Back in the old days of the cold war the US and Russia were trying to develop a Nuclear powered aircraft. Problems with radiation shielding and "safety" held them back.
Let's say that they got this technology to work and developed a large flying fortress type aircraft capable of flying at 600mph at 100,000 feet. Now if you wanted to launch a space payload off that platform (weighing 1kg) how would you adjust the launch equations to calculate how much cryogenic fuel you will need? Also, taking into consideration that the atmosphere up there is very cold and there is very little air pressure. This would mean that the fuel would stay colder longer and need lighter tanks. Also since the pressure is reduced the nozzle velocity of the propellent would be higher at those altitudes.
Edit: Similar to the Pegasus, in the fact that it is launched from 40,000 feet from a carrier aircraft. Inclination: 51.6, Apogee: 421Km, Perigee: 431 (basically the ISS' orbit), launched eastward.
Expanding further: I want to create a model in c# that will calculate and plot the amount of fuel required to launch 1kg into orbit from several altitudes and speeds. Over time I want to add in factors like drag etc, to determine what the performance of the carrier craft should be.