Falcon 9 Q&A

[Jim]

I'm puzzled why Falcon 9 throttles down through Max Q.  I can understand why Falcon 9 Heavy (as well as Shuttle, Delta IV Heavy, etc.) does this to reduce aerodynamic loads on the booster attach points.  But F9 is single stick.  My best guess is that the vehicle velocity approaching Max Q dictates the need for throttle down.  Or this is required by Dragon, not the booster?  Or if Space X built Dragon and Falcon 'beefy' enough to endure full Max Q, there would be a weight penalties.

Sidebar coments:
-  I just watched the last Delta IV Medium (two SRBs) launch.  Didn't hear an RS-68A throttle down comment.
-  I think the Shuttle was first vehicle to utilize throtte down simply because it had the first throttleable engines.
-  The original Atlas rocket is an interesting case.  It was famous (or infamous!) for it's thin-skin construction.  As far as I know it survived Max Q every time with its fixed-thrust engines.
Thanks

Delta IV and Atlas V (and so will Vulcan) also throttle down.  It has nothing to do with "booster attach points", but rather loads on the vehicle and payload.  The shuttle had an additional reason because of the aerosurfaces.  Throttling down prevents the vehicle from accelerating too fast through the lower atmosphere and keeps max q to less than 1000 psf (I think around 700, the same as the Saturn V).    Throttling also reduces the max g which is usually just before core burnout. 

Classic Atlas and Saturn V had lower T/W and accelerated slower through the lower atmosphere.

[AS_501]

Thanks Jim.  Your breath and depth of knowledge is worth umteen times my NSF subscription price.

[DaveS]

keeps max q to less than 1000 psf (I think around 700, the same as the Saturn V).

The 700 lbs/ft² Max Q came later for shuttle. It was a Performance Enhancement. Earlier was around 670/ft².

[ZachS09]

Why does the SpaceX Launch Director give his GO for every Falcon launch at T-45 seconds? Shouldn’t it be earlier in the count?

Also, I noticed before the Falcon Heavy Test Flight, the Launch Director gave his GO at T-25 seconds. That’s cutting it close, right?

[Jim]

Why does the SpaceX Launch Director give his GO for every Falcon launch at T-45 seconds? Shouldn’t it be earlier in the count?

Also, I noticed before the Falcon Heavy Test Flight, the Launch Director gave his GO at T-25 seconds. That’s cutting it close, right?

The range gives him a go late in the count.    As long as there is a place to safely stop the count, it isn't too late.

[Lucjusz]

Hi,
is there any paper/video on YouTube/anywhere else describing what happens if you rotate one/two/all grid fins on Falcon 9 booster? Any video/paper about control authority? I'm just wondering what would have happened if you rotated for example two opposite fins, etc.

[penguin44]

With the recent weather scrubs of Falcon it got me wondering, is there a limit on how many tankings it can do?

[markbike528cbx]

With the recent weather scrubs of Falcon it got me wondering, is there a limit on how many tankings it can do?

Since Elon has said there are no obvious limits to Falcon 9 reuse, then tankings should be included.
The number is high, but not infinite. My guesstimate is about 100?

[AmigaClone]

With the recent weather scrubs of Falcon it got me wondering, is there a limit on how many tankings it can do?

Since Elon has said there are no obvious limits to Falcon 9 reuse, then tankings should be included.
The number is high, but not infinite. My guesstimate is about 100?

I can see the design limits to be above 100 tankings. On the other hand, I can see a much lower number (10?) for the number of times SpaceX would fill the Falcon 9 with propellants and detank before pulling the rocket down to conduct some inspections.

[ddunham]

It appears the external cameras on the first stage have some method of clearing the view after entry burn fouling.  I can think of a few ways this might be accomplished, but not sure what is used.  Rotating shield that moves to a clean spot?  That would require a servo or similar which seems odd to me.  Is there a reference for how this is done?

[edzieba]

It appears the external cameras on the first stage have some method of clearing the view after entry burn fouling.  I can think of a few ways this might be accomplished, but not sure what is used.  Rotating shield that moves to a clean spot?  That would require a servo or similar which seems odd to me.  Is there a reference for how this is done?

No cleaning methods other than the supersonic airflow blasting the camera fairing window that can impinge once the burn ends.
The 'wipe' is shadowing and internal glare from the vehicle moving relative to the sun. The effect of the airflow is most visible for RTLS landings (as the trajectory is more vertical the the time between end of the entry burn and hitting the denser lower atmosphere at speed is much shorter than for the shallower downrange landings) and more dramatic for Falcon Heavy due to the heavy fouling from the separation and boostback (the deposits are visible long before the entry burn starts).

