Nuclear Friday: Second Generation Standoff

Last week The Board took a look at the basic concepts behind standoff range weapons and delved into the history of the AGM-28 Hound Dog missile. The much-loved Hound Dog was a remarkable technological achievement, but it had one major problem. It was simply too big. The B-52 could carry only two AGM-28’s. Even though promising research was underway to give the Dog new capabilities, Air Force experience going up against the Russian-supplied SA-2 air defense system in North Vietnam showed that the AGM-28 would not by itself be adequate to take on Soviet defensive systems. Secretary of Defense Robert McNamara recommended against expanding the Hound Dog system beyond missiles already on order in 1966.

The Hound Dog was going to need the help of smaller and smarter weapons until it could itself be replaced.

The AGM-69 SRAM:

If you do some research on the Short Range Attack Missile you’ll see it described as a replacement for the Hound Dog. That’s not really true as both systems were in service at the same time for several years in the mid-seventies. Both systems were mounted on B-52 bombers and it may have been the case that both may have at times been carried by the same bomber. I can’t confirm this as the temperamental SRAM did not have a lot of fans when it was in service. I haven’t been able to find any nostalgia sites run by Air Force veterans who worked with the AGM-69 even though the missile was in service for nineteen years. How service crews feel about weapons is important.

In spite of the lack of love for the SRAM (maybe it just needed a cuter name) it’s capabilities were impressive. It wasn’t a cruise missile like the Hound Dog, it was really in a class all by itself and conceptually very interesting.

First off it was small:

OSTA-AGM69

I chose this picture of an SRAM being loaded onto a B-1B during the brief period both that aircraft and the SRAM were in service because it shows the size the weapon relative to people. It’s possible to find the exact dimensions of the SRAM on line, but here at The Board we are all about the human experience of nuclear warfare. Knowing that two of these missiles could lay side by side on the floor of a fairly narrow apartment kitchen is more important than numbers.

The AGM-69 took a “more dakka” approach to weapon delivery. The most common SRAM configuration was six missiles each on two pylons mounted under the wings of a B-52 and eight more in a rotary launcher in the weapons bay. That still left room for two smaller bombs in the 52’s bay, though bombers with this configuration usually didn’t carry them in order to save weight. So that’s twenty missiles. You may think that’s a lot but there were a few B-1B’s configured with three rotary launchers, one in each of its three weapons bays for a whopping total of 24! SRAM was also carried by the FB-111, with a maximum load of six missiles. This is a HUGE ability to engage targets. An incoming bomber can afford to use these on radar antennas, communications outposts, and SAM installations.

Despite this impressive strength in numbers, SRAM had some serious limitations. The first was range. It was rocket powered, unlike the air-breathing Hound Dog, so it had to carry oxidizer as well as fuel. If fired in a largely ballistic trajectory it could reach about 200 km away from its craft. But the AGM-69 wasn’t just a miniature version of an ALBM, it could also drop near the ground and cruise in a nose-up orientation, even turning around to attack targets behind the aircraft. This mode of attack limited the range to around 65km. Awesome seconds of video showing a low altitude drop and nose-up attitude at the link.

SRAM carried a dual yield warhead that would either use a tritium boost on its fission primary to detonate at 17 kilotons, that’s about one Nagasaki, or the primary could remain unboosted to initiate the fusion secondary for a 210 kiloton yield. And it really needed the 210kT option as its inertial navigation and radar altimeter systems could only reliably get it to about four football fields away from the target. But of course, few of its targets would be hardened. A typical target might be a remote Siberian village that has very good long distance service because military communications were routed through it. Switchboards are targets.

As I mentioned, the SRAM was not very popular among service crews. A primary reason was that it was considered dangerous. There were a few incidents with cracks and corrosion in the fuel system. At one point parts of all SRAM engines had to be painstakingly replaced in the field. In spite of these repairs, there were still problems, including some Broken Arrows. This is one of the reasons that George HW Bush ordered its withdrawal in 1990 with all missiles removed from service by 1991. We’ll get to one of the other reasons for this order in a moment.

Another unusual feature of the AGM-69 was that it was encased in radar absorbing materials and had fins made of a radar transparent plastic. I have been unable to find out why. There was no anticipated Soviet ABM capability in the seventies that could even track, much less intercept an SRAM. In fact, a few current Russian short range ground based systems use this dual attack mode with a low radar signature as a counter to US BMD systems like Aegis and THAAD. My hypothesis is that SRAM had this stealth design in order to keep the externally mounted missiles on a B-52 from making its radar signature even brighter.

But you never know what people are doing with radar. Its secrets are more closely guarded than the bomb itself.

