SpaceX FH : Falcon Heavy Demo : Feb 6, 2018 : Discussion Thread 2

[the_other_Doug]

Anyone claiming to eyeball video evidence of overshoot or undershoot is fooling themselves.

Your reply reminded me of the old Marx Brothers joke:  ”Who are you going to trust, me or your lying eyes?" 
https://tinyurl.com/y9vm6b8x

So, included is a version with lines added.  The boosters did come in on a path that would have them overshoot a few hundred feet if they came in ballistically. I believe my eyes, if you don’t fine.



Admittedly, the grid fins would do as much as possible to try to keep the booster from overshooting, so the impact point wold not be as far out as projected. But I think it is highly improbable that they could maneuver the booster enough in a few seconds to avoid overshooting altogether. Doesn’t need that kind of maneuverability for the flight phases the fins are designed for. Also, in the case of a planned 1-3-1 landing burn where the outer two do not ignite, the center engine thrust vector would also help to reduce the overshoot.

But there is definitely a ballistic overshoot path when coming in for the landing burn (and this should not be confused with the intentional offshore undershoot that the boostback burn aims for, which is extended across the shore after the re-entry burn, by the grid fin aerodynamic steering).

And again, for the Xth time, this is probably why the center core missed the ASDS by a few hundred feet or whatever. Or do you have some alternative information on why the center core missed the ASDS?

The biggest problem with your "observed" trajectory, as noted on the image above, is that it assumes the stage is heading in directly along its X-axis.  It's not.  It's vector is about one-thirds broadsides to the side of the booster.  It's heading down much moreso than you indicate.

Run it in motion and you'll see what I mean.  Heck, run any of the RTLS boosters coming back in -- they all show the vapor trail from the still-hot and still-venting engines being pushed back, by aerodynamic forces, and away from the booster at about a 30-degree angle from the lee-side of the booster.  Hence, showing the side of the booster about one-thirds broadside to its movement vector.  SpaceX has even stated they get a certain amount of lift from using this angle of attack, taken throughout its return flight.

And finally, I don't believe that SpaceX would *ever* gain permission to do RTLS on the boosters if they targeted them to land ballistically on in amongst CCAFS territory if the landing burn fails to ignite.  If that was the default targeting, they would still be limited (and rightfully so) to landing on barges.

[AJW]

I think the on-board camera may give a better impression of where the stages are heading before and after the landing burn starts.   The approaches may have changed since Thaicom 8, but it sure looks like it is away from the ASDS. (at 0:20 press space bar and '.' or ',' to step forward and back)   

The CRS-12 landing from the on-board camera also looks like it is headed towards the outside of the landing zone before adjusting for the center.  The cores are not on true ballistic trajectories, since they are constantly adjusting the course with the grid fins and this may make it impossible to make a definitive call on just where they would land if the landing burn didn't start.  The grid fins might also be programmed to steer for the target regardless of whether the engine(s) start.

More questions than answers.

[catdlr]

Emphasis mine ... if you follow the path then you zero horizontal when you zero vertical.

No, because the ballistic trajectory isn't vertical at landing. There's still a small horizontal component.

Maybe this will work for the above discussion.  I did see another of these from a side angle that I'm trying to relocate.

https://www.instagram.com/p/Be3ypymhkM0/

https://i.imgur.com/D9BdO86.png

https://www.dailybreeze.com/wp-content/uploads/migration/2015/201512/NEWS_151229939_AR_0_HIEOSEPAZIWJ.jpg?w=620

http://c8.alamy.com/comp/JA1G8E/cape-canaveral-united-states-of-america-03rd-june-2017-the-spacex-JA1G8E.jpg

[Lars-J]

And finally, I don't believe that SpaceX would *ever* gain permission to do RTLS on the boosters if they targeted them to land ballistically on in amongst CCAFS territory if the landing burn fails to ignite.  If that was the default targeting, they would still be limited (and rightfully so) to landing on barges.

Your belief does not matter... Look at the video again: https://www.youtube.com/watch?time_continue=13&v=Z_kfM-BmVzQ

The boosters are already over land - *and heading inland* when the landing burn ignites. I just don't understand why this simple reality is so difficult to accept.

It's like someone speculated that "surely the IIP would always be over water", and then everyone just assumes it is true by group think. It is demonstrably not true! No one from either SpaceX or the Air Force has *EVER* asserted this, yet people are ignoring evidence right in front of them in order to keep believing it. Unreal.

EDIT: I'm adding my composite again that indicates motion vectors. And the top booster has just ignited its engine a few frames prior, again over land.

