The XM246 was the vehicle that lost to the XM247, the vehicle that became the M247 Sergeant York in the DIVAD (Division Air Defense) program of the late 70ties early 80ties. Infamously known as the “good” DIVAD that hit more targets and unfairly lost to the “comically bad” Sergeant York, it is otherwise largely unknown and its info is sparse but we can infer a lot of the relevant specs from the information and requirements known about the DIVAD program.
TLDR
It’s an self-propelled anti-air system similar in performance to the real M247 but with the guns of the Gepard, controlled by the radar from the Phalanx CIWS, protected by the armor of the Bradley with the sight from the XM800, build by General Dynamics.
History:
Like the Sgt. York its development story starts as General Dynamics(GD) entry in the 1972 GLAADS program which Ford won with its dual 25mm gun. GD’s concept featured a triple 27mm Mauser gun installation instead. This triple gun configuration concept was further developed for Army Radar Gun Air Defense system (ARGADS, initial DIVAD program) in 1977. Upgrading to a 30mm Mauser and adding a radar derived from GD’s Phalanx CIWS system. The initial concept used the chassis from the XM723 MICV(Mechanized Infantry Combat Vehicle) the predecessor to the Bradly Fighting Vehicle and looked like this:
Ford, GD, General Electric, Sperry-Rand and Hughes competed for ARGADS with Ford and GD being selected as the contestants. The ARGADS/DIVAD concept was a Anti-Air/Anti Anti-Tank Missile vehicle a modern day M42 buster capable of both suppressing planes, helicopters and vehicles with ATGMS. While the initial requirement had been 3-4km for DIVAD 4km was now the minimum requirement. Hughes had licensed the Gepard Turret and Oerlikon 35mm KDA for the competition, but as the Gepard didn’t fulfill the requirements chose instead to make a deal with GD for the 35mm which had the range required for DIVAD. Another requirement was a M48 chassis. The winning GD concept looked like this:
Changes in the radar design and incorporation of the MICV spaced laminate armor resulted in minor design changes.
Two Prototypes were built for the competition, they rolled of the production line in early 1980:
The gun system did well in the 1980 shoot off against the XM247 hitting more targets directly while firing HEI, but also had some considerable issues due to its prototype state:
Description:
One of the requirements for the DIVAD program was to use “mature” components to build the gun systems. Here is a breakdown of the major components:
Mobility:
The mobility is going to be similar but with marginally better acceleration than the M247 because the vehicle is somewhat lighter, I have not been able to find the precise weight of the XM246, but its known to be less than 60 short tons which was the max weight allowed and it was not an issue, as seen above.
Max speed is going to the same as M247 48km/h forwards 11km/h backwards.
Protection:
DIVAD had the same protection requirements as the MICV and Bradley IFV. Those where all around Protection against 14.5mm AP from 300m. The turret, built by FMC the maker of the Bradley, features the same armor as the Bradley.
Sides are a space laminate armor consisting of 0.26" HHA space by 1" to 0.21" HHA spaced by 3.5" air to 1" of 7039 Al. Top is 1.5" 5083 Al.
Armor Details
Its also equipped with smoke launchers and ESS like all M48’s.
Firepower:
Gun:
The same Oerlikon 35mm KDA L90 gun as used on the Gepard, It also had a similar magazine size but instead of being in the turret basket, the ammo was stored in the side of the turret in two chambers on the outside of the crew compartment
Its magazines held ~600 rounds, about ~250 in each of its two main magazines on the side of the turret and likely about 2x~50 in the gun shield/rotor housing.
Ammo:
There were 3 types of live ammo, HEI-PD(High Explosive Incendiary - Point Detonating), HEI-PX(High Explosive Incendiary - Proximity fuze) and an “armor-piercing” round.
It used belts of interspersed HEI and armor-piercing rounds. Based on M247 magazines would be 66% or 75% HEI to AP:
In game it would be full HEI, HEI + AP , HEI-PX + APHEI-PD round:
The USA adapted the Swiss 35mm HEI but modified it, instead of having a normal point detonating round, it was a delayed point detonating fuze, the difference being that instead of exploding next to the skin of the aircraft the round would travel about 9 inches before exploding, thus delivering its whole charge inside the body instead of outside it like normal point detonating. This 0.03 milliseconds delay gave the projectile a second use, it was now also semi armoring piercing.
