Monday, June 22, 2015

Adventures in Orbital Space

Apollo 8 above Luna

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
(all powered)
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
(all powered)
Lunar orbit to lunar surface and return 3.2 km/s
(all powered)
LEO inclination change by 40 deg* 5.4 km/s
(all powered)
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
(all powered)
LEO to low Mars orbit (LMO) and return 6.1 km/s powered
(plus 5.5 km/s aerobraking)
* Not in source table; delta v estimates are mine.

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.


Nyrath said...

Kudos for chemical propulsion. Somebody else (whose name escapes me at the moment) pointed out that chemical propulsion becomes an incredibly useful interplanetary solution the instant orbital propellant depots become available in low Earth orbit.
That is one way of dealing with the "Halfway to Anywhere" problem.

KraKon said...

In that relatively tiny portion of space that lies between the edge of the atmosphere and lunar orbit, there seems to be a plethora of spaceships designs.

You might even make out a small ecology.

Light lifters carry people into LEO. I see them as two stage rockets. The first is an efficient, reusable stage optimized for burning out of Earth's gravity well. The second is a vacuum optimized LH2/O rocket that finalizes the orbit and docks it with a passenger shuttle.

The passenger shuttle is entirely robotic, efficient electro-thermal drive. Low thrust, high PWR fr an electrical drive at the cost of efficiency. It moves the passengers from LEO and into a docking sequence with a space station or orbital fuel deport. The station/depot could operate a heavy, shielded nuclear reactor to recharge the low orbit shuttle or even beam power to it.

The passengers then move into a High orbit shuttle. This one is very large. It uses the cheapest lunar regolith-derived fuels in large quantities. Likely to be built in space. A 'space' craft through and through. This shuttle either carries the passengers to an interplanetary vessel (it would have a radiation-shielded nose to protect against the harmful nuclear drives the interplanetary ship uses) and back, or to the Moon, where it is refuelled for the return trip.

Cargo takes a different route.

It uses a brutal three-stage ascent craft that places it directly in the final orbit. Advanced technologies that worked for the passenger lifter with a 20 ton payload (beam riding, LANTR, linear accelerators) won't work with a 200 to payload, while those that had enough PWR for the small payloads would become insufficient. Since cargo, especially inert cargo, can be launched from remote locations far away from a practical spaceport, new options open up, such as gas-core nuclear drives, Orion drives or simply massive chemical stages.

The fast and hard ascent is in contrast with the gentle orbital transfer. The cargo is docked to an exceedingly efficient electric thruster, running on nuclear or solar energy. It might even be a scaled down version of interplanetary rockets. This cargo tugs slowly moves the cargo into the desired orbit, such as an orbital workshop, a fuel deport or even something to be dropped off in lunar orbit.

Lots of variety many different roles and requirements. I like it.

fro1797 said...

Chemical rockets can be used for a variety of secondary uses, especially in orbit. Chem-fuel rockets used as boosters for response craft, augmenting their primary engines, or used to boost cargo into a long, slow trajectory are a very real possibility in the near future. Having little steerable rockets strapped to a frame would be the most basic "space taxi", with something on the order of a Gemini capsule being the median passenger vehicle. Something like a TransHab with a rocket engine would be the orbital equivalent of a Metro bus. I think that a lot of cargo or passenger modules would have an array of propulsion options, from a small chemical rocket to an NTR to an electric thruster, depending on the mission. Orbital stations would have as much diversity as the orbital spacecraft. Research stations, space hotels, industrial stations, transfer stations, refueling stations, customs, intelligence gathering, military, and who-knows-what else we find a need for in orbit. At some point, traffic control and enforcement would be needed to keep this impending chaos under control. As more people start working in orbit, the more positive control will be needed, traffic growing exponentially. It has the potential of becoming as complex as the total air traffic control system world wide on Earth.


Aaron said...

This is great! I just discovered this blog, and now it's getting new material too :)

Questions for any of the smart folks that post on here:

Would these types of rockets work for travel to the inner planets e.g. Saturn and Mercury?

Is there such thing as a realistic SSTO craft? Or is multi-stage-to-orbit the way to go?


Brett said...

I'm totally pumped about Local Space now. Chem-fueled rocket-ships flying among the space habitats and mining facilities, scenic views - it's got everything for some good space opera. And the distances involved are still big enough that they can involve days, weeks, and even months of travel if you want a longer duration.

Chemical rockets are just good for that type for that type of distance, as you mention. Solar- and nuclear-electric don't start to turn the tables on travel time until you're heading out into the Outer Solar System, and even then you might be able to try chemical propulsion if you're crazy enough to aero-brake through Jupiter's atmosphere.

