1. Curved Extendable/Retractable Boom-Deployed Bag Asteroid Capture System Concept
Scott Belbin, Mechanical Systems Branch
NASA Langley Research Center
(757)

2. RFI Concept Criteria
Capture an asteroid of unknown composition (possible dust/rubble pile) – Encapsulation bag required
Atlas V Medium Payload Fairing
Asteroid Mass ≤1000 metric tons
Accommodate different asteroid shapes
17.8m x 8.8m x 8.8m prolate spheroid
14m x 14m x7m oblate spheroid

3. Design Drivers Short development time: Launch in 2018
Need High TRL approach with minimal complexity
Deploy, close and retract bag without robot arms
Electrically driven system only to reduce mass (abundant S/C bus capacity)
Need to be able to reliably model
Rigid structures are readily modeled and simulated
Rigid structures to enhance dynamic stability while spinning up to match asteroid
Extendables are predictable and functionally reliable
Need to be able to test
Test in 1G
Vacuum chamber not required for testing (no inflatables), only still air needed
Need to provide crew access
Non-inflatable capture bag – single ply easy for crew to cut through
Non-inflatables allow access panels to be easily incorporated

4. Concept Overview
Proposed system uses extendable booms to deploy a non-inflatable bag to capture and control an asteroid
Cinched up and retracted,
Booms kink and conform to shape of the asteroid, ready to de-spin asteroid
Stowed
Deploy bag then match spin for capture

5. Deployment Storyboard
1) Booms in initial position
2) Bag ready to deploy
3) Booms deploy, pulling bag
4) 1st section taut, pulls next
5) 2nd section taut, pulls next
6) Curved booms minimizes material
8) Booms stop at their limits
7) 3rd section taut, pulls last
9) System ready for spin-up

6. Single drive in stowed position
ConOps
Bag mounted winches drive cinching cables in batten pockets
Booms deploy single-ply Vectran bag
Circumferential cinch cables close bag, pulling in the booms which kink and conform to the asteroid’s shape
Booms retract to draw asteroid against spacecraft
Independent boom drives allow for CG adjustment of asteroid
15 meters
Add cinch motors
Add multiple pics of sequences
500 mm
20 meter booms
Single drive in stowed position

7. Preliminary Sizing Results
Lenticular cross section chosen due to favorable buckling characteristics wrt conforming to the asteroid’s shape
Conducted trades for materials and minimum cross sections
Results:
Titanium: 325mm width, 390mm flattened
MS = 0.059
1756 g/m
36.00 kg per boom
Composite: 250 mm width, 280mm flattened
MS = 0.012
464 g/m
9.51 kg per boom
Composite booms selected for mass estimate

8. Current Best Estimate of Mass
Used Vectran areal density as reference for bag mass calcs
Incorporated catalog selections for space-rated drive motors and gearing
Based on preliminary Pro-E models; included large margins of uncertainty typical of concept level design
Total Boom System Mass
124.87 kg
Enclosure System Mass
69.42
Structure Mass
27.00
Attachments and Cabling
10.00
Capture System Avionics
20.00
Micro Meteorite and Thermal Shielding
30.00
CBE
281.29
Contingency Percentage
30%
Contingency Mass
84.39
CBE + Contingency
365.68

9. Cursory Schedule in RFI Submittal
Timeline: 10/ /2018
Concurrent design and development testing
Includes boom tooling design and procurement
Incorporates Ground Test Article build and test

10. Potential Next Steps Boom cross section design and analysis
Boom section property demonstration (bending, torsion, buckling)
Boom to drive buckling limit demonstration
Bag material demonstration for abrasion and puncture resistance
Cinch cable drive design and testing
Boom tooling design and semi-section bonding demonstration
Boom extension and retraction demonstration

11. Conclusion Dynamically stable in spin-up Medium to High TRLs
Reliable mechanical drives
Leverages abundant electrical power to reduce mass; no gas system
Can demonstrate in 1G environment
CBE Mass meets LV criteria
Improved Crew Access
Scott Belbin, Mechanical Systems Branch
NASA Langley Research Center, Hampton VA
(757)

12. Design Details
Backup

13. Boom Type Trade 1: STEM Boom
Storable Tubular Extendable Member
Extensive flight heritage; TRL 9
Formable as a curved boom
Exerts force during extension for capture bag unfurl
Exerts retraction force to bring asteroid to bear against spacecraft
Drives mount on torsion spring bases to accommodate flexure during capture
Good Bending and torsional capability
Poor Buckling/Crippling characteristics wrt conforming to asteroid; possible reduction in tensile capability; no recovery from buckling/crippling for retraction into spool
Variations of the STEMboom concept [Source: NASA]
Tubular shape collapses and spools without permanent deformation. Section shape is restored when spooled out
STEM TIP Drum antenna boom, self deploying [Source: Northrop Grumman]

