Author Topic: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION  (Read 865115 times)

Offline MarekCyzio

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Jet stream forecasts. It should get better Friday afternoon.

Offline macpacheco

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Re: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION
« Reply #1581 on: 03/02/2016 12:29 AM »
Could someone address succinctly the issue of why (modern) guidance algorithms can't handle wind shear?

It can't really be a structural issue, can it? Just keep the pointy end into the (local relative) wind!?
I don't know this subject well enough to be succint, but here it goes:

Strong variations in winds cause turbulence. Most turbulence experienced at high altitude is either caused by this wind shear Musk refers to, mountain wave turbulence (rotors created downwind of mountains) and convective activity (flying above CB/TS clouds). That critical altitude (32000ft) all of us have flown through with turbofan airliners.

This isn't about the guidance system at all, this is 100% structural risk.
The faster you're flying and the higher the aerodynamic pressure, the more strongly that turbulence will be felt.

In essence there is a force creating a strong wind at some altitude and a lack of force creating a weaker wind at lower altitude. Those forces don't want to mix, but are forced to, through most of the altitudes the wind varies smoothly, but at some point, it changes quickly. That's the phenomena that rockets want to avoid (and that airliners would also like to avoid, but mostly for PAX comfort rather than structural margins). Rockets have far lower structural margins than aircraft. Weight is at a premium, and you can wait and launch when the there's a lesser wind shear risk.

Is there any other effects than wind shear turbulence that put rockets at risk ? The actual variation in wind should be OK as long as it doesn't lead to turbulence, but that's almost a given if wind gradient is too high.

For most of the year, the jetstream core passes further north, but in the winter it can drift into FL and southern TX.
« Last Edit: 03/02/2016 12:31 AM by macpacheco »
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Offline deruch

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Re: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION
« Reply #1582 on: 03/02/2016 12:47 AM »
Could someone address succinctly the issue of why (modern) guidance algorithms can't handle wind shear?

It can't really be a structural issue, can it? Just keep the pointy end into the (local relative) wind!?

F9v1.1 and FT have a very high fineness ratio.  So this vehicle may be more susceptible to structural wind shear effects compared to others.

*edited to provide relevant link to wikipedia.
« Last Edit: 03/02/2016 01:28 AM by deruch »
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Offline John-H

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Re: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION
« Reply #1583 on: 03/02/2016 01:19 AM »
Wasn't wind shear a factor in the Challenger disaster?  One factor of many, of course.

"The steering system (thrust vector control) of the Solid Rocket Booster responded to all commands and wind shear effects. The wind shear caused the steering system to be more active than on any previous flight."

from http://er.jsc.nasa.gov/seh/explode.html

The effect can be really nasty.

John

Offline manoweb

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Re: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION
« Reply #1584 on: 03/02/2016 01:27 AM »
F9v1.1 and FT have a very high fineness ratio.  So this vehicle may be more susceptible to structural wind shear effects compared to others.

Can you explain? I understand fineness when applied to an airplane, but a rocket is... a cylinder, how can the fineness ratio be so different compared to Falcon 9 v 1.0 or to the ULA rocket?

Online Robotbeat

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Re: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION
« Reply #1585 on: 03/02/2016 01:28 AM »
F9v1.1 and FT have a very high fineness ratio.  So this vehicle may be more susceptible to structural wind shear effects compared to others.

Can you explain? I understand fineness when applied to an airplane, but a rocket is... a cylinder, how can the fineness ratio be so different compared to Falcon 9 v 1.0 or to the ULA rocket?
Just means length to diameter ratio. It's long and skinny.
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Offline deruch

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Re: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION
« Reply #1586 on: 03/02/2016 01:47 AM »
F9v1.1 and FT have a very high fineness ratio.  So this vehicle may be more susceptible to structural wind shear effects compared to others.