A Detour into Radar:

Around the time I was born, a typical strategic load for a B-52 would have been two large thermonuclear bombs, and two Hound Dog missiles. That’s not a lot of ability to engage targets. But the B-52 was counting on four little friends to help it get to the target area, the ADM-20 “Quail” decoy.

OSTA-quail

Again, I chose this picture to show the human perspective. I could fit this in my living room if I rearranged my furniture. I could probably turn it sideways and lay it on top of the SRAMs in my kitchen, but then I couldn’t open the refrigerator or oven.

The B-52 could drop up to four of these as it approached Soviet airspace. It would cruise along in a flight path similar to a B-52 for about 700 km with its tiny jet engine prompting air defense aircraft to be scrambled to intercept it and drawing SA-2 fire. The Quail would fall harmlessly to the ground after running out of fuel.

But wait: How can something that can fit in my kitchen be mistaken for a B-52 which can’t fit on my block? The secret is that the Quail was designed to be the opposite of a stealthy missile. It was intended to reflect as much radar as an entire B-52! And that was even more of a challenge be cause the Quail was almost transparent to radar since it was made of fiberglass.

I like to think of radar as trying to figure out what’s far away from you in a large cave while standing in one place with a strong flashlight when you are very nearsighted in one eye and have severe astigmatism in the other. You can tell things are out there, how many there likely are, and how they are moving. Using this model we can sort of understand how a Quail could look like a B-52. Suppose there were two objects in the distance, a large truck, and a glass aquarium with a rotating disco ball in it. In the confused reflections dazzling your defective eyes, you might think that the disco ball was just as bright as the much larger truck. In this analogy the glass of the aquarium is like the fiberglass shell of the Quail. The radiation you are searching with shines through in both cases, but things inside each might reflect brightly. The Quail really did have the radar equivalent of a disco ball in it.

And in case this still sounds implausible, I understand, but you need to keep in mind that aircraft have a natural inclination toward stealth. Curved and streamlined shapes reflect radar away from the source. You might counter with an argument that I am saying that if I’m in a large darkened room with somewhat better eyesight than in the above analogy, that tennis balls are invisible even when you shine a flashlight on them. I agree, tennis balls are indeed visible. This is where the analogy between radar and eyesight breaks down. Light waves are 8 orders of magnitude smaller than width of tennis balls. Light and balls interact in a way that plenty of light will come back to you no matter the shape. Contrast this with aircraft and radar. Planes are tens of meters at most in their longest dimensions, and radar waves are a few centimeters at most. That’s three or sometimes fewer orders of magnitude. Curves and shapes matter when there’s that small a difference. The physics is too complicated for this post.

That’s why I required two bad eyes in my “Allegory of the Cave”. The nearsighted eye represents the weak signal from the curved long dimensions of an aircraft, the astigmatic eye represents the dazzling reinforced interactions of radar waves that happen when there are joints, angles, and concavities on the aircraft that are around the same size as the radar waves or even multiples of under a thousand or so of the wavelength. Radar doesn’t see the plane, it sees engine intakes, wing joints, and control surface seams. That’s why stealth bombers look so weird.

Of course, you may still be skeptical that the Quail really could simulate a B-52. Good! You need to be skeptical when so much of your nuclear capability depends on air defense penetration. By the late sixties, bomber pilots had developed some effective counters to the SA-2 over Vietnam. They wanted a decoy that could build on these successes. It seems someone could have dropped a few Quail over North Vietnam to see if they really would draw fire, but no one did. Tests in 1971 with US radars tuned and pulsed like newer Soviet radars proved the Quail wouldn’t work.

This was somewhat bad news. The new decoy that would actually do what pilots wanted was turning out not to work, so the Quail stayed in service until 1977 when it was replaced by SRAM launchers.

And while missile spamming your way into enemy airspace sounds cool, many were disappointed that the more subtle option of an advanced decoy had been cancelled.

There may not have been a decoy, but soon there would be something better.

AGM-86B. Stealth and Digital TERCOM at Long Last:

The AGM-86B, more commonly known as the ALCM, has been the backbone of US strategic air capability since it went fully into service in 1981. It is still the backbone of the USAF’s nuclear delivery systems to this day. Here’s a picture of a test from December 2016.

OSTA-AGM86test
Obama’s last nuclear middle finger.

The ALCM developed out of a program to make an advanced decoy to replace the Quail. Early on in the Advanced Decoy program planners decided to address the B-52’s limited ability to engage targets by putting small nuclear warheads in the decoys themselves. Thus the program became known as SCAD, Subsonic Cruise Armed Decoy.

