In article ,
"303pilot" brentUNDERSCOREsullivanATbmcDOTcom wrote:

The video show a US fighter jet of some type being towed behind a big
honking tanker-looking airplane.

F5 & C5, I think.

A private company (Kelly Aerospace) bought the F5 and hired the
transport plane in order to access the practicality of aerotowing a
rocket-propelled aircraft in order to act as a 0th stage before they
light the rocket at 40,000 ft and Mach 0.8.

You'll notice that all current and previous rockets that go into orbit
have multiple stages which are destroyed in the process of being used.
It would be much cheaper if you could get into orbit with a single
reusable stage (don't throw anything away). It is theoretically just on
the edge of possibiluty to do this with chemical fuels, but you need
something like 95% of your takeoff weight to be fuel, leaving very
little for structure, engines, pasengers etc. That's hard to do. In
fact, it's a rule of thumb for aircraft designers that sufficiently
strong landing gear alone weighs 3% of takeoff weight.

So a cheap and fully reusable 0th stage make a really big economic
difference and various people have proposed various schemes. You can
carry the rocket on top of or underneath another aircraft. Or you can
aerotow the rocket. Or you can do mid-air refuelling (if you take off
with nearly empty tanks then your wings and landing gear can be smaller
and lighter, and you can use a shorter runway).

That's if you want to go into orbit. If you just want to do an X-Prize
style flight then single stage from the ground is no problem.

-- Bruce