Aegis Ashore vs THAAD (July 27, 2015)

In a comment to my post of July 16 about the THAAD deployment in Guam being made permanent, a question was raised about why THAAD was proposed for South Korea and Aegis Ashore for Romania and Poland (and why not vice versa).

There are two main technical issues that almost certainly drove the decision of which system went where:

(1) Europe can be almost completely covered by two Aegis Ashore sites but achieving similar coverage with THAAD would require a prohibitive number of THAAD batteries.  On the other hand, S. Korea is small enough to be covered by one or two THAAD batteries.

A single Aegis Ashore site (with the Block IIA interceptor) can cover a much larger geographical area than a single THAAD deployment.  The Block IIA interceptor is scheduled to begin deployment in 2018.  This larger coverage area occurs because the Aegis Block II interceptor has a much higher burnout speed (likely about 4.5 km/s) than a THAAD interceptor (likely about 2.6-2.8 km/s) and thus can reach out to make intercepts at much greater ranges.

This is illustrated in two 2007 Missile Defense Agency Briefing slides.  The yellow “footprints” in Figure 1 below shows the area that could be covered by three THAAD batteries in eastern Turkey against Iranian ballistic missiles.  For THAAD, this situation — in which the attacking missiles are launched from a country bordering the country targeted – is closely analogous to the North Korea-South Korea situation.  However, the three THAAD batteries together cover only a small fraction of Turkey.

AegisVsThaad1

Figure 1.  Coverage of Europe against Iranian ballistic missile by THAAD, Aegis (Block IB), and two-stage GBI interceptors.  Slides from MDA Executive Director Patricia Sanders, “Missile Defense Program Overview For The 4th International Conference On Missile Defense,” June 26, 2007.  Available at: https://mostlymissiledefense.files.wordpress.com/2013/06/bmd-overview-sanders-june2007.pdf

What Figure 1 makes clear is that attempting to cover all of Europe using THAAD would require a prohibitive number of THAAD batteries, far more than the U.S. plans to buy.[1]  Current U.S. plans are to buy seven batteries, although there is a stated requirement for nine batteries.

On the other hand, the green shaded area in Figure 1 shows the footprints for Aegis Block IB interceptors deployed on four ships in the Mediterranean Sea (including the Adriatic Sea) and Black Sea.[2]  This slide, which assumes the launch of the interceptors is supported by external radars such TPY-2 X-band radars, shows that four ships can cover a significant fraction of Europe.

The much faster Aegis Block IIA interceptors scheduled to begin deployment in 2018, would allow all of Europe (except eastern Turkey, for reasons discussed below) to be defended from only two interceptor launch sites as shown in Figure 2 below.  Note that both Figures 1 and 2 predate the decision to deploy the European Phased Adaptive Approach (EPAA) system with its land-based Aegis Ashore sites.[3]  Figure 2 also assumes that the Aegis interceptors are supported by external radars.

AegisVsThaad2

Figure 2.  Slide from MDA Deputy Director Major General Patrick O’Reilly, “Missile Defense Program Update For The National Defense University “Road To Bucharest” Conference, February 20, 2008.  (link coming soon.)

Thus is it possible to attempt to defend almost all of Europe (the NATO portion of it, at least) from Iranian missiles using a few Aegis sites, but similar coverage cannot be achieved using any plausible number of THAAD batteries.  (The development of a faster, extended-range version of the THAAD interceptor could change this situation.  Such an extended-range version of the THAAD interceptor is under consideration as part of a THAAD follow-on concept development and risk reduction program that MDA initiated in FY 2016.)

On the other hand, South Korea is small enough that covering it with THAAD is feasible.

