Wednesday, July 1, 2009

Space Warfare IV - Mobility

In space everything is moving, and all motion is relative. Hollywood naturally ignores this, with good reason from their perspective. With a handful of honorable exceptions, such as Babylon 5's Starfuries, media spacecraft move like airplanes or sea vessels, and when their drive engines fail they 'stop.' This Aristotelian physics, in fact, matches the universal human experience, with partial exception for the 499 people who have so far traveled into space.

Like the planet Mercury, real spacecraft do not move according to Newtonian physics. But unless you have an antimatter drive or some such, Einstein can be ignored (at least for navigation and maneuvering). This is fortunate, because dealing with Newton is complicated enough.

Whatever their drive (so long as it is non-magical), spacecraft move not like ships or airplanes, but rather like self-propelled artillery shells. Once they fire themselves into a particular orbit they can change that orbit only by another burst of power, expending more propellant in the process. (With the one exception of using a planetary atmosphere for 'aerobraking.') A task force headed, say, from Earth to Mars probably has reserve fuel for an abort option: an orbit returning to Earth, or to some friendly base. It probably does not have enough reserve to change orbit and attack Ceres instead - at least not without cutting dangerously into reserve margins, and likely not even then.

Fuel supplies and possible orbits will thus dominate every aspect of space warfare. In the familiar expression, amateurs study tactics while professionals study logistics. In space even amateurs ignore logistics at peril of finding themselves on orbit to nowhere. Assuming you avoid this and are on orbit to somewhere, if it is a hostile somewhere you will have a fight on your hands once you arrive. Unless you perform an abort maneuver, arrive there you most certainly will.

And you will arrive at impressive speed, relative to an enemy on a different orbit. Even economical (and sluggish) Hohmann transfer orbits imply typical encounter speeds of a few kilometers per second. If you take a fast, 'steep' interplanetary* orbit, encounter speeds can be dozens of km/s, on up to whatever your drive tech allows.

If you intend to stick around wherever you are attacking (as opposed to a 'drive by' raid), you may perform your orbit-matching maneuver before arriving in fighting range of the enemy, and encounter speed will be lower. But if you are fighting anywhere near a planet, its gravity well also has to be taken into account. Generally, unless the two sides tacitly agree to match orbits before the shooting starts, encounter speeds of at least a few km/s will be the rule. Which brings us to tactical maneuver.

The maneuver characteristics of spacecraft correspond roughly to the mathematical first derivatives of their terrestrial counterparts. Range, in the sense of cruising radius, hardly exists as a concept. Whatever your orbit, you will stay on it indefinitely until you change it or run into something. What a big fuel tank (or potent brand of fuel) gives you is not longer range but greater speed, the first derivative of motion. More precisely it gives you more delta v - the capacity to change your orbit. What a whomping powerful drive engine gives you is not speed but acceleration (the first derivative of speed): the ability to change orbit abruptly.

For a given power output there is a theoretical tradeoff between delta v and acceleration, in engineering terms specific impulse and thrust. For chemfuel this doesn't matter in practice, but for advanced drives it could be important. Some proposed drives, notably VASIMR, can operate in more than one mode, providing the equivalent of afterburning (though based on an entirely different principle). Such drives might have a fuel-efficient but low-acceleration cruise mode for deep space missions, and a combat mode providing short bursts of high acceleration for combat maneuvers.

Alas for Romance**, there is serious question whether dashing combat maneuvers are very useful in space. You can only hope to dodge a laser beam at very long ranges, probably a light-second (300,000 km) or more, and at that range a second or two of jinking will not affect the tactical geometry. If chemfuel missiles are in use they can be outdistanced; while their acceleration is very high their delta v is severely limited, a few km/s, and a missile that has run out of fuel is very easy to dodge. In contrast, missiles fitted with a scaled down version of ship drive - 'torch missiles' - are in Ken Burnside's vivid expression like police dogs. Needing fuel only for a one way trip they can outperform any practical ship. If one is sicced onto you it will hunt you down, and can be stopped only by shooting it. (Or throwing it a bone, perhaps!)

All of which suggests that - absent other complications - space tactics may have somewhat the flavor of 18th century formalism: Constellations of space warcraft approaching each other in well-defined formations, their engagements unfolding with the stately grandeur of a battle between mobile Vauban forts.

Combined with the inexorable quality of space motion - no calling a halt to think things over - this formalism could make space battles rare. If you are outgunned, clever shiphandling probably won't save your bacon. If you are the attacker, think long and hard about that abort orbit. If you are the defender, this might be a good time to seek mutual understanding through dialogue. Perhaps you can talk faster than the other side can shoot.

But what, you may ask, are those 'other complications' that could add a dash of dash to the mix? The most promising of these is a battle in orbital space, amid a clutter of civil spacecraft and stations that neither side is eager to destroy. Why the defender would prefer not to wreck their own infrastructure is obvious. Why the attacker might wish not to destroy these things will, for now, be left as an Exercise for the Reader. Suffice to say here that orbital combat near a planet has a multitude of complications that make it potentially both quite different from and more interesting than battles in deep space.

* For this discussion, enroute to or from a wormhole or other FTL jump point is just another interplanetary orbit.

