[PaulHoule]

Frankly it seemed like an idea that made no sense for multiple reasons. For one thing the density of atmospheric oxygen is a fraction of the density of liquid oxygen so it's hard to picture getting enough oxygen in the thing to make a difference. If you're liquifying it you're going to slow your rocket down by bringing O2 as well as 4 times as much N2 on board, then there is the weight of the liquification plant. Investing in Skylon is like investing in cold fusion.

It was bad enough that Richard Branson discredited private orbital spaceflight with the overly long development process for a vehicle that made the Space Shuttle look like a paragon of safety and low costs -- Skylon was so much worse.

[pfdietz]

Henry Spencer on air breathing launchers (New Scientist, 2009):

https://www.newscientist.com/blogs/shortsharpscience/2009/03...

'Trying to build a spaceship by making aeroplane fly faster and higher is like trying to build an aeroplane by making locomotives faster and lighter - with a lot of effort, perhaps you could get something that more or less works, but it really isn't the right way to proceed. The problems are fundamentally different, and so are the best solutions.

As Mitch Burnside Clapp, former US Air Force test pilot and designer of innovative launcher concepts, once commented: "Air breathing is a privilege that should be reserved for the crew".'

[gridspy]

https://web.archive.org/web/20090727013542/http://www.newsci...

(The original link says "Page is Gone")

And here's some more quoting

Could a single-stage-to-orbit spaceship, something that could operate rather like an aeroplane, be built with just rocket engines? Well, actually, yes. In the 1980s, NASA and the US Air Force spent about $2 billion trying to build the X-30, a single-stage spaceship powered by scramjets (with help from rockets, of course). It never flew. At the same time, for comparison, NASA's Langley Research Center studied building a single-stage pure-rocket spaceship. The results were interesting.

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.

The Langley group's conclusion: if you want a spaceship that operates like an aeroplane, power it with rockets and only rockets.

[PaulHoule]

See https://en.wikipedia.org/wiki/Lockheed_Martin_X-33

There have been some other discussions of this lately, but I would say the pursuit of SSTO resulted in a lost decade for spaceflight in the 1990s.

SSTO is just barely possible, the problem is that you have a big rocket that carries a tiny payload so you are driven to exotic engines, exotic materials, and various risky technologies.

If Musk had any good idea it was not only falling back to two-stage-to-orbit reusable rockets but also recognizing that it was worth just reusing the first stage. A SSTO gets closer to aircraft-like operations in that you don't need to stack two stages on top of each other, but given how much TSTO improves everything else it's probably worth just optimizing the stacking.

[pfdietz]

And I strongly suspect Henry knew the "don't turn an airplane into a launcher" extended to using wings for landing and takeoff as well, although in 2009 that maybe wasn't quite as inescapable a conclusion as it is today.

[KiwiJohnno]

I agree. I've played a LOT of kerbal space program, and yes, this is just a game, with simplified physics, and a MUCH lower orbital velocity required. But the fundamental problems with an air-breating spaceplane are still demonstrated:

1) Orbital velocity is FAST. VERY fast. In KSP orbital velocity for a low orbit is about 2,200 m/s. For earth its about 7,600 m/s 2) An air-breathing engine, by definition can only be used inside the atmosphere. 3) You will struggle to get anywhere close to orbital velocity while still in the atmosphere, due to drag, and heating.

At best, your air-breathing engine will only get you to a small fraction (less than 1/4th) of orbital velocity. Then you will have to a) climb higher, and b) use a different engine to accelerate to the required orbital velocity.

Yes, you will potentially save some weight by not having to carry oxidizer for while you gain that first 1/4 or so of your final velocity. But once your air-breathing engines, and wings and everything else are useless, you still have to carry their weight