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Comment by evgen | Hacker News Reader

If the rocket equation is such an unforgiving bitch then why are you only using two stages to get to orbit? By your own logic you should be using four or five or even ten stages to get to orbit because otherwise you are hauling up too much unnecessary mass on each flight. What makes two such a magic number in this case? Is it perhaps possible that there are in fact real-world tradeoffs between efficiencies in the rocket equation and engineering parameters and that sometimes advances in technique or materials shifts the optimum point?

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The more stages, the less favorable your ratio fuel+propellant / dry mass. When you add stages, you add engines, tanks, systems... that you then have to haul up to the next stage, making your lower stages heavier. Rocket equation again.

Sure it's possible that if you had super lightweight material for all the dry mass, 3 stages would make more sense.

You're right, there is a "real-world tradeoffs between efficiencies in the rocket equation and engineering parameters". This trade-off is unfavorable to SSTOs.

It's because the rocket equation really bites when you get far out at extreme mass ratios, as in an SSTO. Very small changes in mass of systems have a large effect on cargo, and perhaps keep you from even getting to orbit at all.

Viewed another way: if you had an SSTO, you could radically increase its payload to orbit by adding a really dumb first stage. Even a small delta-V from that stage would make the job of the "SSTO" part much easier. And SpaceX has shown that such a first stage can be straightforwardly made reusable, especially if the delta-V allows return to launch site recovery. This basically destroyed any remaining argument for SSTO.

The benefit from going from 2 to 3 stages is much less.

It typically takes two to get the third stage up there, and only the third stage stays in orbit. Alternative shuttle designs, BFR and New Glenn start at two but you wouldn't want to carry around all that extra weight on a second stage unless you had a very specific mission set (launch the ISS in a couple of missions or have a stage that goes and lands on Mars). Not optimized for typical payloads.

IIRC, a lunar probe launched a couple years back on a 5-stage solid rocket. That works well because, essentially, the engine is the fuel.

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