Devices are proposed that would sublime solid propellants to generate small thrusts for maneuvering spacecraft with masses of no more than 15 kg. With solids rather than gases in propellant tanks, there would be no leakage and thus no waste of limited quantities of propellants. There would also be none of the bulk, weight, and cost of plumbing like that needed for handling liquid or gaseous propellants if the propellant tank would be integrated with the subliming solid thruster.

Figure 1. The Concept of the Subliming Solid Microthruster is illustrated in oblique view (a) and side view (b).

The propellant in a subliming thruster would be contained in an aluminum tank (see Figure 1) with an outlet connected to the subliming solid microthruster chip. This chip, micromachined from silicon, contains a nozzle and an integrated filter. Ultimately, a thruster valve will also be integrated into this chip. A wire electric heater could be wrapped around the tank, or else a film electric heater could be deposited on the tank. The propellant material (e.g., ammonium hydrosulfide) would be sublimed on command by activating the heater.

Figure 2. Microthruster Model shows the level of miniaturization that can replace bulkier units.

Opening a valve placed into a flowpath between the nozzle and tank (see Figure 2) will allow the vapor to flow to whichever nozzle faced in the direction opposite the required direction of thrust. The wall of the tank could be as thin as 0.020 in. (0.5 mm) because the vapor pressure that it would have to withstand would be very small; thus, the tank could be very light in weight.

This work was done by Juergen Mueller, Lilac Muller, and Thomas George of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Mechanics category. NPO-19926

Topics:
Mechanical Components On-board energy sources Propellants Solid propellants Spacecraft Vehicles
This Brief includes a Technical Support Package (TSP).
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Subliming solid microthrusters

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NASA Tech Briefs Magazine

This article first appeared in the August, 1998 issue of NASA Tech Briefs Magazine (Vol. 22 No. 8).

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Overview

The document discusses the development of a subliming solid microthruster designed for microspacecraft, specifically those weighing 15 kg or less, as proposed by NASA. The primary motivation behind this innovation is the need for efficient and reliable miniature propulsion systems for small spacecraft, which are increasingly being considered for various missions.

Traditional propulsion systems, such as cold gas propellants, face significant challenges, particularly concerning leakage due to microscopic contaminants on valve seats. This leakage can lead to a depletion of propellant supplies long before the mission's end, which is especially problematic for microspacecraft with limited propellant capacity. To address these issues, the document proposes a subliming solid microthruster that utilizes solid propellants, specifically ammonia hydrosulfide, which minimizes leakage risks.

The proposed microthruster features a compact design, integrating a silicon chip that contains essential components such as a nozzle, isolation valve, and filter assembly. This chip is combined with an aluminum tank, chosen for its high thermal conductivity, which facilitates effective heat transfer to the solid propellant, causing it to sublimate and produce thrust. The design allows for very lightweight tank walls, as thin as 0.5 mm, contributing to the overall compactness and efficiency of the system.

Performance-wise, the subliming solid microthruster is expected to achieve specific impulse values around 50-60 seconds, making it suitable for attitude control and small delta-v maneuvers. The use of microfabrication techniques in the thruster assembly results in a lightweight and compact system, which is crucial for the operational needs of microspacecraft.

Additionally, the document highlights the potential for utilizing waste heat from the microspacecraft to aid in the sublimation process, further reducing power requirements. This innovative approach not only addresses the leakage problem associated with gaseous propellants but also enhances the overall efficiency and reliability of propulsion systems for small spacecraft.

In summary, the subliming solid microthruster represents a significant advancement in propulsion technology for microspacecraft, offering a solution that mitigates leakage issues, reduces weight, and integrates advanced materials and fabrication techniques to meet the demands of future space missions.