I was going over the recent debate of "VTVL wants to be short and
squatty, but in-flight aerodynamics say no", and I'm trying to remember
why the solution that immediately occurs to me - vary the geometry based
on what phase of the flight you're in - isn't the best of ideas (seeing
if maybe I can find solutions to those problems).
The problem seems to boil down into four phases:
1. On the ground (or immediately after takeoff, until you're at least a
few rocket lengths off the ground and all engines are active).
2. Going up and in orbit.
3. Aerobraking.
4. Landing.
Part 2, and to some extent 3, is what the overall design is optimized
for - not surprising, considering as that's most of the mission. That's
also the simplest, since you've got nothing (except maybe air) in the
immediate vicinity to run into. For 1, you can have special support
equipment - like a launch tower - easily set up and recovered (if
necessary) after launch. 4, and to some extent 3, is where it gets
tricky.
However, in 3 and 4, we've already used most of our fuel. I'm thinking
shorten the fuel tank (I can think of a couple not very complex ways to
do this, like making the tank out of multiple pieces which screw
together), and have the rocket engines mechanically linked so that this
action will also pull the side engines to the side, pushing the rocket's
shell aside as needed, and shorten (perhaps disable) the central engine
if there is one. The rocket nozzles would also tilt, to make sure the
thrust vector kept pointing towards the rocket's rear end. I've drawn
up a quick sketch of this at http://www.wingedcat.org/erps/vargeo.gif
This will make the rocket less efficient, but by this point we're almost
out of fuel, so inefficiency here doesn't matter as much. In exchange,
we have a much easier time holding the rocket level as we come down -
and, if this happens before the rocket re-enters the atmosphere, and if
the engines and the insides of the bottom of the rocket's shell are
coated with (or made of, if possible) extremely heat-resistant materials
(moreso than just to resist the temperature of exhaust), this could
expand the rocket's surface area (especially if the shell unfolds into a
parachute or similar, instead of just having disconnected flaps pushed
aside by the side engines), thus making for better aerobraking.
Primary problems with this approach that I see:
* This does add at least a little complexity, moreso than one solid
fixed shape, no matter how much one simplifies the systems.
* Finding a collapsable tank. (It seems that collapsable tanks would
likely be useful for other applications, therefore it seems likely
that someone already makes them.) Ideally, the tank would come with
its own motor - preferably electric (lower probability of igniting
fuel accidentally sprayed on it, and higher probability of simply
working when needed) - to drive the collapsing and expanding.
* Making sure the mechanical links work, even after the stresses
(primarily vibration, so far as likely causes of problems) of launch.
A simple cable and pulley system, for instance, would likely be
extremely susceptible to having the cables shaken off of the pulleys
without something to keep the cables where they are supposed to be.
(If aerobraking melted the links so they fused in the spread out
position, this would also be of concern, but at least the rocket could
probably land like that - though we'd want to make sure that such
failures, if any, tended towards engines spread for landing rather
than engines in launch configuration, so it *could* land like that.)
What am I missing/forgetting?
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