Mon. May 7th, 2812
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