finally after three weeks a new hardware topic :-)
Multiple rocket launchers (MRL) became popular in WW2 when Russia, Germany and later the U.S. introduced such systems in significant quantities. Their accuracy and range was poor, but the ability to fire many warheads at one target in a few seconds had a good impact if the enemy troops were inexperienced.
The same effect was available with well-coordinated fire of several artillery batteries, but that did put a much higher demand on artillery fire control.
Multiple rocket launchers were quite neglected in the West (NATO) during the 50's to 70's, with several R&D projects popping up during the 70's. This did finally lead to the NATO standard MLRS (in Germany: MARS) system of two 227mm rocket sixpacks on a self-propelled launcher vehicle.
The most important ammunitions were a 227mm rocket with bomblets and a 227mm rocket with anti-tank mines. The large ATACMS missiles aren't really MRL ammunition; the MRLS is rather used in a second role as a tactical short-range ballistic missile (~SRBM) equivalent launcher.
The Russians never neglected MRL tech, but they still didn't standardize on a long-range model as for example MLRS, still using many quite short-ranged 122mm MRL.
Several developing countries had their own MRL projects (noteworthy: Brazil and South Africa) and many short-ranged MRL of small calibres (70-110mm) were developed during the Cold War as well.
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Time went by, and the MLRS concept was also used on a lighter, cheaper truck chassis with only one sixpack at a time.
The MRLS ammunition of the 80's became quite useless in the meantime. Many user countries agreed to the DPICM (bomblet) ban that effectively outlawed the bomblet rockets. The anti-tank mine rockets are a quite questionable concept and today much less useful than during the 80's. Their utility is zero in small wars.
One MRLS ammunition that's a real hit is the GMLRS (guided MLRS) rocket - in the XM31 GUMLRS 85 km version (unitary HE warhead, no bomblets). That's about the only MLRS rocket of good value and versatility for the Heer (German army).
The introduction of GUMLRS was planned for this year and the introduction of GMLRS with SmArt submunitions was planned for next year.
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The 227 mm calibre is a poor choice anyway. It was an efficient calibre for the delivery of submunitions, but it's awfully inefficient for fragmentation effects. It's powerful - and inefficient. The fragmentation efficiency in terms of lethally covered area per weight is best with much smaller warheads - that was the point of submunitions in the first place.
The tube artillery uses the 155mm standard calibre, with very few 105mm gun still in service (mostly as a kind of infantry gun).
It would therefore make sense to standardize on 155mm - a calibre that allows for a good range, standardization of warheads and legal submunitions (as SmArt 155), decent fragmentation efficiency and adequate high explosive (object demolition) power.
The Israelis thought of this decades ago and turned during the development towards 160mm and achieved similar ranges as top 155mm gun systems and some export successes.
An even smarter move is to develop a MRL that accepts multiple calibres to match the situation (MRLS accepts 227mm and the much bigger ATACMS, but doesn't exploit the idea in the lower range except in Germany with old 110mm rockets). The Romanians did this with a combination of the Soviet 122m calibre and the Israeli 160mm calibre. The Russians are also working on such a MRL as far as I know - there are several such projects world-wide.
Finally, we could include good passive protection, good off-road agility and the ability to fire very heavy very short range rockets for very large blasts - that would be very handy against minefields and fortified opposition.
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The raison d'être of MRL in competition with tube artillery was always the low cost of the launcher and the good salvo capability. MRL are capable of ripple fire with up to 40 rockets in 20 seconds (BM-21) - compare that to up to 12 shots in 60 seconds by 155mm SPH!
This short + rapid fire lost relevance since we're expecting more artillery strikes on small high-value targets than on troop & tank concentrations nowadays. The expectations may swing back to more conventional war ideas, but a return to 80's expectations is unlikely.
The new raison d'être of MRL is very different (aside from the "we're already in the inventory" argument): It's much easier to develop and produce guided rockets than guided shells. The shells' spin and huge acceleration shock during firing are serious problems that rockets avoid. Fin-stabilized rockets need have a length:calibre ratio of seven or higher, while spin-stabilized shells (and rockets) should not be longer than ratio six.
This enables much longer & heavier rockets than shells for the same calibre - and subsequently offers much more volume for electronics and much greater payload at the same calibre. The more gentle acceleration of a rocket also allows for a thinner casing, which adds even more volume and saves weight.
New MRL systems should therefore be understood as very heavy indirect fire ammunition AND as guided indirect fire ammunition systems. The old salvo and area effect image has to go away.
Mission reports from Afghanistan already cheer the GUMLRS as 70+ km sniper missile for its ability to take out a tiny enemy positions at very long range with a single shot.
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An optimal concept would in my opinion look like this:
* A medium truck-based MRL with a range of munitions of 105, 160 and 227 mm size with emphasis on 160mm (plus ATACMS). This would be part of independent artillery units under corps HQ control.
* A tank-based MRL with the same range of munitions and emphasis on short-range 227 mm thermobaric munitions. This would be part of heavy brigades.
* A simple and extremely inexpensive trailer MRL with the same range of munitions. This would be a rather rare system for expert infantry formations (mountain, airborne...).
Many if not most would disagree with this idea of an optimal MRL system family, but one thing should be obvious by now:
The roles and capabilities of MRLs changed a lot since the 80's and the submunitions ban has a huge effect on MRLs, it turns a huge part of the ammunition inventory obsolescent/illegal.
MLRS is an outdated concept. The environment for MRL use has changed very much and the MRLS concept is nowhere near the optimum.
edit: I forgot to mention that the truck-mounted MRL could be disguised as a normal military truck, thereby improving its survivability a bit.