revenge of space combat physics

My blog had been trucking along with a reliable readership of perhaps a dozen people, when, suddenly, after a slightly stream-of-consciousness post about the physics of space combat, Gizmodo asked to reprint the material from my blog. It was never my intention to get so much attention – but apparently that article turned into the most-commented content on Gizmodo that week! I got lots of questions and comments and emails after that and noticed lots more pingbacks on my blog entries afterward.

I couldn’t help but think, “Wow, if only my research activities would generate this sort of interest! I’m trying to build tractor beams and wrote up my experiences from Vomit Comet flights. How is that not cool enough?!” At least I got to abuse my 15 seconds of Internet fame to plug NASA a bunch!

Well, just a couple weeks ago, Karl Haro von Mogel from the University of Wisconsin, Madison, contacted me to interview me for his radio show, “The Inoculated Mind,” which airs on the student radio station in Madison. This was my first on-air interview, and I had a lot of fun with Karl! You can listen to a podcast of the show on his web site. It sounds from the beginning of his show that Karl and I would get along nicely, and then a little before halfway through he plays the interview. If I sound excited, it’s for good reason!

Many thanks to Karl for having me on his show, and for chatting with me about my research as well as the sci-fi stuff! (Oh, what the heck, my research is practically about science fiction, too!) And great use of Battlestar Galactica music and lead-in with the science of Avatar’s unobtainium!

And, of course, a link to the short story Karl brought up: High Orbit. Enjoy!

Just as a freebie, after the jump I am going to list several common questions and comments I got after Gizmodo picked up my initial blog, and respond to them a little bit. I am falling for exactly the issue that Phil Plait identified in his comment on my post – this could go on ad infinitum! So I’m done with this post now, but if you want even more about space battle physics, click here:

Why do you want all the spacecraft to be spheres? That would be so bo-o-oring!

What I meant, more precisely, is that combat spacecraft ought to have a spherical moment of inertia, because that is the easiest 3D shape to torque around rapidly. Combat spacecraft would likely require agile maneuvering, in the sense that they would need to spin about some axis many times per orbit. A spherical inertia basically means that all the high-mass components of the spacecraft would be as close to the center of mass as possible and as evenly distributed as possible. The actual shape of the spacecraft might not exactly be a sphere. In fact, I’ve been picturing something like the Apollo LM without its descent stage. It might have some bits sticking out here and there, and it might not have perfectly round edges. Not quite boring.

Many people commented that they thought the Starfury from Babylon 5 was pretty much as good as you can get for a combat fighter. I was actually in the middle of going through B5 for the first time when I wrote my first post (oh, the overacting!) so I had thought of that a little. I think Starfuries actually conform pretty well to my guidelines – they are “spheres” with engines stuck out on pods – as long as the inertia of the engines is much smaller than the inertia of the central body. However, it looks like the engines might actually have a lot of inertia in and of themselves, and I have  hard time imagining where all the propellant they constantly spew out gets stored.

There are other possibilities, though. In some online interactions with these guys, who discussed my original post on their “Science…Sort Of” podcast, we hit on an interesting idea…a combat spacecraft with its center of mass outside its physical volume. Picture something shaped like a giant boomerang in space with big pods on the ends. The reason to do that is that the spacecraft could use its maneuvering gyros to flip its body around to avoid incoming missiles and such – which would then pass through the empty center of the spacecraft.

You advocate flak weapons – that is a terrible idea! All this flak is going to make space hugely dangerous when the fight is over.

Yeah, I wouldn’t want to be in a spacecraft in the vicinity of a recent battle. However, the problems with orbital debris are exactly why a military commander might want to outfit his starfighters with flak cannons. His goal is not to make the environment safe, it is to destroy the enemy as efficiently as possible. After the battle is over, maybe we have to find a way to clean up.

And as long as we are having a space battle anyway, we are probably already creating enough debris to get near the debris cascade point just from blowing up fighters and frigates. That point is when the amount of debris floating around in space gets so large that it collides with other debris and makes more debris so fast that nothing is safe. It is a problem that space admirals would have to worry about. But as long as we drive away the alien invaders, maybe we can live with having to sort out the mess afterwards.

Putting radar on your combat spacecraft would be like painting a target on your hull! Any veteran of space computer games knows that active scanners give away your position. You want passive sensors that don’t let the enemy know where you are!

I advocated using radar on combat spacecraft for three reasons: first, we are very good right now at using radar to detect, identify, and track targets; second, because space missions that require rendezvous have a heritage of using radar; and third, because radar is currently used on Earth to identify and track spacecraft in orbit. Radar is very good at finding those other objects in space. Of course, they haven’t been trying to hide. (Here’s a cool view of ISS in radar!)

