> Bigger tank does mean less payload because of the rocket equation (you now have to lift the bigger tank and the extra fuel, which takes more fuel, or less pay

> Bigger tank does mean less payload because of the rocket equation (you now have to lift the bigger tank and the extra fuel, which takes more fuel, or less payload). For fixed gross mass. The point is, for a given payload mass fraction, you can increase the gross mass of the system to increase payload. And indeed for larger systems, you get a subsystem mass amortization effect that tends to decrease the dry mass fraction. > I wonder what the physics looks like for landing a linear spike. One of the problems that SpaceX mentions in their voiceover work for landings is that the thrust for the rockets is so high that even at the lowest thrust it's almost too much to land. The difference in nozzle performance shouldn't change this much at all. To make this problem more tractable, you need more net throttling capability. The problem is the gross mass at liftoff vs. at landing. When you size the engine(s) for liftoff mass, it's difficult to throttle them down enough to keep Thrust / Weight (T/W) low at landing when the system is not much more than first stage dry mass. This is especially true when a propellant is liquid as it leaves the main injector (such as with the Merlin, injecting liquid RP). Gas/gas injection (such as with an expander cycle) can help to some extent. See the CECE testbed engine for an example. As per my comment about the prospective AFRL project, one of the ideas that tends to go along with contemporary aerospike engines is the idea of modular thrust chambers of smaller size. Depending on how granular the turbomachinery is and the chosen cycle, this approach could potentially permit deeper throttling. The XRS-2200 engine for the X-33 was designed with this modular approach. One system-level benefit for the aerospike is the ability to use an "easy" cycle, like gas generator, and use the gas generator exhaust to improve the aerospike performance, such as by plug base pressurization. This is more constructive than what is done in conventional bell nozzle systems with gas generator exhaust. Linear spikes exacerbate the mass inefficiency problem, BTW, and are not likely the way any operational aerospike engine will be designed in the future. Also, when you think of throttling modular thrust chambers for steering, realize that you are either reducing the net thrust coming from the engine, which is not what you want to do, or you are jacking up chamber pressure on some modules while reducing it on others. Also not something you want to do, as if you have the capacity for higher chamber pressure, you want to use it for the whole flight, not just leave that mass margin there for steering. A number of studies have been done on steering using fluidic or hinging/flapped aerospike configurations, and again, it's hard to beat old-fashioned gimbaling. Yet another system-level reason why bells continue to be the status quo.