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Planetary invasions are usually depicted using drop-ships which come straight down from the sky. Pretty much armoured Apollo Era capsules. These can be bigger, for example Warhammer40k has company sized drop-ships, or smaller as in Starship Troopers (the books). However they seem very woundeable coming straight down. Even WW2 AA guns could probably take a significant toll on them.

Assuming I want to take the enemy capital (just an example, any strategically important location counts) instead of the ground zero where the enemy capital used to be after I dropped the Tsar Bomba on it, I've got to send in ground pounders. Assume further that the armies are quite heavy on robots but still require human foot soldiers (Geneva Convention). Nanotechnology is out as well, as it will mostly harm civilians and not combat robots.

My plan to prevent a single Katyusha equipped robot dog with smart antimatter tipped missiles (yield is low, antimatter is there for power density) from ruining my day is to use horizontal drop-ships. While traditional drop-ships come in from above, my idea is to use hypersonic scram jets that enter the atmosphere wherever it is safe to do so, fly low while switching between stealthy glide and acceleration mode until the target is reached where they decelerate hard. Light orbital bombardment and hypersonic missiles that came in with the infantrymen would provide covering fire while the troops deploy.

Is this a more sensitive tactical option? What potential issues have I missed?

Addendum1: This question is purely about the tactical utility of the hypersonic drop-ships vs the classical variant. The strategic context of no specific planetary invasion is considered. Assume that orbital superiority has been achieved and that significant orbital bombardment capabilities exist.

Addendum2: The setting is advanced but hard Sci-Fi. Think Orion's Arm Worldbuilding Project. The only rules of the Universe are no FTL, no aliens and ultimately asymptotically slowing technological growth. Assume no god-tec is involved, just rockets, robots, fusion, lasers, antimatter, AI, mind-uploading, ...

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    $\begingroup$ What's the tech-level - is there FTL, teleportation, other stuff we should know about? $\endgroup$ Commented Apr 17, 2022 at 8:37
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    $\begingroup$ You seem to be mixing up tactics and doctrine. Tactics are the choices made in direct contact with the enemy, As such they're dependent not on facts of your world but specific actions by characters in the world. Doctrine is the term for how a force fights. Doctrine are "We use orbital drops to assault a planet" tactics would describe the actions taken after insertion and contact with the enemy. $\endgroup$
    – sphennings
    Commented Apr 17, 2022 at 16:30
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    $\begingroup$ 1/2 My issue is that the context seems to be incomplete. The cost vs reward. But this stealthy approach is one way to do thing. Finding safe places would probably require dropping them off far far away from the front line. And with your advanced setting. They will be detected anyway. Why not just set some drones to detect and murder small infiltration units dropped like this? W40K for example has it as a slugging match, not joking. Large space ships firing on the planet, atmospheric aircraft, missiles, plasma, lasers...etc are thrown at the enemy. But the context there is clear. $\endgroup$
    – Seallussus
    Commented Apr 17, 2022 at 22:54
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    $\begingroup$ 2/2 large shield nullifying enemy fire. Powerful "wizards" controlling the armies. Wizards opening hell gates to summon demons. Ork bosses that in the case of their death the army falls apart. Those ethereals that command the Tau...etc. So. In many cases there is an actual need to send infantry to assassinate or eliminate a key figure that can once removed the war becomes much easier. Same with disabling shields. That's why I was asking about context. In your world if there is no wizard summoning armies of demons why bother with infantry? Just bomb it. In fact why not siege it...etc $\endgroup$
    – Seallussus
    Commented Apr 17, 2022 at 22:58
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    $\begingroup$ The tactics chosen will depend upon the goal of the attack. There needs to be a compelling reason to send thousands (millions?) of troopers down to fight and die on some shmoo-forsaken rock when seems so much easier and cheaper to smear that city/region/continent from orbit. $\endgroup$
    – user535733
    Commented Apr 18, 2022 at 13:07

7 Answers 7

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A planet is a complicated thing to attack, and I feel a lot of military science fiction vastly oversimplifies this.

Detection on Approach

Your incoming spacecraft are only visible out to about 7.2x their largest dimension in kilometers (for example, a 1 meter spacecraft is only visible out to about 7.2 km). This has nothing to do with radars or absorption. It is a fact of the size of the spacecraft in comparison to the size of the "sky" behind it.

