## 5 Single Satellite Pair System Engineering Requirements
In the all-electric satellite design, the drag compensation can be enabled by a system of gridded ion engine thrusters of two types, drag control thruster (DCT) and fine control thruster (FCT), which operate in different thrust ranges. The engineering requirements envelope has been derived for each type of thruster in terms of thrust throttling range, specific impulse, total impulse, response time, noise and beam divergence, under ceiling requirements on total propellant mass and total power demand. These are reported in Table 8 as derived in Phase 0. These will be critically reviewed together with the optimization of the NGGM/MAGIC orbits during the NGGM Phase A. Thrusters meeting these requirements have already been demonstrated in [53], and FCT backup options encompass flight-proven technologies such as cold-gas thrusters (flown, e.g., on LISA Pathfinder and GAIA) and promising propulsion technologies under development and qualification such as indium-fed FEEP [54]. For the NGGM application, two 15 mN-class DCT thrusters, in cold redundancy, provide the main force components for the in-line in-band (1 to 100 mHz) drag compensation, formation orbiting control (at frequency below 1 mHz) and orbit maintenance. Two candidate implementations compliant with the requirements exist, and both have flight heritage: The first is the GOCE T5 ion thruster, which can serve--in principle--the same purpose in NGGM. The main difference is that GOCE operated in the thrust range from 70 \(\upmu\)N to 20 mN, whereas NGGM requires a thrust range extended at the lower bound since it will orbit considerably higher than GOCE (above 340 km as opposed to 260 km), at the expense of a lower upper thrust limit. Two operating regimes have been specified for the DCT:  
1. Dynamic (throttled) thrust range between 60 \(\upmu\)N and 8 mN for the science operations;
2. Steady-state (unmodulated) thrust of at least 10 mN for the orbit operations such as formation acquisition, collision avoidance and altitude trimming. A cluster of proportional mN-class FCT microthrusters compensate the drag forces into the cross-track and radial directions and the perturbing aerodynamic torque (for angular drag control purposes), as well as providing attitude and pointing control. The minimal configuration of 8 thrusters offers no redundancy, whereas the fully redundant configuration comprises 16 thrusters. Configurations with 8 thrusters plus 1-2 cold-redundant thrusters have been studied, and other options are possible including 10 operating thrusters with compliant operation in case of one failure and gracefully degraded operation in case of two failures. While the specified minimum operating thrust range is between 50 \(\upmu\)N and 1 mN, a reasonably efficient operation below 50 \(\upmu\)N and above 1 mN is highly desirable. The optimal configuration and operation of the FCT system have been intensively studied in the last few years and are still under critical assessment in the MAGIC/NGGM feasibility studies. The following stringent system constraints apply:  
* The propellant mass for DCT and FCT in combination shall not exceed 100 kg;
* The total subsystem peak power for DCT and FCT shall not exceed 350 W;
* Both spacecraft shall have an identical thruster layout, keeping in mind that the forces acting on the leading and trailing spacecraft are not identical, while robustly maintaining the laser link;
* One or two redundant thrusters shall be able to cope with the failure of any one of the nominal eight thrusters;
* The total impulse per thruster shall not exceed the demonstrated lifetime of the candidate technology. The technology readiness of the entire subsystem, as opposed to the thruster alone, will be a crucial selection criterion during the feasibility studies. Details on the configuration, thruster performances and optimal solution for the redundancy problem are provided in [51; 53].