SpaceX F9 : NROL-108 : KSC LC-39A : 19 December 2020 (14:00 UTC)

[FutureSpaceTourist]

https://twitter.com/superclusterhq/status/1340688909588721666

Sound up! SpaceX performs their final booster landing of 2020 at Cape Canaveral.

It was the company's 26th Falcon 9 flight of the year, setting a new annual record.

Produced by @erikkuna for Supercluster

[FutureSpaceTourist]

[wannamoonbase]

GO Searcher is back as well with a fairing.

It's exciting that the reliability in successfully recovering the fairing halves has improved recently.  Seems they have recovered almost all of them lately.

Such great savings, well done SpaceX. 

[Jansen]

GO Searcher is back as well with a fairing.

It's exciting that the reliability in successfully recovering the fairing halves has improved recently.  Seems they have recovered almost all of them lately.

Such great savings, well done SpaceX.

I wonder if they are flaring the parachutes more just before touchdown.

Here’s Scroochy’s excellent list if anyone wants a look: https://www.elonx.net/fairing-recovery-attempts/

[Bean Kenobi]

GO Searcher is back as well with a fairing.

It's exciting that the reliability in successfully recovering the fairing halves has improved recently.  Seems they have recovered almost all of them lately.

Such great savings, well done SpaceX. 

Most are water recoverings, not net recoverings.

[Alter Sachse]

51.35 degree inclined, 520 x 540 km orbit.

https://twitter.com/Marco_Langbroek/status/1340691309548474369

[FutureSpaceTourist]

SpaceX launch photos by Ben Cooper

[wannamoonbase]

GO Searcher is back as well with a fairing.

It's exciting that the reliability in successfully recovering the fairing halves has improved recently.  Seems they have recovered almost all of them lately.

Such great savings, well done SpaceX.

I wonder if they are flaring the parachutes more just before touchdown.

Here’s Scroochy’s excellent list if anyone wants a look: https://www.elonx.net/fairing-recovery-attempts/

That’s a great list, Thanks Scroochy!

Recent success is impressive, they must have still am improving the process.  $6 million a pair, they are really saving some serious money now and importantly enabling a high launch rate. 

[FutureSpaceTourist]

Wow, this is quite a milestone ...

https://twitter.com/launchphoto/status/1340750670165667840

250! Saturday's liftoff of NROL-108 was my 250th launch shot over 19 years. Check them all out here:
http://www.launchphotography.com/Launch_Photos.html

[gongora]

https://twitter.com/Zarya_Info/status/1340765152258498560

NRO L-108 launch

Carried two. satellites - USA 312 and USA 313

Catalogue numbers and designations confirmed

Notes:
https://zarya.info/2020#101

[Star One]

https://twitter.com/stromgade/status/1340700071239757826

Seems to complement USA 305-308 nicely

[LouScheffer]

Here's a closer look at the telemetry for the landing burn.  A couple of things stand out:  First, as the landing burn starts (vertical purple line), the booster starts to fall faster, not slower.   This is likely since it pitches vertical and hence offers less air resistance.   This implies they could (in theory) start the landing burn lower, and let the atmosphere do even more of the deceleration work as Starship does, then flip vertical at the last moment.  I suspect they don't do this since the grid fins given them only limited control, so they need the longer landing burn to drive the residual errors down.

Also, SpaceX telemetry cuts off 18 seconds before landing.  However we can add one more point - at 498 seconds, it contacts the pad at 0 altitude and 0 velocity.  When we add this point we see the booster averages almost exactly -1G over the last 18 seconds.   This makes sense - the booster is thought to mass about 27 tonnes, so that would be 63% throttle for an 845,000N engine.  That gives room for adjustment in both directions.  So it could be the desired landing trajectory is very simple:  -1G straight down, reaching 0 speed at pad altitude, with 0 error in X and Y, and 0 body rates.   Then a more-or-less straightforward feedback loop sets the engine throttle and gimbals to achieve this.  Meanwhile the RCS drives the roll rate to 0 (the actual roll angle does not seem important).  This is all classical Newtonian physics and should be quite straightforward.

The hard and empirical part would be orienting the grid fins to get the booster into the right set of starting conditions so the landing burn has enough control authority to perform the landing.  We saw this directly on one mission where stale wind estimates caused the grid fin coast phase to not get the booster into a state where the landing burn could remove the remaining errors.   Then when the landing burn saw it did not have enough control authority to land, it diverted so as not to harm the barge.

[Herb Schaltegger]

Here's a closer look at the telemetry for the landing burn.  A couple of things stand out:  First, as the landing burn starts (vertical purple line), the booster starts to fall faster, not slower.   This is likely since it pitches vertical and hence offers less air resistance.

Not just lower drag, but LESS LIFT. The booster stage is quite literally flying back to its landing zone at an appreciable angle of attach with a L/D > 0. As the engine starts and the stage pitches bottom-down, lift drops to zero nearly instantly.

[Jansen]

The fairings were offloaded this morning and sent off for refurbishment.

[LouScheffer]

Still more fiddling with the telemetry and re-entry video.   From the video, you can see that the aim stabilizes at a spot between the skid strip and the parallel road, marked on the map below.   This is about 2 km past the pad.  Then, as the booster passes directly above the pad, at an altitude of about 4.7 km, it pivots (quite quickly) to a straight vertical descent.   This looks like a safe aiming strategy as there is nothing between the aim point and the pad, so a failure during landing won't clobber anything important.

The glide slope is therefore atan(4.7/2) = 67 degrees from horizontal, or 23 degrees from vertical, or a glide ratio of 0.42 (0.42 km sideways for every km it drops).  Presumably the rocket can do better than this, as it will need margin around this value to allow for aiming.

[Michael Cassutt]

A bit of a bump here, but for NRO sleuths--

In October 2021 NRO Director Scolese, in a speech at a GEOINT conference, stated that the agency had launched two new intel satellites in the past year, both of them "demos". Further, that both had been developed in the last three years -- and were now providing useful data on North Korea, Afghanistan and other regions.

NROL-101 is an obvious candidate, especially for some kind of GEOINT/SAR vehicle . . . and possibly NROL-108, launched a month earlier but into a much higher, rarely seen orbit (11,000 km) so who knows what it might be.

Anyone possessed of new insight or speculation?

Michael Cassutt