If you want to see airline type operations into orbit forget Falcon, expendable or nominally re-usable - more like refurbishable - launch systems will never get the cost down low enough, they're like throwing 747's away after a single use.
Maybe one day we'll see the laser launcher or a space elevator, but if we do it'll only be after there's already lots of stuff going up via other means, people need to be realistic that
So we're left with the Single Stage To Orbit (SSTO) and Two Stage to orbit (TSTO)fully reusable options that have been proposed again and again over the last 40+ years.
Why hasn't one of the many variations on these methods ever been seen through?
SSTO has the problem with mass ratios, even with the most energy dense rocket propellant in use, 90% of your launch mass has to be propellant and the other 10% is structure, leaving ~0% as payload. You can look to improving that mass ratio with air breathing engines, and if jet engines could be built with the same thrust to weight ratios as rocket engines have, SSTO would be a done deal, but the reality is that the best jet engines only have T/W ratios of about 13:1 (verses about 50:1 for rocket engines), and you end up taking all the mass of those engines to orbit, and one tonne more of engine means one tonne less of payload.
TSTO has the problem of shear size, building a winged orbiter to carry 20 tonnes to orbit would in itself be a manageable financial expense, but to build the ~800 tonne winged booster for a twenty tonne payload isn't quite so manageable. When they were looking for a reusable system to replace expendable launch vehicles in the 70's the designs were often of two stage fully reusable vehicles, and Congress always said "no - too expensive".
If you've seen many of my comments on various blogs recently you probably know that I've become a bit obsessed with Aerial Propellant Transfer for spaceplanes. The reason is that I see it as a route to achieve today what could not be done 40 years ago. APT means you can dispense with that 20 or 30 billion dollar specialized booster and replace it with a $350 million dollar (plus modification) commercial aircraft.
Probably one of the first reactions to the suggestion of a "booster" that only goes to Mach 0.85 and 12,000 meters is that it's not enough to make a difference. But it actually counts for more than commonsense would suggest; when the orbiter begins its climb to orbit its engines are working with far less atmospheric back pressure and so are more efficient, the orbiter can take off from the ground with only enough propellant on board to reach the tanker, and so requires lighter landing gear and possibly smaller wings, and the atmospheric drag from 12,000 meters is far less than that from the ground half the atmosphere is below you.
That start from altitude is worth about 1km/s, it reduces the propellant mass ratio from about 90% to about 85%, and if 11% of your light-off mass is orbiter your payload goes from less than 0% to 4% of the orbiters total mass, even better, aircraft landing gear typically makes up about 2% of lift-off mass, if you're refueling in the air the weight of the landing gear can be halved - so you've increased payload by another 1% of the orbiters total mass to 5%.
If want to get a 20 tonne payload to orbit, how big does the orbiter need to be? 20 tonnes X 100/5 = 400 tonnes, of which 85%, or 340 tonnes is propellant and 40 tonnes orbiter.
So we need a tanker that can transfer up to 340 tonnes (minus any excess that the orbiter has carried up) to the orbiter at an altitude of 12,000 meters.
That's a lot of propellant, it's more than can be carried by a 747-8, and even the A380 freighter only has a take-off weight of 590 tonnes total, of which 250 tonnes is plane, which would leave only 340 tonnes for both rocket propellant and the A380's own fuel.
I've tried sounding aircraft engineers out on this, the A380 as a tanker has far more space in the fuselage than would be required, my thoughts are that perhaps a fuselage based on that of an A340 - some models of which are actually longer than the A380 - could be attached to the flight surfaces of the A380, that would shave about 80 tonnes off the total mass, but even doing this would still require that some of the rocket propellant (LOX) be carried in the tanker aircrafts modified inboard wing tanks as the structural payload (payload that can be carried in the fuselage, even with the lighter fuselage, would still only be 230 tonnes.
So while heavy lift to orbit using APT might be possible now, the aircraft to do it didn't exist just a few years ago, and even the A380 would need big modifications for a heavy lift orbiter.
Another point I want to emphasis is that using even the A380 as a booster for air launch wouldn't put anything like a 20 tonne payload into orbit, even with sticking on an A340 fuselage to reduce weight, the maximum weight of the fueled orbiter would be 230 tonnes and the increased drag and high center of gravity would bring in more complications, maybe 9 tonnes of payload could be carried to orbit.
ATP has an advantage over a system which uses a direct ascent from a runway, there are far more launch windows because the ascent from 12,000 meters can take place anywhere within perhaps a thousand kilometers of the runway.
So how much would each launch cost?
The cost of flying a jumbo on a long haul flight is around $500,000, a third of which is fuel, that's also about the cost of a USAF KC135 flight to refuel other aircraft a thousand km from base. So excluding the cost of the propellant transfered, that's probably a reasonable figure for the tanker.
The cost of the propellant LH2 and LOX will be around $250,000
The cost of the orbiter excluding propellant will be servicing and payback on capital invested, with the servicing aerospace engineers tell me that with the experience gained from the SSME's it's possible to build H2/O2 that can be turned around in with almost no servicing between flights if that applies to all of the orbiters systems, servicing of $450,000/flight seems reasonable (that's about 20 times the servicing cost/tonne of airliners).
With the payback on capital invested, the development of aerospace vehicles that don't require radically new technology is around a hundred million dollars a ton dry mass, the numbers quoted for Skylon are much higher than this, but everything about Skylon is new technology, from the engines, to the active thermal control system, to the truss frame construction. The ATP would weigh about 45 tonnes dry, so I'm putting development cost at $4.5 billion, if 10 are initially built and per unit manufacturing cost is $100 million that's $550 million each. If they're good for one thousand flights that's $550,000/flight.
Adding those all together works out at $1.5 million per flight plus the operators profit. Which comes to $75,000/tonne or $75/kg payload.
That seems ridiculously cheap compared to the SpaceX Falcon, but then they're still throwing hardware away with every flight.
It also seems ridiculously low compared to what REL are talking about for Skylon, by then Skylon is really pushing the envelope with new technology, and it's the last 10% that takes 90% of the effort, an ATP using the Skylon structural system would in theory weigh half as much dry and carry twice the payload.