Self-Assessment Quiz: Power Systems

Twenty questions on solar power, batteries, RTGs, fission, and the power budget. Answer each before opening the key. Aim for 16 or more. Use $S = 1361\ \text{W/m}^2$ and space-cell efficiency $30\%$.

Question 1

The solar constant — the sunlight flux at Earth's distance — is about:

A) $136\ \text{W/m}^2$ B) $1{,}361\ \text{W/m}^2$ C) $13{,}610\ \text{W/m}^2$ D) too variable to quote

Question 2

As a spacecraft moves away from the Sun, the solar flux falls as:

A) $1/d$ B) $1/d^2$ C) $1/d^3$ D) it stays constant

Question 3

Modern multi-junction spacecraft solar cells convert sunlight to electricity at roughly:

A) 6% B) 15% C) 30% D) 90%

Question 4

An array whose normal points $60^\circ$ away from the Sun produces what fraction of its square-on power?

A) 0.87 B) 0.50 C) 0.60 D) 0

Question 5

The array must generate roughly twice the spacecraft's average load mainly because it:

A) is only 30% efficient B) works only in sunlight and must also store energy for the eclipse, with losses C) degrades with radiation D) suffers cosine loss

Question 6

Sizing an array for end of life, rather than beginning of life, makes the array:

A) smaller B) the same size C) larger D) unnecessary

Question 7

The fraction of a typical $500\ \text{km}$ low-Earth orbit spent in Earth's shadow is closest to:

A) 5% B) 38% C) 70% D) 0%

Question 8

A geostationary satellite experiences eclipses:

A) every orbit B) never C) only near the two equinoxes D) only at the solstices

Question 9

Withdrawing $30\ \text{W·h}$ each cycle from a $100\ \text{W·h}$ battery is a depth of discharge of:

A) 30% B) 70% C) 300% D) 3%

Question 10

A low-Earth-orbit battery is cycled to only shallow depth (~25%) chiefly because it:

A) is very cold B) is cycled tens of thousands of times over the mission C) stores little energy D) runs at high voltage

Question 11

The battery chemistry used on essentially all modern spacecraft is:

A) lead-acid B) nickel-cadmium C) lithium-ion D) alkaline

Question 12

A radioisotope thermoelectric generator produces electricity from:

A) sunlight on photovoltaic cells B) the heat of radioactive decay, via thermocouples C) a controlled fission chain reaction D) chemical fuel cells

Question 13

Plutonium-238 is used in RTGs partly because its half-life is about:

A) 8.8 days B) 87.7 years C) 24,000 years D) 4.5 billion years

Question 14

The heat-to-electricity conversion efficiency of an RTG is about:

A) 6–8% B) 30% C) 50% D) 90%

Question 15

The main reason engineers do not build kilowatt-class RTGs is that:

A) they would be too heavy B) plutonium-238 is extremely scarce and expensive to produce C) it is forbidden by treaty D) thermocouples are too efficient

Question 16

KRUSTY, tested in 2018, demonstrated:

A) a new triple-junction solar cell B) a compact space fission reactor C) a lithium-ion battery D) a Hall-effect thruster

Question 17

The fairest single figure of merit for comparing space power sources is:

A) total watts B) bus voltage C) watts per kilogram (specific power) D) conversion efficiency alone

Question 18 (True/False, justify)

"A spacecraft at Jupiter can be solar-powered using the same array area it would use at Earth." True or false? Justify in one sentence.

Question 19 (True/False, justify)

"Raising the bus voltage, for a fixed power delivered, lowers the current and lets the harness be lighter." True or false? Explain briefly.

Question 20 (Short answer)

A $30\%$-efficient array of $5\ \text{m}^2$ faces the Sun square-on at $1\ \text{AU}$. Compute its beginning-of-life electrical output, and state one reason its in-orbit output would be lower.

Answer Key

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