How safe is a hatch in the heat shield?

[Pipcard]

As in, a hatch in the heat shield of the command/re-entry module, connecting it via tunnel to a orbital habitation module.

Certain versions of Gemini (Manned Orbiting Laboratory, Big Gemini) were going to have it, and the TKS with VA capsule was going to have it as well.

Can you re-enter from the Moon with a hatch in the heat shield?

 

[Urwumpe]

As in, a hatch in the heat shield from the command/re-entry module to the orbital habitation module.

Certain versions of Gemini (Manned Orbiting Laboratory, Big Gemini) were going to have it, and the TKS with VA capsule was going to have it as well.

Well, lets call it a manageable risk. It is of course an opening in the heat shield that can fail. It is just pretty safe bet that it won't fail. On an ablative heat shield, the reentry heat actually welds the hatch shut, even if it is not perfectly closed, there is quite some tolerance. For reusable heatshields, you need more accuracy to provide a closed seal and on the Space Shuttle, where you had MANY opening in the heat shield, you also needed overpressure by Helium, to prevent that plasma sneaks into the seals and erodes them.

How thick this heatshield is, and thus how much reentry heating it can absorb, is no problem for a hatch, except that it of course makes it harder to design a hatch, that can get opened and closed, if you have 20 cm of heat shield on one side extra.

 

[Loru]

I was investigating that possibility and thing I came up with was making hatch tapered to the inside so pressure during reentry will support seal.

 

[Pipcard]

My concept (quick sketch):

White section is the command/re-entry module, and is 3.39 m in diameter. It seats four crewmembers.

Below it is the tunnel leading to the habitation module, and the habitation module itself which is 4.39 m in diameter (almost like HTV). Below that is the service module with solar panels, fuel, and engines.

edit: I made this about a year ago; this is more of a Soyuz/Fuji-like design. Diameter (w/o solar arrays) is 3.09 m.

 

[C3PO]

SpaceX wants to put doors for landing legs in Dragon's heatshield, so they think it is a manageable risk.

Both Buran and STS orbiters had doors in the TPS for the landing gear, but they were not opened until after reentry.
STS had fuel line doors that were closed on orbit, so it is proven to be possible.

 

[MaverickSawyer]

Totally manageable risk. They actually flew a Gemini B with the hatch cut in it in 1968... Not a problem. The hatch welded itself shut, just like Urwumpe said.