Author Topic: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion  (Read 394763 times)

Offline mn

  • Full Member
  • **
  • Posts: 250
  • United States
  • Liked: 114
  • Likes Given: 49
Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #620 on: 12/29/2017 04:21 PM »
Now I remember where I first saw this mentioned:

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 Heavy’s 27 engines will not be lit simultaneously like the Falcon 9 engines

So it seems we are dealing with an issue local to each core (worry about damage to the octaweb). so hence my question, why are 3 cores different than one core in this respect.

Suppose we stood 3 cores side by side without any hardware connecting them (obviously would need 3 TELs) would igniting them be different than usual?

Offline NX-0

  • Full Member
  • *
  • Posts: 105
  • USA
  • Liked: 105
  • Likes Given: 158
Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #621 on: 12/29/2017 04:25 PM »

If you watch the the liftoff of the first Falcon 9, it clearly rotates.  For a single stick, that's not really a big deal.  But now bolt three of them together and rather than rotate freely in space they will be torquing the connections.



Waves and waves of nostalgia there.
F9 V1.0 is such a different animal from what we are seeing now.
'stubby' looking even, especially with no legs

3x3 engines looks freaky, now.
Did Octoweb help with some of the roll issues?

Offline mn

  • Full Member
  • **
  • Posts: 250
  • United States
  • Liked: 114
  • Likes Given: 49
Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #622 on: 12/29/2017 04:28 PM »
...

Thanks for explanation, but I'm still stumped by the 1 vs 3 core?

Whatever torque issues are on one core they have obviously solved, so what happens when you have 3 cores side by side? as I said in my question: why is it a larger problem and not the same problem 3 times (with the same solution applied 3 times)?
If you watch the the liftoff of the first Falcon 9, it clearly rotates.  For a single stick, that's not really a big deal.  But now bolt three of them together and rather than rotate freely in space they will be torquing the connections.



The issue they are worried about is during ignition, not during flight.

(and we've seen many launches since then, I don't think they have a problem here)

Offline Gotorah

  • Member
  • Posts: 13
  • New Mexico
  • Liked: 7
  • Likes Given: 358
Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #623 on: 12/29/2017 04:46 PM »
The gimbal of the engines is fully capable of eliminating or inducing any roll wanted or unwanted.

Offline Rocket Science

  • Senior Member
  • *****
  • Posts: 8175
  • NASA Educator Astronaut Candidate Applicant 2002
  • Liked: 2330
  • Likes Given: 5474
Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #624 on: 12/29/2017 05:04 PM »
I'm not concerned by any flight control guidance authority issues, just potential pneumatics and unlatch hangs at  booster sep...
"The laws of physics are unforgiving"
~Rob, Physics instructor, aviator, vintage auto racer

Offline Lars-J

  • Senior Member
  • *****
  • Posts: 3591
  • California
  • Liked: 2836
  • Likes Given: 1787
Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #625 on: 12/29/2017 05:10 PM »
I think the probability of success is pretty high, but not as high as a F9 of course.
I think 50-50 is about right.  That's what Elon suggested, if I recall correctly. 

There are certainly some untested aspects of this vehicle, especially base heating, max-q transonic aerodynamic stresses, pogo, etc.  History is a guide.  The first Delta 4 Heavy failed, as did three of the first six Titan IIIC missions and one of the first two Saturn 5 launches, though only one of those five combined failures was an outright Fail to Orbit.

As Elon said, we'll all be happy if this thing gets off the pad and over the Atlantic without mishap.  I believe that the odds of at least that happening are pretty good.

 - Ed Kyle

I'm amazed that people continue to not understand the expectations game that Elon plays - all the time.

If they truly think that FH only has a 50% chance of succeeding, they are not going to launch. Period. They are going to want to be a lot more confident than that.

Offline drunyan8315

  • Member
  • Posts: 38
  • Seattle
  • Liked: 5
  • Likes Given: 1
Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #626 on: 12/29/2017 05:26 PM »
Due to thrust torque (a thrust-induced rotation)...

