Airspeed,
For a Radar equiped Tower/Approach off the primary airport you have to provide 15 degrees or more lateral seperation between, or three miles (or more for wake turbelence) between departures. Before I can roll the second departure there is also runway seperation that I must apply depending on the aircraft size (weight class).
For example if I'm departing three or more B727's (or other large aircraft) off of runway 23 and it is IMC. Depending on where they are going:
1. Turn right to 320
2. Turn left to 140
3. Turn right to 300 (added 5 extra degrees for my kids)
4. Turn left to 120
5. Turn right to 280
etc. etc. etc.
The key is that I'm providing departure with initial IFR seperation between departures, trurning departures as close to on course within my departure pie that I can and watching that subsequent departures are clear before I turn toward them. Other faciliities have extra rules for noise abatement that they must adhere too.
For departures off of a satelite airport I don't release the second aircraft until the previous departure is airborne and at least 3 miles or more away from the airport, or out of 3000 feet, regardless if it is IMC or VMC.
For a VFR tower the rules change to being required to have 45 degrees or more lateral seperation between subsequent departures released one minute apart. There are some more VFR tower rules but the 45 degree one is the rule used most often.
Mike
FWA NATCA