EDIT2: Attaching long exposure image that catdlr referred to in the post after mine

[catdlr]

Emphasis mine ... if you follow the path then you zero horizontal when you zero vertical.

No, because the ballistic trajectory isn't vertical at landing. There's still a small horizontal component.

Maybe this will work for the above discussion.  I did see another of these from a side angle that I'm trying to relocate.

https://www.instagram.com/p/Be3ypymhkM0/

https://i.imgur.com/D9BdO86.png

https://www.dailybreeze.com/wp-content/uploads/migration/2015/201512/NEWS_151229939_AR_0_HIEOSEPAZIWJ.jpg?w=620

http://c8.alamy.com/comp/JA1G8E/cape-canaveral-united-states-of-america-03rd-june-2017-the-spacex-JA1G8E.jpg

and here is the one I was looking for:  https://www.flickr.com/photos/spacex/39337245575/in/dateposted/

[Kabloona]

And finally, I don't believe that SpaceX would *ever* gain permission to do RTLS on the boosters if they targeted them to land ballistically on in amongst CCAFS territory if the landing burn fails to ignite.  If that was the default targeting, they would still be limited (and rightfully so) to landing on barges.

Your belief does not matter... Look at the video again: https://www.youtube.com/watch?time_continue=13&v=Z_kfM-BmVzQ

The boosters are already over land - *and heading inland* when the landing burn ignites. I just don't understand why this simple reality is so difficult to accept.

It's like someone speculated that "surely the IIP would always be over water", and then everyone just assumes it is true by group think. It is demonstrably not true! No one from either SpaceX or the Air Force has *EVER* asserted this, yet people are ignoring evidence right in front of them in order to keep believing it. Unreal.

Agree with Lars-J on this.

@the_other_Doug, I think the key point made upthread is the distinction between the ballistic IIP  *before* aerodynamic guidance kicks in, which is indeed going to be an undershoot of land.

But *after* aerodynamic guidance kicks in, the stage is side-slipping aerodynamically towards land, as you pointed out. And I believe Lars-J's video analysis shows that, just before landing burn start, the stage would be overshooting the pad due to that aerodynamically-influenced flight path. Not a large overshoot, but just beyond the center of the pad.

And the landing burn appears to correct for that slight aerodynamically-induced overshoot.

So the Air Force can be satisfied that, if both aero steering and landing burn fail, the stage hits water. But after aero steering kicks in, even if the landing burn fails, the aero-induced overshoot is small enough (worst case)  that the stage would impact a safe area. And presumably the grid fins continue to steer towards the pad, reducing or eliminating the minor overshoot even without engine thrust.

[catdlr]

SpaceX Tesla Roadster - orbit raw-simulation

AstronautiCAST
Published on Feb 14, 2018


Ephemerides courtesy of JPL HORIZONS Web-Interface
Simulation software Starry Night 7
Author Riccardo Rossi (ISAA)

[Lars-J]

Additional evidence... I used the Microsoft "image composite editor" to generate a panorama from the video (*), and it also allows you to mark areas of the images as "keep". This allowed me to do a better job illustrating the booster paths:

(* - Microsoft ICE is pretty neat, if you have a video where the videographer is not moving but simply panning with the camera, it can generate great panoramas)

[IntoTheVoid]

One can be certain that the boosters will fall short of where the engines are pointing. If the booster were traveling in the direction of the engines then there would be zero lift, and if the engines were pointed short of the ballistic trajectory the lift would be negative, both of which hurt your propellant requirements. The lift required is only generated if the booster is laid on it's side further than it's direction of travel and will always fall short of where it's pointing.

[LastStarFighter]

And finally, I don't believe that SpaceX would *ever* gain permission to do RTLS on the boosters if they targeted them to land ballistically on in amongst CCAFS territory if the landing burn fails to ignite.  If that was the default targeting, they would still be limited (and rightfully so) to landing on barges.

Your belief does not matter... Look at the video again: https://www.youtube.com/watch?time_continue=13&v=Z_kfM-BmVzQ

The boosters are already over land - *and heading inland* when the landing burn ignites. I just don't understand why this simple reality is so difficult to accept.

It's like someone speculated that "surely the IIP would always be over water", and then everyone just assumes it is true by group think. It is demonstrably not true! No one from either SpaceX or the Air Force has *EVER* asserted this, yet people are ignoring evidence right in front of them in order to keep believing it. Unreal.

Agree with Lars-J on this.

