If the Moon or Mars will we . . .

So how large is the world's largest balloon compared to what would be required to lift even the 65 ton max shuttle pay load to 80,000 ft.? And 65 tons won't get any crewed mission close to the Moon or Mars. Also, many high performance jet air craft now modify their engine geometry to improve efficiency, which can also be done with aerospike and equivalent rocket engines, so one does not need to invent another method to get around fixed rocket engine geometry, which is the point of having two kinds of rocket engines.

Jay Huebner

> Lifting a small rocket by balloon is okay, but small rockets won't
go far or carry much. A typical lift-off mass for the shuttle is
over 2000 tons. At sea level that would require a displaced buoyant
air mass of about 2 million cubic meters. Above sea level,
significantly larger volumes would be required.
>
> Another way to think of this is, how much farther is 80,000 ft.
from the center of Earth? Earth's radius is about 20,718,750 feet,
so 80,000 ft. is less than 1/2 of 1 percent, which is not much
compared to all of the trouble of trying to lift a big rocket?
>
> Jay Huebner

As you point out, the miniscule difference in gravity or potential
energy isn't the advantage of using a balloon. At 80000 feet, you
are above most of the atmosphere, nearly in vacuum. Instead of
needing two types of rocket engine (one for low atmosphere and a
different one for high atmosphere/vacuum) you only need one type of
engine. And your rocket launch will experience much less friction
than a launch from the ground.

Al pointed out that most of the energy required for orbital flight
is to acquire horizontal velocity; he then directly equated this
with fuel. A quick glance at the rocket equation shows that this is
not strictly correct. Most of the fuel is used simply to lift the
fuel itself to 80000 feet.

Ed