The round was produced by Honeywell the delayed detonating fuze was designated XM760 a direct descendent of the XM714 fuze program.
Source
Explosive Mass, 112 g
Total weight 550g
Max flight time is likely the same as M811 as fuzes was developed together so 8.5 ± 2 secondsHEI-PX:
Same round with a proximity fu ze called the Motorola Proximity Fuze(MPF).
Development of the MPF started in 1974, being designed as a drop in replacement for the normal point detonating fuze. It was different than the M822 for the M247 in that it was “Adaptive” meaning instead of just exploding when within a certain rang it would take the targets flight vector into account to deliver the best fragmentation on the target. This was needed because of the comparatively light shell could not deliver nearly as much fragmentation as the 40mm. Flight time is not publicly available AFAIK but it’s lead-acid battery is considerably smaller than that in the M822 which had up to 21 seconds of flight time. So it is probably similar to the regular round. Trigger radius appears to be at least 7m from public photo(see below)
The Armor Piercing round?
Initially it doesn’t seem like a AP round was planned, the midt 1978 report to the US congress only mentions HEI-PD for the 35mm.
But to defeat IFV’s it was required for the belts to feed a mixture of HEI and AP, but what AP round was it actually?
Was it APDS-T, SAPHEI-T or AP-HEI? Both Hunnicutt and Jane’s reference the APDS-T as a round the gun fires and as seen above Oerlikon advertised two AP rounds with the DIVAD.
APDS-T however has a much higher muzzle-velocity, which means either all the HEI or the AP rounds
would miss if fired together and both uses Tracers which is undesirable and was avoided in the Ford proposal. It could also instead have been the earlier version of the SAPHEI-T, the AP-HEI however OERLIKON stopped advertising at the same time it was selected for the program in 1977. Both of the SAPHEI-T and the AP-HEI had identical ballistics to the HEI round they just only had 22g of explosive and could Pen up to 55mm steel, more than the 40mm M811. So they fit the bill. Another option is that plan was to have a setup like the Gepard with the small magazines carrying APDS-T and the big ones carrying HEI. In the only clip of it firing, we don’t see any tracers:
Fire Control:
The XM246 featured both optical and radar tracking like the M247.
Optical:
The optical system was a further evolution of the sight Delco made for the XM800T scout vehicle and the stabalization system was built upon the GM XM1. This means that it was fully stabilized but that it was slaved to the guns.
Radar:
The GD DIVAD Radar is the least publicly available feature of the system. But we can estimate its capabilities from is origin and DIVAD requirements. Its radar system was a derivative of the US Navy’s MK 15 Phalanx CIWS radar, operating in the same J or Ku band, it also featured a close loop control system.
The Phalanx which at the time had just entered production had a radar system based on the AN/VPS-2, same radar as found on the M163 Vulcan, in the Phalanx the radar operated in the J band and featured dedicated antennas. This was needed to allow it to detect its own 20mm ammunition and enable it to operate in a “closed loop” system. A Close-Loop fire control system automatically adjust its aim if it detected that the rounds where not going where intended, this allows the system to negate aiming errors from a wandering zero, barrel drooping etc. The original AN/VPS-2, however, did not fulfill the requirements set out for DIVAD, it only had a max range of 5km while the DIVAD radar range requirement was 10-15km for planes and 7-10 for helicopters. Time to start engaging was 6 seconds max.
Source
This meant so a new search radar antenna had to be used. The best proxy for this new search radar is the one incorporated into the Phalanx Block 1 B/L 0 upgrade program from 1988 which increased search elevation from +5 to +70. Described as a 4 plate back to back radar However like the Sgt York it likely featured full hemisphere coverage so it could track aircraft flying directly above it.
The search radar rotates at 90 RPM roughly twice as fast as the search radar on the M247.Source
The CIWS Tracking radar has an interesting Antenna, it is very tall to create a narrow beam, in an effort to work around multipathing, the antenna is too large however to fit in the ~40" radar dome mounted on the right side of the turret, so another antenna was used likely a round one roughly similar to the SGT Yorks one which is 36" in diameter but a little smaller to allow it to move in the 40" dome.
