Space Solar PowerCritical Mechanical Engineering Technologies

Michael A. Brown, PEUS Naval Research Laboratory (Retired)

Space Solar Power Workshop 2013Baltimore, Maryland

NASA’s 1979 “Reference System Concept”

Rectifying antenna

10km x 13km@ 35 deg. Latitude

1 km dia.

10km

5km50 km2 sun-tracking Photovoltaic array

500m thick support structure for a large flat area

DC to Microwave conversion, antenna

+ Astronaut assembly =$$$$

Microwave antenna

This eliminates slip-rings,and greatly simplifies PMADby grouping the photovoltaicand the microwave systemsin the module. Concentrationreduces the area of(expensive) solar cellsrequired. The problem nowis thermal, illustrated in thenext slide.

“Sandwich” power conversion module

PV Cells

NASA’s “Fresh Look study 1995 – 1999#1: Modular Symmetrical Sandwich Microwave SPS

Off-axis parabolic reflectors

Nadir: Rectifying antenna

Secondary reflectors

Azimuth axis

1,000m

Elevation axis

1,500m

Reduced power cabling

40m?

112C microwave panel

Microwave devices @60% efficiency

3 Suns: 4,140 W/m2

PV panel @ 173.3C, 19.2%,efficiency, 715W/m^2

Heat flow

Solar cells: 29.5% efficiency at 28C

Modular symmetrical sandwich thermal problem

The arrangement of PV andmicrowave support panels providestoo small an exterior area to radiateaway waste heat combined from thetwo systems. The microwave panelmust be below 60C for the devicesmounted on it to function.

At 100% microwave efficiency, PV panel is at 167C, microwave panel at 92C

This is a hybrid design that attempts to solve the thermal problem byseparating PV and microwave functions. This is regarded as a moreexpensive option if the thermal problem of the sandwich module cannot besolved. Problem: It appears this arrangement requires an enormous amountof power cabling as did the 1979 design, and there is some heat exchangedbetween the microwave and PV arrays, and between the two PV arraysthemselves.

NASA’s “Fresh Look Study” 1995-1999#2 “Integrated Symmetrical Concentrator SPS”

Microwave conversion and antenna

1,000m

5,000mPhotovoltaic panel

So my purpose here is to show how this mighty engine can be built for maximum

operating efficiency and with minimum mass and stowage volume.

2. NRL Solution to thermal and power cabling problems

Replace flat sandwich disc in the Modular Sandwich Microwave SPS with an open-topped, hollow cone to increase external thermal radiation area “Step

Module”

The wall is composed of horizontal panels supporting PV and microwave systems (thermally isolated) , and vertical reflective surfaces. (1)This design separates the PV and microwave radiators so neither sees the other. (2)Power cabling is negligible. (3)The cone is a stiffer structure than the flat sandwich.

Step Module Design: Wall section

Thin film reflectors

PV PanelMicrowave system

1,000m

Step Module

Sunlight

Waste heatPanels stacked

Strut

With a 0.58m2

microwave panel, temperature is 60C for approx 60% efficiency

Step Module Thermal Analysis

506W/m2

Microwave Power

ReflectorsPV panel

3 suns: 4,140W/m 2Radiant heat

Step Module

1 m2 PV panel @ 129C vs. 173C, 22.6% efficiency, 843W output

Y

X

1000m dia.

500m

1,105m

1,500m

+24.4

-24.4

2,018m

2,326m

1,358m dia. sunlight

750m2,659m

Step Power conversion module

Off-axis parabolic reflector array

Reflectivity: 95%

Reflector array fill: 90%

Concentration ratio: 3:1

Secondary reflectors

Ellipse minor axis

Ellipse major axis

3. Building the step module space solar power system

Reflectorsupport boom

StructuresVery large structures are usually dependent on a simple and (relatively) inexpensive structural element that can

take compressive and bending loads. But spacecraft, as usual, need an “ultra-light” element.

Spacecraft boom designs NASA TM78687

Isogrid tube

Solid-rod longeron truss

Tube longeron

truss

Column load

Mas

s/L5

/3

To be ultra-light, axial members must be continuous (no hinges). Then for any design, mass is minimized using stiffest (E)graphite composite. But brittleness increases with stiffness—so, how to stow this structure?

D

I = D2ΣA /8

D

Ds

Deployed Skewing Flat

Stowed Rolled-Up

NRL “Superstring” Truss Innovation #1: Roll-Up External Stowage

Top Longeron

Bottom Longeron

Side Battens, Diagonals

Schematic of Cross-section of StowedTruss: Top Longerons are Slightly CloserTogether than Bottom Longerrons

Superstring stows by skewing flat so all longerons are in the same plane. It can then wrap around the outside of a drum or the spacecraft without strain betweenlongerons.

Advantages: (1) Large stowed diameter Ds allows use of very high modulus E (but low allowable strain) graphite rod longerons and (2) deployed diameter D is unrestrained. Result: higher bending siffness metric* and optimized column design for any given length and load.

