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

[OnWithTheShow]

Yes it has. I specifically remember a slide in the weather briefing that talked about recovery weather it showing the limits for go/no-go. Will try to find.

[Kabloona]

It would appear that wind is the main problem to overcome landing on the ASDS.

Wind has never been mentioned as a factor in the two attempted barge landings.

That's what the landing commit criteria (wind speed, wave height, etc) are for. By definition, from analysis, wind speeds below the limit shouldn't be a problem for the stage to land in. Winds above that speed will cause a landing "abort" like the mission in which winds and high seas caused SpaceX to move the barge and let the stage land in the sea.

(For CRS-7 the barge landing criteria were <20kt winds and <10 ft seas.)
http://forum.nasaspaceflight.com/index.php?topic=37739.msg1395299#msg1395299

The main problems so far have been running out of hydraulic fluid and the propellant valve stiction issue. The effects of moderate winds (within the landing criteria limits) on the control system should be straightforward to model and compensate for. The ACS showed itself to be quite robust in that last attempt of the stage to keep itself from toppling over. As long as the wind speed is within limits, the stage should be able to handle it when all systems are working correctly. But proof is in the pudding, of course.

[Karloss12]

Regarding the struts, the two mitigation steps I saw discussed were increasing the strength of the strut and possibly doing 100% testing. However the strut already had a more than adequate 5:1 load factor. The failure occurred because of poor control of the manufacturing process, not a design flaw.

Wrong, it is a design flaw.  Wrong, material and manufacturing technique for the application

In Elons interview he said there was a an issue with the grain structure of the one in thousands of bolts that were tested which caused the head of the bolt to snap off.  This isn't a wrong design, material or fabrication technique.

If 10000 pressure vessels are fabricated not pressure tested, then a very small number of those vessels will fail due to a material anomaly specific to that vessel.  If however all of these pressure vessels are pressure tested then the bad ones can be removed preventing them from catastrophically failing in service.

The failed pressure vessels don't have a design flaw, wrong material or incorrect fabrication technique.  They have a material anomaly that can only be detected by testing or NDT.

Likewise, if 10000 COPV strut bolts are fabricated and not load tested, a grain structure anomaly will be present in a very small number of bolts (bolt forging is not a perfect process).  This is not a design flaw, wrong material or incorrect fabrication technique.  This is a material anomaly that can be detected by testing or NDT.

I don't see that the design, material or fabrication technique are "wrong".  At worst the design is "less than optimum" as in my experience it is wise for anything to do with fastening with bolts to have a redundant second bolt built into the design.  But appropriate testing can replace this redundancy, as I understand is the case for the struts (from Elons interview).

[kraisee]

http://www.oocities.org/rbt_kraisee/Map.html

Wow Jim, that map is a blast from the past

That's my version from 2002.   I actually kept that updated thru 2010, which might help a bit.   Here you go...

I really should do a modern version, but it looks like I may not need to

Ross.

[shooter6947]

Where's the SpaceX landing pad facility on your map kraisee?  Maybe I'm just not looking hard enough to find it . . .

[WHAP]

Where's the SpaceX landing pad facility on your map kraisee?  Maybe I'm just not looking hard enough to find it . . .

LC-13

[sdsds]

As regards RTLS and weather: do the lightning experts know how having a vehicle return through the same airspace its ascent ionized several minutes earlier will effect the likelihood of a ground path forming between the in-flight vehicle and the pad below?

[wxmeddler]

As regards RTLS and weather: do the lightning experts know how having a vehicle return through the same airspace its ascent ionized several minutes earlier will effect the likelihood of a ground path forming between the in-flight vehicle and the pad below?

I can try and answer that. So, the concern you bring up doesn't have to ionized particles, ionized particles occur when a lightning strike occurs when a strike actually happens. Ionisation isn't triggered by the rocket or the plume. What is of concern to the rocket is the ice particles that are hitting the rocket when it ascends, this causes a static build up on the rocket. Lightning actually has many that go out and find the easiest way to the ground. The huge metal body of the rocket and the built up static charge, makes the "path of least resistance" much easier to find if there is a lightning discharge nearby.

Launch criteria/holds are geared towards this never happening by virtue of the "Cumulus Cloud Rule", "Anvil Rule" and the "Nearby Thunderstorm Rule". The cumulus cloud rule says that the launch may not proceed if there is more than 2000ft of convective type clouds above the freezing level. This prevents the rocket from traveling through a region where forming ice would be present (ie. static buildup). Anvil rule pretty much does the same thing except it's ice farther up near 9-12km ASL. I think the "Nearby thunderstorm rule" is pretty self explanatory.

Regarding RTLS, the troposphere (where weather is) is only about 12km deep, 15km at most. Looking at these graphics the 1st stage only spends about 60 seconds in this region on ascent and only about 45 seconds on decent. So the rocket actually spends more time potentially building a charge on ascent then return.Essentially, with the range safety rules in place, if it is clear to launch, it is clear to land. Lightning wise anyway.

