Aerobraking Without the Aero

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:

Dragwheel de-orbiting method
Figure 1.

  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.

  2. 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.

  3. 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.)

  4. 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.

Figure 2. In orbit:

Figure 3. Touchdown at near-orbital speed:

September 28 2004