Flak on the Tender K4T30

2cm Flakvierling on the Tender K4T30

The Myth and How it is Easy to Show it Never Existed on BR 52s – An Analysis

Many scale models show flak on the tender of the infamous BR 52 locomotive. It looks great and adds some nice details as well as more depth to models. Modelers around the globe depict it this way and common vibes appear to legitimize this by the need for AA capabilities on trains to fight CAS and attack aircraft. While this might make sense, at first sight, a quick look into the matter shows why no photos nor documents for this solution exist. We will now discuss the recoil forces, tunnel clearance profiles, crew space and capabilities as well as center of gravity.

The result should not surprise.

Why would the DRG put flak guns on a Tender, especially the K4T30? The obvious answer is to save space and waste wagon volume. The more wagons (for AA) you safe, the more goods you can transport. If you mount an AA on the water tank, you free up wagon space, in theory. However, the construction of wartime tenders and coal cars minimized every gram of steel used and all parts were minimized to the absolute minimum thickness and strength to save weight.

This alone makes a heavy AA gun on a tender a thing of impossibility. Tenders were purpose-built, to keep water and coal inside. Add pressure and catastrophe happens, aka the can opener effect.

2cm Flakvierling 38

Most models show the 2cm Flakvierling 38 on top of the infamous K4T30 tender coal car. The K4T30 is a rigid frame tender with four fixed axles in its frame. It weighs 23.8tons, can hold a maximum of 8t of coal and 30t of water. It is 2.9m wide and the top of the water tank is 2900mm by 3624mm. The 2cm Flak 38 is 3624mm by 2336mm according to official tech dimensions. As such, the gun and barrels are longer than the available space, and additionally, much wider when the guns point sidewards.

This leads to huge problems as we will see later.

Thanks to the University of California, we have access to war department documents about the 2cm Flakvierling 38 here. The three images from the manual are from the source linked in the previous sentence.

General Gun Placement

Graphic 1 of the gun on the tender shows that the gun must be mounted backward onto the tender. The guns stick out too much, roughly 1m to the front at zero degrees elevation. If it is mounted with the two tripod spikes to the front, the guns would need to be elevated high above the coal bunker, which would make any tunnel passage impossible. The center of rotation of the guns is around 170mm below the top of the coal bunker. As such, the two spike side needs to face the rear, the one spike side to the front.

This placement blocks the water infill openings and renders the vehicle useless, but they could be modified, so it’s ok.

2cm Flakvierling on K4T30 Tender Side Profile
2cm Flakvierling 38 on K4T30 Tender – by ISA Models
Flak on K4T30 Tender
K4T30 tender schematics (source unknown, would love to find it again and add it and contact the owner)

The Poor Crew

Ammunition storage and resupply would suck, as the constant fear of falling down or the gun throwing you down while turning would make this job one of the most hated. How would the crew get to the tender in the first place, where would they store extra ammunition other than on the tender roof (suicidal), where could they sleep? Gregory D. Heuer also pointed this out in recent discussions, so I add his name here as I place his input in high regard. But I must stress that it is possible to put this thing up there. It could rotate freely, spit out brass, smoke and sound and elevate up and down without problems. Enter the tunnel!

Tunnel Standardization and Clearance Profiles

Tunnels, as well as most railway-related infrastructure, show a high level of standardization nowadays. However, the creation of common standards and unification of railway systems started as early as 1890. However, standardization remained unique for infrastructure only till the 1920s. Then, during the following 2 decades, the LONA [Lokomotivbau Normen Ausschuss], RZA [Reichsbahn Zentralamt] and many other entities worked under ever-increasing pressure to standardize everything from boilers to nails, rails tunnels and so on.

Lichtraumprofil Flak on the Tender
Tunnel clearance profile standard from Reichsgesetzblatt 1892 Source: Wikipedia

Resultingly, we can analyze a typical tunnel of the era and see what it would do to any 2cm Flakvierling and crew positioned on a tender. You guessed it. When not placed and turned correctly, a tunnel would knock the flak right off any many ppl would die from concrete and stone attacks as a result. See the K4T30 example with the 2cm Flakvierling.

Flak on K4T30 Tender Tunnel
Flakvierling on K4T30 tunnel clearance profiles – by ISA Models

The clearance profile of a tunnel of the period is 4m wide at the base, 3.4m wide at a height of 3.05m and had a maximum height of 4.8m in the center only. As a result, a 2cm Flakvierling pointed to the side or up would not clear any tunnel or other common things, like bridges cables and so on. It would simply be too high or wide. The above schematics show this clearly. All are based on technical drawings and thorough research.