Each of the three THAAD batteries’ footprints in Figure 1 are roughly 430 km long (east-west in this geometry) by 340 km wide.  This is larger than the dimensions of mainland (leaving out the island of Cheju) South Korea – which is about 370 km north-south and 270 km east-west.  Thus it should be possible to cover South Korea with no more than one or two THAAD batteries.  This is consistent with defended footprints, reportedly provided by THAAD manufacturer Lockheed-Martin, published in a South Korean newspaper last month.  These footprints are shown in Figure 3 below.

AegisVsThaad3

Figure 3. Coverage of South Korea by one (left figure) or two THAAD batteries.   The coverage is said to be for missiles with ranges between 300 and 1,000 km. Image Source: “Simulation Shows How THAAD Would Defend S. Korea, The Chosunilbo (English Edition), June 15, 2015. Available at: http://english.chosun.com/site/data/html_dir/2015/03/25/2015032500896.html.

(The distinction between the green and blue coverage areas was not specified. One possibility is that green area is against missiles with ranges close to the lower limit of the 300-1,000 km attacking missile range band, while the larger blue area is for coverage against longer-range missiles for which a longer interceptor flyout time is available.)

(2) Due to the short distances involved, Aegis BMD cannot defend South Korea against short range ballistic missile attack. This problem arises because the Aegis SM-3 interceptor (all versions) can only intercept above the atmosphere (exo-atmospheric). While the precise lower altitude limit for the SM-3 interceptor is not publicly available, it is generally taken to be at about 100 km or higher.

Since no part of mainland South Korea is more than about 380 km from North Korea, most of South Korea can be targeted by 300 km Scud-class missiles and all of it can be targeted by 600 km extended-range Scuds. The Scud missiles never rise as high as the SM-3 interceptor’s lower altitude limit and thus cannot be engaged by Aegis BMD. An extended-range Scud might be high enough to be engagable over part of its trajectory, but the possibility of engaging such a missile can be reduced or eliminated by flying it on a lower-altitude depressed trajectory.

On the other hand, THAAD could attempt to intercept missiles with ranges as short as a Scud, since it is able to conduct intercepts in the upper layers of the atmospheres, as well as above it. The lower altitude limit of THAAD is also not publicly available, but is generally taken to be about 40 km, well below much of the trajectory of a Scud.

This capability to intercept at lower altitudes is the main technical reason that THAAD is more appropriate than Aegis BMD for defending South Korea against North Korean ballistic missiles. This is the same reason why Figure 1 includes THAAD batteries covering eastern Turkey, since eastern Turkey could be targeted by shorter-range Iranian missiles that Aegis SM-3 interceptors cannot operate low enough to engage.

GUAM

The decision to base THAAD on Guam, however, was not determined by these two technical issues. From a technical perspective, Guam could have been covered by either a THAAD battery or an Aegis Ashore facility. Thus other issues such as cost and availability likely determined this decision. In particular, the original deployment to Guam in 2013 was made in response to North Korean threats and subsequent questions about whether or not the GMD national missile defense system could cover Guam (It can’t). It was possible to deploy a THAAD battery to Guam within days, since a THAAD battery’s equipment is air-transportable and can be operational with four hours of arrival. Building an Aegis Ashore facility there would likely have taken a year or more, particularly if this was to be done without disrupting the schedule of the European Phased Adaptive Approach. An Aegis BMD-equipped ship could have been deployed there much more quickly than an Aegis Ashore, but the cost (both financial and in terms of the limited numbers of Aegis BMD ships) of maintaining an Aegis ship permanently off Guam would have been much greater than that of maintaining a THAAD battery there.

—————————————————————

[1] THAAD batteries deployed further from Iran could attempt to defend somewhat larger areas, particularly if supported by external radars, but such larger areas would not change the conclusion that the required number of batteries would much greater than the total the U.S plans to procure.

[2] The blue-shaded area shows the coverage of the now-cancelled plan to deploy two-stage versions of the U.S. national missile defense Ground-Based Interceptors in Poland.

[3] Although Figure 2 shows northern Scandinavia uncovered, this area would be covered by the planned Aegis Ashore site in Poland.

