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I've read in @geoff's answer that SpaceX is doing some experimentation that may lead to fairing recovery capability. In fact I remember hearing Elon Musk mentioning a few years ago that it was being seriously looked at.

Originally I hadn't given them much consideration, and in the back of my head I probably thought of them as big fiberglass shells that kept the wind off of the payload. However within the first minute of each launch there's always the mention of max-Q, and in some un-manned launch cases thrust is reduced during max-Q specifically to decrease stresses in the rocket's frame - thrust is pushing up on the frame while aerodynamic drag on the noise is simultaneously pushing down on it. And the "nose" which experiences this stress is actually the fairing, something that has to separate into pieces and reliably clear the area shortly thereafter.

So I am wondering, what goes into the manufacture of a 21st century fairing that might make it expensive enough to recover and refurbish, and to then convince a customer your going to protect their payload with a used fairing? Expensive high strength alloys? Expensive manufacturing techniques? Integrated sensors and actuators?

The "Skybox" and the "Atlantis" - selected for their SXSE sounding names. 21st century fairings are much more than passive rooftop boxes to "keep the wind off of the customer's stuff."

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uhoh
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    I can not remember in which cases thrust is reduced around max-Q to limit structural stresses in the rocket body, allowing some weight reduction. A comment or an edit is appreciated. – uhoh Feb 02 '17 at 17:58
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    You may be thinking of the way the Shuttle reduced thrust in the "thrust bucket" around max-Q, but of course the Shuttle no longer flies and wasn't unmanned anyway. – Nathan Tuggy Feb 02 '17 at 19:30
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    They don't have to be "so" valuable. They just need to be a little more valuable than it would cost to recover and refurbish them. – Mark Adler Feb 02 '17 at 22:06
  • @MarkAdler Improved wording most welcome! I may have used a bit of SE license. I struggled with several variants; "valuable enough", "sufficiently valuable", settled for something that at at least got the job done. "Why might fairing recovery be economically viable?" just doesn't have the same pizzaz, nor flag the fairing itself as my central focus (rather than economic theory of cost-effectiveness). – uhoh Feb 02 '17 at 22:39
  • I've asked this follow-up question. – uhoh Feb 03 '17 at 01:54
  • If the crux of the question is what makes it "expensive enough to recover and refurbish", this can be boiled down to: "Is the cost of recovery and refurbishment less than the cost of manufacture?" That is, as yet, an unanswered question in the industry, and moreover it has little to do with the absolute cost of manufacture. – ruief Feb 03 '17 at 20:00
  • @ruief The reason I wrote "What makes 21st century fairings so valuable that..." is because the crux of my question is exactly that - the value of the fairing. If that wasn't clear enough, I gave several example: "Expensive high strength alloys? Expensive manufacturing techniques? Integrated sensors and actuators?" I've asked the question that I wanted answered. – uhoh Feb 04 '17 at 03:36
  • @NathanTuggy I think what I'm thinking about in my first comment is something like this. – uhoh Feb 04 '17 at 05:26
  • @OrganicMarble in this comment above, I'm looking to add something that shows that around max-Q thrust is sometimes reduced for purely mechanical stress reasons in unmanned missions. I just noticed your comment here - is there some place I can read about that? – uhoh Feb 04 '17 at 05:31
  • @uhoh If the cost of recovery and refurbishment comes in at $5000, then the fairing doesn't need to have any of your "example" expenses to be worth recovering. If you're asking about absolute cost, why ask a question about relative cost? – ruief Feb 04 '17 at 07:28
  • @ruief life is a mystery! Enjoy your stackexchange experience. – uhoh Feb 04 '17 at 08:50
  • Imagine a briefcase filled with 6 Million Dollars falling from the sky. Would you try to catch it? – Ben Bai Apr 10 '17 at 19:23

3 Answers3

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It is not so much that in the 21st century it is hard or expensive to make a fairing.

Rather they are just REALLY REALLY big. 13 meters by 4.6 meters.

Falcon 9 Fairing dimensions That is about 40 feet long, and 14 feet wide. The common description is that a school bus would fit in it. (Sort of like the Space Shuttle cargo bay size).

These need to be very light, as every gram/pound of mass, is a reduction in chargeable payload, but they need to be strong, since they are the aerodynamic shell around a very fragile payload as it accelerates from a dead stop at sea level to Mach 25 (I do not know what speed they are at, when they ditch the fairing, but orbital is Mach 25 or so) in orbit.

