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Comment by WilliamDhalgren | Hacker News Reader

Why? Its supposed to be significantly more efficient, given that its breathing air for launch which enables much higher specific impulse than anything a chemical rocket even could do, and it'd have the operational advantages of being a pure single stage to orbit skyplane, which SpaceX also can't do.

Hard to imagine what else one could even want a launch system to do (except perhaps to be a nuclear turborocket ala http://www.youtube.com/watch?v=C46Dt-X0V8c , if that were politically feasable). And you can hardly say that of anything SpaceX is doing, given that those are simply incremental improvements of existing rocketry tech.

Presuming they ever manage to make it work (which is prob the big question), it should bring significantly more to cheap launch than SpaceX ever could on their current trajectory.

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The real question is why would you want to waste payload capacity hauling an aeroplane into orbit.

Skylon uses that extra efficiency to haul extra components into space. Such as the wings, landing gear etc. Based on the rocket equation it will only have usable payload if the mass of the craft can be kept very low. This would require lots of new technology and materials. Even with a better engine that would be very difficult.

The thing is that all that new technology could also be used by Spacex to improve a conventional rocket. And without the extra mass you would have have much higher payload. And the payload could be used by customers and not wasted on wings.

The best way to use new engine is probably to strap it onto a conventional rocket. Less complex and more payload. And maybe that extra payload could allow second stage reuse. That would make things even more competitive.

"Based on the rocket equation"

I understand the whole point of this is that the rocket equation, assuming as it does that you'll be carrying all the fuel you need with you right from the start, doesn't apply until you get to about 28 km up, and once you get to that point you're not starting from zero speed; you're starting at about mach 5.

Anyway, on the the answer to your question. The projected outcome is that the cost per KG of payload into orbit is less. So that's why. Because it will be cheaper.

The difference to a conventional rocket is that you have more fuel left at 28 km up. A vertical launch rocket with this kind of engine would also have much of the same benefit. And with none of the extra weight, complexity, and cost of wings, landing gear etc. And that first stage could be reusable based on the SpaceX model.

So why not use the extra fuel to increase payload or have a reusable second stage? This could reduce costs for conventional launch companies, and make Skylon less viable.

The projections would have to make assumptions about development costs. That seems rather hard to predict with any accuracy. Especially considering how revolutionary every part of the craft would need to be.

> And with none of the extra weight, complexity, and cost of wings, landing gear etc.

> And that first stage could be reusable based on the SpaceX model.

Do you see a contradiction in these statements?

No?

Specifically, "landing gear" vs. "SpaceX model"?

Don't you think SpaceX uses landing gear?.. You see what I mean?

A payload hit is acceptable if the reusability is very great: they can just truck up bits of spacecraft, fuel for depots, crew modules and so on in many runs.

A fully-featured spaceplane is a paradigm-shifter. SpaceX decided not to try because the tech was too far down the line. Novel propulsion takes decades to prove out, and Reaction are working through it reasonably. The sticking point was the hypersonic precooler, and they've cracked that. A whole-engine scale demonstrator should be a relatively straightforward next step.

Why is a space plane a paradigm-shifter?

The question isn't whether they could reduce operating costs per launch vs. SpaceX. It's whether they could reduce them by enough to pay back the R&D costs, and the cost of building a much more complicated spacecraft than SpaceX's current line of boosters. (I wouldn't say that about SpaceX's planned ITS -- but that has capabilities that _Skylon_ can't touch; Skylon itself, last I checked, was never getting out of low Earth orbit, so even GTO comsats required a second stage mated to the payload in the cargo bay, a la the old Shuttle-Centaur plans.)

I don't exactly see what ITS brings to the table here, being a multi-stage rocket as anything else that puts stuff in GTO.

But anyhow yeah - the physics should favor them by practically an order of magnitude, but if they can ever survive to reap any percentage of that potential - they could just as likely just go bust at any further point in r&d, and they don't seem to have either much funding nor are they advancing particularly rapidly...

The spacecraft portion of ITS is being designed for interplanetary travel. Skylon, as I already said, is confined to Earth orbit, with the air-breathing space-plane portion confined to LEO. (It only gets stuff to GTO or GEO with the payload-bay booster, which is a purely conventional rocket.)

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