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Actually, liquid oxygen is almost 40% denser than kerosene (1.14 vs 0.82 g/cm^2). And this is the reason most rockets [1] put oxygen on top/forward: they want t
by yaakov34 11y ago
Actually, liquid oxygen is almost 40% denser than kerosene (1.14 vs 0.82 g/cm^2). And this is the reason most rockets [1] put oxygen on top/forward: they want the center of gravity to be as far forward as possible, ahead of the aerodynamic pressure center, so that the rocket will be as stable as possible when ascending through the atmosphere. Just as we would put a weight on the front of the arrow, and feathers in the back, to make it more stable.
For the vertical landing, this would seem to work against them, but I guess the mostly empty tanks don't dominate the considerations. Falcon has oxidizer on top in the conventional manner, as far as I know (they mention LOX feed lines going through the fuel tank).
[1] The N1 rocket had oxidizer below the fuel in every stage, apparently because they wished to use spherical tanks, and this allowed packing them in a conical shape - the oxidizer tanks are much larger.
- JshWright 11y agoYeah, you're absolutely right. Not only is it denser, but they carry more of it (due to oxidizer:fuel ratios and to account for boil off in-flight). No idea what I meant by "lighter"... I'll blame it on a pre-caffeinated state. It is however true that they can vent it pretty quickly though, leaving the only remaining propellant in the bottom tank.
- stcredzero 11y agoFor the vertical landing, this would seem to work against them Actually, an inverted pendulum -- where most of the weight is on the top -- is easiest to balance on top of the rocket thrust. Coming down at high speed, this would be aerodynamically unstable, but this is why they have the grid fins, to move the center of aerodynamic pressure behind the center of gravity.