NASA.gov
A new era of space exploration is being planned.
Manned exploration architectures under consideration require the long term storage of cryogenic propellants in space, and larger science mission directorate payloads can be delivered using cryogenic propulsion stages.
Several architecture studies have shown that in-space cryogenic propulsion depots offer benefits including lower launch costs, smaller launch vehicles, and enhanced mission flexibility.
NASA is currently planning a Cryogenic Propellant Storage and Transfer (CPST) technology demonstration mission that will use existing technology to demonstrate long duration storage, acquisition, mass gauging, and transfer of liquid hydrogen in low Earth orbit.
This mission will demonstrate key technologies, but the CPST architecture is not designed for optimal mission operations for a true propellant depot.
This paper will consider cryogenic propellant depots that are designed for operability. The operability principles considered are reusability, commonality, designing for the unique environment of space, and use of active control systems, both thermal and fluid.
After considering these operability principles, a proposed depot architecture will be presented that uses water launch and on orbit electrolysis and liquefaction.
This could serve as the first true space factory. Critical technologies needed for this depot architecture, including on orbit electrolysis, zero-g liquefaction and storage, rendezvous and docking, and propellant transfer, will be discussed and a developmental path forward will be presented.
Finally, use of the depot to support the NASA Science Mission Directorate exploration goals will be presented.
I. Introduction
Past and current studies have shown that propellant depots in space enable sustainable exploration of the solar system.
The advantages of cryogenic propellant depots in space have been discussed since early in the space program 2.
Werhner Von Braun used in-space refueling as a component of his famous human exploration of space articles in Colliers magazines in the 1950' s.
Most early concepts called for large orbiting complexes requiring multiple launches to assemble the depot in orbit, followed by multiple resupply missions to transport large quantities of cryogens to the orbital storage complex.
Depot customers then rendezvous with the station and purchase propellants in whatever quantities they need.
This architecture is analogous to a terrestrial gas station; semi-permanent facilities built to provide a range of refueling capabilities.
The scale and complexity of these types of depots will probably prohibit their deployment, at least until launch costs are decreased significantly.
Other smaller depot studies have been performed over the years.
A simpler depot concept that uses a single launch to provide refueling capability to a single dedicated mission 3 is one such proposal.
The Augustine committee report 4 identified in-space refueling as a "significant potential benefit to the in-space transportation system beyond low-Earth orbit", and considers both models of depots as feasible.
More recently, a controversial NASA study showed the total program cost of both Lunar and Mars exploration would drop significantly if the mission architecture could benefit from in-space refueling using propellant depots 5.
However, perceived complexity and launch costs have helped limited the development of propellant depots, and critical technology demonstrations for microgravity handling of cryogens must be achieved before these concepts will gain acceptance by NASA mission planners.
The main advantage of propellant depots is the possibility of stationing a large quantity of propellant along the path of travel.
This advantage allows the reduction of initial propellant mass and the reduction (mainly in propellant tank size, but also in engine mass) and reuse of system inert mass.
Since the specific impulse of a given rocket engine is fixed, one easy way to increase the "mass efficiency" of a vehicle is to refill it.
Equation 1 shows the rocket equation modified for refueling a stage N times, by refilling the propellant tanks once you can double the delta v of the stage.
In this context the analogy of a gas station is appropriate.
By refueling a stage at a single or multiple locations, the stage can be made much smaller to achieve the same 1'1 V.
However, the gas station infrastructure already exists on the Earth; it must be created beyond the Earth.
More:
https://ntrs.nasa.gov/api/citations/20120015764/downloads/20120015764.pdf