[Kaputnik]

What baffles me is that using high performance fibers (like dyneema) you can have the same strenght for one tenth of the mass; that's worth something, specially on second stage.

Would you want to embed that inside a LOX tank?

Cheers, Martin

Why not?
It has amazing cryo properties.
As to flammability, do you realize that SpaceX is already embedding carbon fiber inside LOX tank?
Lot of mitigation factors can be considered.

Do we know that the strut operates only in tension? I would speculate that the strut may need a degree of stiffness that dyneema obviously cannot provide.
Suitable terminals would also be required at the metal-polymer connection point, which would be metal, and which would go some way towards negating the mass savings.

Did you realized the difference between strenght/mass ratio of steel and dyneema?
More than ten times.
Which way a terminal (if needed at all) would negate this kind of mass savings?

Yes, I fully appreciate the difference in strength/mass of dyneema and steel (I fly paragliders and sail racing dinghies, both rely on dyneema).
In the context of a strut, You can't just tie the dyneema onto something. Some sort of terminal will be needed to ensure that it is fitted without loss of strength and, more importantly, at exactly the required length. Not saying it would work out the same weight as an all-steel strut, far from it, but just pointing out that it's not a straightforward swap.

SpaceX will have heard of dyneema. We can speculate that they took a rational decision to use steel instead. I would conjecture that this is because of some combination of the loads on the strut and manufacturing/installation reasons, but we can't know for sure.

[Garrett]

As regards RTLS and weather: do the lightning experts know how having a vehicle return through the same airspace its ascent ionized several minutes earlier will effect the likelihood of a ground path forming between the in-flight vehicle and the pad below?

Ions and electrons have very small lifetimes in the lower atmosphere before they recombine. On the order of microseconds or less. So to answer your question, there should be no issue.

[LouScheffer]

As regards RTLS and weather: do the lightning experts know how having a vehicle return through the same airspace its ascent ionized several minutes earlier will effect the likelihood of a ground path forming between the in-flight vehicle and the pad below?

Ions and electrons have very small lifetimes in the lower atmosphere before they recombine. On the order of microseconds or less. So to answer your question, there should be no issue.

In particular, even a lightning strike at a given place does not leave a residue that can be used by some other lightning strike minutes later.  The atmosphere has a pretty short "memory" for such events.  Also, since lightning usually happens in conjunction with wind, even if there was some trace it would be quite some distance away 10 minutes later.

[rokan2003]

As regards RTLS and weather: do the lightning experts know how having a vehicle return through the same airspace its ascent ionized several minutes earlier will effect the likelihood of a ground path forming between the in-flight vehicle and the pad below?

I can try and answer that. So, the concern you bring up doesn't have to ionized particles, ionized particles occur when a lightning strike occurs when a strike actually happens. Ionisation isn't triggered by the rocket or the plume. What is of concern to the rocket is the ice particles that are hitting the rocket when it ascends, this causes a static build up on the rocket. Lightning actually has many that go out and find the easiest way to the ground. The huge metal body of the rocket and the built up static charge, makes the "path of least resistance" much easier to find if there is a lightning discharge nearby.

Launch criteria/holds are geared towards this never happening by virtue of the "Cumulus Cloud Rule", "Anvil Rule" and the "Nearby Thunderstorm Rule". The cumulus cloud rule says that the launch may not proceed if there is more than 2000ft of convective type clouds above the freezing level. This prevents the rocket from traveling through a region where forming ice would be present (ie. static buildup). Anvil rule pretty much does the same thing except it's ice farther up near 9-12km ASL. I think the "Nearby thunderstorm rule" is pretty self explanatory.

Regarding RTLS, the troposphere (where weather is) is only about 12km deep, 15km at most. Looking at these graphics the 1st stage only spends about 60 seconds in this region on ascent and only about 45 seconds on decent. So the rocket actually spends more time potentially building a charge on ascent then return.Essentially, with the range safety rules in place, if it is clear to launch, it is clear to land. Lightning wise anyway.

What a great explanation. Thanks very much.

[JasonAW3]

As regards RTLS and weather: do the lightning experts know how having a vehicle return through the same airspace its ascent ionized several minutes earlier will effect the likelihood of a ground path forming between the in-flight vehicle and the pad below?

Ions and electrons have very small lifetimes in the lower atmosphere before they recombine. On the order of microseconds or less. So to answer your question, there should be no issue.

In particular, even a lightning strike at a given place does not leave a residue that can be used by some other lightning strike minutes later.  The atmosphere has a pretty short "memory" for such events.  Also, since lightning usually happens in conjunction with wind, even if there was some trace it would be quite some distance away 10 minutes later.

      Oddly enough, though, lightning has been known to strike the same location twice or more times in rapid succession.  (Usually within less than five minutes).

