Author Topic: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion Thread 1  (Read 593360 times)

Offline Jarnis

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #460 on: 12/22/2017 02:59 PM »
Any estimates of the time between the static fire of Falcon Heavy (assuming successful) and the initial launch attempt?

1-2 weeks would be my guess. Zuma is I think January 4th and I don't think they're fully equipped for two east coast teams, so I do not think they can launch before that. Might not even get a static fire before that.

...and the next east coast one after that is very late in January, so... somewhere in between those, assuming everything "just works". If they see any issues during WDR and/or static fire, all depends on how long the fixes take.

Offline CyndyC

Been meaning to ask a (rather trivial) question but keep forgetting: does anyone know why some (but not all) of the engine bells on the left booster are shiny/reflective, while all the other engine bells are matte/sooty?  Any significance to that?

(Best seen from the overhead views)

SpaceX stopped shining the nozzles at some point and switched to a matte finish.  Since they are reusing old boosters, the full switch must have happened between the original launch dates for the side cores.  Before they went to all engines on a booster matte, they experimented with a mix of both.  Well, first I think they experimented with a few only partially shined bells (they looked like they had stripes running up and down), then went forward incrementally from there.

I did some detective work last night while Abaddon's original post was the last one just sitting there for hours. First of all you can tell the shiny bells are not brand new replacements because the larger photo of the underside shows TEA/TEB trails inside every bell. Next, it's clear the shiny bells are on core 23 which was the Thaicom 8 booster, one of the earliest successful landings at #4 and the "Leaning Tower of Thaicom". Yes SpaceX was removing & replacing bells early on, adding ample opportunity to clean a few before a decision that cleaning the rest was unnecessary, or just to give an intern or someone with some downtime something to work on temporarily, but take your pick from a wealth of landing videos. It looks like those bells might have come in that clean, or cleaner enough to wipe down more easily than the others. My guess is the shiny bells were on the high side opposite the crushed leg and residual flaming, and the sooty exception inbetween them was part of the 1-3-1 landing burn.
« Last Edit: 12/22/2017 04:56 PM by CyndyC »
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Offline envy887

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #462 on: 12/22/2017 04:47 PM »
What do we think the biggest risks are for this mission? If I had to guess, in order:

1. Side booster sep
2. Liftoff
3. MaxQ
4. Faring sep
5. S/C sep

Anyone else?

So, in other words, everything except engine startup.  I have to put engine startup at the top of the list.

They will be testing that in the static fire, probably starting with a few engines and building up as they get some data. If it works there it's low risk for the launch.

Offline CyndyC

What do we think the biggest risks are for this mission? If I had to guess, in order:

1. Side booster sep
2. Liftoff
3. MaxQ
4. Faring sep
5. S/C sep

Anyone else?

So, in other words, everything except engine startup.  I have to put engine startup at the top of the list.

They will be testing that in the static fire, probably starting with a few engines and building up as they get some data. If it works there it's low risk for the launch.

From Chris Gebhardt's latest FH article dated Nov 1st at https://www.nasaspaceflight.com/2017/11/spacex-aims-december-launch-falcon-heavy/

Quote
Due to thrust torque (a thrust-induced rotation) scenario that could destroy or severely damage the octawebs at the base of each Falcon core that house the Merlin 1D engines, the Falcon Heavyís 27 engines will not be lit simultaneously like the Falcon 9 engines. Instead, Falcon Heavy will take a page from the Space Shuttleís book and will employ a staggered start sequence Ė like main engine start on Shuttle where each main engine started 120 milliseconds after the previous. For Falcon Heavy, it is understood that two engines will be lit simultaneously followed by the next twoÖ and so on until all 27 are up and running.
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Offline Paul_G

Due to the turbulence produced by the nose cone of the side boosters while it is traveling backwards more control authority is needed by the side booster than the centre stage. Hence they have the larger Ti grid fins.

This I can't picture. The tails of aircraft aren't open cylinders, they taper to points.
I get that the two are different and have different control responses. But why does the nosecone produce
worse turbulence? Is it not long enough or something?

Edit:  I guess this belongs in an FH thread.

My guess - the open cylinder of a standard F9 stage creates drag, that helps keeps the stage pointing the right way. The cone ends of the side boosters do not create so much drag, meaning the engine end could end up being more 'dragy' than the top of the stage. Larger grid find help create the drag to keep the side boosters facing the right way?

Paul

Online cppetrie

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #465 on: 12/22/2017 06:02 PM »
Due to the turbulence produced by the nose cone of the side boosters while it is traveling backwards more control authority is needed by the side booster than the centre stage. Hence they have the larger Ti grid fins.

This I can't picture. The tails of aircraft aren't open cylinders, they taper to points.
I get that the two are different and have different control responses. But why does the nosecone produce
worse turbulence? Is it not long enough or something?

Edit:  I guess this belongs in an FH thread.

