SpaceX Falcon 9 : Iridium NEXT Flight 2 (June 25, 2017) : Discussion

[tvg98]

Concur with those that observed it seemed to reach 0 velocity a few feet above the deck and drop the last couple feet after cut off.

Impressive sleuthing from a terrible angle. :p Don't quit your day jobs, people.

Pause the YouTube video (<space>), then advance slowly with > (go back with >) and you'll see the engine glow stop about 0.5 seconds before the legs move on impact with the deck.

The legs move out after landing to absorb impact/mass as well.

Based on some oversimplified assumptions for some calculations, it looks like that stage may have dropped 2-4 meters before hitting the deck. Does that sound right?

[ulm_atms]

Just curious, but what is this many, many layered circular-ish patch?  Since it is below the interstage connection, I would assume it is an access port for the top tank...but I always thought the tank access areas were at the tops of the tanks for structural reasons since the tank sides support most of the rockets structural loading.

It's just weird with all the streamline looking areas and see that big pimple 

[meekGee]

You can crash any landing, but vertical ones are more forgiving.

With respect to rockets given the current state of the art, I *might* agree.   However, as a general statement this fails.  You might want to do an FMEA before pronouncing vertical landings as less risky.  There are a lot of variables in play, of which energy-at-touchdown is only one.  Otherwise we would all be traveling on VL commercial aircraft.

This is veering towards a general conversation, but since it's somewhat relevant and the mission is over...

Airplanes land horizontally, since long-duration atmospheric flight dictates the necessity of wings.

Other than that little thing, even in airplanes, landings are harrowing, dangerous, and require a landing gear which is large, heavy and greatly interferes with the chassis design.

Aircraft designers would have gotten rid of the landing gear a long time ago, it's just that nobody has been able to come up with an elegant way to have a winged flyer that lands vertically.

Vertical landing is so appealing that contraptions have been designed in which the wings actually fly in a circle around the airplane, just in order to enable it, resulting in much simpler landing gear designs.  Sadly, the rotating wings introduce a lot more complexity than they save - but still those contraptions are proving very useful.

With spacecraft, the winged requirement does not exist, and the only reason to re-introduce it is that the concept of "space planes" is appealing to some.

[cscott]

Elon has some free time to spend on twitter:

Slightly heavier than shielded aluminum, but more control authority and can be reused indefinitely with no touch ups

No, but shielding got fragged every flight. More control authority is for Falcon Heavy, but also enables Falcon 9 to land in heavier winds.

Interesting that they need more control authority.  On landings to date you can barely see the grid fins move, much less approach maximum travel for any period of time.   Must be planning for quite a bit worse conditions than we've seen them land in so far.

More control authority allows flying at a better angle of attack, for a greater L/D coefficient and greater crossrange.  That's probably also more important for FH, but it would allow a few more RTLS launches that would otherwise be ASDS.

[cscott]

Stage 1 boostback burn has started; the three restartable engines will fire until the impact point lies directly on the drone ship.

I don't believe this is true.  There are two more burns to go: an entry burn and a landing burn. Both of these will inevitably shift the IIP.

Has our fabricated launch commentary been vetted by anyone?

[Herb Schaltegger]

Elon has some free time to spend on twitter:

Slightly heavier than shielded aluminum, but more control authority and can be reused indefinitely with no touch ups

No, but shielding got fragged every flight. More control authority is for Falcon Heavy, but also enables Falcon 9 to land in heavier winds.

Interesting that they need more control authority.  On landings to date you can barely see the grid fins move, much less approach maximum travel for any period of time.   Must be planning for quite a bit worse conditions than we've seen them land in so far.

More control authority allows flying at a better angle of attack, for a greater L/D coefficient and greater crossrange.  That's probably also more important for FH, but it would allow a few more RTLS launches that would otherwise be ASDS.

Yep.

Further, more control authority - at specific points in the flight regime, such as during transonic flight and slower - can be key to higher tolerance of wind gusts during terminal descent.

[StuffOfInterest]

Based on some oversimplified assumptions for some calculations, it looks like that stage may have dropped 2-4 meters before hitting the deck. Does that sound right?

Any landing you can walk away from...

It will be interesting to see the deckside camera footage of the landing when it becomes available.  Both today and Friday may have been "interesting" landings, and both should make for some good discussions once the video is out.

[Tuts36]

I expected to see some visual indication of grid fin heating during landing, but didn't see anything.  Very impressive. Wonder if we've seen the last aluminum fins.

I'm assuming the fancier titanium fins are much more expensive.  How much more I wonder?

To me this signifies their confidence in first stage recovery, that they can start making refinements that increase initial costs, but re-use will make economical.

[AC in NC]

Based on some oversimplified assumptions for some calculations, it looks like that stage may have dropped 2-4 meters before hitting the deck. Does that sound right?

I don't believe it does.  I once searched and found the maximum design impact velocity.  It was really difficult number to find and I doubt I saved the link or have it handy.  I want to say it was 2 m/s but it may have been that the landing I was looking at actually landed a 2 m/s and the design was for 3 m/s max.

I was curious if that was like jump off the couch or something.  Running a calculator showed 2 m/s is like a drop from 8 inches.  2-4 meters sounds like it would be ugly.