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8 Comments

  1. THOMAS BARTON JD

     /  July 29, 2015

    FINAL SCORECARD: EXCELLENT ANALYSIS, SOUND MILITARY DECISIONS 2, INTER-SERVICE RIVALRY 0. HIT THE SHOWERS AND ENJOY A COLD ONE.

    Reply
  2. Allen Thomson

     /  August 2, 2015

    Seguing from Aegis Ashore to Aegis Afloat, this bit is interesting if true. (I’ve not been able to find any confirmation of it as of now.)

    http://www.ibtimes.com/navy-cruisers-should-be-stripped-ballistic-missile-defense-us-admiral-says-2033743

    Navy Cruisers Should Be Stripped Of Ballistic Missile Defense, US Admiral Says
    By Eben Blake on July 31 2015 11:55 AM EDT

    Another high-profile naval officer is speaking out against having the U.S. Navy’s fleet of cruisers equipped with ballistic-missile defense capabilities. Adm. John Richardson, the current nominee for chief of naval operations, said he supported stripping Navy cruisers of the sea-based missile defense system, Politico reported.

    [snip]

    Reply
  3. THOMAS BARTON JD

     /  August 2, 2015

    I THINK I READ THAT HE OBJECTS TO THE COST AND TIME AND RANGE CONSTRAINTS THAT ARE PLACED ON THE AEGIS CRUISERS AND IT ROBS HIM OF FLEXIBILITY. PERHAPS OUR ESTEEMED WRITER MAY POST ON THIS CONUNDRUM.

    Reply
    • I haven’t followed Aegis closely recently. I think this is mostly about cost. It is odd that an argument can be made that increasing capability reduces flexibility, It may have to do with who makes the decisions about what roles a ship will play — the Navy or the regional commanders. I suspect the Navy would be happy not to retrofit any more ships with BMD capabilities, and just let the fleet’s BMD capability increase as two new construction BMD destroyers enter service each year.
      George

      Reply
  4. THOMAS BARTON JD

     /  August 4, 2015

    SO WE HAVE AEGIS ASHORE IN ROMANIA AND SOON POLAND. IS THERE A FACTION IN THE NAVY THAT WOULD LIKE TO SEE AN AEGIS ASHORE IN THE SOUTH CHINA SEA AREA ? THE STATE OF POLITICS IN JAPAN REGARDING DEFENSE EXPANSION AWAY FROM DECADES OF FIRMLY ENTRENCHED SELF-DEFENSE ALONE AT ALL COSTS SEEMS TO BE IN PLAY ALTHOUGH I THINK MR ABE IS OUTGUNNED 60 TO 40 PERCENT PARTLY BECAUSE HE IS A LOUSY POLITICAL ADVOCATE. ARE THERE CALLS INTERNALLY TO PUSH FOR AEGIS ASHORE TO PLACED IN OKINAWA ? IS IT AT ALL POLITICALLY FEASIBLE ? WOULD MULTIPLE THAAD SITES BE MORE PALATABLE AND YET BE EFFECTIVE AS A DETERRENT ? I MUST FIND THE TIME TO READ ALL YOUR THOUGHTFUL WORK AND I MUST READ MORE AT THE WEBSITE DIPLOMAT WHICH HAS AN EXTENSIVE ARRAY OF ASSETS ALIGNED TO THE SOUTH CHINA SEA AREA AND THE ASEAN REGION IN GENERAL. THANKS FOR YOUR THOUGHTFUL RESPONSES.

    Reply
    • There is at least some interest in deploying an Aegis Ashore system in Japan in both japan and the US, see articles such as these:

      http://news.usni.org/2014/09/16/report-japan-interested-aegis-ashore-ballistic-missile-defense

      and

      http://news.usni.org/2015/05/18/house-paves-the-way-for-japan-to-buy-aegis-ashore-adds-anti-air-warfare-to-european-sites

      I don’t know anything about Japanese politics, but given that AA is a defensive weapon and that Japan already operates four Aegis BMD ships I don’t see why it would be a big problem. It could relieve some pressure on the four (eventually to be eight) Japanese Aegis ships. US could see benefits from it as well, as it would relieve burden on our Aegis BMD ships homeported in Japan.

