Japanese jet engines
D. The Ne-20.
Development of the Ne-20 was carried out by the Naval Air Technical Arsenal. The design is based on the German BMW 003 engine and began in May 1944 upon the receipt of BMW 003 prints. the Ne-20 is about three fourths the size of the 003. Two Ne-20's were installed in Kikka and flight tested in early August 1945. The first flight on 7 August 1945 was successful for a duration of 21 minutes. In the second flight on 12 August the plane crashed in the sea on take off due to pilot error (presumably). Twenty one engines were constructed, nine at the Air Technical Arsenal and twelve at the Yokosuka Navy Yard. Production was scheduled to start at this latter agency in late summer r 1945. Photographs of the Ne-20 are summer shown in Figures 1 to 4 and drawings in Figures 5 to 7.
The Ne-20 is 106.5 inches long., 24.4 inches in diameter and 32' inches over the accessories. The dry weight is 1000 lbs. The static thrust is given as from 1050 to 1100 lbs. at 11000 rpm and 1.36 lbs/hr lb.thrust specific fuel consumption. 1100 lbs. thrust is believed to be low in relation to the engine size and mass flow of 31 lbs/sec. 1300 lbs. thrust could probably have been realized from a more fully developed Ne-20. It should be noted, however, that the Japanese had virtually no back log of information on high temperature materials. Japanese turbo superchargers were only in the experimental stage and were largely based on German and captured U.S equipment. The nozzle box temperature of the Ne-20 is listed as 1300 1380 F. which is somewhat lower than present U.S. and British turbo jets.
An eight stage axial compressor is employed (the 003 has seven stages). The compressor rotor has a constant O.D. of 18.7 inches and is 21.3 inches long. Approximate rotor blade heights by stages are:
The means of rotor blade attachment is not known except that blades are tongued into peripheral grooves in the rotor disks. The first two compressor stages each have 28 blades. The number of blades for succeeding stages is not known.
Compressor rotor disks are apparently of aluminum alloy, shrunk and pinned on steel sleeves. These sleeves in turn act as spacers and are keyed to the compressor shaft. A shoulder is machined on the shaft between the fourth and fifth stages and the rotor discs are assembled from either end against this shoulder.
Labyrinth seals are employed at the forward and after end of the compressor rotor. A balance line is observed on each side of the engine apparently connected to these seals.
The combustion chamber is of the BMW type, being annular in design, using 12 BMW 003 burners and having sandwich mixers. The 003 has 16 of these burners. In scaling down the BIN chamber it is evident that the Japs used the same size burners but just three fourths as many'.. thus eliminating additional experimentation by retaining the same flame length, airflow characteristics, etc.
The temperature of combustion is 22000F, the nozzle box temperature 1300 13800F, and the tail pipe temperature is 1020 10800F.
The turbine is single stage axial flow and has solid blades. The turbine diameter is 22.1 inches and effective blade height is approximately 3.6 inches. Blades are tongued into the turbine disk. The turbine wheel is overhung on the turbine shaft and is joined to a face coupling on the shaft by through bolts.
The rotating masses are supported in a similar fashion to that of the 003. Compressor thrust is taken by a single row ball bearing forward. The after end of the compressor shaft is supported by a single row roller bearing. The turbine shaft is supported by a single raw ball bearing for ward taking end thrust and a single row roller bearing aft (just forward of the turbine wheel). The two turbine bearings are pressure oiled from the main oil pump and scavenged by a pump mounted on the forward end of the turbine shaft. The turbine shaft extends through the combustion chamber. Both the turbine and compressor shafts are necked down to receive a short., internally splined, sleeve coupling.
The distance between turbine bearing centers is 21.8 inches, compressor bear Ing centers 25.3 inches and the distance between the after compressor bearing and the forward turbine bearing is 7.7 inches.
The tail cone is adjustable on the ground for variation in operating conditions and the total travel of the cone is 4 inches. Minimum exhaust area is estimated to be 105 sq. in. in the extended position and 180 sq. in. in the retracted position.
The engine housing from the primary combustion chamber section, forward appears to be made from light alloy castings and the housing aft of this section is apparently constructed of welded sheet ferritic metal. 18 8 stainless steel was tried for the combustion chamber but later aluminum dipped mild steel was used here.
Accessories are driven from two vertical tower shafts connected to a single bevel gear. This gear is driven by a bevel gear on the compressor shaft.
Starting is effected by an electric starter mounted within the compressor spinner and drives through double 4:1 reduction spur gears to a bevel gear. This bevel gear drives the engine through the accessory drive bevel gear. The engine is usually started at 1500 rpm. Japanese sources state that the starter is capable of turning the engine over at 6000 rpm which is 24000 rpm starter speed and is considered dubious. The Japanese perfected 15000 rpm a.c. and 10000 rpm d.c. motors of about 1 bhp but none at higher rpms.
Accessories are geared into the top and bottom of the vertical tower shafts. For the top accessories a gear box only is shown and it is believed that these take offs are provided for generator., vacuum pump, tachometer and other aircraft accessories. The bottom group of accessories is presumed to consist of pressure oil pumps, dual fuel pump., governor and booster coil. On the lower right side of the engine is what appears to be a vapor separator of the pendulum type. An oil tank is located concentrically below the engine entry duct.
The engine is mounted by means of two vertical pads cast on either side of the compressor housing centering over the eighth stator stage. The compressor housing is parted on a horizontal plane. Stator ring halves are bolted to the compressor casing on each side of the parting surface.
In all the Ne-20 appears to be a fairly well and simply designed turbojet. The BMW 003 was probably the best of the smaller German turbo jets and the Japanese apparently didn't sacrifice any of the 003's good features. It should be noted that a Japanese plane with two Ne-20's would have equivalent performance to a German plane with two 003's due to the light construction employed by the Japanese in their aircraft.