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

[speedevil]

Hopefully we don't have to wait two years for another blooper film.  Actually, hopefully they don't have enough bloopers in the next two years to make a film.

'If you're not having explosions, you're not testing hard enough'. (Paraphrased Elon, I think).

Another explosion or two on landing would not wholly surprise.
Especially if they are flying non-block-5 and can think of any interesting extreme tests to do with them.

Hopefully lots of nice explosions of test BFR/S bits after doing their full test program.

Maybe even fairing recovery fail crashes.

[Jcc]

The fact that it hit the ocean and not the droneship is fortunate, perhaps shows that the stage aims ballistically to miss the target, and redirects under engine thrust to the landing point.

Did any of the 3 engines light for the landing?

[jpo234]

Did any of the 3 engines light for the landing?

The center engine fired.

[su27k]

I wonder if/when we'll get a video of the center core smacking into the ocean at 300 mph.  Elon mentioned that they may have captured video of it.  Should be quite interesting.  Hopefully we don't have to wait two years for another blooper film.  Actually, hopefully they don't have enough bloopers in the next two years to make a film.

They can add new footage to old blooper film and make it v1.1

But I'm not sure they would release this high speed impact, they didn't release the video for SES-9 which should be similar.

[jpo234]

I wonder if/when we'll get a video of the center core smacking into the ocean at 300 mph.  Elon mentioned that they may have captured video of it.  Should be quite interesting.  Hopefully we don't have to wait two years for another blooper film.  Actually, hopefully they don't have enough bloopers in the next two years to make a film.

They can add new footage to old blooper film and make it v1.1

But I'm not sure they would release this high speed impact, they didn't release the video for SES-9 which should be similar.

Elon in the FH press conference:

If we got the footage … that sounds like some pretty fun footage if the cameras didn’t get blown up as well, then we’ll put that up … for — you know — just the blooper reel

[JimO]

Fascinating conversation re visible interference patterns -- does this explain why the visible streamers from a Soyuz launch are NOT exhaust trails but are between the exhaust trails?

[Robotbeat]

Combustion requires mixing and compression. Where the exhaust streams impinge on each other, you get higher pressures and mixing, thus combustion.

[marsbase]

Somehow I have missed this fact in all the discussion of FH.  Can the center core be re-used as a single stick F9?

[whitelancer64]

Forgive the crappy hand drawing, as oddly I couldn't find any pictures of 3 point interference patterns with a quick google search.

3 point interference produces two distinct "bars" of intense interaction, which is exactly what we see.

Naturally, it's not perfect, there's other aerodynamic interactions going on.

The problem is the relevant physics are different. We are talking about fluid flows here, not sound waves.

Sound is pressure waves flowing through a fluid (the air). At any rate, I'm saying it's analogous, I did not saying identical physics are at play.

[whitelancer64]

Somehow I have missed this fact in all the discussion of FH.  Can the center core be re-used as a single stick F9?

It could, but there's no reason to, since it's much heavier than a stock Falcon 9 first stage.

[Prettz]

The fact that it hit the ocean and not the droneship is fortunate, perhaps shows that the stage aims ballistically to miss the target, and redirects under engine thrust to the landing point.

This is what they've said it does since the earliest barge landing attempts. Exactly what happened.

[Hominans Kosmos]

Fascinating conversation re visible interference patterns -- does this explain why the visible streamers from a Soyuz launch are NOT exhaust trails but are between the exhaust trails?

The text in  the image is mistaken, you see the same phenomenon during night launches of Falcon9, Atlas5 and Antares.

[georgegassaway]

The fact that it hit the ocean and not the droneship is fortunate, perhaps shows that the stage aims ballistically to miss the target, and redirects under engine thrust to the landing point.

This is what they've said it does since the earliest barge landing attempts. Exactly what happened.

I had speculated that a long time ago, the sense it would make to program the booster to veer off and splash itself if it was too far out of the landing profile to land safely on the ASDS (as compared to the hopeless attempt of the CRS-5 booster, with its 45 degree Kamikaze crash).  Some replies hand-waved that away as though impossible to program (as though that would be so hard to do compared to a suicide burn landing within a few  meters on an ASDS when things are working right). 

So, I have never heard an official or authorative source confirm that indeed the boosters were later programmed to avoid the ASDS if a safe landing was not possible. Is there a source?

