Author Topic: SpaceX Falcon 9 : Iridium NEXT Flight 4 : December 22/23, 2017 : Discussion  (Read 101012 times)

Online FutureSpaceTourist

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Is it just me or are those gridfins?

Maybe the patch is accurate and they're trying some entry lifting testing or something along those lines?

Yeah, looks like grid fins.

I'm guessing they're doing a Cassiope-style water landing with grid fins just to get more data and precision.

Presumably SpaceX would then need to look for and retrieve any significant pieces left floating to avoid being a shipping hazard? Maybe another job for Mr Steven, or is the fairing recovery area likely to be too far form the booster 'landing' position?

Offline JamesH65

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well if you're expending an older version stage, you might as well expend a few old style aluminum grid fins aswell  ;)

Why?

Even just leaving them off they have scrap value.

I presume they'll be testing something that requires guidance, but likely loss of booster. So just a booster you were going to lose anyway.

Offline vanoord

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Presumably SpaceX would then need to look for and retrieve any significant pieces left floating to avoid being a shipping hazard? Maybe another job for Mr Steven, or is the fairing recovery area likely to be too far form the booster 'landing' position?

It's mostly metal, so it'll sink.

The faster it hits, the more pieces there will be, but even if it soft-lands it'll blow up once it tips over (see previous landing attempts for further information!).

The fairings are composite, so float - and indeed have washed up in various places.

As for locations of where things will land, in theory the fairings would go further, but if they're fitted with steerable parafoils and if the core stage performs any sort of boost after separation, the locations can be altered as required.


Offline Ben the Space Brit

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Grid fins but no legs....

I wonder what they are planning to test that means they are not recovering the booster, but still need guidance.

Are we sure that those are the aluminium ones and not the titanium ones? If it's the latter, they may want to get some data on any difference in performance and guidance authority from the new fins through different atmospheric layers (perhaps due to different responses to heating from different materials) without a drone ship or landing pad in the way to suffer if they've got it wrong.
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Online RocketLover0119

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No, they are not titanium. And why waist Titanium into the ocean?

Grid fins but no legs....

I wonder what they are planning to test that means they are not recovering the booster, but still need guidance.

Are we sure that those are the aluminium ones and not the titanium ones? If it's the latter, they may want to get some data on any difference in performance and guidance authority from the new fins through different atmospheric layers (perhaps due to different responses to heating from different materials) without a drone ship or landing pad in the way to suffer if they've got it wrong.
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Offline StuffOfInterest

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Ok, everyone is hung up on grid fins but no landing legs or the lack of soot. What about the unpainted interstage?  I don't recall seeing that before, but I have missed details a few times.

Offline nacnud

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It looks painted to me, just dirty from its last flight.

Offline ketivab

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What are these yellow things on the TEL? They are on 39A, but I am sure they weren't present during previous launches from California.


Online KaiFarrimond

What are these yellow things on the TEL? They are on 39A, but I am sure they weren't present during previous launches from California.



They extend and support the payload while Falcon is horizontal. :)
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Offline garcianc

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Regarding the grid fins, I was wondering if this would be a good mission to test any software changes necessary for the returning stage to fly a side-booster profile trajectory in preparation for the FH mission.

Offline vanoord

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Regarding the grid fins, I was wondering if this would be a good mission to test any software changes necessary for the returning stage to fly a side-booster profile trajectory in preparation for the FH mission.

Yes, but...

The biggest change to the flight back is going to be the requirement to get the boosters apart immediately after separation, ie rather than pointing them directly back they need to fly away from each other - and at that point in the flight the manoeuvring is done using the thrusters (the fins pop out a little later),

Offline quagmire

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It will be interesting if SpaceX will still cover the 1st stage during its soft landing into the pacific on their stream. I know on their other expendable launches this year, they didn’t, but I believe those lacked grid fins as well and was dropped into the ocean the old fashioned way.

Offline garcianc

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Regarding the grid fins, I was wondering if this would be a good mission to test any software changes necessary for the returning stage to fly a side-booster profile trajectory in preparation for the FH mission.

