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

[Hauerg]

What the duck!
Does not the fact that we "have" to discuss some lost paint and coating show just HOW successful this flight has been ?

[clongton]

(snip)
He said "no damage" and based on his history of truthfulness and informed statements I believe him. There was no "damage".

I agree with you.
What was the source of the first of the two images in the comparison?
There are odd differences, including an obvious element that seems to have switched sides of the N2 thruster cluster, which is in the red circles below.
They are definitely both FT interstages, with the lowered N2 thrusters and no cowlings over the grid fin pivots.
Could they be of opposite sides of the rocket?  Is there a logo on the "back" side?

I believe they are opposite sides of the rocket. If it is rotated 180 degrees everything drops into the identical spot.

[cscott]

Well, I'm also on the optimistic side.  I'm just saying not to completely shoot down the pessimists, they have an okay point--although I wish the pessimists would concentrate on engine inspection, etc, rather than trying to make a big deal out of chipped paint.  There was a minor fuel leak during landing, it was visible on the video, that's not a nominal event.

My take: Elon's tweet is a little simplistic, but basically correct. Nothing major is broken that wouldn't be normal to address on the usual course of pre-launch preps.  (Without getting into specifics, "fuel leaks" have occurred before during the normal flow; they are repaired without undue drama. Presumably paint touch ups are also not a big deal.) The pessimists are also basically correct: there's no way you'd want to just stand this rocket up on the stands and launch it again right away.  But no one is seriously proposing to do that.  The point of these initial recoveries is to find weak spots and remedy them.  By all accounts that is exactly what is happening.

[Rocket Science]

Here is a diagram showing shock wave interactions inside and around the grid-fin these may produce the erosion we see... I posted this info link on another thread.

 "the airflow through the cells of the fin can be accelerated to supersonic speeds creating normal shock waves within the grid. This behavior is often referred to as a choked flow that blocks incoming air from passing through and increases drag. The drag grows even worse for a weapon flying slightly faster than Mach 1 since a detached normal shock wave, also called a bow shock, can form in front of the fin. This bow shock worsens the effect of the choked flow by forcing additional air to spill around the fin further increasing drag and reducing control effectiveness.

This situation improves as the weapon accelerates beyond the transonic region, approximately Mach 0.8 to 1.3. At higher speeds, the normal shock is "swallowed" and shock waves are instead formed off the leading edges of the lattice at an oblique angle. The oblique angle is still fairly large at low Mach numbers, causing the shock waves to reflect off the downstream lattice structure. These reflections can create a large number of shock waves within the grid fin resulting in high drag. As Mach number increases, however, the smaller the oblique shock angle becomes until the shock passes through the structure without intersecting it."

http://www.aerospaceweb.org/question/weapons/q0261.shtml

[Andy USA]

Thread clean up as random people on the internet are not in a position to question if Elon is "lying" about the condition of the core stage or not, because some paint is missing.

[meekGee]

Here is a diagram showing shock wave interactions inside and around the grid-fin these may produce the erosion we see... I posted this info link on another thread.

 "the airflow through the cells of the fin can be accelerated to supersonic speeds creating normal shock waves within the grid. This behavior is often referred to as a choked flow that blocks incoming air from passing through and increases drag. The drag grows even worse for a weapon flying slightly faster than Mach 1 since a detached normal shock wave, also called a bow shock, can form in front of the fin. This bow shock worsens the effect of the choked flow by forcing additional air to spill around the fin further increasing drag and reducing control effectiveness.

This situation improves as the weapon accelerates beyond the transonic region, approximately Mach 0.8 to 1.3. At higher speeds, the normal shock is "swallowed" and shock waves are instead formed off the leading edges of the lattice at an oblique angle. The oblique angle is still fairly large at low Mach numbers, causing the shock waves to reflect off the downstream lattice structure. These reflections can create a large number of shock waves within the grid fin resulting in high drag. As Mach number increases, however, the smaller the oblique shock angle becomes until the shock passes through the structure without intersecting it."

http://www.aerospaceweb.org/question/weapons/q0261.shtml

Yup.  Though of course the damage we see is on the outside wall, but adjacent the other structures (that whole shoulder joint)

Hey, maybe every 10 flights they'll have to rotate the fins...

[meekGee]

This is a replay of similar past episodes which have always in retrospect amounted to nothing.

Nothing to see here, move along.

[docmordrid]

From SpaceX FB:

Looks like someone hit the engine nozzle at 6 o'clock with a Scotch-Brite pad.

[cartman]

Reposting image from update thread. Unsurprisingly the engine area is much sootier than the rest of the rocket.
1) Can we guess which are the outer engines that fired for the boost back burns? my guess is the 1o clock and 7o clock ones.
2) Has SpaceX released any pictures of the engine area after a full duration static fire? it would be interesting to compare.
3) Under the engines to the right is a small pipe that leads to a cart with 2 large gas cylinders.