** In the older, broader sense of Romance; the issue has no obvious bearing on love affairs.


Related links: A general discussion of interplanetary movement. And as always, Winch Chung's Atomic Rockets pages are a treasure trove of relevant information and commentary (plus a century of SF/space related eye candy).

58 comments:

Marktheother said...

It seems to me that most deep space engagements would be an attacking force on its way to a planetary orbit, vs fairly simple interceptors launched from said orbit. When the defenders realizes a constellation of spacecraft are aggressive it should be easy to lob heat seeking missiles on a reciprocal orbit. the attackers could mix themselves in with civilian cargo haulers, which would probably require a manned attack much closer to the defending orbit on an intercepting orbit.

I figure most fighting will happen in planetary orbits, or to keep people out of orbits, because there isn't really anything in deep space to fight over.

-Mark

Sundog said...

That depends on what the enemy is trying to do. An invasion force can be engaged close to the target planet fairly safely; but a raiding force is a different story. If they want to obliterate your orbital infrastructure and do surgical strikes on selected planetary targets, they won't be interested in fighting you orbital fortresses and planetary defence centers - they'll just blow past the planet firing as they go. In that case, you want to engage as far from the planet as possible, to give your point defence maximum time to engage any weapons they DO send at your infrastructure.

Worse, in the case of a Genocide Strike (mega-impactors being the obvious such)you're going to need to go out and deflect that big rock - while the attackers attempt to escort it.

quietmagpie said...

You mention Babylon 5.
If I might ask... what was your view of the realism or otherwise of space combat as it was depicted in the new version of Battlestar Galactica (which had muted sound effects to impart a sense of vacuum) ?

Anonymous said...

The part mentioned in Marktheother's comment about space combat being primarily fought around planets and other such Celestial Objects does make sense. Empty space, that is to say a part of the vacuum in between planets that is only occupied by the occasional micrometeorid, is effectively empty. There are no resources to extract, no colony or settlement to protect, absolutely nothing. The only way Empty Space would be worth fighting for would be if someone put something valuable there like an asteroid or derelict constellation of combat craft.

Interceptions would be the most common form of Deep Space combat if only because the defenders of that strategic location would want to cause as much damage as casualties as possible en route so that the attackers are not at full strength when they left their own Territory-Astrotory-Aerospace. Of course intent would be hard to detect unless combat spacecraft have a specific "look" that would distinguish them from civilian transport craft. The best way to disguise a Combat Constellation en route to a possible Battle Orbit outside the questionable Q-Ship idea would be political and diplomatic deception. That is to say, the so-called humanitarian constellation sent by Faction A to help with Faction B's space station is actually an occupation force whose true nature is revealed when the Espaciers have pacified the station crew.

And for those who think that strategic orbital bombardment against planetary infrastructure and defenses are as just as easy a kill as orbital targets, you may want to re-read the June 2009 entry on the Gravity Well. Even bombardment from higher orbit wouldn't be that easy without adding in the defense constellation since an hour flight time for your missiles and what nots equates to an hour that the planetary defenses can track and take out the weapon. Directed Energy Weapons might shorten the targeting window, but there are things such as Blooming and Atmosphere that might limit those system's usage that high above the clouds.

Add in more space faring nations than Faction A and B and it becomes a situation where even though no nations would want to get into a war with anyone, no one wants any one side to have complete orbital dominance. I believe I got the idea from this site: http://www.projectrho.com/rocket/ThroughStruggleSample.html

Now that I think about it, the paroling Constellations would be more akin to B-52 bombers back during the 1950s in which these aircraft would make orbits near the USSR so that whenever nuclear war was declared, they were already close enough to their targets. The only difference being that the Constellations can stay in their orbits indefinitely and continually resupplied by transport craft.

- Sabersonic

Rick Robinson said...

I tend to agree that engagements will usually happen near some objective, not in the middle of nowhere. But 'near' in strategic terms, not necessarily tactical. As Sundog said, defenders might choose to engage at the apoapsis of a long elliptical orbit, positioning themselves well 'in front' of the objective they are defending.

This also depends on the attacker's objectives and the culture of warfare. On the one hand, the clutter of orbital infrastructure can provide the defender with concealment; on the other hand, that puts civil infrastructure in the middle of the shooting gallery.

It is rather analogous to modern urban warfare - do you try to protect a city from a forward position, or accept the damage of urban warfare in turn for posing a more difficult environment for the attacker?

The one case where I can see an engagement in the middle of nowhere is if a force en route from A to B is intercepted by a defender from C. Especially if kinetics are predominant, the steeper the intercept orbit, the harder the punch.


Quietmagpie - Confession that (gasp!) I've never watched the new Battlestar Galactica. When it first came on I took a pass because the original show was so lame. ('Range 30 microns and closing!') And while the new show is highly regarded by a lot of people, they also say that the flavor is very much contemporary USN. My bias is that stories set in a different era should have the flavor of a different era (something Firefly did quite well).

Jim Baerg said...