@the_other_Doug, I think the key point made upthread is the distinction between the ballistic IIP  *before* aerodynamic guidance kicks in, which is indeed going to be an undershoot of land.

But *after* aerodynamic guidance kicks in, the stage is side-slipping aerodynamically towards land, as you pointed out. And I believe Lars-J's video analysis shows that, just before landing burn start, the stage would be overshooting the pad due to that aerodynamically-influenced flight path. Not a large overshoot, but just beyond the center of the pad.

And the landing burn appears to correct for that slight aerodynamically-induced overshoot.

So the Air Force can be satisfied that, if both aero steering and landing burn fail, the stage hits water. But after aero steering kicks in, even if the landing burn fails, the aero-induced overshoot is small enough (worst case)  that the stage would impact a safe area. And presumably the grid fins continue to steer towards the pad, reducing or eliminating the minor overshoot even without engine thrust.

Agreed. I always thought this graphic told that exact story.

http://spacenews.com/wp-content/uploads/2015/04/SpaceX_falcon_landing_graphic-879x485.gif

[Lars-J]

One can be certain that the boosters will fall short of where the engines are pointing. If the booster were traveling in the direction of the engines then there would be zero lift, and if the engines were pointed short of the ballistic trajectory the lift would be negative, both of which hurt your propellant requirements. The lift required is only generated if the booster is laid on it's side further than it's direction of travel and will always fall short of where it's pointing.

Right. But the whole question being debated is where people feel that the booster would land if the engines *failed* to start. They would indeed fall in a parabolic arc, but a longer arc than one with a landing burn. Burning along your direction of travel (a reverse gravity turn) is the most optimal use of propellant, just as a gravity turn is the most optimal use of propellant during ascent.
 
But the point with my last image was to prove that the IIP (instantaneous(?) impact point) at the time landing burn ignition was on land, because they were even right above the beach when the landing burn started.

Perhaps this one as well?

Not a SpaceX graphic. It is again using assumptions that appear to be false.

[IntoTheVoid]

One can be certain that the boosters will fall short of where the engines are pointing. If the booster were traveling in the direction of the engines then there would be zero lift, and if the engines were pointed short of the ballistic trajectory the lift would be negative, both of which hurt your propellant requirements. The lift required is only generated if the booster is laid on it's side further than it's direction of travel and will always fall short of where it's pointing.

Right. But the whole question being debated is where people feel that the booster would land if the engines *failed* to start. They would indeed fall in a parabolic arc, but a longer arc than one with a landing burn. Burning along your direction of travel (a reverse gravity turn) is the most optimal use of propellant, just as a gravity turn is the most optimal use of propellant during ascent.

The booster was never retrograde orbital. The booster starts (on ascent) east, and throughout the ascent the IIP is east of the launch site. If you ever drive the IIP west of the intended landing site, then you have wasted propulsion in the horizontal direction. Sure, on the way down, gravity is working to draw the IIP further east, but the upward component of the engine(s) counters a portion of that and extents the flight time compared to a ballistic flight. The extended flight time allows the horizontal velocity to act longer and moves the IIP west (in the dir of travel) as compared to ballistic flight.

[Hominans Kosmos]

We've seen that SpaceX trajectory graphic (in docmordrid's post) before, so that's how I imagined the trajectory design. But Lars-J's photo analysis upthread seems to show that, without the landing burn, there would be some overshoot of the pad.

It's possible that they're getting more horizontal component of lift due to improved grid fin control authority that enables a significant angle of attack from the rocket body, moreso than they expected when that graphic was made.

In which case, they may have changed their trajectory design to get more cross-range velocity from the aerodynamics, which then would need to be canceled by the horizontal component of the landing burn.

Not necessarily a mistake, between the re-entry burn and landing burn the aerodynamic flight switches the trajectory from an inaccurate undershoot to a very precise overshoot to line up for the final burn. The whole debate here is about the last two pixels of trajectory in the company diagram.

[chuck34]

I am going to make a lot of assumptions here, so please forgive me that.  But I think a lot of people are over complicating this.  As the booster comes back it has vertical and horizontal velocities.  A simplification would be to think about the vertical component more like time.  So now you have horizontal velocity and time.  Therefore at the start of the landing burn lets take nice round numbers and say there is 1 second to impact and 10 meters per second horizontal velocity.  That means the impact will be 10 meters from this start point (short of the pad).  But now we have killed some of that vertical velocity, basically buying time.  So now we have 2 seconds to impact.  Therefore the stage will now be 20 meters from the initial point (nicely over the pad). 

