SpaceX Falcon 9 FT - ORBCOMM-2 - Dec. 21, 2015 (Return To Flight) DISCUSSION

[Jim]

As SpaceX returns to being a company actively launching payloads I think they deserve a lot of praise. They are approaching the point where they will declare their first Full Thrust vehicle to be ready for flight. But not only is this a substantially different vehicle from those they have flown before, they are attempting to reach flight readiness along a path different from the paths they took with prior vehicles.

That's because this is their first flight since they have changed their corporate "risk posture." Previously they were comfortable with the level of risk associated with using parts like the strut that failed; now they are not. So even though they've previously brought three launch vehicles to flight readiness (F1, F9 1.0, F9 1.1) this time they're asking not just "are we on the path" but also "does this path truly lead to readiness?"

I wish them good fortune, and hope they will be a shining example of how to "do it right!"

Others have being "doing it right" from the beginning.

[Lars-J]

A question about the MOOG dispenser for the 11 satellites - I assume it stays attached to the 2nd stage, right?

And do we know if the upper stage restart test after deployment be a de-orbit disposal burn?

[Semmel]

Others have being "doing it right" from the beginning.

Who?

[Karloss12]

I'm surprised people are focusing on QA/QC for this strut and yet no discussion on fault tolerance. I mean, the F9 doesn't need nine Merlin 1D engines to get off the pad, rather there's that many for fault tolerance.

Personally I find it amazing that one strut could fail and take out the entire shooting match. I would think if a strut holding a tank failed at 1/5 rated load then in a properly designed fault tolerant system the surrounding strut work should be able to take on the added load.

Just thinking out loud...

When you look at things like hand rails that run next to foot paths and stairways around schools and offices, you will never find the bolting plate that fastens the rail to the vertical ground post having a single bolt in it (it is just to risky).  It always has 2 or 4 bolts for redundancy or is welded.

If a bolt plate has a single bolt then there is about a 1 in 5000 chance of a hand rail failing causing injury.  If there is 2 dual bolts then there is a much more reasonable 1 in 5000×50000 = 25000000 of dual bolts failing causing injury.

If SpaceX retains this single bolt without redundancy design and introduces testing, then can anyone estimate what the new chances of strut failure will be?  1 in 5000 improving to 1 in 50000 maybe???  This sounds like a silly thing to say, but as the F9 is man rated, doesn't logic say the struts need a similar level of safety as a stairway hand rail?  Maybe not, as a walkway handrail doesn't have the luxury of an LAS. :-)

I don't work in the rocket business so don't know how precious weight saving is.  Would the "long term" redesign solution possibly involve adding a second redundant bolt even if it added a few kg's to the weight rocket?

[OxCartMark]

Who?

Suess sez best practice is to not hear a who.

I'm surprised people are focusing on QA/QC for this strut and yet no discussion on fault tolerance. I mean, the F9 doesn't need nine Merlin 1D engines to get off the pad, rather there's that many for fault tolerance.

Personally I find it amazing that one strut could fail and take out the entire shooting match. I would think if a strut holding a tank failed at 1/5 rated load then in a properly designed fault tolerant system the surrounding strut work should be able to take on the added load.

Just thinking out loud...

When you look at things like hand rails that run next to foot paths and stairways around schools and offices, you will never find the bolting plate that fastens the rail to the vertical ground post having a single bolt in it (it is just to risky).  It always has 2 or 4 bolts for redundancy or is welded.

If a bolt plate has a single bolt then there is about a 1 in 5000 chance of a hand rail failing causing injury.  If there is 2 dual bolts then there is a much more reasonable 1 in 5000×50000 = 25000000 of dual bolts failing causing injury.

If SpaceX retains this single bolt without redundancy design and introduces testing, then can anyone estimate what the new chances of strut failure will be?  1 in 5000 improving to 1 in 50000 maybe???  This sounds like a silly thing to say, but as the F9 is man rated, doesn't logic say the struts need a similar level of safety as a stairway hand rail?  Maybe not, as a walkway handrail doesn't have the luxury of an LAS. :-)

I don't work in the rocket business so don't know how precious weight saving is.  Would the "long term" redesign solution possibly involve adding a second redundant bolt even if it added a few kg's to the weight rocket?

