PAGE CONTENTS
Objectives
Despite technology improvements, the current generation of European bi-propellant reaction control thrusters are not optimized to meet the increasingly stringent spacecraft pointing requirements. Significant improvements to the performances of the current sub-systems may be obtained with a lower thrust reaction control thruster with a smaller and more robust minimum impulse bit capability.
The minimum impulse that can be delivered by a thruster is a function of the minimum achievable pulse duration at a certain thrust level. The minimum impulse bit (MIB) is a measure of the smallest control torque that can be commanded to the satellite using the thruster. The steady state thrust level of a typical European bi-propellant RCTs is 10N. These engines have been widely used with on-times as low as 5 ms to give a MIB for fine pointing control. In this regime however the engine combustion is incomplete. This leads to a disturbing increased build-up of plume contaminants. However, the trend towards increasing antenna size and smaller spot beans in telecommunication platforms is driving the need for a continuous improvement in the spacecraft pointing accuracy.
Two options are available at system level to address this issue: Change the accommodation of the current 10N RCTs to reduce the moment arm, or implement a lower thrust RCT with a lower impulse bit in the same accommodation. From the system point of view the latter option is to be preferred.
As the present study demonstrates, current RCTs can be down scaled to meet this need. Low thrust RCTs have been validated in the past. Therefore, thrusters of this kind are expected to represent the natural evolution of current designs. Indeed mixed thruster configuration in general is a re-emerging issue as the first generation telecoms satellites had mixed thrust level RCTs. Optimizing the thruster selection for its location and required operations on the spacecraft (Station keeping, torque control, apogee back-up or pointing) could present opportunities to optimize budgets with a small impact on the spacecraft configuration.
Plan
In agreement with ESA’s SOW, the work was divided in following main tasks and phases:
Current Status
Demonstration test completed (vacuum hot firing).
Figure: S4-3 EM1’s maiden ignition
The S4-3 EM1 design demonstrated a very effective and steady behaviour over a wide inlet pressure range, for both, steady state and pulse mode firing. The thruster reached a specific impulse of up to 298 s at 4.8N steady state thrust. In its lowest impulse range, the S4 demonstrated up to twice the efficiency of a S10 thruster for a 15 mNs impulse.