Here's the math : $ \delta = { D \over d }$, which gives you an angular size, $\delta$, in radians. And takes your ship size, D, and distance, d, in whatever units you choose (as long as you use the same units for both).

Conveniently, one arc minute is approximately ${{1} \over {3600}}^{th}$ ($1 \over {60^2}$) of a radian. Just in case your want to experiment with designs, the number of pixels of information, n, available to a receiver with a gap size, K, is this -- $n = ( {{\delta} \over {2K}}) - 1$

Human visual acuity, for a benchmark is 0.6 arc minutes.

What about active detection (radar)? The equation governing this is :

$S = P_{tx} - G_{tx} + A - L + G_{rx} - db (R \cdot A) - N - 2r$

Which is a lot of math for saying that the signal at a radar receiver is, at best, ${1 \over {4 \times distance}}^{th}$ the power at the transmitter, and subject to massive losses if you coat the craft with anti-reflective material or a geometry that angles most of the transmitter power somewhere else.

Bearing in mind that transmitters start to melt in the mega-joule range ($1 \times 10^6$), the usefulness of radar and other active sensing measures starts to drop off dramatically at around 250 kilometers.

Back to strategy : the sky is big. One hundred kilometers to the arbitrary "edge" of the atmosphere (on Earth), and it would be impractical for a defender to have receiving stations on every square kilometer of a planet looking straight up for attackers.

Practically speaking, there's a lot we don't see in space. Look at the Philly airburst this year, or the airburst above Russia a few years ago.

SOMEtimes, a small target is spotted and tracked. But most of the time, they are not.

Additionally, planets are big. If you were to put one detection station on each square kilometer of Earth's surface, you would need to station 510 million such stations.

And despite the standard military sci-fi trope, planets have a wide variety of peaks, and volcanoes, and deep oceans, and shifting ice packs. There are places where it is extremely impractical to maintain an observation / planetary defense center.

So, if your attacker has good intelligence on the terrain and placement of forces, several pathways through the upper atmosphere are completely expected to exist.

Re-entry heating: Re-entry heating is a function of hitting the air at many multiples the speed of sound. If your invaders are willing to trade time for stealth, going a few hundred meters per second instead of a few thousand makes a fractional difference in your time-to-arrival, and gains you both no sonic boom announcing your presence, nor a thermal train announcing your position.

Exhaust plumes: for calculating how visible exhaust energy is, we have to go back to the signal path equation, but formulated a little differently :

$P_{rx} = P_{tx} + G_{tx} + A_{tx} - L + G_{rx}$

Where $P_{tx}$ is the power being emitted by the exhaust, $G_{tx}$ and $A_{tx}$ are the gain and area of emission, both of which a designer will be trying to minimize.

L is the loss over distance, which is :

$L = 20 log_{10}{{4 \pi d} \over {0.3 f}}$,

where f is the frequency. Thermal radiation is spread over a broad spectrum, but the peak is at about 600 nanometers ($5 \times 10^{14}$ Hertz).

For a sci-fi torch drive ship producing, say, a combat 6 gees of thrust to a 100 ton vehicle, and sending that over the rails at about the speed of light for maximum fuel efficiency. That'd be about 0.0196 kilograms of material per second, emitting about 1.76 petaWatts ($1.76 \times 10^{15})$

So, $P_{tx} = 15$, $L = 20 \times (-14 + db(d))$. Armed with these values, we can see that a torch ship with a one meter-squared exit plume, and no attempt to somehow suppress the exhaust signature.

Power received has no context, unless compared to the thermal noise, N, around it.

$N = -198.6 + 10 log_{10} T F$

Where T is the temperature of the space around you, in Kelvin. In the outer solar system, this is close to 30 K; but closer to Earth 283 K. Noise near a body in the habitable region (like Earth) is around -196.1

With these values in hand, we can go back to our signal-to-noise, S, calculation :

$S = P_{tx} - G_{tx} + A - L + G_{rx} - db (R \cdot A) - N - 2r$

$ S = 15 - 1 + 1 + 28 - 20 log(d) + G_{rx} - (-196) $

Unbaffled, a torch ship in this configuration would be visible out to one billion kilometers out.

Back to strategy : However, let's put this into perspective. At Earth's 11.7 kilometer escape velocity, a torch ship capable of 6 gees only needs to burn for three minutes (183 seconds) to brake completely, and this can all be done in the last thousand kilometers to atmosphere.

