# Binary system travel time for war

To set the stage, 2 opposing groups exist in the binary Alpha Centauri system. The 2 stars have a roughly 80 year orbit that brings them as close at 11 AU together and as far as 36. A ship at a 1g acceleration will take 9.3 days at closest approach and 17.0 days at furthest.

Previously, I had the history of these 2 nations basically going to war every 80 years as they get close to one another as the travel time is more feasible for this. The close orbit is present for roughly 12 years before they get further and further apart.

My question is in a an age of a torch drive starship and fusion economy, would this 7.7 day difference cause such a problem that most wars only happen during the close approach because the logistics behind backing anything further away would impede the war effort too much?

I'm probably overthinking it and that the necessary war effort would be difficult either way and a large command and control ship would need to go with the travelling ear group due to light lag time any way.

• Travel between planets is not going to involve a constant thrust. At a constant 1g you could travel 11 AU in 6.7 days, but then you would promptly crash-land at 6000 km/s. Plausible for missiles, not for personnel. You could accelerate half the distance and decelerate the other half, and arrive in 10.3 days with a safe landing. But this is still very expensive in terms of fuel. If you only accelerate for 1 day, and decelerate for 1 day, cruising the distance in between, you will cross the distance in 23.5 days, but use 80% less fuel. Jul 11, 2022 at 14:02
• This war does not sound profitable. Jul 11, 2022 at 15:23
• @Markitect What do you mean by "torch drive starshps"? Travelling a few AU between the planetary systems of two stars in a binary system is not star travel and doesn't make the ships starships. And what do you mean by "torch drive"? You write that like you foolishly expect everyone to know what that means. Jul 11, 2022 at 18:27
• I don't think it's foolish to expect a sci-fi writer to know what a torchship is, it's imho a rather established term. Torchship is a ship with both large thrust AND specific impulse, i.e. a "point and shoot" (Brachistochrone) ship rather than one that needs to bother with Hohmann orbits. See projectrho.com/public_html/rocket/torchships.php Jul 12, 2022 at 8:45

In a war doing what the enemy expects you to do might not be a smart idea. I have read that at the approach of the D day the Germans were expecting the allies to land during high tide, because it would have given the infantry a shorter walk on the beach, and so they packed the beaches with Rommel's asparagi and other defenses which would have worked well in high tide. Apparently the allies opted for landing during low tide, making all those defenses less effective.

In your case it's also true that the closest approach seems to be the best option for attacking, therefore it's also what the defender choose as a moment to strengthen their defenses, and as an attacker you want to maximize the surprise effect.

All in all, if those 7 days will make a difference depends on the specifics of the attacking forces: are they launching sort of a routine attack where they are well within the boundaries of their technology so that there are no big differences in choosing the long or the short path attack, or are they pushing those boundaries and even the savings related to those 7 days can make the difference between a completed and a failed mission? You can use this an element for your story, if it helps the plot.

Here's the biggest problem:

A ship at a 1g acceleration will take 9.3 days at closest approach and 17.0 days at furthest.

If your ship can maintain a 1G thrust for nearly twice as long, there's clearly no problem, but it requires you to handwave away some fairly serious technological hurdles.

You've nearly doubled your delta-V requirement for each leg of the journey, and remember that unless you can guarantee victory or the ability to safely refuel at your destination you will need to carry enough go-juice to get you there and back again.

Running two 1G brachistochrones across 11AU and back requires a delta-V of approximately .1c. Across 36AU it'll be more like .2c. It is expensive (in terms of fuel and reaction mass) to have a delta-V that exceeds your exhaust velocity. If you're running fusion torches, then your exhaust velocity is unlikely to exceed 0.09c (you are familiar with Atomic Rockets, I assume? If not, read it all, ASAP!). The mass ratio of your rocket is $$R = e^{\Delta_v/\Delta_e}$$. Lets imagine, for mathematical convenience, that your rockets have an exhaust velocity of 0.1c, which means that a delta-V of 0.1c gives you a mass ratio of e, or about 2.7 (equivalent to ~63% of the "takeoff" mass being go-juice). To achieve a delta-V of 0.2c with the same rockets gives you a mass ratio of more like 7.4, equivalent to ~86% of the "takeoff" weight being burnt to get you there and back.

Just think, at the closest approach your ships can carry 2.7 times more payload mass. That adds up to quite a lot of munitions, or even troops if you were in to that sort of thing. "I know", you might be thinking, "just make the ships 2.7 times bigger!". Well, now they're going to be a lot more expensive, aren't they? Harder and pricier to build, harder and pricier to fuel. If you waited til the closest approach, you could have 2.7 times more ships instead. Doesn't that sound like a better deal?

If your torch ships use some other technique for propulsion with a much higher exhaust velocity, such as antimatter, then this constraint doesn't apply (though you still need to build up a stockpile of antimatter twice as large, which is likely to be expensive and/or time consuming). This lets you tweak your tech to fit your story needs.

(Personally I'd just dial down your rocket performance until doing a 36AU trip promptly seems impractical. 1G sustained brachistochrones require ridiculously powerful rockets, after all, and if they're easy to achieve then just having a rocket artillery duel seems likely to be the easiest means of waging war...)

It is certainly cheaper to wage war when both civilizations are close to each other. So unless they have unlimited resources that could be a strong reason for such a scenario to develop.

The wars between the two nations could be somewhat ritualized, like the wars of the Aztecs, the so called "Flower Wars", which did not have the purpose to destroy the enemy but to capture some of them in order to sacrifice them to the gods.

I do not assume that those two advanced nations do alien sacrifices, but I could imagine that they use a kind of limited warfare to settle disputes.