[matthewkantar]

From SpaceX FB:

Looks like someone hit the engine nozzle at 6 o'clock with a Scotch-Brite pad.

I am thinking the 6 O'clock engine and the one across from it are the boost back engines, therefore less sooty. just a guess.

Matthew

*edit* the center one is awfully sooty though, so maybe not.

[Johnnyhinbos]

Again, as I've said before, I am questioning the center engine being used during the three engine burn (as in, I think it's only used for final burn). For supersonic retrograde you want the engines on the periphery to not impact aero drag. A center engine would deform the shockwave, thereby reducing drag and therefore associated slowing...

[Lars-J]

From the update thread:

Again, as I've said before, I am questioning the center engine being used during the three engine burn (as in, I think it's only used for final burn). For supersonic retrograde you want the engines on the periphery to not impact aero drag. A center engine would deform the shockwave, thereby reducing drag and therefore associated slowing...

It is certainly used in the boost-back burn. And visual evidence from the stage 1 rocket cam footages that have been released certainly make it look like 3 parallel engines burning during the reentry/braking burn. (Exhaust interaction)

And the drag reduction properties of a center engine depends on the thrust level (low thrust has that effect) and the reentry speed and atmospheric density. It also assumes a single engine, the combination with outer engines is far less obvious.

[OxCartMark]

Reposting image from update thread. Unsurprisingly the engine area is much sootier than the rest of the rocket.

Engine in 6:00 position looks clean(ish).  Perhaps cleaned up for better inspection?  Perhaps swapped out?  Perhaps those of you who have a handle on the coordinate system of the stage can see if that engine corresponds to where we saw the flameyness after landing.

edit:
Related post for those that visit L2: http://forum.nasaspaceflight.com/index.php?topic=38667.msg1462838#msg1462838

[John Alan]

My opinion... engine bell on 6 o'clock position had interior clean a bit... likely to check condition...

I'm looking at the building... 
That is the first pic I have seen released of the inside on that building... 

[Dante80]

A question. What is that blue capped mechanism on the side of the octoweb (looking around 10 o clock)?

Also, pretty cool HIF view.

[meekGee]

Another wonderment - what's the instrument in the background, visible under the rocket under the blue slash of the X of the SpaceX logo?

[Steven Pietrobon]

1) Can we guess which are the outer engines that fired for the boost back burns? my guess is the 1o clock and 7o clock ones.

In the enhanced image below, the insides of those engines definitely appear to be lighter in colour.

[llanitedave]

From the update thread:

Again, as I've said before, I am questioning the center engine being used during the three engine burn (as in, I think it's only used for final burn). For supersonic retrograde you want the engines on the periphery to not impact aero drag. A center engine would deform the shockwave, thereby reducing drag and therefore associated slowing...

It is certainly used in the boost-back burn. And visual evidence from the stage 1 rocket cam footages that have been released certainly make it look like 3 parallel engines burning during the reentry/braking burn. (Exhaust interaction)

And the drag reduction properties of a center engine depends on the thrust level (low thrust has that effect) and the reentry speed and atmospheric density. It also assumes a single engine, the combination with outer engines is far less obvious.

The other problem with using three peripheral engines is that the octoweb design doesn't allow for even spacing between them.  Each engine is spaced at 45 degrees, so if two engines are fired at 90 degrees, the third will be at 135 degrees from those two.  The vector triangle will not be equilateral, and the resulting shape of the shock wave... who knows?

[deruch]

Again, as I've said before, I am questioning the center engine being used during the three engine burn (as in, I think it's only used for final burn). For supersonic retrograde you want the engines on the periphery to not impact aero drag. A center engine would deform the shockwave, thereby reducing drag and therefore associated slowing...

SpaceX has repeatedly and explicitly stated that they use 3 engines (including the center engine) for both the boostback and re-entry burns.  End discussion.  All in all, I'm going to assume that they considered pretty much every variation on which engines to use in which burns and determined that for whatever reason that configuration was the best for them.

[llanitedave]

Again, as I've said before, I am questioning the center engine being used during the three engine burn (as in, I think it's only used for final burn). For supersonic retrograde you want the engines on the periphery to not impact aero drag. A center engine would deform the shockwave, thereby reducing drag and therefore associated slowing...

As I mentioned elsewhere, a problem with using three peripheral engines is that the octoweb design doesn't allow for even spacing between them.  Each engine is spaced at 45 degrees, so if two engines are fired at 90 degrees, the third will be at 135 degrees from those two.  The vector triangle will not be equilateral, and the resulting shape of the shock wave... who knows?