Or, the burn down could be broken up into small single-second bursts.

And none of this includes alternatives for baffling the waste heat, like a sacrificial blast shield that's allowed to fall free of the invasion ship.

Once You Get Close

Once you get close, the horizon equation becomes your friend.$ 3.57 \times \sqrt{h}$ is the distance (in kilometers), an observer at height h (in meters) can see until the horizon gets in the way. Typical single-story observers won't see the coming fleet (if they stay low) until they pass within 30 to 100 kilometers.

Since large cities are spaced about 90 kilometers from one another, if you have good maps, you can transition to your target and remain out of sight save, perhaps, by some rural community.

Assault

If everthing goes sideways, and you are detected, it is still nearly impossible to hit you with a weapon if you are careful. Again, this is all about relative size.

Should you be seen, and seen enough times for your course to be calculated, you merely need to be accelerating or decelerating continuously at random intervals. Within $ \sqrt{d \over {2a}}$ seconds the ship has completely accelerated out of the region where an observer would predict it to be.

You must either be coasting, or surprised for the defender to hit you. That's why most air defense is to load the sky full of artillery hoping to hit something. That has a cost in damage when that flak returns to the ground, and it costs something to be saturating the air with explosive shots... A defender will likely not open up the flak until they are very confident you are close.

What about lasers? if each city had a ready orbital defense laser, and an observer caught the invaders during atmospheric operations (about 45 to 100 km away, let's use 100 km) it would take 0.0003 seconds for the spotter to notify the orbital defense network, a negligible amount of time to transmit the 9 parameters of interest ($x,y,z,v_x,v_y,v_z,a_x,a_y,a_z$) at enough precision to be useful (64 bit) using a gigahertz channel, and another 0.0003 seconds for the laser to come back (assuming the laser was at full power and aimed in 0 seconds). In that time, zig-zagging has done almost nothing to foil the firing solution. The drop ship is toast.

But if aiming and coming to full power takes as little as a whole second, then a drop ship with as big a profile as 30 meters wide can accelerate or decelerate completely out of the location it is expected to be in when the defense laser arrives on target.

Short answer : hypersonic atmospheric "drop pods" are VERY reasonable. Possibly more reasonable if they are subsonic, but its your story.

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  • $\begingroup$ Thanks for the answer. While I'm personally convinced that stealth in space is possible, putting a Giga or even Terrawatt torch drive on a vessel is going to make it slightly more visible if it moves. Big telescopes are probably helping the defenders, though those are big targets as well. How does the air defence change if high energy lasers are involved? $\endgroup$ Commented Apr 18, 2022 at 17:08
  • $\begingroup$ @TheDyingOfLight I added a section on torch ships and thermal radiation. I also added a section near the end on laser defenses. $\endgroup$ Commented Apr 18, 2022 at 18:46
  • $\begingroup$ This is a crazy good answer! @TheDyingOfLight keep in mind that apart from Tsar Bombas you can also drop some pebbles on top of telescopes and probably orbital defense lasers. $\endgroup$ Commented Apr 19, 2022 at 19:31
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    $\begingroup$ @Blueriver. It's amazing how little damage you can do to temporarily decommission complex equipment. Pebbles are nice, since you'll want to use those telescopes after some minimal repairs. $\endgroup$ Commented Apr 19, 2022 at 20:33
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In terms of penetrating through air defenses, you aren't going to be able to do better than a reentry vehicle in the first place.

It's easy to miss this, because there isn't much frame of reference up in space, but reentry vehicles travel insanely fast. Apollo 11 hit the reentry interface traveling about 11 km/s, or above Mach 30. A Soyuz from the ISS, which is "only" entering from orbital velocity, hits atmospheric entry at a comparatively low 7.6 km/s, or Mach 22-ish. Pinpointing an object traveling at many times hypersonic speeds is difficult enough, let alone hitting it with countering munitions. Any weapon capable of that job will have no trouble tagging "mere" hypersonic jet fighters.

The question of what happens closer to the ground is trickier, because our space vehicles are optimized for low vehicle weight (= greater payload weight) and cost. Nobody is building reentry systems that are optimized for short flight times, but we can extrapolate.

