SpaceX Falcon Heavy Demo - Discussion and Speculation

[Jim]

I don't understand why people think this is a problem

[Jim]

This was 60 years ago.

[StarGeezer]

I assume the greatest 'torque' or 'mis-alignment' of thrust would occur at engine startup. Is this the reason that most if not all launches worldwide are 'held back' until 'proper' thrust is achieved?

[abaddon]

I don't understand why people think this is a problem [...]
This was 60 years ago.

Well, one difference is we are talking about 27 engines.  Presumably there could be some unique properties associated with that which would not exist with one engine per booster.

But in general, agree there's a lot of hand-wringing here, of course SpaceX could fail, but it's not really that unique a thing they are doing.  The most unique thing about it is ironically also the thing that is most proven, i.e. boosters post-sep coming back to land successfully.

[rcoppola]

This was 60 years ago.

But do any of these examples approach what they are trying to achieve with FH? High capability, low cost, re-use? Sure, tri-cores have been with us for a long time. But a tricore that costs sub 100million, throwing up 56t to LEO and returns all of it's boosters for reuse? Not so much. But hey, you're the one on the ground. So if new cost, capability and reuse aren't driving this complexity train then what is? (and I include managing 27 engines in this since part of their cost savings is using an existing booster design)

[M_Puckett]

This was 60 years ago.

50 Jim.

[Jim]

This was 60 years ago.

But do any of these examples approach what they are trying to achieve with FH? High capability, low cost, re-use? Sure, tri-cores have been with us for a long time. But a tricore that costs sub 100million, throwing up 56t to LEO and returns all of it's boosters for reuse? Not so much. But hey, you're the one on the ground. So if new cost, capability and reuse aren't driving this complexity train then what is? (and I include managing 27 engines in this since part of their cost savings is using an existing booster design)

Meaningless points.  They are all rockets.  Cheap or expensive, it doesn't change the engineering.  Just like a cake, the same things have to be done to make and fly one.

They don't have to worry about "high" capability and  re-use on the first one.  They can increase their margins if they want.

[Rocket Science]

Delta IVH uses laser alignment during the attachment of the side boosters and the remainder is up to the guidance system...

[rcoppola]

Come now Jim...they weren't points. More like honest questions.
But I am very interested in this flying cake of yours.

[Space Ghost 1962]

They don't have to worry about "high" capability and  re-use on the first one.  They can increase their margins if they want.

Yes.

(He's referring to "pinching" the stack with the side boosters to have the relative TO's push against each other through the central core. Deals with much of it except for the axial torques, which need to be taken up through the thrust structures on the base.)

Still, each of the vehicles you posted were difficult, worthy accomplishments, each on their own.

Good reminder of never let your pursuit of the "perfect" cause you to lose sight of the "good enough". Point taken.

[llanitedave]

Taken from the FH mission 3 thread:

At some point leading up to MaxQ, the torques between the three boosters will attempt to tear apart the stack.

Please clarify my ignorance.  I have heard on these forums several times now about torques being present between the cores of the Falcon Heavy stack.  There was a reference to startup torques requiring a staggered ignition of the engines.  Given that these rockets are symmetrical, I fail to see where this "torque", or twisting force, is coming from.  Why would there be such, apart from random vibrations of the ignited motors?

LREs don't "combust" uniformly, more like loosely controlled chaos. Also, they are all gimballed, and the gimbals have certain variation/misalignment/compliance as well. Now, imagine you've got 3 sets of 9 each resultant vectors, each pulling/pushing/pitching/yawing/rolling at the thrust structures, which in turn are linked together. Oh, and also add in roll for each of these three groups as well. And axial thrust oscillations of each mass/thrust group per booster. (These forces are nontrivial, a significant percentage of the engine's thrust.)

You're goal is to keep the aggregate thrusts mostly aligned (for performance), and the thrust structures (plus combinations) from twisting/shearing/pivoting such that aggregate peak loads don't fatigue/fracture the thrust structures. You've got a big problem if multiple ones just happen to synchronize and pull apart (or together) in phase. You can't just assume that the randomness of the vectors will solve the problem for you.

And I haven't gone into the resonance effects of these long fluid filled cylinders, the effects of deep cryogens, the placement of separation hardware, the need to have certain passive behavior on shutdown, and still more (like fluid displacement, sloshing(!), and altitude). You've got limited controls to affect this and certain latencies to when the signaled control actually affects the deviation. It's like playing an enormous pipe organ, offsetting vector accumulations with an induced opposing "note"/phase to diminish the deviation. Gradually.

If combustion/engines were ideal, this would be considerably easier and like how you seem to believe it would work. Rest assured its a bit more than that.

(If it were easy, you'd see more clustered vehicles.)