I think you have analyzed a whole nuther animal and come to a correct conclusion for that animal but a flawed conclusion for the critter being examined. 

Let's simplify your railing down to a single vertical post with a sideways load applied at the top which has a plate welded to the bottom with a bolt at each corner of the plate.  Down at the plate there is a substantial bending moment.  Since the bolt taking the load needs to be on the side of the plate where the bolt will be in tension to do much good and since you don't know the direction of the force at the top of the post you have to have multiple bolts in multiple positions to accommodate for all load cases.  Or put another way, if you put all four bolts in the same corner of the base plate it would be very easy to tip the post over in one applied force direction.

In the case of the F9 strut (and struts in general) its a two force member, meaning that the forces applied at either end are equal and in opposite directions so that the line of force is always going to be straight down the axis of the strut.  With a single bolt at each end of each strut allowing pivoting around the bolt and with a properly designed system of multiple struts all applied forces will automatically be resolved to be along the axis of strut(s) with no significant bending moment.

Counterintuitively, adding extra bolts to the end of the strut could cause failure because it would allow bending moments to be introduced to the strut which could cause buckling failure.   Also counterintuitively, adding additional struts could increase the loads being carried by the struts rather than reduce the loads.

[meekGee]

Who?

Suess sez best practice is to not hear a who.

I'm surprised people are focusing on QA/QC for this strut and yet no discussion on fault tolerance. I mean, the F9 doesn't need nine Merlin 1D engines to get off the pad, rather there's that many for fault tolerance.

Personally I find it amazing that one strut could fail and take out the entire shooting match. I would think if a strut holding a tank failed at 1/5 rated load then in a properly designed fault tolerant system the surrounding strut work should be able to take on the added load.

Just thinking out loud...

When you look at things like hand rails that run next to foot paths and stairways around schools and offices, you will never find the bolting plate that fastens the rail to the vertical ground post having a single bolt in it (it is just to risky).  It always has 2 or 4 bolts for redundancy or is welded.

If a bolt plate has a single bolt then there is about a 1 in 5000 chance of a hand rail failing causing injury.  If there is 2 dual bolts then there is a much more reasonable 1 in 5000×50000 = 25000000 of dual bolts failing causing injury.

If SpaceX retains this single bolt without redundancy design and introduces testing, then can anyone estimate what the new chances of strut failure will be?  1 in 5000 improving to 1 in 50000 maybe???  This sounds like a silly thing to say, but as the F9 is man rated, doesn't logic say the struts need a similar level of safety as a stairway hand rail?  Maybe not, as a walkway handrail doesn't have the luxury of an LAS. :-)

I don't work in the rocket business so don't know how precious weight saving is.  Would the "long term" redesign solution possibly involve adding a second redundant bolt even if it added a few kg's to the weight rocket?

I think you have analyzed a whole nuther animal and come to a correct conclusion for that animal but a flawed conclusion for the critter being examined. 

Let's simplify your railing down to a single vertical post with a sideways load applied at the top which has a plate welded to the bottom with a bolt at each corner of the plate.  Down at the plate there is a substantial bending moment.  Since the bolt taking the load needs to be on the side of the plate where the bolt will be in tension to do much good and since you don't know the direction of the force at the top of the post you have to have multiple bolts in multiple positions to accommodate for all load cases.  Or put another way, if you put all four bolts in the same corner of the base plate it would be very easy to tip the post over in one applied force direction.

In the case of the F9 strut (and struts in general) its a two force member, meaning that the forces applied at either end are equal and in opposite directions so that the line of force is always going to be straight down the axis of the strut.  With a single bolt at each end of each strut allowing pivoting around the bolt and with a properly designed system of multiple struts all applied forces will automatically be resolved to be along the axis of strut(s) with no significant bending moment.

Counterintuitively, adding extra bolts to the end of the strut could cause failure because it would allow bending moments to be introduced to the strut which could cause buckling failure.   Also counterintuitively, adding additional struts could increase the loads being carried by the struts rather than reduce the loads.

In short, it's a kinematic design because of the thermal and dynamic environments, and kinematic designs are generally not fault tolerant.

[StuffOfInterest]

The updates thread has a photo of the carrier structure for the satellites.  Being that the carrier has 12 positions symmetrically located and there are only going to be 11 satellites, how will they handle the weight imbalance?  Can the rocket fly with a slightly off balance weight loading or will a dead weight be put in the 12th position?

