At this point the idea of soldiers, equipment, weapons, etc. being deployed straight from close orbit is fairly popular (for some reason). The idea is pretty much that you have a ship in orbit in the upper atmosphere, high enough up that you can push through the atmospheric drag with low-power engines but low enough that the ship can launch small pods containing {tactically valuable thing} and have them reach the ground in a reasonable amount of time, usually taken to be just a few seconds or minutes.

The benefits to such an orbital deployment system are clear:

  • You can carry much more weaponry and equipment than on a standard aircraft, because you’re on an engine-powered space station in the upper atmosphere that’s technically in orbit rather than having to obey the pesky laws of aerodynamics and having to have big wings to keep your stuff afloat
  • You can get pretty much anywhere anytime, since in low orbits of Earth-like planets you can circumnavigate in under two hours versus upwards of 15 hours for supersonic fighter jets, let alone bombers or anything bigger; just boost into low orbit to enter “transit” and then descend into the upper atmosphere to get within your pods’ deployment range of the ground

There still are issues that, a lot of the time, aren’t really addressed:

  • To get an accurate and quick shot, your pods will have to be rocket-powered to deorbit them faster than the atmosphere normally would; since they’d be reentering at a nearly 90° angle, this means that the atmosphere won’t slow them down as much as if they renetered as a shallower angle and they’ll be hitting the ground very very fast. Then either you have to use parachutes to slow down the pod on reentry, opening it up to getting shot down by anti-air, or whatever’s in the pods will be smooth paste as it decelerates from Mach 25 to 0 in the millisecond it spends digging straight into the ground
  • The pod is likely to hit the ground and then immediately explode anyway, because by that point it’s carrying kilotons of TNT’s worth of kinetic energy and dispersing that energy is the way that orbital railguns are supposed to work to vaporize cities at a time

So here’s a question: is there a way for a ship in orbit to safely get equipment down to the ground in a short time (<1min) without either turning the payload into paste or getting it shot down by anti-air?

The reason I’m asking is because it seems like orbital deployment is a fun science-action thing to do, but without inertial dampening (which I refuse to use in any of my work) there doesn’t seem to be a way to make it fast, accurate, and safe all at the same time.

Let’s assume that we have as much miscellaneous supplies as we need, and that we can actually build military torchships that

  • run on super-efficient hydrogen fusion engines (virtually unlimited propellant/efficiency/thrust) that can keep the ship in low orbit by firing the engines to counteract atmospheric drag,
  • can fire equipment, would-be soldiers, bombs in pods of whatever design you can think of on chemical or solid-fuel rockets that have enough $\mathrm{d}v$ to get the pod out of orbit, and
  • the ships can have whatever other facilities are necessary to get the stuff to the ground in under one minute.

For the record, this question has at this point been closed twice as a duplicate of this question (which I have answered myself). That is a good question, but it asks more generally about how you get troops from orbit at all; here I'm trying to ask if there's some mechanism by which specifically drop pods could be used to safely transport troops and equipment to the ground, knowing that there are other safer but slower methods (i.e. standard reentry pods with heat shields and parachutes, spaceplanes of some sort, etc). The goal here isn't to ideate how to get troops to the ground, but specifically how to get troops (or whatever we can, really) to survive in drop pods.

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    $\begingroup$ Pick one between science based and internal consistency: they are mutually exclusive $\endgroup$
    – L.Dutch
    Commented May 7 at 15:13
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    $\begingroup$ Under a minute probably isn't reasonable. It would require a downward acceleration of 6 gs, constantly, without deceleration. With deceleration, the forces get more ridiculous(15 gs acceleration and deceleration), as the power of the engine which is burning straight down increases in order to get it under a minute. 5 minutes might be doable though. $\endgroup$
    – Bubbles
    Commented May 7 at 15:26
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    $\begingroup$ Just for a real world comparison. Felix Baumgartner jumped from a capsule suspended from a helium balloon in 14 October 2012 . He fell 24.21mi. 135,890 ft and was in freefall for 4 minutes 19 seconds and reached a speed of 843.6 mph (Mach 1.25) in a pressure suite. Clearly to reach the ground in under a minute, some thrust assistance will be necessary. along with a hell of a lot of luck balls and imagination. $\endgroup$
    – Gillgamesh
    Commented May 7 at 15:39
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    $\begingroup$ Orbit could just be a common language misnomer for "way up high but not out in interplanetary space". The ships from which people shoot themselves at planets do tend to have gravity without rotating, which something in orbit can't have. $\endgroup$
    – g s
    Commented May 7 at 17:27
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    $\begingroup$ Voting to reopen, because this is clearly not a duplicate of the linked question. The alleged duplicate's top answer is explicitly not accepted by this question (no intertial dampeners allowed here) and the other answers do not deal with the issue of time, which is what this question is concerned about. $\endgroup$ Commented May 7 at 18:29