My guess - the open cylinder of a standard F9 stage creates drag, that helps keeps the stage pointing the right way. The cone ends of the side boosters do not create so much drag, meaning the engine end could end up being more 'dragy' than the top of the stage. Larger grid find help create the drag to keep the side boosters facing the right way?

Paul
There was a fairly lengthy discussion in L2 when the design and size of the new grid fins was first discovered. Iím was not (and still am not) very knowledgeable on the topic so I found the explanation counterintuitive. My laymanís summary is that the grid fins require turbulent airflow through them to work. The open cylinder of the interstage creates said turbulent airflow. The smooth nose cone creates less turbulent flow and decreases the effectiveness of the grid fins, which is why the side boosters require larger ones with scalloped leading edges that result in greater control authority. Iíve probably oversimplified the explanation so if someone more knowledgeable wants to chime in go ahead, but that was my take away. Itís counterintuitive to me and the physics of ailerons and tails on the back end of a cone-shaped plane operating at subsonic speed arenít applicable.

Offline Norm38

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #466 on: 12/22/2017 07:30 PM »
That makes sense, but the post I responded to implied that the nosecones produce more turbulence, not less.
At least, that's how I read it.

Offline CyndyC

There was a fairly lengthy discussion in L2 when the design and size of the new grid fins were first discovered. Iím was not (and still am not) very knowledgeable on the topic so I found the explanation counterintuitive. My laymanís summary is that the grid fins require turbulent airflow through them to work. The open cylinder of the interstage creates said turbulent airflow. The smooth nose cone creates less turbulent flow and decreases the effectiveness of the grid fins, which is why the side boosters require larger ones with scalloped leading edges that result in greater control authority. Iíve probably oversimplified the explanation so if someone more knowledgeable wants to chime in go ahead, but that was my take away. Itís counterintuitive to me and the physics of ailerons and tails on the back end of a cone-shaped plane operating at subsonic speed arenít applicable.

That seems logical, and so did a comment on SFN that said the larger surface area of an interstage helps provide more lift than a nosecone's, relieving the grid fins of part of their duty. So maybe say "stronger" turbulence & structural lift from an interstage to put the 2 ideas together if both are accurate.
« Last Edit: 12/22/2017 09:45 PM by CyndyC »
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Offline TripD

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #468 on: 12/22/2017 11:31 PM »
Quote
The open cylinder of the interstage creates said turbulent airflow.

This doesn't seem correct.  Since the engine end is leading on the final return, all airflow will have passed through the fins before experiencing the open end of the interstage.  Or did I miss the context?

Edit/  I didn't include the part of the quote about the effect on the fins.
« Last Edit: 12/22/2017 11:49 PM by TripD »

Offline zhangmdev

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #469 on: 12/22/2017 11:46 PM »
Remember this?

https://www.nasa.gov/centers/dryden/multimedia/imagegallery/Shuttle/EC05-0166-27.html

Having nose/tail cone to smooth the airflow and reduce drag is not necessarily good to landing.
« Last Edit: 12/22/2017 11:57 PM by zhangmdev »

Offline cscott

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #470 on: 12/22/2017 11:46 PM »
As I understand it, it's actually the fact that the smooth laminar airflow tends to *mostly* "stick" to the tapered nosecone --- but can detach suddenly, which causes a strong side force if/when it happens. What's worse, if the stage is allowed to "lean in" to the force generated, the airflow will reattach and then separate on the opposite side, leading to oscillation. (There are videos of this vortex-shedding oscillation floating around youtube, maybe even posted earlier in this thread.) It's "better" to have an open cylinder end, where the airflow detaches immediately in a predictable way.  Since the end can't be blunt on the booster when it's pointing up during ascent, instead the larger fins were added to ensure that they have sufficient control authority to counteract any vortex-shedding and prevent oscillation.

EDIT: the vortex-shedding oscillation is what caused the Tacoma narrows bridge to collapse, and I encountered an example in a walk near my house not too long ago (attached).
« Last Edit: 12/22/2017 11:55 PM by cscott »

Offline Herb Schaltegger

Quote
The open cylinder of the interstage creates said turbulent airflow.

This doesn't seem correct.  Since the engine end is leading on the final return, all airflow will have passed through the fins before experiencing the open end of the interstage.  Or did I miss the context?

In a very simplified nutshell (and admittedly it's been a couple decades-plus since undergrad aerodynamics), the open end creates turbulent flow around the now-aft end of the falling/descending stage body. The grid fins have marginal overall effect on this.  But as a result of that turbulent flow, the grid fins have greater general control of the attitude (and thus flight path) of the stage than they would otherwise. With a nosecone on the now-aft end of the falling/descending stage body, the nosecone is acting in essence as a streamlined tailcone. That results in vastly different aerodynamics as opposed to a stage with an open-ended blunt cylinder interstage.  SpaceX's analysis has found that as a result, the grid fins on the boosters must be larger and have greater overall control authority to handle the changed aerodynamics.
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Offline TripD

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #472 on: 12/23/2017 12:03 AM »
That seems straight forward.  I was actually responding to cppetrie and wondering if the turbulence from the open interstage could actually provide turbulence to the fins for better control.  This was all pertaining to the issue of laminar vs. turbulent flow through the fins.