      This is because of structural height, in the case of launch towers and tall buildings.  The height lessens the arc distance between the negatively charged clouds and the positively charged ground.

      As these structures are typically made mostly of steel, this further allows for a much higher likelyhood of static discharge of this nature, than would a structure made of plastics.  (Although a wet plastic structure would still allow a pretty good discharge to occur, I imagine).

[rickyramjet]

Regarding the struts, the two mitigation steps I saw discussed were increasing the strength of the strut and possibly doing 100% testing. However the strut already had a more than adequate 5:1 load factor. The failure occurred because of poor control of the manufacturing process, not a design flaw.

Wrong, it is a design flaw.  Wrong, material and manufacturing technique for the application

In Elons interview he said there was a an issue with the grain structure of the one in thousands of bolts that were tested which caused the head of the bolt to snap off.  This isn't a wrong design, material or fabrication technique.

If 10000 pressure vessels are fabricated not pressure tested, then a very small number of those vessels will fail due to a material anomaly specific to that vessel.  If however all of these pressure vessels are pressure tested then the bad ones can be removed preventing them from catastrophically failing in service.

The failed pressure vessels don't have a design flaw, wrong material or incorrect fabrication technique.  They have a material anomaly that can only be detected by testing or NDT.

Likewise, if 10000 COPV strut bolts are fabricated and not load tested, a grain structure anomaly will be present in a very small number of bolts (bolt forging is not a perfect process).  This is not a design flaw, wrong material or incorrect fabrication technique.  This is a material anomaly that can be detected by testing or NDT.

I don't see that the design, material or fabrication technique are "wrong".  At worst the design is "less than optimum" as in my experience it is wise for anything to do with fastening with bolts to have a redundant second bolt built into the design.  But appropriate testing can replace this redundancy, as I understand is the case for the struts (from Elons interview).

With a single bolt, no matter how it is tested and verified to be of the desired strength you still have a single point of failure and a certain probability of failure that still exists.  Given this is such a mission critical bolt, wouldn't redesigning the struts slightly to allow two bolts be a good solution?  In any case, it would certainly be interesting to know what solution SpaceX came up with!

[Senex]

With a single bolt, no matter how it is tested and verified to be of the desired strength you still have a single point of failure and a certain probability of failure that still exists.  Given this is such a mission critical bolt, wouldn't redesigning the struts slightly to allow two bolts be a good solution?  In any case, it would certainly be interesting to know what solution SpaceX came up with!

It's not that simple.  Stop to think about how many "mission critical" components there are.  Double the number of "bolts" and that thing is never getting off the ground. 

We are operating at the margins of the possible.  That's why it's hard.

[cambrianera]

Yes, I fully appreciate the difference in strength/mass of dyneema and steel (I fly paragliders and sail racing dinghies, both rely on dyneema).

Not a given: lot of people uses high tech and can't appreciate the subtleties of design and engineering (true for myself in many fields  )

In the context of a strut, You can't just tie the dyneema onto something. Some sort of terminal will be needed to ensure that it is fitted without loss of strength and, more importantly, at exactly the required length. Not saying it would work out the same weight as an all-steel strut, far from it, but just pointing out that it's not a straightforward swap.

Designing something into a rocket seldom is a straightforward swap. Nevertheless keeping dry mass low is a must for rocket design.

SpaceX will have heard of dyneema. We can speculate that they took a rational decision to use steel instead. I would conjecture that this is because of some combination of the loads on the strut and manufacturing/installation reasons, but we can't know for sure.

They took a lot of rational decision (alblative engine, parachute reuse of first stage, single engine on first stage, tic-tac-toe arrangement of engines....) then changed to get higher performances.
And yes, we can speculate they had some manufacturing / installation reasons, otherwise... 

[WHAP]

Cross posting from the updates thread. 

Mark Eisenberg 25min ago:

"All satellites fully fueled and attached to the rings.  Waiting on SpaceX to confirm launch date."

Static fire must be imminent.

[Greg Hullender]

One "solution" I haven't heard discussed (maybe I've missed it) is that if the Dragon is capable of rescuing the payload, then SpaceX could tolerate a higher percentage of failed launches.

[guckyfan]

One "solution" I haven't heard discussed (maybe I've missed it) is that if the Dragon is capable of rescuing the payload, then SpaceX could tolerate a higher percentage of failed launches.

There is no way SpaceX can tolerate a higher percentage of failed launches. They cannot count on 0, but they have clearly said they want the most reliable launch vehicles of all providers. I believe them.

[Lars-J]

One "solution" I haven't heard discussed (maybe I've missed it) is that if the Dragon is capable of rescuing the payload, then SpaceX could tolerate a higher percentage of failed launches.

It has been discussed, and dismissed/debunked elsewhere. Most flights have no Dragon, so what could it do? And the Dragon 2 LAS abort is only there to rescue the crew. Not a payload.

And it is very off topic for this thread.