      Japan is well positioned for Aegis Ashore. It will be coproducing the SM-3 Block IIA missiles, so would want to wait for the AA version that can incorporate them, which will be available by 2018. AA with Block IIA needs to be supported by external radars, such as TPY-2 X-band radars, in order to fulfill its potential, and Japan already has two U.S. TPY-2s deployed that could provide this additional radar coverage.

      I don’t see why THAAD would be politically any more or less palatable than AA. For Okinawa, could cover either with AA or THAAD. However, might need additional radar if choice is AA (or possibly wait for an AMDR version of AA with more powerful radar which might be available in early to mid 2020s).

      AA has the advantage of being able to cover with a single site an area that would require multiple THAAD batteries. Also Japan is very familiar with Aegis BMD. THAAD has the advantage that it could provide a second, somewhat independent layer under existing or additional Aegis coverage. However, Patriot already provides an independent second layer, and Japan already has a substantial investment in Patriot (16 batteries , I think).

      Reply
      • THOMAS BARTON JD

         /  August 6, 2015

        YOUR ANALYSIS IS AS ALWAYS A MODEL OF EFFICIENCY. I ASSUME THAT YOU SEE NO LARGER PHENOMENON AT PLAY RIGHT NOW. I WAS A BIT STARTLED BY THE DIPLOMAT’S PORTRAYAL OF MCDEVITT’S REPORT. I DO NOT HAVE THE TIME TO FIND OR READ IT BUT IT WAS A SEA-CHANGE FOR MY PERCEPTION OF CHINESE CAPABILITY AND AMBITION. IF THEY WANT TO PLAY IN THE DEEP BLUE SEA OF THE VAST PACIFIC AND NOT JUST THUNDER AROUND IN THEIR NEIGHBORHOOD POND THEN SURELY THIS WILL HAVE RAMIFICATIONS FOR POLITICAL DISCUSSION THIS FALL AFTER THE SILLY SEASON OF SUMMER HAS FINALLY RUN ITS COURSE. IS THIS AN ALARMIST RESPONSE, IS THIS COMMON KNOWLEDGE TO THOSE OF YOUR ANALYTICAL BENT THAT IS JUST NOW REACHING THE REALM OF THE NEARLY EVERYDAY POLITICAL DISCOURSE. ALSO I AM PUZZLED AS TO WHY JAPAN IS SO EAGER TO HAVE BOTH AEGIS SHIPS AND THE LAYERING OF AA. I CONFESS THAT I THINK JAPAN IS BADLY ADRIFT IN ITS DEFENSE AMBITIONS VERSUS WHAT SEEMS LIKE FIERCE POLITICAL OPPOSITION. YOU ARE VERY KIND TO ALL YOUR READER INQUIRIES AND I HOPE YOU ENJOY THE SUNNY DAYS OF AUGUST. SUMMER IS SO BRIEF WHEN THE DECADES HAVE ROLLED PAST.

  5. THOMAS BARTON JD

     /  August 5, 2015

    I COMMEND TO YOUR ATTENTION AT ARTICLE AT THE DIPLOMAT DATED 30 JULY 2015 DETAILING THE FINDINGS OF REAR ADMIRAL MICHAEL MCDEVITT ON THE ASTONISHING PROJECTED EXPANSION OF THE CHINESE NAVY WITH REACH AND SCOPE FAR BEYOND THE LIMITS OF THE SOUTH CHINA SEA. PERHAPS THIS IS ALL PART OF A LARGER REASSESSMENT OF OUR TOTALITY OF RESPONSE IN THE REGION AND BEYOND.

    Reply

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