Pocket Rocket Research Articles

An Inductively-Coupled Plasma Electrothermal Radiofrequency Thruster

The “cubesat” form factor has been adopted as the defacto standard for a cost effective and modular, nano-satellite platform. Many commercial options exist for nearly all components re- quired to build such satellite; however, there is a limited range of thruster options that suit the power and size restrictions of a cubesat. Based on the prior work on the “Pocket Rocket” electro- thermal capacitively-coupled radiofrequency (RF) plasma thruster operating at 13.56 MHz, a new design is proposed which is based on an inductively-coupled radiofrequency plasma system operat- ing at 40.68 MHz. The new thruster design, which includes a compact and efficient radiofrequency matching network adjacent to the plasma cavity is presented, together with the first direct thrust measurements using argon as the propellant with the thruster immersed in vacuum and attached to a calibrated thrust balance. These initial results of the unoptimised first prototype indicate an up to 40% instantaneous thrust gain from the plasma compared to the cold gas thrust: typical total thrust at 100 SCCM of argon and 50 W RF power is ∼ 1.1 mN.

Performance modelling of plasma microthruster nozzles in vacuum

Computational fluid dynamics and plasma simulations of three geometrical variations of the Pocket Rocket radiofrequency plasma electrothermal microthruster are conducted, comparing pulsed plasma to steady state cold gas operation. While numerical limitations prevent plasma modelling in a vacuum environment, results may be obtained by extrapolating from plasma simulations performed in a pressurised environment, using the performance delta from cold gas simulations performed in both environments. Slip regime boundary layer effects are significant at these operating conditions. The present investigation targets a power budget of ∼10 W for applications on CubeSats. During plasma operation, the thrust force increases by ∼30% with a power efficiency of ∼30 μNW−1. These performance metrics represent instantaneous or pulsed operation and will increase over time as the discharge chamber attains thermal equilibrium with the heated propellant. Additionally, the sculpted nozzle geometry achieves plasma confinement facilitated by the formation of a plasma sheath at the nozzle throat, and fast recombination ensures a neutral exhaust plume that avoids the contamination of solar panels and interference with externally mounted instruments.

Vacuum Testing of a Miniaturized Switch Mode Amplifier Powering an Electrothermal Plasma Micro-Thruster

A structurally supportive miniaturised low-weight ($\le$150~g) radiofrequency switch mode amplifier developed to power the small diameter {\it Pocket Rocket} electrothermal plasma micro-thruster called {\it MiniPR} is tested in vacuum conditions representative of space to demonstrate its suitability for use on nano-satellites such as `CubeSats'. Argon plasma characterisation is carried out by measuring the optical emission signal seen through the plenum window versus frequency (12.8-13.8~MHz) and the plenum cavity pressure increase (indicative of thrust generation from volumetric gas heating in the plasma cavity) versus power (1-15~Watts) with the amplifier operating at atmospheric pressure and a constant flow rate of 20~sccm. Vacuum testing is subsequently performed by measuring the operational frequency range of the amplifier as a function of gas flow rate. The switch mode amplifier design is finely tuned to the input impedance of the thruster ($\sim$16~pF) to provide a power efficiency of 88 $\%$ at the resonant frequency and a direct feed to a low-loss ($\sim 10 \%$) impedance matching network. This system provides successful plasma coupling at 1.54~Watts for all investigated flow rates (10-130 sccm) for cryogenic pumping speeds of the order of 6000~l.s$^{-1}$ and a vacuum pressure of the order of $\sim$ 2x10$^{-5}$~Torr during operation. Interestingly, the frequency bandwidth for which a plasma can be coupled increases from 0.04 to 0.4~MHz when the gas flow rate is increased, probably as a result of changes in the plasma impedance.

A Comprehensive Cold Gas Performance Study of the Pocket Rocket Radiofrequency Electrothermal Microthruster

This paper presents computational fluid dynamics simulations of the cold gas operation of Pocket Rocket and Mini Pocket Rocket radiofrequency electrothermal microthrusters, replicating experiments performed in both sub-Torr and vacuum environments. This work takes advantage of flow velocity choking to circumvent the invalidity of modelling vacuum regions within a CFD simulation, while still preserving the accuracy of the desired results in the internal regions of the microthrusters. Simulated results of the plenum stagnation pressure is in precise agreement with experimental measurements when slip boundary conditions with the correct tangential momentum accommodation coefficients for each gas are used. Thrust and specific impulse is calculated by integrating the flow profiles at the exit of the microthrusters, and are in good agreement with experimental pendulum thrust balance measurements and theoretical expectations. For low thrust conditions where experimental instruments are not sufficiently sensitive, these cold gas simulations provide additional data points against which experimental results can be verified and extrapolated. The cold gas simulations presented in this paper will be used as a benchmark to compare with future plasma simulations of the Pocket Rocket microthruster.

A Short Review of Experimental and Computational Diagnostics for Radiofrequency Plasma Micro-thrusters

Experimental and computational diagnostics for radiofrequency plasma micro-thrusters are presented, based on the low power (10–100 W) electrothermal thruster prototype, Mini Pocket Rocket, developed for use on the Cubesat nanosatellite platform. Computer simulations include computer fluid dynamics simulations and particle in cell simulations while experimental results are obtained using a variety of electrostatic, optical and momentum probes. The output and limitations of each diagnostic are discussed within the context of device development for space use.

The power of product integrity

In the dictionary, integrity means wholeness, completeness, soundness. In products, integrity is the source of sustainable competitive advantage. Products with integrity perform superbly, provide good value, and satisfy customers' expectations in every respect, including such intangibles as their look and feel. Consider this example from the auto industry. In 1987, Mazda put a racy four-wheel steering system in a five-door family hatchback. Honda introduced a comparable system in the Prelude, a sporty, two-door coupe. Most of Honda's customers installed the new technology; Mazda's system sold poorly. Potential customers felt the fit--or misfit--between the car and the new component, and they responded accordingly. Companies that consistently develop products with integrity are coherent, integrated organizations. This internal integrity is visible at the level of strategy and structure, in management and organization, and in the skills, attitudes, and behavior of individual designers, engineers, and operators. Moreover, these companies are integrated externally: customers become part of the development organization. Integrity starts with a product concept that describes the new product from the potential customer's perspective--"pocket rocket" for a sporty, subcompact car, for example. Whether the final product has integrity will depend on two things: how well the concept satisfies potential customers' wants and needs and how completely the concept has been embodied in the product's details. In the most successful development organizations, "heavyweight" product managers are responsible for leading both tasks, as well as for guiding the creation of a strong product concept.