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

Ballistic hops tend to use less propellant than hover hops, since the duration of powered flight is
shorter, but performing precision hops with a ballistic trajectory may be more difficult because of the
narrow window of time available to make powered maneuvers. By contrast, hover hops tend to
consume more propellant because the entire trajectory is powered, but this also allows the hopper to
perform attitude control and course corrections mid-hop. A detailed analysis of the tradeoffs between
these two types of hops is available in [18].
Selection of a hop trajectory influences the propulsion options available to a hopper. For example,
designs have been proposed for hoppers using mechanical methods such as springs or pistons to exert a
reaction force on a surface; see for instance [19,20,21,22]. However, mechanical hopping can only be
ballistic, while the current concept of operations for the GLXP lunar hop calls for covering the mandated
500 m distance by hover hopping [18]. Furthermore, as described in Chapter 1, there are potential
benefits to using the same propulsion system for initial landing on a planetary surface and later hopping
travel. A mechanical method of hopping would not be very useful for an initial landing; rocket engines
are much more appropriate for this purpose. Additionally, an impulsive rocket propulsion system
capable of performing a hover hop could also be used to perform a ballistic hop if desired by simply
firing the rockets only at the beginning and end of the flight. Thus, the TALARIS vehicle is designed to use
rocket propulsion, in order to provide the most accurate simulation of the planned GLXP trajectory as
well as to allow TALARIS to be a testbed for the greatest possible range of hopping applications.
There are many types of rocket propulsion systems, but not all are appropriate for hoppers. Because the
propulsion system for a hover hop is required to perform attitude control as well as to provide
acceleration to make the vehicle travel, it must be able to stop, restart, and deliver small impulse bits.
Even ballistic hoppers, if they are designed to make multiple hops, require stop and restart capability for
their propulsion systems. This rules out solid rockets. Hybrid rockets are a theoretical possibility, but
they have much less flight heritage, and hybrid rocket development has generally been focused on high-
thrust applications like launch vehicles and orbit insertion stages [23]. The remaining feasible options,
for both the GLXP and TALARIS hoppers, are cold gas, monopropellant, and bipropellant propulsion.
Cold gas, monopropellant, and bipropellant systems all have extensive flight heritage. Of the three, cold
gas systems are the least complex and costly, but they generally have the lowest thrust and specific
impulse; bipropellant systems have the highest complexity and cost, but also the highest thrust and
specific impulse; and monopropellant systems fall between the other two [24]. Higher complexity means
that more hardware is necessary, and the additional hardware mass outweighs the performance gains
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