>>11592600For launching into low Earth orbit, you only want two stages. I'll get into the complexities if you want to read them, but rest assured that three stage to orbit doesn't get you any gains.
Okay so, some basic rocket science.
Due to the nature of chemical reaction energy densities, propellant volumetric densities, engine design etc, it gets really hard to build a single rocket stage that can push its own dry mass plus a payload mass fraction of 5% up to a delta V of beyond 6 km/s. Getting into orbit around Earth requires a minimum of about 9.5 km/s. Therefore, a single stage launch to Earth orbit is totally impractical, let alone a reusable SSTO, which is full retard. However, with two stages, not only can you get into orbit, you can actually get into orbit using easier to build (more massive) structures, and with a bigger payload mass fraction. Considering three stages however, each stage could have a 60% propellant mass ratio and you'd still be putting comparable payloads into orbit as the 2STO. The problem is, you're now building and stacking three stages, and nobody is so hampered by manufacturing problems that they need to build a rocket that is 40% dry mass.
So without considering anything else, 3STO is not optimized for launching into Earth orbit. If Earth required more delta V to get into orbit, then 3STO would make sense. There are other reasons why 3STO isn't as big an improvement as 2STO, though. For one, in the 2STO case, the second stage is already pretty much in vacuum once it ignites, meaning you're already using engines optimized for vacuum. That's one reason why 2nd stages tend to impart much more delta V than first stages. Another reason is that since 2nd stages are dropped off already moving fast, they don't need high TWR to work, so it makes sense to give them oversized propellant tanks to increase burn time. The list of things goes on.
Where a 3STO works is if you're going significantly beyond low Earth orbit. Pic related.