[Citabria]

Does SpaceX ever put previously flown engines on unflown expendable cores?

[whitelancer64]

Does SpaceX ever put previously flown engines on unflown expendable cores?

I presume you are really asking if SpaceX puts older engines they are willing to lose onto new, expended Falcon Heavy cores. There is no way to know this outside of someone at SpaceX explicitly saying they installed older, previously flown engines on an expended core.

However, SpaceX does engine swaps all the time, and there is no particular reason why a previously flown engine could not be installed on a new booster. The Merlin engines are effectively interchangeable and can be installed on any first stage.

Unless the customer specifies that they want completely new engines to fly on their booster, SpaceX can assign engines to that launch as they please.

[deltaV]

Why does NASA Launch Services Program's Launch Vehicle Performance Website (https://elvperf.ksc.nasa.gov/) not show Falcon performance for many orbits? For example the orbits (i) 200 km 28.5 degree LEO and (ii) C3 = -2 km^2 / s^2 high energy show performance numbers for other launchers but not for Falcon 9 or Falcon Heavy.

Edit: Falcon 9 and Falcon Heavy do show up for some orbits, e.g. C3 = -1.5 km^2 / s^2.

[markbike528cbx]

What is the use of a entry burn for RTLS, especially FH boosters?

The RTLS entry burn start speed (4000km/hr) is less than the ASDS (barge) landing entry burn end speed (~6000km/hr).


Info I've gathered from webcast telemetry.

RTLS One Web 16  Jan 9 2023
Payload mass kg   5900      40x 147.5 = 5900 plus dispenser (1000kg??)

Entry Burn start speed 4473
Entry Burn end speed  2926

FH side booster RTLS  USSF-67  Jan 2023
Payload mass kg  to GEO

Entry Burn start speed 4472
Entry Burn end speed  3690

F9 RTLS ISI EROS C-3 Dec 29 2022 Vandenburg
Payload mass kg 470

Entry Burn start speed  4651
Entry Burn end speed   2616



F9 ASDS  Starlink  Dec 17th 2022  Cape
Payload mass kg

Entry Burn start speed  8152
Entry Burn end speed   6074   

Distance to ASDS km est 656

F9 ASDS  Starlink 5-1  Dec 28th 2022  Cape
Payload mass kg 16200   54x300kg Starlink 1.5

Entry Burn start speed  8026
Entry Burn end speed   5627

Distance to ASDS km  660

[John Santos]

What is the use of a entry burn for RTLS, especially FH boosters?

The RTLS entry burn start speed (4000km/hr) is less than the ASDS (barge) landing entry burn end speed (~6000km/hr).


Info I've gathered from webcast telemetry.

RTLS One Web 16  Jan 9 2023
Payload mass kg   5900      40x 147.5 = 5900 plus dispenser (1000kg??)

Entry Burn start speed 4473
Entry Burn end speed  2926

FH side booster RTLS  USSF-67  Jan 2023
Payload mass kg  to GEO

Entry Burn start speed 4472
Entry Burn end speed  3690

F9 RTLS ISI EROS C-3 Dec 29 2022 Vandenburg
Payload mass kg 470

Entry Burn start speed  4651
Entry Burn end speed   2616



F9 ASDS  Starlink  Dec 17th 2022  Cape
Payload mass kg

Entry Burn start speed  8152
Entry Burn end speed   6074   

Distance to ASDS km est 656

F9 ASDS  Starlink 5-1  Dec 28th 2022  Cape
Payload mass kg 16200   54x300kg Starlink 1.5

Entry Burn start speed  8026
Entry Burn end speed   5627

Distance to ASDS km  660

Entry burns are to reduce velocity and therefore aerodynamic heating of the booster during re-entry.  This is needed whether the booster is heading for a drone ship landing or an RTLS landing.  The altitude and duration of the entry burn is carefully calculated to use the least possible amount of fuel while keeping the heating of the rocket below the safe limit.

[markbike528cbx]

The previous answer deftly avoided the question.

WHY does Spaces even bother with a RTLS entry burn since the ENDING velocity of an ASDS is greater than the START velocity of an RTLS entry burn.

The altitude and duration of the entry burn is carefully calculated to use the least possible amount of fuel while keeping the heating of the rocket below the safe limit.

This doesn't hold water since the entry burn time is identical and altitude  is very close between RTLS and ASDS.



