Author Topic: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion Thread 1  (Read 561806 times)

Online IanThePineapple

My guess is it's around 30 seconds, give or take 10 seconds.  That's the moment on the mission clock that I'll be watching for ...

If you look at this fabulous Falcon Heavy launch simulation vid by Zach and freeze it at the right time you should get an idea of when the stack wouldn't tumble back on the launch pad ... Looks like around 15 seconds elapsed mission time to me.

Thanks Oersted for the link to that GREAT simulation.

Zach, could you run that with a view from the side, showing the IIP during the first 30 seconds?  (in other words, keep the ground in view, and we can use the stack height to estimate horizontal distance.

Per the simulation, the pitchover doesn't even start until +15 seconds, so +15 secs is definitely NOT long enough (it will fall straight back down, a la Antares as I mentioned).  By my guess, looking at downrange distance, altitude and apogee, it's not until about +25 seconds (+/- 5 secs) that the IIP would clear the LC-39A fenceline, 400 meters from the pad.  We are converging on a solution :)
Antares didnt activate the flight termination system, so it came down as two full tanks.

Hopefully SpaceX will be faster on the trigger if it happens.

Propellant dispersal is really important in these situations.

Every gallon of propellant burned in the air is a gallon of propellant not burning the pad up.

Online nacnud

  • Extreme Veteran
  • Senior Member
  • *****
  • Posts: 2211
  • Liked: 369
  • Likes Given: 177
Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #701 on: 12/31/2017 04:51 PM »
It's 975m from the pad due east to the ocean. Assuming that if the vehicle fails it travels ballistically from that point onwards the sim suggests that any time after 30 seconds the pad is safe. After 40 seconds or so the remains would land in the water and after a minute the vehicle is flying over the sea.

Sources: the above linked sim and the ruler buried in the left click menu of google maps.

Offline Space Ghost 1962

  • Senior Member
  • *****
  • Posts: 2783
  • Whatcha gonna do when the Ghost zaps you?
  • Liked: 2896
  • Likes Given: 2249
Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #702 on: 12/31/2017 04:57 PM »
If you look at this fabulous Falcon Heavy launch simulation vid by Zach and freeze it at the right time you should get an idea of when the stack wouldn't tumble back on the launch pad:



Looks like around 15 seconds elapsed mission time to me.
Watch the bouncing IIP as it suddenly has a horizontal component. I'd say a half minute.

(Zach's video does a fine job. If I were to add anything it would be corrected sways/bends in combined flight, then minor twists during booster sep. Great work!)

One might fire it up on actual FH launch in side by side - then you'll notice the little differences more easily in the actual.

Offline Greg Hullender

  • Member
  • Posts: 60
  • Seattle
    • Rocket Stack Rank
  • Liked: 60
  • Likes Given: 72
Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #703 on: 12/31/2017 04:59 PM »
I thought the center core was only going to fire until shortly after liftoff and then reignite when the side boosters separated. Has that changed?

Offline jak Kennedy

  • Full Member
  • *
  • Posts: 112
  • Liked: 22
  • Likes Given: 33
Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #704 on: 12/31/2017 05:05 PM »
I seem to recall some rockets start moving actively move away from directly above the pad as soon as they clear the tower. ie sliding in a horizontal direction for a few seconds so that any debris would clear the pad.

Offline mme

  • Full Member
  • ****
  • Posts: 1231
  • Santa Barbara, CA, USA, Earth, Solar System, Milky Way Galaxy, Virgo Supercluster
  • Liked: 1482
  • Likes Given: 4055
Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #705 on: 12/31/2017 05:16 PM »
The center core will throttle downn but it will not shutdown its engines.
Space is not Highlander.  There can, and will, be more than one.

Offline ZachS09

My guess is it's around 30 seconds, give or take 10 seconds.  That's the moment on the mission clock that I'll be watching for ...

If you look at this fabulous Falcon Heavy launch simulation vid by Zach and freeze it at the right time you should get an idea of when the stack wouldn't tumble back on the launch pad ... Looks like around 15 seconds elapsed mission time to me.

Thanks Oersted for the link to that GREAT simulation.

Zach (was that ZachS09?), could you run that with a view from the side, showing the IIP during the first 30 seconds?  (in other words, keep the ground in view, and we can use the stack height to estimate horizontal distance.

Per the simulation, the pitchover doesn't even start until +15 seconds, so +15 secs is definitely NOT long enough (it will fall straight back down, a la Antares as I mentioned).  By my guess, looking at downrange distance, altitude and apogee, it's not until about +25 seconds (+/- 5 secs) that the IIP would clear the LC-39A fenceline, 400 meters from the pad.  We are converging on a solution :)

It was a different Zach that ran that simulation. I had nothing to do with it.
"Liftoff of Falcon 9: the world's first reflight of an orbital-class rocket."

Offline Oersted

  • Member
  • Full Member
  • ****
  • Posts: 873
  • Liked: 465
  • Likes Given: 277
Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #707 on: 12/31/2017 05:59 PM »
Watch the bouncing IIP as it suddenly has a horizontal component. I'd say a half minute.

