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Comment by jccooper | Hacker News Reader

Propulsive landing has been possible since the 70s. But cheap and easily available sensors (especially but not exclusively navigation--gyroscopes, accelerators, altimeters, GPS), cheap and light high-performance compute hardware, and existing packaged software and other advances in stuff like convex optimization and fluid dynamics make a real difference in practicality of doing so, both on the rocket and during design.

There's also significant improvements in metallurgy that make it easier to do things like long-life turbopumps, carbon-fiber composites help with mass fraction, CNC machining and additive manufacturing that make complicated parts affordable, and other such things that also help. But really it's pocket-sized sense-and-compute that really kicked off the low-cost retro-propulsion thing.

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Also, SpaceX being the first to demonstrate it was feasible. There were naysayers even after they landed their first booster in December of 2015. ("refurbishment will cost too much!")

Now that SpaceX regularly flying a booster 5 times or more and have at least 10 boosters in the fleet, the only people arguing that reusable space hardware is not worth doing are people with a vested interest in old-space-hardware designs.

That argument really doesn't work. Most of those censors are needed in orbital rocket too.

Shuttle software was far more complex then Falcon 9 and it runs on 70s computers.

GPS is not really needed, its just there as a backup. Launching on a ship might be tricky, but on land you can target a pretty big landing area.

> advances in stuff like convex optimization and fluid dynamics make a real difference in practicality of doing so, both on the rocket and during design.

True, but if you do more iterations on first stages that you drop into the ocean anyway, you can build design and iterate.

Shuttle didn't fail because of they could figure out reentry conditions.

> There's also significant improvements in metallurgy that make it easier to do things like long-life turbopumps, carbon-fiber composites help with mass fraction,

SpaceX uses very little carbon fiber.

There is truth to the improvements in metallurgy but its not metal issue that caused the RS-25 to not work well in practice is not really the metal.

> CNC machining and additive manufacturing that make complicated parts affordable

While true, that makes the benefit of making it reusable even more worth it.

It seems to me that SpaceX simple had the right design and the right setup to test and iterate. There was nothing fundamentally stopping NASA from doing this instead of the Shuttle. Nobody else even tried to make a reusable rocket.

I would argue this could have been done in the 70s and it would have worked nearly as well.

Thanks - sounds like it was a lot of things but mainly low cost compute and sensors. Not to mention Elon's deep pockets and appetite for risk.

Yes; someone willing and able to try was also a missing ingredient. None of the incumbents seemed interested in trying anything particularly different (and for the most part, still aren't) so who knows how long we'd have waited for a new stab at reusable launch without SpaceX.

Nit: propulsive landing was not only feasible but actually done in the late 1960s. On a human-rated Single Stage To Orbit vehicle, no less.

Okay, fine, so it wasn't to Earth orbit, but still...!

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