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Comment by JumpCrisscross | Hacker News Reader

> about...2,000 Saturn V rockets

Or about 670 BFRs [1].

It would take 6 cycles of tripling size and/or performance to get a single system with enough thrust to meet your 7/42 year schedule. The Saturn V first flew in 1967 [1]; SpaceX plans to launch its Interplanetary Transport System (ITS) in 2022 or 2024 [2]. That's an expected 60 years to triple performance.

[1] http://waitbutwhy.com/2016/09/spacexs-big-fking-rocket-the-f...

[2] https://en.wikipedia.org/wiki/Saturn_V

[3] https://en.wikipedia.org/wiki/Interplanetary_Transport_Syste...

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Though that kind of performance is probably not going to come about from improvements to chemical rockets but moving to fusion. There are some promising developments on fusion reactors (see Breakthrough in Nuclear Fusion? - Prof. Dennis Whyte https://www.youtube.com/watch?v=KkpqA8yG9T4)

Guess something like that with the 100 million C gas doing the propulsion maybe?

There may have to be some clever system where you don't take a lot of fuel with you.

Imagine a type of particle accumulator and aggregator - call it cosmic flypaper. If you're going say 100 km/s, things seemingly insignificant and rare you may be able to collect a lot of pretty quickly.

It sounds like you're talking about the Bussard Ramjet. It collects interstellar hydrogen gas for use in fusion engines. The problem is that it's unlikely there's enough hydrogen floating around for it to really work.

Are there places that would have high concentrations of floating hydrogen? You would just need to "fly" through those.

Sure: stars. I don't think flying through those would work out well.

There's also nebulae. But that's only going to be useful when you're actually in a nebula, which isn't most of the time.

> Are there places that would have high concentrations of floating hydrogen?

IIRC, a Bussard Ramjet is at least potentially viable at the typical density in the galaxy, but unfortunately for any use by humans to get away from our solar system, or local region of space (to a distance, IIRC, on the order of 1000 light years in any direction) is a pocket of relatively low density.

Running a direct output fusion rocket in the atmosphere would be tough-- all the reactors we know how to build, or can vaguely guess how to build, run at small fractions of one atmospheric pressure.

If you have fusion engines, you would just be doing all the construction in orbit anyway, rather than boosting stuff from the surface of the orbit.

To put that 60 years into perspective: It was 66 years between the first powered airplane flight and landing people on the moon.

These kinds of realizations make me sad that life is so relatively short.

It's also exciting, isn't it? That every generation is likely to experience some unique achievement?

Don't forget about space debris. Hitting anything at the posted max speed is just gonna dril a hole straight through your ship. If it's big enough, it'll add some nice shockwaves to really mess up your ship.

Note that GP was talking about the mass of two thousand Saturn V's, not the payload. It would take 20,000 BFR launches to put 6 million tons in orbit.

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