In a conventional (Apollo, Soyuz) landing, the capsule sheds the last of its velocity by parachute. This means that it spends the last 10 or 15 minutes of its descent "slowly" falling the last 10km or so. We can shave a lot of those margins by using propulsive landing, which is hideously expensive but what is the military for if not throwing vast amounts of resources at the problem? Propulsive landings of Falcon 9 boosters give us some idea of what to expect. The booster's greatest velocity before landing is typically in the neighborhood of 2,500 km/h (0.7 km/s) at ~20km above the ground. That's actually not far off the Soyuz numbers at that altitude, so let's assume we can staple the two descent profiles together, wave our hands, and end up in the right neighborhood at least.

This gives us an overall descent profile with two regimes, ballistic and propulsive:

  • The ballistic descent takes about 9 minutes or so and drops from 100km to about 5km. During this period, velocity drops from really gratuitously hypersonic (10 km/s or more) to barely supersonic (0.3 km/s). This is similar to your hypersonic scramjets' preferred operating mode: high and vulnerable but hard to address because of how fast it's going. The most likely time to be intercepted is the non-hypersonic period at the end, which lasts about a minute.

  • The propulsive descent is short and sharp, about 30 seconds of high Gs before a "soft" touchdown. Starting at a mere 5km above the ground, you'll quickly drop below the altitude that theater-scale air defenses are meant to cover. Any personal-scale air defenses will almost certainly not be able to track you in the few seconds they have before you travel out of their line of sight.

Put another way, any air defense system capable of either destroying your drop pods as they reenter at hypersonic speeds or tagging them in the last 30 seconds of wildly decelerating propulsive flight is more than capable of handling hypersonic fighters.

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    $\begingroup$ A great addition to this strategy would be lots and lots and lots of decoys. The enemy can't shoot all your dropships down, and if there are many more dropships, most of which are decoys, there will be even less AA munition per one of your real dropships $\endgroup$
    – vsz
    Commented Apr 17, 2022 at 18:54
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    $\begingroup$ Your argument about AA defenses not being able to track an incoming ship seems to assume a lack of systems designed specifically to counter this kind of near-vertical approach. With a sensor network passing data around, a missile designed to hit within the last 5 km can be tracking the incoming ships from higher up, ready to fire as they enter the range. This is future tech that's had plenty of development time, so we can assume that the necessary software and HW to relay data from high-altitude sensors down to all local ground-based fire-control stations exists in a city with fixed defenses $\endgroup$ Commented Apr 18, 2022 at 5:12
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    $\begingroup$ @PeterCordes It's a potentially solvable problem, to be sure. (It's basically an anti-ballistic-missile system with a few different parameters.) But a system that can track and intercept high-altitude, high-velocity drop pods can do the same to high-altitude, high-velocity fighters. My argument isn't so much that you can't defend against drop pods, it's that the OP's proposal isn't harder to defend against. $\endgroup$
    – Cadence
    Commented Apr 18, 2022 at 5:28
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    $\begingroup$ Note also that there's absolutely no reason that a drop capsule has to fall in a straight line. A few control surfaces or maneuvering jets and it could be corkscrewing in just like a fighter, only much, much faster. $\endgroup$
    – Perkins
    Commented Apr 18, 2022 at 16:31
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    $\begingroup$ @Alexander the question is not, "are purely ballistic flight profiles good"? It's "is ballistically dropping low and then engaging powered hypersonic flight the best way to land from orbit?" Maneuvering while falling from orbit is an option, and doesn't use nearly as much fuel (or more importantly, time) as the OP's scenario. $\endgroup$
    – Harabeck
    Commented Apr 18, 2022 at 22:18
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Dropships would ordinarily fulfill a specific role just like paratroopers would. You want to be able to simply fly a plane or VTOL craft (or in this case a dropship) in and drop fully equipped people in the area, but in some cases you'll have to use (relatively) lightly equipped units in drop pods instead.

Most media actually shows that in-combat drops with drop-pods is a bad idea. Units land without organization, no cover and generally only survive because of them being highly trained or biomechanical supersoldiers. A smart way to deploy them would be a little away from their objectives (unless you can surprise your enemy or they are already engaged). Drop-pods would be smaller, less detectable targets and let them land closer to their objectives than other options.

The goal of drop-pods would be to land troops and clear landing zones for larger transports to bring the big guns. Taking out the very AA emplacements and anti-orbital guns that threaten the rest of your forces. Other options are to land them behind enemy lines in order to pincer them or to take out important things like supply lines, communications and the command structure.

