IAD Local Controller described a minimal arrival/departure separation event listing adding increased arrival spacing is required during tailwind operations.

Narrative

The runway configuration was south; landing Runways 19L 19R & 12; departing Runway 19L. I was working Runway 19L arrivals/departures. My plan in this situation was to get Air Carrier Y; off in a 5 mile gap between two successive arrivals; the succeeding arrival being Air Carrier X. Upon initial contact with Air Carrier X I instructed him to reduce speed and advised him about the departure. A few seconds later I instructed Air Carrier X to make an 'S' turn. When the previous arrival crossed the landing threshold; I put Air Carrier Y in position on Runway 19L. While the previous arrival was rolling out; I issued landing clearance to Air Carrier X as it appeared spacing would be adequate. When the arrival cleared the runway; I cleared Air Carrier Y for an immediate take-off. The departure did not appear to be gaining speed as quickly as I had anticipated and I thought I may have to send Air Carrier X around. I issued go-around instructions to Air Carrier X and then immediately rescinded them and cleared him to land again as the departure had rotated. Air Carrier X landed. Recommendation; unfortunately; operating with a tailwind to increase airport capacity is all too common at IAD. The problem is not that aircraft land and depart with the tailwind component; the problem is in using the same runway to land & depart given the additional runway occupancy time per operation. That said; in this situation; attention to aircraft performance characteristics; outside air temperature (hot day); and ground speeds on final are very important. With three runways available for landing; I recommend additional spacing be provided to the single departure runway; especially when a tailwind is present. Also; better utilization of the other two arrival runways would allow for a more efficient overall operation.