Ed GaddyMarch 21, 2011

The Real Value of Spacecraft Solar Array

Technology and Slow and Steady Does Good

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un-solicited proposal for replacement of spacecraft

solar arrays

History repeats (maybe)

This presentation will review the now settled question of whether to switch from silicon to GaAs based solar cells.

This highlights a successful strategy for solar array development and implies that a similar strategy should be successful for the future.

The numbers I will use in this presentation are about 15 years old. I believe the point of the presentation should still be valid.

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To switch from silicon to GaAs?

The cost to switch from 15% silicon solar cells to 18% GaAs cells caused some discussion.

GaAs cells were:

More expensive

Less robust mechanically

Without heritage

Presented the opportunity of future improvement.

In those days, I wrote a series of papers discussing, what I believed, were the substantial cost benefits of GaAs.

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Start with:

Silicon: $1.25/cm2 14.8%

GaAs: $20.83/cm2 18.5%

MJ: $26.04/cm2 24.0%

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What’s it worth to save a kg on the array?

Launch : $11,000/kg

Payload: $600,000/kg

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Payload expenses

Launch

Spacecraft

Operations

The payload itself

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Well, not quite $600,000

The computation is more complex than save $600,000 when a kg is taken out of the array

If a 1 kg on the array is saved, not the entire 1 kg can go into the payload

To support the additional payload size, it is necessary to increase the capability of the some of the spacecraft “overhead” –such as C & DH, Communication, Electrical, Battery and PSE

The benefit after making these corrections is: At $250,000/kg, the MJ cells are without question a bargain.

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$250,000 or $11,000 per kg?

$250,000 per kilogram made the MJ cells a bargain at 20 time the cost of the silicon cells.

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Encore on what a kg is worth

Is the value of weight saved being accurately valued?

$250,000 per kg versus $11,000 per kg?

First recommendation is to re-study the value of a kg saved. If the higher value is near correct, the implication is that advanced technology is being severely under appreciated.

Of course, this is true for every technology used on the spacecraft - - - not just solar arrays.

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What the investment in MJ did.

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The investment from 1980 through 2010 was critical for the development of this game changing and enabling technology improvement.

Note that the improvement is gradual, Since 1980, only 2.6% absolute per year.

Would not have happened without government investment.

The investment in the MJ technology worked a wonder. The investment worked a wonder.

The roadmap says it isn’t going to happen anymore.

I want the US to be a great competitor.

I want to have NASA have access to the most advanced cells in the United States.

I recommend that even though most of the funding for this MJ development is done through other agencies, that NASA not drop out of this effort that has proven so successful.

Gradual steady improvement has been very fruitful. NASA should continue support it.

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The investment in the MJ technology worked a wonder.

The next step is the IMM solar cell. Again the roadmap is saying no.

Second recommendation: Support the development of the next generation technology, the IMM cell.

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Array Technology

Array technology is available that produces 150 W/ kg.

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It’s not being used so much by Goddard (hence NASA) and its subcontractors.They possibly perceive it as high riskThey possibly do not value it accurately

So a re-emphasis on the first part of the presentation, which is to determine how much a kg is worth.

Third recommendation: provide knowledge or incentive to projects to use weight saving techniques that are at a high TRL level but that are not commonly used.

Summary

Formally evaluate what a kg is worth.

Invest in slow, and steady, which is presently the IMM cell.

Implement incentives to utilize existing technology that is effective.

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