Frequently Asked Questions
Q1. What is the HERMES Quick disconnect "QD" ?.
This is a special fluid line connector designed to interface on the one side with the “Fill and Drain Valve” of a satellite and on the other side with the fuel connector of the service vehicle (Utility Agent), for enabling fluid transfer without any leakage during the transfer, and also importantly no leakage during the disconnection process (when the service vehicle disengages from the client).
Q2. Why the HERMES Quick disconnect is so important ?
There are two reasons:
A: The standard "fill and drain valves" of the satellites have a safety cup which under the space environment (high vacuum) becomes extremely tough to unscrew. Friction between metals in space becomes extremely harsh (10 to 30 times that on Earth) and that would require excessively powerful activation motors on the side of the service vehicle. To make the problem worse the cup usually has a safety pin that would need to be cut and further secured into a safe place to avoid debris. However, even that is not the end of the problems, as prolonged stay of the satellite in space may result in the so called "cold-welding", ie the welding of the similar metal parts of the housing of the valve with the safety cup. This is a desired effect from safety perspective and valves are engineered so they may employ this characteristic, despite that the cup is required only during the difficulties (high vibration) of the launch.
B: The standard "fill and drain valves" of the satellites have a void as large as a 1 cm3 that fills with the fuel during the filling process, and this volume is isolated beyond the closing gate of the valve. This fuel volume needs to be purged before disconnection occur, otherwise spillage will occur. On the ground, at the filling facilities there are special equipment employed for the purging exercise but that equipment although simple can not be afforded in space due to their large mass. Failing to contain the spillage and instead flushing that amount of fuel in space would contaminate the sensitive optics of the servicing vehicle.
Q3. How many times one can refuel a satellite ?
A limitation is imposed by the Quick Disconnect, and this is 8 times. However, this is a theoretical upper limit. In practice a more modest number is envisaged. See below for recommended practice.
Q4. What are the benefits from fueling a satellite in many rounds ?
There are a number of direct and indirect benefits.
In the direct benefits included: The sizing of the fuel storage subsystem may be divided by 8 (or so), with respect to common sizing. From the financial perspective this means that one does not have to pay upfront, far ahead of utilization, the costs associated to an oversized fuel storage subsystem (fuel cost, tank size cost, supportive structure cost, launch cost, insurance cost).
As regards the indirect benefits, we observe that a smaller fuel storage subsystem and smaller satellite mass will require smaller MGSE and facilities for manufacturing and testing (vibration bench, Thermal Vacuum chamber, transport container), delivering savings at construction and transportation. Furthermore, the design budgets of the torquers become leaner as regards their mass and power requirements. Lastly the various direct and indirect savings in mass (on fuel storage subsystem, structure, torquers, power subsystem) have a saving effect on fuel consumption during operation.
Q5. What is the optimum number of fuelling rounds ?
Due to the fixed costs associated with an in-orbit refuelling mission the fuel budget should not be split excessively unless consumed for unforeseen reasons. Ideally on the 10th year of operation a satellite shall receive fuel equalling for additional 5 years, and that could be further repeated. However, circumstances (imperfect launch) may require refuelling at earlier stage, even at day 0 of a satellites life in GEO, like we have witnessed recently with the needs of RASCOM-QAF1, AMC-14, and earlier with Astra-K1, ARABSAT-1A etc. On average one satellite per 9.5 months suffers from premature fuel depletion due to an unforeseen event.
Q6. What are the unforeseen events that refuelling may be the "show saver" of ?
A. Imperfect launch. Any stage of the launcher may malfunction or underperform. This could result to a minor or significant deviation from the planned apogee of the GTO transfer. Consequently the satellite shall itself compensate during the GTO to GEO transfer firings, consuming more fuel than planned. Occasionally the required compensation depletes the totality of the fuel of the satellite resulting to having a brand new satellite on orbit without fuel for routine operations.
B. Imperfect performance of the propulsion subsystem of the satellite. Due to the fact that the satellite has itself to perform the GTO to GEO transfer any underperformance will result to higher fuel consumption. We have seen recently the case of RASCOM-QAF1 which had to use the attitude control thrusters for this (GTO to GEO) manoeuvre, due to problem in the high pressure regulator, which precluded the operation of the more efficient LAM (Liquid Apogee Motor). Result of this deviation to normal was the higher consumption of fuel by the attitude control thrusters and premature deplition. The satellite claimed a total-loss compensation from insurance.