So at the start of the burn the stage must be undershooting.  It's only going to make to 10 more meters if you don't buy time.  But the burn buys time, letting the horizontal velocity drift you over the pad.  Seems fuel efficient that way.  If you initially targeted an overshoot you would have to reverse this velocity vector AGAIN,  seems like a terrible waste of fuel to me.

At the same time you also have to be bleeding off horizontal velocity, which slightly complicates this.  But you do not want to reverse it's vector.

I may be oversimplifying too much, but this makes a lot of sense at least to my mind. 

[Jcc]

One bit of information we don't know, in the case of the center core failure to ignite, is whether it did a 100 m overshoot or undershoot. My impression was that it was an undershoot, but is there any actual report of where it hit?

[AncientU]

One bit of information we don't know, in the case of the center core failure to ignite, is whether it did a 100 m overshoot or undershoot. My impression was that it was an undershoot, but is there any actual report of where it hit?

By the model proposed by chuck34 above, which I think is right on the mark, it would be an undershoot.

[marsbase]

Additional evidence... I used the Microsoft "image composite editor" to generate a panorama from the video (*), and it also allows you to mark areas of the images as "keep". This allowed me to do a better job illustrating the booster paths:

(* - Microsoft ICE is pretty neat, if you have a video where the videographer is not moving but simply panning with the camera, it can generate great panoramas)

Nice work Lars.  The question is what is the trajectory *before* the burn starts.  You show only the trajectory after the burn starts.  Can you go back further in the video using ICE and find the images of the boosters as they are falling with no propulsion?

[Lars-J]

One can be certain that the boosters will fall short of where the engines are pointing. If the booster were traveling in the direction of the engines then there would be zero lift, and if the engines were pointed short of the ballistic trajectory the lift would be negative, both of which hurt your propellant requirements. The lift required is only generated if the booster is laid on it's side further than it's direction of travel and will always fall short of where it's pointing.

Right. But the whole question being debated is where people feel that the booster would land if the engines *failed* to start. They would indeed fall in a parabolic arc, but a longer arc than one with a landing burn. Burning along your direction of travel (a reverse gravity turn) is the most optimal use of propellant, just as a gravity turn is the most optimal use of propellant during ascent.

The booster was never retrograde orbital. The booster starts (on ascent) east, and throughout the ascent the IIP is east of the launch site. If you ever drive the IIP west of the intended landing site, then you have wasted propulsion in the horizontal direction. Sure, on the way down, gravity is working to draw the IIP further east, but the upward component of the engine(s) counters a portion of that and extents the flight time compared to a ballistic flight. The extended flight time allows the horizontal velocity to act longer and moves the IIP west (in the dir of travel) as compared to ballistic flight.

This is completely irrelevant in the time frame being discussed here. (less than 10 seconds from ignition to landing) And don't think you understand at all what I mean by a "reverse gravity turn", it has nothing to do with launching retrograde. I'm just trying to encourage people to think of a propulsive landing as a launch in reverse.

Additional evidence... I used the Microsoft "image composite editor" to generate a panorama from the video (*), and it also allows you to mark areas of the images as "keep". This allowed me to do a better job illustrating the booster paths:

(* - Microsoft ICE is pretty neat, if you have a video where the videographer is not moving but simply panning with the camera, it can generate great panoramas)

Nice work Lars.  The question is what is the trajectory *before* the burn starts.  You show only the trajectory after the burn starts.  Can you go back further in the video using ICE and find the images of the boosters as they are falling with no propulsion?

Sure...  Although there is less cloud definition so more difficult to stitch a panorama that high up (accuracy suffers). But here is an attempt, this time focusing on the first booster. (although the 2nd booster did sneak into one shot) 

But I'm really not sure what this is going to change. I'm finding myself surprised by the lack of physics knowledge (and use of their eyes) among some here - no amount of evidence will convince them. And people really should go play Kerbal Space Program to get the feel for propulsive landings targeting a spot. There is no greater learning tool for orbital mechanics and physics from a space travel perspective.

[marsbase]

That is amazing!  I wonder why nobody did this before.   Lars, if you fit a (hyperbolic?) curve to those pre-ignition images, you should get the position of the crash site if the engines had failed.

[Herb Schaltegger]

That is amazing!  I wonder why nobody did this before.   Lars, if you fit a (hyperbolic?) curve to those pre-ignition images, you should get the position of the crash site if the engines had failed.

No you won’t, unless you also posit a failure of the grid fins, ignore winds and aerodynamic drag, and presume some unknowns about SpaceX’s control system laws and how they are programmed to deal with failure to ignite.