The
ESA's SMART-1 is a
spacecraft that has almost touched the surface of the Moon and sent
back excellent detailed photos such as these.
The really astonishing thing about this mission is that SMART-1 is
little bigger than a washing machine: a radical savings of mass and
money achieved thanks to the use of an ion drive. Ion propulsion
allows spacecraft to reach very high speeds on teaspoonfuls of fuel
mass, with the only
tradeoff that accelleration is very low, and top
speed can take weeks to reach.
Well now that
SMART-1 is so very close to the Moon, wouldn't it be nice if
something that small and affordable could go ahead and land there and
do much, much more? Alas, you can't land with an ion drive; it is so
weak that it would just crash. And conventional decelleration rockets
would add tons of mass to our spacecraft, putting us right back into
the high tax bracket of space travel. What to do? Here's an idea.
When
a spacecraft lands on Earth, it doesn't need rockets to decellerate
itself all the way down to the surface. Instead, it only uses rockets
to bring itself to the lowest possible orbit, and then atmospheric
friction brakes it down to landing speed. It would be very convenient
if you could land on the Moon this way, except that (d'oh!) the Moon
doesn't have an atmosphere.
So
let's create our own frictional interaction between satellite and
surface:
Figure
1.
Use
the ion drive to decellerate the spacecraft to the lowest
possible circular lunar orbit, nearly touching the mountaintops.
At this level, almost all of the momentum left to dissipate is
in the horizontal direction.
The
craft then unwinds long threads made of something maximally
strong and light, e.g. carbon fiber. These threads are long
enough to just brush the peaks of the terrain.
As
each thread makes contact, the satellite starts rolling with the
surface. Drag causes it to decellerate and descend more. The
tips of the threads grind off as they touch the moonscape,
making the “wheel” shrink as its hub gets closer to
the ground. (Alternatively the threads
could be reeled in to regulate drag.)
Just
before the hub impacts each thread is jettisoned (“popped
off”) on the downward pass, so the threads will be left
behind and clear of the landing area (or, alternatively, reeled
all the way in). A conventional crash balloon could be deployed
after the last thread is gone, for final impact with the
surface.
Some
points:
Really,
this is more like a giant crash balloon than aerobraking.
The wheel "bounces" like a rolling balloon whenever
it descends too quickly, or hits a bump, etc. This is because
whenever the terrain pushes up against a thread, some
downward-pulling tension is relieved from the hub, and the net
upward-pulling tension of the remaining threads lifts the hub until
the wheel is in equilibrium again. Generally this would keep the hub
at a protective distance above the terrain long after it ground down
from orbital speed, but before it stopped completely.
The
system is self-stabilizing. That is even if the threads are deployed
at the wrong angle, as long as one thread touches the terrain the
craft will be dragged into a rolling motion whose plane of rotation
is parallel to the plane of the craft's orbit.
The
longer the threads are, the lower the G's are at the tips. This is
because when the radius of a wheel is larger the curvature of
centripedal accelleration is gentler (assuming linear speed at the
rim is the same, which it is). A slower spin is also easier on the
payload, or on whatever axle allows the threads to turn around the
payload. So paradoxically, a bigger structure might be more feasible
than a smaller one.
I
admit I don't count the fuel mass of an ion drive as significant
here. An ion drive has such a high impulse that it can get almost
anywhere on very little fuel. However the thrust of an ion drive is
too poor for takeoffs or landings. So, this system is really meant as
a bridge from a lightweight solar-ion flight to a tolerable landing
on an airless moon.
And
of course this whole idea is moot if a dragwheel is more massive than
ordinary de-orbiting rockets + fuel!
.
.
.
To
visualize the physics of the thing, imagine that all of the
mass of each thread is at its endpoint. While spinning freely,
tension on the hub is equal in all directions. But as soon as a
thread-tip touches a surface, tension is relieved slightly on that
thread, allowing a net upward tension. So even though the system has
only tensile strength with no compressive strength, it can still
bounce and roll on a surface. Friction against the surface is
near-zero because the lower weights are moving at zero velocity
relative to the Moon's surface. The small amount of friction that is
permitted slowly saps the vehicle's lateral velocity, lowering it
from orbit until it eventually becomes an ordinary rolling wheel
supported by the surface of the Moon.
Figure notes: All speeds relative to
surface. Satellite is pre-spun before touchdown. Not to scale;
contact exaggerated.