Advantages: (1) Enables precise, controlled deployment for beams of extreme lengths and (2) coupled with external stowage, beams may be stowed and deployed integral with power cables, dynamic control devices and mission hardware all along their lengths. (3) Fixed nodes increase boom strength

Innovation #2: Mechanical Deployment

Superstring is deployed by a single-axis shuttle mechanism that pulls the truss off the stowed roll and through a frame where the node junctions of longerons, battens and diagonals are snapped together

Deployment Demonstration at University of Maryland

Innovation #3: Fixed Nodes, Increased HierarchyIncreasing hierarchy means increasing the moment of inertia of a beam’s longerons*. If the mass of the longerons and the beam’s diameter do not change, the global buckling strength remains the same but the local buckling strength increases. This is of great importance to beams of extreme L/D ratio, as it has been shown** that waviness, either local or global, significantly reduces axial strength. Fixed nodes enable the battens to resist longeron deformation.

*“Some Performance Trends in Hierarchical Truss Structures,” Murphy, T, and Hinkle, J. AIAA-2003-1903

**Strength of Initially Wavy Lattice Columns,” Crawford, R., and Benton, M. AIAA Journal, Article 79-0753R

Superstring increases hierarchy by replacing rod longerons with thin graphite fiber ribbons that on heating at launch reform into a U-shape to increase moment of inertia an order of magnitude. The material is “Shape memory Composite.” The (thin) ribbon design allows use of ultra-high modulus graphite and reduces beam’s stowage volume

Ribbon Longeron

Side Batten and Diagonal

Beam Stowed

Beam Deployed

Longeron Reformed

Latch: Pushnut on Threaded Shaft

Nodes are rigid after deployment

Properties of 1,500 meter reflector support4-longeron truss boom

E = 60msi (4.18E11 N/m2, free-free)

Compressive load P’ = 100N P’ = 1,000N

Longeron diameter 0.0046m (0.183”) 0.01m (0.394”)

Boom diameter 2.52m (99”) 3.7m (146”)

EI(Nm2) 2.28E7 2.28E8

Kg/m 0.178 0.814

Total mass (kg) 268 1,222Stowed outer diameter (m)

Stowed inner diameter

3.94 6.22*

1.46 3.14*

*Much smaller if ribbon longeronsused

Deployable #2: The isogrid tube beam

A problem with the truss beam is cost, in particular because of the machined parts and hand assembly required. The isogridtube would be significantly lighter, but the one NASA attempt to develop a deployable beam was not successful.* NRL has now developed an isogrid beam concept that could possibly become the “Inexpensive I Beam” of large space structures.

*Allred, et.al., “UV rigidizable carbon-reinforced isogrid inflatable booms.” AIAA 2002-1202.

The Isogrid TubeM #axial ribs

h

60o

A A

b

b

Section A-A

Fig. 1 Isogrid Geometry

“Open” isogrid means no skin between ribs

R

The isogrid tube is composed of three sets of ribs: one set axial, the other two at opposite angles to the first. If that angle is 60 degrees, the structure will have isotropic mechanical properties. There are three independent dimensions: tube radius R, rib dimension b,* and the number (must be even) of axial ribs M.

*This approach is from NASA TM-78687, cited above.

Comparison of isogrid and rod trussP’

L

r

Optimization of an isogrid column for minimum massrequires three equal failure modes to go with the threecritical dimensions: Euler buckling, local buckling ofaxial ribs, and wall crippling. This produces an unusualdesign, compared with trusses (truss batten radius is”r”):

For L = 100m, P’ = 10N and E = 60msi

D #M r,b g/m4-longeron truss 0.36m 4 0.87mm 9.63Isogrid tube 0.50m 86 0.093mm 3.83

Check out the numbers in bold red: they are the key to SSP’s success!

Axial ribs

Diagonal ribsFeed

Rib spools

Edge treatment

Fabric stowed in roll

Edges joined

The isogrid beam starts as an isogrid “fabric” made in a machine process like any other fabric, then rolled up for shipping and stowage in the launch vehicle.

At deployment the fabric edges are curled crosswise and joined to make a tube. The tube is then cut to required length in space. Because of the fabric’s small thickness (the “b” dimension), Material to make 10,000m of the tube in previous slide can be in a single roll 1m in diameter x 1.6m long (38kg).

Isigrid Fabrication & Deployment

A tube assembly method for an isogrid fabricThe preferred approach is to use square ribs with notches machined to allow the ribs to interlock with epoxy bonds. The intersection of the three ribs directions is offset slightly to allow the isogrid fabric to have a minimum thickness “b” of the rib..

Junction of two ribs, bonded

Axial rib

0.45mm

(0.018”)

13cm

Machined rib

3mm

0.51mm dia. rods

74mm

Alternative assembly method

This approach uses round ribs, joined with nodes. This could be made with an automated process, but would have a larger mass and greater thickness than the design using notched ribs.

Joining edges of isogrid fabric to make a tube

Edge of fabric

Shape-memory graphite tab

Tab locks around axial rib of opposite edge.

