Was Stealth Technology Used In Bin Laden Raid?

Helicopter parts left behind after the raid on Osama bin Laden's hide-out may offer some clues about the stealth technology used in the mission. Bill Sweetman, editor-in-chief of Aviation Week's Defense Technology International, describes some features of the downed 'craft.

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IRA FLATOW, host:

You're listening to SCIENCE FRIDAY. I'm Ira Flatow.

We're going to be talking now about something really unusual. The raid on Osama bin Laden's compound revealed more than where the terrorist leader was hiding. Of course, you saw the picture, the very strange picture that came out of that. Military analysts are looking at that helicopter, you know, trying to figure out what was that helicopter? What was it made out of? Why - you know, funny-looking stuff on it. It left behind some tantalizing clues when they left that helicopter behind.

And talking - joining me now to talk about it is Bill Sweetman, editor-in-chief of Aviation Week's Defense Technology International. If you want to tweet us and talk about it, our number is 1-800-989-8255. You can tweet us @scifri, @S-C-I-F-R-I. Welcome to SCIENCE FRIDAY, Bill.

Mr. BILL SWEETMAN (Editor-in-Chief, Aviation Week's Defense Technology International): Hi. Good afternoon.

FLATOW: Were you thrilled when you saw those pictures of the helicopter?

Mr. SWEETMAN: Well, yeah. I think for anybody who takes an interest in these things, it's always a great moment when you actually see something new for the first time. And it was one of those sort of instant moments where you just looked at something and you go, oh, that's - that was why we were having trouble identifying it, because it's not a helicopter we've seen before.

FLATOW: And it was immediately assumed that this was stealth technology.

Mr. SWEETMAN: It was fairly obvious. The shaping that you could see around the mast that carries the tail rotor, the stabilizing rotor on the back, and the shape that you could see on the back of - the rear end of the helicopter's tail boom was very reminiscent of the sort of shaping you see on other stealth aircraft like the F-22 or the F-35 Joint Strike Fighter. It's a combination of, kind of, gradual curves, flat planes and the occasional - and a few knife edges where those join.

FLATOW: Mm-hmm. And from the picture, what functions did those extra bits of equipment or edges have? What were they on the helicopter for?

Mr. SWEETMAN: Well, you could see, really, two sets of things. One was a tail rotor that was modified with more blades than usual, which is - which you would do to make the helicopter quieter and make its noise less obtrusive, to take some of the very strong beats out of the helicopter's noise signature.

FLATOW: How would that be accomplished?

Mr. SWEETMAN: Well, the sort of beats you hear when you hear a helicopter, the sort of rhythmic pounding...

FLATOW: Right.

Mr. SWEETMAN: ...is actually associated with the rotor blades. And quite often, as the wake from the rotor blades - from the main rotor cuts through the wake from the tail rotor, you get the sort of - you get a very strong noise signature. So if you can break that down, if you have more blades, the beats are a little weaker and the frequencies higher, and so it's going to blend in to the background noise a little better.

FLATOW: Mm-hmm. And so we didn't get a look at the main rotor because that was blown up, right, the front part.

Mr. SWEETMAN: That's right, yes.

FLATOW: Would you suspect there might be shorter, more blades on that one also?

Mr. SWEETMAN: I suspect that they've added a blade or two to the main rotor. It looks like this is a modified version of a Sikorsky Black Hawk, which has four main rotor blades. And I would guess, quite possibly, they would have gone to five. And that's, you know, that's something that we've seen in a lot of programs, a lot of declassified programs over the years. Even - I mean, even back to Vietnam, the CIA was modifying helicopters to reduce the noise and doing exactly that sort of thing.

FLATOW: Uh-huh. And I guess, because you want it to be a quieter one in terms of stealth, you're talking about not people knowing that you're lurking around or coming so close to your location, you want to make less noise. But, what about also the ability to evade radar, so you can't see on radar that the helicopter is coming?

