Installation of the GM-1 System in the Bf 109

Text: Pavel Raška

It is widely known that the GM-1 was a system devised to increase engine power at high altitude by injecting liquid nitrous oxide into the engine's intake compressor. Here, I will explain how the system was installed in the Bf 109 and the principle on which it was based.

Nitrous oxide, commonly referred to as laughing gas, can be stored in liquid state in two ways:

1. at room temperature in special bottles under a pressure of 80 bar or about 1160psi.

2. at normal atmospheric pressure at a temperature of -90°C.

Both of these methods were used in the following versions of the Bf 109:

 1. Bf 109 E-7/Z. The first ever GM-1 installation in the Bf 109 employed the first method using three pressure cylinders installed in the fuselage behind the pilot. It was called ‘MONA’, but it is not clear what the acronym stood for. I can only assume that this is a typical German abbreviation, and may stand for  ‘MOtorleistungsNAchschub’. Externally, the aircraft differed from the standard E version by the presence of two filler caps for filling the pressure bottles on the left side of the fuselage behind the cockpit.

 2. Bf 109 F-4/Z. Here, the pressure bottles were moved from the fuselage to the wing. It should be mentioned that the Bf 109 from the F version on had newly designed wings that had two free spaces for the installation of additional equipment, one just behind the landing gear bays and the other further towards the wing tip. Four nitrous oxide pressure bottles were installed just behind the wheel wells with access to filling these from within them. The F-4/Z carried a total of eight pressure bottles. It was distinguished from the outside by a deeper oil cooler, and apparently most of these aircraft had wider propeller blades. It must be noted that this installation eliminated the possibility of underwing cannon pods.

 3. Bf 109G/U2. Because the pressurized nitrous oxide cylinders were vulnerable to enemy fire and tended to explode when hit, the GM-1 system was redesigned for the Bf 109 G version. A thermally insulated tank was installed in the fuselage behind the cockpit, filled at normal pressure with liquefied nitrous oxide cooled to a temperature of -90°C. Compressed air, stored in pressure bottles in the outer part of the right wing, was used to expel nitrous oxide from the tank into the engine intake. To maintain the aircraft's balance, pressurized oxygen bottles for the pilot's breathing apparatus were moved to the outer part of the left wing. Below the right wing, a cap for filling compressed air (not nitrous oxide!) was added, and on the right side of the fuselage a cap for filling the cooled nitrous oxide (in the same place as the cap for the MW-50 system) was added. The cap for the GM-1 system was marked with a white triangle with two blue stripes, possibly with the inscription ‘Sonderstoff’. Because the space in behind the wheel wells remained vacant, it was possible to mount cannon pods under the wings. This system was not as vulnerable to hits from enemy fire and at the same time had the advantage of even better efficiency (increased engine power) than the pressurized method, since the low temperature of nitrous oxide cooled the fuel mixture further before entering the engine cylinders. This installation was designated U2 and was installed in the Bf 109 G-5 and Bf 109 G-6. It is likely that it was not used on earlier versions of the Bf 109 G, as it was not yet available at the time of their production. Unfortunately, I have not been able to find out if the early versions of the Bf 109 G were equipped with the same GM-1 system as the F-4/Z. I believe it wasn’t, which would mean that the Bf 109 G equipped with the GM-1 did not appear until after considerable delay. Douglas E. Calum states that universal installation of the GM-1/MW-50 system was considered, however, the GM-1 system required a thermally insulated tank unlike the MW-50. This idea was probably abandoned due to technical complications. If the Bf 109 was equipped with the MW-50 system, the GM-1 installation of this third version was impossible.

 The GM-1 installation was nicknamed the ‘Ha-Ha Prozess’, which is quite fitting given the effects of nitrous oxide.

 There are a quite a lot of inaccuracies, myths and misconceptions on this topic in published references. I will try to put the main ones in perspective, which some may find contradictory to what they have already read:

 1. H.H.Vogt states in several places that the installation of the GM-1 made it impossible to mount the guns under the wings of the Bf 109. However, this statement does not apply to the G version, but only to the F-4/Z.

 2. Robert Michulec (AJ-Press) reports that the F-4/Z version had either pressure bottles or a cylindrical tank for compressed nitrous oxide in the wing. I believe that the cylindrical tank for the GM-1 system was not present in the wing and there was a visual confusion with this item and the circular ammunition drum for the underwing cannon.

 3. Prien/Rodeike states, without any context, that the Bf 109 G-5/U2's GM-1 pressure bottles were located in the right wing, unlike the Bf 109 G-6/U2, which had these installed in the fuselage. This claim is not supported by any drawing or detailed description and can therefore be considered unproven. I find it highly unlikely that the GM-1 system compressed air bottles in the Bf 109 G-6 could be installed in the fuselage where the nitrous oxide tank was already fitted because the bottles simply wouldn’t fit.

Sources:

English: Douglas, Calum E., Mortons Media Group : The Secret Horsepower Race

German: Vogt, H. H., VDM Verlag: Messerschmitt Bf 109 Einsatzmaschinen - Das Nachschlagwerk

Czech: Robert Michulec, AJ Press: Messerschmitt Bf 109, Volumes 1, 2 and 4.

English: Richard A. Franks, Valiant Wings: The Messerschmitt Bf 109 Late Series

English: Prien, Jochen / Rodeike, Peter: Messerschmitt Bf 109 F, G, & K Series - An Illustrated Study

Online Sources:

http://kurfurst.org/Performance_tests/109G_GM1_instructions/kBetrA_GM1_109G.html