Why is the descent module of the Soyuz in the middle of its stack?

Question

The Soyuz is made up of three major components/modules.

The Orbital module on top, basically a sphere, which is basically extra room, and contains the toilet, which allows for some modicum of privacy. It is discarded when reentering. (The Shenzou approach leaves its analog to this module orbiting with experiments, which is quite clever)

The Descent module, in the middle, sort of gumdrop shaped, is where the astronauts reside for launch and reentry. This is reported to be very cramped and tight on space.

The Propulsion module at the bottom contains the engines, solar panels, etc.

However, during a launch abort, the first two modules need to be pulled away from the launch vehicle. Why is the manned component in the middle? Did it not make more sense to have the people at the very top, thus allowing a simpler abort (less mass to pull away)?

I am pretty sure Soyuz uses a tractor (not pusher) abort system.

Answer 1

From Encyclopedia Astronautica (emphasis by me):

This configuration was selected only after considerable engineering angst. From the point of view of pulling the capsule away from the rocket in an emergency, positioning the capsule at the top of the spacecraft was ideal. But to use this layout with the living module concept, a hatch would have to be put through the heat shield to connect the two living areas. Korolev's engineers just could not accept the idea of violating the integrity of the shield (and would later get in bitter battles with other design bureaus when competing manned spacecraft - Kozlov's Soyuz VI and Chelomei's TKS - used such hatches

There are a couple of other designs which come to mind, but have also significant problems. I don't know whether such ideas have ever been considered:

1. Put the orbital module below the reentry module during startup, without any way to enter it at this time. After reaching orbit, the configuration could then be changed to the present configuration, similar to how the Apollo lunar module was moved to the front of the spacecraft at this time. His would have been rather complex and therefore more prone to failure. Also it would probably require some redundant systems, and additional fuel for this maneuver.

2. Reverse the orientation of the reentry module during launch, so that the top hatch is pointing downward. But this would have required a way to rotate the seats, and everything pointing to them.

Answer 2

Consider the Soyuz spacecraft vs the Apollo spacecraft. Both are 3 man spacecraft originally designed for the lunar mission. Both use tractor rocket escape systems for launch abort.

In the Apollo spacecraft, the reentry craft, living area and docking functions are combined into the same module. The Soyuz separates those functions into two modules.

The Soyuz design, by minimizing the size of the reentry module also minimizes the mass of heatshielding and parachute system needed. The lightweight structure of the orbital module allows it to be made larger, providing the Soyuz a greater total volume of living space for the crew than the Apollo. Because of the lightweight design the combined 4,300 kg mass of the Soyuz descent module and orbital module is less than the 5,800 kg mass of the Apollo command module.

During launch abort, either spacecraft must jettison the launch escape tower and payload shroud before parachutes can be deployed for landing. With the Soyuz, the tower/shroud separates along with the orbital module. For all practical purposes the launch abort sequence is not any more complicated with the Soyuz than with the Apollo.

The primary reason the Soyuz orbital module is ahead of the descent module is so the docking equipment can be placed on the orbital module. That helps to minimize the mass of the descent module, and also allows the orbital module to function as an airlock.