I believe the torque mentioned is around a horizontal axis. A single stick might use an asymmetric engine ignition sequence without overstressing its own octaweb. But placed side by side, the asymmetric startup thrusts could overstress one or both webs. Now think of that happening on both sides of the center booster web simultaneously... complex.

I thought the problem would actually be in any mismatch in the overall thrust startup of the two side boosters, which could result (say) in the links on one side of the center booster being lift-loaded before the other. Could cause a center tube or link failure, I would imagine. Fireball.

Online abaddon

  • Full Member
  • ****
  • Posts: 1548
  • Liked: 989
  • Likes Given: 810
Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #627 on: 12/29/2017 05:27 PM »
If you watch the the liftoff of the first Falcon 9, it clearly rotates.  For a single stick, that's not really a big deal.  But now bolt three of them together and rather than rotate freely in space they will be torquing the connections.
I'm sure that would be a problem if SpaceX was trying to launch three of the very first Falcon 9 1.0 rockets together... given that none of the 45 subsequent Falcon 9's have done this, I'd have to think it isn't a factor today.

Offline Lars-J

  • Senior Member
  • *****
  • Posts: 3591
  • California
  • Liked: 2836
  • Likes Given: 1787
Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #628 on: 12/29/2017 05:29 PM »
Due to thrust torque (a thrust-induced rotation)...

I believe the torque mentioned is around a horizontal axis. A single stick might use an asymmetric engine ignition sequence without overstressing its own octaweb. But placed side by side, the asymmetric startup thrusts could overstress one or both webs. Now think of that happening on both sides of the center booster web simultaneously... complex.

Yep, those are all potential failure points. BUT... Given that we have seen absolutely ZERO evidence of any torque induced roll in any flight after F9 flight 1, I think they are completely aware and it will not be an issue that I have any concerns about.

I thought the problem would actually be in any mismatch in the overall thrust startup of the two side boosters, which could result (say) in the links on one side of the center booster being lift-loaded before the other. Could cause a center tube or link failure, I would imagine. Fireball.

And this is why they are going to be doing hotfires of the 3 cores together (perhaps more than one), and why they have hold-downs that won't release FH unless the thrust is good on all cores.
« Last Edit: 12/29/2017 05:48 PM by Lars-J »

Offline Rocket Science

  • Senior Member
  • *****
  • Posts: 8175
  • NASA Educator Astronaut Candidate Applicant 2002
  • Liked: 2330
  • Likes Given: 5474
Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #629 on: 12/29/2017 05:39 PM »
Due to thrust torque (a thrust-induced rotation)...

I believe the torque mentioned is around a horizontal axis. A single stick might use an asymmetric engine ignition sequence without overstressing its own octaweb. But placed side by side, the asymmetric startup thrusts could overstress one or both webs. Now think of that happening on both sides of the center booster web simultaneously... complex.

I thought the problem would actually be in any mismatch in the overall thrust startup of the two side boosters, which could result (say) in the links on one side of the center booster being lift-loaded before the other. Could cause a center tube or link failure, I would imagine. Fireball.
My thoughts are that it would then result in a pad abort shutdown and then safing the vehicle...
« Last Edit: 12/29/2017 05:40 PM by Rocket Science »
"The laws of physics are unforgiving"
~Rob, Physics instructor, aviator, vintage auto racer

Offline topopesto

  • Member
  • Posts: 87
  • Liked: 15
  • Likes Given: 0
Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #630 on: 12/29/2017 05:49 PM »
I think that this mission it's be fine without problem.

Offline mhenderson

  • Member
  • Posts: 69
  • USA
  • Liked: 101
  • Likes Given: 18
Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #631 on: 12/29/2017 06:12 PM »
How much heavier is this experimental FH than future commercial versions? 

The first prototype is likely to be heavier than future vehicles because of higher margins allowed for unknown stresses, the ongoing march of materials technology, and the use of extra instrumentation on the first flight that becomes irrelevant after a few flight experiences.