One bit of good news for stealthiness: A single radar ping would immediately tell you the distance and direction of enemy spacecraft, as well as their speed towards or away from you. If the enemy ship is watching for your radar, then that single ping tells them only the direction to your ship. More pings will give them more information to piece together the rest – but there are ways around this. You could put out a radar drone on an independent spacecraft or a tether, or you could time your radar pings so that the enemy doesn’t get the most up-to-date information about your fighter’s dynamics.

But yes, radar would work both ways.

Combat spacecraft with people inside have to have a cabin at a temperature of 290 K, so these spacecraft will shine like beacons against the 2.7 K background of space. IR would detect everything instantly! There’s no hiding, and every spacecraft must have thermal imaging systems.

Well…yes and no. I definitely forgot all about thermal issues when I wrote my original post – and thermal management is a complicated problem that is not negligible!

Some good news for those of you who want to design stealth combat spacecraft is that spacecraft engineers these days are pretty good at shunting heat around. Combinations of sunshades, cooling systems, and radiators give us the ability to cool down just about any part of the spacecraft we like to temperatures as low as that of liquid helium, and shoot the excess thermal energy over to some other part of the spacecraft or out to space. So it’s possible you could design a combat spacecraft that would have a cool (stealthy) side to face the enemy, and a hot (visible) side that would just point off to empty space.

Another bit of good news for stealthy spacecraft is that infrared detectors can be challenging to make in space. Detector optics are usually intended to gather radiation – but the more IR radiation the optics get, the warmer they get…they then radiate their own thermal radiation, which hits the detectors. In order to detect a signal, the radiation from the detector optics needs to be much less intense at the detector than the radiation from the signal you are looking for – that is, the thermal radiation from the spacecraft you are trying to find. Since the detector optics are probably much closer to the detector than the enemy spacecraft is, they have to be much, much colder than the target spacecraft. So if you want a good IR detector on your spacecraft, that has to come with cryocooling systems anyway. The upshot is that if you cool your spacecraft (or the side of your spacecraft facing the enemy) to, say, 100 K, then it will be very unlikely that all their combat ships have all the delicate optics and cooling systems necessary to see you.

Some of them might, though.

Lasers don’t actually travel forever in space!

Correct. I was wrong, and I absolutely should have known better since my undergraduate physics thesis was on laser pulse propagation!

Let’s neglect dispersion for now, and just figure that the laser beam will diverge once it leaves the last optical element in the laser system. If the laser light were to travel forever in a straight line, the radial velocity of all the photons in the beam would have to be exactly 0. But the Heisenberg uncertainty principle says that I can’t know the velocity of the photons exactly! They will diverge, at a rate that depends on the beam intensity profile when it leaves the optical system. Eventually, the peak intensity will be too small to use as a reliable weapon.

Fortunately for us sci-fi geeks, “eventually” is still a long distance away. The Air Force has tested an airborne laser platform that can destroy missiles from 300-600 km away! That range ought to be even longer in vacuum, assuming the USAF platform doesn’t rely on atmospheric self-focusing to maintain that range. For comparison, low-Earth orbital velocity is about 7 km/s, so it would take a minute and a half to orbit that distance.

Why didn’t you talk about robotic drones, which could take crazy gees that people can’t / When we invent engines based on Crackpot Physics Theory, we won’t have to obey orbital dynamics / What about Energy Proton Gamma Beam Plasma Weapon / Why do you keep talking about “space fighters” that are about the size of Apollo capsules and not battleships?

I made the – albeit arbitrary – decision to think about how combat spacecraft would look and behave if we were to start designing them today, using technologies that are currently available or just on the horizon. Propulsion will come from something that either works on the rocket principle (throwing stuff out your spacecraft in one direction so that your craft gains momentum in the opposite direction) or some high-specific-impulse but very low-thrust principle like solar sailing. Weapons will be based on weapon systems that exist nowadays. Spacecraft will be about the same size as spacecraft that we have historically launched, for economic reasons. And we’ll need human-in-the-loop control.

Still, with the way things are going, we can make a few good projections. We are getting better at designing spacecraft that can do autonomous proximity operations. We are getting better at constructing or deploying large structures in space. And we could conceivably build very large things out of materials on asteroids, where a little push is all they need to get off the surface. It is certainly possible that my speculations will turn out to seem incredibly naive and shortsighted ten or twenty years from now; just as many of Arthur Clarke or Robert Heinlein or Wernher von Braun’s predictions do to us now. (I mean, running everything on magnetic tapes? Come on!)

When we invent engines based on Crackpot Physics Theory, we won’t have to obey orbital dynamics.

I separated this one out of the list from the last point because it deserves special consideration.

