stability and energy absorption of a lunar lander by 1/6 scale drop testing and full-scale...

7/27/2019 Stability and Energy Absorption of a Lunar Lander by 1/6 scale Drop Testing and Full-scale Simulation

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Proceedings of the X on Mechanical Engineering

December 6, 2012, Pittsburgh, PA, USA

STABILITY AND ENERGY ABSORPTION EVALUATION OF A LUNAR LANDERBY 1/6 SCALED DROP TESTING AND FULL-SCALE SIMULATION

Ahmet SahinozMechanical Engineering

Carnegie Mellon UniversityPittsburgh, PA 15213

[email protected]

Mikio DavidMechanical Engineering

Carnegie Mellon UniversityPittsburgh, PA 15213

[email protected]

ABSTRACTStable soft landing is crucial for planetary missions. Shock and

tip over are risks that are mitigated by spacecraft landing gear.This research evaluates stability and energy considerations for

landing gear. 1/6 scaled prototype and full-scale simulation of

an 850kg lander were used to determine landing characteristics

for various touchdown scenarios.

A 1/6 scale lander model is designed and prototyped using

appropriate scaling factors to simulate full scale landing on the

moon. Drop tests are performed in comparison with simulations

to investigate landing loads, honeycomb stroke and tip over

stability on different slopes and surface materials. Remarkable

correlation is achieved. The system provides a safe landing

under the worst case scenarios that are based on the current

requirements and previous lunar missions.

Scaled prototype stably landed on a lunar simulant at 30deg

downhill slope with 4m/s vertical and 1m/s horizontal velocity.

Lunar simulant absorbed 50% of total energy on average.

1 INTRODUCTION

Planetary landers require compliant legs to touchdown

undamaged in a stable position, ready for operation. The proper

understanding of the mission requirements, reduced gravity

forces, lander mass properties, worst case touchdown scenarios

and the stowing space limitations of the launch vehicle are of

great importance in order to design an optimal landing system.Challenges are to design for uncertain landing conditions and to

perform tests on Earth to simulate lunar gravity. Landing gear

must cope with the expected mass, velocity and orientation of

the lander at touchdown, in the expected range of terrains, and

doing so with minimal mass and a margin of safety.

Uncertainties include the mechanical properties of regolith,

slope of the surface and rock distribution.

Fig 1: Scaled model with Astrobotic Griffin lander (mock up

legs) and Red rover

During final descent, the vertical velocity of the lander wil

be reduced nearly to zero as a result of the deceleration

provided by the main thruster. Hazard detection identifies

obstacles larger than a threshold value and selects a landing site

A horizontal velocity component may be present due to

detection errors or hazard avoidance maneuvers. The main

engine cuts off at a predetermined altitude in the order of a few

meters to prevent instability due to surface effects. Thetouchdown occurs following a short free fall phase. Resulting

kinetic energy has to be dissipated over a finite distance while

providing sufficient clearance and a stable landing [4].

In this paper, landing conditions and spacecraft landing

system are described first; and then, landing loads, energy

absorption and stability are investigated through full scale

simulation and scaled drop experiment results.


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4 SPACECRAFT LANDING SYSTEM

The primary structure of the lander consists of a deck,

upper cone, lower cone and bulkheads illustrated in gold. A

rover sits on top of the upper cone; fuel tanks are located within

four holes on the deck; weight of the tanks is transferred to the

bottom ring through bulkheads. Rectangular plates near the

bottom ring between bulkheads are the hinge connection pointsfor the lower struts of legs.

Fig 5: Lander structure with telescoping legs

Landing legs are inverted tripods with telescoping main

struts. The motion during landing is a rotation about the hinge

axis, shown in Fig 6, and the energy is absorbed by a

honeycomb cartridge which provides constant force throughout

the stroke. Assuming the lower struts provide high stiffness in

the lateral plane, the main strut is under pure axial compression.

Fig 6: Landing gear schematic

Aluminum honeycomb with 360psi crush strength is used

in 63.5mm diameter main strut of each leg, providing constant

force of 12kN during the crush with 10% deviation [7].

