Inverted Aerobraking

ANTIcarrot wrote:

>Something similar involves filling a long tube with gasious fuel and oxidiser and then 'flying' an inside-out ram-jet down it to achieve high-speeds. The vehicle resembles two cones with their bases stuck together, with the fuel/oxidiser igniting at the midpoint where the mix is at its most compressed.Intended to used as a 'zeroth' stage, like JATO rockets or magnetic liniar accelerators.
>
Sounds like a sophisticated version of the "launch by cannon" approach.
Well, ... this is something _completely_ different from inverted
aerobraking.

The problem with the cannon and catapult approaches is that they can
help only a little bit. You just can't go very fast in the lower
atmosphere. It's not a coincidence that space ships start with low
acceleration and go straight up through the first dozens of kilometers.
At the speed of sound, rain drops become dangerous. Not to mention the
problem of insulation parts being ripped off by air and being
accelerated against the hull. And orbital speed is the equivalent of 24
times the speed of sound.

Due to the exponential growth of the amount of conventional rocket fuel
(with exhaust speeds of 4-5 km/s) needed to go to orbit, a catapult- or
sled- or air-launch might indeed turn out to be a worthwhile difference
in the end, sometimes. But with inverted aerobraking, zeroth stage
catapults will not be very helpful, because the exponential growth
problem is mostly eliminated due to the usage of highly energetic fuel
from space. This fuel needs not be carried up to orbital height. (If you
care about a physical explanation, think of it as opening up a new,
moving, but non-rotating coordinate system at the moment when inverted
aerobraking starts. (Details depend on wether all reaction mass will be
supplied from outer space or wether some onboard fuel must be used for
cooling and additional reaction mass during inverted aerobraking. But
you will get the idea.))

Axel