Going in Style - The Aldrin Cycler

My last post talked about Starship, the new spacecraft under development by SpaceX. If SpaceX can make it work, Starship will certainly be a winner, sporting the capability to ferry up to 100 people on the 6-month journey to Mars. But what if it doesn't work out? Or what if SpaceX doesn't permit you to use it for whatever reason? It's a private company, after all.  Or what if the US government forbids it for security reasons? Is there another way to make the journey more comfortable? Yes, there is. Let's talk about the Cycler, an interesting and high-potential concept.

A Cycler is both an orbit and a vehicle. Building, to be more accurate, but since some people recoil from the idea of buildings in space, we'll call it a vehicle. It turns out that there are eccentric transfer orbits between Earth and Mars that I don't even pretend to understand, but they are multiples of the Earth-Mars synodic period of 780 days. A synodic period is the number of days it takes for two planets to repeat their positions relative to each other, and of course that relates directly to Hohmann Transfer Orbits; there is exactly one Hohmann Transfer Orbit in each Synodic Period. The first Cycler Orbits 'discovered' took 2 or more synodic periods, but way back in 1985, Buzz Aldrin investigated the possibility of a single-period Cycler Orbit. And he found it, with a lot of help from a very capable young lady, but that's a story for another day. A vehicle placed in this orbit leaves Earth, passes Mars in 146 days, loops out towards the asteroid belt, passes back through Mars orbit in 488 days (no planet there then, sorry), and then loops back around Earth in another 146 days. Once placed in this orbit, a building (sorry, vehicle) only needs occasional minor course corrections or assists to remain there indefinitely, for free. Sounds like a great place for a hotel/cruise ship, no?

So, could we do that if we wanted to? Build one and put it into orbit with existing tech? Most of the bits and pieces already exist. It would just be a matter of putting them together to fit the purpose, and any part that didn't exist would be well within our manufacturing capabilities. Here's one hypothetical that I'm sure people much smarter than me can vastly improve on, but it at least allows us to consider the issues involved.

The International Space Station (ISS) is rapidly approaching its Best Before Date. Not because there's anything wrong with it (well, except the Russian parts maybe, they've been springing a lot of leaks lately) but because it has largely served its purpose and both Congress and NASA have other uses for the money spent keeping it aloft and habitable. Slated for the Great Pacific Ocean Scrapyard in 2024, it recently received a reprieve until 2030. NASA expects by that time to replace the ISS with private space stations, Starships, etc, and the ISS will then move into private hands in some fashion, or the member nations will take their pieces and go home, or it will simply be de-orbited.

It has components that would serve elegantly as the basis for a Cycler vehicle, in particular Harmony and Tranquility (US Nodes 2 and 3) and Svezda, the Russian Service Module. These three units are all designed to provide basic life support services (heat, power, air, water, toilets), Tranquility has medical facilities, and Svezda has engine capability plus a docking port for external boosting (think course adjustments). The three modules have a total of 11 berthing ports. Joining them together via their existing docking ports would create a roughly thirty-meter habitat with lots of circulation room and ample life support. These units likely cost more than a billion dollars each to build, and another billion dollars each to place into orbit. Hopefully, NASA won't do anything insane like de-orbiting them, because we should snap them up when they show up in the Low-Orbit Yard Sale!

Harmony (Node 2) Outside View

And an Inside View

Those pre-owned ISS components give us the basis for a pretty good building, but let's not forget the eleven docking ports! We could, for example, permanently attach three Dragon XLs for extra living space, leaving four ports for the Crew Dragons that will carry our Leonards to Mars Surface, plus four ports for Cargo Dragons full of stores. The existing solar panels on the eight Dragons would provide 100-150 kilowatts of power. And we could spin that all up to 0.2-0.3G to make everyone more comfortable during their trip.

When something sounds too good to be true, it usually is. If you carefully read the fine print, the Cycler zips past Earth at 6.7 kilometers per second, so you can hitch a ride for free but first you have to catch it. It's hustling pretty fast once it reaches Mars as well, but the Crew Dragons can burn off that extra speed by aerobraking. So, can we get all the bits and pieces up to our Cycler with that Falcon Heavy second stage? We established earlier that a Falcon Heavy with no payload can reach LEO with 85.5 tons of fuel onboard. The Crew and Cargo Dragons both weigh just under 4 tons empty, and the Rocket Equation Calculator tells us that the Falcon Heavy second stage can accelerate 13.5 tons 'payload' to 6.8 kilometers per second, plenty to reach the Cycler. That 'payload' includes the Falcon Heavy second stage itself (4 tons) plus the Dragon (4 tons), so the real payload is 5.5 tons, plenty to keep four Leonards alive for 5 months. So it will cost two Falcon Nines to get the four Leonards, a Crew Dragon, and a Cargo Dragon to LEO, plus an expendable Falcon Heavy to get the fuel up. Multiply that by four and you have a 16-person expedition travelling in relative comfort to Mars!

That sounds like an awful lot of effort and money, eight Falcon Nines plus four Falcon Heavy Expendables for each Cycler-trip, but the 'customer cost' for this would only be $67M*8=$536M plus $160M*4=$640M, or 1.176 billion dollars. That's peanuts to put 16 colonists on Mars Surface, much less than what NASA paid to deliver the two Mars Exploration Rovers.

This will all get much, much easier if SpaceX can sort Starship out. A Starship in LEO with full fuel and a 100-ton payload can generate 6.9 kilometers per second (this will almost certainly get better, but that's the minimum at present). Trip costs for the much larger spacecraft go down, as the fully recoverable spacecraft delivers payload for basically the price of the fuel. Now you could, for example, insert four Starships into a Cycler-orbit; one as a permanent base, one with 50-100 Leonards, and two for equipment and stores. And the beauty of putting them into a Cycler-orbit is that it's so much safer. If anything goes wrong, just sit and wait, and 780 days later you're back circling Earth!

Travelling to Mars will be dangerous for the foreseeable future. There are just so many things that can go wrong and kill a ship-full of colonists. We understand most of those dangers, and we have the technology and the plans to address them. Our next few posts will discuss the danger that we don't know enough about, and that we haven't yet worked out exactly how to address. The scary radiation thing.

Stay tuned, and thanks for reading along!