10g maximum acceleration requirement for a 650kg (lower

limit) lander results in an approximate total maximum force of

64kN, 16kN for each leg. The energy that needs to be absorbed

is equal to the kinetic energy of an 850kg (upper limit) lander

with 4m/s velocity.

Kinematic analysis showed that for a vertical landing on a

flat rigid surface with friction coefficient of 0.3 (min expected)

the maximum total energy can be dissipated over 10cm vertica

displacement of the center of mass with 12kN honeycomb in

each leg. The honeycomb crush corresponding to this

displacement is 8cm. This is related to the angle of the main

strut with respect to the vertical axis; higher the angle, lower thestroke value. Although 8cm crush length is sufficient for an

ideal landing, 25cm honeycomb (crush length 17.5cm, 70%) is

used to provide sufficient stroke for the worst case of landing

on a rock.

Fig 7: Main strut (transparent) with honeycomb insert

The wider the stance compared to the center of mass

height, the more stable the lander is in the presence of

horizontal velocity, orientation errors and uneven terrain

Footpad placement is done such that the static stability angle is

42deg, 10deg less than Apollo, providing a lower profile. Base

width is 3.6m, equal to the diagonal across the deck. Top of the

footpad is located 40cm below the bottom ring, providing 30cm

clearance after a maximum displacement of 10cm for crush

The height of the footpad is not added into this clearance

because it is expected to penetrate into the lunar soil.

Fig 8: Illustration of lander parameters

Footpad diameter is 45cm to support landing forces with

penetration less than footpad height which is about 7.5cm

considering the bearing strength data of the lunar soil from

Surveyor landings. The top section of the footpad is a domeproviding remarkable strength with the same mass compared to

a flat surface. Honeycomb with 30psi crush strength is placed

under the dome. The round profile with a thin sheet of

aluminum bonded at the bottom minimizes penetration and

friction.


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Fig 9: Footpad cross sections (w/o honeycomb)

Honeycomb footpad stays intact if the landing is on

regolith, but partially crushes if there are rocks on the surface,

creating an even contact. It also deforms to soften the impact on

the leg when excessive side loading is present, in case of hitting

a rock while sliding on the ground.

5 METHODOLOGY

5.1 Simulation

Honeycomb stroke, landing loads and tip over stability are

investigated in a 2D motion simulation program. Two separate

models are created for 2-2 and 1-2-1 scenarios. In simulationenvironment, lander structure and the ground are rigid. Leg

angles are identical to the original design. Honeycomb is

modeled as a constant force spring. Mass, center of mass and

rotational inertia of the lander are defined. Parameters varied

are lander velocities, slope and friction coefficient.

Fig 10: Simulation model (2-2 configuration)

5.2 Scaled Lander

The need is to create a representative and a viable scaled

test platform because a full scale drop test requires extensive

use of resources. To achieve dynamic similarity to full scale

landing on the moon, a 1/6 scale model is designed according to

a set of scaling factors used in Apollo testing methods [10]. The

reason behind the match is the similarity of acceleration/gravity

ratios. Important parameters in design are leg geometry, total

mass, center of mass height and rotational inertia. Scaling

factors and parameters used are specified in Appendix A.

Legs and the body are made of aluminum with bronze

bushings and plastic footpads. A steel counterweight cylinder is

placed on top and large diameter holes are drilled to the body to

satisfy the desired mass property values within 20% error.

After the complete assembly, total mass is measured to be 4kg

which corresponds to 850kg in full scale. Other values are

evaluated in software and are not physically measured.

Fig 11: Scaled lander prototype

The inner diameter of the scaled main strut is equal to

12.7mm and the length is 35.6mm in which the crush material

must be inserted. For this purpose, samples from 360psi and

690psi honeycomb are prepared and crushed with an Instron

compression tester. 360psi samples crushed with an average

constant force of 325N and the 690psi samples with 650N, both

by 70% of their initial length before the force increased rapidly

Although the samples had small number of unit cells, they

crushed very consistently. Test plots are located in Appendix B.

Fig 12: Progressive crush of 360psi cartridge (3 unit cells)

Struts have venting holes to minimize force increase due to

air compression for the drop tests.

