Some form of orbital maneuvering engine is a must, you can't use the main propulsion system for thouse kinds of maneuvers. Draco (not super) might do the very fine maneuvers for docking and such but you will still want something stronger for a de-orbit burn.Lets compare to shuttle, it massed around 75 mt on orbit and had OMS with 53 kN thrust. Scale up to a likely 200+ mt mass in orbit and your looking at the thrust of 2 Super Draco engines for a de-orbit burn. A Raptor engine would need to throttle down to 6% to give that thrust. Still we would be looking at about a 6 fold increase over a reasonable orbital maneuvering system so it's not free. Their may be additional benefits to these engines, their position high on the vehicle gives them huge leverage on the vehicle useful in EDL if a flip over is needed.
Quote from: Impaler on 07/14/2016 11:32 pmSome form of orbital maneuvering engine is a must, you can't use the main propulsion system for thouse kinds of maneuvers. Draco (not super) might do the very fine maneuvers for docking and such but you will still want something stronger for a de-orbit burn.Lets compare to shuttle, it massed around 75 mt on orbit and had OMS with 53 kN thrust. Scale up to a likely 200+ mt mass in orbit and your looking at the thrust of 2 Super Draco engines for a de-orbit burn. A Raptor engine would need to throttle down to 6% to give that thrust. Still we would be looking at about a 6 fold increase over a reasonable orbital maneuvering system so it's not free. Their may be additional benefits to these engines, their position high on the vehicle gives them huge leverage on the vehicle useful in EDL if a flip over is needed.Couldn't you just use a single Raptor and fire it for a shorter time period? What type of delta V is needed for a de-orbit burn typically?
Another concept I just considered, with a small BFS and a larger second stage without using SEP. Launch with just cargo in the BFS and all propellant in the 2nd stage, the two stay together and reach orbit much like a Dragon capsule and it's trunk. Then both are refueled to full, TMI is conducted by firing the 2nd stage for ~2 km/s of acceleration then the BFS separates and performs the remaining boost. This leaves the 2nd stage far short of Earth escape and it will be in an elliptical orbit which can easily allow it to land again.Another 2nd stage without a BFS attached is also placed in orbit and refueled to make a TMI on it's own with a propulsive insertion at Mars, by utilizing a slow hohoman transfer the propellant delivery is much more efficient. BFS then rendezvouses with it in orbit and takes on the necessary propellant for TEI. The 2nd stage now very light now returns to Earth via a slow transfer and aero-captures at Earth. The 2nd stage would be capable of this kind of total DeltaV because it is almost nothing but tank so a 4-5% dry mass fraction is reasonable for it (far lower then what the BFS could achieve). The main challenge is endurance and maintaining propellants against boil-off for that length of time, but any vehicle that waits in LEO while it is being filled up will need considerable insulation so this 2nd stage can perform the job of tanker to LEO, depot in LEO, tanker to Mars and heavy lifter to LEO for the BFS and other payloads.I see this stage being around 60 mt in dry mass with 1200 mt of propellant capacity and equipped with solar and radiators on the surface as on the Dragon 2 capsule.
It is not "considered dangerous", it elevates the risks for mission success. It's a statistics and systems analysis thing and prior performance of the general principle of docking is probably only marginally relevant to a specific system, in this context. Three docking events present more risk than two identical docking events, even if you eventually manage to pull both flawlessly, 100 times each.