Henry Spencer (who I expect will be at Space Access this year, after missing last year for the first time ever) explains (once again) why the future of space launch continues to lie with rockets, despite the superficial appeal of not having to carry oxidizer.

This is an important point that I’d never thought about explicitly:

The pure-rocket design was more than twice as heavy as X-30 at takeoff, because of all that LOX. On the other hand, its empty weight – the part you have to build and maintain – was 40% less than X-30’s. It was about half the size. Its fuel and oxidiser together cost less than half as much per flight as X-30’s fuel. And finally, because it quickly climbed out of the atmosphere and did its accelerating in vacuum, it had to endure rather lower stresses and less than 1% of X-30’s friction heating. Which approach would be easier and cheaper to operate was pretty obvious.

This implies that a rocket powered vehicle will have much better off-design (higher delta V, such as more altitude or higher inclination) performance than the air breather, because its dry mass that has to be given the additional velocity is much less. It also means that it will be cheaper to deorbit, and the thermal load will be less, for a given wing area (assuming that it has wings, which an air breather certainly would). I suspect that no matter what the technology level, air-breathing launchers are doomed to remain the equivalent of flying cars — interesting in theory, but never achieved in practice.

I see that Brian Wang is continuing to post on the potential benefits of nuclear-explosion propulsion, here and here (where he takes on Charlie Stross), and here, where he talks about it in the context of unmanned Mars missions and a high-speed asteroid interceptor.

I do think that there’s potential for this vehicle off planet, but I remain highly skeptical that it will ever launch payloads from earth, regardless of how theoretically cheap it might be. Particularly in the Age of Obama.

And frankly, when I read things like:

Nuclear Orion can achieve launch costs of less than $1/kg and perhaps a tiny fraction of that.

…it reminds me of the old claims from the early days of nuclear power that it would be “too cheap to meter.”

Actually, he understates Shuttle costs as being “$5000 to $6000 per pound,” even if it is an “accepted figure.” At current flight rates, I would guess that (at least to ISS), the current costs are about a billion per flight for about 40,000 (or less) lbs, or more like $25,000/lb (or more, depending on the payload). Which makes Orion look even better, of course. But it also displays my long-standing claim that the single most sensitive variable with regard to launch costs is flight rate, and any vehicle design consideration is a secondary matter.

I think that Brian’s mistake is demonstrated in the false choice of the title of this post which was a response to this one of mine:

Small and Expensive Versus Big and Possibly Infrequent Space Launch

The implication is that small is intrinsically expensive. But it’s not.

Small is only expensive when a) you throw the vehicle away and b) you don’t fly it very much. I would suggest that Brian read this piece on the subject of the reasons for high launch costs, which I wrote over four years ago to allay exactly this kind of misunderstanding, and (if he can afford the time and money — it’s really a bargain at the cost if one can get to Phoenix) attend the Space Access conference a month from now, where he can get up to speed on the current state of chemical-rocket launch technology (and its economics and business prospects).