Offline deruch

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #473 on: 12/23/2017 12:03 AM »
In a very simplified nutshell (and admittedly it's been a couple decades-plus since undergrad aerodynamics), the open end creates turbulent flow around the now-aft end of the falling/descending stage body. The grid fins have marginal overall effect on this.  But as a result of that turbulent flow, the grid fins have greater general control of the attitude (and thus flight path) of the stage than they would otherwise. With a nosecone on the now-aft end of the falling/descending stage body, the nosecone is acting in essence as a streamlined tailcone. That results in vastly different aerodynamics as opposed to a stage with an open-ended blunt cylinder interstage.  SpaceX's analysis has found that as a result, the grid fins on the boosters must be larger and have greater overall control authority to handle the changed aerodynamics.

I wonder how close the trade on jettisoning the nose cones post booster separation vs enhancing the gridfin system was.
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Online cppetrie

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #474 on: 12/23/2017 12:13 AM »
In a very simplified nutshell (and admittedly it's been a couple decades-plus since undergrad aerodynamics), the open end creates turbulent flow around the now-aft end of the falling/descending stage body. The grid fins have marginal overall effect on this.  But as a result of that turbulent flow, the grid fins have greater general control of the attitude (and thus flight path) of the stage than they would otherwise. With a nosecone on the now-aft end of the falling/descending stage body, the nosecone is acting in essence as a streamlined tailcone. That results in vastly different aerodynamics as opposed to a stage with an open-ended blunt cylinder interstage.  SpaceX's analysis has found that as a result, the grid fins on the boosters must be larger and have greater overall control authority to handle the changed aerodynamics.

I wonder how close the trade on jettisoning the nose cones post booster separation vs enhancing the gridfin system was.
My recollection from the discussion in L2 at the time was that having no nose cone didnít solve the problem because the location of the turbulent flow wasnít right when no cone or interstage was present. There were some ideas on having the cone open to extend the cylinder in a fashion to replicate the aero effects of the interstage but it appears SpaceX just elected to use larger fins to solve the problem.

Offline Space Ghost 1962

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #475 on: 12/23/2017 12:21 AM »
In a very simplified nutshell (and admittedly it's been a couple decades-plus since undergrad aerodynamics), the open end creates turbulent flow around the now-aft end of the falling/descending stage body. The grid fins have marginal overall effect on this.  But as a result of that turbulent flow, the grid fins have greater general control of the attitude (and thus flight path) of the stage than they would otherwise. With a nosecone on the now-aft end of the falling/descending stage body, the nosecone is acting in essence as a streamlined tailcone. That results in vastly different aerodynamics as opposed to a stage with an open-ended blunt cylinder interstage.  SpaceX's analysis has found that as a result, the grid fins on the boosters must be larger and have greater overall control authority to handle the changed aerodynamics.

The laminar airflow reconnects around the nose (now tail cone). The region close to the booster passing though the grid fins minimally deviates and so functions to lose control authority. The larger grid fin's area includes the more turbulent outer radius flow and so regains the lost control authority.

None of this matters after the stage goes transonic.

I wonder how close the trade on jettisoning the nose cones post booster separation vs enhancing the gridfin system was.
Passive system improvements have much lower risk - it may be that larger fins for the core as well might become standard for other reasons.

Active separation systems are another separation event to fail, thus more costly. Worst parts are the increased weight/drag of the passive improvement. Perhaps also flight rate/erosion might matter if they intend high frequency FH flight rate?

Also, they've assembled the vehicle faster than expected. Interesting to see if they are as quick to make it fit on the TE, and have fit checks erect on the launch mount.

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #476 on: 12/23/2017 01:11 AM »
An ejectable/jettisonable nose cone is another part that has to be replaced each time. More expensive grid fins were likely a clearly better trade.
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Offline zhangmdev

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #477 on: 12/23/2017 01:25 AM »
Will there be camera on the side booster? Consider where the nose cone begins, it is hard for a downward facing camera to observe those gird fins.

Offline old_sellsword

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #478 on: 12/23/2017 01:55 AM »
An ejectable/jettisonable nose cone is another part that has to be replaced each time. More expensive grid fins were likely a clearly better trade.

Not to mention the fact you canít even jettison the nosecone because the grid fins are inside it...

Offline deruch

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #479 on: 12/23/2017 02:10 AM »
An ejectable/jettisonable nose cone is another part that has to be replaced each time. More expensive grid fins were likely a clearly better trade.

Not to mention the fact you canít even jettison the nosecone because the grid fins are inside it...

They could have made a 2-part assembly with a base/barrel section that housed the fins and control systems and a detachable upper/cone section, but clearly they chose a different route.  I'm sure lots of things played into that discussion beyond just reusability.  I was wondering on if it was an easy/clear choice or a more "could have gone either way" one.  Given they're already flying a reusable system, the improved grid fins seem like an easy choice. 
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