Arcane Game Lore

Roll for Initiative!

On Space Combat

I listen to several podcasts during the week on my way to and from work and when working around the house.  Most of them are RPG podcasts and one of those is Fear the Boot.  Last week in their bonus episode #60, Dan had a wonderful solo episode where he talked about Capital Ship Combat.  It was more an episode on general space combat and it was great.  I’ve been thinking about writing posts on this topic but Dan hit most of the points and covered it thoroughly. Everything that follows was sparked by comments Dan made.  If you haven’t listened to the episode, you should do so now.  (It’s a little long at ~90 minutes but it is well worth the time.)  Go ahead, I’ll wait…


Back?  I told you it was worth it.  Now on to my thoughts.  I was actually listening to this while out grocery shopping over the weekend so my notes are just short jotted down thoughts on my week’s grocery list.  I will probably be revisiting this again in the future but these are the thoughts that stood out.  And I’ll admit up front I’m one of those guys Dan mentioned that is very much into and comfortable with the physics and math involved (B.S. in Physics, M.S. & Ph.D. in Astronomy).

Beam Diffusion

Dan talks about ranges in space combat.  One of the things about lasers is that they spread out.  They don’t stay a tight beam of light forever.  I touched on this a bit in the laser rifle post a couple of weeks ago when I talked about focusing the laser beam at the proper range for the target.  This effect will tend to limit the range of your laser weapons.  So while in theory you could shoot a ship that is orbiting Mars from Earth as Dan talks about, the laser may not have much power when it arrives.  At least not concentrated enough to do any damage.

We use lasers to regularly measure the distance between the Earth and the Moon.  The lasers are fired from Earth to the Moon where they bounce off retroreflectors left by the Apollo missions and the reflected photons are measured to determine the roundtrip time (you can read more about it in this Wikipedia article and this one about a specific experiment at Apache Point Observatory in New Mexico, where I went to school).  The relevant point is, that even over just the distance from the Earth to the Moon, about 385,000 km (or 38 hexes on the Knight Hawks map for you Star Frontiers fans out there), the beam has spread out to be about 6.5 kilometers (4 miles) in diameter.  So while you can hit targets at great distances, there may not be much power left in your beam when you do.

Long Laser Cavity

Which brings us to another topic.  In order to get around the beam diffusion issue you need really long lasing cavities.  This means that the laser weapons inside your ship are going to be long and skinny if you want longer ranges.  Dan talked about the fact that you can have your weapons pointing out any side of your ship and why you might want to do that in his discussions about “broadsides” and he’s completely right.

However, this may be a reason to have long skinny ships with a forward firing laser cannon.  You need that entire length of the ship to hold the long lasing cavity to generate a tight enough beam to get the long range you want.  And depending on how your ship is designed, you might have that equipment running through all your decks.

Using All Your Weapons

That relates to another point talked about, namely bringing all your weapons to bear on a target when the weapons are spaced out all around your ship.  Battery type weapons (that have a 180 degree field of fire) are much better for this.  Half of your weapons can swivel in their mounts to come to bear on the target and to get the others into position, all it takes is a roll of the ship around the thrust axis which can typically be done fairly easily.  So weapons are fired in a 1-2 punch but this can be fairly close together.

A fixed weapon is not so easy as it requires you to orient your ship so that the weapons is aimed at the target which may not be along the direction of thrust.  However, all is not lost.  As Dan points out your ship can point any direction and doesn’t have to be pointed along the direction of motion.  As long as you’re not thrusting to attempt to change your velocity (speed and/or direction) you can have your ship pointing in any direction you want to fire your weapons.

It’s only when you’re trying to maneuver and line up a fixed weapon that you might run into problems. Basically what happens in this situation is that you have to rotate your ship, fire, and rotate back to the proper maneuvering position.  This can be done with or without turning off the main engines.  Turning them off provides more exact maneuvering but reduces the effective thrust you have for that time period.  Leaving them on introduces some randomness into your velocity depending on how you perform the maneuver (which may be a good thing, see below) and still reduces the effective thrust available as you’re now using it in a direction that isn’t where you want to go.

