When it comes to alternative engine and frame types, has anyone here been keeping tabs on Reaction Engines' work with the SABRE engine (and its proposed Scimitar derivative), or have any thoughts regarding the proposed Skylon and LAPCAT A2?
IIRC, the jury is out on SABRE until at least 2019. SABRE's clever precooler has been proven, but there won't be a full SABRE engine through test firings until 2019 (again, IIRC) at the earliest. You wouldn't say that a new automobile engine (or the automobile itself) is ready to go based only on fuel injector tests. That's kind of where SABRE is right now.
Setting aside the technical challenges to SABRE or Skylon, with a $12 billion estimate, Skylon will cost at least 3x-10x more than other launch vehicles to develop. Ariane 6 has a $3.6 billion development pricetag. The Atlas V and Delta IV launch vehicle families were developed for $5 billion total (DOD and company cost-shares), or around $2.5 billion each. And Falcon 9 and Dragon combined were south of $1 billion (actually $850 million) total (NASA and SpaceX cost-shares) to develop.
Maybe that 3x-10x higher development cost will make sense if Skylon delivers a huge operational advantage. Skylon promises to reduce launch costs to 650 British pounds per kilogram, or about $850 US per kilogram to LEO. By comparison, the expendable Falcon 9 FT does $2,720 US per kilogram to LEO today, and the expendable Falcon Heavy is supposed to hit $2,200 US per kilogram to LEO or better. So payloads on Skylon could enjoy a cost savings of ~3x over today's lowest cost expendable launchers.
But it's hard to say with certainty now what Skylon's actual future launch costs will be. Does Skylon development really come in at $12 billion or does it experience large overruns like nearly every other launch vehicle development in history? Does the private sector have to amortize all of that $12 billion or will there be a government cost share? Will Skylon be as simple and cost efficient to operate as its proponents estimate or will it suffer exponential operational complexity and cost growth like the Space Shuttle did? Will the market will generate enough demand to justify a high launch rate for Skylon to keep its per launch cost down?
Every launch vehicle development has to deal with these tough questions. But these questions are easier to answer and the uncertainties and risks much smaller for a vehicle that costs $1-3 billion to develop versus one that costs $12 billion to develop. In that light -- higher development costs leading to higher risks versus competitors -- Skylon doesn't make much sense to me.
And what if SpaceX or Blue Origin or someone else develops a reusable first stage or launch vehicle that is roughly competitive with Skylon for a fraction of what Skylon costs to develop? What if they do it before Skylon is flying? That is something that past launch vehicle development decision makers did not have to consider, but it must be a consideration going forward. Under that competitive threat, which raises risks even higher, Skylon makes even less sense to me.
I'm not an aircraft/airline guy, but LAPCAT seems like a bridge too far given where the market is today. The business case for supersonic jetliners never closed for Concorde, and I don't know of a credible/substantive entrant trying to make that business case close today. If a supersonic passenger transport doesn't make sense, then a hypersonic, antipodal passenger transport like LAPCAT makes even less sense.
An important but largely intractable problem that faster airline travel runs up against is door-to-door trip time. Airports are not destinations. It matters little if you halve my air time from New York to London if I still spend hours stuck in New York or London traffic getting to my actual home, vacation destination, or meeting. Forget hypersonic or supersonic transport. Airlines have difficulty just ridding themselves of the economics behind the awful hub-and-spoke system in favor of more point-to-point subsonic travel for this and related reasons.
This is perhaps an overly broad generalization, but if you look at the history of aircraft or launch vehicle propulsion, new engine or rocket technology is usually proven out first in military applications before migrating to the civil or commercial markets. Given the paucity of complex, multi-mode, air-breathing rocket engines in military use today, it would require a huge break with history to see SABRE-derived civil launch vehicles or SCIMITAR-derived passenger transports anytime soon. I'm not saying it will never happen, but I think we're decades, not years, away.
Some folks are stuck on air-breathing propulsion for launch vehicles, and I direct them to water-injection (or mass-injection) pre-compressor cooling technology as a realistic, near-term step. It's little more than spraying cool water in front of a jet engine to densify the incoming air to allow the aircraft to fly faster and higher than it would otherwise be able to. F-4s were actually modified with this technology for the Israelis back in the 1970s. The Navy's drone F-4s today could be similarly modified and equipped with a simple F-15 ASAT-like upper stage for launching smallsats. It's a much more achievable first step than multi-mode air-breathing rocket engines like SABRE. Here's a NASA paper on it:
https://www.nas.nasa.gov/assets/pdf/papers/Mehta_U_February2015.pdf[Disclosure: I directed a little funding to the work behind the paper in a past life at NASA HQ.]
While the Skylon itself seems to be aimed at low orbit work only, it would seem to offer a useful means of building up the kind of orbital infrastructure useful for long-range expeditions, were it to prove viable.
A lot of space-based infrastructure only makes sense if we're committed to going over and over again. If it's a limited Apollo-like effort, it makes little sense to invest billions or tens of billions of dollars on infrastructure that will only be used once or a handful of times.
If infrastructure does make sense, Earth-to-orbit launchers aren't really the bottleneck. Other technologies, like long-term cryogenic propellant storage and transfer, are more critical. Unfortunately, we're not investing in them.