Can you explain? I understand fineness when applied to an airplane, but a rocket is... a cylinder, how can the fineness ratio be so different compared to Falcon 9 v 1.0 or to the ULA rocket?

Compared to F9v1.0, it's the same diameter but significantly longer.  Technically, the F9 FT is a little longer than the v1.1 as well.  But the difference is pretty minor, which is why I grouped them in my original comment.

WRT the Atlas V, the F9 FT is both longer and thinner.  WRT to Delta IV, F9 is always thinner, but only longer than some variants.  But the width difference means that regardless of Delta variant, the F9 FT will have a higher fineness ratio than the Delta IV.
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Offline WHAP

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Re: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION
« Reply #1587 on: 03/02/2016 02:36 AM »
In a long career of launching rockets, I can't recall seeing a vehicle wait for days for upper level winds to subside before attempting to launch.  It may have happened, but I don't remember it.  What I DO remember is sitting through countdowns on more than one mission waiting for the next balloon data to come in, hoping that the smart guys could develop a solution for the wind profile.  Sometimes we launched (successfully, of course  :) ), and sometimes we scrubbed.  The forecasts are obviously important, but they're not a guarantee.  It says a lot that SpaceX isn't even willing to try.  I'm struggling to understand the logic that says a multiple day delay is worth it for such a low probability of success for booster recovery.  Unless that isn't what's driving the decision here.
« Last Edit: 03/02/2016 02:37 AM by WHAP »
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Offline manoweb

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Re: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION
« Reply #1588 on: 03/02/2016 02:37 AM »
And is this because the SpaceX company transports rockets by truck instead of ${EXPENSIVE_TRANSPORT} so they have diameter limitation? Or because the higher fineness ratio has intrinsic performance advantages?

Offline speedevil

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Re: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION
« Reply #1589 on: 03/02/2016 02:56 AM »
And is this because the SpaceX company transports rockets by truck instead of ${EXPENSIVE_TRANSPORT} so they have diameter limitation? Or because the higher fineness ratio has intrinsic performance advantages?

It somewhat reduces aerodynamic drag.
The optimal shape for a rocket on an airless earth would be rather closer to a sphere than the current long cylinders.
Better structurally, less heat flow into/out due to better surface-area/volume, ...
A 500m^3 cylinder 3.5m in diameter and 50m tall will have about 500m^2 of surface area.
If it was a sphere, it'd be under half that.
(approximate numbers).

Offline Kabloona

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Re: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION
« Reply #1590 on: 03/02/2016 03:01 AM »
  I'm struggling to understand the logic that says a multiple day delay is worth it for such a low probability of success for booster recovery.  Unless that isn't what's driving the decision here.

It has nothing to do with booster recovery. It has everything to do with not breaking the vehicle as it flies through high wind shear. Upper level winds are forecast to be too high for the vehicle structure to handle until Friday. And they must have enough confidence in the forecast not to want to waste time and effort on an earlier attempt that would likely scrub.

As others have pointed out, F9 has a high fineness ratio compared to other LV's, and so may be more stressed by Q-alpha loading than some of the vehicles you've launched, and thus require a bit more patience for better wind conditions.

IIRC, Taurus was so "long and bendy" that Orbital had to program active control of the first bending mode into the guidance algorithm, so that TVC commands didn't excite the first bending mode. Not quite the same thing as F9, but just an example of long, skinny rockets being especially vulnerable to bending moments.
« Last Edit: 03/02/2016 03:27 AM by Kabloona »

Online HMXHMX

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Re: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION
« Reply #1591 on: 03/02/2016 03:30 AM »
  I'm struggling to understand the logic that says a multiple day delay is worth it for such a low probability of success for booster recovery.  Unless that isn't what's driving the decision here.

It has nothing to do with booster recovery. It has everything to do with not breaking the vehicle as it flies through high wind shear. Upper level winds are forecast to be too high for the vehicle structure to handle until Friday. And they must have enough confidence in the forecast not to want to waste time and effort on an earlier attempt that would likely scrub.

