I found these lines in the Apollo 14 Lunar Surface Journal:

116:07:37 Mitchell: Houston, the current reading is 8.

116:07:42 McCandless: Understand 8 amperes before pressing the (shorting) switch.

116:07:49 Mitchell: That's affirmative.

RealVideo Clip (3 min 44 sec)

116:07:50 McCandless: Roger.

    [Comm Break. The shorting switch helps keep the RTG from overheating before current is applied to the experiment packages. In essence, the shorting switch provides a resistance equivalent to the full array. Ed's report of the ampere reading is the fourth item in the 2+00 paragraph in his checklist.]


The ALSEP RTG had 1480 W thermal power and only 70 W electrical power at 16 V DC. I doubt that shorting the RTG would prevent an overheating of the RTG itself.

So I applied the basics of electric sources, their internal resistance and the maximum power transfer matching to the ALSEP and its RTG.

To use the maximal electric power of the RTG, the load resistance should be matched (equal) to the internal resistance of the RTG. Without any load, the RTG voltage is two times the voltage at a matched load. So the open circuit voltage was 32 V.

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AS14-67-9366 (OF300) ( 150k or 978k ) 117:17:41 Down-Sun picture of the RTG with the Central Station in the background. The fins on the RTG provide radiative cooling. Note the relatively large amount of dust that has been kicked on to the RTG. The smaller object in the background is the LRRR. The mortar pack is at the right edge.

When ALSEP was installed on the Moon, the RTG was connected to the central station and the several experiments were connected to the central.

Here is my interpretation of the shorting switch:

When the RTG was connected to the central station only but without any experiment, the output voltage of the RTG would have been too high and might damage the central station. To prevent a damage, the RTG was shorted at first and the shorting switch was released after all experiments were connected and nominal load to the RTG was connected.

Any other thoughts of the shorting switch and its purpose?

  • $\begingroup$ Note that the current will drive heat across the junctions as it does in a Peltier cooler...the effective thermal conductivity is higher when the junctions are short circuited. And the thermoelectric properties aren't necessarily all that linear. The effect might be greater than you'd expect from ~70 W not being removed in the form of electrical power. $\endgroup$ Commented May 8, 2022 at 1:33

1 Answer 1


I found some information:

Short-circuiting of the RTG provides a cooling effect on the RTG' s thermal junction and slows the rise to full operational temperature. Figure 1 is a plot of no-load voltage vs. time for warm-up of a typical RTG under open-circuited and under short-circuited conditions. As the curves indicate, the effect of the short circuit is progressively greater beyond a period.of about 11 minutes. There- fore, it is important that the shorting switch be opened as soon after 11 minutes after fueling as is practical (or before this time), to ensure rapid warm-up of the RTG


enter image description here

So the RTG heats up faster after fueling when the short-circuit plug is removed, as Christopher James Huff commented. The shorting plug should be removed 11 minutes after fueling.

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The RTG needs about 50 minutes warm up to reach 70 W.

I was wrong, the short circuit plug should not protect the load against overvoltage.

The short-circuit current reading is important as an indicator of RTG performance during the early minutes after fueling. The reading is reported by the LM crewman in a simple voice message, e. g., "RTG current reads 6" , which is acknowledged by MCC and logged. NOTE: Meter reads directly in amperes

So the short-circuit plug and the current meter was used to indicate the RTG performance during the first minutes after fueling. An early indication of RTG performance was important to verify the correct insertion of plutonium fuel.


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