As for your drop-pods: normal drop pods wouldnt be dropped straight down either. They would use the atmosphere to slow down first, meaning they'll first angle themselves into the rotation of the atmosphere to slow their orbital speed before falling "down". By changing the amount of slowing down you do before you go into down mode you can have a higher horizontal velocity, and land in a diagonal line (more like a slight curve but this describes it better I think).

Going full horizontal would be bad, as anything like buildings, trees, hills and mountains would limit where you can land. Also you would have to go either FAST (orbital speeds) or simply use flight to cover most of the distance anyway, making it less a drop-pod and more a dropship.

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If you want to conquer a planet, you don't invade it, you attack it.

The best way to attack a planet is to show up and schedule a demonstration of your weapons, and then give the natives a chance to surrender instead of being totally exterminated.

So maybe you tell the natives when and we here to look and then explode a million megaton bomb in space, or maybe warn them to look at their moon (if any) and ram a space vehicle travelling at relativisitic speeds into it to make a big explosion.

Or maybe you drop ordinary atomic bombs from orbit, taking out every military base, every missile launching site, every naval vessel, and every undwerwater submarine at once.

Or maybe you don't want to explode that many atomic bombs and make too much fallout on the planet.

If you can put enough ray guns into orbit undetected, maybe you can aim an undetected ray gun at every native soldier you see outside, and aim wider angle rays at every occupied military building and vehicle, and fire them all at once, vaporizing the entire planetary military in one instant.

Or maybe you use millions and millions of war robots who drop from orbit similtaneously and attack all the armed forces on the planet, automatically shooting all the soldiers, or maybe stunning them and taking them prisoners to use as hostages. And other attack robots will go to all the government buildings and capture all the government personnel and torture the leaders until they agree to total, absolute, complete, and uncondidtional surrender.

Anyway, once the planet surrenders, you send down the drop ships. And naturally your assume that the natives of the planets will have most of the military assects hidden and still active to shoot down your dropships, so you will use all the precautions and methods suggested in other answers to avoid having the dropsips shot down. And the drop ships will contain only robot troops, just in case.

And after the robot drops capture and replace all the police personnel on the planet and take total control of all the major communities, then you can send down living military and civilians to rule the planet.

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    $\begingroup$ Theres a slight problem: there isnt FTL. Your ships likely dont have the fuel for a return trip and will require the planet as a resource. Keeping the infrastructure and their operators alive is paramount to the survival of the fleet itself. And the planet knows it. A simple MAD strategy would be to say "if you nuke us we'll start demolishing all infrastructure you need to keep your fleet in working order". This leaves only relatively clean invasion to succeed. $\endgroup$
    – Demigan
    Commented Apr 18, 2022 at 10:16
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    $\begingroup$ This answer just feels like you read the title and ignored the actual content of the question. It doesn't seem to be a response to the scenario OP described in any way. $\endgroup$ Commented Apr 18, 2022 at 18:08
  • $\begingroup$ @Demigan But the target solar system should have several planets & moons beside the inhabited target planet, as well as many asteroids, comets, & other small objects. Thus the invasion fleet can easily mine the small objects for raw materials for the factor ships n the invasion fleet to make enough war robots, ships, atomic bombs, space lasers, etc. needed for the conquest. The motive for invading another solar system should not be the infrastructure on a habitable planet itself, but to control the resources on the other worlds so that the natives can never invade your system. Continued $\endgroup$ Commented Apr 18, 2022 at 20:29
  • $\begingroup$ @Demigan Continued The sensible and rational motive for an interstellar conquest would be peace - adding the new solar system to your realm and controling it, thus preventing it from ever attacking you in the future. The natives will be equally harmless if they surrender or if they die. And by definition their infrasttucture will be too primitive to serve the conquerors and only serve to keep the natives alive. So if the natives threaten to commit suicide by destroying their infrastructure the conquerors are like to say go ahead it's all the same to us. $\endgroup$ Commented Apr 18, 2022 at 20:37
  • $\begingroup$ Why are we talking about atomics at all? Why use nuclear weapons if you can just drop a rock from orbit and get an equivalent explosion? $\endgroup$
    – Harabeck
    Commented Apr 18, 2022 at 22:25
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Your invasion is easy to hard counter with missile trucks.

Even after aerial superiority is achieved, it's easy for a planet to hide a bunch of trucks with missiles under tarps, forests, caves, buildings or loads of things.

