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I was thinking, How much of an advantage is it to have engines capable of very high acceleration in space combat with an opponent who cannot accelerate so quickly? Specifically, if you have a spaceship that can accelerate at 120gs(effectively indefinitely), and theirs can only accelerate at 20gs(also indefinitely) all other things being equal, is that enough of an advantage to win easily, or just a slight edge? Weaponry consists of missiles(the slow ship's missiles can pull 200gs for a short time, the fast one has missiles that can pull 250), really powerful railguns(that fire at 0.2c), and macron guns.

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    $\begingroup$ If this is science-based, then the question of how long either ship can pull 20 gs or 120 gs is critical. For example, chemical rockets only burn for a handful of minutes total before the fuel is gone, and they can't pull anywhere near 20 gs for that amount of time. On the other hand, if your ship can pull 120 gs for an hour, this would have far-reaching consequences in your setting, because it means your rocket's power plant is measured in petawatts (hundreds of times larger than the current world electrical power generation). What else can you do with those petawatts? $\endgroup$
    – causative
    Commented Jun 11 at 3:58
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    $\begingroup$ Also please be specific about the weaponry: how much acceleration can the missiles pull and for how long, and how fast are the railgun projectiles? And how big of a target are these ships and missiles? $\endgroup$
    – causative
    Commented Jun 11 at 4:16
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    $\begingroup$ In real life, superior acceleration helps aircraft defeat air-to-air missiles with powerful, unexpected evasive maneuvers. So if your space missiles can pull 1500g (they are not manned, after all), the target pulling 120g is no help. $\endgroup$ Commented Jun 11 at 12:18
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    $\begingroup$ If you want 'science-based', consider rethinking your ship weaponry. 100 G is barely possible for a missile. Having ships, with other design constraints, do this is absurd, even besides killing whatever lives inside them and Δv limits. Also, a 0.2c railgun is probably not possible, and anyway would be a million km long with the projectile spending actual minutes inside the barrel accelerating. Instead consider lasers (X-ray free-electron) or particle beams (particularly highly relativistic electron beams), which we can build with current/near-future tech and would work much better besides. $\endgroup$
    – geometrian
    Commented Jun 11 at 16:36
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    $\begingroup$ if you have a power source technology able to accelerate a space ship at 120g indefinitely that means you have in your hand a weapon of planet shattering magnitude. Acceleration is the least of your problems. $\endgroup$
    – armand
    Commented Jun 13 at 6:42

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Absolutely dominant advantage

You have total control over the range of combat (at this ratio of acceleration, they might as well be just standing still, since you can outpace them to a ludicrous degree). For a sense of scale, at that acceleration, starting from zero velocity in earth orbit, it would take the 20g ship 33 minutes to get to the moon. (That's pretty zippy!).

The 120g ship could give them a fifteen minute head start, and beat them to the moon with almost five minutes to spare, because they can do it in ~13.5.

That's a monstrous advantage. Weapons typically have an optimal range, and the higher acc ship gets to pick when it wants to be in range, how long, and what to do while it's there. Especially with missiles, since it means that the higher acceleration ship can almost definitely outrun whatever missiles the slow ship can throw at them, and can give their own missiles a pretty major initial velocity boost. The higher acceleration ship will, at its leisure, get to the optimal range for firing its own missiles, drop a salvo, escape any counter battery, then rinse and repeat.

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    $\begingroup$ What if the missiles from both ships can pull 240 gs? $\endgroup$
    – causative
    Commented Jun 11 at 3:59
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    $\begingroup$ @causative if one side has 6 times better engines, I would assume that they have an advantage also for the missile acceleration, not only for the ships. $\endgroup$
    – Peteris
    Commented Jun 11 at 11:32
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    $\begingroup$ @Peteris Not necessarily. If the fast ship is a fighter packing small short range bombs and the slow ship is a cruiser packing large, long range missiles, then they could both be of similar tech levels and carrying inversely capable weapons. The fastest boats in the world are 9 times as fast as an aircraft carrier, but that does not mean they can also hold better weapon systems. $\endgroup$
    – Nosajimiki
    Commented Jun 11 at 13:44
  • $\begingroup$ Since velocity (speed) is relative, you can indeed consider the slower ships to be "standing still" and the faster ones as just moving relative to them. $\endgroup$
    – T.E.D.
    Commented Jun 11 at 14:54
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    $\begingroup$ Alternately: Pin the faster force down against a target they have to defend. $\endgroup$ Commented Jun 11 at 17:25
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Frame Challenge: As primary weapons, Railguns and Macron Guns are Useless

