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Comment by diggernet | Hacker News Reader

T+2:41: "Phil, also as a reminder, we also have that retro-thrust system on the bottom of the crew capsule to make sure that the touchdown is even smoother for our astronauts."

T+2:54: "Just as a reminder, that's also going to kick up quite a bit of dust out here in Texas."

T+3:08: "It's going to come down at just about 3 miles an hour by the time it touches down."

T+4:15: Capsule smacks into the desert at 16mph, with no sign of retro-thrust or dust kick up before impact.

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Seriously? You don't think that it could happen faster than you could see it with your eyes easily? Go watch a Soyuz capsule touchdown, same deal. Also, it's very hard to spot the retro-rocket firing unless you pause the video and go frame by frame. The New Shepard retro fire is even harder to spot. Which is why they tell you about it.

Here are some shots from the video:

First puff of dust from under the capsule: http://i.imgur.com/u055Gjs.jpg

Fully settled: http://i.imgur.com/u53t96Q.jpg

Between the first and second images, the capsule settles about 1/4 of its height. This is because the first image occurs when the capsule is still that high of the ground, and firing its retro thruster.

I don't know the specifics of when the Blue Origin capsule's rockets fire, but for the Soyuz capsule it's 70cm above the ground. The time between the rockets firing and impact with the ground is far too fast to see in real time.

The dust cloud that gets kicked up is kicked up by the rockets firing, not the capsule slamming into the ground (which is the point the commentators were trying to make).

That's what I thought as well, but watching some other videos on their site, they say the boost happens in "the last second" https://youtu.be/xYYTuZCjZcE?t=2m34s. Any sign of thrust before landing really isn't visible in that video either. So unless they've never actually fired it successfully and we just don't know what it looks like, it's possible that it's just so close to touchdown that we can't distinguish the thrust from the landing.

I'm assuming that decelerating by 13mph over < 1s is still quite a bit better than by 15mph instantly? But I really don't know.

I thought 20mph was maximum survivable rate of decent?

I suppose that a last-instant firing is a plausible explanation, and would greatly reduce physical damage to the capsule. But then the talk of a "gentle" 3mph touchdown is conveniently ignoring the 16->13 deceleration a split second before. As you say, it's likely preferable to 15->0, but I suspect not a whole lot more pleasant to experience. The astronauts would probably not even notice the actual touchdown an instant later.

The goal is to stretch out the deceleration. Instead of going from 16 to 0 instantly (probably more like 1/10th of a second when you count the capsule deforming), it's going from 16 to 0 over a much larger fraction of a second.

It's probably the difference between running full speed into a brick wall and running full speed into a large foam pad.

The foam pad decelerates you in less than a second, but it's a whole lot more comfortable than the brick wall.

I'm sure there's no deceleration at all at the beginning of the last second, there's no slackening of the lines for example, but it does look as if there's some sort of thrust brake in the last tenth of a second or so, somewhat in lieu of a crumple zone I suppose.

http://rowvid.com/?v=bqUIX3Z4r3k&t=4232&s=0.25

Thanks. I always forget about RowVid...

In frame-by-frame, you can see that the capsule is still descending when the first dust appears. To my eye, seems to be about 1/3 to 1/2 second later that the capsule stops. So, ok, retro-thrust. (But still giving quite a jolt to the poor meat-bags.)

Note that in previous videos of landings, there was just a biggish last-second dust cloud -- people on HN guessed that there was no retro-thrust and that it landed hard, when actually there was retro-thrust and a nominal landing.

Yeah, I was curious about that part too...

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