Re: [GZG] Thrusters

On Thu, Dec 11, 2008 at 9:26 AM, Tom B <kaladorn@gmail.com> wrote:
> One possible defense of thrusters...
>
> Imagine your reaction mass/reaction chamber is in the center of the
ship.
> Let's say you have ducting on all three major axes. You just open the
> ducting from the main reaction chamber whenever you want to thrust.
The
> thrust goes out any of the six directions (or combination of).
>
> Thus you have equal thrust capability in all directions without the
need to
> install 'a second engine in the nose' and justifying *full*
turn/retrothrust
> capability.
>
> Of course, the best design for this is a big spherical ship.
>
> But there are ways to imagine drive systems that don't necessarily
mean
> you've only got thrust to the rear.
>
> --

A few big flies in the ointment are the thrust nozzles.  If the
nozzles extend from nearly the center of the ship to the surface, they
will consume huge volumes (but not too much mass), they cannot overlap
(nozzles for use in the lowest ambient pressures have the widest
bells*), and the whole nozzle must be cooled.  If there is ducting
before the nozzles, pumping losses to get from the main reaction
chamber to the nozzle will make its presence known as heat, so it will
need more cooling than the really big nozzles.

Guide vanes in the thrust plume are fine for single-use boosters with
low burn-out times, or units that already have intensive maintenance
shedules, as they tend to be ablative.

* Not as serious an issue for aerospike nozzles, but the aerospike
engine reaction chamber is a surface mount, or you go back to the
ductwork with serious cooling needs.  If you built a large disk shaped
vessel with an aerospike built around the edge, you might be able to
thrust along the main axis in either direction, at full power.

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