- PROFAC (Propulsion Fluid Accumulator) by Demetriades
- LOXLEO ( Liquid Oxygen Low Earth Orbit) by Klinkmann et al
- PHARO ( Propellant Harvesting of Atmospheric Resources in Orbit) by Jones, et. al
Demetriades' concept was first proposed during the Apollo Era. LOXLEO and PHARO have been recent attempts to revive the idea.
The PROFAC concept relies on nuclear power, which wasn't in disfavor at the time. The most recent versions attempt to work around the nuclear issue. What distinguishes LOXLEO from PHARO is that PHARO proposes to use beamed energy. LOXLEO is uncertain about its proposed energy source, as it could be beamed energy or a dynamic tether setup.
The PHARO concept is a comprehensive analysis of proposed system to implement gas harvesting. The study claims that there are no other ways to provide energy besides beamed energy. The energy requirements are such that all others ( besides nuclear ) aren't power dense enough to do the job. This would rule out the idea of using hydrogen fuel cells combined with a VASIMR, however fuel cells were not explicitly mentioned. VASIMR wasn't ruled out either, but MHD propulsion system appears to be preferred instead.
The conclusion is as follows:
Implementation of propellant harvesting
has the potential to dramatically reduce the
launch mass for a multi-mission Mars
architecture. Additionally, introducing such
a technology can reduce the life cycle cost
of the campaign. Although such an
architecture does lead to a reliability penalty
relative to a DRA 5 Derived, non-harvesting
concept, this penalty could be mitigated by
further improvements in technology. This
study shows the potential of atmospheric
propellant harvesting as an enabling technology
for future humans-to-Mars missions.
The term DRA 5 Derived is most likely referring to the now canceled Constellation program.
I like the proposition. It is better than using the same old technology that has been in place for over 50 years. Any experimental technology will have additional risks as it is being perfected, but the risk would be worth the gain.