"Whatever their drive (so long as it is non-magical), spacecraft move not like ships or airplanes, but rather like self-propelled artillery shells. Once they fire themselves into a particular orbit they can change that orbit only by another burst of power, expending more propellant in the process. (With the one exception of using a planetary atmosphere for 'aerobraking.') A task force headed, say, from Earth to Mars probably has reserve fuel for an abort option: an orbit returning to Earth, or to some friendly base. It probably does not have enough reserve to change orbit and attack Ceres instead - at least not without cutting dangerously into reserve margins, and likely not even then."

An exception can be made for spacecraft propelled by solar sails or magsails ( http://en.wikipedia.org/wiki/Magsail ) Since they get their propulsive force from sunlight pressure or the solar wind their delta v is not limited by propellant supply. They are limited to rather low accelerations, so perhaps warships would use a solar or mag sail that can be furled in or dropped if necessary plus some propellant using high acceleration system for use when needed.

quietmagpie said...

Oh.. well... the new Galactica is nothing like the old one. They took the premise and created something completely new, and in my humble opinion it's the one of the best things I've ever seen. Yes the technology and mood is very USN. As to whether a "different era should have the flavor of a different era" ... well the show speaks to that in rather profound way but I'd be giving you a huge spoiler if I spill it out.
Besides the show can also be interpreted as an allegory of the post 9-11 era. But we just talked about allegories didn't we...?

Rick Robinson said...

Jim - Magsails have one other limitation: They can't sail to windward. At least I don't think they can. They can tilt their sails to get lateral vector, but the only way for them to go closer to the sun is to cancel their solar orbital velocity and 'fall.'


Quietmagpie - I'm not making an actual, substantive criticism of BSG. (Which would be pretty stupid, never having watched it.) Only saying why I wasn't tempted by it. I know that it has gotten quite a lot of critical acclaim! But alas, not having seen it I can't comment on how it handles space combat.

Calsir said...

Rick: actually a magsail can only be used as a "tacking" sail :). While it seems to me that the wikipedia article has some flaws and wrong analogies, we have to remember that a magnetic force is perpendicular to both the magnetic field and the velocity of the charged particles.

Solar wind is radial, so the magnetic force won't have any component in the radial direction. This means that the magsail can be used to increase or decrease orbital velocity, effectively letting the vehicle go "upwind".

Of course, as the vehicle speed increases, the particles won't be moving radially in the vehicle's reference frame, but that is a minor effect.

Rick Robinson said...

Calsir - Don't blame Wikipedia, blame an irresistable but inexact analogy. :-) But the magsail still has the limitation that it cannot directly accelerate inward; it can only reduce your orbital velocity so that you 'fall' inward. Which at Earth's orbit is a gentle fractional milligee.

Admittedly we are so accustomed to space boosters fighting their way up through a 1-g field (not to mention living in a 1-g field) that we tend to think of milligee acceleration as nearly insignificant. But 25 km/s in a month is an impressive head of speed.

Doug said...

Magsail vessels may still have the same restrictions on other consummables that could affect possible orbits. Unless life-support is fully self-contained they are going to need to take on supplies evewntualy, even if they can tack around the solar system annoying the enemy to no end.



The new BSG's space combats made some commitment to believable physics, at least in the graphics and visualization. Maneuver was accompanied by thrusters firing, braking without turning around, and the like.

At the same time a Viper's miniscule fuel/propellant tank contains give enough juice to cross a solar system in a matter of hours, and fly against the pull of gravity in a planet's atmosphere for a prolonged period. While there were attempts to make the spaceflight look more realitstic the ideas behind it were still in a Top Gun/Star Wars idiom. There was never a shred of explanation of why Vipers would even be tactically useful, given that the diffenreces of an airborn palne versus surface born ship, don't apply in a space setting. You'll notice that that in spite of the amount of coverage the pilots got the Galactica's gunners never got an episode; and the ground troops who fought a number of battles against the Cylons were never worth their own officers, they were commanded by pilots.

I've heard arguments that the Cylon's technical expertise would have made unmanned fighters useless, but that ignores the question of what good a fighter is anyway. Given the weight of fire that the Galactica's kinetic weapons could put out there doesn't seem to be much call for little flippy ships with vastly inferior weapons, when the two massive launching areas on either side of the Galactica could have been put to far better use as fuel or ammo storage.

Carlo said...

"Calsir - Don't blame Wikipedia, blame an irresistable but inexact analogy. :-) But the magsail still has the limitation that it cannot directly accelerate inward; it can only reduce your orbital velocity so that you 'fall' inward. Which at Earth's orbit is a gentle fractional milligee."

True, but it's "free". And Doug is correct about other limitations. But that is the usual trade-off system of engineering.

In that same wikipedia entry, check the part about using the magsail as "keel" for normal solar sails... I felt sick... Why do people fail to understand that the sailing analogy can only go so far?!

Citizen Joe said...

This probably deserves a topic for itself, but from the grand scheme of mobility of ships down to the microcosm of moving inside a ship with microgee acceleration, there are issues there. Humans are used to 1 G acceleration in the down direction. Working in essentially free-fall is tricky. You're essentially a bunch of marbles rattling around in a tin can. How do you go about solving the working environment issue?

The most fanciful way would be something like artificial gravity generators. But that is essentially magic.