Railguns expose their projectile to such intense magnetic fields that you can't really deploy them with any sort of self-guidance system like you can with a missile. Railguns also can't fire much faster than 16km/s because iron loses its magnetic attraction as it melts, and railguns heat up the projectile as they are accelerated. To put this into perspective, if you have 2 ships orbiting a planet and one peeks up over the horizon, your ships will be thousands of km apart, and that is the bare minimum range you can expect 2 ships to notice each other in space. Depending on orbital altitudes, it could take anywhere from 3-30 minute for a railgun to hit a target at those ranges meaning that something the size of a Nimitz Aircraft Carrier could dodge any shot with less than 0.01g of acceleration. Even the slower ship at 20g of acceleration could probably dodge pretty much any shot from more that a fraction of a second of flight time away. So, unless your ships are passing within 10km of each other, chances are no one is getting hit by a railgun.

As for Macron Guns, space has dust in it, not a lot, but it is there, and it for the most part hits you really really fast. You can not build a spaceship if it's not designed to shrug off really fast moving dust; so, there is no reason to think a ship will ever be destroyed by a dust gun. And even if it could, going back to the whole effective ranges of unguided munitions thing, it would be way too easy to dodge. If a targeted ship has a maneuverability of just 20g, and you were to try firing a dust gun from a range of just a few hundred km away, the targeted ship could evade by dozens, if not hundreds of km in any given direction to get out of harm's way.

The only possible use case I can see for these would be as point defense weapons for shooting down missiles on thier terminal approach. That said, if they are useful in point defense, you MIGHT treat them kind of like bayonets in modern warfare. Many armies still issue them because soldiers need general utility knives anyway, but on the rare occasion a soldier gets the order to "fix bayonets", about 8 other things have already gone wrong. So when your captain gives the order "lock railguns", the whole crew would understand they are probably not going home that day.

A Better Approach to Direct Fire Weapons

When people talk about railguns as space weapons, what they normally mean is some sort of RKKV (Relativistic Kinetic Kill Vehicle). There is no known science by which we can make a cannon instantly accelerate a macroscopic object to any appreciable fraction of the speed of light, but if such science exists, it's not going to be with a Railgun or Macron Gun... but good science fiction knows when to describe the effect and not the mechanics. You don't really need to explain how a cannon imparts so much momentum on an object, just like you don't really need to explain how a ship can have 120g of acceleration without liquifying the crew. Sometimes it is enough to simply say that this is what the technology can do and let the science behind it remain a mystery. In this since, it is probably better to stick to weapon names like "Mass Drivers" because there is no inferred technological limitations in the name. If you say that a Mass Driver can fire a projectile at speeds of 0.1-0.9c, no one can say "that is impossible because mass drivers..."

That said, if you want to use direct fire weapons in space that are explainable using modern science, lasers are the only things that actually make any since, especially against such insanely maneuverable ships. As an enemy ship comes over the horizon, you can hit the target in somewhere between 0.01-0.1 seconds instead of 3-30 minutes, and the enemy ship can not see the shot coming before it actually hits him due to the limited speed of light making it much harder to dodge a laser. While there are certainly engineering hurdles to overcome with making a laser that can project a focused and well aimed shot over 10s of thousands of km, there are a lot less hard science barriers that make them totally unbelievable.

If you go with relativistic direct fire weapons like this, the faster ship will have a an advantage because it can do randomized evasions. Even relativistic weapons have time delays of significant fractions of seconds and 120g of acceleration is enough to move 10s to 100s of meters out of your projected flight path between when the enemy ship sees you, fires its weapon, and the shot reaches you. So if you are dodging randomly as opposed to reactively, you could still make the enemy miss you more often than not, or at least make it impossible to hold a laser in one place long enough to cause significant damage.

If you don't make your direct fire weapons relativistic, it all comes down to how nimble the missiles are in your setting

In space, a lot of the more complex physics that are involved with dodging don't apply. With all else being the same, the effectiveness of being the faster ship will have everything to do with how fast the missiles are. If missiles have an acceleration of less than 20g, then no one will be able to hit each other. If they both have missiles with well over the other ship's acceleration, then they should both be able to hit each other just fine, but the faster ship will have the tactical advantage of range control. The more nimble ship only has a major advantage if he is fast enough to reliably evade 20g guy's missiles, but 20g guy is not fast enough to reliably evade 120g guy's missiles.