I think launches in the Plausible Far Future will be a combination of rockets and some type of mass driver system if you need high volume of travel up. Building a mass driver on Earth necessary to accelerate a ship to orbital speeds is a tall order, but building one that merely helps to frontload some of the velocity change isn't so bad. And once you're up there amidst the propellant factories/depots, you can have your ships slow down as much as you want for easy landing back on Earth.

Brett said...

Welcome Aaron!

Would these types of rockets work for travel to the inner planets e.g. Saturn and Mercury?

For Mercury, definitely. They'd be great for travel to anywhere in the Inner Solar System and Local Space around Earth.

For Saturn . . . you could use them to get to the planets in the Outer Solar System, but you'll lose in travel time to solar- and nuclear-electric propulsion, like ion drives. Since the electric propulsion also has much better fuel efficiency, you'd probably want to use that instead - although once you're around those planets, you'd definitely use chemical propulsion to move between their moons.

We can see that with the robotic spacecraft launched on direct flybys of Jupiters with commercial rockets versus the ones using ion drives. Dawn passed Jupiter after launching on Earth about 13 months earlier IIRC, while the Voyager spacecraft mostly took about 22-24 months.

Is there such thing as a realistic SSTO craft? Or is multi-stage-to-orbit the way to go?

We'll have to wait and find out. The US military is experimenting with some of the technology that might make it possible (scramjets for missiles), but it's an open question so far.

Jim Baerg said...

Rick: you mentioned aerobraking for getting into low orbit around any planet with an atmosphere.
Atomic Rockets mentions a neat way to do that that *may* turn out to have major advantages.
Atomic Rockets links to this with more detailed information.

Another blog with lots of discussion of techniques for getting around the solar system in the Plausible Mid Future is:

Hollister David said...

I'm happy to see a conversation on the earth-moon neighborhood. Some intereating regions are Earth Moon L1 and L2. If getting to Mars or other inner system destinations is the goal, EML2 is on the 10 yard line, energetically speaking.

Quite close to EML2 are the lunar cold traps (2.5 km/s delta V) or a Near Earth Asteroid in a lunar retrograde orbit (.4 km/s from EML2). The cold traps or a parked NEA could be sources of propellent, water and oxygen.

It is quite plausible to park small NEAs with earth like orbits in lunar orbit.

Not only is EML2 closer to destinations beyond the earth moon neighborhood, but it's also close to important orbits such as geosynch. As our appetite for bandwidith soars, I hope to see a lot more going on with comm satellites.

Jim Baerg said...

Brett: Re the mass driver system for launches from earth.
Have you heard of the Lofstrom loop?

Assuming the thing can be built, I would put in on the equator (probably near New Guinea) launching east. I would also put an orbital tether in synchronous orbit over a point about 120° east of the east end of the launch loop. The tether would only extend down just low enough to catch spacecraft sent from the launchloop, rather than down to the ground.

Most cargo sent from the launch loop would be given the right velocity to reach the tether, but occasionally some other destination will be reachable with a different exit speed from the launchloop.

Traffic going down from the tether can be dropped from the end to enter the earth's atmosphere about 120° from both the tether & the launch loop. Note: 120° rather than 180° because the earth rotates about 60° in the time it takes something to do half a geosynchronous transfer orbit.

Most traffic from earth to space would go via the launch loop & tether. Traffic from space to earth could go either via the tether or directly enter the atmosphere. Given this system there wouldn't be much below synchronous orbit except automated satellites for earth observation, GPS, signal relay etc.

The tether would need some sort of propulsion system to keep it in place even when the mass going up & down is unequal.

Krakonfour said...

Hollister: how much deltaV is required to reach EML2 from LEO? And from Low lunar orbit? I'm trying to visualise the mass ratios of the ships making the refuelling runs, and from there the propellant quantities required.

If I multiply that by the average number of trips, I can then estimate the amounts of lunar resources being lifted from the lunar surface and what level of industry would be required to provide those amounts.

For example, a solid core nuclear thermal Rocket using water propellant for 4km/s eV and a 100 ton payload would require about 300 tons of propellant to be able to reach the moon from LEO and circularise at destination. If we have 10 trips a month, we'll need at least 3000 tons production on the lunar surface, likely more if we include the propellant cost of bringing it into lunar orbit.

Water content on some parts of the moon rises to 2% by mass, so a well placed mining industry would have to sift through 150000 tons per month, or 5000 per day. Look at this:

Rick said...

Aaron - welcome to the discussion threads! You can see why Winch and others say that the comments are the most valuable part of this blog.