Slightly less fanciful would be the use of magnetism. Either magnetic boots (fairly hard science) but that doesn't solve the floating marbles problem. Or you could create a magnetic field between two plates and anything that you WANT to be 'gravitized' would be doped with magnetically oriented materials. So people would wear suits with rare earth magnetic lining. The pens and clipboard would have magnetic linings. Water would float about in globules though. It is technically STILL microgee environment, but there's a constant force pulling on you through your clothes.

Next more hard scifi would be to accelerate the whole ship at near earth gravity. Without a reactionless drive, that doesn't seem to be continual.

The most hard scifi would be some sort of habitat ring that spins. That has its own problems but should be doable.

One thing to note is that we use Nitrogen as a buffer gas. 100% oxygen has caused some deadly fires in early space ships. The reason we use nitrogen is that it is relatively close to the same density as oxygen, so it will not easily separate out to cause a stratified atmosphere. It is also inert relative to our respiratory system. In a microgee environment, density has little bearing on the mixture of gasses. Instead of Nitrogen, you could use a more readily available (in space) Helium (He4). It is lighter and doesn't need to be shipped up from the surface.

So command decks, where you don't want to be spinning around like crazy, could use the mag-plate system with helium buffer gas. Meanwhile the nitrogen is reserved for the habitat ring, which would otherwise stratify dangerously. Helium also doesn't cause the bends from pressure changes like what happens with deep sea divers and nitrogen. So it makes sense to use Helium for EVA and microgee activities and save the nitrogen for where it is really needed.

Likewise, in space habitats. You may need extra gas in the center to provide more pressure. If people don't interact with it, Helium is much easier to use. You just don't want to fall out of the central axis tram and fall through the core, since it would be devoid of oxygen and be pure helium. You'd be dead before you drifted to the ground.

Anyway, how about some discussion about life support?

Rick Robinson said...

Doug - Magsail craft would still have the consumables issue, but on the whole that is a lot less mass than propellant requires. (Of course there's the mass of the magsail ring, of which I haven't a clue.)

It sounds as if BSG's designers were struggling, understandably, between a desire to get things somewhat right and the imperatives of making an audience (and network suits!) happy. Everyone in Hollywood remembers 'Top Gun,' and space fighters are an excuse to put studly/babelicious 20-somethings in prominent action roles. (Sign of changing times: In the Tough Guide, originally written c. 2000, I only mentioned studly fighter jocks, not babelicious ones!)

Incidentally, another reason I was biased against BSG is that I'm a battleship fan, thus a bit disapproving of carriers on principle. Even sea carriers. :-)


Carlo - using the magsail as "keel" for normal solar sails ... oh, groan. What you said about analogies taken too far!


Citizen Joe - You're thinking of the Apollo 100 fire, in 1967 I believe.

Nitrogen is a serious problem in high pressure - the bends - but I don't think it's a concern for spacecraft pressure levels. And one problem with helium is making people sound like Mickey Mouse. (Silly, yes, but just as with gravity, we are wired for the normal human voice range.)

I tend to think that long-voyage spacecraft will be spun. This has structural inconveniences. The spin radius has to be large or people may suffer nausea, and having both spinning and nonspinning portions of a spacecraft means a rather problematic connection between the two sections.

Life support is certainly worth a discussion!

Anonymous said...

One thing that I've always thought was remiss during discussions on military spacecraft was what kind of drives they might have; the difference between civilian ships and military ones is that civilians' need to worry about efficentcy and military not so much. Military spacecraft may have more than one set of engines; one for cruise and one for combat or intercept. At long range (a few light-seconds), combat spacecraft may look like civilian ones...right up until they lightoff those 'battle boosters' and change trajectories at 1000x that of a civilian ship...hopefully leaving enemy interceptors out of position. Also; a high-acceleration supply vessel would be esentially the same as a high-acceceration Missile Crusier/Heavy Bomber/Gun-Ship, depending on the loadout. Jinking at long range and just barroling in at short range is the opposite of air or naval warfare. Space warfare may well resemble shooting at each other from railroad cars than anything else.(of course, in space you can 'jump the tracks' instead just staying on the one rail line; or, maybe, some weird hybrid of trains and zepplins):)
Ferrell

Anonymous said...

As I was writing my last post, someone posted a reply! oh, well.

"Citizen Joe - You're thinking of the Apollo 100 fire, in 1967 I believe." Too many zeros Rick; a Fraudian slip?

"I tend to think that long-voyage spacecraft will be spun. This has structural inconveniences. The spin radius has to be large or people may suffer nausea, and having both spinning and nonspinning portions of a spacecraft means a rather problematic connection between the two sections." Why not put everything that the crew needs inside the spin section and then put everything else (mission equipment, engines, ect) on a non-rotating hull or scaffling that surrounds the spin section? Just a thought!
Ferrell

Calsir said...

oups I used the wrong signature:

Carlo == Calsir :-)

Rick Robinson said...

Calsir/Carlo - LOL!


Ferrell - Note to myself to do something on drive engines and life support. On drive engines there are, I think, four conceptual options:

1) A fully separate combat drive.

2) A drive fitted with 'afterburning,' e.g. variable ISP.

3) A bigger drive relative to ship mass (loosely analogous to a sports car).

4) Little or no difference (think sailing ships*, or WW II era bombers compared to contemporary airliners).