[cscott]

They are targeting 0 m/s.  The question is, how much can the emergency crush cores take in the worst case? The answer is clearly, "more than BulgariaSat or Iridium 2", although perhaps not much more than that.

[mgeagon]

A ship at sea will rise and fall with the average swell of its cross-section. In heavy seas, swells can easily reach several meters and have periods of seconds to minutes. The rocket starts its landing burn and throttles to achieve 0 m/s at what it hopes will be the deck of the ASDS. If level off is achieved and the deck has heaved 2 - 4 meters below, the results must be engine shutdown and a hope for the best. Impact in this case seemed to provide a short bounce, but no particularly noticeable ill effects.

[Ben the Space Brit]

Interestingly, the 'bounce' in this case seems to have taken the booster closer to the centre mark so I'm thinking that SpaceX may have rolled a 'seven' on this one.

[IntoTheVoid]

Elon has some free time to spend on twitter:

Slightly heavier than shielded aluminum, but more control authority and can be reused indefinitely with no touch ups

No, but shielding got fragged every flight. More control authority is for Falcon Heavy, but also enables Falcon 9 to land in heavier winds.

Interesting that they need more control authority.  On landings to date you can barely see the grid fins move, much less approach maximum travel for any period of time.   Must be planning for quite a bit worse conditions than we've seen them land in so far.

More control authority allows flying at a better angle of attack, for a greater L/D coefficient and greater crossrange.  That's probably also more important for FH, but it would allow a few more RTLS launches that would otherwise be ASDS.

I figured that the most straight forward answer, especially since Elon mentioned FH, is that the extra control authority is required to make up for the extra dry weight of the FH center core, to provide at least equivalent performance to what they've been using for F9.

[TorenAltair]

As my old account seems to gone, I made a new one.

I checked the landing video I have and got to about 7 frames (of 30 fps) from engine out to touchdown on a quick and dirty "measurement". That would make 0.233 seconds. Accordingly to AC in NC calculations above, that would make a little above 0.2 meters although I admit it looks much higher.

[virnin]

I expected to see some visual indication of grid fin heating during landing, but didn't see anything.  Very impressive. Wonder if we've seen the last aluminum fins.

I'm assuming the fancier titanium fins are much more expensive.  How much more I wonder?

To me this signifies their confidence in first stage recovery, that they can start making refinements that increase initial costs, but re-use will make economical.

I'm expecting grid fins to be reused more often than booster cores, just due to the elimination of refurbishment.  Any stockpiles of aluminium fins could soon be relegated to expendable launches.

[hpras]

I expected to see some visual indication of grid fin heating during landing, but didn't see anything.  Very impressive. Wonder if we've seen the last aluminum fins.

I'm assuming the fancier titanium fins are much more expensive.  How much more I wonder?

To me this signifies their confidence in first stage recovery, that they can start making refinements that increase initial costs, but re-use will make economical.

I'm expecting grid fins to be reused more often than booster cores, just due to the elimination of refurbishment.  Any stockpiles of aluminium fins could soon be relegated to expendable launches.

Why would you have grid fins on an expendable launch?

[Kenp51d]

Aluminum grid fins.
Expendable launch has no need of grid finds. It just drops in the ocean. Correct me if I'm wrong.
Muesium might like 'em. Or high quality scrap metal.

Sent from my XT1565 using Tapatalk

[Nomadd]

A ship at sea will rise and fall with the average swell of its cross-section. In heavy seas, swells can easily reach several meters and have periods of seconds to minutes. The rocket starts its landing burn and throttles to achieve 0 m/s at what it hopes will be the deck of the ASDS. If level off is achieved and the deck has heaved 2 - 4 meters below, the results must be engine shutdown and a hope for the best. Impact in this case seemed to provide a short bounce, but no particularly noticeable ill effects.

I think the booster has an radar altimeter and should be able to measure rate of approach. It wouldn't even be that hard a piece of software to figure vertical barge movements into the decent if they were regular. Problem is of course, barge movement from swells isn't always a smooth oscillation and can be erratic, and different parts of the deck move at different rates depending on how the barge is pivoting. They could ballast the barge before landing to reduce movement, but you'd have to balance that against the risk of waves washing over the deck. And discharging ballast at sea has it's own complications and would add time to the operation.
 They're going to have a less than optimal landing eventually, and I can't help but think they really need a spare ASDS to keep the schedule up. There are too may reasons one might not be available for a launch, and that would mean underwater boosters until it was back online.

[Jcc]

I expected to see some visual indication of grid fin heating during landing, but didn't see anything.  Very impressive. Wonder if we've seen the last aluminum fins.

I'm assuming the fancier titanium fins are much more expensive.  How much more I wonder?

To me this signifies their confidence in first stage recovery, that they can start making refinements that increase initial costs, but re-use will make economical.

Even if they don't refly that stage, they can take off the grid fans and use them on another.

[CyndyC]

I'm expecting grid fins to be reused more often than booster cores, just due to the elimination of refurbishment. 

Someone made a comment on SpaceflightNow that was funny but partly true. He said SpaceX will only need one set of titanium grid fins. Let's say 3, one for each launch site.