Propellant resupply of orbiting spacecraft is no longer in the realm of high risk development. The recently concluded
Orbital Express (OE) mission included a fluid transfer demonstration that operated the hardware and control logic in
space, bringing the Technology Readiness Level to a solid TRL 7 (demonstration of a system prototype in an operational
Orbital Express (funded by the Defense Advanced Research Projects Agency, DARPA) was launched aboard an Atlas-V
rocket on March 9th, 2007. The mission had the objective of demonstrating technologies needed for routine servicing of
spacecraft, namely autonomous rendezvous and docking, propellant resupply, and orbital replacement unit transfer. The
demonstration system used two spacecraft. A servicing vehicle (ASTRO) performed multiple dockings with the client
(NextSat) spacecraft, and performed a variety of propellant transfers in addition to exchanges of a battery and computer.
The fluid transfer and propulsion system onboard ASTRO, in addition to providing the six degree-of-freedom (6 DOF)
thruster system for rendezvous and docking, demonstrated autonomous transfer of monopropellant hydrazine to or from
the NextSat spacecraft 15 times while on orbit. The fluid transfer system aboard the NextSat vehicle was designed to
simulate a variety of client systems, including both blowdown pressurization and pressure regulated propulsion systems.
The fluid transfer demonstrations started with a low level of autonomy, where ground controllers were allowed to review
the status of the demonstration at numerous points before authorizing the next steps to be performed. The final transfers
were performed at a full autonomy level where the ground authorized the start of a transfer sequence and then monitored
data as the transfer proceeded. The major steps of a fluid transfer included the following: mate of the coupling, leak
check of the coupling, venting of the coupling, priming of the coupling, fluid transfer, gauging of receiving tank, purging
of coupling and de-mate of the coupling.