Reusable Launch to LEO - The craft designed to provide the service between Earth's surface and LEO needs to be designed to handle the demands of transferring each way through our atmosphere, and of course such vehicles already exist (eg Soyuz, Dragon, etc). Ongoing developments at this state for the Earth to LEO craft are approaching reusability.
There is a need for a continuing LEO station, such as the ISS, as the LEO node of the architecture. Space travelers will take two effective shuttle rides to reach their interplanetary craft at Gateway Earth complex; first they ride up to the ISS node - then they transfer to the LEO-GEO tug. And of course the process is repeated on returning from their interplanetary trip; first they dock at Gateway Earth, then they take a tug down to LEO, finally they take the last leg to the surface using the traditional aero-thermal braking approach for atmospheric re-entry.
LEO refueling component is an important part of the logistical infrastructure. This might be provided as a separate commercial venture, or partly supported by government funding.
The main purpose of the regular tug service between LEO and GEO is to take astronauts and space tourists, and their associated supplies, to their GEO station. The tugs are in fact a business operation in their own right. They are refuelled, and restocked, in LEO each time before they make their next trip to GEO. These tugs could also be used to deliver GEO comsats to their appointed orbit.
We propose to install Gateway Earth station in the geostationary orbit (GEO) more or less near the rim of Earth's gravity well, which then becomes the effective start and end point for interplanetary missions. It serves as a Space Hotel and a construction and fuelling hub for interplanetary vehicles.
The Gateway is also the place to where the vehicles return after their journeys. Therefore, the interplanetary vehicles themselves do not need to be built to withstand the rigors of a launch from Earth's surface, either from the point of view of structural strength or aerodynamics. And furthermore, they do not need to possess the weighty thermal control system associated with atmospheric re-entry, or the powerful motors needed to provide the energy for getting from the Earth to GEO.
Another way of thinking from the "gravity well" perspective is to view the "getting to the edge" part of the operation as "powered flight", and traversing the vast interplanetary spaces as much more like "gliding". Much of the structure for the interplanetary "glider" craft can therefore be built using additive manufacturing techniques.
In furthering the reach of Space Exploration missions, Landers and ISRUs could be built at the Gateway Earth Complex to complement orbital missions. Given the fact that size and shape are less relevant due to in-orbit construction, these vehicles could be far more advanced than the current proposals.