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Comment by wiz21c | Hacker News Reader

FTA :

>>> The crew had to return in "ballistic descent mode", Nasa tweeted, which it explained was "a sharper angle of landing compared to normal".

is it me or is it a very diplomatic qay to sell : "fallen vertically" ?

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My understanding is that "ballistic" means without any form of propulsion (except the initial one sending it on its way). In other words, like a projectile (or a ball), just subject to the forces of gravity and air resistance.

In other words: falling, yup.

(But note that one could "fall" upwards or sideways initially - not like Arthur Dent [1], but like a bullet (or indeed rocket) fired upwards).

[1] The Hitchhiker's Guide to the Galaxy states: "There is an art to flying, or rather a knack. The knack lies in learning how to throw yourself at the ground and miss. ... Clearly, it is this second part, the missing, that presents the difficulties."

I believe the Soyuz SAS has engine thrusters for the landing sequence. So ballistic may mean the thrusters were not operable and they may have landed a bit harder than normal (normal for an abort sequence at least). Meaning they were still under canopy, just falling at a higher impact velocity than intended. So maybe rather than a gentle landing at 5m/s it was more like a car crash at 20 m/s. Beats blowing up with the rocket or hitting the ground at terminal velocity, though, and presumably the reports of "good condition" mean there are at most some concussions and broken bones.

>there are at most some concussions and broken bones.

If you're strapped into a bucket seat with your back to the impact it's gonna take a fuckton of Gs to break bones. The astronauts would not be in "good condition" if they experienced that kind of a landing. They would be calling it a crash at that point.

Wikipedia has some pertinent (and interesting) details:

> The Descent Module (Russian: Спуска́емый Аппара́т, tr. Spuskáyemy Apparát), also known as a reentry capsule, is used for launch and the journey back to Earth. Half of the Descent Module is covered by a heat-resistant covering to protect it during reentry; this half faces the Earth during re-entry. It is slowed initially by the atmosphere, then by a braking parachute, followed by the main parachute which slows the craft for landing. At one meter above the ground, solid-fuel braking engines mounted behind the heat shield are fired to give a soft landing. One of the design requirements for the Descent Module was for it to have the highest possible volumetric efficiency (internal volume divided by hull area). The best shape for this is a sphere — as the pioneering Vostok spacecraft's Descent Module used — but such a shape can provide no lift, which results in a purely ballistic reentry. Ballistic reentries are hard on the occupants due to high deceleration and cannot be steered beyond their initial deorbit burn. That is why it was decided to go with the "headlight" shape that the Soyuz uses—a hemispherical forward area joined by a barely angled (seven degrees) conical section to a classic spherical section heat shield. This shape allows a small amount of lift to be generated due to the unequal weight distribution.

So, maybe ballistic refers to the fact that no lift was generated? (A glider is not considered ballistic, even though it has no propulsion.)

An, and indeed, that's what Wiktionary says:

> ballistic entry: when an entry vehicle has only drag with no apparent lift. An axisymmetric entry vehicle would have no apparent lift if its angle-of-attack time averaged out to zero, e.g. sinusoid angle-of-attack centered or trimmed about zero lift.

https://en.wikipedia.org/wiki/Soyuz_(spacecraft)#Descent_mod...

https://en.wiktionary.org/wiki/Appendix:Glossary_of_atmosphe...

Here's a diagram of the descent module, if you are having trouble with the description:

https://i.stack.imgur.com/BUYME.png

Edit: BUYME?

The standard landing experience is described in an ESA video: https://youtu.be/-l7MM9yoxII?t=1086

An interview on BBC Newshour said that a normal Soyuz landing could go as high as 6 G's, and max acceleration this time was 6.7 G's. So certainly on the rough side (6 G's is bad enough!) but not that far out of normal. Even a shock of 9 G's is expected to be survivable, although that would take a lot more recovery.

Ballistic in the Soyuz context means not using any aerodynamic lift, as oppposed to gliding reentry which is using lift, is controllable, and imposes much less Gs on the crew. Neither one uses any propulsion (expect for a specialized RCS which is used to control the roll while gliding).

It's not entirely clear to me why there's less acceleration on the gliding entry. It can't really be that the speed is bled off over a longer distance, right (in other words, I assume that the high-g phases are rather punctual than extended), and intuitively it would seem that, coming in at high speed, "gliding" (ie with lift) would lead to quicker deceleration than just "bleeding off" speed in the ballistic fall, ie higher g. Could you elaborate on the difference between the usual and ballistic mode, and when the high g-load occurs?

Fallen in a parabolic arc. They did have considerable horizontal velocity already at that point. The usual Soyuz descent involves a more shallow angle, with some aerodynamic lift generated by the capsule’s flat bottom, as well as (very) limited controllability using eight hydrazine thrusters around the capsule. But the great thing about the Soyuz is that even without control it can land safely if somewhat uncomfortably.

...From the creators of "unscheduled rapid disassembly" a.k.a. explosion on the launchpad.

Please... We can do RUDs in space too!

Space x and Boeing must have designed for failure at all points during launch, including this, does anyone have the details? It's impressive that the original design from so many years ago still works! There may be quality and manufacturing problems but I salute the designers, the engineers!

> designed for failure at all points during launch

If you want to investigate this, the term to look for is "black zone".

I would interpret it as "fallen horizontally" with a lot of braking happening lately in the atmosphere.

Yep. 8g's apparently.

The part when they are falling is only at an acceleration of 1g (that's what falling is). If they experienced 8g acceleration, it was before the ballistic part of the descent.

8g was probably from the parachutes deploying, although you do experience very slightly more than 1g acceleration while in freefall, as you encounter increasing air resistance (and significantly more in reentry).

I suspect the high G period is mostly when the escape rocket fires.

edit: Never mind. Chris Hadfield says it's hitting the atmosphere. https://twitter.com/Cmdr_Hadfield/status/1050385343625740289

Although that didn't happen this time - the escape rocket had already been jettisoned before the failure occurred.

The high g period was then they ran into the thicker part of the atmosphere at faster than terminal velocity (so the deceleration caused by the air is what led to the g forces).

Atmospheric reentry is more than 1g

https://www.quora.com/What-is-the-acceleration-of-the-space-...

The reentry forces could definitely exceed 1g and usually do from orbital velocity. You're moving sideways however fast the rocket left you going, and you have fallen at 1g acceleration for some amount of time before hitting thicker parts of the atmosphere. Your velocity is now way higher than terminal velocity for that altitude and you experience acceleration (deceleration).

Whey you are falling in a vacuum, it's not 1g, but 0g. That's what they experience on orbit. When you are falling with atmosphere resistance, it can be anything. Consider yourself jumping into water. You feel weightless when you are falling, but what do you feel when you hit the water? That's what happens when you hit atmosphere at 8km/sec.

The difference is between acceleration and g-force. Falling towards earth in a vacuum (or orbiting earth in a vacuum) you are experiencing 1g of acceleration but zero-g of g-force because you and your spaceship are accelerating together and you aren't meeting any mechanical resistance to the acceleration.

Once you start hitting atmo the velocity you've built up from your 1g of freefall meeting the resistance of the atmosphere creates g-forces.

Not vertically (they'll have had some horizontal momentum), but falling like a stone yeah. On the Soyuz it's a designed-in failsafe mode though.

Much in the same way a disaster is often called an "off-nominal aviation event".

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