As others have pointed out, F9 has a high fineness ratio compared to other LV's, and so may be more stressed by Q-alpha loading than some of the vehicles you've launched, and thus require a bit more patience for better wind conditions.

Anecdote: In 2005, a long time friend and senior NASA guy at JSC (who shall remain nameless) took me aside at an AIAA conference and said "...von Braun used to say to me – never build a rocket with a  fineness ratio greater than 10!"

At the time, he was criticizing Ares I, but the words apply to the current F9, too.  And to the old Titan IV, which would occasionally have to wait on the pad at WTR waiting for a break in the high altitude wind shear to be able to fly safely.

Offline Craftyatom

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Re: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION
« Reply #1592 on: 03/02/2016 03:36 AM »
Most recent scrub due to high-altitude winds was, I believe, SMAP on the Delta II (another rather spindly rocket): http://spacecoastdaily.com/2015/01/upper-level-wind-shear-scrubs-smap-launch/

So yes, it happens on occasion, just another weather constraint to take into account.
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Offline OxCartMark

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Re: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION
« Reply #1593 on: 03/02/2016 03:47 AM »
Could someone address succinctly the issue of why (modern) guidance algorithms can't handle wind shear?

It can't really be a structural issue, can it? Just keep the pointy end into the (local relative) wind!?
I don't know this subject well enough to be succint, but here it goes:

Strong variations in winds cause turbulence. Most turbulence experienced at high altitude is either caused by this wind shear Musk refers to, mountain wave turbulence (rotors created downwind of mountains) and convective activity (flying above CB/TS clouds). That critical altitude (32000ft) all of us have flown through with turbofan airliners.

I want to correct or at least comment on a small point here.  Mountain wave is very smooth and rotor is a different phenomenon (though they often exist side by side) which is very turbulent.

Now moving on I want to give a slightly different parallel explanation to what is quoted above.  When you are in a sailplane or paraglider on a good thermic day you'll find yourself flying around at a relatively low speed and finding that the air is moving up in some areas and down in others (and sideways as well) which you'll experience as locations that cause you to go up and locations that cause you to go down.  The transition between these areas happens so gradually at those speeds that you may need instruments or significant experience to tell which is which.  Days like that make glider pilots smile.  But on that same day in that same area the pilot and passengers of an airliner are likely to experience significant turbulence which will cause them to seek another flight path to avoid the thermic air.  That's despite the airliner having a much heavier wing loading.  The reason for the difference between what the glider experiences and what the airliner experiences due to the difference in airspeeds.  A glider may move from an area where the air is decending at -10m/sec to an area where its ascending at +10m/sec over a 20 second period.  The airliner may go between those two areas in 2 seconds.  That's quite a transition.  Now turn that whole scenario 90 degrees and you have a rocket going more or less vertically through air thats moving more or less horizontally but that horizontal air velocity changes as you go up.  And it changes very rapidly as you go up rapidly.  Which could be experienced as turbulence.  But it could be experienced in other ways.  As the original poster mused, all you need to do is to keep it pointed into the relative wind.  But doing that won't exactly get you to the point that you want to be in space with the velocity you want to have when you get there, at least not without burning additional propellant.  And the rate at which you need to adjust the attitude to achieve alignment with the relative wind may be too great. The fact that the nose end is lighter than the tail end and the nose end presents more area to a side wind component due to the larger diameter payload fairing means that a side wind component will tend to turn the rocket further out of alignment with the relative wind not into it which creates additional need for correction.  Someone somewhere in Hawthorne has put numbers to this and more and after some pondering and calculating they came up with what they think is a reasonable wind limit or a reasonable rate of change in wind limit and I'm going to go with their answer on this.
« Last Edit: 03/02/2016 05:39 PM by OxCartMark »