Now at first this sounds like a terrible idea. The purpose of the Quail was to misdirect interceptors and draw surface to air missile fire. It doesn’t make sense to put a nuclear weapon into a decoy missile when its purpose is to draw fire. That’s just wasting a nuke. But the SCAD concept was based on real experience in Vietnam. SCAD wasn’t going to really be a decoy, it wasn’t going to misdirect air defenses. It was planned to be an automated ECM platform to suppress air defenses. The idea was for SCAD to have ECM capabilities similar to the B-52 itself. It would fly around in the area of the bomber, effectively extending the bomber’s ECM range to hundreds of kilometers.  And then when the SCAD was low on fuel, it could go nuke something.

Brilliant concept in principle, but it soon proved to be beyond the capabilities of late 60’s early 70’s computers. ECM had worked as well as it did in Vietnam because there’s plenty of room in a manned aircraft for a bulky computer. More importantly, ECM usually had a crew member dedicated solely to this task. Computers could be made more compact, but simulating the expertise of a dedicated ECM crew member proved impossible. In 1973 the original version of the SCAD was cancelled, but its primary contractor, Boeing, was commissioned to make a revised prototype of a new cruise missile to replace the Hound Dog out of existing parts they had already developed for SCAD. By 1976 they had made the AGM-86A test vehicle. It’s navigation system was the fulfilment of a decades-long dream. The 86A had automated TERCOM.

Terrain contour matching, TERCOM, had been around since WWII. The first TERCOM system was the RAF’s H2S bomb aiming system. H2S worked just fine, but it’s main problem was that it needed a human crew member to sit in front of a radar scope and lay transparent maps of the target area over the radar screen and then give verbal instructions to pilot to make course corrections. Later versions of H2S automated some of these functions, but still relied on a human radar operator.

Early attempts to fully automate TERCOM in the 1950s were stone knives and bearskins. Terrain images were stored on film strips, which were then converted to an NTSC analog video signal and run through a video mixer and combined with radar signals also converted to NTSC video. The control surfaces of the drone aircraft they were tested on would then move until the peaks in the two signals overlapped, thus correcting altitude and course. Analog TERCOM worked. And guess what, it would fit in my kitchen. Just barely. Obviously this is too big to be an appropriate missile guidance system.

In 1976 the AGM-86A successfully tested a new fully digital TERCOM system. The results were so impressive that in 1977 Carter cancelled the B-1A bomber program reasoning that ALCM would more reliably give the same capabilities at much lower cost. It would still be a few years before the ALCM was deployed. The Navy also had a cruise missile program with successful TERCOM, the BGM-109 Tomahawk. The DoD decided that both missiles should use the same engine and TERCOM computer. The DoD also preferred that there should be only one type of missile, so a study of an air launched Tomahawk was made. As it turned out, putting the BGM-109’s engine in the ALCM improved its range and the ALCM became more accurate when using the Tomahawk’s computer. While there were some proponents of the air-launched version of Tomahawk, the ALCM program was saved because it had been shaped such that it could fit into modified SRAM launchers. The air version of the Tomahawk would have needed an entirely new launching system.

Digital TERCOM and careful design made the ALCM a stealth standoff weapon. TERCOM allows it to fly at altitudes so low that it is fully lost in radar scatter from the ground, and it was designed to have a low radar signature from almost every direction. That’s why the elevator vanes are pointed down at a slight angle. The only way to spot it is by shining strong radars down from above to get backscatter from the engine intake. It’s almost as compact as the SRAM, though it’s too long to fit in my kitchen, and has ten times the SRAM’s range.

It has a much smaller warhead than either earlier standoff nuke. The maximum yield of its W80 warhead is around 150kT, 3/4ths that of SRAM, and only 1/8th that of Hound Dog. This is fairly typical of US weapons at the time. Accuracy improves and yields get lower. Since the Super Bowl was in Houston last month I like to think of it this way. At maximum yield Hound Dog could be launched near Dallas, get vaguely near Fannin and the South Loop, and flatten NRG Stadium along with the Medical Center and the inner southern suburbs. Maybe downtown too if it drifted too far north. SRAM could be launched from near Beaumont and detonate within a short-ish walk from the stadium and flatten it and the Medical Center. ALCM could be launched from Mississippi, follow South Loop at low altitude, take the Fannin exit while climbing to clear the sides of the stadium, and then detonate between the sidelines and the thirty yard lines while not completely flattening the entire Med center. I highly recommend trying these simulated missile attacks on your own community with NUKEMAP.

The AGM-86B has been almost unbeatably good for almost 40 years. The question is, does it need to be replaced?

NEXT WEEK:

The Board will take a look at how cruise missile technology took a very different path in the Soviet Union, the current missile race between India and Pakistan, and finally; how I think airborne standoff capability fits into current and likely future strategic needs of the US.

 

 

 

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