Seems straitforward enough. What could go wrong?

[redliox]

I don't understand why people think this is a problem

Well this is SpaceX's first time with large, multiple boosters.  They've mastered and (perhaps more accurately) wowed us with 1st stage reusability.  This is a new challenge even Elon is wary about.  Remember how many landing failures were suffered before SpaceX stuck a decent landing; SpaceX is in THAT phase of FH for the moment.  I'm confident SpaceX is mature enough to handle the FH difficulties, but this is new all the same.  Otherwise all we can do is wait for November and prepare to see FH's unveiling.

[wannamoonbase]

I don't understand why people think this is a problem

Well this is SpaceX's first time with large, multiple boosters.  They've mastered and (perhaps more accurately) wowed us with 1st stage reusability.  This is a new challenge even Elon is wary about.  Remember how many landing failures were suffered before SpaceX stuck a decent landing; SpaceX is in THAT phase of FH for the moment.  I'm confident SpaceX is mature enough to handle the FH difficulties, but this is new all the same.  Otherwise all we can do is wait for November and prepare to see FH's unveiling.

A new rocket from a new-ish company.  But they aren't rookies anymore and no question they have modeled and designed and checked over and over.  I'm confident it will work, but there is uncertainty in every rocket launch, especially the first flight of a rocket.

Fingers crossed it flies this year.

[ChrML]

What is the reason for the side by side rocket arrangement, as opposed to example a triangle arrangement with the payload mounted the same way on one of them?

Would seem the triangle arrangement would allow for better/stiffer attachment, thus be less sensitive to thrust differences. It's not like it's easy to mount two cylinders in a stiff matter next to each other.

[jabe]

my $0.02  can throttle the two outside cores higher and keep center core more fueled at staging..  a "triangle" makes that harder..
jb

[RotoSequence]

I don't understand why people think this is a problem

I think most people perceive SpaceX as a company that employs simple solutions to complex problems (whether that's warranted or not). What SpaceX discovered is that tri-core rockets are a relatively complex solution to a simple problem. I don't think people trust SpaceX in that domain yet.

[Kenp51d]

What do you think are the highest risk events that will occur during the launch? And when they get past one nail biting event, what would the next event be that is of course.
I know everything has to be like clockwork during any launch.

Sent from my XT1565 using Tapatalk

[Kenp51d]

From the other thread:

It's logistics/operations that one should fear before T0. So much has to go right before you make it to ignition. Could sit on the pad for a month. Complex beast, more like 5x the trouble for three boosters.

Next, it's the hold down time and validating vehicle before launch and after ignition. Static fire?

Then its that all the clamps go. Otherwise engine shutdown.

After off the pad, very likely to clear the tower, and find out how well the acoustics worked. Very loud as things don't scale linearly (overtones).

At some point leading up to MaxQ, the torques between the three boosters will attempt to tear apart the stack. But likely the oscillations will be damped and fall by flight software (to be later analyzed to improve vehicle performance). This starts where the most critical phase begins, ending with engine shutdown and side booster separation.
...

So risks and what to expect.

In short, if they make it off the pad, they'll not have a disaster, and that's likely.

Thanks for posting that.

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[oldAtlas_Eguy]

From the other thread:

It's logistics/operations that one should fear before T0. So much has to go right before you make it to ignition. Could sit on the pad for a month. Complex beast, more like 5x the trouble for three boosters.

Next, it's the hold down time and validating vehicle before launch and after ignition. Static fire?

Then its that all the clamps go. Otherwise engine shutdown.

After off the pad, very likely to clear the tower, and find out how well the acoustics worked. Very loud as things don't scale linearly (overtones).

At some point leading up to MaxQ, the torques between the three boosters will attempt to tear apart the stack. But likely the oscillations will be damped and fall by flight software (to be later analyzed to improve vehicle performance). This starts where the most critical phase begins, ending with engine shutdown and side booster separation.
...

So risks and what to expect.

In short, if they make it off the pad, they'll not have a disaster, and that's likely.

If the hot-fire test is a success then they will make it off the pad. Remember this thing has a high T/W >1.2 probably as high as 1.4 at liftoff. Once it clears the pad the center core will likely start throttling back in order to manage the MAX Q event. Too much acceleration too low in the atmosphere can create a destructive MAX Q. So a throttle back of the center core will likely happen sometime after the first 10 seconds. By 10 seconds the velocity would be ~40m/s, altitude ~200m. Prop consumed ~76mt.

[ChrML]

my $0.02  can throttle the two outside cores higher and keep center core more fueled at staging..  a "triangle" makes that harder..
jb

Most likely, but down-throttling is more of an optimization than neccessary. With cross-feed implemented, down-throttling is not neccessary.