starship-design: Vapor Trail Fuel

["bugzapper" <bugzappr@bellsouth.net>]

Externally Fueled Boost Vehicles

A notion I came up with for an air breathing boost vehicle, is to scavenge a
deliberately laid trail of vapor phase fuel in free air. I have been struck
by aerial photos of smoke, dust, etc., how in favorable conditions they can
cohere as a long linear trail, for a very great distance before ultimately
mixing with the surrounding air by diffusion. Now this is a recipe for air
pollution, definitely, for the vehicle can by no means scoop up all the fuel
released. The fuel molecules, though, would be relatively innocuous
hydrocarbons or alcohols, which are found in the air anyway, and are rapidly
broken down by natural processes. We are also talking about a relatively
small controlled release from a couple thousand feet altitude, most of which
will be burned off in a matter of seconds.

The obvious advantage in gleaning fuel out of a vapor stream in free air, is
weight savings in the vehicle. For this phase of the flight, only a scoop
and combustion chamber are in use. Plowing through the thick air down at a
couple thousand feet, in level flight over an ocean, is not normally a good
place to try to build high velocity. This leg of the flight would be limited
to achieving high supersonic velocity, no more. Still, use of the technique
should save much of the weight of a ground-launched air breather which
carries its own fuel, which is in turn only half the weight of a
ground-launched rocket vehicle which must also tote its own oxidizer.

The fuel stream should be composed of a vapor which approximates the density
of air, so it will neither sink fast nor rise fast. Hydrocarbons like
hexane, heptane and octane fill the bill here. These are the major
constituents of gasoline, which is cheap. The fuel should be released from a
tethered floating aerostat, into a steady westerly breeze, so the fuel
streamer extends straight East. The fuel is very visible in the infrared
bands, so guesswork is not involved. The launch may be timed to catch an
overhead passage of the Jet Stream, though this 200-300 mph extra speed is
really negligible when reaching for orbital velocity. The vehicle may be
lifted up into the fuel stream either under its own power, or by various
launch assistance techniques.

This segment of level flight might seem almost a detour, for a vehicle
headed way up yonder, but it should boost the vehicle to Mach 4 practically
for free, in terms of onboard expendables. A steep change of angle, after
the vapor trail has been practically consumed, will cost little of the
kinetic energy built up, won't slow it down much.

My proposal is to synchronize the ascent phase with another externally
fueled flight concept. This would be the hydrogen shock tube, which I have
mentioned here before. A large hydrogen aerostat positioned under the
diffusion end of the vapor trail, is angled to align with the projected
trajectory of the ascent phase of the oncoming lift vehicle. At just the
proper time, the explosive fabric composing this gas bag is ignited from the
rear. This creates a progressive implosion of the aerostat, which drives the
contained hydrogen out in a hypersonic jet, to lay fresh fuel into the
flight trajectory. It may seem the hydrogen would burn off at once, but such
combustion fronts travel at the speed of sound, and the vehicle on entering
it is already traveling four times faster.

But my hydrogen shock tube wasn't received favorably here last time,
evidently makes people nervous. Count it as an option. This post is about
the vapor trail as external fuel, so just assume you're going on internal
fuel after you pull the nose up. Still you've saved quite a lot of weight in
your vehicle. Lighter means faster and way cheaper, more access. Actually I
was thinking in kind of survivalist terms, a last-ditch way to haul lots of
people off a planet in hazard, when I came up with a gasoline streamer
method of flight vehicle acceleration. That to my mind, mitigated the
obvious pollution drawback of this method. But people should consider the
concept of externally-fueled air breathing boosters, because they seem to me
about the best way to go if we ever do get the method right. There's a way
to shoot fuel up just right so a ship can use it, a ship with just scoop,
engine and payload.

Johnny Thunderbird