John Carmack,
Have you considered the van control system used by Robert H. Goddard? The advantage to his system is the vanes would be most of the time out of the jet, thus less erosion and less ISP hit. The only disadvantage is it would only work for pitch and yaw control, no roll. However for a multiple engine system the vanes could be configured to also then control roll.
Roll control is critical for a longer burn time vehicle. Multiple engines is something we are trying to get away from. If we have another engine, we want to get full redundancy out of it, not make it a necessary part of the control system.
Erosion is a non-issue for our current propellant, and if we move to a hotter propellant later on, I would be inclined to make refractory jet vanes. Cutting and machining those would be a lot easier than the refractory chamber we made for our biprop work, and the temperatures are a lot cooler past the nozzle, so I expect they could run indefinitely. Good refractory coatings are something that was developed in the 60's, after launchers had already moved away from jet vanes.
Also for anti slosh baffels have you considered using some telescoping tubing mounted on the end plates of the big tank and on the outer tube mount some rolled sheet metal with perforations so that the rolled parts would spring out when the assembly was slid into the access port? This would allow a large area baffle to be installed without having to deal with the access problems of the wound tank. My concern is that sloshing will couple with the flight control in a distructive open loop condition with a significant partial fuel load.
Since we are reworking the tank bulkhead for a central engine, we are going to weld some small (6" tall) anti-swirl vanes on the bulkhead tomorrow. If we find slosh to be an issue, our easiest correction will be to float a bunch of stainless steel balls in the tank, which is effectively what they did on Vanguard.
John Carmack
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