7 Answers 7


For Space Shuttle missions, NASA defined the "atmospheric entry" altitude to be 400000 ft, or ~120 km. Ignoring the whole orbit thing, and starting and finishing at 0 velocity, you can get an approximation of the acceleration needed with the good old brachistochrone equations (thankyou, project rho), with $A = {4D \over T^2}$ where $A$ is the acceleration, $D$ is the distance (120 km) and $T$ is the time (60 seconds) meaning you need to sustain an acceleration $A$ of nearly 14 gravities. I'm going to go out on a limb here and claim that sustaining +14 gees for 30 seconds followed immediately by -14 gees for the next 30 seconds won't actually turn the crew to paste, but it is going to be hazardous, and the "flipover" point is going to be traumatic.

Problem is, that's just the vertical component of your velocity. That doesn't include decelerating from orbital speed to stationary relative to the surface, which involves shedding another ~7.3 km⁠/⁠s in that same minute, yielding an extra 12 gees at right-angles, giving a total acceleration of ~18 gees. The additional acceleration required will definitely tenderize your starship troopers, though they probably won't be completely reduced to soup. They're likely to spend much of the flight unconscious, and whilst fit, healthy adults will probably survive they risk (brief) amnesia and seizures on recovery. This means that if the flight doesn't kill them, defending ground forces probably will.

So much for that. How about anti-air?

enter image description here

(image credit US Army via wikimedia)

Well, lets visit the scifi future of (checks notes) the 1970s. Consider: the Sprint missile. Carrying a nuclear warhead, it was designed to intercept missile re-entry vehicles travelling inbound at ~5km⁠/⁠s, which is actually slightly faster than the peak speed of my simple brachistochrone estimate above. To quote wikipedia:

Interception at an altitude of 1 to 19 miles (1.5 to 30 km) took at most 15 seconds.

That's with 1970s tech (and remember, this thing actually flew). Your future scifi missiles will have a better performance envelope. If there's a willingness to operate powerful weapons (say, a nice laser-triggered pure fusion warhead) in the atmosphere near the target, your drop-pods are gonna get fried, because a 1 minute flight time is just too slow.

But that's not all that you need to worry about. What about the launcher? This needs a little bit of geometry rather than kinematics:

enter image description here

$a$ is the altitude of your launcher, which is 120 km in this case. $r$ is the radius of the Earth, $R$ is the radius of orbit of the launcher. The points $H_1$ and $H_2$ are the earliest and latest points in your launcher's orbit that an observer at $I$ could see them (this is a simplified horizon calculation, which assumes the Earth is perfectly smooth and spherical, and the observer is flush with the surface).

The angle of arc is $\theta = 2\arccos \left( {r \over R} \right)$ and from $\theta R$ you can get the length of the arc $H_1 D H_2$, which is ~2500 km. At orbital speed at 120 km altitude, it'll take ~320 seconds for your launcher to cross from horizon to horizon. (it might be shorter for an eccentric orbit where the launcher drops at its perigee, but that means the drop-pods need even higher accelerations to break out of orbit!)

In an era where people can just conjure up fusion torch ships, that's a long time, and longer than your projected human-crushing landing plan. Your launcher would need to get in-and-out pretty fast to avoid issues with interceptor weapons from below. If the enemy have effective interceptor weapons for your drop pods, you might not be safe. If they have torch-drive-equipped antisatellite weapons, you're in serious trouble.

What's to be done?

Get you some of these instead:

Artist impression of Raytheon's hypersonic boost glide vehicle (Artist rendering of Raytheon's Tactical Boost Glide flight system. Image credit: Raytheon)

Hypersonic re-entry vehicles can't reach a target in 1 minute, but they can be launched from a spacecraft over the horizon that can then take evasive measures, and it can approach from much lower altitude and manoever as it comes, which makes getting a safe firing solution for nuclear-tipped interceptors somewhat more tricky. The total flight time will be longer, but the chances of the payload being able to get out and walk away once their vessel has touched down is significantly higher.

