SpaceX recently tried to recover fairings from falcon 9, which landed intact in the ocean. How come do these not burn up during reentry?
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3 Answers
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As a followup to Rory's excellent answer, I thought I'd just note one thing:
Ok, let me ask other way around. Why does the booster need a reentry burn to slow it down, but fairing is fine without it. Half-Fairing seems to be rather a large and fragile piece.
Fairings have a few things going for them.
First of all, they're incredibly light for their size, giving them a tiny ballistic coefficient. I don't have exact numbers, but I've heard that they're around 900 kg. Compare that with the mass of an empty F9, at 23,000 kg. At 1,600 m/s, that's ~30 GJ vs ~1 GJ of energy. Already a significant difference.
Secondly, area. The F9 fairing has a surface area of (very roughly) 106 m^2. The rear end of an F9 has an area of 10 m^2. If we assume the deceleration takes 1 minute (naive, of course), we get 181 kw per m^2 for the fairing and 4.9 MW per m^2 on the rear end of a F9. Broad daylight delivers about a kilowatt per m^2, for reference. This means the material will have an easier time dissipating the heat non-destructively.
To put this in perspective a bit more, the heat dissipated in the F9 would heat the 9 Merlin engines (assuming they're made of aluminum, naively of course again) to 8600 degrees. A full aluminum fairing would heat to just 1400 degrees.
This all neglects the insanely complex dynamics of hypersonic compressive heating. Some proportion of the heat is typically dissipated in the air itself, turning it into plasma - I don't currently have time to research what this coefficient would be. It also neglects the ablative nature of all the materials involved.
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2$\begingroup$ The lower ballistic coefficient means the fairing slows higher in the atmosphere, so the compressive heating is also less $\endgroup$– user20636Commented Feb 24, 2018 at 10:08
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1$\begingroup$ Great answer. You should know better than talking about “8600 degrees”, shouldn't you? What are those, Celsius, Fahrenheit, $\tfrac{\pi}{180}$ radians...? $\endgroup$ Commented Jul 31, 2018 at 14:56
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2$\begingroup$ @leftaroundabout that's degrees Rankine. $\endgroup$– 0xDBFB7Commented Jul 31, 2018 at 19:09
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$\begingroup$ Also, why thank you! I'd add units now, but I've forgotten what I used... $\endgroup$– 0xDBFB7Commented Jul 31, 2018 at 19:09
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Burning up happens through heat generated through friction* between an object and the atmosphere.
When the fairing was deployed, the rocket was only travelling at 6500km/h. (As an example, the Chelyabinsk meteor was estimated at ten times that speed)
The fairing had been on the rocket on the way up, accelerating to that speed, and on its way down at only slowed through drag with the atmosphere, and with a parafoil towards the end.
There was nothing that would generate enough heat for it to burn up at that speed.
*In technical terms this comes actually from the shockwave in front of an object entering the atmosphere. It compresses the air in front of it, and this compression can dramatically heat the air, which then heats the object.
Looks pretty good after coming back as well, a little toasted.
On board ship:
answered Feb 23, 2018 at 10:06
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$\begingroup$ And the fairing is very light too. $\endgroup$– jkavalikCommented Feb 23, 2018 at 10:26
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$\begingroup$ Yeah - it does slow pretty fast once it gets back down into denser atmosphere $\endgroup$– Rory Alsop ♦Commented Feb 23, 2018 at 10:27
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1$\begingroup$ The fairing is separated as early as possible when the payload needs no longer protection against the remaining air. The payload needs much more speed to be inserted into an orbit. Separatíng the fairing as early as possible saves weight and a lot of fuel for acceleration. $\endgroup$– UweCommented Feb 23, 2018 at 10:28
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2$\begingroup$ "The term 'friction' is a misnomer" Radiative and convective heating from the bow shock would be a better way to describe the source of heat than "friction" when things "burning up" during reentry. $\endgroup$– uhohCommented Feb 23, 2018 at 10:46
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1$\begingroup$ Ok, let me ask other way around. Why does the booster need a reentry burn to slow it down, but fairing is fine without it. Half-Fairing seems to be rather a large and fragile piece. $\endgroup$ Commented Feb 23, 2018 at 12:41
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Wikipedia: "The fairing is 13 m (43 ft) long, 5.2 m (17 ft) in diameter, weighs approximately 1,900 kg, and is constructed of carbon fiber skin overlaid on an aluminum honeycomb core". Each fairing half is relatively light with a low ballistic coefficient, which means they are easily slowed down by the atmosphere without generating a huge amount of heat...
