Tail engines add more weight. Placing the engines in pods underneath the wing relieves the bending moment in flight, allowing for lighter wing root construction. Placing them at the tail requires local beefy frame construction.
Note that the upwards bending moment in flight is the limit design factor:
- In flight all of the weight of the aeroplane times the ultimate load factor of 3.5, and this is the bending moment relieved by the weight of engines mounted at the wing.
- On the ground, the undercarriage supports the weight of the aeroplane at load factor 1, with a smaller moment arm. Way smaller than the ultimate bending load.
- At touchdown, the spring/dampers are dimensioned such that the bending moment at maximum sink rate is below the ultimate design bending moment.
Torenbeek 1 mentions the following on this subject:
The following factors have to be considered:
- A wing structure weight saving is possible with wing-mounted engines due to the mass relief effect on the bending moment on the inner wing.
- Engines placed too far outboard increase the landing impact loads and necessitate a large vertical tailplane.
- Engines at the rear of the fuselage require local "beef-up" and lead to loss of useful space in the tail, resulting in added structure weight and a larger fuselage for the same payload.
- Differences in weïght of the tail surfaces depend on various factors which do not permit a general conclusion.
Summing up, we may say without too much emphasis that the empty weight of a CaraveIle-type layout will typically be 2 to 4% more than that of a comparable design with the engines on the wing.
As mentioned in this answer, at the time of B737 design Boeing had two competing designs from different teams, and the wing mount won. For an Order Of Magnitude check we can compare the wing and fuselage weights as a percentage of OEW for the DC9 and the B737-100. The weight of the wing & fuselage groups as per the table in this answer, the OEW from Wikipedia:
So indeed if we look at the weight only, the underwing configuration wins. The tail configuration compensates with aerodynamic pluses, mainly from the clean wing.
Note that these considerations were of value at the time of DC9 construction, with the narrow engines of the time. A modern 200 pax plane with high bypass turbofans has no good reasons anymore to opt for the tail mount.
1 E. Torenbeek, Synthesis of Subsonic Airplane Design