In article om,
says...

On 2017-06-24 22:58, Jeff Findley wrote:

Pretty much. But this stage started with a higher velocity than normal
due to very limited fuel reserves.

And this explains the "re-entry" being hotter and more stressful than
normal. I get that.

But by the time, the stage has fallen through atmosphere, wouldn't its
vertical speed be comparable to other landings?


hotter than normal). Also, due to lack of fuel reserves, this was a
three engine landing, which is necessarily harder than a single engine
landing.

If you have X fuel left, is there an advantage of running 3 engines for
2 seconds instead of 1 engine for 6 seconds?

Yes. You're neglecting gravity losses. Firing for 6 seconds means the
stage also has gravity acting on it for 6 second too. Only firing for 2
seconds means you're saving those 4 seconds fighting gravity. Hence the
term gravity losses.

Isn't there some fuel wasted during engine start-up? As such, woudln't
starting a single engine and running it longer be more efficient?

I doubt there is much "wasted" during startup. The turbopumps spin up
very fast.

Also, how does re-entry burn get started? Since the stage is sort of
free floating at that point, I take it the fins are not very useful? Do
the nitrogen thrusters turn the stage around such that the engines are
in the front, so when they fire, they decelerate the rocket's orbital
velocity?

Yes, watch the video closely. You can clearly see the nitrogen
thrusters firing in order to slowly turn the stage to get it ready for
reentry.

Or does de-orbit burn happen once the rocket has fallen sufficiently
that the fins are holding it vertical and the firing slows vertical
descent rate?

The fins are deployed by then, but aren't nearly effective enough, hence
the nitrogen thrusters firing.

(from the video, attitude isn't so obvious relative to direction of
travel).

The camera is at the top of the booster looking down which is why you
can see the engines firing for the reentry burn. Since you can see the
earth below, the burn is mostly to get rid of the stage's velocity. The
barge is pore-positioned at the optimal location so there is no need for
a "boost back" burn.

Jeff
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