Mr. SWEETMAN: Oh, you want - yes, you want to do both those things. Noise is important for a helicopter because that's generally the first sign you have even before a helicopter is within line of sight. So, you know, you may not be able to see it on radar, maybe behind trees or behind a hill, but you'll still hear it. So the idea then, is to bring the radar signature down kind of to match the detection range that you have acoustically, but also just to protect you against, perhaps, airborne radar. The Pakistanis have some of those.

FLATOW: Can you change the shape of the aircraft to make it better elude the radar?

Mr. SWEETMAN: Yeah. Primarily what you're trying to do with radar stealth is to change the shapes so that radar waves will tend to bounce off anywhere except towards the radar itself. So shape is the number one concern. And what you could see on this photograph was that a cover(ph), looks like - I call it the dishpan - that goes over the hub of the rotor. And if you look at a helicopter's rotor hub, either the main or tail, it's a lot of very complicated metal machinery. And so it has - it actually sends(ph) a very strong radar signature, so anything you can do to cover that up is helpful.

FLATOW: Yeah. Some people describe the cover as a hub cap.

Mr. SWEETMAN: Yeah.

FLATOW: Sort of looks like a hub cap.

Mr. SWEETMAN: It looks like that too, yes.

FLATOW: It looks like a hub cap on a car. What about making the helicopter not visible or less visible in the heat range, in the infrared range so you don't, you know, maybe it shows up on infrared detectors or maybe heat-seeking missiles or things like that?

Mr. SWEETMAN: There was some suggestion - it certainly appeared that the tail rotor section had some kind of - a sort of silvery color to it. And we've seen something similar to that on a few V-22 tilt rotors. And it has been described as an anti-infrared coating.

I suspect that what you're trying to do is actually shift the waveband of the infrared signature. I mean, you're not going to make it go away. But if you can shift it into a different waveband, there are just about two parts of the infrared spectrum where IR goes - travels well through the atmosphere. Otherwise it tends to be soaked up by moisture in the atmosphere.

So if you can shift - so one result of that is that the infrared detection system that - systems that are out there usually operate in just two specific parts of the IR band. Now - so, if you can shift some of the energy out of that band, then you're less visible.

FLATOW: Let's go the phones. Darryl(ph) at Austin Texas, hi. Welcome to SCIENCE FRIDAY.

DARRYL (Caller): Hi, Ira.

FLATOW: Hi, there.

DARRYL: I'm in active duty Army Special Forces. We pretty much use the same type of explosive devices that the SEALs use. And I just wanted to make the comment that when they destroyed that aircraft, they destroyed the aircraft. There's a special munitions package that the aircraft commander has on that helicopter - all of the crew is trained with it.

And in the event something like that happens, those packages are placed specifically near all the classified and highly technological instrumentation and devices. And when it's set off, it vaporizes it. It's not a situation where it just blows it up into little pieces.

Those devices no longer exists, so no one could take a look at them and see how they were made or what they were made of or get any information out of them. And that's pretty much standard operating procedure these days.

FLATOW: So why were pieces left over?

DARRYL: Well, the - it's due to the type of explosive that's used. Parts of the hull and rotor assemblies may stay intact, but where the packages and these are very, very small packages, about the size of a pack of cigarettes.

When those are detonated, they instantaneously produce temperatures that are classified but whatever their next to no longer exists after the detonation. It just - literally it will melt metal and dissolve any kind of composites or whatever. There's not even ash left.

FLATOW: So there may be avionics or electronics that was even more valuable intelligence-wise that was destroyed than just the mere shell of what was left in the helicopter.

DARRYL: Oh, absolutely. The shell of the helicopter was probably secondary to the systems that were inside the aircraft.

FLATOW: Yeah. All right. Thanks for calling, Darryl. Any comment?

Mr. SWEETMAN: Yeah. I think from where the wreckage was sighted, it looks actually as though the tail escaped destruction, because as the aircraft came down, it seems to have struck the compound wall with its tail. So the tail, it looked like, was outside the compound. The rest of the aircraft was inside it.

