Development of a Cold Gas Propulsion System for the ... - SSL - MIT

accurate conditions for testing GNC algorithms, and it had proven remarkably successful in the past with
the LLRV and LLTV. Furthermore, keeping the simulated lunar propulsion entirely separate from weight
relief meant that theoretically, software written for the TALARIS testbed could be moved directly over to
control the actual GLXP hopper. Overall, the selected architecture was deemed to make TALARIS the
most effective possible testbed for the GLXP hopper and a robust demonstrator of hopping in general.
In the 16.898 class, electric ducted fans (EDFs) were selected for the weight relief propulsion system,
required to lift 5/6 of the vehicle’s Earth weight at all times [17]. This decision clarifies some of the
priorities of the TALARIS project. For instance, another option for weight relief might have been to
devise some system of pulleys and counterweights from which to suspend the hopper. This would have
had the benefit of little to no reliance on expendable propellant, thus allowing for longer flight times.
However, it also would have had the drawbacks of restricting the flight range of the vehicle, introducing
dynamics from the suspension tethers, and requiring a large amount of infrastructure to be moved with
the vehicle for tests or demonstrations at a different site. The decision to make TALARIS a free-flying
vehicle reveals that simulation fidelity and flexibility in operations are more important to the project
than maximizing flight times at all costs. At the same time, it is important that flight times not be so
short as to preclude the collection of useful data or make it difficult to demonstrate all the phases of a
hop. The selection of a weight-relief propulsion system for TALARIS that makes use of the air through
which it flies was made to maximize efficiency and flight times as much as possible, creating a
propulsion architecture more similar to that of the USC LEAPFROG testbed with its flight times of several
minutes as opposed to the NASA cold gas vehicles limited to flights of just seconds. Finally, the choice of
the EDFs over other aerodynamic propulsion methods (such as rotors or small jet engines like the JetCat
used by LEAPFROG) illustrates that beyond performance metrics such as propellant usage, thrust output,
and system mass, other factors such as cost, safety, and ease of implementation were also considered.
The 16.898 class also selected cold gas propulsion to emulate the actual GLXP spacecraft propulsion
system while operating in an artificial lunar gravity environment created by the constant weight offset
from the EDFs. After the 16.898 class concluded in fall 2008, the 16.83/89 Space Systems Engineering
class held in spring 2009 developed the design of the TALARIS testbed further, but due to limited
resources they were only able to focus on one of the propulsion systems. They chose to work on the
EDFs, and by the end of the semester they had built a first-generation vehicle that flew in a test stand
constraining it to linear vertical motion only [17]. However, the cold gas propulsion system remained at
the conceptual design stage when the work described in this thesis began.
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