Spotters with binoculars, advanced radar systems, nanite clouds, and lots of unpleasant things on the surface can spot your airplanes which are flying for a while, and shoot them down. Stealth jets are slower than normal speeds, and work best when your enemy has no idea you're there. They don't work that well when you fly down from orbit at immense speeds and the enemy can focus all their detection stuff on you.

A radar dish and a couple missiles are much cheaper than a stealth jet. Nanite swarms are an option as well, since they offer active detection of stealth jets that fly low for some reason when the stealth jets smash through them like a million bugs and paint a target on their back since you have a rough idea of where the jet is going.

Your plan works better once you have a foothold.

Once you have invaded the planet then you can use stealth jets. You can move jets quietly on land (remove the wings, stick the wings in a truck and the body in a truck), and fly them along routes where any detection methods are degraded.

When doing an orbital invasion everything you do is obvious. People can see the jets flying in from orbit. They can track you from there on. To use stealth effectively you need to have people not know where you are.

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Atmospheric troop ships will fail to reach the enemy's capital city for the same reason that orbital drop ships will fail: if there is a defense against X, the one place you can guarantee the enemy has deployed that defense is their capital city.

If you want to say the invaders have decent anti-aircraft defenses, but the invaders' stealth is just unbeatable, it's the same as just giving the invaders indestructible drop ships: you're asserting that the invaders' tech is so far superior that strategy is essentially irrelevant. (FWIW, sometimes that's just how it is. European invaders didn't steamroll Native Americans because European culture was better.)

If you want strategy to matter, you have to acknowledge that the most appealing avenues of attack can and will be deliberately neutralized. Otherwise, it's just a huge bully pulverizing a tiny weakling. If that's the setting you want, you can do it, but that setting has no room for great military leaders, heroes, or meaningful sacrifices among the invaders.

So, you can either assert that the enemy is technologically inferior to Earth's WWII-era defenses, or you have to admit that there is no way to move ground troops from orbit directly into a maximally-fortified location in a single move.

I think your best bet for a direct assault on the capital city will involve sabotage. Get some spies into the capital so they can reconnoiter and then hamstring the defenses. Throw in some last-minute assassinations of key military and political figures to ensure that the target is organizationally paralyzed.

Before it starts to rain space marines, you send down a bunch of spies and saboteurs, along with a small armed force. They land as close to the capital as possible without risking detection, which could very well mean hundreds of kilometers away. (For reference, Normandy is ~1200km from Berlin.) The team observes the nearest settlement, learns how to blend in, and, eventually, sends spies on the long journey to the capital.

This will probably take months, or even years, depending on the difficulty of learning the language and making your spies appear to be the same species as the locals. Military invasion is not something that happens overnight.

But look on the bright side: this is actually the easy way! For comparison: back in WWII, it took a full calendar year for the Allies to get to Berlin from the landing at Normandy, and it was a brutal, bloody slog that saw the deaths of something like a quarter million Allied soldiers wiki. By contrast, this campaign will be a slow burn where nothing much happens for 12-16 months, and then suddenly the entire war is fought to its conclusion in 2-3 weeks, ending with the unconditional surrender of not just the city but the entire nation. You could even run the same scam on the planet's other great powers simultaneously.


One more note: because planets rotate, drop ships aren't going to descend in a vertical line. Yes, cartoons and video games often make it look that way, but it's BS. If you've ever wondered why NASA's rockets always take off in a big curve instead of going straight up, it's for the same reason: the main challenge isn't to gain altitude, it's to gain orbital velocity. Things in orbit will need to shed orbital velocity on their way down, so they'll come streaking in like comets.

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  • $\begingroup$ You seem to use hyperbole most as an answer. Stealth immediately makes the pods invisible, rather than reduce the range at which they can be detected and locked on to for example. The idea of a horizontally approaching pod has merit as it means the pod has to be picked up among terrain features, and with the gliding aspect you can even approach from below the horizon, reducing the distance you can be detected from. This has its own set of problems, but its not the same as dropping it in vertically. $\endgroup$
    – Demigan
    Commented Apr 17, 2022 at 10:46
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There are three better options, one only available if the assaulter is a bioengineered hivemind that consumes everything:

Global kinetic orbital bombardment and an ultimatum

Just accelerate a couple of blocks of tungsten towards any military installation the assaulters can see. The resulting smoking craters of some hundred meters in diameter are a very potent sight of your power, and the ultimatum demands total surrender or total annihilation - at nearly no cost. Even better: due to the nature of orbital bombardment, the heavy objects, aka rods from gods can not be intercepted in any way: the moment you detect them, it is already too late to prevent an impact in the killing zone, and even if you could, you'd lose a lot of the target zone from shrapnell.

