Almost all early manned spacecraft have been modular, like the two-module Apollo Command/Service Module or the three-module Gemini spacecraft. Mercury had no modules.
All Russian spacecraft were modular. Vostoks were two module spacecraft, with the cosmonaut having little to do with the central functions of the spacecraft largely automated or controlled from the ground. After reaching a safe altitude for parachutes, the sole crewman ejected, landing separately from the descent module.
The Voshkod spacecraft were uprated 2-man Vostoks with an extra retrorocket for landing (the crew stayed inside the descent module on landing) with options for an inflatable airlock for the first-ever spacewalks.
The Soyuz, still in use today, used three modules. The cylindrical Orbital Module is a storage and living habitat area while in-orbit, and also houses the docking mechanism and other instrumentation. The Descent Module is the central command module where the crew sits during ascent, and is also the crew deorbit and reentry module. The last is the Propulsion Module (also called the Instrumentation or Service Module), a non-crewed, non-pressurized area akin to the Apollo Service Module with propulsion, power and life support functions.
The typical sequence of events for re-entry calls for sealing off of the Orbital Module, reorienting the spacecraft for the deorbit burn, completing the burn and then jettisoning the Orbital Module and then the Propulsion Module. For a while, the procedure to jettison the Orbital Module before retrofire was used, but when one Soyuz experienced a retrofire malfunction that left the crew stranded in space for a day, that procedure was rescinded.
When de-orbiting, both American and Russian spacecraft had to jettison all but their entry modules, of course. Imagine if, on lunar re-entry, an Apollo Command Module failed to completely separate from the Service Module.
With the vehicle roaring towards Earth at nearly 25,000 MPH, the Command Module must be at the proper entry angle. Dragging a Service Module along (say, the guillotine mechanism that cuts the power, data and water umbilical between the SM and CM failed) would certainly destabilize the entry angle, not to mention a probability of the SM causing severe damage by collision with the CM. In short, a bad re-entry angle equaled a bad day.
Such a scenario with the Apollo CM was highly unlikely because the guillotine was redundantly designed with two blades to ensure separation, as well as redundancies included on the three connective tie straps at the base of the Command Module that held the CM to the SM to that point. The guillotine blade was non-metallic to ensure that it wouldn’t short any CM system wiring as the connections were severed.
NASA thought long and hard on the reliability of all pyrotechnic systems on the Saturns and Apollo spacecraft, as seen in this detailed technical document. No incidents were recorded of any pyrotechnic malfunction on any Apollo mission save one: Skylab’s launch in May, 1973. The second stage’s interstage failed to separate (its mechanisms damaged by the micrometeoroid shield destruction). The S-II stage engines were in grave danger of overheating and failure but somehow made it through to loft the crippled space station into orbit.
For the Russians, however, failure of their service modules to separate from the reentry modules, in all three of their manned space vehicles since 1961, has happened. Seven times.
Thankfully, all of the crews in these incidents survived, although not without some injury, at least most with shaken nerves.
No introduction should be needed for the great Yuri Gagarin, traveling aboard the very first manned spaceflight on April 12, 1961.
With one orbit completed, the spacecraft was commanded to begin deorbit. After the separation commands to jettison the Vostok’s service module were sent for re-entry, the service module clung perilously to the descent module by some wiring that wasn’t cut with the separation.
Despite the danger, Gagarin, illustrating the size of his own Russian brass balls, simply indicated to ground that all was OK on descent, all while the spacecraft tumbled rather uncontrollably into the outer atmosphere as he endured at least 8G deceleration. The cosmonaut also believed the rough ride was natural, given the cylindrical shape of the module. Before long, the wires melted away to separate the two modules, and the descent module stabilized for a safe reentry.
By design, Gagarin ejected around 23,000 feet (7 km) and parachuted separately from the descent module.
The suave, handsome talented party animal and statesman known as Gherman Titov experienced a similar problem to Gagarin on his mission four months later, in August 1961.
His mission time dwarfed Gagarin’s, after completing 17 orbits. To date, Titov remains the youngest person to ever fly in space, not quite 26 years old.
With the separation command, Titov heard the separation, but indicator lights on his panel, which were powered by the service module’s power, said otherwise.
After a similar tumbling of the descent module with the service module dragged along on a few wires, the wires soon burned through for Titov to reenter safely.
Launched on June 19, 1963, Valery Bykovsky’s spacecraft failed to reach a sufficiently high orbit, so the mission would end earlier than planned.
