May 22 2018

Extremis Phase Two Reconsiderations

Today mission planners for Extremis belatedly realized that their initial round of selection for Phase Two trajectories failed to take into account the fact that all trajectories were calculated from a starting equatorial 200km parking orbit. This was done for the sake of simplicity as the multi-flyby maneuver sequencer that ran various possibilities could easily eject the spacecraft from this orbit in any direction needed. In most cases for interplanetary travel the spacecraft will leave Kerbin at an inclined angle. Changing the inclination of an orbit is a very costly maneuver, especially when it is done deep in the gravity well close to the planet. Ideally, the spacecraft would be launched into an inclined orbit suitable from which to perform an escape burn with only a prograde component.

What this all means is there is a large segment of trajectories that have been overlooked for Phase Two because their v requirements appeared to exceed the 3km/s requirement when they did not actually need that much energy! Take, for example, a route that goes from Kerbin to Dres to Eve and then out to Urlum in the span of just over 5 years. The initial requirement for this trajectory was 3.513km/s of Δv, however looking at the departure burn requirement from Kerbin it defines a radial and normal burn component totaling 1.68km/s of Δv. If the spacecraft were launched into the properly-inclined orbit this portion of the burn could be almost entirely removed (this all depends on the capabilities of the launcher). Therefore the actual power requirements for this mission could be as low as 1.833km/s, which makes it feasible under the Phase Two considerations of  less than 3km/s Δv and less than 10 years of travel time.

The Extremis team will be going back and taking a closer look at trajectories with initial requirements of 3-5km/s to see how many drop below 3km/s when the radial/normal burn component is factored out. Those that remain and cover the gaps left by the one chosen mission will go through the Phase Two 500-iteration route stress test.