Analysis of modern electric propulsion thrusters

with high specific impulse

V.V.Gopanchuk, M.Yu.Potapenko

EDB "Fakel"

Spacecraft (S/C) improvement is achieved by developing new elements including also a creation of more effective electric propulsion (EP) thrusters. The increase of active life of the modern and prospective geostationary satellites, complication of the tasks being solved by the on-board propulsion system, toughening of requirements for thrusters in terms of effectiveness, reliability, and compatibility with S/C electronics assume creation of new-generation EP thrusters.

The purpose of this paper is a comparative generalizing analysis of main performances and characteristics of various modern EP thrusters, determination of the possibility to widen the area of Hall effect thrusters application with achievement of high values of specific impulse that would be comparable with those of ion thrusters, and selection of a preferable design lay-out to develop prospective high-voltage Hall effect thrusters.

During the last years there was an attempt to create a new type of Hall effect thrusters designed using a hybrid scheme [[1]].

The thrusters operating with a combination of principles, mechanisms, and processes of ions formation and acceleration acting as in both Stationary Plasma Thrusters (SPT) and Thrusters with Anode Layer (TAL) [[2]] can rightly be attributed to Hybrid Plasma Thrusters (HPT).

The main design distinguishing features of the Hybrid Plasma Thruster is that its discharge chamber is made as a combined element: its outlet part is formed by dielectric rings while its bottom part is made of metal due to the walls of the adjoining hollow anode/gas distributor [[3]].

Analysis of modern EP thrusters of different types (both Hall effect and ion) and of different geometrical dimensions allows us to estimate the level of main performances achieved in the wide range of powers. Ion thrusters provide higher specific impulses. Hall effect thrusters operating at higher power and at sufficiently high discharge voltages, as for example SPT-1 model, are capable of achieving high values of specific impulse of 3200 s and more, what, in its turn, allows coming close to the area of specific impulses of ion thrusters.

For any model of Hall effect thrusters with fixed dimensions and selected geometry the typical behavior is that at a constant power its specific impulse increases as the discharge voltage increases, but thruster thrust decreases. Thus, during the demonstration test of SPT-1 two modes of its operation were revealed:

Ø           mode of operation with high specific impulse (Ud=800 V, F=89.8 mN, h=0.57, I_sp=2974 s);

Ø           mode of operation with high thrust (Ud=600 V, F=104.2 mN, h=0.59, I_sp=2651 s).

The propellant utilization factor depends on the flow rate value and the acceleration channel cross section area. Thus, the main parameter that has effect on the propellant utilization factor value is specific flow rate.

As is known, when developing thrusters, one of the main input parameters is power consumption. The analysis of the known Hall effect thrusters shows that the required specific impulse is achieved by various types of Hall effect thrusters at different specific xenon flow rates. The SPTs as compared to TALs mostly operate at lower specific flow rates. But the TAL models provide higher specific impulse at relatively high specific flow rates. The SPT-1 hybrid thruster, operating at specific flow rates typical for SPT, achieves higher values of specific impulses than those of SPTs and TALs.

Significant influence of specific flow rate on the specific impulse becomes apparent in high-voltage modes as, for example, for the NASA-103M.XL HiVHAC thruster when operating at a discharge voltage of 700 V the increase of specific flow rate from 0.4×10^-3 to 1.4×10^‑3 mg/(s×mm²) provides an increase of specific impulse by ~600 s.

So, when operating in high-voltage modes, to achieve the high specific impulse it is necessary to try to provide as high specific flow rate as possible. The comparative analysis of high-voltage Hall effect thrusters performed demonstrates the possibility of their competing with ion thrusters in terms of main integral parameters and specific characteristics.

From the analysis of the well-known EP thrusters it follows that a thruster that has high integral parameters and characteristics can be created if the following requirements are met and if design is improved in the following directions:

- for previously developed Hall effect thrusters (without changing their size), in order to increase their specific impulse to a value of more than 2500 s, their operation must be forced at the expense of increase of the operating power to a value of more than 2 kW with a provision of the guaranteed life;

- to achieve high specific impulses of more than 2500 s when operating in high-voltage modes with the voltage higher than 700 V, high specific flow rates of 0.8×10^-3 to 1.6×10^-3 mg/(s×mm²) must be provided in the acceleration channel;

- it would be more preferable to develop new prospective high-voltage Hall effect thrusters with the specific impulse higher than 3200 s on the basis a hybrid design that accumulates main advantages of various designs and physical processes of SPTs and TALs as well as a number of innovative technical solutions.