On specifics, I suspect that two stages to orbit, TSTO, wins out for at least a long time to come. In a nutshell, staging is inconvenient and has risks, but enormously increases the payload you can take up for a given launch weight.

Everyone -

I was actually startled by the power and versatility of chemical rockets, once I looked closely. The image of huge vehicles, mostly fuel tankage, and tiny payloads is so deeply rooted (and valid, for Earth lift!) that we forget how much chemfuel can do once you are in orbit. True, interplanetary travel on Hohmann orbits is long, slow, and tedious, but if you have the patience, chemfuel has the punch. And in Earth orbital space it is hard to beat.

My bias, for now, is to stick heavily to chemfuel rockets at least for local space, versus alternative techs (such as tethers). Rockets are nearly speculation-free: we know what they can do because they have done it. Even nuclear thermal is not quite in the same class; we bench tested it once, with NERVA, but don't (yet) have operational or development experience.

That said, once we are up there on a substantial scale, we can and will test the other options. Some, perhaps most, will go back on the shelf; others will become mainstays, and people will wonder how we ever got along without it.

One curious example is aerobraking. Back in the rocketpunk era it seems not to have been thought of - everyone knew you had to re-enter the atmosphere to get back to Earth, but I don't recall it being used for slowing down to enter orbit. Real spaceships were supposed to be completely unstreamlined. Aerobraking ships may not be 'streamlined,' exactly, but they are aerodynamic vehicles that don't have odd bits sticking out when they hit the atmosphere.

Great information on mining costs, highly relevant to my upcoming post.

And an odd little question about the Lagrange points. Do EML4/5 really have much practical advantage over other lunar-distance orbits? Objects adrift in the Lagrange zones stay in the zone, instead of drifting all over and getting perturbed into radically different orbits. But active ships and stations don't drift: they stationkeep. Is stationkeeping significantly easier at the Lagrange points?

If not, I can think of a slightly macabre use for them - as dumping ground for things that ARE set adrift - such as decommissioned nuclear engines, that you don't want around but are hard to get rid of. So L4 becomes a Sargasso of abandoned hulks ...

Rick said...

... And there's more!

The Weekly Moonship

Hollister David said...

Krakonfour, Using Farquhar's 9 day route, LEO to EML2 takes 3.4 km/s. Taking a 70 day route, LEO to EML2 can be as low as 3.1 km/s. I go over this in my post: .

From an asteroid parked in a 40,000 km retrograde lunar orbit, it would be about .4 km/s to EML2.

There may or may not be rich volatile deposits at the lunar poles. Spudis believes some lunar cold traps have two meter thick sheets of relatively pure water ice. If the water deposits are only 2% by mass, I am not sure it would be worthwhile to mine lunar water.

Hollister David said...


In an ideal 3 body system, L4 and L5 are very stable. However the earth-moon system is subtantially influenced by the sun. In my sims, EML4 and EML5 are not long term stable when the sun is included. Even these locations would need some station keeping.

The 5 earth moon Lagrange regions all need some station keeping but not as much as non Lagrange regions in high earth orbit.

Of the Lagrange regions, L1 and L2 are the most interesting. These are hubs of many low delta V routes.

Rick said...

Hollister/Hop - Welcome to the discussion threads!

Almost a bummer about the EML points - that Sargasso of abandoned space hulks would be a cool story element. :(

On lunar ice, my first take is that a lot depends on what you have to do to get at that 2 percent (if that turns out to be the figure). My mental image is Moon-dozers scraping up rime-rich regolith and dumping it into a pressure hopper where you warm the stuff enough to melt the ice (or mildly bake it to vapor).

The Moon-dozers strike me as the toughest part - even more than the cracking plant. They may look like ordinary bulldozers, might even be built by Caterpiller, but they are spacecraft that operate under lunar conditions, and a long long ways from your friendly equipment-parts dealer. See my brief discussion in the Daily Moonship post.

Great blog you have! But lordy, that fine-grained orbital mechanics is serious advanced workshop. I am okay (usually, I hope) at avoiding the really stupid mistakes like forgetting or completely bolluxing the Oberth effect. But when it gets to near navigation-level analysis ... yikes!

Tony said...

