JF Mezei wrote:
On 2020-07-25 14:49, Alain Fournier wrote:
Not nearly as important as recovering the first stage, but still
recovering the nose cone reduces yet again launch costs for SpaceX.
Their fairing recovery ships caught both halves of the nose cone on
their launch Monday.
[...]

is there logic on the fairing to control parachutes to steeer to the
location of ship with the net?

Or does the ship with the net have high speed capability to position
itself under the arriving fairing?

AFAIK they do both, it's not exactly hard to find quite a bit of
details (though the exact details is obviously SpaceX confidential).

The fairings have attitude thrusters for correct orientation during
re-entry (firings can sometimes be seen on stage 2 camera footage) and
then deploy a steerable parachute.

The boats used are capable of both high speed and high maneuverability
and the videos make it obvious that it's moving and maneuvering to get
the fairing to land in the large catching net.


In the first case, I assume the awaiting ships are pre-positioned to
area where the fairings are expected to drop based on wind etc.

If they weren't pre-positioned no amount of steering and boat speed
would help, unless you posit super-sonic boats :-)

Obviously the boats leave their base early enough to at least cover as
much of the expected landing area as possible. I don't know if that
mean they start in the middle of the most likely zone (becaue they
can't cover it all) or whether the likely zone is small enough that
they can afford to start at an offset (don't need to leave the port as
early which saves slightly on man-hours).

If you care you about that aspect you may well be able to make an
educated guess on it based on crunching public data for all the
fairing recoveries attempted so far. But I'm not going to do your job
for you.


From orbital mechanics point of view, was this a question of SpaceX
learning how long it takes for the fairings to tumble in space before
they "hit" atmosphere and start to fall? Or it is a calculated thing?

As mentioned above they have control systems on the fairing assembly
for both before and after re-entry but it's a very large and light
item so even very light wind variations make significant

Skydivers with steerable parachutes can make very precise landings
but... they're probably at least one order of magnitude less
susceptible to wind deviations than this (if not more) and also SpaceX
can't use TOO heavy equipment on the fairing or it might not be worth
it any longer (or at least not all launches might have recoverable
fairings).

It's important to remember that like suicide burn this kind of fairing
recover was still fairly recently considered sheer lunacy...

I doubt the fairing recoveries required enough complicated math to
result in a Math paper published in prestigious journals (the
hoverslam/suicide burn did) but that doesn't mean it's easy.

In fact, a lot of observers though SpaceX was going to give up on "net
on a fast boat" fairing recover based on their success with recovering
fairings from the sea.

Yes, it obviously saves LESS money and time than a non-water recover
but it's still worthwhile and much cheaper/simpler and they've had
already proven that to work.

I suspect they might have been right in light of a planned future that
includes Starship+SuperHeavy if it hadn't been for the sheer NUMBER of
launches they're going to need for Starlink before S+SH will be
available. Still, Starlink launches have already used water recovered
fairings so it wasn't a silly guess.