6 LANDING LOADS

A set of experiments are conducted in CMUs Motion

Capture Room, equipped with high speed infrared camerasdistributed around the room that can track coordinates of the

markers on the lander at 480fps with 0.1mm resolution.

A total number of 10 landing scenarios with differen

configurations, velocities and pitch angles are tested. A four bar

mechanism (Fig 4) with pitch adjustment is used with an

electromagnet as a quick release to drop the lander. Real pitch

angle values were different than predetermined values due to

the imperfection of the drop rig. Thus, after experiment results


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Fig 14: Scaled lander on JSC-1A (soft)

The most stroke intensive scenario is one leg landing on a

large rock. This case is tested by placing a rock on GRC-1 with

the maximum expected height, and dropping the scaled lander

in a way that one footpad lands on it. The leg that hit the rock

crushed 8mm and the second leg (diagonal) crushed 24mm

where the other pair of legs barely made contact with simulant.

The result is parallel with simulation.

Fig 15: Rock drop test on GRC-1

The reason why the second leg absorbs the most impact is

related to the ratio between honeycomb crush force and lander

rotational inertia. Honeycomb force is high enough to convert

the linear velocity of the lander into rotation with moderate

crush; the lander has a small period of time where it rotates

about the footpad, then the second leg makes contact with

ground, absorbing the remaining energy.

8 STABILITY

Tip over stability is first investigated by simulation. Surface

plot of marginal stability is created for a vertical landing with

zero pitch angle on a sloped surface for 2-2 orientation. Vertical

velocity (0-4m/s), slope (0-45deg) and friction coefficient (0-1)

are varied; data points are collected and plotted by linear

interpolation. Color represents the velocity. Shape of the plotted

surface is a tilted u-channel that is wider on top with increasing

velocity, meaning it becomes more susceptible to tipping.

Fig 16: Surface plot of marginal stability from simulation

Internal region of the surface towards the left side in stableand external region towards the right side is unstable. A vertica

landing at maximum velocity is always stable if the friction

coefficient is below 0.5 and in all cases when slope is less than

20deg. At a friction coefficient of 0.8, front legs stick to the

ground, and increasing the friction further practically makes no

difference as far as tip over stability is concerned.

Fig 17: Landing stability boundaries for planar vertica

landings on slopes, 2-2 spacecraft orientation

The cross section plot of the 3D surface illustrates the two

high velocity curves which have the same colors on both

figures. Orange represents 3m/s and red corresponds to 4m/s

Simulation results are continuous lines with cubic interpolation

and experiment results are shown with markers. To have

comparison with simulations, drop tests are performed on a stiff

plywood surface covered with carpet, and high friction is

achieved by using double sided tape on lander footpads. These

tests produced similar results to simulations.


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Fig 18: Marginally stable landing on a stiff surface

To get more realistic results for landing on the moon, drop

tests are conducted on a lunar simulant, JSC-1A. No lift-off is

observed on trailing leg pair on 20deg and 30deg slope

landings, and this indicated an equivalent friction coefficient of

0.5 in simulations.

Fig 19: Touchdown instant on JSC-1A 30deg slope

9 CONCLUSION

Landing characteristics are investigated in a 2D motion

simulation program. Results provided valuable insight on the

kinematics and dynamics of the problem. Simplicity of the

method is novel, minimizing runtime.

A scaled mass model of the lander is prototyped and drop

tests are conducted in a motion capture room with various

landing conditions to verify full scale simulation results.

Pitching motions, center of mass velocities and accelerations

were in remarkable agreement. The scaled prototype adequately

reproduces landing dynamics and it is suitable for detailed

studies.

GRC-1 tests provided preliminary insight about the energy

absorption characteristics on a soft surface. 50% of the energy

was absorbed by GRC-1 compared to landing on plywood. JSC

1A absorbed 30-70% of the energy depending on compactness.