* Though men of war had much larger crews, and so could do more with their rig.

Apollo 100 - a memory glitch. For some reason that's the designation I've remembered all these years, but a google shows no basis for it!

Why not put everything that the crew needs inside the spin section and then put everything else (mission equipment, engines, ect) on a non-rotating hull or scaffling ...

Why not simply spin the entire vehicle? The only type for which this would definitely not work is solar electric, because of the enormous solar wings that need to face the sun.

Jim Baerg said...

Ciizen Joe said:
"One thing to note is that we use Nitrogen as a buffer gas. 100% oxygen has caused some deadly fires in early space ships. The reason we use nitrogen is that it is relatively close to the same density as oxygen, so it will not easily separate out to cause a stratified atmosphere. It is also inert relative to our respiratory system. In a microgee environment, density has little bearing on the mixture of gasses. Instead of Nitrogen, you could use a more readily available (in space) Helium (He4). It is lighter and doesn't need to be shipped up from the surface."

Actually I don't think the helium would separate out any more than the 1% argon in the earth's atmosphere does. We don't get argon forming a layer at ground level with the oxygen & nitrogen above it.

However would helium really be all that easy to get in space?
Sure there's lots in the gas giants, but it's harder to get stuff from them than from Earth.
We can get some from solar wind that has been adsorbed into the lunar & mercurian regolith, but I suspect that would be too rare & valuable for such things as cryogenics to be used as a buffer gas in space habitats.

If it's hard to get enough nitrogen then argon in potassium rich minerals from the decay of K40 may be used.

Rick Robinson said...

Jim - Good point about the difficulty of obtaining helium. Argon, if conveniently available, would be excellent, without even the relatively minor complications of nitrogen. Plus it doesn't make people sound like cartoon characters. :-)

Calsir said...

@Ferrel and Rick. Spinning a whole spacecraft rises complications, since at least some equipment (thinking of the sails that rick mentioned and communication antennas) needs to be in inertial reference frame.
Also, Ferrel's proposal about caging the spinning section with a non-spinning outer hull rises some issues:
1) the wear of external gear is higher (higher speeds)
2) you cannot use attitude jets to set the habitat in motion... No, you cannot use jets in a confined space :).

IMO, military spacecraft should forget about artificial gravity... they are manned by soldiers, after all. You can provide them with personal centrifuges for exercising, if needed.

Marktheother said...

One problem with using helium is that it leaks. because the atoms are smaller than those of other gases (and hydrogen gas is diatomic) it's really hard to make things helium tight even when they're otherwise gas tight.

I Like Zubrin's nuclear salt water rocket for military spacecraft, since it has both a high specific impulse and high thrust, but it has it's disadvantages, like having high speed radioactive debris for exhaust, and it's fuel explodes if there's too much of it in one place. so it's probably not something you would want to use near population centers. like earth.

What actually get used for engines will probably depend on a number of things, like whether it's a purpose built war craft or an orbital patrol craft that's been hastily modified, or a cargo carrier carrying drones.

Magsails and lightsails could work for unmanned bulk cargo (like jovian He4) you don't have to worry about life support, and it doesn't really matter if it takes years to get there as long as there is a constant supply.

-Mark

Rick Robinson said...

Calzir - IMO, military spacecraft should forget about artificial gravity... they are manned by soldiers, after all. You can provide them with personal centrifuges for exercising, if needed.

I don't think you can dispense with artificial gravity for most deep space missions, unless you have near-magitech drives. Prolonged micrograv isn't just inconvenient, it is debilitating. Exercise alone has not helped, and though we won't know till we test it, I suspect that an hour a day in a centrifuge won't be enough.

A few months in micrograv might be acceptable for control-panel jockeys, but not for troops who may have to get physical, e.g. SWAT type boarding parties.

One possible solution is simply to detach the crew module during the unpowered phases of flight. Another is to design the crew transport to spin, and let some other spacecraft in the constellation carry antennas, etc.

Or, I may be overestimating the problem of coupling spinning and nonspinning sections, especially if constant pressurized access between them is not required.


Mark - one major problem with NSWR is some serious doubt whether it would work as claimed. I'm highly skeptical that the claimed exhaust velocities can be achieved without melting the thrust chamber, since any thermal high specific impulse drive involves temperatures of 100,000 K or more. For that you need some kind of electromagnetic containment.

Anonymous said...

"Or, I may be overestimating the problem of coupling spinning and nonspinning sections, especially if constant pressurized access between them is not required."

I think that even a small electric motor/flywheel would be enough to spin a relitively large pressurized section; the use of magnetic bearings should, at least partially, reduse ware-and-tear on the spin/nonspin interface. Place the primary airlocks on the spin section and not have any pressurized access between the spin and nonspin sections.

Ferrell

Rick Robinson said...

Ferrell - Any motor/flywheel arrangement would set both sections spinning, in opposite directions. :-D

The more I think about it, the spin coupling is probably not that big an issue, so long as you don't have a continual passageway between sections. If the spacecraft is big, design the coupling so that if it freezes up it breaks - better to have to two halves of the spacecraft drift apart than suddenly torquing each other.