Offline deruch

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Re: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION
« Reply #1594 on: 03/02/2016 03:56 AM »
Most recent scrub due to high-altitude winds was, I believe, SMAP on the Delta II (another rather spindly rocket): http://spacecoastdaily.com/2015/01/upper-level-wind-shear-scrubs-smap-launch/

So yes, it happens on occasion, just another weather constraint to take into account.
SpaceX's launch attempt #2 of DSCOVR, ~10 days later on 2015-02-10, also had a scrub for high level winds and shear.

http://www.nasaspaceflight.com/2015/02/spacex-falcon-9-dscovr-mission/


« Last Edit: 03/02/2016 03:57 AM by deruch »
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Offline deruch

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Re: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION
« Reply #1595 on: 03/02/2016 04:28 AM »
  I'm struggling to understand the logic that says a multiple day delay is worth it for such a low probability of success for booster recovery.  Unless that isn't what's driving the decision here.

It has nothing to do with booster recovery. It has everything to do with not breaking the vehicle as it flies through high wind shear. Upper level winds are forecast to be too high for the vehicle structure to handle until Friday. And they must have enough confidence in the forecast not to want to waste time and effort on an earlier attempt that would likely scrub.

As others have pointed out, F9 has a high fineness ratio compared to other LV's, and so may be more stressed by Q-alpha loading than some of the vehicles you've launched, and thus require a bit more patience for better wind conditions.

Anecdote: In 2005, a long time friend and senior NASA guy at JSC (who shall remain nameless) took me aside at an AIAA conference and said "...von Braun used to say to me – never build a rocket with a  fineness ratio greater than 10!"

At the time, he was criticizing Ares I, but the words apply to the current F9, too.  And to the old Titan IV, which would occasionally have to wait on the pad at WTR waiting for a break in the high altitude wind shear to be able to fly safely.

Both Atlas V and Delta IV also have fineness ratios greater than 10.  Without knowing von Braun's age at the time of the statements, I hesitate to invoke a bastardized form of Clarke's first law, but, while the challenges and hazards of successfully building/launching rockets with fineness ratios greater than 10 are undoubtedly the same as they were in his day, great improvements since then in materials science, fabrication methods, computer modeling, and raw computing power for guidance have likely done much to reduce or eliminate them.  This is not to say that it's easy/simple to do it today, only that some of the foundations upon which von Braun's concerns were built should be less formidable obstacles to today's rocket designers/engineers.
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Offline georgegassaway

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Re: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION
« Reply #1596 on: 03/02/2016 04:35 AM »
To visit a different topic that is unique to this mission's landing plans.....

If the reports of a multi-engine landing are correct, it will be coming down on at least two outer engines, or probably three engines ( such as two outer plus core as normally done for Boostback), with FAR more thrust to weight ratio for landing than they've ever tested.

Well, the more that I think on that, I wonder if perhaps they might be using say three engines for MOST of the landing burn, to help slow it down a LOT, but then at a few hundred feet up (or lower), at a critical velocity and altitude criteria, shut down two outer engines and do the actual landing on the center engine. That would make a lot more sense than doing a "hover-slam" landing on two or three engines, two to three times more thrust than ever tested for touching down.

Or is there some more definitive info out there as for what “multi-engine landing burn” really means?

 I know it implies more than one firing at touchdown, but man that sounds too far out from anything they have tested or worked towards. While shutting down outer engines shortly before the actual landing, and doing the descent of the last few hundred (or last few dozens) of feet on the center engine would seem more plausible and with lots of experience and testing to support a single engine landing.

- George Gassaway

Offline WHAP

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Re: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION
« Reply #1597 on: 03/02/2016 04:38 AM »
I'm struggling to understand the logic that says a multiple day delay is worth it for such a low probability of success for booster recovery.  Unless that isn't what's driving the decision here.

It has nothing to do with booster recovery. It has everything to do with not breaking the vehicle as it flies through high wind shear. Upper level winds are forecast to be too high for the vehicle structure to handle until Friday. And they must have enough confidence in the forecast not to want to waste time and effort on an earlier attempt that would likely scrub.