Thus the aero loads on it can be quite high. The size is quite large. The need to manage weight is quite critical.

All these items conspire to make it cost enough to matter, that recovering it, if possible to do cheaply enough, is a good idea.

In terms of actual construction it is mostly carbon fibre reinforced where needed.

geoffc
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    +1, light plus strong = expensive. The fairing is usually ejected when the vehicle reaches the free molecular flow regime, I think. – Organic Marble Feb 02 '17 at 18:52
  • @OrganicMarble Any idea how fast or high that is? – geoffc Feb 02 '17 at 20:36
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    ISTR it's when the mean free path is bigger than the vehicle. This paper gives mean free path vs altitude. science.widener.edu/~svanbram/chem332/pdf/menfpath.pdf – Organic Marble Feb 02 '17 at 21:15
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    FYI, since Mach number refers to speed of sound, as you get to higher altitudes with lower pressures your airspeed drops and must travel faster to maintain same Mach number. At orbital velocity, you would actually have Mach 0 speed even though traveling at 7.8 km/s. You were spot on with the numbers tho. – IT Bear Feb 02 '17 at 23:26
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    I've just check five videos and confirmed times with the corresponding press kits; all fairing separations are tightly clustered around an altitude of 110 km. If the plots in @OrganicMarble 's pdf are correct, that's a mean free path of 2 centimeters. However, using their scale height of about 8.4km, the pressure is 2E-06 bar, maybe that's low enough? – uhoh Feb 02 '17 at 23:26
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    @ITBear no that's wrong. Speed of sound scales as the average molecular velocity, which scales as the square root of temperature. The effect of pressure is relatively small in comparison, until it's so low that effective sound propagation stops. – uhoh Feb 02 '17 at 23:28
  • @uhoh Ah you are right, I stand corrected. Speed of sound is mostly dependent on temperature. Technically sound needs a medium to travel through, so once you reach LEO your airspeed would still be zero? – IT Bear Feb 02 '17 at 23:36
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    Sorry I sent you off on a incorrect tangent! :( – Organic Marble Feb 03 '17 at 00:11
  • @OrganicMarble I live for tangents! Thus this follow-up question. – uhoh Feb 03 '17 at 01:54
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    @OrganicMarble Good news! Instead of using the simple scale height there, I found some tabulated data from the 1976 standard atmosphere. https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19770009539.pdf On page 56 they show the density at 110 km of about 9.7E-08 of standard. MFP scales inversely with pressure, so 70 nanometers at standard atmosphere becomes 72 centimeters, not the 3cm from the simplified model, and the MFP is almost doubling every 5km, so you are basically right, within a few km. – uhoh Feb 03 '17 at 13:52
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    A fairing should not only be very light and strong, it should be reliable too. Very bad for the reputation of a launcher if a payload is lost due to a failure of the fairing. – Uwe Feb 06 '17 at 19:52
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    I'd just like to add that from the launch vehicle manuals I've read the time the fairing is jettisoned is determined so that the free molecular aerothermal flux remains below 1135 W/m^2 – awksp Dec 10 '17 at 01:33
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The fairings are large structures, made in a way that's difficult to automate. The structure consists of an aluminium honeycomb core with carbon fibre inner and outer panels laminated onto it. Carbon fibre is a labor-intensive material. Then there's the quality assurance that makes everything rocket-related expensive.

Roof boxes, OTOH are injection moulded plastic so the entire shell goes from plastic granules to finished product in 10 seconds. And they are made by the million so it's cost-effective to automate the process.

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It's also possible that they don't want the negative publicity of these farings floating on the surface of the ocean downrange and being either navigational hazards, or a target for environmental concerns. I'd guess they're light enough that they wouldn't burn up and probably would just float after hitting the water.

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    Hmmm.... aluminum honeycomb and carbon fiber - wow they might float! I never thought of that. Considering they are "the size of a bus" and there are two of them, and they would probably sit quite low in the water and be harder to spot visually (and perhaps radar) until fairly close, they might be candidates for some kind of navigation hazard. – uhoh Feb 07 '17 at 03:07
  • So I've asked this. – uhoh Feb 07 '17 at 03:29
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    It seems that finding these on beaches isn't too uncommon http://www.collectspace.com/ubb/Forum14/HTML/001305.html – Tyler Apr 11 '17 at 19:45