Barring some fundamentally new development in physics – I mean really fundamental – we are always going to have to expend energy to change orbits and we are always going to be limited in the amount of energy a spacecraft has at its disposal to do so. Our spacecraft will have to use up propellant to effect maneuvers, which means they must carry fuel around with them, which means that they must move that fuel reserve around, which means that they will have to carry a limited amount of fuel, which means that they must budget that fuel carefully. Even the most efficient rockets developed to date are not very efficient; and the most efficient (in “miles per gallon” terms) engines do not put that much thrust on a spacecraft. As long as orbit dynamics exist, we might as well use them instead of fighting with them and using up all our propellant! (Here, I’ve used “fuel” and “propellant” synonymously, though in general they are not – ion engines being a good example of such a case. However, high-thrust maneuvers mean rocket engines, for which they are the same.)

Don’t worry, though – having to obey orbit dynamics doesn’t mean that spacecraft won’t be able to attack each other from any direction they like, or have any relative velocity they like, or fire guns in any direction they like. It just means that our common intuition, built around things moving in pretty much straight lines, would have to be re-trained if we are to be starfighter pilots. Instead of thinking about air-hockey-table physics with cardinal directions like left and right and up and down, we’ll have to think in terms of orbit periods, eccentricities, inclinations, and pericenters!

You should have done your background research. I found a few websites that already thought of everything, and here’s a list of books that get physics exactly right.

This was just an interesting way to exercise my brain-muscles. I had no idea the attention I would get! There definitely are some good resources out there. Some of your comments have pointed me in the direction of novels and movies and stuff that look really cool, so thank you very much.

I didn’t do any background research. I didn’t run through any numbers. (I could, but I also have to do the space technology research that pays me!) My original blog was written in one go, with one addendum a day later, and is basically just the result of me batting these ideas around in my head after an evening of batting them around with some of my fellow aerospace engineering grad students (considering, of course, the general grounding in real physics). But a few people seemed a bit annoyed that I didn’t exactly agree with them.

There are some things that we’re all missing. First, I have never seen any detailed trade studies for combat spacecraft – though I have no doubt that such things exist in some classified folders on some Air Force base or other. Second, there are economic considerations here that are not negligible; they will contribute to the size of space fighters, the amount of weapons, engines, and fuel that go on those space fighters, and the numbers of them that get launched.

Third, and most importantly, we don’t have any live opponents against which to play out these scenarios. Let me say outright that that is a very good thing. However, if we were actively engaged in a space war, then even with only currently available technology and our current knowledge of physics, we’d likely come up against or develop tactics and spacecraft designs that deviate radically from any current predictions. Why am I confident of this? Because that’s been true in just about every war that has ever been fought: the adversaries do unexpected things to each other. That doesn’t mean that nobody can think about these things; it just means that nobody will think of everything. To give another, non-combat-related example, in the early space program of the 1960’s, some reputable scientists thought that human eyes might not function without gravity pulling them downward – despite what any kid on a playground hanging upside-down from the monkey bars could tell them.

Anyway, I certainly haven’t thought of everything, nor was it my intention to write an exhaustive article. So go on, sci-fi enthusiasts, and think up your own arguments!

5 thoughts on “revenge of space combat physics”

  1. Oh, it’s strictly a spare-time affair! I have no way to make money off it, and I keep busy enough as it is. Hence my relatively infrequent posts. 😉

  2. I really liked these posts, so I’m going to throw my two-cents’ worth in as well;

    Making a spacefighter very spherical might make it as manouverable as a gnat, but weapon design always involves tailoring the tool to the job, which might not require that level of manouvreability. Thats’ pretty much the same reason we don’t have Mach 6 jet fighters; though very possible to build, they would prove quite useless except in very narrow tactical situations. Your Starfury example show this very well. Very agile, but no fuel to do anything useful!

    I love the concept of using debris to deny an enemy “orbital superiority”. Unlike other discussed topics related to space warfare, this is actually one that we can actually study as a real phenomenon, since it’s causing problems for space agencies right now!

    Your view of stealth is a lot more balanced than that of both the stealth-naysayers (who doesn’t seem to understand the conditional situations that allow stealth in warfare) and the wannabe-Romulans who want stealth as simply a button to be pushed.

  3. Thanks, Dries!

    You’re right, there are a lot of factors that come into play here. If you mount weapons and thrusters on gimbaled appendages, for instance, then you don’t have to worry about turning the whole spacecraft bulk around to point them in some desired direction. I suppose it’s all about what our future space admirals want to achieve!

    My opinion on stealth is basically that it’s a balance between your camoflaging ability and your enemy’s sensing capability. You don’t have to make yourself undetectable to be stealthy – you just have to make yourself hard enough to detect that you can loose your missiles before the opponent finds you. Most of the stealth naysayers I’ve seen on the Internet completely neglect the challenges of sensing, and just assume they have a device at their disposal that will detect everything (something I think is unfeasible – and I recently took some more flak for that opinion! 😉 ). On the other hand, to the wannabe-Romulans, I have to note that every cloaking device invented for a sci-fi plotline invariably seems to fail all the time, or encounter adversaries who find a way around it.

    Thanks for the comments!

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