The downside of haveing only one system is that you can end up granting a monopoly to one company and end up paying more money for lesser products in the end. Multipule companys that can both do the work sends clear message to companys that if they screw up are just dont put enough effort into project that they can be replaced.

ESSM was never and is never going to be a navalized AMRAAM: The ESSM RIM-162 (Evolved, or Enhanced, SeaSparrow Missile) is a development/follow-on of the RIM-7 SeaSparrow family which has been in service since before AMRAAM was even conceived. And the 8inch diameter body of RIM/AIM-7 affords greater growth potential and usable volume (range + warhead) over the 7inch diameter of the AIM-120 series. Besides, ships aren't as limited by orcnance weight as aircraft (a key point in going to a smaller, lighter system when AMRAAM was considered to supercede Sparrow, which never really lived up to expectations anyway. That's the reason the UK developed the Skyflash "copy", and Italy developed the Aspide. They both look identical to Sparrow, but the innards are considerably better.) I believe that Norway was developing a navalized AMRAAM for lightweight air defense, but I'll have to look for it. And also, the ESSM current body planform vaguely resembles the "40 year old" Tartar airframe, just as the current Standard SM-2 series (the ill-informed may think ESSM is a scaled-down SM-2) The airframe isn't obsolete, but is an evolutionary design of several decades of US missile research. And ESSM uses thrust-vector control to augment its maneuverability. http://www.raytheon.com/products/essm/ http://www.designation-systems.net/dusrm/m-162.html And considering how much Patriot PAC-2/3 tech went into the Standard SM-3 (or maybe the other way around...), it is interesting to note that both systems are still being procured. Of course, Raytheon owns the US missile monopoly, so anything to keep their bank rolls in the black keeps them happy... even if most of these systems overlap each other considerably (which technically, a truly effective ADA network, land or sea, should be capable of.).

A couple of points.... 1) The SM-2 is getting rather long in the tooth. The airframe aside, it lacks a few key features that define the current generation of medium weight ship-to-air missile -- an active terminal seeker, track-via-missile 2-way data link and aerodynamically independent lateral jet attitude control (ala Aster, PAC-3). The AEGIS system has always been one with a great radar (for its time anyway and a so-so missile). 2) The AMRAAM is not just a reduced form factor sparrow. Other than aerodynamic differences, it is an active missile. ESSM can benefit from an active seeker which will relieve the missile directors of a lot of work and allow a much higher salvo rate against sea skimmers. 3) The SM-3 is a 3 stage rocket that basically uses the SM-2 AEGIS ER booster and missile to deploy a terminal kill vehicle payload. The THAAD missile is designed from the ground up as a 2-stage kinetic energy exo-atmospheric interceptor and it fits into a Mk41 cell with room to spare. Not considering a navalized version of the missile seems at the very least wasteful.

I don't intend to imply that I think AMRRAM is a "reduced form factor Sparrow". AMRAAM airframe/weight envelope was suggested/preferred over a product-improved Sparrow because of the necessity of weight constraints and larger numbers to be carried by (US) aircraft. The F-16 can and does carry AIM-120s on its wingtip rails, something it can't do with the 500+ lb Sparrow series. Plus, it can deploy a twin-rail underwing system for AIM-120, where it can only carry a Sparrow singly (typically, and F-16 could carry 4 Sparrow and 2 Sidewinder, or 8-10 AMRAAMs). And fortunately, all US and NATO aircraft who carry AIM-7 semi-conformally can also fit the AIM-120 (F/A-18, F-15, F-14, Tornado). There is still plenty of "room to grow" in both the ESSM and Standard SM/ER series missiles. The surface-to-air systems just have not been as upgrade-favored because we have foolish-thinking politicians and brass who may be implying our naval air assets will be effective enough, so our heavyweight naval SAMs have not gotten the top-of-the-line upgrade packages that could keep them at the top of the capabilities list (although the latest-produced SM-2s are far more superior than the first generations). I do agree that we do need a navalized missile that is on par with the suggested performances of the S-400 system, if not better (more capable against all-altitude targets, including ballistic and sea-skimming capabilities...and it shouldn't need to be the seriously expensive SM-3 variant, which would not be used against very-low-level threats...hence the need for ESSM). The 13.5inch diameter main body of Standard is certainly sufficient to pack in the necessary technology. http://www.raytheon.com/products/ There are sufficient sites available with unbiased capabilities statements to both the ESSM and latest Standard missile systems. Agrreably, both would be far superior with further upgrades. But a naval AMRAAM does not offer the growth potential of the ESSM body, even if it has such a promising-performing seeker/guidance section (no reason it couldn't be retorfitted into the ESSM at some future date). Besides, ESSM's primary requirement was very low level/anti-sea skimmer operation (which again, the SM-3 won't be used for); AMRAAM was designed for more low-to-medium altitude work and shoot-down modes (not becoming its own sea-skimming anti-missile missile like ESSM can do)..

