fta: Falcon 9 uses an open hydraulic system that has a separate tank of RP-1 (which is used as the hydraulic fluid) pressurized by Nitrogen near the interstage, which, after use, drains down back into the main RP-1 tank for reuse by the engines. There are a variety of reasons that SpaceX made decisions to use this design: 1. A closed hydraulic system separate of the main RP-1 tank would require a pump to re-pressurize the RP-1 for reuse. This adds weight and complexity to something which you really dont want to make much heavier or more complex. 2. A closed hydraulic system that uses the RP-1 from the main tank is also infeasible as it would require a pump to push RP-1 up from the tank near the engines right to the top of the vehicle. There is no easy way of doing this. 3. Switching to an electromechanical system would require a very large amount of power to operate, which would require an impractical amount of batteries. All three of the above solutions require pumps to be active or some energy storage mechanism - not very attractive or really suitable for Falcon 9 as the engines are only burning over a subset of the return trajectory. The grid fins are deployed at approximately T+5 minutes. This is before the reentry burn takes place, which does not last for very long. For the majority of the time the grid fins are deployed, Falcon 9 is in free flight. SpaceXs solution is rather clever, actually: 1. It does not require power from the engines to operate. 2. It doesnt involve a complex plumbing solution which adds weight to lift the RP-1 up from the bottom tank 3. It doesnt add much mass beyond the pressure vessel, since the RP-1 is free as it can be reused by the engines. 4. Overall, it weighs less than a similar amount of pressurized Nitrogen cold gas to steer.
Posted on: Sat, 17 Jan 2015 03:46:14 +0000
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