Stinky Cyclers

Considering interplanetary transport, I vaguely recalled alternatives to Hohmann and to the wildly overpriced nukey ride to glory. I went back through my copies of Koski / Grcevichs' Vacation Guide to the Solar System and of Zubrin's How To Live On Mars. Zubrin floats Type 1 Conjunction, and Cycler transit.

So here is my marker for Zubrin's Stinky Cyclers.

The Venus-Earth route has a major advantage over other planets', in that their orbits align with Earth's so nicely. A Hohmann Cycler is possible. It departs from Earth every five synods... yeah, it's the VISIT-2. But, as Hop David points out, you fix that up by building five of 'em. And you build five more for the Venus-to-Earth trip. You just let each of 'em drift for 2800 days.

But suppose you haven't built so many. For Mars/Earth Zubrin didn't want to say the name, which Koski and Grcevich did: the Aldrin Cycler. It was Buzz himself who'd floated the notion, back in the mid 1980s. Aldrin's Cycler, sometimes labeled "Castle", is a transport pushed into a Free Return Orbit. This such an orbit as would hit planet A and planet B (and maybe C &c), back to A, at regular intervals. For Aldrin, the energy costs of speeding up and of slowing down would be nil: hence, the return fare is free. His cycler would need only enough energy to stay in that orbit, upon receiving and delivering freight (=mass). In 2002 T. Troy McConaghy, James Longuski, and Dennis Byrnes found others from the Lambert equations (pdf).

Classical Aldrin Earth-Mars "1L1" takes 146 days / five months from the respective "station" (we'll get to why that's a misnomer), boarding every 2.135 years - the TerraMartian synod; we contrast that also-synodal Hohmann which after boarding takes 259 days, more like eight-an'-half months.

One of Zubrin's complaints pp. 3-5 was that the re-use of this transport would make everyone in it, physically ill. We'll get to that. We should take more seriously, Zubrin's note: they take you to the cleaners for your seat on the taxi capsule; he also mentions insurance. Zubrin is telling us that getting on and getting off this thing is... hard. I believe we've just read, for Earth-Mars, that Aldrin's 146 day journey is shorter than is Hohmann's 259. That means Aldrin is, at its bus-stations, going faster (6.54 km/s Earth, 9.75 km/s Mars).

And Aldrin's cycler doesn't stop - in fact, it's got 84 degrees of angular momentum to resolve which (per McConaghy) he must apply with propellant.

Ultimately Aldrin's 1L1 is a there-and-back-ONCE deal, for rodents. For regular freight McConaghy preferred a "S1L1" boarding every 2.828 years and departing to Mars every other aphelion.

Aldrin and McConaghy (like for that matter Hohmann for Earth / Venus) were talking about the cycler itself, here. "The Train Is Fine." . . . The other Lambert cyclers have other tradeoffs. None sum up to Hohmann's 2.95 km/s at Earth and 2.65 km/s at Mars, not even S1L1 at 4.7 and 5.0. That "station"? Not a station.

To fill this in, I went off to Reddit (yeah, I know). Here we read Elon Musk, also, poo-poo'ing the Stinky Cycler. Indeed: the window to dock with the thing is narrow; and the delta-V (read, G-force acceleration and fuel cost) is formidable. You may as well send your craft to Mars or whitherever alongside the cycler. I've certainly been mooting as much for Venus.

But I think a decent design for the cycler can mitigate some of these problems.

For one, Zubrin was writing in 2008, those three nerds in 2002 (Aldrin in 1985); we're in 2019. Other trainspotters have proposed other routes. Including - relevant to my interests - Venus flybys.

The Earth/Venus Hohmann cyclers also work only for those 146 days - less! - out of the 2919.6 days they run in their orbits between hitting their departure-planet. Stink isn't their problem. Although, maintenance and radiation ...

Also: a savvy engineer can make the Cycler non-stinky. Namely: the Cycler's "disadvantage" can be an advantage, in that it only has one job, to deliver mass. So the Cycler's main "payload" is a big shield against Bremsstrahlung (I'd been assuming plain-ice, but I hear good things about lithium hydride), covering a rotating inner piston, and sufficient internal Ceres-gravity spaces around that, for the tech-support to sleep in.

In which case: the main habitats for passengers are... the shuttles themselves. The shuttles get attached to the central piston, via umbilicals. Recall that big against the sun's radiation; the umbilicals don't need to be all that protective, except from micrometeorites.

The shuttles leave when their passengers leave. Their stink is their passengers' problem.

I should also note that energy costs are lower in the inner System, Venus most of all. I'd not worry too much about Venereans boarding a Cycler. Where the Cycler is at aphelion, likely somewhere in the Asteroid Belt, it is at its slowest and there the Belters (h/t "James AC Corey" again) won't have too much trouble boarding, either.

UPDATE 12/9 - for the Venus region, that'd have to start with the Hollister 1969 cycler, emphatically not the Aldrinlike. Here I am mainly thinking of Hilda cyclers, skating between Libration points L5-3 bypassing Venus - but in position for Mercury.

UPDATE 1/6/2020 - consider the Rama and the Pre-Rama.

UPDATE 2/27/2021 - Lambert.