On many pictures (maybe on most of the pictures) part of nose looks hammered or slightly smashed, or bent. Please note these 2 symmetric dents just behind nose spike:

enter image description here source

enter image description here source

On other pictures these dents are missing:

enter image description here source

Why? Is this a feature or damage?

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    $\begingroup$ You don't normally see perfectly symmetrical damage. They look like fairings that are installed over sensors that have to be mounted in the chines, or, it was decided to put a step along the edge of the chine for an aerodynamic reason, and the shapes are transition fairings. $\endgroup$ – John K Oct 18 '19 at 11:35
  • $\begingroup$ Good info here:flightglobal.com/pdfarchive/view/1973/1973%20-%202291.html $\endgroup$ – Mark Jones Jr. Oct 19 '19 at 17:24
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    $\begingroup$ Micrometeorites and bits of smashed satellite? $\endgroup$ – Harper - Reinstate Monica Oct 19 '19 at 17:51
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    $\begingroup$ High-speed mosquito strikes. $\endgroup$ – Mark Oct 20 '19 at 20:09

The SR-71 had a detachable nose (photo #2), and could change between three different nose cones depending on the mission. I haven't found a lot of information on the three, other than what this site and this site mentions:

  • Training nose with dead weights
  • Radar nose containing a Side Looking Radar. This is probably the bulge you saw on the photos.
  • Photo nose containing the optical bar camera, taking very wide photos of the ground.
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    $\begingroup$ The linked sites are good references, but they don't explain the leading edge bulges. As those links explain, the sensors are located elsewhere. $\endgroup$ – mike Oct 19 '19 at 8:28
  • $\begingroup$ From SR-71.us: "The antenna assembly/motion sensing unit, transmitter, receiver and related equipment are installed in a specially-configured detachable nose." $\endgroup$ – bjelleklang Oct 19 '19 at 18:03
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    $\begingroup$ TIL the mission payload is in the nose of SR71. $\endgroup$ – user3528438 Oct 20 '19 at 2:25

Got it. Those indents are cut-outs for the "DEF A2" radar warning and ECM system.

Text from pdf of (purported) SR-71 Flight Manual describing components of the DEF A2 defensive electronics system

Relevant quote:

The receive antennas are located aft of cut-outs on the left and right nose chines.

The transmit antennas may also be visible in the OP's picture on the underside of the A/C, but hard to say.

This manual is linked from the wikipedia article on the SR-71 as refs 72-75. Direct link to the imaged page from the manual: https://www.sr-71.org/blackbird/manual/4/4-124.php

Note that the manual is silent on rear-facing sensors. This is consistent with the SR-71's primary defense characteristic, that being speed to outrun all threats. To the SR-71, a threat not already in the forward cone is simply not a threat. Also alluded to in this article about a one-off SR-71 development for countering an eventual hole in the defense strategy:

There was also the possible threat that future ground defenses would someday have the ability to reach the SR-71 from behind since it carried no aft facing defensive countermeasures.

There's an illustration without explanation on a reddit thread:

The picture there is not attributed, and at any rate, is too large to upload here. As mentioned in the reddit thread, the SR-71 itself did not carry the large radar illustrated -- the picture is actually of a YF-12, but close enough for our purposes here, as the other element in the picture is clearly identifiable as lying within the cut-out, and consistent with the manual's description of the DEF A2 system receive antenna. It shows the fairing removed from the area in question, revealing a light/off-white/silvery disc positioned at the aft end of the cutout.

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    $\begingroup$ Does it seem like there are two different sets of chine indentations available? I think the photo shows indents that are further forward than the DEF A2 referenced in the manul. Or maybe the position was changed for some installs. $\endgroup$ – user2298 Oct 20 '19 at 12:32
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    $\begingroup$ Nice find !!!!! $\endgroup$ – Fattie Oct 20 '19 at 18:32

Bjelleklang is right, the nose is detachable and would be switched out according to the mission's needs. The bulges are a key component of the (for that time) very sophisticated radar imaging system, ASARS-1. There's more information here.