But I'd still put the main airlock in the nonspin section - egress from the spin section would be problematic. People won't be going constantly back and forth, so just put a 'spin-o-vator' at the coupling. Go in one end, spin/de-spin, and go out the other.

Anonymous said...

"But I'd still put the main airlock in the nonspin section - egress from the spin section would be problematic. People won't be going constantly back and forth, so just put a 'spin-o-vator' at the coupling. Go in one end, spin/de-spin, and go out the other."
After thinking about it, maybe if you completely enclose the spin section inside the nonspin section (e.g. hull), that may simplify the design.
Ferrell

Citizen Joe said...

I went with a rigid system within a very large ship. Paired counter-rotating habitat rings spun around the central spine of the ship. Those were also enclosed within a pressurized hull, using helium as the bulk gas (since is was readily available). Transfer from the spin section to the central core (and the rest of the ship) was via pods that would engage the drive cog and quickly lift you up and around to the core. There it disengages from the cog and you disembark into the core. Some of those sections were used for fabrication, either for ships or station components. So, after construction was complete within a pressurized simulated gravity area, that pair of spin sections would slow down and position the constructed module in front of the bay doors where the module would then be 'dropped' into the microgee staging area. Then the deck and pressure bubble would be laid for the next project and the sections would be spun up again. I think they were like fifty meters in diameter but the whole ship was also five miles long.

Sundog said...

Spin is easy, provided you have two ships.

Just stick a hawser between the two and rotate around the common centre of gravity. When you approach the combat zone, or want to maneuver, one disconnects and the other reels in the hawser. It also gets around the requirement for a minimum size to the rotational section.

Rick said...

There are two (fairly minor) disadvantages to rotating section enclosed in a nonrotating shell. One is that air friction will tend to slow down the spin section (and spin up the rest) unless you keep feeding power, which ultimately will have to be dumped as waste heat. Also you'll have some extra mass: the pressure shell plus the spin structure itself.

Does spinning up two counter-rotating spin sections cause odd precession effects? (I have no idea.) But if so, a solution would be three spin sections, a thicker one flanked by two thinner ones spinning the other direction.

Ease of access between spin and nonspin sections could easily outweigh these considerations.

Sundog's suggestion, tethering, can equally well be applied between two sections of one spacecraft.

Anonymous said...

"There are two (fairly minor) disadvantages to rotating section enclosed in a nonrotating shell. One is that air friction will tend to slow down the spin section (and spin up the rest) unless you keep feeding power, which ultimately will have to be dumped as waste heat. Also you'll have some extra mass: the pressure shell plus the spin structure itself. "

Ok, you've confused me; are you talking about the air inside the pressurized spin section? Because the space between the inside of the ridged hull(nonspinning) and the outer surface of the inflateable spin section is vacuum. The only source of friction would be where the airlocks on either end fit together. Those two spots would also be where the internal and external components of the heat management system conect.

Ferrell

Rick said...

Ferrell - Yes, I misunderstood your proposal. But I don't really see the need/advantage of a nonpressurized shell structure surrounding the spin structure. If the spin structure is 'amidships' you need some connection of the nonspinning sections, but I'd provide a center 'axle' that the spin section revolves around.

Jim Baerg said...

Rick said:
"Mark - one major problem with NSWR is some serious doubt whether it would work as claimed. I'm highly skeptical that the claimed exhaust velocities can be achieved without melting the thrust chamber, since any thermal high specific impulse drive involves temperatures of 100,000 K or more. For that you need some kind of electromagnetic containment."

The electromagnetic containment is probably required, but I don't see that it would be terribly difficult to do.

IIRC Zubrin's original proposal included electromagnetic coils around the thrust chamber that would push the plasma away from the walls & out the back. This would be much easier than the sort of confinement required for a fusion reactor.

To the extent that I have followed the argument about the NSWR, it seems a more serious difficulty is the mechanical stresses on the thrust chamber.

Rick said...

I guess my main exposure to NSWR was in the development days of Ken Burnside's game, Attack Vector. Ships (then) had an NSWR drive, which I had the clear impression had no magnetic containment.

Magnetic containment should indeed be easier than for fusion, with the one proviso that NSWR drives are necessarily beaucoup big, since they must sustain a critical mass in the reaction core. Which I suppose goes to your point about mechanical stresses.

Citizen Joe said...

I realize that atmospheric friction could slow things down, but compared to the energy needed for the drive, a little bit of friction is negligible. My proposed rings within a shell had the greater purpose of providing a construction environment. Working in zero-g is difficult. Working in zero-g vacuum requires a bulky space suit. If you pressurize the shell, you only need respirators (assuming you pressurize with non-breathable). Also pressurizing the shell means that you need LESS material to pressurize the actual habitat rings. So the weight requirement is a wash. The rotating rings also provide gyro stabilization for the whole ship. Which is useful since it is basically a gigantic laser. This sort of arrangement is admittedly unfeasible in sizes under 'Mobile space station'.

With the two tethered ships idea, you pretty much have to expend reaction mass to increase or decrease spin.

Jim Baerg said...

"Working in zero-g is difficult. Working in zero-g vacuum requires a bulky space suit."

Though considerably less bulky if this works:
http://en.wikipedia.org/wiki/Space_activity_suit

Rick said...

