SF-related Physics Question

[Zaune]

What effect would a rifled barrel have on a rail/coilgun projectile fired in space? I'm working on a Homeworld fanfic series, so I may at some point have to deal with how the setting's space fighters cope with combat in an atmosphere. Friction from rifling would also be a handy excuse for mass-driver rounds in hard vacuum resembling tracer rounds.

[Dave]

Disclaimer: I am, for all intents and purposes, a layperson when it comes to this topic.

If you're asking if it works or not, yes, I would think so, so long as you use non-magnetic rifling, since you don't want that accelerating.

Will it help you much? Remember that the purpose of rifling is to spin the bullet so that is stabilized gyroscopically -- the spin resists tumbling, which reduces range and accuracy when traveling in a fluid such as air. Is this going to help you? Probably not in space, since, well, there isn't any air to affect the trajectory of the bullet. Even in a nebula (which is really not very dense anyway), and especially at the ranges that Homeworld ships typically enter engagements (< 5km), there just isn't going to be enough stuff in the way to appreciably impact the path of the bullet. Even if it was tumbling, it wouldn't matter.

The only possible benefit I can think of from rifling in space is that if your rounds are meant to be rifled (and thus rotating in contact with the inside of the barrel), you basically know the round is supposed to come out dead center of the barrel, moving in a straight line parallel to the length of the barrel (a good thing). I seem to recall hearing a rumor that with old smoothbore ball muskets, there was usually such a difference between the ball and the barrel diameter that you could hear/feel the ball bounce around on its way out the business end of the gun. This is probably not what you want.

[Zaune]

Thanks for that. The only reason I bring it up is because rifling would obviously be of benefit in an atmosphere, but most Hiigaran fighters don't spend much time dogfighting inside one and stripping out and replacing the barrels every time there was a chance they'd have to would be a lot of wasted extra effort. This of course only applies to the ones that have more aerodynamic lift than an iPad...

[Dave]

stripping out and replacing the barrels every time there was a chance they'd have to would be a lot of wasted extra effort.

You know, given the apparent ability to extract resources at will and the apparent ability to build a fighter (well, scout) in ~10 seconds using a carrier, I'm wondering if it wouldn't be a pain in the ass. I mean, given the technology we're looking at here (cheap fusion power, resource extraction in seconds, and the ability to crank out ships in minutes (granted, I'm using in-game time here)), I'm wondering if we're looking at a nearly post-scarcity society, and it might actually be easier to just chop off the front half of the fighter in question and rebuild whatever gun/cockpit/sensors or whatever you wanted.

Of course, you could just say that Homeworld uses time compression and it takes hours to get fighters going and weeks to build a new ship.

[Simon_Jester]

There is no reason to rifle coilguns whatsoever. Not in space. Among other things, friction between the slug and the barrel would burn the barrel up so fast it would be insane. As in, you'd be getting serious barrel wear during combat, not just replacing barrels between combats.

[Dave]

There is no reason to rifle coilguns whatsoever. Not in space. Among other things, friction between the slug and the barrel would burn the barrel up so fast it would be insane. As in, you'd be getting serious barrel wear during combat, not just replacing barrels between combats.

Ok, so I fired up my copy of Homeworld to mess around with. My bad, my numbers were off. I had scout ships engaging a frigate at ~5 km, with projectiles from the fighters transversing the distance in ~0.5 seconds or less. Thus projectile velocity would be 10 000 m/s, and yeah, I can see how that would cause significant barrel wear. Those are conservative estimates, by the way.

[Admiral Valdemar]

Railgun designs would be using APFSDS anyway. Rifling on a coilgun is utterly redundant and negates the one advantage a coilgun offers over a railgun.

[khursed]

Might as well riffle your laser guns while you're at it

[Zaune]

@Dave: The story is being written on the basis that the games are just that; games, with all the usual compromises between historical accuracy and player enjoyment. Haven't got as far as needing to deal with new fighter construction yet, but it'll probably be something along the lines of six hours to fabricate each fighter's components, two more to bolt them together and a final two for ground and inflight testing.

@everyone else: Thanks for the input, I'll think of another justification for the rounds to be leaving the barrel visibly glowing with heat.
And is anyone interested enough to want the story cross-posted from the Relicnews forum?

[Admiral Valdemar]

Why not put phosphorus in said rounds to act as tracers? Like... tracers. Although why you'd want to do this is another matter. A cold, fast moving slug is a better way to wake your enemy up than loads of highly visible IR signatures approaching en masse.

[Zaune]

The tracers are for the pilot's benefit, so they can be certain their rounds are actually going where the reticule says they will and compensate if they aren't.
And phosphorous won't be much use in a vacuum; getting it to combust in a vacuum would be a considerable design challenge to say the least, and probably inconveniently complicated to fabricate. Tritium might work, though it'd have to produce a pretty strong glow to be visible at the outer limits of effective coilgun range.

[Admiral Valdemar]

That's just it. Either you add expense by having a pyrotechnic with an oxygen supply to give a tracer, or you use some meagre luminescent chemical or radioisotope or you use electronic beacons. It's all expensive and overly complex for what you'd be using it for. The operators could just use their own optical tracking systems or radar to keep tabs on the projectiles. It's not like this is really analogous to modern users of tracers, who are GPMG or HMG users for the most part without any fancy sensory equipment like that on a starship.

[Zaune]

True. That's why I decided that the tracer effect visible in the games was a result of friction. It's the least complicated, lowest-tech way of achieving the necessary effect. It makes sense to have a backup method if said fancy sensors pack up from battle-damage, and to be honest, I kind of need it for storytelling reasons; incoming fire that's completely invisible to the naked eye would make dogfights extremely short and turn the entire cast into Redshirts.

Radar/electro-optical shot tracking is a good idea, though it might be pushing it a bit for a single-seat fighter. I'll probably incorporate it for larger craft when they turn up.

[Sarevok]

CIWS turrets on warships relied on radar tracking for adjusting aim. That technology is from 1970s. Why would a spaceship pilot need to track shots with naked eye ? Especially given that at space combat speeds any aiming would be done by computers anyway.