One possibility for the glow seen post-boostback might be combustion of
residual fuel purged out of the engine cooling passages post-burn. It'd
have to accumulate somewhere it'd then gradually burn to account for
what was observed - implausible but not utterly impossible; the
airstream is probably doing interesting things around the aft end.
Or, yeah, they might just be pumping a small amount of fuel through the
engine to cool it.
Or as the engine heats up, fuel pooled in the cooling passages expands,
dribbles out the injector, and ignites. Hmm. I'd give modest odds that
one's it.
I'm pretty sure the small fire under the booster post-landing was
something similar - some number of quarts of kerosene purged from the
center engine post-shutdown, igniting on the hot pavement.
Henry
On 12/22/2015 2:11 PM, Ben Brockert wrote:
There was a delay for the boostback burn, I imagine they don't want to
blow exhaust at the upper stage. I don't know that it was minutes
though. Someone has probably extracted the timings on /r/spacex.
The high overhead 3 engine burn is to burn off vertical velocity, not
horizontal, to my understanding. Falling from 200km near vertical is
an incredibly tough thermal environment. A ballistic fall from 150km
was used back in the day to test heat shield materials for orbital
reentry. (This is also why I always cringe when people talk about
suborbital point to point as the next step after suborbital: for any
useful range it's harder than orbital.)
I agree about the glow after boostback. I was wondering if they were
dumping a bit of fuel to keep the center cooler. As far as I know the
engines can only throttle to something like 80%.
I've watched launches from where you watched, that's where I took my
Mom and where I took Jon's brother to watch ULA launches, both of
which had incredible twilight effects. For this one I tried a new
place further south, the parking ramp of the hotel next to Ron Jon in
Cocoa Beach. While a bit further away, it has the advantage of being
six stories or so off the ground, to be able to see over the
vegetation of CCAFS. From where I was the booms hit a few moments
after landing and the glow dissipating, and it was a very distinct
double boom, though not as loud as Shuttle.
On Tue, Dec 22, 2015 at 3:22 PM, Steve Traugott <stevegt@xxxxxxx> wrote:
Visually and anecdotally, if I understood what I was seeing, while standing
at Port Canaveral (https://goo.gl/NqW5h5), the first stage boost was more
vertical than usual, MECO and staging was still nearly overhead, only
slightly out to sea (maybe 30 degrees off vertical, 100 km altitude, 60 km
downrange). I'm guessing someone here will jump in with better numbers.
I may be completely misunderstanding the flight profile, but following MECO,
there appeared to be a long wait (minutes) with the first stage in
free-fall, no visible light.
We then saw what looked like a boostback burn out there at about 45 degrees
-- I was struck by how short that burn appeared to be; a few seconds. (I
could have the sequencing wrong -- it makes sense to me that the boostback
should be before free-fall, not the other way around. But I remember it
after. Ben, did you note how that went?)
The boostback was followed by another long pause (a minute or so), and
another short burn straight overhead, I'm guessing to halt the uprange
velocity resulting from the boostback.
After that, we had a clear view up the tailpipes with the booster in
free-fall right over the Cape; I was struck by the fact that it looked like
they were keeping the fires lit at something like 10% throttle (I don't know
how deep the Merlins can actually go); just a flickering glow.
Finally, at only 30 degrees or so above the horizon, maybe 4 miles up, we
got throttle up, and a nice deceleration all the way to the pad -- looked
completely unreal. Hollywood.
After touchdown and the glow fading, there were a tense few seconds there,
some folks cheering and the rest of us waiting for a fireball... Right then
the sonic booms reached us, a rippling tear rather than a clear single or
double. I heard later that even Elon got tripped up by that -- the timing
of the shock waves was perfect for fooling the brain into thinking Something
Bad had happened. The cheers really started a few seconds later when we got
the video of the booster sitting solidly on its legs.
I'm still processing it. Beyond the rain, the humid Florida breeze,
standing there in the dark watching all this... The thing that most made it
surreal was the agility, the dynamic behavior of that vehicle. I grew up in
Florida and once lived near the Cape. Instead of the plodding, predictable,
near ballistic, conservative trajectory we've all been used to with large
vehicles, there was this huge heavy thing bouncing all over the sky. Big
rockets just don't act that way. ;-) Completely different world.
Steve
On Tue, Dec 22, 2015 at 10:17 AM, John Dom <johndom@xxxxxxxxx> wrote:
http://www.parabolicarc.com/2015/12/21/spacex-falcon-9-orbcomm2-mission-overview/#more-57125
Table in text: does boostback burn mean sort of a translation burn to the
pad?
jd
From: arocket-bounce@xxxxxxxxxxxxx [mailto:arocket-bounce@xxxxxxxxxxxxx]
On Behalf Of David McMillan
Sent: dinsdag 22 december 2015 18:42
To: arocket@xxxxxxxxxxxxx
Subject: [AR] Re: SpaceX F9 Launch/Update -- Live Link
Well, like they say, the best way to get a correct answer on the
internet... :)
On 12/22/2015 12:27 PM, Lars Osborne wrote:
David,
I was told by one of my coworkers that the simulation you posted is old.
This one is supposed to be more accurate (and way more detailed):
http://www.flightclub.io/results.php?id=0490d68b-62a1-4a2a-b39a-f47bacadc6e3&code=OG22
Thanks,
Lars Osborne
On Tue, Dec 22, 2015 at 9:11 AM, David McMillan <skyefire@xxxxxxxxxxxx>
wrote:
On 12/22/2015 12:04 PM, John Dom wrote:
As to the successful F9 v1.01 stage 1 return yesterday, again I’d love to
see the 3D graph (km scale) of the booster *return* trajectory. The gravity
turn to orbit as shown on ascent footage must have sent it far down from the
pad. How high did it fly? Range at separation?
Not entirely sure of the provenance, but here's one (2D) I came
across.