While a neat intellectual toy, I'm not seeing the real world motivation. Even space tourism is probably not the answer. Even the manned exploration of the inner solar system is probably done better without much (if any) infrastructure in orbit, given that the payload tonnage one might expend on a large orbital infrastructure is probably better spent on the exploration vehicles and payloads themselves. In the next topic, Rick imagines a baseline 300,000 tons of on-orbit and lunar mining infrastructure, plus 30% annual replacement. If that much effort were all dedicated to going to Mars or perhaps the asteroids, it would probably be best, for practical reasons alone. But there are more than practical reasons. Since the primary motivation of people in space pretty much has to be sending them places where they can expand the frontiers of human existence, it's really tough to make a moral case for a manned orbital environment.

Now, all is not lost. There is one orbital platform that might make sense, both practically and morally, but it's not in Earth or Lunar orbit. It's a Mars cycler. There could be real value in reusing habitat and power systems for Hohmann-like transits to Mars and back.

Hollister David said...

Rick, if you want Sargasso, Jupiter's Trojans fill the bill.

The typical 3 body scenario has a central body with the most mass, an orbiting body with less but still substantial mass and a 3rd body of neglible mass.

Except for the sun, Jupiter is by far the biggest frog in our solar system pond. So Sun-Jupiter-Trojan asteroid is a pretty approximation of the ideal 3 body scenario. The other puny planets don't exert enough gravity to destabilize Jupiter's Trojans.

As evidenced by the subsantial populations of rocks at the Sun-Jupiter L4 and L5.

Neptune and Uranus are pretty big and their orbits might be far enough from Jupiter that they too might have substantial sargassos 60 degrees in front and behind. I like to imagine these planets also have healthy populations of ancient solar system detritus.

Hollister David said...

Tony, Mars isn't the only interesting destination in the solar system.

For a big moon or a planet, we can only dig so deep and pressure and heat bar us from burrowing deeper. So available real esate and resources are measured in area

I suspect the asteroids have more surface area than the big moons and rocky planets of our solar system. Further, their entire volume is accessible.

So if the goal is real estate and resources, the asteroids are the most interesting goal.

Further, with their shallow gravity wells, asteroids are easier to land on and/or depart.

In the near term, there a some near earth asteroids that could be parked in lunar orbit with very plausible space craft. See the Keck Report on asteroid retrieval.

aaaronnn said...
This comment has been removed by the author.
aaaronnn said...

Rick and Brett -- thanks so much for the replies, really making me feel welcome :)

Building on what was said here, I'm wondering what these chem fuel rockets would look like?

Lets assume that they're already off the earth (halfway to anywhere) -- what would the volume of propellant to cargo/crew habitat look like?

For travel between LEO/Orbit, Lagrange points, the moon etc.?

Rick said...

Aaaronnn - I will be saying more about ship configuration - probably a lot more - but a preview:

Ships in orbital space fall into two broad classes, those designed for aerobraking and those that are not.* The latter will look like spacecraft, no streamlining and probably no single 'hull', just tanks and modules joined together. But less flimsy than low-thrust deep space craft. Figure, for the size range I'm discussing, 100 tons of spacecraft structure plus payload, and 100-300 tons of propellant. If you are using H2-O2 propellants (let alone pure hydrogen for electric or nuke thermal ships), the fuel tankage will still dominate their appearance, because hydrogen is so bulky.

Aerobraking ships are aircraft, with a unitary airframe structure and aerodynamic configuration. This does not mean they look like airplanes, though some probably will. There are, I think, two basic configurations for rapid aerobraking, a stubby cone that hits the atmosphere base-end forward, like a space capsule, and wedge-shaped like the Shuttle, coming in on one side (the belly, in aviation terms). The wedge-shaped ships would be roughly the size of the Shuttle, but with a bigger fuselage, much like the external tank. The cones (for the 100 ton range) would be much bigger than today's capsules - about the size of a 3-story townhouse. Both configurations have advantages, but the jumbo cones may be harder to launch on top of a booster rocket. And fabricating fuselage structures in space is advanced workshop, not like snapping Legos together.

*There is also a gradual type of aerobraking, higher up, using many perigee passes to shed speed bit by bit. It requires no heat shield or aerodynamic form, but is much too slow for human travel.

Jim Baerg said...

"Aerobraking ships are aircraft, with a unitary airframe structure and aerodynamic configuration."

Or maybe not see the links I posted up thread to magnetohydrodynamic aerobraking.

Rick said...

I did see the links - demanding stuff, so I only skimmed at this point!

My first take was that it would probably involve a somewhat higher techlevel, or at any rate substantial electrical generator capacity aboard the spacecraft. For this series of discussions I'm taking a very conservative perspective, basically things we already know how to do.

fro1797 said...

You might need a second MHD to power the aerobraking MHD...a big bite out of your payload. But if you don't have a heavy heat shield, it might not be that bad. I'm not sure what the numbers on that would be.