Yup, true, 15 seconds is a bit too early. At half a minute into the flight the projected path has moved quite a bit away from the launch complex, in my estimation. Still a bit too early for comfort, though!

Here is a screenie of the dots you need to keep an eye on...

(Please note that the lift-off is 20 seconds in from the beginning of the movie. This screenie is consequently 11 seconds into the flight, where - if the engines were to stop immediately - the rocket would just rise by a bit more than its own length before it would fall back down.)
« Last Edit: 12/31/2017 06:01 PM by Oersted »

Online nacnud

  • Extreme Veteran
  • Senior Member
  • *****
  • Posts: 2211
  • Liked: 369
  • Likes Given: 177
Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #708 on: 12/31/2017 06:07 PM »
It's easier just to keep a note of the downrange distance displayed in the top left of the sim.

Offline speedevil

  • Senior Member
  • *****
  • Posts: 2048
  • Fife
  • Liked: 1006
  • Likes Given: 1123
Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #709 on: 12/31/2017 07:01 PM »
It's easier just to keep a note of the downrange distance displayed in the top left of the sim.

(altitude/downrange) gives a rough estimate of the current launch angle from vertical.
perigee*2/(altitude/downrange) is a conservative estimate of how far it will travel horizontally.

At 100m altitude, it's gone 20m, apogee is 87m.

200 24 233 55
400 24 540 64
1000 36 1562 112
1500 54 2300 165
2500 124 4200 416
3500 200 6000 685
5000 350 8400 1176

So, ~5km, ~40s in means the IIP is likely to be on water.

But, I don't believe this at all for a few reasons. Unless the stage just stops thrusting, and falls intact, it's really, really not going to be ballistic.
Once apogee hits ~1km or so, 12s in, if the FTS triggers, you're getting shards, not any part of the rocket.
So, hold your breath, and when you let it out it's fine.
If it explodes, once you're over a few hundred meters, most of the cloud of debris is going to be shards of sooty fuel tank, with the COPVs and engines falling intact.
Very small tweaks to the trajectory that are essentially free variables and have no impact on launch performance can change the impact dramatically.

Still, a minute in is certainly pad safe,
« Last Edit: 12/31/2017 07:20 PM by speedevil »

Online wannamoonbase

  • Elite Veteran
  • Senior Member
  • *****
  • Posts: 2975
  • Denver, CO
    • U.S. Metric Association
  • Liked: 682
  • Likes Given: 1120
Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #710 on: 12/31/2017 09:26 PM »
If it makes it past the first few seconds itís likely fine till staging. 

Then, hey, who knows, if it survives staging the side boosters I think it will be completely successful.
SpaceX, just a few things planned for 2018: FH, Starlink Prototypes, Block 5, Dragon 2, Increased launch rate.

Offline zlynn1990

  • Member
  • Posts: 6
  • United States
  • Liked: 16
  • Likes Given: 8
Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #711 on: 12/31/2017 10:29 PM »
Oersted I'm the Zach who made the sim :) Most of that trajectory was done by OneSpeed, and he has the gravity turn starting at 10 seconds. Take the entire thing was a large grain of salt because there are a lot of unknowns right now.

Offline Surfdaddy

  • Member
  • Posts: 43
  • Liked: 82
  • Likes Given: 94
Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #712 on: 12/31/2017 11:11 PM »
If it makes it past the first few seconds itís likely fine till staging. 

Then, hey, who knows, if it survives staging the side boosters I think it will be completely successful.

I want to believe you, but I've seen a lot of launch videos where problems happen in your "safe" time period.

Offline Oersted

  • Member
  • Full Member
  • ****
  • Posts: 873
  • Liked: 465
  • Likes Given: 277
Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #713 on: 12/31/2017 11:27 PM »
Oersted I'm the Zach who made the sim :) Most of that trajectory was done by OneSpeed, and he has the gravity turn starting at 10 seconds. Take the entire thing was a large grain of salt because there are a lot of unknowns right now.

Yes, I realise it is a good guess more than anything else. Nevertheless, splendid effort. Thanks so much for putting the vid together. It is clear that a lot of love went into it. Kudos and happy new year!

Offline meekGee

  • Senior Member
  • *****
  • Posts: 8173
  • N. California
  • Liked: 4411
  • Likes Given: 868
Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #714 on: 01/01/2018 12:56 AM »
My guess is it's around 30 seconds, give or take 10 seconds.  That's the moment on the mission clock that I'll be watching for ...

If you look at this fabulous Falcon Heavy launch simulation vid by Zach and freeze it at the right time you should get an idea of when the stack wouldn't tumble back on the launch pad ... Looks like around 15 seconds elapsed mission time to me.

Thanks Oersted for the link to that GREAT simulation.

Zach (was that ZachS09?), could you run that with a view from the side, showing the IIP during the first 30 seconds?  (in other words, keep the ground in view, and we can use the stack height to estimate horizontal distance.

