Haiving gained experience with the NOTAR (no tail rotor) system on the MD520N in the early 1990, McDonnell Douglas Helicopters developed an all new helicopter with this anti-torque feature. This is the MD900 Explorer. The Explorer has a rather high fuselage, that slopes up after the cabin. Clam-shell doors provide access to the rear of the cabin. Above is a quite thick tail boom that widens at the end. The end has an exhaust downward that can be tilted for steering. On top of the tail boom, where the tail boom starts to be come wider, are horizontal stabilisers with large swept vertical fins at the end, pointing up and down. The sweep angle of the upper part is back ward and that of the lower part forward. The two turbo­shaft engines are placed on top of the cabin, just behind the main rotor. They power a five blade main rotor. Compared to other helicopters, the Explorer has few side windows, just one for the cabin, in the door. Of the cockpit windows, the lower front ones are relatively large. The skids have short legs.

The different versions of the MD Helicopters Explorer family can be recognised by looking at the air intake.

MD900 is the designation of the first version of the Explorer. It has all the basic features described above. To distinguish it from the MD902 you have to look at the air intakes. On the MD900 these are clearly visible; there is only a fine roster on them, to prevent foreign objects to be sucked in.

Here is an original Explorer, the MD900, recognised by the clearly visible air intakes (photo: James/WikiMedia)

Here is a detail photo of the engine hub, where you can easily look into the air intakes. (photo: James/WikiMedia, modified)

The improved version of the MD900 is officially still called MD900, but marketed as MD902. The only externally clearly visible difference is in the air intakes. On the MD902 these have a roster that is more closed than on the MD900. Look at the photos and you'll see want we mean.