If you do not have strong rules which both belligerents agree on, wars between technologically advanced nations should be very short and end disastrous. For example, a surprise attack against a planet with a strong laser weapon can wipe out a civilization in seconds. The laser is as fast as any signal you could send as a warning, so as soon as it is fired there is nothing you can do.

In fact our conflicts are already somewhat ritualized. There is, for example, a broad agreement not to use nuclear bombs, because it would lead to mutually assured destraction.

My claim is that every war between advanced civilizations of equal technological possibilities would lead to mutual destruction unless there are very specific rules both civilizations have agreed on. One of those agreements could be that military operations are only conducted during the years when both stars are closest to each other and when costs and energy expenditure is the lowest. They could wage war at any time, they just don't.

Why don't they just live peacefully together? I don't know. Why don't we?

• Very important point indeed : We have to remember that some of the old wars have been managed very "ritualistically". If the two planets are bound by a code of honor of some sort or follows some noble/monarchy style of politics, it's definitely plausible and believable to have them fight each other only at the closest distance. Jul 11, 2022 at 14:13
• Don't know why you need a monarchy to have a code of honor. Jul 11, 2022 at 14:34
• It's only a common trope (and history bias, I believe), where because it's medieval political systems, you need to have samurais doing the bushido or knights following courtship rules, whether that actually makes sense or not :p. It's not compulsory at all to follow it. Jul 11, 2022 at 14:49
• I see. -medieval societies just with fancy weapons and two suns. The usual lazy starwars stuff. :-{} Jul 11, 2022 at 16:02

### The two sides of the star

Nine days means a lot. If your armies are going all out pew-pew and blasters out, a whole squadron could get smashed down to star dust in a quarter of the time. It's even worse with fuel : if we dare to take a comparison with some modern battle tanks, their fuel autonomy is at the very most one day1. This makes fuel, ammo and ship management an hell to predict, so picture it now for a 17 days travel. It looks like it's an impossible task for your space generals.

But look it another way : If your travel time is already that bad, is it really important if it's longer? Can't we just work around that and nullify most ofthe issue? Yes, we can! When using land warfare as an example, we're not looking the right way. It's actually closer to modern naval warfare : Ships travel for days there, and they have autonomy for a good while2. What your armies need to do is to focus on self-autonomy rather than day-to-day resupplies strategies.

Very big fuel and ammo tankers, flagships which can hold smaller ships with less autonomy (akin to aircraft carriers), distance-for-fuel efficient engines, independent directives on a tactical level followed by rendez-vous points when the time to resupply comes... All these can make your troops more independent of logistics, making the issue of a 9 days trip much less critical, and a 7 day longer trip at most an hinderance on top of the initial issue. In other words : Initial costs of adapting to long travels are heavy, but adding a bit more once the warfare culture is adapted to it is more... Secondary.

### Conclusion

So if we sum up, yes, adding travel time is an issue, and usually an important one. But knowing your troops still needs at the very least 9 days of travel, they would have gone towards a more consistent, more autonomous way of getting resupplied. And knowing they would have adapted for the ideal situation, adding a single week of travel is much less important.

1 : For instance the first version of the M1 Abrams has a 500 US gallons tank, while it consumes 60 gallons per hour while moving. This means... (count on fingers), 7h of autonomy. Not accounting start time (10 gallons!) and idle time (10 gallons/hour). If that's not the love for good fuel, what is it ⛽?
2 : Here, we can see that the aircraft carrier "Charles de Gaulle" can hold 45 days of food and moves at most at 50km/h with virtually infinite autonomy, meaning it needs 20 hours for 1,000 km. In contrast, the greatest East-West width of the Pacific Ocean is about 19,800 km, so naval travel times around the world are counted in days.

You ask whether logistical considerations "would impede the war effort too much". This is impossible to answer until you define "too much".

Armies have historically travelled for months across hostile territory under severe weather conditions, so if the question is "will an invasion become unfeasible if it takes an extra week" then the answer is obviously "no". In fact I don't expect there would be that much of a necessity of getting across as fast as possible and many logistical issues can be mitigated by simply taking it slower (Tortiliena's answer goes into more detail as to why).

But it comes down to: why do they fight? It's hard to imagine a motivation serious enough to cause otherwise independent, self-sufficient civilisations to commit significant resources and lives to fighting one another, yet that motivation somehow expires once travel takes 11 days instead of 10 (especially since the change would be extremely gradual). Any incentives developed during wartime, e.g. an expansion of the military industry or generals gaining political power, would tend to self-reinforce, making it hard to exit the war cycle.

One option is ritual wars, as suggested by Avun Jahei. It would work technically, but I'm a bit skeptical about using this in a narrative, because "my aliens do this illogical thing because their culture/religion says they Must" is a bit of a lazy trope. People object to their children dying in war. You'd have to have a very fanatical (and therefore stereotypical) society, and even then you'd probably want an underlying reason: humans, at least, tend to apply "moral" or "ritual" framing devices post-hoc to justify wars fought for "normal" reasons - resources, expansion, power.

Maybe think about your story constraints. Do you specifically want this war to be fought for only ~12 years for every 80 year cycle? Find a motivation that only applies during those 12 years. The planets encroach into each other's space debris field? There is a resource of some sort that becomes inaccessible when the two planets are very close? Proximity perturbs the planets' geology such that each planet internally experiences significant geopolitical strife (due to displacement, crop failure etc) and every time someone comes up with the idea to channel these destructive circumstances into a war of expansion? These are trivial examples, but something along those lines would justify your setting and give it more depth.