To avoid collisions, the Flak crew would need special spotters to tell them to stop firing and retract the gun, to avoid collisions, quite often. So, while it is still possible to have a Flak on top of a tender, its clearance profile vastly limits its use and operation. But it gets worse.

Center of Gravity Analysis with Flak on the Tender

We have already established some minor issues with the gun placement on top of the tender. To understand the following sections, a detailed look at the fact sheets of Wilhelm Hermes Stahlbau, a K4T30 tender manufacturer in the Cologne area, will help. Weights of the wagon itself and its load are the key here.

K4T30 Description
K4T30 description and manufacturing details – Source sadly unknown

I have plugged most weight information into the graph below. It shows the tender and the flak. The fact sheet of the 2cm Flakvierling 38 lists 1517kg with full ammo load. Adding the crew and the spotters and additional ammo would leave it above 2100kg or more. It is quite suicidal to add 2.1 tons of weight on top of a structure that only supports itself. Let us assume for now that we added additional 6 tons of steel for reinforcement of the casemate. (This is not possible due to the load on the axles, wheels, frame and tracks below, but let’s ignore it for now).

Flak on tender center of gravity
Center of gravity analysis for 2cm Flakvierling 38 on K4T30 – by ISA Models

Such massive weight high up, raises the center of gravity significantly, as shown by the green lines and yellow crosses. The last column shows a flak on a tender without a full load of coal and water during a right turn. Moving water already pushes the car to the side but due to the gun on top, the leverage and rotation point is way out of balance now. The car might derail or fall over. Going through a turn at 5km/h might solve this problem, but speed is of the essence in logistics, especially during the war. The flak on the tender is a myth and impossible already now, but we have more.

Recoil Forces for Flak on the Tender

Recoil forces increase with the caliber, velocity and power usage of the gun. A single 2cm bullet of the Flakvierling produces a recoil on the structure of 1,000kg. Fire a burst of 4 shots and this is 4 tons of recoil in half a second. The gun recoil system absorbs some of that force, but the tender roof would also bear a huge load of the recoil. The sides of the tender would, most likely, split open like a can from a can opener.

If angled sideways, the 90degree angle of the roof and sidewall bear this pressure. They could never withstand this force as their design only calls for water and coal transport, not a fighting platform. Do this during a turn, and you would also knock over the vehicle during its own explosion.

Recoil Flak 38 Vierling on K4T30
Recoil analysis for 2cm Flak 38 Vierling on K4T30

The green and red arrows show simplified directional forces and their results. Firing a gun towards the interior of the turning circle lifts its inner side. This exacerbates the effect of the already worsened center of gravity. As a result, you can not shoot the flak

  • Before or in a tunnel
  • Towards the turning circle
  • To the side
  • Before passing bridges
  • Around cables and wires
  • Actually never, as mounting it is already impossible, due to weight limits.

The Flak on the Tender Myth

I stood on top of the K4T30, and the idea of a gun on it is just off. It looks nice on display but does not work in practice. There are photos of all types of wagons and modifications of railroad vehicles. Ultimately there are no photos of guns on tenders, simply because they never existed. Furthermore, the LONA and RZA archives in Berlin show no such discussions. Furthermore, the detailed reports of field modifications of armored trains and logistics trains in the East in the OKW books, do not mention anything alike, either. Feel free to check them all here and in the German national archives invenio.

The guys at Schöneweide also state its impossible and they work with it on a daily basis. Additionally, my grandpa’s memoirs of a flak commander protecting railroad yards and oil fields never mentioned them. He commanded an AA battery, also in Cologne, and would have mentioned such a weird sight. He mentioned every AA train in all his logs as they usually drew more CAS and attack aircraft to his position. As a result, he disliked AA on trains.

If you want to know which details the OKH files show and why Flakguns on tenders would be present, look no further than here. For the joy of modelling, it makes sense to add the flak on top of the tender. But this is pure fiction, albeit beautiful fiction. Keep on modelling and enjoy the hobby. This is not do discourage anyone from adding them, but rather an informative and educational article for enthusiasts and knowledge-seeking modellers. A BR 52 with flak on the tender is a lovely myth.

Alex Emde – ISA Models


Were 2cm Flakvierling 38 mounted on a tender?

No, never and it’s also not possible. Enjoy the structured analysis of this article to find out why.

Were other guns used on tenders?

Big guns certainly not. A light mg on a tripod maybe.

Why do many models have a flak on the tender?

It looks good and adds some more detail to a model. Its great but historically absolutely incorrect.