The first space shuttle to reach orbit (Columbia) weighed about 4000 kg more than its successors.  Part of this (~500 kg) was due to instrumentation used on the first four test flights and not needed on successive flights.  Columbia was "put on a diet" and updated to replace gear with lighter advanced materials / designs, or to remove those unnecessary instruments.  https://spaceflightnow.com/shuttle/features/000414overhaul/weightloss.html

In this case, much of the center FH booster core, the interstage and upper stage have been refined through dozens of launches of single stick rockets.  The outer cores on this vehicle are "block 3" (or whatever the heck they are calling them this week) cores.

SpaceX has also announced that they intend to freeze Falcon and redirect resource to BFR development, leaving less manpower for successive improvements to FH. 

So, how close is this beast to the "final" FH?   


Offline Lars-J

  • Senior Member
  • *****
  • Posts: 3591
  • California
  • Liked: 2836
  • Likes Given: 1787
Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #632 on: 12/29/2017 06:29 PM »
So, how close is this beast to the "final" FH?

F9 and FH will continue to evolve, so I'm not sure what you are asking for. (For example, every single F9 that has launched has likely had some unique tweak, this is common)

But I do not expect any significant mass changes. Sure some mass might be removed for added efficiency, but mass might also be added for recovery hardware. And FH has so much performance margin that there isn't much need for reducing mass.

Offline meekGee

  • Senior Member
  • *****
  • Posts: 7894
  • N. California
  • Liked: 4141
  • Likes Given: 843
Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #633 on: 12/29/2017 07:01 PM »
I think the probability of success is pretty high, but not as high as a F9 of course.
I think 50-50 is about right.  That's what Elon suggested, if I recall correctly. 

There are certainly some untested aspects of this vehicle, especially base heating, max-q transonic aerodynamic stresses, pogo, etc.  History is a guide.  The first Delta 4 Heavy failed, as did three of the first six Titan IIIC missions and one of the first two Saturn 5 launches, though only one of those five combined failures was an outright Fail to Orbit.

As Elon said, we'll all be happy if this thing gets off the pad and over the Atlantic without mishap.  I believe that the odds of at least that happening are pretty good.

 - Ed Kyle

I'm amazed that people continue to not understand the expectations game that Elon plays - all the time.

If they truly think that FH only has a 50% chance of succeeding, they are not going to launch. Period. They are going to want to be a lot more confident than that.

If you take Ed's tradition definition of failure, which includes inaccurate orbital insertions at the very least, then 50-50 is not grotesque (though still pessimistic IMO).

I'd say the odds for a picture-perfect mission, correct trajectory, two good landings - that's 50-50.
ABCD - Always Be Counting Down

Online edkyle99

  • Expert
  • Senior Member
  • *****
  • Posts: 12957
    • Space Launch Report
  • Liked: 3710
  • Likes Given: 634
Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #634 on: 12/29/2017 07:14 PM »
I'm amazed that people continue to not understand the expectations game that Elon plays - all the time.

If they truly think that FH only has a 50% chance of succeeding, they are not going to launch. Period. They are going to want to be a lot more confident than that.
Above all else, SpaceX wants a useful test flight.  That doesn't necessarily require a "success" by the strictest definitions.  I'm certain that the company has high expectations (much greater than 50%) of at least getting to Max-Q, which would provide substantial development data regardless of the ultimate mission outcome.

 - Ed Kyle
« Last Edit: 12/29/2017 07:15 PM by edkyle99 »

Offline biosehnsucht

  • Full Member
  • *
  • Posts: 199
  • Liked: 52
  • Likes Given: 71
Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #635 on: 12/29/2017 07:30 PM »
Re: start up torque

don't forget that there's more hold downs on the FH than the F9, so there's more paths for any torque to have the force removed than just via the octowebs. It should also be trivial to design the startup sequence to balance the torque across a given core, and with only a few engines starting at a time, the maximum imbalance across all cores even if there are initial thrust variations before thrust settles for a given set of engines starting, should be trivial compared to a full F9 startup.

I think that torque during startup is a non-issue, and for launch mainly because they'll wait until thrust is balanced out before launching (and abort otherwise).

I'd be far more concerned with aerodynamic and thrust (in a non-torque sense) loads and the separation sequence.