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    $\begingroup$ regarding the macron gun, I suspect that you might have missed the point of it somewhat. According to tough sf you could stuff fusion fuel in it in order to boost it's energy output several hundred times over. Also, a macron is much less massive than a railgun slug so it could reach velocities that are unimaginable for a railgun slug of the same size, It's effective range would likely be huge in comparison to the railgun toughsf.blogspot.com/2019/11/… $\endgroup$ Commented Jun 11 at 6:51
  • $\begingroup$ @General_Ripper compared to a railgun, yes... but good enough, probably not. If you look at Figure 1, you will see that the dust grains in interstellar flight have a much higher mass to velocity ratio than than any of the proposed options. If you assume a ship can be designed to survive interstellar flight, then you have to assume it is already designed against such a weapon. $\endgroup$
    – Nosajimiki
    Commented Jun 11 at 18:07
  • $\begingroup$ Also, if you look at the section about converting hit chance into hit rate, you'll see that it's talking about 2000-3000km hit ranges against ships with 0.5g of thrust. The OP's setting involves ships with 20-120g of thrust which still drops these weapons' effective ranges to a under 100km. $\endgroup$
    – Nosajimiki
    Commented Jun 11 at 18:07
  • $\begingroup$ you're right about the hit rates, though on the other hant the missile might not work well either. The missile have to have a few moments where they're flying directly into the target to intercept it (unless it's a bomb-pumped laser, EFP or casaba howitzer), therefore there would be a window of oppertunity where the missile have effectively no acceleration from the target's POV. In this window the macron gun might be absurdly effective at point defence so the conflict might still necessate extreme close range combat. On the other hand you're probably right on the dust immunity part $\endgroup$ Commented Jun 12 at 3:42
  • $\begingroup$ @General_Ripper I was thinking something similar yesterday. I've added a paragraph about maybe using them as point defense weapons, and how if you already have them, they could be seen kind of like your bayonet if things really get bad. $\endgroup$
    – Nosajimiki
    Commented Jun 12 at 13:47
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The higher the acceleration, the more rapidly one can change course and go away from the calculated impact point of any projectile thrown at them, compatibly with the limitation of the flesh-ware hosted aboard, of course: dodging an attack and having all the crew with shattered bones is a Pyrrhic victory.

On the other hand having a structure which can withstand such loads will make the ship bulkier and will negatively affect the payload, offensive weapons in this case.

The actual balance will depend on the details of the design of the various elements.

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I'm going to make an assumption that those missile systems have roughly the same effective range*. Being the side that's able to dart into effective range, fire, then dart back out, would mean that side is the only one that actually gets to realistically fire at all.

Note that this was effectively the strategy employed by the Mongol horse archers in the Middle Ages. The only thing that (perhaps) slowed them down was very well-fortified habitations (castles).


*- I could definitely see where this might not be the case. Having a higher acceleration on the missiles with otherwise similar tech might mean greater fuel consumption so an actual smaller range under power.

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  • $\begingroup$ In a scenario like this you have both the effective range of the missile (how far it can go while still under power--once it's motor is gone it becomes ballistic and easy to avoid) and the no escape range (where your target is incapable of outrunning it. The fraction of the total range that is the no escape range is a function of missile acceleration vs target acceleration. There's a huge difference here, the slow ships could keep the fast ones at bay for a while but their magazines will run out. $\endgroup$ Commented Jun 12 at 14:51
  • $\begingroup$ And if fuel is an issue--there was a "kill" in Desert Storm while the missile never hit it's target. The target was outside the no escape envelope and successfully beat the missile--but the time on afterburner had used too much fuel, they didn't make it back to a runway. $\endgroup$ Commented Jun 12 at 14:55
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Just to throw a wildcard into the mix, and take this further into the sci-fi genre, other effects of speed are definitely worth considering. Case in point would be the Picard Manoeuvre, a domination that allowed Jean Luc Picard's ship to appear in two places at the same time.

This was based on the rate at which sensors could differentiate between what was there and what wasn't.

This attack strategy was only countered by Lt Cmdr Data, using tractor beams.

Of course this is all dependent upon the level of technology available - FTL travel, tractor beams, etc. All very Trekkie.

Other things to consider would be:

  • The mass of both ships and proximity to gravity wells. (Also, would a larger mass, even of the 120g ship, make it more of a target when setting off/stopping?)
  • Targeting and distance sensors.
  • If the missiles are doing 200g then the game could be up for both parties, depending on distance.
  • What kind of warheads do the missiles carry?

Finally, there must be a limit to the number of missiles each ship can carry, and, of course, counter-measures and their effectiveness - would a slower ship knowingly carry more counter-measures?

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