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

[Kaputnik]

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. 

That's essentially the same as my understanding, however Lars-J's excellent image analysis persuades me that this isn't what was going on for the most recent flight. I still remain open minded to the possibility that earlier landings aimed to undershoot.
The one point I would pick you up on is that an undershoot trajectory is the most fuel efficient. I don't think it is, rather a 'reverse launch' is, where the incoming trajectory aims for the pad itself.

[marsbase]

It's a safe assumption that all of those unknowns (unknown to us, not to SpaceX)  operate to move the booster toward the landing site.  If they did not, at least taken together, move toward the landing site, SpaceX would have brought the boosters down somewhere else.  You are right about post-failure measures but those do not affect the images Lars has documented.  So an extrapolation of the known position of the pre-ignition boosters is the best we can hope for.  At least this approach would tend to err on the side of undershoot so if we still get overshoot it's  likely correct.

[FutureSpaceTourist]

Congratulations to John Kraus for his FH shot being picked up:

Here it is — I’m very honored to share that one of my Falcon Heavy images graces this issue of Aviation Week magazine!

https://twitter.com/johnkrausphotos/status/963566960314339328

[Lars-J]

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.

Yep. And this long exposure I found from the first F9 landing does seem to support such an idea... Look at the entry burn and landing burn... not a straight line, the grid fins have steered the stage to aim near the landing pad.

(Be aware of the extreme wide angle in the photo, the landing burn in this photo is close to vertical, compare with buildings in the low right corner)

Based on this, I think it is safe to assume the following:
 - the boostback and entry burns place the IIP far short of the landing pad
 - after the entry burn, the grid fins is steer to aim to the landing pad
 - when the landing burn begins, the IIP is either close to the landing point or just overshooting it

[speedevil]

- the boostback and entry burns place the IIP far short of the landing pad...

Is anyone aware of attempts at using the many vantage points of landings to reconstruct a 3d scene?

[CJ]

Sorry to veer so sharply off the current topic in this thread, but I'd like to ask a question about the Falcon Heavy Demo flight.

Do we, yet, have any idea why the earth-escape burn occurred well before perigee? Perigee, as I recall, was over Venezuela, but the burn apparently began (per sightings) over Southern California.

[Lars-J]

Sorry to veer so sharply off the current topic in this thread, but I'd like to ask a question about the Falcon Heavy Demo flight.

Do we, yet, have any idea why the earth-escape burn occurred well before perigee? Perigee, as I recall, was over Venezuela, but the burn apparently began (per sightings) over Southern California.

They wanted the burn to happen where they had a good telemetry link? If this burn after a 6 hr coast was going to fail, they would have needed all the data they could get to figure out why. That’s probably why, but I’m only speculation.

[CJ]

Sorry to veer so sharply off the current topic in this thread, but I'd like to ask a question about the Falcon Heavy Demo flight.

Do we, yet, have any idea why the earth-escape burn occurred well before perigee? Perigee, as I recall, was over Venezuela, but the burn apparently began (per sightings) over Southern California.

They wanted the burn to happen where they had a good telemetry link? If this burn after a 6 hr coast was going to fail, they would have needed all the data they could get to figure out why. That’s probably why, but I’m only speculation.

Thank you, that makes sense. I looked for the locations of SpaceX ground station (per Jim in another thread,
https://forum.nasaspaceflight.com/index.php?topic=44889.msg1785888#msg1785888
 they use USN, which has a network called PrioraNet) and it does look like they have a coverage gap at low altitude over northern South America.
https://earth.esa.int/documents/1656065/1664726/21-Evolved_Global_SSC_Ground_Station_Network
On page 3 of that PDF is a map of their network, the only one I can find.

My hypothesis was that the burn was done to enable an out-of-the-ecliptic aphelion, but, looks to me like they'd have burned after perigee if that had been their reason.

[georgegassaway]

What I was referring to was the LANDING BURN part. NOT the trajectory before the re-entry burn which of course aims for an ocean impact before the grid fins cause it to aerodynamically  extend its path across the shore.

But the whole nature of that is that when the landing burn begins, the path is aimed to overshoot. That’s the nature of the whole thing.

Here’s a photo (Zuma landing) that is one “heck” of a lot more valid to show this effect during the landing burn than the one a few messages up.  First short streak is the entry burn. Then the landing burn is the long streak  And this one was (IIRC) a conventional single engine landing burn. A planned 1-3-1 burn’s trajectory would be a bit different, in the videos the horizontal velocity at the start of the 3 engine burn was significantly noticeable, by the time of shutting down the outer two the horizontal velocity was very very low. Failure to ignite the outer two, would not be able to overcome the remaining horizontal velocity (that is supposed to have 3X the thrust for Y amount of duration) in time to prevent an overshoot. The grid fin steering and 1X thrust with far less time to stop the horizontal velocity than aimed for, would not be able to completely prevent an overshoot.

