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Comment by pfdietz | Hacker News Reader

Making those things easier is a lot simpler than making SSTO work.

We thought that reusing the shuttle would be easy and that relanding boosters would be impossible and, yet, we were wrong both times.

Some things are impossible until they aren't.

No these are fundamentally different. SSTOs make things expensive because the rocket equation is unforgiving: every kilogram you haul into orbit needs fuel and propellant, which in turn makes the rocket bigger and heavier, which in turns requires more fuel and propellant, which in turns... So, you don't want to bring up anything unnecessary, like taking your first stage into orbit SSTO style.

Landing boosters is an incredible engineering feat. The rocket equation is just derived from the laws of physics, it's not something you can change with great engineering.

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?

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.

I didn't say it'd be easy, but that's why people continue developing new materials and engines. There is a point where it becomes practical.

Technically, making those things easier also makes SSTOs work since optimized multi stage orbits let us get to the moon where SSTOs are the default option to get off of the lunar surface. In a sense SSTOs are part of the current earth to moonbase to Mars strategy.

SSTO on the Moon or Mars is a lot easier.

And also already accomplished on the moon. The Apollo lunar module was SSTO during takeoff from the lunar surface.

And make a ton of sense since there be zero to none surface support for relaunching.

You'd probably still need to carry propellants, but yes, an Earth-capable SSTO should be able to hop around a lot on the Moon before needing to refuel.

Land-anywhere and launch-anywhere are also very attractive features for places with sparse support.

I wonder if SpaceX could build a demonstration landing pad, ferry some liquid methane and liquid oxygen to, say, Guam (which is kind of part of the US, making things a bit easier) and do a suborbital hop across the globe with the starship alone without the booster, refuel, and fly back to Florida.

Or, a second stage of a two stage earth-to LEO launcher. There's no reason to make the earth-to-orbit part single stage, just because reuse in space of that upper stage would be SSTO.

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