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

2 Propulsion System Architecture
Although most major architecture decisions for TALARIS were made before the work described in this
thesis began, it is useful to discuss other approaches for the propulsion of prototype or simulator
landers and exploration vehicles. For one thing, a brief review of the spacecraft emulator propulsion
system was conducted before detailed design work began, to ensure that the decisions made for the
original concept were still valid even though the TALARIS design had evolved in other areas.
Furthermore, understanding the strengths and weaknesses of cold gas propulsion compared to the
other options that might have been selected provides insight into the design decisions made.
2.1 Vehicles Other Than TALARIS
Since hopping is not a common method of exploring planetary surfaces, there is little precedent for
prototype or testbed vehicles built for this purpose. The closest analogues to TALARIS are Earth-based
lunar lander testing vehicles. Several different concepts have been developed for vehicles of this type,
but they all face the problem of predicting lunar performance from tests performed on Earth. From a
propulsion perspective, the major difference between Earth and the Moon is gravity; about six times
more force is needed to lift an object on the Earth as compared to lifting an object of equal mass on the
Moon. The atmosphere on Earth also introduces forces that are not present on the Moon, but the
effects of wind can be mitigated by operating on calm days or even indoors, and drag forces for most
vehicles are small compared to the forces needed to overcome gravity and to accelerate the vehicle for
horizontal transit.
One approach to testing a lunar vehicle on the Earth is to simply operate the system as designed for the
Moon, as long as it is powerful enough to operate in the higher gravity of Earth. Mission profiles may be
scaled in terms of distance or time such that they use an equivalent amount of delta-V for a scaled flight
on Earth as they would for actual operations on the Moon. This was the approach taken for the
Northrop Grumman Lunar Lander Challenge, for which the goal was to simulate a descent from lunar
orbit, refueling, and return to lunar orbit [7]. An equivalent task for demonstration on Earth was
designed to involve taking off from a concrete pad, ascending to an altitude of approximately 50 m,
flying 50 m horizontally, and landing on a second concrete pad, with refueling and a return flight within
two hours and fifteen minutes. The vehicle had to remain aloft for at least 90 s on each flight for the first
level of competition and 180 s on each flight for the second level [8]. The benefit of this method is that it
allows for testing of both hardware and software exactly as they will be used in the real lunar mission.
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