And I think, you know, a tactical decision was taken, you know, they're not going to try and destroy the tail. And as Daryl says, this - you know, even -any military aircraft, particularly one that's equipped for special operations, you've also got a lot of gear on board - communications, cryptographic, things like that - that is in itself very sensitive.

FLATOW: So that was a smart decision by the president to include those extra helicopters along for the ride.

Mr. SWEETMAN: Yeah. I think it was a smart decision. I think it was, you know, I'm actually surprised anybody thought about doing it any other way. You know, any time that you're operating helicopters tactically and, you know, going into a - trying to land or do fast roping(ph) in a complicated situation on the ground, you there's all sorts of hazards involved.

FLATOW: Yeah. Let's go John(ph) in Bethesda, Maryland. Hi, John.

JOHN (Caller): Hi. I heard that density altitude might have been one of the problems that led to the helicopter crashing, and so I was curious to know whether any of the helicopter blade designs you guys talked about earlier might have contributed to the copters not being able to deal with this thin air. And I'm happy to take my answer off the air.

FLATOW: Yeah. Because it was hot air there, the altitude is actually equivalent to being higher because of its thinner air. So he's asking the density altitude. He's asking whether that might have been a contributing factor on this.

Mr. SWEETMAN: Yeah. I mean, a couple of things here, one is the weather report that the temperatures in the landing zone were higher than expected. And when you're coming in, you know, coming in to try to decelerate into (unintelligible) the helicopter, it's (unintelligible) you're sort of trying to anticipate what the temperature is, so that you have the right amount of power and the right amount of lift as you slow down. So that could have been a factor. Some of the, you know, possible low-level atmospheric effects just caused by, you know, rotor wash being trapped in the compound and falling on (unintelligible) rising column could have been a problem too.

I don't know necessarily that the stealth modifications sort of affected that very much. The only possibility would be that - there has been discussion over the years that one way of reducing noise is actually to run the rotor a little bit slower than max power. And that might have meant that it didn't quite have the power reserve instantly available it normally would have done. But that's, you know, that's getting very far into the realm of speculation.

FLATOW: This is SCIENCE FRIDAY from NPR. I'm Ira Flatow, talking with Bill Sweetman, editor-in-chief of Aviation Week's Defense Technology International.

Can anyone really learn anything, any - you know, any of our, quote-unquote, rivals or enemies learn anything from these photos that they've seen?

Mr. SWEETMAN: Frankly, Ira, I'd say that the biggest secret about this was the fact that it existed and that was it operational, and the decision was taken to accept the risk that that would be compromised. As for the - as for learning about the technology from photographs, I don't think - I think that the most -the thing that you learn from that is that, yeah, not only does this exist, but it is pushing for a fairly significant level of reduction in radar image. But it doesn't - it's not really going to help you very much to design your own stealth helicopter. To do...

FLATOW: I guess that's was my question, because...

Mr. SWEETMAN: Yeah.

FLATOW: ...anybody in the helicopter business would probably have thought about these things.

Mr. SWEETMAN: Yeah. And stealth is a pretty complicated discipline. It's not just a question of, you know, copying a shape. It's a question of figuring out what your, you know, figuring out what your - what systems, what sensors are looking for you, what their detection range will be, modeling that against all your, you know - everything that you can do to increase the stealth of the system and - and using a lot of computer simulation in the first place, then doing, you know, a lot of very painstaking testing and detailed design, because, well, one thing about stealth is it doesn't take a physically big details to be wrong to pretty much give the game away.

FLATOW: All right, Bill. Thank you very much for taking time to be with us today, and good luck and have a good weekend.

Mr. SWEETMAN: OK. Thank you very much.

FLATOW: Bill Sweetman is editor-in-chief of Aviation Week's Defense Technology International. And he was talking with us today about those photos of that stealth helicopter, or we think it's a stealth helicopter, left at the raid for Osama bin Laden.

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