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    $\begingroup$ I don't normally comment just to say "this is a beautiful answer" ... but this is a beautiful answer. :) $\endgroup$
    – Graham
    Commented May 8 at 6:48
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    $\begingroup$ Agreed. This ultimately is something I think I might implement in-world - hypersonic spaceplane-like reentry vehicles that are launched from over the horizon to outmaneuver interceptors and anti-air, then even operate under aerodynamic rather than ballistic flight to actually fly down to the surface and land to deploy troops and equipment in reasonable time. (If they have efficient engines, maybe they can go back up and dock with the ship too for extraction!) $\endgroup$ Commented May 8 at 14:50
  • $\begingroup$ @Graham thankyou 🙂 $\endgroup$ Commented May 8 at 15:14
  • $\begingroup$ @controlgroup there's definitely scope for hybrid engines driven by fusion reactors that can be air-breathing at low altitudes, and rockets at high altitudes. You might still have reaction mass issues (because you need high thrust, and that's problematic without high mass flows) but a single-stage down-and-back shuttle which doesn't need a refuel on the surface is certainly possible. $\endgroup$ Commented May 8 at 15:20
  • $\begingroup$ Thats the kind of engine I already have implemented - nuclear-driven hybrid ramjets that can switch to injecting fuel in regular air breathing mode to burn at low speed rather than using air as propellant at high speed in ramming mode. More of a hand wave than anything else - still doesn’t exist in full today - but seems reasonable enough, given I’m also waving torchships in. $\endgroup$ Commented May 8 at 15:23

the simplest answer, the kind that military would likely go to, is this sequence:

  1. A military vessel parks itself into orbit.
  2. It launches a hail of sky rods all over the intended drop territory to absolutely vaporize any kind of anti-air devices in the whole region.
  3. Repeat point 2 until the floor is lava, or at the very least, there is a little sign of anti-aircraft defense still existing.
  4. Deploy drop pods to slowly glide down on parachutes, so that your space marines can mop up survivors and crack open the deepest bunkers that "rods from god" could not pierce.

The gist is, any kind of pod deployment follows only after the enemy at the drop site, and anywhere within missile range is glassed. If the Space Navy did it job well, the only life the drop marines would encounter down the gravity well would be bacteria and the rare cockroach.

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    $\begingroup$ If you've glassed it, why bother deploying? The whole point of deploying is that there's something there you want to keep intact $\endgroup$
    – Separatrix
    Commented May 8 at 10:01
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    $\begingroup$ @Separatrix "Glassing it" is probably overkill - a few (dozen) targeted strikes at any anti-air installations should be enough to ensure a safe deployment, while keeping anything capture-worthy (mostly) intact. $\endgroup$
    – zovits
    Commented May 8 at 13:00
  • $\begingroup$ It would still make sense to use high-speed landing vehicles instead of parachutes (if possible). In a high-tech scenario, you can expect fast-response troops (e.g. helicopters) to move in after the orbital bombardment and secure the area again. $\endgroup$ Commented May 8 at 13:01
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    $\begingroup$ This is how an Amphibious Operation would work today. The Navy would pound the LZ with 5 inch guns and cruise missiles, then the Marines hit the beach. I'd remove the part about "lava" since I think it's a distraction. $\endgroup$
    – codeMonkey
    Commented May 8 at 14:42
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    $\begingroup$ @zovits : the problem with striking anti-air installations is that many of them are not installations, and are highly mobile. Or they are hidden and they only reveal themselves a few seconds before firing. Just like how the Russians learned in Ukraine, that even with superior firepower, you can't wipe out all the enemy anti-air defenses solely from the air, if your enemy has a comparable technology level to yours. $\endgroup$
    – vsz
    Commented May 10 at 4:50

Instead of doing everything literally from orbit, you might achieve the essence of your goals with less extreme physics by deploying a secondary, smaller staging vehicle at the start of a mission. It would have to be able to enter the atmosphere, maintain altitude throughout the mission, carry all of the resources needed for the mission, deploy them balistically, and wield sufficient weaponry and countermeasures of its own to provide defensive cover and supporting fire. At the end of the mission it can deploy something like shuttlecraft to recover critical resources before ascending to orbit and rejoining the main vessel.

The main drawback of this approach is it can't afford as large of a service area as delivering everything from orbit could, but that's not part of your requirements. It does move resource staging closer to the ground making it easier for the enemy to reach, but with the technology level you describe I assume the weaponry in play on both sides makes the difference minimal.

It also has the benefit of constant service, unlike a low orbit which would only pass over the mission area once every 90 minutes on an Earth-sized world.


Focus on airspace control, stealth, and combined arms tactics to get your dropships in safely, not faster drop times.

You will never drop troops directly onto a well defended military base. Safe landing requires airspace control, not just for your landing ships, but for your mothership as well. Before you can even put a landing ship in orbit, your battle fleet will already need to take a lot of steps to blind and control the planet's airspace.