Honeycomb stroke is tested by landing one leg on a rock as

this was the worst case. The first leg landed on the rock

absorbed 25% of the energy and the opposite leg absorbed the

remaining 75%. Two other legs did not make ground contact as

expected. The second leg crushed 80% of its total stroke.Tip over stability is examined with drop tests on slopes

with worst case velocities, first on stiff surfaces to relate to

simulations, then on lunar simulants to generate realistic results

A surface plot of marginal stability is created with simulation

Drop tests on stiff surfaces with sticky footpads produced

results that are similar. Then, JSC-1A is used to represen

bearing strength and friction properties of lunar soil with high

fidelity. Lander stably landed on a 30deg slope at maximum

velocities on the simulant. The trailing legs did not lift-off and

sliding was significant. From these observations, equivalent

friction coefficient is estimated to be 0.5. Tip over stability is

notable with the current design under the selected worst case

conditions.Future work includes fabricating and testing full scale legs

and full scale drop testing to conclude the study.

ACKNOWLEDGMENTS

The authors thank Red Whittaker, Uriel Eisen, Justin

Macey, Steve Huber, Kevin Peterson, William Pingitore, Jason

Hallack, Eric Benson and Kevin Fulton for their support.

REFERENCES

[1] A. Ball, J. Garry, R. Lorenz and V. Kerzhanovich, 2007

Planetary Landers and Entry Probes, Part I, Chap. 7.[2] NASA, Surveyor Program Results, pp. 141-163

[3] Bryan, C., Strasburger, W., Lunar Module Structures

Handout IM-5, NASA LSG 770-154-10, May 1969

[4] Buchwald, R., Witte, L., Schroder, S., Verification of

Landing System Touchdown Dynamics, IAC-11.A.3.1.3, 2011

[5] Rogers, W.F., Apollo Experience Report - Lunar Module

Landing Gear Subsystem, NASA TN D-6850, June 1972

[6] Astrobotic Technology, System Definition Review, 2010

[7] Plascore, 2012, Crushlite Lightweight Energy Absorption,

http://www.plascore.com/pdf/Plascore_CrushLite.pdf

[8] F. Sperling, J. Galba, A Treatise on Surveyor Landing

Dynamics and an Evaluation of Pertinent Telemetry DataReturned by Surveyor I, NASA N67-34177, August 1967

[9] Simulant Working Group, Status of Lunar Regolith

Simulants and Demand for Apollo Samples, December 2010

[10] Blanchard, U., Evaluation of a Full-Scale Lunar-Gravity

Simulator by Comparison of Landing-Impact Tests of a Full-

Scale and a1/6-Scale Model,NASA TN D-4474, June 1968


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APPENDIX A: SCALING, DIMENSIONS AND TESTS

Table 1: Scaling factors [2]

= Geometric model scale, = Gravitational ratio

Table 2: Lander parameters (1/6 scale model, corresponding full scale, real lander values)


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Table 3: Landing scenarios of motion captured drop tests (pitch angle is affected by drop rig)

MOCAP Results

Fig 20: Illustration of pitch angle plot for Landing 5

Impact 1

Impact 2

Lift-off

Impact 3


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Fig 21: Landing 1 (Flat, = 0.5, Vx = 0, Vy = 3.3m/s)


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Fig 22: Landing 2 (Flat, = 0.3, Vx = 0, Vy = 3.3m/s)


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Fig 23: Landing 3 (Configuration: 2-2, Pitch = 7.5deg, = 0.5, Vx = 0, Vy = 3.25m/s)


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Fig 24: Landing 4 (Configuration: 1-2-1, Pitch = 7.5deg, = 0.5, Vx = 0, Vy = 3.25m/s)


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Fig 25: Landing 5 (Configuration: 2-2, Pitch = 11deg, = 0.5, Vx = 0.4, Vy = 3.25m/s)


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APPENDIX B: HONEYCOMB BEHAVIOR

Fig 26: Representative aluminum honeycomb behavior [7]

Fig 27: Progressive crush of 360psi cartridge (3 unit cells)


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In-strut Crush Test Results

Initial length: 31.5mm, Force (N), Displacement (mm)

Fig 28: Instron compression test result of 360psi honeycomb sample


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Fig 29: Instron compression test result of 690psi honeycomb sample

stability and energy absorption of a lunar lander by 1/6 scale drop testing and full-scale...

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