Author Topic: Falcon 9 Q&A  (Read 40546 times)

Offline Jim

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Re: Falcon 9 Q&A
« Reply #60 on: 04/22/2021 10:29 pm »
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.
« Last Edit: 04/22/2021 10:33 pm by Jim »

Offline AS_501

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Re: Falcon 9 Q&A
« Reply #61 on: 04/22/2021 10:39 pm »
Thanks Jim.  Your breath and depth of knowledge is worth umteen times my NSF subscription price.
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Offline DaveS

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Re: Falcon 9 Q&A
« Reply #62 on: 04/22/2021 10:51 pm »
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.
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Offline ZachS09

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Re: Falcon 9 Q&A
« Reply #63 on: 05/24/2021 03:21 pm »
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?
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Offline Jim

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Re: Falcon 9 Q&A
« Reply #64 on: 05/24/2021 04:07 pm »
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.

Offline Lucjusz

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Re: Falcon 9 Q&A
« Reply #65 on: 11/03/2021 09:53 pm »
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.

Offline penguin44

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Re: Falcon 9 Q&A
« Reply #66 on: 09/17/2022 02:41 am »
With the recent weather scrubs of Falcon it got me wondering, is there a limit on how many tankings it can do?

Offline markbike528cbx

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Re: Falcon 9 Q&A
« Reply #67 on: 09/17/2022 05:08 pm »
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?

Offline AmigaClone

Re: Falcon 9 Q&A
« Reply #68 on: 09/17/2022 10:58 pm »
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.

Offline ddunham

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Re: Falcon 9 Q&A
« Reply #69 on: 01/16/2023 07:11 am »
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?

Offline edzieba

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Re: Falcon 9 Q&A
« Reply #70 on: 01/16/2023 11:32 am »
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).

Offline Citabria

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Re: Falcon 9 Q&A
« Reply #71 on: 01/31/2023 07:07 pm »
Does SpaceX ever put previously flown engines on unflown expendable cores?

Offline whitelancer64

Re: Falcon 9 Q&A
« Reply #72 on: 02/08/2023 05:59 pm »
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.
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Offline deltaV

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Re: Falcon 9 Q&A
« Reply #73 on: 03/28/2023 02:46 am »
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.
« Last Edit: 03/28/2023 02:59 am by deltaV »

Offline markbike528cbx

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Re: Falcon 9 Q&A
« Reply #74 on: 04/05/2023 07:43 pm »

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

Offline John Santos

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Re: Falcon 9 Q&A
« Reply #75 on: 04/05/2023 08:36 pm »

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.

Offline markbike528cbx

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Re: Falcon 9 Q&A
« Reply #76 on: 04/13/2023 07:04 pm »
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.

Quote
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.

 

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