Apr 11, 2017

6 min read

Polar Aerocycler Teaser

It’s rare that I stumble on a truly good idea, but I lately encountered one that is absolutely irresistible.

The concept is an energy-generating and propellant-collecting orbit. Its orbital phases start out with a highly elliptical polar orbit around Earth, the orbit tilts to become flatter with atmospheric interaction (mostly perpendicular to direction of motion), then a lunar flyby restores it to a polar orbit. I wrote a previous post on this subject, but many of the details from that I’ve now realized are wrong. It’s definitely not as easy as my first hack at it represented. There’s a hill to climb, for sure, but the demands of unproven tech are relatively reasonable for the industrial output that it offers.

This article is only offered as a teaser. At first, I started out trying to write a comprehensive article about it, then realized that was too much. Then I started out trying to scribble and describe the basic design of it, and this was also more content than I can put together quickly. So let me outline what I hope to do:

  • Design of the Polar Aerocycler — goal here is to show that the problem really does turn into an empirical engineering optimization
  • Tidal energy extraction design family, and how the Polar Aerocycler fits into that
  • Electromagnetic plasma-driven lift and propellant collector design

Beyond covering these, one would reasonably go into actual parameter tuning, but it would be pointless to try to go about organizing that material at this point.

In this article, alone, I do not think I can credibly make the case for feasibility. So you will be completely fair to doubt that it is, all all, possible. Simmer on the ideas, and see where it takes you.

Nope. No perpetual motion here. The energy this uses has been inside of the Earth for billions of years in the form of rotational energy of the planet. But angular momentum does squat unless you have another reference frame to interact with.

This design transfers Earth’s rotational momentum to the moon. This is not an easy task, and to validate the design that we come up with, a good litmus tests is “would it still work if the Earth stopped spinning?” If the answer is “yes”, then something in the design must be outright incorrect, or else get energy from another source (more likely the former).

The design could go north → south as well, but I have to pick just one to describe. The sun sets in the West, so if you look at the globe from the North Pole you will see it rotate counter-clockwise. Our spacecraft will be hitting the upper edges of the atmosphere to 1) scoop some atmosphere and 2) to gain some forward momentum.

Motion of the spacecraft in atmosphere is relative to the airspeed that it flies through, and here is where the difficult engineering comes in. Traveling at around 11 km/s directly north, the air is traveling at 0.5 km/s directly to the west. The ship’s front (aft) is pointed in something like a NNNW direction in order to get the incoming air head-on. If the lift-to-drag ratio in this regime is greater than 20, then it will leave this stage of the orbit actually going faster than what it started at — and incredible thing.

Relatively ordinary aircraft can have lift-to-drag ratios of 20-to-1, however, the values which can be achieved for this coefficient degrade at higher speeds. The Space Shuttle had a ratio of maybe 2-to-1 during reentry, for an example, although something like the Dreamchaser might be more like 4–to-1. Radical new-ish Waverider designs can have ratios as much as 8-to-1 at the cost of an extremely elongated aerofoil design.

But these numbers don’t even matter, because they are only dealing with re-entry type speeds. For what we’re doing, we need a close approach to the moon, which means we need a highly elliptical orbit. That means we’d be going much faster than the examples I have allow, and we need a much higher lift-to-drag ratio. So, stick a fork in it, this design is toast, right?

I’m not willing to give up here, and the reason is because I’ve read about the various crazy designs for Earth-orbiting atmospheric scoops.

Presumably I wouldn’t still be writing this if I didn’t think there was some loophole out there. Lo and behold, there is! You have to dig into the trenches of the atmospheric scoop designs (not the first page in that link, keeeeep on clicking) to realize that electronic acceleration of ions was a relatively common proposal. A failed proposal, but incredibly attractive in its own way.

But before getting into that, there is a fundamental prerequisite fact that must be established. That is that the upper atmosphere is almost entirely ions. Normal gas is bound up in molecules that satisfies both the electron orbitals as well as net electric charge. High enough, above about 150 km, ions become much more numerous than stable molecules. Go ahead, see for yourself, check out the ratio of O/O2 for example in the standard models:

The atmospheric scoops (also look up lifters) fail to leverage this to provide station-keeping thrust because it had to supply current in order to keep the thrust going. This becomes non-viable extremely fast.

Perhaps you already see the trollishly simple place I’m going with this. The electromagnetic propulsion idea could be very effective if it wasn’t paddling against the current, so to speak. So instead, I offer a simple mechanism that is actually a passive one — apply a magnetic field.

A simple electromagnet in the thermosphere (high-up region of the atmosphere) will generate lift. The problem for something like the ISS is that this lift isn’t in any useful direction, being neutral at best. But we don’t need station-keeping in this part of the design. The station keeping can come from the rotation of the Earth itself. All we need is efficient force directed perpendicular to the direction of motion — and all signs point to magnetic lift doing this fantastically.

These claims are provocative. The use is relatively immediately apparent when you consider the larger context of a space transportation infrastructure. Since this would produce surplus momentum (to a varying degree, depending on the exact lift-to-drag ratio you can generate), it can afford to pick up some extra atmospheric gas from Earth and liquefy it.

This liquefied gas would be located in possibly the most convenient location in the universe, just on the doorstep of Earth’s gravity well, and this would happen without any direct energy input for the bulk of it. Not only would its location be very nice, but its composition would be highly skewed toward pure Hydrogen, which is a rare commodity in the inner solar system.

I am very optimistic about propellant production at the moon’s poles, but I’m quite skeptical of people who see that production fitting into a larger system of movement. Propellant factories on the moon would do best to ship goods and people off of the moon. Going from Earth-moon Lagrange points to other places in the solar system would be best supplied by propellant from this system. Because transfer burns to get it there would be very reasonable. This would absolutely demolish the rocket equation.

Tidal energy contains a relatively large reservoir of energy, and even if our machines for the Polar Aerocycler were relatively inefficient, the scheme would last for quite a long time at a large scale. Anyway, it could be argued that we only need to get the ball rolling on a space-faring civilization and if we do, much more advanced technology like launchers and catchers could provide propellant-less means of transport in the vastly distant future.

It’s more important that the design could scale down, and as far as I can see, this orbit could be traversed by a nano-sat. That makes this a tantalizing possibility, not in the centuries time scale, but in the decades time scale.

Obligatory analytical writing, online participation account for Medium. Engineering, software, books, space, constant daydreaming.

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