Ballistic Re-entry

So the Russians had another Soyuz spacecraft switch to ballistic re-entry, the third one in about two years. What does that mean, and it is important?

First, what it means. The Soyuz spacecraft consists of three major chunks, but only the bell-shaped Descent Module (often called the "DM" to the presumable confusion of Dungeons & Dragons fans) makes it back to earth. Because of its shape, it is capable of generating a small amount of lift. Unlike a rock that simply falls straight to the ground, the Soyuz DM (like most manned spacecraft) can be flown, though it has to be said that the amount of lift is really quite small.

What's the point then, if the amount of lift is so small? One reason to do this "lifting re-entry", as it is sometimes called, is because it reduces the strain on the crew. When the DM is flying in its half-assed way, the gee-forces on the crew are substantially reduced. Another good reason to have lifting re-entry is that you can steer the DM to a precise touchdown - during the Gemini and Apollo programs, some feared (and not entirely irrationally) that the lifting re-entry technique made the Gemini and Apollo capsules so accurate they might actually hit their recovery aircraft carrier, which would be simultaneously very impressive and very sad.

So those are the advantages. Lifting re-entry is comfortable for the crew, especially for a crew that's been aboard a space station for a while and isn't fully adapted to gravity. Lifting re-entry is also accurate and you can in principle plop the capsule down right next to the caterer's van.

But it requires that the spacecraft be under positive control. Lifting re-entry doesn't just happen; you have to control the spacecraft using its attitude control jets to make it happen. And that means that the spacecraft needs a functional computer control system, functional attitude control jets, and functional other bits like a navigation system, an inertial reference system and so forth.

What is some part of this system fails? What if you can't control the spacecraft and can't do that groovy lifting re-entry? Ah, then you go for the backup ballistic re-entry, where you don't need a control system; the spacecraft is designed in such a way that it'll fly a ballistic re-entry even if the computers are all turned off (indeed the word ballistic implies that there is no control and guidance; it's as uncontrolled as the flight of a bullet). As it happens, this ballistic re-entry is much more vigorous than lifting re-entry. The angle is steeper, the deceleration is harder, and the spacecraft lands much sooner than it would have. In the case of the Soyuz, uncontrolled ballistic re-entry caused it to land about 250 miles "upstream" of where it was expected to land.

Is any of this bad? Well, yes and no. Nobody wants to make the crew uncomfortable, and a control system failure of any kind is always worrisome. And coming down 250 miles away from the recovery people is annoying, especially when nobody knows where you are and you have to literally call home and tell them where you are. But none of this is all that much more dangerous than normal spaceflight. Annoying, uncomfy, unexpected, sure, but not particularly dangerous.

The real question is WHY.

The last time this happened, the Russians tried to blame the crew - I remember reading that the first reaction of the controllers when they heard that the Soyuz had gone in ballistic was What did you touch? Up until recently, "crew error"was responsible for everything, including the time the Progress rammed Mir when Tsibliev was supposed to fly it to a manual docking with no range or range-rate information - insert sound of forehead being gripped here. Later they concluded that an electrical problem had disabled the flight control computer, and the capsule reverted by heavenly routine to ballistic re-entry, safe enough though inconvenient and uncomfortable.

But this time? There are reports, thus far unconfirmed, that an explosive bolt holding either the Service Block or the Orbital Module to the DM didn't blow, or didn't blow properly, which caused the DM to tumble during the early part of re-entry presumably until aerodynamic forces built up to the point that the bolt sheared. This has happened before, at least once, and it's potentially deadly, an issue much, MUCH more serious than the mere ballistic re-entry.

The Soyuz spacecraft and booster have been admirably safe and effective and have given the Russians sterling service over the years. There have been accidents, sure, but so far only four men have died in Soyuz spacecraft in almost forty years of use. So I'm not necessarily one of those Safety Cassandras that insists that the Soyuz be scrapped because something went haywire on the last mission. But this business with the explosive bolt, if that's true, that's very bad juju, and I for one wouldn't take a seat on a Soyuz until the Russians presented me with some pretty compelling failure-analysis and corrective-action information. Ballistic re-entry? I can hack it. But a Service Block trailing my DM all the way down? Negative. Moscow, we have a problem.