Or did you simply mean that when the outer two engines did not ignite, the slightly angled ballistic path (not vertical descent when the landing burn begins) caused an overshoot?  Lacking any info that SpaceX ever has added an "anti-Kamikaze" veer-off programming for hopeless landing attempts, I figured that's the most likely reason why it crashed that far off from the ASDS.   

Look some of the better spectator videos of the 1-3-1 landing burns of the two FH  boosters that landed. They were at a significant non-vertical angle when the single engine burn began, but got  lot more vertical from the time the outer 2 engines ignited, to the time they shut down to a single engine again. Had those suffered the same fate as the center core, outer two engines not igniting, then their ballistic path to impact would have overshot and  extended several hundred feet westward. The Grid Fins could try to steer it, and would to some extent, but not be maneuverable enough to stop the ballistic overshoot.

[Lars-J]

The fact that it hit the ocean and not the droneship is fortunate, perhaps shows that the stage aims ballistically to miss the target, and redirects under engine thrust to the landing point.

This is what they've said it does since the earliest barge landing attempts. Exactly what happened.

I had speculated that a long time ago, the sense it would make to program the booster to veer off and splash itself if it was too far out of the landing profile to land safely on the ASDS (as compared to the hopeless attempt of the CRS-5 booster, with its 45 degree Kamikaze crash).  Some replies hand-waved that away as though impossible to program (as though that would be so hard to do compared to a suicide burn landing within a few  meters on an ASDS when things are working right). 

So, I have never heard an official or authorative source confirm that indeed the boosters were later programmed to avoid the ASDS if a safe landing was not possible. Is there a source?

Or did you simply mean that when the outer two engines did not ignite, the slightly angled ballistic path (not vertical descent when the landing burn begins) caused an overshoot? Lacking any info that SpaceX ever has added an "anti-Kamikaze" veer-off programming for hopeless landing attempts, I figured that's the most likely reason why it crashed that far off from the ASDS.

That's a bingo... The core comes it at an angle, not from above. If the final engine burn fails to ignite (or not be strong enough), it  will naturally overshoot the landing spot and impact the water (or land) there. It's not any more complicated than that.

If you look at this video of the side boosters landing, you get an idea of how much of a horizontal velocity they have before the landing burn start. If these landing burns had failed, they would have hit the ground hard, a couple of hundred feet inland of the landing spots.

http://www.youtube.com/watch?time_continue=13&v=Z_kfM-BmVzQ

[JonathanD]

Sorry if this has been clarified already but I've heard a few different versions.  Did they plan to have the center core have a 3 engine reentry burn AND a 3 engine landing burn?  I'm just curious given the conservative flight profile why they would have a 3 engine landing burn. 

If it had a single engine landing burn planned, which they would presumably have plenty of fuel for, then it would suggest the outer two boosters didn't ignite for the reentry burn then.

[Prettz]

Sorry if this has been clarified already but I've heard a few different versions.  Did they plan to have the center core have a 3 engine reentry burn AND a 3 engine landing burn?  I'm just curious given the conservative flight profile why they would have a 3 engine landing burn. 

All boostback burns and entry burns are always 3 engine. The center core was planned for a 3 engine landing burn as well.
Your last question is one of two questions I haven't seen a good answer to yet in all the discussion -- why did they need a 3 engine landing for the center core, and why was adding in a boostback burn preferable to a longer entry burn?

[mme]

Sorry if this has been clarified already but I've heard a few different versions.  Did they plan to have the center core have a 3 engine reentry burn AND a 3 engine landing burn?  I'm just curious given the conservative flight profile why they would have a 3 engine landing burn. 

All boostback burns and entry burns are always 3 engine. The center core was planned for a 3 engine landing burn as well.
Your last question is one of two questions I haven't seen a good answer to yet in all the discussion -- why did they need a 3 engine landing for the center core, and why was adding in a boostback burn preferable to a longer entry burn?

My assumption is that it's the same reason they did a "water landing" for GovSat-1. To experiment.  The mission had a tonne of margin and they don't need the core back so I imagine they were pushing some boundary condition.  The 3 engine landing burn of GovSat-1 worked, I bet they were trying to collect more data to model the landing envelope.