You never know what countermeasures the enemy will come up with, so it makes sense not to depend on a single system, as any one system may prove very vulnerable to a particular unforseen countermeasure.

Funny thing... They are actually planning a Standard with the AMRAAM seeker and the AIM-9X/ASRAAM IR seeker. Tentatively called the SM-6 though that is weird given that SM-4 and SM-5 are yet unassigned. In essence it'll do what the SM-2MR Block IIIB does at greater ranges, over land or under the radar horizon. The active radar and IR seekers allow for speculative launches at targets that cannot be illuminated by the SPG-62 directors -- such as approaching sea skimmers that are under the horizon. The long ranged ones normally fly a hi-lo trajectory where they fly at high altitude for range performance but pop down to wave height for the last 100km or so to avoid interception. Normally airbourne or ship bourne sensors can pick them up after launch, but they pop down off the screens before weapons and be employed against them. So rather than waiting for them to break radar horizon at 40km, salvos of "SM-6" missiles can be hurled in their direction. The missiles will not only seek and destroy the incoming missiles if they find them, but also provide radar info on what is over the horizon by relaying basic info from their active seekers back to the ship via a low-bandwidth, SW, 2-way radio link.

There is no real redundancy. SM-3 has three stage and the kill vehicle has significantly higher speed than THAAD. PAC-3 is not even in the same category for atbm purpose. In fact a larger diamter block 1 and bloack 2 SM-3 will be build to increase the speed further In order to intercept longer range , higher reentry speed ballistic missile at largert standoff range and higher attitude, you need BIGGER missile with better kinematic. SM-3 block 1A is 3309 pound. A Thaad is only 1200 pound. The kill vehicle kinematic performance is determind by speed and its lateral maneuvrability. A high speed heavy kill vehicle has more fuel to maneuvre and more electronic to discriminate target. SM-3 block 3 may use a 27 inch motor and will be a far heavier missile that cannot even fit into the current mk41 launcher. Probaly the navy will only build the block2 missile which has a full body 533 mm diamter airframe. ESSM has almost nothing to do with sparrow in terms of airframe despite the name. The new missile has a motor that is one inch larger in diameter and ur weighs 600 plus to 700 pound. Its 9inch motor with a 8 inch seeker missile. This missile use better airframe, higher energy propellant and modern electronic. almost nothing in common with sparrow except maybe the seeker. This missile could easily use a AMRAAM active seeker or some new AESA radar or huge diameter IR seeker (scale up version of aim-9x). AMRAAM when launch from ship when have a range of 18km. Maybe agianst slow flying incoming plane, it could intercept the target at 40km. ESSM could destroy fat agile target at 50 plus km. There is no way a tiny AMRAAM could have that kind of kinematic performance. If cost is not a issue, ESSM could be fitted with AMRAAM seeker without much hassle PAC-3 can intercept both short range ballistic missile and also AIRCRAFT. Its function is different from SM-3 which is used only for high attitude interception of missile. The kill vehicle in THAAD and SM-3 cannot be used in low attitude and is not design to kill aircraft although it some special situation, you can try using it to hit a very high flying MIG-31. It may work but its not design for this task. What US need is an equivalent of the S-400 to kill aircraft at 400km range. SM-6 fits the bill. US Army should procure some SM-6 to enable it to destroy enemy aircraft at very long range. For landbase application, SM-6 could have a lengthened booster. The current mk-41 does not permit SM-6 to e longer than 6.55 meter. A 5 meter booster could extend SM-6 max range to 600-700km and NO ESCAPE ZONE to 150km against all known aircraft (excluding SR-71). MK-41 cell is very small compare to the tube that house SA-12B giant missile. Thats the tyranny of dimension and constraint. Using modern technology, US could easily creat a 700km missile that weigh no more than 2.5 ton. Still much lighter than SA-12b which is rated at between 3300kg to 4500kg

Just a suggestion.  People will get brassed off if you ask questions that basic research could provide. 

I just skimmed this thread from the dead. The basic idiocy contained herein brasses me off. (Not you stinger.)