Offline drunyan8315

  • Member
  • Posts: 38
  • Seattle
  • Liked: 5
  • Likes Given: 1
Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #636 on: 12/29/2017 07:45 PM »
Ideally you are correct. I suppose it depends on the precise engineering of the hold-downs and structure as to whether any torques or displacements can exist prior to release, and be transmitted from one core to another. Hopefully the answer is NO.

Online mme

  • Full Member
  • ****
  • Posts: 1093
  • Santa Barbara, CA, USA, Earth, Solar System, Milky Way Galaxy, Virgo Supercluster
  • Liked: 1315
  • Likes Given: 3454
Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #637 on: 12/29/2017 07:50 PM »
If you watch the the liftoff of the first Falcon 9, it clearly rotates.  For a single stick, that's not really a big deal.  But now bolt three of them together and rather than rotate freely in space they will be torquing the connections.
I'm sure that would be a problem if SpaceX was trying to launch three of the very first Falcon 9 1.0 rockets together... given that none of the 45 subsequent Falcon 9's have done this, I'd have to think it isn't a factor today.
We had pages of "what torsional forces?" and I was trying to provide a clear example where one could visualize the forces acting on a single booster. The fact that you don't "see" the rotation after launch does not mean there are not extreme torsional forces during start up nor that there are not small constant corrections in flight.

With the F9 as you gimbal the engines any vectoring differences go into the single octaweb.  With the FH you have the sum of the vectors going into the octaweb of each booster and then the resulting forces being transmitted into the attachment points of the octawebs.

The GNC needs to vector/throttle 27 engines in a way that controls a rocket as a whole and it needs to do it in a way that does not over-stress the attachment points. Depending on the information available to the GNC that may be trickier than it sounds.

I have no doubt that SpaceX has done their very best to model all of this. I am optimistic that this mission will succeed. But I think people are over simplifying the dampening and feedback effects of bolting three really powerful rockets together.
Space is not Highlander.  There can, and will, be more than one.

Offline Karloss12

  • Full Member
  • **
  • Posts: 200
  • Liked: 29
  • Likes Given: 0
Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #638 on: 12/29/2017 09:08 PM »
I'm amazed that people continue to not understand the expectations game that Elon plays - all the time.

If they truly think that FH only has a 50% chance of succeeding, they are not going to launch. Period. They are going to want to be a lot more confident than that.

How do you suggest they become more confident?  You can do Engineering calculations until the cows come home, however they will only ever get you so far.  Sometimes you just have to be brave to go for it and learn from a 50:50 failure.  It's not like the Block 3 cores are going to be used again if it is a success.
The cores operating independently have now been proven with F9 launches, so I don't see any issues with individual cores failing.
As there is a very light payload, the axial forces will be low on the centre core, therefore I don't expect the centre core to fail.
In the side cores, everything above the octaweb is over designed as they are designed to lift a payload positioned on top of them.  For FH, the side cores are lifting the centre core from the base.
I think the primary purpose of the FH demo launch will be to test the core connection mechanisms during lift off, flight and separation under light loading.
If they can confirm that then additional weight will be added for the next test.
« Last Edit: 12/29/2017 09:12 PM by Karloss12 »

Online cscott

  • Senior Member
  • *****
  • Posts: 2637
  • Liked: 1843
  • Likes Given: 658
Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #639 on: 12/29/2017 09:31 PM »
They are doing a staggered start. They would not be adding that complexity if there weren't a startup situation that would cause a Bad Day otherwise.  So some combination of ignition delays and thrust asymmetries can overtorque the octowebs. The only question is whether the staggered start is sufficient to address the issue, or whether we will find out the hard way about another bad coupling mode between the engines.  With N engines, there are N^2/2 possible interactions between them...

I agree with others that any bad effect here would most likely be found during static fire and would be resolved before the hold downs were released.  But it doesn't seem impossible to me that the first all-up static fire will torque the FH in such a way that they need to stand down and send the FH back to the shed to repair/replace components. I'm sure SpaceX has done all the modelling possible to ensure this doesn't occur.  But there are unknown unknowns... and also always the possibility of human error, as in the test stand failure that resulted in damaging 8 of 9 engine bells at McGregor.  Murphy is always fond of discovering ways to do "impossible" things.

Tags: