90mm or even 75-76 seem like sweet spots to me.

The AAI rdf LT was essentially the precursor to the M8 that was for a marine program iirc and had an ares 75 or 90mm CT autocannon originally slated to fire at 120 rpm and an ability to fire at +80 degree elscao in a turret bookended by two 4 packs of either Hydras or rbs 70 missiles.

It was meant to be air transportable and have an adiabatic diesel engine which was later swapped out for a turbine because the diesel never materialized.

The turret was fully unmanned in some versions.

I feel like with the threats we currently face, and which will only get worse from here forward, a relatively straightforward modernization of this basic platform could serve us quite well.

I imagine a modernized version would likely weigh in at more like 30 tons, have a crew of 3, and be tied into the new American battle management system pretty heavily while also sporting something like the rada radars we're seeing installed on other platforms.

You could also probably do a modern version on Bradley components etc even.

roguetechie said:

AAI rdf LT

Having now looked it up, it does look rather interesting. And that would be a ood starting chassis too.

graylion said:

As regards 105 being too big, what about 90mm? What I find compelling is the assortment of ammo and the indirect fire option, where would not want much smaller than 105mm

105 mm is not too big per se. It depends on what you want to shoot, what effect you want and how much ammo you want/need to carry.

The ammo for the 105 mm L7 is pretty big. Because the gun is designed to get high KE for armor penetration. For firesupport this is unnecessary. Unfortunately nowadays the armor piercing capability of the 105 is limited. So chances are high you get a gun and ammo combo that is to big for what you actually want and need.

With most 90 mm guns its more or less the same. Allthough there are lightweigh 90 mm guns that are not optimised for KE around. The HE shells of these guns are pretty powerfull.

Traditionally the sweet spot for direct fire support purposes ist 75 mm/3 inch. The shells are powerfull enough to deal with pretty much every kind of field fortification and have a good area of effect.
 

roguetechie said:

The AAI rdf LT was essentially the precursor to the M8 that was for a marine program iirc and had an ares 75 or 90mm CT autocannon originally slated to fire at 120 rpm and an ability to fire at +80 degree elscao in a turret bookended by two 4 packs of either Hydras or rbs 70 missiles.

If I recall correct the last version carried two pods of four Stinger SAMs each.
70 mm rockts IMHO make little sense for a 75 or 90 mm main gun. The rockets add nothing the gun can not do.

graylion said:

it does look rather interesting.

Yes it does.

graylion said:

And that would be a ood starting chassis too.

Keep in mind the vehicle and concept are more than 30 years old now. At best it can give very basic inspiration.

Part of the RDF LT concept was to have very thin armor. The vehicle was supposed to move all the time and be hard to hit. This way it should not need armor.
The rapid fire 75 mm was directly linked to this concept. It should fire 3 round bursts that would achieve the same accuracy as a single shot 105 mm (then standard MBT armament) fired from a standing tank only from the fast moving LT.

These concepts are rather... bold. Allready back then. Nowadays they are unsustainable. Especially with modern FCS the idea of being hard to hit because of high speeds is outright rediculous. The concept to saturate an area of opperation with lots of light tanks so losses can be sustained while the LTs punch above their weight class also is not sustainable. Survivability trumps everything nowadays.

While the 75 mm ARES gun has comparale penetration with APFSDS of its time to the 105 L7 with APs its no where near the level of power required today for AT work.

The concept would have to be reworked conciderably.

graylion said:

What I like about the 3105 is the maximum angle of 42° which gives it rudimentary indirect fire capability.

I don't think that is a good idea.
To be effective at indirect fire it takes a lot more than just being able to raise the gun up.
A high velocity piece also if pretty bad at most indirect fire missions.

For indirect fire a purpose build vehicle is the better choice IMHO. Direct fire capability is secondary in this case.
Somthing like this is an intresting concept:

schnuersi said:

With most 90 mm guns its more or less the same. Allthough there are lightweigh 90 mm guns that are not optimised for KE around. The HE shells of these guns are pretty powerfull. Traditionally the sweet spot for direct fire support purposes ist 75 mm/3 inch. The shells are powerfull enough to deal with pretty much every kind of field fortification and have a good area of effect.

again with the indirect fire option? or what about Cockerill 90 LP?

graylion said:

again with the indirect fire option? or what about Cockerill 90 LP?

In theory any gun that can be raised enough can be fired indirect. For this the gun elevation doesn't have to be high.

The problem is with a single fixed charge only one trajectory is possible (two if the gun can be raised above 45° but most of these will be of limited use for a medium velocity piece). Which limits the possible beaten zone. The Cockerill 90LP fires its HE shell at 700 m/s. So its comparable in its trajectory to WW2 era 90 mm guns. These are usually a little faster but the difference is not that big.

To really be indirect fire capable a gun needs an elevation of 45° and more plus adjustable charges. But this directly interferes with the usefullness in the direct fire role. Its also more complicated, requires different FCSs, need a link into the artillery communication network and different crew training.
For a vehicle that is intended for direct fire this would result in conciderable more expensive system. While most of the investment is seldom used. There also is the conflict of mission problem. If the vehicle is linked into the arty network it will get fire missions all the time. Which will interfere with the crew observing the surounding area and look for targets as well as keep a close linke to the elements its supposed to give direct support to.

cool, thanks. About 105mm: Challenger reputedly has a rifled gun b/c of HESH. So the UK clearly sees/saw a necessity for this in infantry support. Challenger 3 seems to be pivoting to anti tank. Reversely, I don't see the point of an anti tank gun for an FSV. France OTOP moved from 90mm LP to 40mm CTA for the role.

Thinking about the Boxer and its famed modularity. Why is the RCH still so incredibly top heavy high? It looks positively absurd.

graylion said:

So I am wondering whether a FSV turret would not be better off with a 105mm gun.

Well the US 'Mobile Protected Firepower' Light Tank is reportedly going to have a 105mm cannon:

https://www.defensenews.com/land/2021/10/11/ausa-us-armys-light-tank-competition-enters-final-stretch/

Looking at 105mm shells, warhead weight and HE fill look pretty comparable to the 70mm rocket M229 '17 pounder' which has 17lb warhead with 4.8lb HE fill. 

To me, if the goal is basically 'vehicle lighter then a tank that provides multi shot direct fire ability with tank like destruction' a 'Mini MLRS' firing 70mm makes a lot more sense.

A big gun requires a big heavy vehicle, as well as cost and complexity. Its also a harder system to make Unmanned - necessitating armor to protect the human gun operators.

Rockets solve that issue. They're light and recoilless, and fired by simple electrical impulse. 

The result is one can have a Jeep sized unmanned vehicle capable of delivering 14-24 shots of laser guided 70mm, ~equivalent to the same number of 105mm shells from a light tank.

Something like this is what I'd like to see as a Fire Support Vehicle.