EC3230 - SPACE POWER AND RADIATION EFFECTS

IndividuaL Mini-Project

Objective

The requirements for this assignment were to design and select a power system for a spinstabilized satellite to meet the objectives of a 5 year mission life, end of life (EOL) power of

75 watts, mission design to accommodate Van Allen belt transitions, a circular, 18 , 888 NM I zero inclination and a satellite of cylinder shape with dimensions of 188 em length and 25 em radius.

Calculations

The calculations made utilized the tables and figures from the NASA Radiation Handbook, which are attached to this document. The first step in the process was to calculate the surface area of the cylindrical spacecraft in the following manner:

A = 2tcrl = 2tc(25cm)(l00 cm) = 15707.96cm 2 ~ II5708cm 2 1

The effective area is a function of the · product of the length and diameter, and was found to be: Aeff

= 2rl = dl = (50 cm )(1 OOcm) = lsoOOcm

2

1

Based on the altitude of 18,888 NM and an inclination of 8°, when using TabLes 6.6 and 6. 7, the sum of the equivalent 1 MeV fluence due both electrons and protons was calculated as (when using 6 mils of cover glass):

FLuence = 1.49 *10 14 + 2.67*1013 = 1.757*101 4 MeV/ year

FLuence/ year * 5 Year Mission Life = 8.785 *1014 TotaL FLuence (5-Year Mission)

Using Figure 3.103, the maximum power produced by each cm2 of Silicon and Gallium Arsenide was

13.5 mW/cm 2 and 16.3 mW/cm 2 , respectively.

Next, the power available at EOL due to the total fluence was calculated with respect to the effective surface area for both the Silicon and Gallium Arsenide cases. The Si option only provided approximately 66 watts at EOL, and thus could not be considered further for a design of this type. In the AlGaAs case, there would be 81.5 watts of power available at the end of 5 years; as such , this remained a viable option. The RTG option even with a 38% power reduction due to fuel exhaustion, would have 84 watts of power available at end of life and had to be compared to the AlGaAs cells.

In order to down-select between the Gallium Arsenide cells and the RTG power subsystem, cost factor metrics had to be computed. The total available area of the cylinder was used to compute cost with the 4cm 2 AlGaAs cell size in the following manner:

157081m

2

cm 2

4

cell

=l3, 927cellsl

With a cost of $388 per AlGaAs cell, a total cost of $1,178,188 was necessary for this option .

Conclusions

Due to the Silicon cell not being a non-available option - the only remaining choices were the

AlGaAs cell type and the RTG power subsystem. Since both meet the EOL power objective, cost became the deciding factor in choice. Going with the AlGaAs solar cell system saved approximately

$621 , 988 over the RTG power subsystem... As such, Gallium Arsenide solar cells ar e the obvious choice. Table 6.7.

Annual Equivalent 1 MeV Electron Fluence from Trapped Protons {Voc ' Pmax),

(Infinite Backshielding)

0° Inclination

PROTONS - VOC AND PMAX

EQUIV. 1 I'£V ELECTRON FLUENCE FOR VOC AND PfoW< CIRCULAR ORBIT

DUE TO GEOHAGNETICALLY TRAPPED PROTONS, HODEL APSHAX

ALTITUDE

0'\

I

N

0

8

(8)

2.64E-3

7.64E-3

168

268

388

468

688

888

1888

1268

1688

1768

2888

2268

2688

2768

3888

3688

4888

4688

6888

6688

6888

7998

8889

9888

18988

11998

12898

13898

14998

16888

16999

17999

18888

19327

277

463

666

833

8.88

8.88

&.98

2.87+12

3.88+13

3.27+14

1.48+16

6.61+16

2.11+16

6.17+16

1.89+17

2.36+17

4.26+17

7.89+17

1.88+18

2.61+18

4.98+18

8.48+18

1.28+19

1.79+19

2.26+19

2.83+19

2.71+19

1.66+19

1.81+19

6.48+18

4.67+18

3.29+18

2.22+18

1.68+18

1.21+18

9.48+17

6.86+17

2.66+17

8.89

8.89

9.89

2.26+12

3.39+13

2.88+14

1.27+16

6.48+16

1.76+16

4.22+16

8.62+16

1.71+17

2.83+17

4.13+17

6.41+17

8.69+17

1.16+18

1.34+18

1.46+18

1.49+18

1.37+18

8.41+17

3.72+17

1.81+17

2.37+16

4.61+16…