Citizen Joe - The one real question I'd have is why not simply pressurize the whole volume with a breathable mix? The spin sections are then just Really Big counterparts of the Discovery centrifuge in '2001', and you can dispense with pressure hatches entirely. (Of course you'll have emergency gastight hatches, but any large spacecraft will have those.


Jim - Presumably the entire suit does not have to be on this principle (the helmet obviously isn't. That would alleviate the complications of fitting it to the torso, and the struggle of getting into it.

At minimum this approach would be great for the gloves, which seem to be much the worst aspect of working in present day spacesuits - imagine trying to do handwork wearing a baseball mitt or boxing gloves. Arms and shoulders are the next priority. I don't think the hips are a big issue in space, though more of one for people working on planets.

Citizen Joe said...

Yep, I've seen those. I extrapolated on that suit for the command crew of the ship I've suggested. The command pressure suit includes biomonitoring, cooling via perspiration, magnetic linings and it relaxes when a charge is applied. Thus, it is easier to get into. Without the charge, the fibers tension up to apply pressure. The command crew operate with configurable gloves that operate the controls virtually. In case of an emergency, the command crew need only don their helmets rather than try to suit up entirely. Meanwhile, operating between two magnetic plates simulates gravity for operation in an otherwise microgee environment.

Citizen Joe said...

Rick, I would pressurize that area with a breathable mixture, but I'd use helium as a buffer gas since it is more readily available than nitrogen in space. But I wouldn't use helium in the habitat section due to the centripetal force causing it to stratify. The helium will want to leak towards vacuum not into the pressurized nitrogen/oxygen areas. Losses of helium and oxygen aren't that big a deal, since oxygen can be cracked out of the ice stores and helium is easy enough to find in space. Nitrogen is harder to find (at least in our system) and requires lifting it off the surface or penetrating into gas giants to crush depths (Although I have concept ships for that too). That leaves Earth, Titan (at Saturn) and Triton (at Neptune) as sources of nitrogen.

Rick said...

How about argon as a nitrogen substitute? It's not dirt common, but probably not too hard to find locked into ices anywhere beyond the snow line. I'm nearly certain it is easier to come by than helium.

Jim Baerg said...

Joe:

I think nitrogen would be much easier to get than helium. IINM there is reason to think the ices on the moons of Jupiter & Saturn would include ammonia (NH3) ice. Also the organic crud in carbonaceous chondrite meteors probably includes nitrogen bonded to carbon atoms.

Meanwhile unless you have some way to get stuff out of the atmosphere of gas giants, helium will be only available from gas absorbed from the solar wind in regolith on airless bodies like the moon & will be a minor byproduct of mining the regolith.

Citizen Joe said...

One of the driving forces in my proposed setting was fusion power, which requires Deuterium and He3. Diving into the upper atmospheres of the gas giants was to be the primary source. That of course requires a specialized ship. However, once you're in the atmosphere, you can use it for propulsion rather than burning your remass. I had even envisioned deep plunge vessels that used multiple pressure hulls and the crew operated in hardshell suits to harvest ammonia from Uranus at around 100 atm pressure. That is an incredibly dangerous job that requires people on board because the dense atmosphere pretty much cuts off external communications, like during the blackout phase of re-entry. But the nitrogen is needed to support plants, so in the end, it is worth it.

The operations around Saturn were intended as an attempt to have a self sustaining ecosystem in case we found another habitable system. Titan was the primary source of nitrogen for the agriculture industry there. Meanwhile large chucks of frozen ammonia were lofted into space and then chucked towards Mars. With some course corrections near the end, these ammonia asteroids were driven into thin atmosphere of Mars in an attempt to build it up, part of the terraforming project there.

All of this was part of a scifi setting and not something I truly expect to happen. Much of it was designed around the notion of creating a compelling story and explaining why people would be out there doing this stuff when you could otherwise just send a robot. So, yes, I'm invoking the Zeroth Law of SciFi a bit, but I do try to keep the rest of physics intact.

Now, if you get into the likely events, space warfare drops off the playing field entirely in favor of OMG! We just hit WHAT?

Citizen Joe said...

Argon is 25% denser than oxygen, which means that leaks from the microgee environment into the spin habs could result in a deadly stratification as the argon builds up on the outer edge of the habitat. Should you pass out on the floor, that means no oxygen.

That being said, there are many uses for argon. I just don't see it as being easier to get and use than nitrogen, thus less likely to be used as a buffer gas.

Helium-4 is about 50 times more abundant than oxygen, and 250 times more abundant than nitrogen and almost 3600 times more abundant than Argon. If you can solve that atmospheric dive problem it just stands to reason that Helium-4 is well suited to the microgee buffer gas. Even more so when you consider that it is the impurity you're pulling out of the hydrogen scoops, and thus would otherwise be waste.

Jim Baerg said...

Joe:

If you have some technology that can get stuff out of the atmospheres of the gas giants in large quantities at reasonable cost, that technology can more easily get stuff from Earth or Venus. I note that Venus has over 3x as much nitrogen as earth in its atmosphere (mixed in with a *lot* of CO2) http://en.wikipedia.org/wiki/Atmosphere_of_Venus

If helium is readily available, so is nitrogen.

Rick said...