Overlapped edges

Axial direction

2.Ares V: fairing ID 8.8m x 9.7m,12m, 18.7m high + 6.2m, 7.5m high cone x 4.44m diameter; 183,000kg to LEO. Making an Isogrid Beam

To be useful, the isogrid tube must be formed into a beam that can be integrated into a structural system. The beam has three elements: an isogridtube, end caps and “joiners.” End caps are thin-walled graphite cones—these are stackable for compact stowage for launch.

Isogrid tube

End cap Joiner

Isogrid beam

Locked

Leading edge of tube trimmed

Shape-memory composite rod

Rod reshapedEnd cap

A A

Section A - A

Assembling the beamWhen the isogrid tube isplaced into the end cap, shape-memory graphite composite rods located within the cap are heated. This causes them to rotate, from the position in the center sketch of section A-A, and curl around the tube’s leading edge diagonal ribs.

Joining

Section D - D

JoinerClamp open

Clamp Closed

Joining beams into a structural system

D

D

“T” Junction Grooves in T junction crossbar

Main beams are joined by locking the “joiner” ends of the isogrid beams into” junctions.” The joiner grooves fit into grooves in the junctions,

E

E

Section E - E rotated

Axial boom

Saddle

StrapsTransverse beam

Junction

Junction of “axial” beam to a transverse beam

The problem here is how to join the end of one beam to the isogrid tube portion of another, considering the very thin ribs in the structure. The design here is to strap a “saddle” with an integral junction to the tube with Kevlar straps and Velcro binding. The underside of the saddle would have a number of mall protrusions that would lock into the wall ribs of the tube it is mounted on.

496m

1,000m

700m

1 of 16 flat segments

Transverse booms 25m to 50m long

200m

C

C

Hinge

Reflector film

PV panel

Strut

10m ?

View C-C

Transverse Boom, strap

Axial boom

8.5m

End of a panel stack attach to transverse beams

SSP structural conceptMicrowave

4. Step module Worksite transport

This is concerned with moving construction material and equipment from a docking area to the point where it is integrated in the SSP structure.

The isogrid beam has a wall of very fine graphite composite ribs, and this will require a novel means of holding it and moving it around the SSP worksite. The “handler” is a clamshell device containing a number of “brushes,” each controlled by a two axis drive. With this the handler can move the isogrid beam in an axial and/or a rotation about its axis. A handler is part of the isogrid tube deployment machine.

Handler2-axis brush driveView through handler

Isogridbeam

Isogrid tube

Brush drives

Handling the isogrid beam

. Mobile Work Platform

Platform Hinge

Work platform

Clamshell hinge

PV array

Independent rotation axes

Isogrid tube

Brush drives

The mobile platform consists of two isogrid tube handlers joined by a hinge structure that includes power and control systems, and a work platform to mount robotic tools* or another handler for moving tubes around the assembled structures.

*Reference: NRL’s FREND space robotics program

Mobile platform mobility

Clamshells open

The hinged structure is able to move about the structural framework

Material transfer: dock to worksite via hoist

Docking depot at module base

Worksite

1,000m

Cable drivencart

3 to 4 m

Cross-section

Two or more hoists per assembly

MOBILE WORK PLATFORM Attachment

Hoist moves on Deployable tube longron trussIsogrid fabric roll

A

A

Section A-A

3 to 4 meters

The tube longeron truss provides strength against bending loads, and the “b” dimension of the axial ribs can be increased to support the cart’s wheels

NRL solid-rod longeron truss

3. Building the reflector

Cart

Hoist

Reflector deployed position

To construct the reflectors, the supporting boom is rotated down toward the module. Motion by the boom and the hoist cart, and rotations of the hub/reflector joint, allow robotic arms on the cart to assemble the reflector structure.

Hoist feed

Reflector assembly position

1,500m Reflector support boom

Secondary reflectors

Hub

133m Reflector

78m Boom

Force

2E-6m thick membrane at 1psi

Compression in isogrid rim = 133m x 2E-6m thick membrane x6894N/m2/2 = 0.92N. For a factor of safety = 3:Boom dimensions: diameter = 13.7” (0.35m), mass = 1.43kg

Stress = 1psi (6894N/m2)

Isogrid tubes in reflector system hexagonal rims

Rim joint

Isogrid tube

Reflector membrane attachment point

Isogrid rim joints

Tube rims

Isogrid fabric strip 78m long: one of six per rim

Links

Deployment direction

Stowed Deployed

Flange formedFlange, stowed configuration

Joint details

Isogrid tube

Joint flangeRim

Velco joint

Joining

NRL Reflector Design

The reflective membrane is CP1 polyimide,invented by NASA/Goddard and marketed by NeXolve Corp. of Huntsville, Alabama. Typical properties are 2 micron thickness, 3 grams per square meter, reflectivity to 97%. Glass transition temperature is 263C, enabling it to be used to approximately 5:1 concentration ratio.

NASA 20m sail

NRL concentrator reflectorNRL’s compliant border gives perfectly flat surface

Power out

High Currentflow

The problem of Gaussian distribution

lens1,000m

Boom

Power @ radius