Comment by creshal | Hacker News Reader

Comment by creshal | Hacker News Reader

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All SpaceX did was building a normal rocket that can land again.

Which, as it turned out, was anything but trivial and took 14 years. Their main competitors – ULA and Arianespace – already announced they won't be able to manage similar capabilities this decade; and their plans for the next decade are still not as sophisticated as SpaceX's approach (they only want to recover the engines and build a wholly new stage around them).

Skylon is a massively more ambitious undertaking compared to that – if it works, it'll be the first practical SSTO, the first horizontal takeoff spaceplane, the first vehicle to use multi-mode engines, the first practical space plane to use a novel heat shield technology, …

That's an awful lot of challenges to tackle. If it turns out to work, and if it then turns out to be as re-usable as hoped (something that did never panned out for the Shuttle, and which SpaceX will only learn over the next years for their Falcon), then it could beat SpaceX… in a decade or two.

The trouble with Skylon is that all of these things you mention make it more ambitious, but they don't really make it more useful.

The main advantage Skylon would have over the Falcon 9 is the fact that the Falcon 9 is not fully reusable because they don't and can't recover the second stage, while the entire Skylon is recovered.

That's significant, of course. The second stage makes up about 25% the cost of the rocket, so that places a ceiling of about 75% on how much Falcon 9 reusability can save..

But then, starting from where the Falcon 9 is now, building a new rocket that can also recover the second stage doesn't seem too difficult. It's a big challenge, to be sure, but relative to Skylon it's not too bad.

Skylon is certainly much more ambitious in terms of how it works, but it doesn't seem to be very ambitious in terms of what it can do, which is not a promising combination.

Skylon does have the advantage of burning atmospheric oxygen at low altitudes, meaning it would have a significantly higher payload fraction than the Falcon 9.

It's somewhat higher, but not dramatically so. Skylon is projected to be 325 tons loaded with a LEO payload of 15 tons, Falcon 9 Full Thrust a.k.a. Falcon 9 1.2 is 541 tons loaded with a LEO payload of 13-17 tons depending on who you believe. And that's comparing paper hardware with flying hardware. Spacecraft tend to get heavier as they go from paper to reality.

Plus, payload fraction is another thing like SSTO, horizontal takeoff, novel heat shields, and all the rest that are cool but just don't matter except for how they affect the end result. All that matters in the end is how much you can launch, and how much it costs.

>And that's comparing paper hardware with flying hardware. Spacecraft tend to get heavier as they go from paper to reality.

That's the critical point. Skylon is still vaporware, and they've really assumed the best case in published numbers.

According to specs it'll reach 1600 m/s on atmospheric oxygen.

So, afterwards, it needs another 8000 m/s Δv on rocket engines to reach LEO.

Skylon is a neat trick, but it's not magical.

I think the Shuttle design get's a bad rap. The cost savings from scrubbing a mission where minimal and if it had flown more missions then it's cost per mission would have dropped. Cost per mission would have also dropped if they had canceled it sooner and not waited 3 years after challenger. IMO, the real issue was Congress turned the thing into a jobs program.

NASA on the other hand had lot's of issues.

Challenger: "[W]e're only qualified to 40 degrees ...'what business does anyone even have thinking about 18 degrees, we're in no man's land.'"[2]

Columbia had 27 successful missions and went though several redesigns one of which resulted in failure.

> if it had flown more missions then it's cost per mission would have dropped.

How, when the cost was limited by the refurbishing costs after each launch?

Costs where not only based on refurbishing costs, as they included R&D and ongoing capital costs. See: upgrades https://en.wikipedia.org/wiki/Space_Shuttle_main_engine Not to mention building but never using an extra engine, Ground crew etc.

https://en.wikipedia.org/wiki/STS-61-B Had a 54 day turnaround time which is not 'great' still. That's 6+ short missions per year per orbiter call it 5 including missions and some unscheduled down time.

They started with 6 orbiters * 5 missions a year ~= 30 missions per year. The shuttle actually did 135 over 30 years or 4.5 per year * (~0.75 per orbiter per year).

PS: At even 1/2 that 30 mission per year rate they would have finished 135 missions in under 10 years.

> as they included R&D and ongoing capital costs. See: upgrades https://en.wikipedia.org/wiki/Space_Shuttle_main_engine

If your engines burn out after 5 instead of the planned 55 flights, I damn well hope you're going to invest into more R&D on them!

> Had a 54 day turnaround time which is not 'great' still.

And the pressure to keep up that break-neck pace directly led to the Challenger disaster.

I am not going to defend Challenger. IMO, it was more an issue with NASA than the shuttles design.

Still, the point is they did not need to wait over a year reusing launches from design constraints. Instead, NASA simply did not need that many manned launches per year.

Now, I am not saying it was a great or even that good of a design. It was simply a poor fit for what they used it for. NASA would have been better off with either a much smaller shuttle that only sent crew up, or an unmanned system for hauling cargo. Instead, they tried to do both which caused a lot of problems.

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