Space is vast. But most of it is empty, and we pass through only to get somewhere. The people who live and work in space will mainly do so somewhere, in some region of local space, most often a planet's orbital space, including the moon systems of giant planets.
So to celebrate my return to regular blogging (touch wood!), some old fashioned goodness: ships and travel in orbital space, mainly Earth's: exactly what it says on the tin.
But first of all I want to thank all of you who have visited Rocketpunk Manifesto during my prolonged absence. Especially I thank the commenters here for keeping the conversation going, and in exemplary fashion. You are why I am back here to talk more about space.
Orbital and local space get lip service, with most of our attention drawn to the grandeur of interplanetary or interstellar travel. But orbital space, and the ships that ply it, deserve more attention.
Every journey from world to world passes through orbital space; indeed begins and ends there, unless your starships land directly on planets. A routinely spacefaring future will surely have many stations and other habitats in orbital space, or on the Moon or a counterpart. And every world's orbital space is unique, shaped by its particular circumstances. Mars has two tiny moons, close in; Earth has a single enormous one at the far fringes of its orbital space.
Local space may also emerge in regions far from any large body, perhaps because of interesting concentrations of small objects, e.g. asteroids, or simply because habitats have congregated there. Wherever people gather in space, with regular traffic among them, there is a region of local space.
This traffic has a tempo and flavor quite different from deep space travel. Travel times are short: four hours to geosynch, the popular geosynchronous 24-hour orbit; three or four days to the Moon.
Spacecraft in orbital service will range from moonships down to what I call taxis, minimal space capsules used to move between larger spacecraft that have made rendezvous but are not docked together. Most local craft will be fairly small, because they can be. Passengers can be accommodated coach fashion, in airline type seats (or just above them, loosely strapped in). Crews may have a little more room to float around, but probably do not live aboard their craft between missions.
Maximum design endurance is perhaps two weeks, the current standard. The distinction between ships and stations, which can be a bit blurred in deep space, is sharp in local space: stations and habs you live in versus craft you travel in.
Passenger ships surely have viewports, because the views are spectacular. Orbital space itself is vast, a thousand times a thousand miles across, but it does not quite share the chill loneliness of deep space, weeks and many millions of kilometers from anywhere. In all, there is something comfortably human about travel in local space, especially a world's orbital space.
And this travel will most likely be aboard plain old chemical-fuel rocket ships, surely into the midfuture, and even in what the commenter community here has dubbed the PFF, the plausible far future.
My text for this sermon is the set of delta v maps, especially the first of them, at the still ever-growing Atomic Rockets site. These maps show the combined speed changes, delta v in the biz, that you need to carry out common missions in Earth and Mars orbital space, such as going from low Earth orbit to lunar orbit and back.
Here is a table showing some of the missions from the delta v maps, plus a few others that I have guesstimated myself:
|Low earth orbit (LEO) to geosynch and return||5.7 km/s powered
(plus 2.5 km/s aerobraking)
|LEO to lunar surface (one way)||5.5 km/s
|LEO to lunar L4/L5 and return*||4.8 km/s powered
(plus 3.2 km/s aerobraking)
|LEO to low lunar orbit and return||4.6 km/s powered
(plus 3.2 km/s aerobraking)
|Geosynch to low lunar orbit and return*||4.2 km/s
|Lunar orbit to lunar surface and return||3.2 km/s
|LEO inclination change by 40 deg*||5.4 km/s
|LEO to circle the Moon and return retrograde*||3.2 km/s powered
(plus 3.2 km/s aerobraking)
|Mars surface to Deimos (one way)||6.0 km/s
|LEO to low Mars orbit (LMO) and return||6.1 km/s powered
(plus 5.5 km/s aerobraking)
Two things stand out in this list. One is how helpful aerobraking can be if you are inbound toward Earth, or any world with a substantial atmosphere. Many craft in orbital space will be true aerospace vehicles, built to burn off excess speed by streaking through the upper atmosphere at Mach 25 up to Mach 35.
But what really stands out is how easily within the reach of chemical fuels these missions are. Chemfuel has a poor reputation among space geeks because it barely manages the most important mission of all, from Earth to low orbit. Once in orbit, however, chemfuel has acceptable fuel economy for speeds of a few kilometers per second, and rocket engines put out enormous thrust for their weight.
In fact, transport class rocket ships working routes in orbital space can have mass proportions not far different from transport aircraft flying the longest nonstop global routes.
A jetliner taking off on a maximum-range flight may carry 40 percent of its total weight in fuel, with 45 percent for the plane itself and 15 percent in payload. A moonship, the one that gets you to lunar orbit, might be 60 percent propellant on departure from low Earth orbit, with 25 percent for the spacecraft and the same 15 percent payload. The lander that takes you to the lunar surface and back gets away with 55 percent propellant, 25 percent for the spacecraft, and 20 percent payload.
(These figures are for hydrogen and oxygen as propellants, currently somewhat out of favor because liquid hydrogen is bulky, hard to work with, and boils away so readily. But H2-O2 is the best performer, and may be available on the Moon if lunar ice appears in concentrations that can be shoveled into a hopper. Increase propellant load by about half for kerosene and oxygen, or 'storable' propellants.)
Propellant thirstiness does impose odd logistics and economics, because every ton of payload needs three or four tons of propellant to dispatch it on each leg of major trips. Inter-orbit tankers and other bulk cargo can ride slow solar-electric kites, taking a couple of weeks to spiral up and down to geosynch, a month or more to the Moon and back. But these are not for human travel; besides being slow, they spend days at a time in the Van Allen belts.
Nuclear thermal rockets, NTRs - the original atomic rockets - are one alternative, but a limited one. For local operations their engines must have all-around shielding, because an unshielded nuclear reactor poses a low-level but significant long term radiation hazard out to an amazing distance in space - about 100,000 km radius for a gigawatt reactor. This would not make for good neighbors in local space.
Heavy shielding limits nuclear propulsion to larger spacecraft, probably in the thousand-ton class, and with relatively sluggish performance: landing even on the Moon is problematic. Big ships do get the most saving from halved propellant consumption, but nuclear propulsion is not a panacea for travel in local space. Torch-level drives would be worse; torchships must normally stay out at the fringes of a world's orbital space, met by rockets to ferry passengers up and down.
Other possible options - laser propulsion or other beamed power, mass drivers, and so on - have their own constraints. And even outright magitech drives will be hard put to match the flexible power of rockets for people in a hurry. Not to mention that if you want opera, big rockets are positively Wagnerian.
This is (almost) the final thing to note about travel in orbital and local space: how operatic and rocketpunk it is. Rocket ships! A world swelling up in the viewport, becoming a landscape below you as your ship arcs down to a surface landing ...
Aboard a ship that might even be streamlined, with wings or fins, built for aerobraking as well as landing on airless worlds. Ordinary transport types would not combine these features, but emergency response craft, built for versatility rather than economy, might well do so. We will look more closely at such ships in our next exciting episode.
But the most astonishing thing about travel in local space is that it not only is operatic and rocketpunk, it is also real. Forty-six years ago next month we carried out the combined lunar orbit and lunar landing missions, returning safely to Earth. So the only real matter for speculation is not whether we can cross orbital space to the Moon, or even (details aside) how, but only when we will decide to go back again.
The image comes from a blog post reflecting on Apollo 8, which as the author says deserves to be more remembered. I remember looking up at the half moon at twilight on a clear Christmas Eve, in awe that there were people up there.