                                                                          Entry Burn start   Entry burn end   Entry burn time   Altitude at End of Entry burn
                                                                       km/hr                      km/hr                       seconds                km
F9 RTLS One Web 16  Jan 9 2023                        4473                       2926                            19                      32.4
FH side booster RTLS  USSF-67  Jan 2023             4472                     3690                            21                       34.5
F9 RTLS ISI EROS C-3 Dec 29 2022 Vandy             4551                    2616                            21                       31
Average RTLS                                                     4499                    3077                             20                       33
            
F9 ASDS  Starlink  Dec 17th 2022  Cape                 8155                       6074                            18                      36.1
F9 ASDS  Starlink 5-1  Dec 28th 2022  Cape           8026                       5627                            23                      41.6
F9 ASDS Starlink  January 19 Vandy                       8110                       5843                            19                      35.8
Average ASDS                                                     8097                        5848                            20                      38
            
Difference                                                             3598                        2771                               0                        5


Entry burn starts range from 6:07 to 6:51 with RTLS starts tending to be a little earlier.

Speculation on reasons for this, in the hopes of invoking Cunningham's Law https://en.wiktionary.org/wiki/Cunningham%27s_Law

Commonality of Operations?
Doesn't make sense, SpaceX seems to have a pretty darn good grasp on writing real-time adaptive control software.

Vehicle control ? 
But the same question applies to the relative ENDING velocity.

RTLS is probably more nearly vertical than ASDS?

Perhaps just making the later entry less stressfull for RTLS, as in you might be able to have lesser structural and heating loads. 

Reducing cold-gas RCS thruster requirements?

An on a non-sequitor note:  Does anybody have suggestions for a good way to insert spreasheet tables that you don't have to  hand-tweek?  A PM and or a pointer to an answering thread would be appreciated.

[duh]

The previous answer deftly avoided the question.

WHY does Spaces even bother with a RTLS entry burn since the ENDING velocity of an ASDS is greater than the START velocity of an RTLS entry burn.

The altitude and duration of the entry burn is carefully calculated to use the least possible amount of fuel while keeping the heating of the rocket below the safe limit.

This doesn't hold water since the entry burn time is identical and altitude  is very close between RTLS and ASDS.



                                                                          Entry Burn start   Entry burn end   Entry burn time   Altitude at End of Entry burn
                                                                       km/hr                      km/hr                       seconds                km
F9 RTLS One Web 16  Jan 9 2023                        4473                       2926                            19                      32.4
FH side booster RTLS  USSF-67  Jan 2023             4472                     3690                            21                       34.5
F9 RTLS ISI EROS C-3 Dec 29 2022 Vandy             4551                    2616                            21                       31
Average RTLS                                                     4499                    3077                             20                       33
            
F9 ASDS  Starlink  Dec 17th 2022  Cape                 8155                       6074                            18                      36.1
F9 ASDS  Starlink 5-1  Dec 28th 2022  Cape           8026                       5627                            23                      41.6
F9 ASDS Starlink  January 19 Vandy                       8110                       5843                            19                      35.8
Average ASDS                                                     8097                        5848                            20                      38
            
Difference                                                             3598                        2771                               0                        5


Entry burn starts range from 6:07 to 6:51 with RTLS starts tending to be a little earlier.

Speculation on reasons for this, in the hopes of invoking Cunningham's Law https://en.wiktionary.org/wiki/Cunningham%27s_Law

Commonality of Operations?
Doesn't make sense, SpaceX seems to have a pretty darn good grasp on writing real-time adaptive control software.

Vehicle control ? 
But the same question applies to the relative ENDING velocity.

RTLS is probably more nearly vertical than ASDS?

Perhaps just making the later entry less stressfull for RTLS, as in you might be able to have lesser structural and heating loads. 

Reducing cold-gas RCS thruster requirements?

An on a non-sequitor note:  Does anybody have suggestions for a good way to insert spreasheet tables that you don't have to  hand-tweek?  A PM and or a pointer to an answering thread would be appreciated.

This bring up from a different perspective discussed elsewhere on this site (don't ask me -- don't remember where) when comparisons have been made with Falcon 9 launches as whether near launch time the earth's surface (the launch pad ) is the reference point but in the last portion of the rocket burn fixed inertial space is the reference. Rephrase that as a prograde equatorial circular orbit would have the same velocity as a retrograde equatorial circular orbit in a fixed inertial orbit (like astronomers use) for mapping the sky, but the velocity would be significantly higher for the earth based attached rotating reference system for the retrograde orbit (add the earth's surface velocity [use vector arithmetic please] and subtract the earth's surface velocity for the earth's  surface velocity [again using vector arithmetic] for the prograde orbit.  {EDITED THIS SENTENCE]

Just a guess on my part but this thread has slept long enough (says I) or not (is that what the screaming in the background is about?)

[penguin44]

I thought RTLS used most propellant for the boostback and then entry burn to use the remaining fuel or it would be too heavy on landing.