This image from here: Does the Dragonfly project (quadcopters on Titan) envision attached RTG's or would they be static and revisited for charging?

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and this image

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show that the RTGs on these two recent missions are tilted at a stylish angle relative to the main body of the vehicle.

Why is that? There's a (to put it nicely) speculative thread on reddit here but the only guess that seems vaguely plausible is the one about packaging volume in the aeroshell.

  • 2
    $\begingroup$ Seems like just keeping the body out of "line of fire" of heat radiating off the radiator blades. $\endgroup$
    – SF.
    Feb 21, 2019 at 14:20
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    $\begingroup$ Wouldn't having the RTG axis parallel to the main body axis be even better for that? $\endgroup$ Feb 21, 2019 at 14:28
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    $\begingroup$ Looking at the the first image, which includes the outline of the hatch used to insert the RTG just before launch: if the RTG were horizontal, there would have to be a hatch in the heat shield. So I think the top comment in that Reddit thread nails it. Now to find references... $\endgroup$
    – Hobbes
    Feb 21, 2019 at 14:42
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    $\begingroup$ It's a standard configuration. $\endgroup$
    – uhoh
    Feb 21, 2019 at 15:04
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    $\begingroup$ @uhoh i.imgflip.com/2u96vy.jpg $\endgroup$ Feb 21, 2019 at 15:26

1 Answer 1


The Dragonfly cruise stage looks rather similar to Curiosity's cruise stage, so I've looked at Curiosity. Curiosity's backshell contains a hatch. This was used to install the RTG at the last possible moment (a few days before launch, long after the rover had been packaged in its backshell plus heat shield.

Here is Curiosity's backshell being prepared for encapsulating the rover. You can see the hatch.

enter image description here

The MMRTG won't be installed until just a couple of days before the launch, because it throws off so much heat. Once the rover has launched, a cooling system will vent the excess heat to space, but between now and then it takes a pretty powerful cooling system to keep the MMRTG from heating up the interior of the spacecraft to hazardous levels. I was told a while ago that they have three redundant cooling systems working to keep the spacecraft at a safe temperature between MMRTG installation and launch.

For Curiosity, this means the RTG must be installed so it's accessible from the backshell. If it were installed horizontally, the hatch would have to go through the heat shield, and that's something spacecraft designers try to avoid as much as possible.

In the drawing you can see Dragonfly has the same packaging issue: if the RTG were horizontal, the hatch would have to go through the heat shield.


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