations when compared to the measured value. As before, the vertical force, FY is small because of the high rake angle. The horizontal force, FX calculated by Luth-Wismer is roughly 10 times the force from the experimental data. Unlike the Balovnev equations, the force calculated from Luth-Wismer is a function of vehicle velocity whereas velocity is not a variable in Balovnev. Horizontal Blade Force = y g w dL’2 I’ l’ (d / (I sin3))°77 (1.05 (d / w)’ + 1.26 V2 / g / I + 3.91)(6) DESCRIPTIONVARIABLEUNITS blade widthwmeter blade lengthI.meter blade heightImeter vertical cut depthdmeter rake angledegrees external friction angle6degrees gravitational constantgmeter/second2 soil densityykilogramlmeter3 cohesioncNewton / meter2 internal friction angledegrees surcharge massqkilogram/meter2 blunt edge thicknesse,meter blunt edge angleczbdegrees side plate thicknesssmeter vehicle velocityvmeter/second blade radiusrmeter soil prism heighthmeter Table 2 – Input Data to Blade Force EquationsAlthough the Balovnev bucket force equations performed the best compared to the data obtained, there were too many un-controlled circumstances in the GRC-1 simulant test since it was performed outdoors in Houston, Texas 9American Institute of Aeronautics and Astronautics
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and without the instrumentation desired which would be typical of a laboratory soil bin facility. In addition, the quality and simulant fidelity of the GRC-1 is questionable. Therefore further testing was desired with a higher fidelity simulant. GRC-3 is a higher fidelity simulant (sand with river silt mix) developed at the Glenn Research Center which, when prepared correctly, attempts to mimic the cohesion of lunar regolith on the moon. A further round of testing was performed at Johnson Space Center in November 2008 with GRC-3 in order to attempt to verify the predicted values from the classical methods presented here. These results will be the subject of a future paper. Overall, it was concluded that the prediction capability of the Balovnev bucket equations was promising but not accurate enough to use for design load cases, so further work is required. Subsequently, empirical test values from Chariot draw bar pull were used to generate load cases for the LANCE blade design. B. Finite Element Analysis LANCE structural stresses were modeled using the COSMOSWorks finite element analysis software. The Pathfinder blade is constructed primarily of machined aluminum parts that are welded together. In order to estimate the stresses that the Pathfinder blade experienced during usage analyses were performed in two scenarios. The first examined a worst-case scenario by applying a 5000-lbf load to the outside of the blade to simulate the condition where the blade contacts a large rock on the lower corner. This is an appropriate scenario to analyze as there could be large rocks buried in the lunar soil. NASA reported approximately 5000 lbf pushing (draw bar) force at Chariot wheel slip on a firm surface. The second scenario used forces measured during a series of experimental tests performed by NASA to model stresses in the blade. In addition to experimental testing of the LANCE blade with GRC, engineers at the Colorado School of Mines (CSM) completed analytical work on the expected draft forces and structural analysis. The CSN4 team utilized finite element anaylsis (FEA) to study the interaction between the cutting edge of the LANCE blade and the surface of soil. FEA was also used to examine various load cases and their effect on the lightweight structure of the LANCE blade. The current analytical models do not allow for a time varying accumulation of soil in front of the blade, nor can they calculate the force distribution over the surface of the blade. Finite element analysis (FEA) methods can help analyze the horizontal force increase with forward motion over time while varying the surcharge from zero to the maximum steady state surcharge. 3D Finite element analysis will also allow for a stress/force distribution to be calculated on the blade at any time during the simulation. The total horizontal force on the blade, calculated with FEA, could then be compared to the analytical models and the experimental data. FEA also allows for a more accurate constitutive soil model which describes the soil behavior in much more accurate detail than assumed in the analytical models. FEA will thus allow a more accurate prediction of the expected excavation forces which will result in the lightest weight, most reliable blade design. I) Stress Equations and Formulas For the maximum stress calculations the von Mises stresses criterion was used. The von Mises stress equation takes into account stresses in all locations to calculate a combined stress. The equation for von Misses stress is: 10American Institute of Aeronautics and Astronautics
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222(222 –+ (o – c) +-+++ 2 vm = von Mises stress = stress in x direction = stress in y direction = stress in z direction xy = shear stress in xy direction= shear stress in xz directionyz = shear stress in yz direction The reported displacement is the URES value. The URES displacement is the displacement between an unloaded and loaded model. The equation for the URES displacement is: URES =+ 52 + = displacement in x directionS = displacement in y direction= displacement in z direction The reported factor of safety values are based on the yield strength of the material and the stress at the location. The factor of safety equation is:S FOS =vm Sy = yield strength vm = von Mises stressIIAmerican Institute of Aeronautics and Astronautics
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