The main objective of the “Dual Mode and Electric Propulsion System Study” is to assess the impact of this propulsion system combination w.r.t. operational, system and programmatic aspects in the context of telecommunication satellites. In the study the OHB consortium has analysed and traded chemical Dual Mode system with classical bi-prop systems (UPS) in combination with different electric propulsion systems. This exercise followed a staggered approach:
- Market analyses
- Feasibility Study
- Identify potential propulsion system combinations
► Preliminary performance characterisation of different propulsion combinations for a mass range from 2 t to 8 t wet masses
► Identification of reference mission
- Subsystem Design and Operational Impact Analyses
► Validation of reference mission by
-AOCS performance analyses
► Identification of key technology
► Cost and MAIT Assessment
Key objective of the Feasibility study is the identification of a reference mission. Based on a computational based field study a platform has been identified for which a Dual Mode System in combination with an electrical propulsion system yield satisfactory results and which is further investigated. In conclusion, the reference mission will be a platform specific in size and propulsion system layout.
Key objective of the Subsystem Design and Operational Impact Analyses is the validation of the previously identified reference mission. This is done by evaluating the potential accommodation and by evaluating the AOCS performance taking the specific centre of mass shift into account. The main premise of this task is to allow only for minor modifications of current platform design since it is believed that the acceptance to change the propulsion system of a given platform is higher if changes in platform architecture (like AOCS concept) are minimal.
Key objective of the programmatic consideration is the characterisation of the hardware cost implications (RC). Despite potential benefits, a new propulsion system will only be attractive if recurrent costs can be significantly reduced.
The following key issues could be identified:
- A propulsion system consisting of Dual Mode in combination with Arcjets (for NSSK) is most competitive for small spacecraft wet masses between 3000 kg and 4000 kg.
- Hall Effect Thruster, either in combination with MON/Hydrazine or MON/MMH outperform Arcjets for higher spacecraft wet mass.
- The MON/Hydrazine propellant combination has a nominal mixture ratio of 0.85 which in combination with the respective propellant density which leads in all cases to disparate volume ratios (MON/Hydrazine):
- The ratio is 0.6, if the chemical propulsion system task is restricted to the apogee firing.
- The ratio is 0.5, if the chemical propulsion system task is restricted to the apogee firing and NSSK is performed with Arcjets.
- The ratio is 0.3, if the chemical propulsion system includes apogee firing as well as station keeping tasks (i.e. full chemical platform).
Current European platforms are not designed to comply with this disparate volume ratio and a detailed accommodation check is necessary.
- Due to the high thrust level of Arcjets, NSSK manoeuvre are only required once a week. For the studied reference mission a single manoeuvre lasts about an hour and can be powered by the battery without causing a redesign.
The use of a Dual Mode System in combination with Arcjets for NSSK has the following benefits:
- Arcjets and Dual Mode chemical systems are both propelled with Hydrazine, leading to a simplified propellant management system.
- Arcjet operation is less complex than that for HETs or GITs.
- Reduced development costs.
- Reduced integration costs.
- Weekly operation reduces operation cost.
- First estimate indicates that the recurrent costs of an Arcjet system are lower than for a HET system.
The selected reference configuration consists of a spacecraft wet mass of 3 - 3.5 t and a Dual Mode propulsion system in combination with 4 Arcjets for NSSK. The wet mass regime of the reference mission is congruent to OHB’s SGEO platform.
The study has identified a range of tank sizes that only partially exist in Europe. For instance, equatorially mounted MON-tanks in this volume regime and the required diameter do not exist currently in Europe.
A spherical MON tank from ATK could be identified that fulfils the requirement regarding volume and partially regarding the diameter.
Both the chemical as well as the electrical propulsion system use Hydrazine as propellant. Considering that the current SGEO design foresees additional Xenon tanks outside the cylinder, there is the possibility to accommodate additional Hydrazine tanks and thus increasing the spacecraft’s life-time.
In order to comply with the study objectives and to not pre-exclude a platform mass or a propulsion combination, a staggered evaluation approach has been applied by OHB System:
- Definition of different conceivable propulsion systems which are in line with the statement of work: DM and Arcjets, DM and HET, DM and GIT
- Definition of mission tasks to be performed by each propulsion system. Orbit topping by the electrical propulsion system was excluded in any case, leaving the following options: EP for all GEO tasks (NSSK, EWSK, RW off-loading) and EP only for NSSK
- Evaluation of a wide range of spacecraft wet masses (2 t – 8 t).
The first three steps yield a large parameter space consisting of propulsions system combinations, use of EP system and spacecraft wet mass. Each configuration has been evaluated with respect to the potentially achievable payload mass. As a result, a promising configuration (hereafter called reference platform) was selected which was considered sufficiently competitive.
- Accommodation check of propellant mass into the selected reference platform.
- Operational performance check of the reference platform.
- Programmatic considerations.
The study has shown that a Dual Mode (MON/Hydrazine) propulsion system is a feasible propulsion combination for a platform range of 3000 kg to 4000 kg. It leads to similar performance figure regarding wet mass and payload mass. A ROM cost estimate revealed a remarkable advantage in recurrent cost for the studied propulsion system.
Furthermore, several areas of improvement could be identified bearing the potential to increase the technical performance and reduce cost. Therefore, follow-on activities are recommended:
- Integrated PCDU-PPU design
- MON tank with suitable load capacity
- Configuration adaption to increase lifetime medium size telecommunication platform with Dual Mode & Arcjets to 18 years
- Investigation of orbit raising (GTO to GEO) with Arcjets