Here what is possible comes down to the details of the game mechanics.  If you’re playing with short time scales and large ships, maybe there will be a significant  loss of capability.  For example, you can still fire your fixed weapon anywhere you want but can only use half of your available thrust.  If your target is in a direction close to where your ship is pointed based on your desired maneuver, then there is no loss.  Of course this requires understanding the physics and orientation of the ship so maybe just a rule that reduces effective thrust when firing the fixed weapon regardless of target location would be easier to play.

On the other hand, if you’re playing a game with long time scales and/or smaller ships, you can just allow the weapon to be fired with no loss of capability.  Once upon a time I was developing a Star Frontiers combat plugin for the Orbiter Space Flight Simulator program.  The plugin was very nearly fully functional but we never released it as we were waiting on a usable multi-player plugin.  All it was lacking was automating the turn, fire, return to orientation maneuver.  In the process of developing that, I looked at what was required to pull that off.  And what I found that even for the largest ship in the Star Frontiers game, the UPF battleship (length 600 meters) you could turn the ship to any direction, fire, and return to your original orientation in less than 90 seconds and never experience more than 0.5g sideways acceleration at the extremities of the ship (I’m pretty sure it was 0.5g and not 0.5m/s/s or 0.05g.  It’s been many years and my memory is fuzzy).  The smaller ships could rotate even faster.  Since the Star Frontiers combat turns are 10 minutes long, this represents a relatively small part of the turn (<15%).  It would have real impact in the computer simulator game but for the table top version, using real vector physics (Star Frontiersman issue 11) we could ignore it and allow the fixed weapons to fire any direction without affecting the capabilities of the ship.

By the way, if you really want to get a feel for what maneuvering in space with real physics is like, grab Orbiter and play with it.  I highly recommend it.

Leading the Target

Dan also talked about leading your target and having to shoot where it will be and not where it currently is or where your sensors say it was when they got a detection.  This is very true and something that modern militaries deal with already.  And while it can be compounded as Dan described by the longer range distances, there is actually a mitigating factor with space combat that makes it easier.  And that goes back to the fundamental difference between the difference between movement in space and movement in an atmosphere or on the ground: no friction.

Unless your universe has ships capable of unbelievably high accelerations without smashing the crews to jelly, they can’t change direction or velocity very quickly and the way they do so is somewhat predictable.  To the first point, let me give an example.  A ship that fires it engines non-stop for one minute at 1g (where I’m using 1g = 10m/s/s instead of 9.8 to keep the math simple) can change it’s position by 18 kilometers (11.25 miles). That may seem like a lot but remember, space is big, and that change is the same regardless of what it’s initial velocity was.  Basically, if you knew it’s original velocity, you know know that it will be somewhere within a sphere 18 km in radius centered on it’s projected position.

But it’s not even as bad as that because of the second point.  If you have some idea of the ship’s design (i.e. you know where the engines are mounted) and you can determine roughly the ship’s orientation, you know which direction it’s thrusting and the number of possibilities for its ending position are greatly reduced because the ship has to obey the laws of physics, specifically Newton’s laws of motion.  The better you can estimate the acceleration being used and the direction the ship is pointing, the easier it is to lead your target accurately.  There will still be some error but it’s not really as hard it you might think.

Sensor Lag

Which brings me to the final point I wanted to mention.  And this isn’t so much commentary (as Dan covered it quite well talking about the time delays involved) but a reading suggestion.  What’s needed is some way to represent the uncertainty involved.  There is an example of this that I have always loved and it is the way I envision it when I think about it.  And that is the sensor displays from C. J. Cherryh’s Downbelow Station.  If you’ve never read this book, I highly recommend it.  The ships are moving at near light speed in the book but the same type of error display would work at the velocities and ranges we’re talking about here as well.

Last Words

Actually I don’t really have much else to add at the moment.  I just wanted a section heading to separate this bit from the section above.  Dan did a great podcast.  Hopefully you enjoyed my ramblings as well.  Feel free to share your thoughts in the comment section below.

Categorised as: General

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