Previously, I thought maybe the carrier would have two rings of four and one ring of three to keep the loading balanced.

[MechE31]

The updates thread has a photo of the carrier structure for the satellites.  Being that the carrier has 12 positions symmetrically located and there are only going to be 11 satellites, how will they handle the weight imbalance?  Can the rocket fly with a slightly off balance weight loading or will a dead weight be put in the 12th position?

Previously, I thought maybe the carrier would have two rings of four and one ring of three to keep the loading balanced.

It will most likely have a mass simulator in all unoccupied positions like the previous OG2 flight.

[Skyrocket]

Here's an image of the mass simulators from the previous flight:

[edkyle99]

I mean, the F9 doesn't need nine Merlin 1D engines to get off the pad, rather there's that many for fault tolerance.

It does need nine to liftoff.  Eight wouldn't provide enough T/W at liftoff (only 1.05 or so for v1.1).

 - Ed Kyle

[rpapo]

I mean, the F9 doesn't need nine Merlin 1D engines to get off the pad, rather there's that many for fault tolerance.

It does need nine to liftoff.  Eight wouldn't provide enough T/W at liftoff (only 1.05 or so for v1.1).

 - Ed Kyle

Which is why the launch pad has hold-downs.  If any engine isn't performing up to snuff at right that instant, they don't have to let it go.  The risk is in the first minute or so.  If you lose any one engine too soon, you're still going to lose the flight.

[Skyrocket]

why not some cubesats everyone else does?

The OG2 orbit is too high to satisfy the 25 years rule for the cubesats and apparently SpaceX wants to do some tests after OG2 deployment, which might end also in an orbit not desirable for cubesats.

[Hywel1995]

SpaceX FCC application for Orbcomm launch communications approved (F9-21)

EXPERIMENTAL SPECIAL TEMPORARY AUTHORIZATION - https://apps.fcc.gov/els/GetAtt.html?id=169199&x=.

With a NET of 23 November 2015 - https://apps.fcc.gov/els/GetAtt.html?id=168689&x=.

[WHAP]

As Chris mentioned in the updates thread, the launch won't occur until December.  But what's the current speculation?  I've seen info that suggests as early as 12/4, but the Range typically requires a 48 hour turnaround so the earliest after OA-4 would be 12/5.  That's only a little more than 3 weeks away, with a holiday in between.  No indication of a booster test at McGregor, and no indication of status of the upper stage.  Some of those earlier single engine tests may have been of the upper stage (?).  What's the fastest that SpaceX has turned around from stage testing in Texas to being ready to launch?  With so many changes and this being an RTF mission, I can't see them trying to set records getting back on the pad.

[OnWithTheShow]

The booster was tested at McGregor on Monday.  No word on result but it was tested.

[abaddon]

Has SpaceX ever done a full-duration burn on a stage and then used that stage?  My (admittedly unreliable) memory was that the qual stage for F9 became Grasshopper, and the qual stage for F91.1 became F9R-Dev1.

[guckyfan]

Has SpaceX ever done a full-duration burn on a stage and then used that stage?  My (admittedly unreliable) memory was that the qual stage for F9 became Grasshopper, and the qual stage for F91.1 became F9R-Dev1.

I am quite sure Grasshopper was the structural test article. The first  first stage of each type was used for a full duration test fire and then flown.

[Chris Bergin]

Booster is still at McGregor (F9 FT, Stage 1 for this mission), so the wait continues the milestone of McGregor testing complete and the road trip for the stage.

[LastStarFighter]

Has SpaceX ever done a full-duration burn on a stage and then used that stage?  My (admittedly unreliable) memory was that the qual stage for F9 became Grasshopper, and the qual stage for F91.1 became F9R-Dev1.

Do they only do full duration tests on the qual stage? How long are the tests for each flight booster?

[abaddon]

Has SpaceX ever done a full-duration burn on a stage and then used that stage?  My (admittedly unreliable) memory was that the qual stage for F9 became Grasshopper, and the qual stage for F91.1 became F9R-Dev1.

Do they only do full duration tests on the qual stage? How long are the tests for each flight booster?

Actually... that's a great question, because they have had a lot of "louder/longer than usual" test warnings over the years.  So I guess it's actually the norm and I just forgot .