#planet modding

https://forum.kerbalspaceprogram.com/index.php?/topic/211156-v10-central-fire-planet-jam-2-armstrongs-limit-homeworld-in-the-ilio-pyri-planetary-system/

Welcome to Armstrong

Radius: 950,000m (1.5 Kerbin Radii)

Surface Gravity: 1.75G

Atmospheric Pressure: 0.068atm

Orbital parent: Pyri, a red dwarf star.

Orbital and rotation period: 6 hours.

System Layout: The Ilio System is a large and complex system. Counting stars, planets, moons, and asteroids, there's a total of 101 objects.

The Ilio System is divided into an Inner System, with four gas giant planets, the Pyri System with its planets and trojans, and an Outer System, with three gas giants on two orbits with associated coorbitals, and a distant dwarf planet.

The Inner Pyri System. This is your home in space. Note the prevalence of asteroids in the Arriprit Belt. What may look like clutter in the map view is actually vital early-space-program scientific targets, and refuelling infrastructure that can be used to climb out of Pyri's steep gravity well. Zhandar is a mid-size gas giant that can be used to provide powerful gravity assists, these trans-Zhandar trajectories are some of the most important in any deep space interplanetary mission. 

Zooming out to see the Outer Pyri System. Bifröst is a superjupiter. Its magnetodisk is visible as a dim rainbow ring in the night sky. Its enormous mass also makes it extremely useful for providing gravity assists. If you can get out to Zhandar, you can get out to Bifrost. And if you can get out to Bifrost you can go anywhere. Beyond Bifrost, but much less massive, is Sinacin-Glice.

Zooming all the way out, we can see the Outer Ilio System. Pyri is so small compared to its double-sun parent that it can support trojans, leading and trailing 60 degrees away from Pyri but in the same orbit. Beyond the destabilizing influence of Pyri are Oynag and its trojans, and the Tobalk-Turtlestar binary. There's also four dwarf planets in this part of the system.

Armstrong's Limit is the altitude and atmospheric pressure at which water boils at body temperature, a property discovered by General H.G. Armstrong (not the Apollo astronaut Neil). Put simply, above that altitude, the air is so thin that your blood would boil, and you could not survive without a space suit. It corresponds to an atmospheric pressure of 0.0618 atm. The planet Armstrong's atmospheric pressure at sea level is barely above that, at 0.068atm; and much of its glacial highlands are below that pressure.

From an in-universe perspective, Armstrong is named after Harriet "Arm Strong" Kerman, an arm wrestler and amateur pilot who... erm... 'discovered' the limit, prior to the development of the Kerbal Space Program. Armstrong's atmosphere is so thin because it orbits point blank around a red dwarf flare star--it probably started out as a very thick water-hydrogen envelope before that got blown away by intense solar wind and ultraviolet flares.

From an IRL design perspective, Armstrong's atmosphere is so thin to make designing re-entry vehicles intentionally difficult. Returning to Armstrong requires slowing down from a 3.5 km/s orbital velocity using an atmosphere that doesn't really want to do that, but which is still thick enough to cause shock heating damage to your craft. Armstrong is like the real life Mars in that respect, with reentries resembling the 7 minutes of terror that occurs during Entry Descent & Landing sequences for Martian spacecraft.

Why make it so difficult? Armstrong is the sequel to Mesbin in my mod Whirligig World, a totally airless super-earth.

There, care must still be taken to design landers if you want to return your crew home, but you dont have to worry about aerodynamics at all (unless you're flying to another planet.)

As difficult as Mesbin can be sometimes due to its sheer mass (its rapid rotation gives you a major head-start into orbit, but climbing from low orbit to its moons or escape velocity is tough), but after playing Armstrong, not needing to worry about aerodynamics feels almost luxurious.