The planet will have hundreds, if not thousands, of communications, surveillance, targeting, and weapon-platform satellites. These will need to be taken out first and foremost before you try to establish a Low "Earth" Orbit. On top of this, the planet could have thousands of long range missiles and hundreds of thousands of shorter range missiles. So, destroying all of the ground defenses from orbit would be REALLY hard. Instead you need to clear a path. Most defenses will be around important stuff like state capitals, military bases, etc. So, if you can find an orbital path that keeps you mostly over the planet's oceans, deserts, polar ice caps, etc, then you will only need to hit a few ground stations to create a safe orbital and entry zone to park around. This still leaves the planet's major strongholds which will be way too defended by too many cheaper shorter ranged weapons to land your troops near or to be able to take out from orbit

This takes us to the meat of the question: how to zoom down to a fortified location with drop pods. Unfortunately, there is no effective way to bypass modern or near future defenses simply by going fast. All the recent talk in the news about hypersonic speeds being a good way to get past missile defense systems has turned out to be misinformed media hype. In the current Ukraine conflict, Russia's already had nearly 20 of thier hypersonic missiles get intercepted. Much to many people's surprise, there is not a statistically significant difference between the success rate for shooting down hypersonic missiles vs thier slower cruise missile counter parts; so, even if you could drop a troop transport straight onto the planet at Mach-20, chances are you'd still get shot down if you tried deploying directly into a militarized zone.

The only defenses that have proven to be reliably effective against a missile defense system are stealth and numerical superiority. So, instead of trying to do a straight drop, you use your secured orbital path to drop a swarm of stealth transports. On Earth, a space shuttle needs about 7000km of airspace and 30 minutes to bleed off its re-entry speed; so, presumably, your drop ships will need about the same. So, you go for the slow re-entry, and when you get down to a good cruising speed, you drop your heat shield revealing a stealth aircraft that can carry your troops out of the secured orbital zone to reach the militarized zones at speeds that barely exceed the sound barrier to prevent atmospheric friction from revealing your location. By using lots of little stealth shuttles, it means that if a few happen to fly too close one of the planet's remaining ground based surveillance systems for thier stealth to keep them hidden, you can still skirt thier defense grid with most of your forces in tact to land within a relatively close proximity to the enemy.

These shuttles may also be accompanied by a swarm of drone bombers that can move out ahead of the dropships to target and destroy defense radars allowing you to get in even closer. Not only does a slower combined arms approach do a way better job on realism, it also makes the whole process of landing troops on a hostile world far more intriguing.


No for soldiers, maybe for bombs or cargo.

If the mothership is in anything remotely like an orbit, it will move at very roughly 10 kps. Just making up for that requires about 17 G for 60 seconds (17×9.81 m/s2×60 s = 10.006 m/s). Maneuvering jet fighters pull less than that for shorter periods.

An unmanned drop pod can endure this acceleration, so the question would be if it the engine can provide that much thrust for a minute and if the shell can withstand the thermal stresses. If you can build the drive, you can probably build the heat shield.

Note that I'm assuming that your magical fusion engines are small enough to fit on pods.

Of course such a pod would get very hot, which makes stealth difficult.


Just going to leave some quick math here...

Orbital speed for low Earth orbit is ~8000 m/s.

You want to decelerate to a stop in 60 seconds, that means we need to decelerate at (8000 m/s) / (60 s) so a deceleration of ~133 m/s^2 this is about 13.5g.

This is not going to be comfortable, but is generally survivable. Note this is comparable to deceleration from a car crash. Don't expect to be in great shape, broken bones, passed out, loss of vision, or otherwise incapacitated is a definite possibility.

For further reference an answer to this question references a NASA study graph which indicates for one minute exposure greater than 10g is probably not a great idea.

What is the highest acceleration that a normal human could safely endure for (say) 8 hours?

So this time is probably right on the edge of possible given the human bodies limitations.

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    $\begingroup$ Interesting, I just wrote almost the same answer, with slightly different rounding assumptions, and came to a "no." 10 kps is a little high for the very lowest orbit, but we both approximated other maneuvering away, so I rounded up. $\endgroup$
    – o.m.
    Commented May 7 at 16:17

For the succesfull troop deployment you need to estabilish dominance to protect them.

In the early modern era naval dominance was enough. Since WW1 the air and naval dominance is mandatory. Your case goes further.

First step is to estabilish stellar dominance meaning your orbital station(s) is/are safe from counter attacks.
Second stage is estabilishing air dominance by taking down air defenses and airports.

After that you can deploy troops to estabilish ground dominance.

Even one minute deploy - arrival time is too long for safe invasion when air defenses are working and ready.

Succesful invasion against enemy that was not blinded beforehand is plausible only using rusiian tactics - overwhelm the defenses by plenty. But I suppose this is not feasible in space combat.


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