[rpapo]

...this one was (IIRC) a conventional single engine landing burn.

How little time it took for radical to become conventional...

[Lar]

...this one was (IIRC) a conventional single engine landing burn.

How little time it took for radical to become conventional...

Let's draw a line under the under/overshoot subthread and call it done. Most readers are just wishing it was over, and rather underwhelmed at the over and over back and forth... it's starting to get to the point where some are asking mods to shoot the thread.

[Nomadd]

Let's draw a line under the under/overshoot subthread and call it done. Most readers are just wishing it was over, and rather underwhelmed at the over and over back and forth... it's starting to get to the point where some are asking mods to shoot the thread.

Try not to overshoot and hit the neighboring thread.

[Journeyman]

Well, for me all that matters are that the two side boosters landed successfully! The center core didn't and the reason for this and the fix for it is well understood by SpaceX.

The overshoot / undershoot discussion belongs in a separate thread for those interested in discussing all the details that goes into landing the first stage

[Herb Schaltegger]

I guess this will the final view of the Tesla Roadster & Starman. It was taken on Feb 13, at a distance of 2 mln km (magnitude 19).

source: http://spaceweathergallery.com/indiv_upload.php?upload_id=142629&PHPSESSID=avsdr9chil55mcr1f8p0q60i04

Thanks for sharing this here!  I took this image set at the McCarthy Observatory, thinking it was close to as deep as our little 406mm telescope could see (at 72m elevation in a river valley, with the roadster transiting at 20 degrees in altitude).  We went after the roadster again Saturday morning, and found it again at magnitude 19.7!

Monday early morning we're going to take one more shot at it (projected magnitude 19.84).  One of our volunteers was at the Cape for the launch, and it's been a blast tracking Stage 2's journey through the solar system so far.  The public interest this has generated is fantastic, it has really captured the imaginations of so many people!

Very cool. I see you're in New Milford; that's where I was born. Small world.

[speedevil]

Thanks for sharing this here!  I took this image set at the McCarthy Observatory, thinking it was close to as deep as our little 406mm telescope could see (at 72m elevation in a river valley, with the roadster transiting at 20 degrees in altitude).  We went after the roadster again Saturday morning, and found it again at magnitude 19.7!

That looks like you could get a couple magnitudes more even before getting into co-adding frames!

Awesome.
I guess the big telescopes will have it for quite a while yet.

[cscott]

Sorry to veer so sharply off the current topic in this thread, but I'd like to ask a question about the Falcon Heavy Demo flight.

Do we, yet, have any idea why the earth-escape burn occurred well before perigee? Perigee, as I recall, was over Venezuela, but the burn apparently began (per sightings) over Southern California.

They wanted the burn to happen where they had a good telemetry link? If this burn after a 6 hr coast was going to fail, they would have needed all the data they could get to figure out why. That’s probably why, but I’m only speculation.

There may also be launch window compensation involved.  The burn may have indeed occurred exactly at perigee for some optimal launch time (start of the window? center of the window?) but since the launch was at the end of the window they made adjustments resulting in a slightly suboptimal burn before perigee.

Not surprising that a launch at the end of a launch window would result in suboptimal burns.  Since "reaches Mars" was not the primary objective of the mission, the real question is what factor dictated the size of the launch window.  I think you've made a compelling case above that telemetry coverage during the departure burn may have been that gating factor.

[cscott]

I ran over to KSCVC and took a few pics of the side core booster on display.

https://flic.kr/s/aHsmeswVkb

Very cool! They'll need that trailer back, for sure, though.  And I suspect they are going to want to tear down the engines on the booster, too.  So presumably a limited time engagement?

Pretty audacious to park it blocking the entrance to the shuttle exhibit.  The other option would block the space ice cream stand, and clearly KSC can't bear the loss of revenue.

[deruch]

I ran over to KSCVC and took a few pics of the side core booster on display.

https://flic.kr/s/aHsmeswVkb

Very cool! They'll need that trailer back, for sure, though.  And I suspect they are going to want to tear down the engines on the booster, too.  So presumably a limited time engagement?

Pretty audacious to park it blocking the entrance to the shuttle exhibit.  The other option would block the space ice cream stand, and clearly KSC can't bear the loss of revenue.

According to KSC Visitor's Center twitter it's only on display there til Tuesday.

[cscott]

The FH booster's appearance apparently has to do with the National Space Council reception at KSC on Tuesday.
https://twitter.com/NASAWatch/status/965331995076198401

[StuffOfInterest]

Notice how straight all of the engine covers are on the booster being displayed?  Definitely more attention to detail since they are showing this one off for a few days.

For some reason, seeing that booster on the carrier, and with that nose cone, my first thought is "mobile ICBM".