Biogenetic Warfare

You just crossed space, so you totally mastered genetics. Just drop a couple of genetic weapons that alter the genetics to kill 50% of the population 3 years after getting infected. If you are merciful, offer a counteragent for total surrender. Otherwise, you wait for the collapse of society and only land then, when no resistance is possible anymore.

Hive Fleet Invasion

The scouts of the hive fleet already discovered the planet and have left a living ship on it. This is both their scout and homing beacon. The strength of the signal is modulated to the size of its hive: the larger, the stronger. They also infiltrate the population and create some kind of secret cult, to spring to action once the fleet arrives.

The moment the ships arrive, they inject genetic manipulation virii and parasites into the atmosphere, which bond to the planetary population and generate a subspecies of not-quite hivemind-soldiers that are basically mind-controlled to kill and maim everything that doesn't secrete the right protective pheromones indicating a member of the hive.

While the turned population is enveloped in chaos from the activated sleeper cells and infected population, dedicated combat drones are dropped right into zones. These start to kill every living being not part of the hive, quickly overrunning any defenses by sheer mass assault. Losses in the trillion bodies are acceptable since there is little loss to the hive fleet: dead beings and any biomatter alike are thrown into pools where they are liquified and used to feed new invasion soldiers directed by the hive fleet.

Within a short time frame, resistance on the planet is starting to wane as survives are pretty much herded and encircled by the hive swarms, and then overrun to be fed to the reproduction machinery.

Once no fauna is left but the own hive beings, the ground beings start to gather around the liquefication places, harvesting the fauna and then becoming food for the next generation themselves. The food gel produced this way is then brought back to the hive fleet. Once nothing is left on the planet to feed on, the fleet sets its sights on the next largest homing beacon, and the cycle repeats itself.

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  • $\begingroup$ You just crossed interstellar distances without FTL. You either need a supply line of interstellar distances which is unlikely or you need to use the planets infrastructure to maintain your fleet on arrival. The population can say "you nuke us/our militaries, we destroy the things you need for your ships and survival". Not to mention that hiding military bases would be priority number 1 and having an interplanetary space fleet to murder the invaders (or interstellar MAD by sending their own fleet to nuke their planet) would be basic defense. Dont use WMD's remains true in the future. $\endgroup$
    – Demigan
    Commented Apr 18, 2022 at 10:38
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    $\begingroup$ @Demigan if you can safely cross the space without FTL, you don't need something as pinky as planetary infrastructure - the ships have the infrastructure you need. And just bombing a planet into nothing or a few years is not more than a tiny delay. As for the hivemind: They eat the planet and leave an empty husk. They have no supply line, everything is in the ships already. $\endgroup$
    – Trish
    Commented Apr 18, 2022 at 10:53
  • $\begingroup$ you would definitely need planetary infrastructure. Your fuel has been depleting, carrying enough ammo for multiple years of bombing would increase the fuel requirements, you will most definitely not be able to bomb them safely as they will do this thing we call "retaliate" and a MAD strategy where a ship of the planet you bomb moves to your planet without braking would be far too easy to accomplish (assuming ships are so stealthy, which they arent). Just like modern day: you use them, you will have them used against you. So only non-WMD attacks are really useful in the long run. $\endgroup$
    – Demigan
    Commented Apr 18, 2022 at 12:01
  • $\begingroup$ @Demigan Once you are in orbit, you do need exactly 0 fuel to stay there. $\endgroup$
    – Trish
    Commented Apr 18, 2022 at 12:05
  • $\begingroup$ You mean you'll never use your fleet again and just hang there doing nothing? Or will you refuel and reuse those ships at some point? And wouldnt it be exceedingly easy for interplanetary ships to murder the hell out of those hostiles in orbit? Keep in mind that interplanetary ships require magnitudes less fuel and can use that space for weapons, detection and armor. Not to mention that the deceleration phase of the attackers would be visible weeks to months in advance, making it easy to get your forces ready and waiting. $\endgroup$
    – Demigan
    Commented Apr 18, 2022 at 12:17

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