Citizen Joe - Just considering the tech issues by themselves, I still think the simplest approach would be to use the breathing mix for the whole pressurized volume. The 'dead space' between the pressure shell and the spin structures inside is is probably small compared to the volume of the drums themselves, plus the volume of nonspun micrograv working spaces.

But story requirements do pretty much trump all other considerations!

Citizen Joe said...

Plus, it is really funny to have the command crew talking like Mickey Mouse.

I suppose on a more serious note, what kind of ships would have some sort of artificial gravity and why? What sort of ships would make do without? What sort of ship run with the full pressure Nitrogen/Oxygen mix and which use a 'lesser quality' air?

Rick said...

Plus, it is really funny to have the command crew talking like Mickey Mouse. ... Well, yes. :-)

As for which spacecraft will use spin, I suspect the main factor will be mission duration. For missions of a few days it is hardly needed, but for spacecraft intended for permanent habitation (stations, or ships that function as 'mobile stations') it is essential. Where the dividing line is will depend on a host of secondary features, but I'm inclined to think that mature interplanetary craft will have spin.

I suspect that most spacecraft will use the full mix, because it is the simplest approach, e.g., you can open both doors to the docking collar. Spacesuits will probably use pure O2 to minimize pressure and resulting joint stiffness, but other than that I think the standard mix is preferred.

Anonymous said...

Future spacecraft might be classified into low, medium, and high endurance... no spin, spinable-when-needed, and permanently spun...

Ferrell

Rick said...

Yes, though size would also be a factor, especially between 'spinnable' and 'permanently spun'.

Anonymous said...

"Yes, though size would also be a factor, especially between 'spinnable' and 'permanently spun'."

Yes, or maybe thrust...a military interceptor or a Space Watch Asteroid Deflector may be spun-when-needed AND be a High Endurence space crusier...
Ferrell

Rick said...

Ferrell - also attitude-change maneuvers, if those are combat relevant, as for Citizen Joe's 5-mile ship with the badass X-ray laser. You pretty much have to de-spin a ship for attitude maneuvers, or it will go all haywire on you. (Which could be a good way to make Bad Things happen to spin bearings.)

Anonymous said...

"Ferrell - also attitude-change maneuvers, if those are combat relevant, as for Citizen Joe's 5-mile ship with the badass X-ray laser. You pretty much have to de-spin a ship for attitude maneuvers, or it will go all haywire on you. (Which could be a good way to make Bad Things happen to spin bearings.)"

You're right, a military or "space watch" space crusier should have a huge set of maneuvering thrusters...with lots of fuel, perhaps even a seperate supply from the main engine supply...

Ferrell

Citizen Joe said...

Large internal rotating bodies can act as a gyro. They normally stabilize things, but if you apply some friction, you can rotate your ship along the axis of the gyro. You can also push perpendicular to the axis in order to turn the ship. Now the forces we are talking about are minuscule compared to what is needed for combat maneuvering, but they don't require re-mass to maneuver.

The 5 mile xray laser ship has ranges in the order of light minutes, so even microradians of adjustment are thousands of miles. Fortunately, the fine tuning and aiming is done with an array at the end of the ship, so once you get in the right area, you don't have to re-orient the ship. Likewise, being able to target at that range means that you don't have to worry about maneuvering from return fire.

Rick said...

Rapid attitude changes probably go hand in hand with high combat acceleration, but might well require a separate fuel supply. (Probably the same chemfuel as ordinary attitude thrusters, but a lot more of it.)

True that the super X-ray ship does not have to do a lot of 'gyrating' in combat, given its enormous weapon range.

Citizen Joe said...

I just watched the show Defying Gravity on Hulu and sure enough, they used the magnetic clothes trick that I described earlier to simulate gravity.

Rick said...

As Mr. Spock would say, fascinating! This seems to be my day for interesting news/media items. (See my latest post.)

Jean Remy said...

I guess I am new here so I am unfamiliar with the 5-mile ship with the "badass Xaser" that has a range in the light minutes but...

It has a range in the light-minutes? Unless the object you are targeting has no way of creating Delta-V for itself, and therefore follows a predictable orbit, how are you supposed to aim and hit anything when it takes several minutes to spot the object. Let's say that you even have FTL sensors (gasp!) it also takes several more minutes for the weapon to hit. It seems like a good weapon to dislodge asteroids from inconvenient orbits (ie: on a collision course with a certain planet whose ecosystem we have a vested interest in) but utterly useless against a hostile foe who presumably knows about said "badass Xaser" and is very likely to alter its course, and at those ranges they wouldn't even need to eat to deep into their Delta-V budget. Close might count in horseshoes and hand grenades, but definitely not with a laser.

Rick said...

Jean - The uber-range laser is Citizen Joe's idea, not mine, but my guess: You can hold down the firing button so long as your power supply holds out, forcing any target within Incredible Range to maneuver constantly, gradually eating away at its delta v.

And if the laser is powerful enough you can sweep it through the target's evasion cross section like a hose, and sooner or later you will hit it.

Turbo10k said...

Hosing down an enemy ship with micro-second pulses?! Okay. We have GW lasers but only enough heatsink and radiator to absorb half a second of 'hosing'. You do the maths.

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