Only a few of the launch system concepts likely to be explored over the Bifrost program will have true GEO capability by themselves and many will be quite limited in payload capacity precluding the addition of booster systems. Thus their use for supporting GEO facilities will depend on the additional use a reusable inter-orbital vehicle for transfer from a LEO rendezvous point. Early on the use of dual-launch missions where separate launch vehicles supply booster and payload for mutual rendezvous on orbit before transfer will be a likely strategy and may often be preferred for the largest one-off unit payloads and quick transits. But eventually a permanent transfer facility may become necessary for more routine transit. Hence the need for an Inter-Orbital Shuttle.
The earliest form of IOS vehicle may be derived from the ‘carrier pallet’ chassis employed with Modular Unmanned Orbital Laboratory - MUOL, being of roughly similar form, using the same attitude control systems, but employing additional systems in the form of deployable solar panels or electrodynamic tether and a powerful plasma thruster which can be recharged with removable gas propellant cartridges. It would be designed to provide transfer for individual MUOL carrier pallets which it would intercept and link with. A variant carrier pallet would be used to replace its propellant cartridges and perform other plug-in component swaps.
This vehicle would eventually evolve into a much larger IOS based on a much larger assortment of the MUOL components, employ its own telerobots, and which would carry many carrier pallets at once on a long truss boom equipped with self-engaging self-ejecting quick-connect interfaces. With its power and thruster systems arrayed at one end and its payload boom on the other in an Earth-ward pointing configuration, this IOS would represent one of the first of the class of built-on-orbit spacecraft known as ‘beamships’ (for their truss beam core structures) and would set the path of design evolution for the much larger trans-lunar and inter-planetary Inter-Orbital Transport of the Asgard phase of development. Though most likely relegated to the role of unmanned vehicle, a manned version of larger IOS would also be possible, employing a single TransHab style crew module instead of a payload boom.