I never suggested that SpaceX should launch if the conditions exceeded the capability of the vehicle.  I questioned SpaceX's rationale for not making an attempt.

Subsequent posts provided supporting evidence for my argument.  The examples given were vehicles that tried to launch on a given day - including Falcon 9.  They didn't use forecasts, they used actual data on launch day to determine if they could meet their criteria.
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Online HMXHMX

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Re: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION
« Reply #1598 on: 03/02/2016 04:43 AM »
  I'm struggling to understand the logic that says a multiple day delay is worth it for such a low probability of success for booster recovery.  Unless that isn't what's driving the decision here.

It has nothing to do with booster recovery. It has everything to do with not breaking the vehicle as it flies through high wind shear. Upper level winds are forecast to be too high for the vehicle structure to handle until Friday. And they must have enough confidence in the forecast not to want to waste time and effort on an earlier attempt that would likely scrub.

As others have pointed out, F9 has a high fineness ratio compared to other LV's, and so may be more stressed by Q-alpha loading than some of the vehicles you've launched, and thus require a bit more patience for better wind conditions.

Anecdote: In 2005, a long time friend and senior NASA guy at JSC (who shall remain nameless) took me aside at an AIAA conference and said "...von Braun used to say to me – never build a rocket with a  fineness ratio greater than 10!"

At the time, he was criticizing Ares I, but the words apply to the current F9, too.  And to the old Titan IV, which would occasionally have to wait on the pad at WTR waiting for a break in the high altitude wind shear to be able to fly safely.

Both Atlas V and Delta IV also have fineness ratios greater than 10.  Without knowing von Braun's age at the time of the statements, I hesitate to invoke a bastardized form of Clarke's first law, but, while the challenges and hazards of successfully building/launching rockets with fineness ratios greater than 10 are undoubtedly the same as they were in his day, great improvements since then in materials science, fabrication methods, computer modeling, and raw computing power for guidance have likely done much to reduce or eliminate them.  This is not to say that it's easy/simple to do it today, only that some of the foundations upon which von Braun's concerns were built should be less formidable obstacles to today's rocket designers/engineers.

No question but that you are correct in your assessment of technology advancements, yet in rocketry as in much other engineering, simplicity leads to reliability (at least in my opinion).  I am a firm believer in Vicomte de Saint-Exupéry's admonishment:

"Perfection is achieved not when there is nothing more to be added, but when there is nothing more to be taken away."

I'd put SpaceX's foray into sub-cooled LOX in the same category as the high fineness ratio issue.  Some decisions increase operational cost and aren't worth the effort.  Launch vehicles are especially unforgiving due to their very nature.  Of course, you may fairly charge me with succumbing to Clarke's first law, too.  I have reached Medicare age so my experience makes me suspect.  ;)

Offline cscott

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Re: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION
« Reply #1599 on: 03/02/2016 05:02 AM »
I'm struggling to understand the logic that says a multiple day delay is worth it for such a low probability of success for booster recovery.  Unless that isn't what's driving the decision here.

It has nothing to do with booster recovery. It has everything to do with not breaking the vehicle as it flies through high wind shear. Upper level winds are forecast to be too high for the vehicle structure to handle until Friday. And they must have enough confidence in the forecast not to want to waste time and effort on an earlier attempt that would likely scrub.

I never suggested that SpaceX should launch if the conditions exceeded the capability of the vehicle.  I questioned SpaceX's rationale for not making an attempt.

Subsequent posts provided supporting evidence for my argument.  The examples given were vehicles that tried to launch on a given day - including Falcon 9.  They didn't use forecasts, they used actual data on launch day to determine if they could meet their criteria.
We'll see on Friday if the forecasts were right.  You seem quite convinced they will not be.  Since I am not a meteorologist, I will defer to those who are.
« Last Edit: 03/02/2016 01:02 PM by cscott »

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