Per the simulation, the pitchover doesn't even start until +15 seconds, so +15 secs is definitely NOT long enough (it will fall straight back down, a la Antares as I mentioned).  By my guess, looking at downrange distance, altitude and apogee, it's not until about +25 seconds (+/- 5 secs) that the IIP would clear the LC-39A fenceline, 400 meters from the pad.  We are converging on a solution :)

Usually the IIP is the thing to watch, but this assumes the rocket continues as an inert solid body - as if the engines are shut off and nothing else breaks.

In an early failure scenario, two things will likely happen - off-axis thrust will move the IIP very quickly in some random directions, and then FTS will make the whole thing "stop cold" and fall straight down, but influenced by wind.

At some altitude, the liquid fuel will have time to burn on the way down, or disperse enough that it doesn't pool on the ground, and that will already be a "safe" situation.
ABCD - Always Be Counting Down

Offline joek

  • Senior Member
  • *****
  • Posts: 2917
  • Liked: 676
  • Likes Given: 363
Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #715 on: 01/01/2018 02:44 AM »
Usually the IIP is the thing to watch, but this assumes the rocket continues as an inert solid body - as if the engines are shut off and nothing else breaks.

IIP is a point spread function based on numerous parameters.  You can only state "probability of IIP at point X,Y is Z based on parameters T, P, Q, R, ...".  Constrain the parameters sufficiently and you can assert that it "continues as an inert solid body".  However, most real-world models (probabilistic-time-based) do not apply such constraints or make such assumptions--unless the parameters are well bounded and understood  (e.g., close to lift off).  Which is why the models tend to vary their parameters over the flight path (in particular, probability vs. time, as spread-uncertainty tends to increase with time).

Offline meekGee

  • Senior Member
  • *****
  • Posts: 8173
  • N. California
  • Liked: 4411
  • Likes Given: 868
Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #716 on: 01/01/2018 05:16 AM »
Usually the IIP is the thing to watch, but this assumes the rocket continues as an inert solid body - as if the engines are shut off and nothing else breaks.

IIP is a point spread function based on numerous parameters.  You can only state "probability of IIP at point X,Y is Z based on parameters T, P, Q, R, ...".  Constrain the parameters sufficiently and you can assert that it "continues as an inert solid body".  However, most real-world models (probabilistic-time-based) do not apply such constraints or make such assumptions--unless the parameters are well bounded and understood  (e.g., close to lift off).  Which is why the models tend to vary their parameters over the flight path (in particular, probability vs. time, as spread-uncertainty tends to increase with time).
I forget where I saw a realtime IIP displayed, but I got the impression it was simply ballistic.

You're correct that a true IIP estimate is a stochastic thing.
ABCD - Always Be Counting Down

Offline gadgetmind

  • Full Member
  • *
  • Posts: 115
  • Liked: 109
  • Likes Given: 236
Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #717 on: 01/01/2018 09:47 AM »
I wonder how FH would handle an engine failure on a side booster? I guess it would need to throttle down the other side? Throw in scenarios of failures of two engines in various configurations and it could all get complicated.

Offline SLC

  • Full Member
  • *
  • Posts: 106
  • London, UK
  • Liked: 89
  • Likes Given: 651
Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #718 on: 01/01/2018 01:57 PM »
I wonder how FH would handle an engine failure on a side booster? I guess it would need to throttle down the other side? Throw in scenarios of failures of two engines in various configurations and it could all get complicated.
Would it be too off-topic to give this link to the last time a rocket with this many engines made its (short) first flight -  the Soviet N1 moon rocket 49 years ago next month:

http://www.russianspaceweb.com/n1_3l.html

On that occasion the KORD control system did indeed diagnose an engine failure on one side, it did shut down the symmetrically opposite engine to compensate, and then it all got complicated ...

Offline speedevil

  • Senior Member
  • *****
  • Posts: 2048
  • Fife
  • Liked: 1006
  • Likes Given: 1123
Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #719 on: 01/01/2018 02:11 PM »
I wonder how FH would handle an engine failure on a side booster? I guess it would need to throttle down the other side? Throw in scenarios of failures of two engines in various configurations and it could all get complicated.

Maybe.
From memory, it has been said that the first launch will be at 82% thrust.
This is considerably more than 9/8ths, so you can lose certain engines, and simply throttle up on others with no external changes in thrust vector.
For example, lose an inboard engine on a booster, and if you throttle the remaining outboard ones to 92%, the centre of thrust moves out from the core by perhaps half an engine diameter.

If you throttle the outer engine to nearly minimum, the ones next to the core to ~105%, then it may not move at all.

Only once you can no longer keep a booster or cores thrust vector within narrow limits would you need to make changes to the thrust profile of the stack as a whole.

Would it be too off-topic to give this link to the last time a rocket with this many engines made its (short) first flight -  the Soviet N1 moon rocket 49 years ago next month:
Yes.

More seriously, N1 failures are not comparable in so many ways, as overall QC and management was so terrible that it loses all meaning as a reasonable comparator.

« Last Edit: 01/01/2018 02:16 PM by speedevil »

Tags: