In article , says...

When I first went to college there was the dream that a space shuttle
would have a large first stage air breathing launch vehicle. It would
have turbine and/or ramjet engines. My initial major was mechanical
engineering targeting turbine machines because I wanted to work on that
project, so I'm both biased and obsolete on this topic but I knew the
issues in the late 1970s early 1980s. (Then I discovered that every
assignment that had computer work was an assignment I liked more than
others. So I switched to computers and have been doing them ever
since).

A launch stage doesn't need to be hypersonic to be worth it. Much of
the energy of a ground launch rocket is spent getting the vehicle up
into thin air. A subsonic jet can do that. Some of the energy of a
ground rocket is spent getting the rocket to subsonic horizontal speeds
before going supersonic. A subsonic jet can do that.

Given this, to me the price difference of a supersonic air breathing
launch stage versus a subsonic air breathing launch stage is larger
than the payload difference justifies. We can go as far back as
dropping the X-15 from the wing of a B-52 to demonstrate this. Now at
least Spaceship-One uses a subsonic launch stage. Get it up as high as
feasable into thin air. Put the plane on an arc that gives some more
up plus as much forward as it can. Do the math to see what arc works
best as the trade off between verticle loft and horizontal speed. Do
that arc and release at the point described in the math. The launch
vehicle might or might not stall, might or might not flame out but
it's plenty high enough to recover.

Add rocket packs that burn out and drop down on chutes ater a suborbital
arc? If the math and engineering compromises say that's the way to go
given any one specific design.

Given current engineering I figure subsonic air breathing launch stage
is one of the better ways to go. How long before it's beat by direct
rocket launch? We have already seen direct rocket launch in action for
decades. It's led to very large disposable first stages.

So far, subsonic air launch hasn't been terribly successful due to the
need for a *very* large carrier aircraft for what I'd consider to be a
"reasonable" sized payload to orbit. Stratolaunch plans to build its
subsonic carrier out of two Boeing 747's.

http://en.wikipedia.org/wiki/Stratolaunch_Systems

SpaceX recently announced that it has parted ways with the Stratolaunch
Systems's air launch scheme. Stratolaunch has switched to Orbital
Sciences as the provider of the rocket stages.

It's nice to see SpaceX and Stratolaunch are taking two different
approaches to "first stage" reusability. This is exactly the sort of
competition the industry needs, IMHO. Time will tell if either (or
both) of these approaches proves successful.

Jeff
--
"the perennial claim that hypersonic airbreathing propulsion would
magically make space launch cheaper is nonsense -- LOX is much cheaper
than advanced airbreathing engines, and so are the tanks to put it in
and the extra thrust to carry it." - Henry Spencer