Yes, but...

The biggest change to the flight back is going to be the requirement to get the boosters apart immediately after separation, ie rather than pointing them directly back they need to fly away from each other - and at that point in the flight the manoeuvring is done using the thrusters (the fins pop out a little later),

Emphasis mine. That's the point. They do not just need to fly away from each other, they need to get back on a track that will bring them back closer again. It will be a somewhat heart-shape mirrored flight pattern and not a parallel formation flight pattern. At no time before has a booster needed to fly in this pattern, so I would expect a software change or, even if the change is a delay maneuver to put both boosters on the same track, at least a test of the flight parameters.

Mods, sorry if this is off-topic. Feel free to move or remove.

Offline smoliarm

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Regarding the grid fins left on expendable booster,
100% speculation:

There were no attempts so far of RTLS landings in VAFB. My guess - the reason is the place is much more dangerous with wildfires than Florida.
Therefore it seems reasonable that VAFB put a requirement - like *N successful demonstrations* of pinpoint accuracy in stage return.
I may be wrong, but it seems to me, that a malfunction resulting in just inaccurate landing - this mishap is much more dangerous in VAFB - it can result if forest fire.

If my guess is right, and there is indeed such a requirement - then this expendable flight still gives an opportunity to demonstrate the required accurate landing (at predefined spot in ocean) and add another *one* towards that *N*.

Offline deruch

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Regarding the grid fins, I was wondering if this would be a good mission to test any software changes necessary for the returning stage to fly a side-booster profile trajectory in preparation for the FH mission.
The biggest change to the flight back is going to be the requirement to get the boosters apart immediately after separation, ie rather than pointing them directly back they need to fly away from each other - and at that point in the flight the manoeuvring is done using the thrusters (the fins pop out a little later),

Boosters fly in 3-dimensional space, not 2D.  So, a simple solution is to loft/depress one of the boostback trajectories.  You still aim both directly for the LZs but with non-trivial vertical separation there's basically no chance to collide.

To bring this digression back on topic, [speculation]more likely that they just want to see if they can really find the edge of the envelope with reentry.  They can basically test to destruction because they aren't going to try to recover.  And they don't need the extra performance like they do on expendable flights so they have the margin to afford to add all the mass of the grid fin system.  But there's no need to waste a perfectly good set of legs, so those were left off.  [/speculation]
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Offline envy887

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Maybe they are testing how far they can push the angle of attack on return. The more gliding it can do, the less fuel needed for entry and landing burns.

Edit: or maybe they need the fins to make sure they don't hit the fairing recovery boat with flying booster pieces...
« Last Edit: 12/22/2017 01:26 PM by envy887 »

Offline vanoord

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To bring this digression back on topic, [speculation]more likely that they just want to see if they can really find the edge of the envelope with reentry.  They can basically test to destruction because they aren't going to try to recover.  And they don't need the extra performance like they do on expendable flights so they have the margin to afford to add all the mass of the grid fin system.  But there's no need to waste a perfectly good set of legs, so those were left off.  [/speculation]

There are probably half a dozen reasons why this is a one-way flight, but it makes every sense to use the remaining propellants to try something different on the way down - whether it's a test for different a trajectory or something like an experiment to see if the vehicle can fly with one grid fin not deployed remains to be seen.

Offline Johnnyhinbos

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Perhaps they’re testing a failure mode - such as the loss of one of the fins. In light of the rampant California fires as mentioned above, I would think that proving adequate control authority with such a failure would be a requirement for RTLS.
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Offline deruch

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1. Maybe they are testing how far they can push the angle of attack on return. The more gliding it can do, the less fuel needed for entry and landing burns.

2. maybe they need the fins to make sure they don't hit the fairing recovery boat with flying booster pieces...

1. That was basically my first thought. 

2. Better to use the engines and boost away from the recovery area.  That would create much more room to ensure safe operations.  Only danger would be a burn failure.  With the grid fins only, something could go wrong after reentry which might cause problems.
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Online abaddon

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I think @envy887 has it right:  with the fairing recovery boat out there they want to control the booster disposal area.

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