The spacecraft was able to make a close encounter with Vostok 6 with Valentina Tereshkova, the first woman in space.
Like Vostok 1 and 2, Vostok 5’s instrumentation module didn’t cleanly separate, and the two modules gyrated wildly into the upper atmosphere until burn-through of the wayward connections separated the modules.
Crewmembers Alexei Leonov, the first spacewalker, and Pavel Belyayev had a spacecraft with a few gremlins due to thermal issues. Leonov’s spacewalk was also not without complications, requiring him to ultimately partially depressurize his own suit to re-enter the airlock.
As with Vostok 1, 2 and 5, the descent module spun wildly with the partially-connected instrument module until the two modules finally severed with reentry heating.
The fate of the two cosmonauts was unknown to recovery crews for a time. Landing in a heavily forested area, it took two days for the crew to be recovered.
The Soyuz spacecraft began operation in 1967 and have been used (with some variations) ever since for sending one to three people into low Earth orbit.
Boris Volynov just completed his rendezvous and crew transfer mission with Soyuz 4.
Now alone, he prepared the spacecraft for reentry on January 18, 1969, first jettisoning the orbital module and, after retrofire, activating the pyrotechnics to separate the propulsion module from the descent module.
Volynov was horrified to see, out of his window, that the propulsion module was still attached. When ground crews heard of the news, helpless to rectify the problem, they grimly and quietly began a small monetary collection for the cosmonaut’s family.
As the vehicle reached the atmosphere, the vehicle first slowly tumbled before incorrectly stabilizing to a nose-first re-entry posture, the nearly unprotected upper hatch facing forward, the descent module’s heat shield trapped by the clinging propulsion module behind him.
The forward hatch buckled slightly from the heat and smoke entered the cabin. But just as hope seemed to completely fade, the re-entry forces and heating sheared the propulsion module away, and the descent module’s center-of-gravity and shape naturally re-oriented the module to its proper blunt-end forward entry attitude.
Landing was violent as the parachute lines tangled and the landing rockets failed. Volynov was hurled about, breaking a few teeth. He lived with the capacity to tell the tale, but not the ability: The Soviet government would keep the incident secret for nearly two decades.
The harrowing re-entry malfunction of Soyuz 5 would be repeated decades later as a crew from the International Space Station returned to earth.
It’s October 21, 2007, and three members of ISS Expedition 15 depart from the station. At the time, the Space Shuttle was still in operation, so American astronauts Suni Williams and Clayton Anderson arrived and departed on Discovery and Atlantis, respectively. The returning Soyuz was occupied by two Russians, Oleg Kotov, Fyodor Yurchikhin and the first Malaysian astronaut, Sheikh Muszaphar Shukor.
Due in part to a software fault, the pyrotechnic bolts connecting the instrumentation module to the descent module failed. Like Soyuz 5, the spacecraft entered nose-first, threatening the lives of the crew before re-entry heating caused the malfunctioning connections to break apart.
Because of the wayward orientation with both modules connected for a time, the descent module had been pulled off-course. Once free of the instrumentation module, the descent module made an emergency high-deceleration ballistic re-entry, pulling 9G on descent. The crew landed unharmed.
Russian officials kept the incident secret to their American partners. That was a mistake.
The Expedition 16 crew of Yuri Malenchenko, Yi So-yeon (first Korean astronaut) and NASA astronaut Peggy Whitson were returning home on April 19, 2008.
The same scenario that occurred on the last Soyuz, TMA-10, played out again. The instrumentation module failed to separate, and the ship careened nose-first into the atmosphere until the instrumentation module was forcibly separated. The crew endured a high-G ballistic re-entry as well, but landed safely, although Yi suffered some back injuries.
NASA was angered not only because of this malfunction but also because of the Russian’s reticence to notify NASA of the TMA-10 incident. One Russian official even went so far as to claim that the problem was exacerbated by having two females aboard.
NASA held Russia’s feet to the fire with a commission and investigations to ensure that their flight computers and hardware became more reliable to ensure that future module separations would occur normally, especially in light that the Americans were soon to become dependent on flying American crews solely on Soyuz vehicles once the Space Shuttle program ended in July 2011, three years later.
None the worse for wear, Peggy Whitson returned to the ISS on Expedition 51/52 (currently aboard as of this writing) as station commander. She is (for now) the third most experienced spacewalker, holds the record as the oldest woman to fly in space, and holds the record for the longest spaceflight by a woman. On departure in September 2017, she would be in space for 290 days.