I'm a bit surprised they put the ASDS at risk as their flight rate ramps up.  But I think they are still largely in "throw out old boosters" mode so maybe they have some buffer regarding required recoveries.

[whitelancer64]

Sorry if this has been clarified already but I've heard a few different versions.  Did they plan to have the center core have a 3 engine reentry burn AND a 3 engine landing burn?  I'm just curious given the conservative flight profile why they would have a 3 engine landing burn. 

All boostback burns and entry burns are always 3 engine. The center core was planned for a 3 engine landing burn as well.
Your last question is one of two questions I haven't seen a good answer to yet in all the discussion -- why did they need a 3 engine landing for the center core, and why was adding in a boostback burn preferable to a longer entry burn?

A 3 engine landing is more fuel efficient than 1-3-1 burn. Faster "slam on the brakes" means less dV lost to gravity.

Boostback burn because sending out the ASDS 200 km is easier than sending it out 600 km (numbers pulled from the aether).

[Kaputnik]

The fact that it hit the ocean and not the droneship is fortunate, perhaps shows that the stage aims ballistically to miss the target, and redirects under engine thrust to the landing point.

This is what they've said it does since the earliest barge landing attempts. Exactly what happened.

I had speculated that a long time ago, the sense it would make to program the booster to veer off and splash itself if it was too far out of the landing profile to land safely on the ASDS (as compared to the hopeless attempt of the CRS-5 booster, with its 45 degree Kamikaze crash).  Some replies hand-waved that away as though impossible to program (as though that would be so hard to do compared to a suicide burn landing within a few  meters on an ASDS when things are working right). 

So, I have never heard an official or authorative source confirm that indeed the boosters were later programmed to avoid the ASDS if a safe landing was not possible. Is there a source?

Or did you simply mean that when the outer two engines did not ignite, the slightly angled ballistic path (not vertical descent when the landing burn begins) caused an overshoot? Lacking any info that SpaceX ever has added an "anti-Kamikaze" veer-off programming for hopeless landing attempts, I figured that's the most likely reason why it crashed that far off from the ASDS.

That's a bingo... The core comes it at an angle, not from above. If the final engine burn fails to ignite (or not be strong enough), it  will naturally overshoot the landing spot and impact the water (or land) there. It's not any more complicated than that.

If you look at this video of the side boosters landing, you get an idea of how much of a horizontal velocity they have before the landing burn start. If these landing burns had failed, they would have hit the ground hard, a couple of hundred feet inland of the landing spots.

http://www.youtube.com/watch?time_continue=13&v=Z_kfM-BmVzQ

My understanding is that the booster would under shoot, not overshoot, the pad/ship if the landing burn impulse fell short.
This is because the booster is approaching on a ballistic trajectory that falls short of the pad. Most of the velocity is vertical, and some is horizontal. In making the landing burn, the vertical rate is rapidly reduced, prolonging the length of time that the booster remains airborne, and allowing it to cover a greater horizontal distance and thus reaching the pad.

I actually made a simple sim in scratch a few years ago that captures this pretty well.

[Lars-J]

That's a bingo... The core comes it at an angle, not from above. If the final engine burn fails to ignite (or not be strong enough), it  will naturally overshoot the landing spot and impact the water (or land) there. It's not any more complicated than that.

If you look at this video of the side boosters landing, you get an idea of how much of a horizontal velocity they have before the landing burn start. If these landing burns had failed, they would have hit the ground hard, a couple of hundred feet inland of the landing spots.

http://www.youtube.com/watch?time_continue=13&v=Z_kfM-BmVzQ

My understanding is that the booster would under shoot, not overshoot, the pad/ship if the landing burn impulse fell short.
This is because the booster is approaching on a ballistic trajectory that falls short of the pad. Most of the velocity is vertical, and some is horizontal. In making the landing burn, the vertical rate is rapidly reduced, prolonging the length of time that the booster remains airborne, and allowing it to cover a greater horizontal distance and thus reaching the pad.

I actually made a simple sim in scratch a few years ago that captures this pretty well.

That may be your understanding, but it does not match reality of what they are doing. (It would also waste propellant) Watch several landings and you'll see. (Did you watch the video I linked to?)

Or do you think that the boosters in the video would have undershot the landing area without the landing burn?