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Rutgers Symposium on Lunar Settlements, June 4-8, 2007

A Solar Electric Propulsion Mission with Lunar Power Beaming

Henry W. Brandhorst, Jr., Julie A. Rodiek and Michael S. Crumpler

Space Research Institute, Auburn University

Mark J. O’Neill

ENTECH, Inc.

June 4, 2007

Space Research Institute

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Rutgers Symposium on Lunar Settlements, June 4-8, 2007

Outline

Introduction

Getting to the moon

Lunar Orbit» Equatorial Orbit

Orbital analysis Power delivery

» Polar Orbit Orbital analysis Power delivery

Summary and Conclusions

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Rutgers Symposium on Lunar Settlements, June 4-8, 2007

Rationale

NASA’s Vision for Space Exploration to return to the moon» Lunar south pole desired location

Up to 70% sunlight per year

» Other locations being driven by science Equatorial and high latitude locations

Energy storage for the lunar night is massive» Solar array/RFC system – 20 kWe

» Weight ~6 MT with cryogenic storage Can power beaming reduce the mass of night time storage?

» Orbital options to provide power to locations within ±45º of the equator» Molniya-type orbits for polar -90 to 45º S (or +90 to 45º N)» Two year orbital analysis

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Rutgers Symposium on Lunar Settlements, June 4-8, 2007

Getting to the Moon

Solar electric propulsion mission» ~4343 kg spacecraft

1400 kg Xe propellant Hall thrusters – 10 to 50 kW (3)

» 100 kW Stretched Lens Solar Array 300 W/kg, 300 W/m2, TJ cells

No detailed design of spacecraft

Spiral out through Van Allen belts» 500 km initial orbit, 28º inclination

89 day trip time, plane change at moon

Compared to 272 day trip time from previous SEP space tug analysis

» Radiation dose calculated with SPENVIS

Provides solar array degradation

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Coverglass Thickness (mils)

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SEP Laser Mission

600 kW Tug Mission

Selected shielding

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Rutgers Symposium on Lunar Settlements, June 4-8, 2007

Lunar Orbit – Single Spacecraft

Wide range of elliptical equatorial orbits examined

» Chose 500 x 30,000 km orbit

Ran STK 7.1 for arbitrary 2-year period

» July 1, 2008 to June 30, 2010

Determined when surface sites between ±45º were in view of the satellite

» AND the satellite was in sunlight

Times without satellite coverage were up to 164 hours (6.83 days)

» Offers no major reduction in energy storage mass

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Rutgers Symposium on Lunar Settlements, June 4-8, 2007

Lunar Orbit – Two Spacecraft(500 x 30,000 km equatorial orbits)

2nd satellite access times with orbital location

Equatorial satellite orbits about the moon with beaming

Satellite 2 Base Access Time vs. Satellite 1 Argument of Perigee Delta

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Rutgers Symposium on Lunar Settlements, June 4-8, 2007

Dual Satellite View Times

Adding second satellite had a major impact on view times

» Adjusting orbital relationship between the satellites boosted view times

Satellite 2: 3870 hrs access» Power beaming times increased

significantly: Only 8 periods of 84 hrs (3.5

days) with no access Rest of the time it’s lower than 54

hrs (2.25 days)

Substantially reduces the mass of surface storage system

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Rutgers Symposium on Lunar Settlements, June 4-8, 2007

Overlapping Coverage

Because the satellites are in equatorial orbits, their view times often overlap

Provides an opportunity for substantial power increases

» Beaming laser power to a planar GaAs (1-J) photovoltaic array on surface

» Monochromatic laser beam cannot excite multiple junction solar cells

Can’t use tracking concentrator array to simultaneously view two satellites

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Rutgers Symposium on Lunar Settlements, June 4-8, 2007

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Laser Power Beaming

Uses 850 nm diode pumped laser, 4 m2 area (2.26 m dia) beaming aperture

» Aperture controls surface beam size» ~90 kW satellite power available

Laser beam incidence angles determined by satellite orbit

» Surface array may track in the E-W axis when in sunlight

Laser intensity varies due to view angles and orbital elevation

» Satellite near moon when beaming starts – high intensity

» At 30,000 km, beam intensity drops to ~0.2 AM0 sunlight

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Rutgers Symposium on Lunar Settlements, June 4-8, 2007

Surface Solar Array/Laser Beam

Assuming 1-J GaAs cell surface solar array

» Nominal 40 kW surface power» ~18% efficient GaAs cells» Temperature corrected

Size of laser beam on surface GaAs solar array determined

» Less than total array area for maximum power delivery

» Largest beam size is at 30,000 km distance

About 60% of GaAs surface solar array is covered

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Rutgers Symposium on Lunar Settlements, June 4-8, 2007

Lunar Power Produced by Laser

Chose 45º N site for calculation» Most difficult case

Calculated laser beam power from satellite

» 50% conversion of orbital electricity into laser beam

» Plus 12% mirror losses Calculated laser power received

by surface GaAs solar array» 45% conversion efficiency

18 kW power delivered to site» Adequate for night time power

needs» Storage for 64 hrs maximum

However, NASA’s interest is in a south polar location, so…

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Rutgers Symposium on Lunar Settlements, June 4-8, 2007

Polar Power Beaming Satellites

Two satellites in polar elliptical orbit

» Offset by ~180º» 500 x 5,000 km orbit» ~7.5 hr orbital time» Apogee over the south pole

850 nm laser beam» 1.5 m2 aperture (1.38 m dia)

Increases beam size on surface vs previous case

Uses 1-J GaAs tracking array on surface

» Can track only one satellite» Or can use fixed array

Reduces surface power

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Rutgers Symposium on Lunar Settlements, June 4-8, 2007

Satellite Parameters – 8/23-24/08(500 x 5,000 km Polar Orbit)

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Rutgers Symposium on Lunar Settlements, June 4-8, 2007

Access Times for Polar Orbits

Polar orbits give excellent access times

» From the pole to ~30º

» 5,000 km apogee has least time Requires the second satellite Both satellite access times are

comparable Access time depends on satellite

altitude» Higher provides more access

Longer beam distance reduces power received

» Second satellite can provide more power

If it can also be tracked Or use planar array

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Rutgers Symposium on Lunar Settlements, June 4-8, 2007

Power Delivered to Surface

With a tracking array, power to the surface is essentially constant

» ~16.8 kW per satellite» 50% power conversion to laser beam» 45% conversion of laser into power

Includes other losses as well

» Assumes a 15 kW surface array (in sunlight, 62 m2 in area)

Neither receiving array area nor laser beam intensity is excessive

Can also adjust beaming parameters

With two satellites, the longest time a receiver at 45º does not receive power is:

» Only 1.5 hours maximum, less for a polar site» Substantially reduces storage!

Beaming is a very plausible option!

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Rutgers Symposium on Lunar Settlements, June 4-8, 2007

Summary

Two cases of lunar power beaming were studied» Equatorial orbit, ±45º N-S, two satellites, 500 x 30,000 km (2 year)

850 nm laser, 4 m2 beaming aperture Delivers up to 18 kW with two satellites to GaAs surface array

› Partial tracking

Eight times with storage times of 84 hrs, rest of time <54 hrs

» Polar orbit, -90 to 45º S two satellites, 500 x 5,000 km (same for N) 850 nm laser, 1.5 m2 beaming aperture Delivers 16.8 kW with either satellites Maximum dark time of only 1.5 hrs

› Insignificant storage time

Laser power beaming to lunar surface seems feasible» Multiple orbits are possible» Substantial reduction in energy storage times for any location» Can yield significant mass savings for exploration architecture