14. Boom Type Trade 2: Lenticular Section
TRL 5 – tested in zero-G
Formable as a curved boom (two halves construction, continuous length members)
Exerts force during extension for capture bag unfurl
Exerts retraction force to bring asteroid to bear against spacecraft
Drives mount on torsion spring bases to accommodate flexure during capture
Good Bending and torsional capability
Good Buckling/Crippling characteristics wrt conforming to asteroid; no appreciable tensile capability reduction; recoverable from buckling/crippling for re-spool
Lenticular shape readily collapses and spools without permanent deformation. Section shape restores when spooled out

15. Preliminary Lenticular Boom Sizing
Examined load case from de-spin of asteroid
Used loads for JPL2 case (worst case)
7500N tangential de-spin load (6 booms, 1250N per boom)
Assumed contact at half-diameter (10 meters) but more likely ~6 meters (at second cinch cable); conservative
Analyzed for simple cantilever load case in strong axis
Safety Factors used: 1.4 on ultimate and 1.25 on yield
Three materials analyzed: AL 6061 T6, Ti-Al6-4V, Carbon Composite (T500 12k/976)
Geometry optimized until MS=>0.0 to determine minimum section size
Results: Aluminum min. section too large to fit volumetric constraints; Titanium and Composite are viable candidates FT

16. Preliminary Lenticular Boom Sizing, Min. Cross Section
Examined for lateral loads in strong axis direction
Safety Factors applied
Beam section properties from Pro-E
Composite properties from Mil-Hbk-17
Case shown here for T1-6Al-4V, 325mm section width
Design Data
SF, ultimate
1.40
SF, yield
1.25
Force at half Ø contact pts, total
7500 N
No. of Booms 6
Beam Geometry
Beam Length
20.50 m
Beam Length to half Ø contact pts. 10
End Force, Tangential
1250
Neutral axis dist., strong axis, c
0.1625
MOI, strong axis
3.18E+06
mm^4
Beam Calcs
Max Moment, M
12500 Nm
Max Stress, δ
639.55
MPa
Ti-6Al-4V, .020" thk
Young's Mod, E
113.8
GPa
Ftu
951.48
Mpa
Fty
882.53
MS, Ftu
0.059
MS, Fty
0.094
Beam mass/length
1.756
kg/m
Beam mass (1)
36.00 kg
Max Displacement, w
1153 mm
T500 12k/976 unidirectional tape, Mil 17, V2, Pg "thk layup
Tensile Mod, E
141.34
0.495
0.464
9.51
928
Flattened Width =390mm
I= mm^4
Solving for minimum composite cross section = 250mm width
Flattened Width =280mm
I= mm^4

17. Packaging Atlas V Medium Fairing Payload Volume
Conical Instrumentation Volume provides unobstructed forward view
Capture Bag Stow Volume
Boom Mechanism
Volume
Volume trades allowed as design matures

18. Design Details
Cinch Cables
Bag made of six 4-panel gores with Velcro deployment staging (can deploy well in advance of asteroid encounter)
Bag-mounted winches driving cinch cables in batten pockets close the bag in sequence around the asteroid, pulling the booms inward
Booms buckle/cripple by design as they contact the asteroid and conform to asteroid’s shape without losing tensile capability
Booms make a rigid connection for de-spin
Booms retract to draw asteroid against spacecraft
Independent boom drives allow for CG adjustment of asteroid

19. Internal View Bag Dimensions Instrumentation Volume Bag Volume
15 meters across flats at mouth, 18 meters across apices
20 meter booms
Thin Vectran construction w/reinforcement doubling in critical areas
Instrumentation Volume
Unobstructed view for asteroid characterization and bag deployment
Provides rigid coupling to captured asteroid while protecting boom drives
Bag Volume
Facilitates bag packaging and unfurling by mimicking bag shape
Boom Drive Volume
Allows for boom drive flexure motion
Provides volume for electrical harnesses and system to bus attachments

20. Boom Drive and Bag Attachments
Bag Extraction Attach Point attaches to mouth of bag
Cable Rings attach to bag via straps
Boom unfurls first segment
Subsequent segments Velcro retained
Booms slide through rings; bag segments become taut then pulls subsequent segments
Drive with Torsion Spring Base
Decreases risk of local boom buckling at boom exit point
Internal guides and compliant members further decrease local crippling risk