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    $\begingroup$ Welcome, Mitch! IMHO, the link provided does not identify the "notches" referred to in the OP as specific to ASARS. Your link shows ASARS components to be in the L and C bays, which are not in the location mentioned by the OP. There is a sensor described as in the "detachable ASARS nose", but its location is not indicated beyond that. I'm no expert -- just working to improve your answer, which so far is the only one to propose a specific answer which is not just plain wrong, i.e., there is no way that this is deformation or damage. $\endgroup$ – user2298 Oct 20 '19 at 7:02
  • $\begingroup$ As it turns out, the notches have nothing to do with the ASARS, see Haakon Dahl's answer. $\endgroup$ – Jpe61 Oct 21 '19 at 9:52
  • $\begingroup$ The indentations have nothing to do with ASARS-1, based upon first hand knowledge. $\endgroup$ – mongo Oct 22 '19 at 3:16

The dents are there to provide a proper thickness window for DEF systems. Ferry noses had no need for DEF systems and so didn't have the bulges. If you look carefully at the noses you will see that there is a band just aft of the pitot mast that is all composite and it was called an isolation something or other. There were several noses. OBC is optical bar camera and could take a horizon to horizon panoramic picture like every second - in stereo if you wanted to use twice as much film. The SLR or side looking radar, which was replaced by the ASARS advanced synthetic aperture radar. The Asars nose looks pregnant and is quite distinct. Those are the operational noses of which I know. There may have been others before my time. There was a capre nose, but I don't know what CAPRE stands for or if that nose is distinct from the ferry nose. The SR was originally intended to have not only swappable noses but also swappable forebodies and they would latch them at the fs 715 break. you would have an interceptor forebody, a penetrating bomber forebody, a recon forebody, which are way cheaper than the whole proulsion system, which would be swapped out according to mission requirements I was on the project in Burbank from 1987 to 1990

  • $\begingroup$ Welcome to Av.SE! Thanks for the added perspective about the nose sections, and about how so much was intended to be swappable. Hadn't come across that before, though it makes total sense. What a cool aircraft to have been involved with - can only imagine all the fascinating stories from that era. $\endgroup$ – Ralph J Apr 22 '20 at 3:19

When looking at a picture of a Blackbird, you might be looking at either of 2 different aircraft - the YF12 and the SR71. One of the differences between them is the nose - the SR71 has a smooth continuous chine, whereas the YF12 has the indentations that you've spotted but with usually a forward-looking sensor there (possibly IR?).

So it depends which aircraft you're looking at.

  • $\begingroup$ SR-71 and YF-12 have disdinctly differend noses, the aircraft in the pictures of this question definitely are not of an YF-12. It might be possible, even easy, to confuse SR-71 and its predecessor A-12: migflug.com/jetflights/a-12_oxcart-vs-sr-71_blackbird $\endgroup$ – Jpe61 Oct 22 '19 at 15:00

I don't have a copy with me right now, but the book Skunk Works is Ben Rich's account of taking over the Lockheed special-projects group from Kelly Johnson, and includes quite a lot of description of the SR-71 development and operation.

As I recall, he described that the SR-71 nose would often soften and deform in flight from the heat of very high speeds. I suspect that the bulges are a product of that.

As to why they are so symmetrical, I especially notice on your second picture, where the bulges appear to be propagating back from the join in sections.

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    $\begingroup$ If a nosecone would deform like this from heat, you'd have a serious problem. $\endgroup$ – Mast Oct 19 '19 at 17:42
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    $\begingroup$ @Mast According to this site about the SR-71 the "Temperatures ranged from 450 degrees F (232 C) near the back part of the aircraft to 950 degrees F (510 C) near the engine exhaust. The cockpit had special heat-resistant glass to withstand surface temperatures of 640 degrees F (338 C).". Dunno if that's enough to deform a nosecone. Probably not. $\endgroup$ – PerlDuck Oct 19 '19 at 18:25
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    $\begingroup$ @PerlDuck Considering the entire thing is built to withstand that heat, yea, don't think deformation is the answer here. The other answer(s) look(s) much better. $\endgroup$ – Mast Oct 19 '19 at 19:01

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