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Context

I'm helping a friend with a hard sci-fi story and we ran into an issue. In this setting the technology for absurdly powerful and efficient fusion drives which seem not to have heat managment issues is ubiquitous across the solar system.

Reading a lot of The Expanse and ProjectRHO has told me that running an absurdly powerful engine is akin to detonating a modestly sized nuclear bomb in a pillar directly under your ship, continuously. Which would probably play hell with an atmosphere, should you try to use said engine in one.

I can count half a dozen reasons why, but mostly from shockwaves as the exhaust slams into the surrounding air, turbulence from the speed and temperature difference, and reflected heat energy screwing with the magic thermal managment system which seems to simultaniously not exist and keep the engine from melting.

I'll pretend that radiation isn't a concern here because this is a direct 'lensed' fusion drive burning some nondescript aneutronic fuel, so I'll leave that here.

The Question

Earth's navy force is built and maintained mostly in orbit, but rarely ships are built or brought down planet-side for repairs or secretive operations. So as to not go into detail let's just pull an any-ship.tm to use as an example. Let's pick the Leonidas-class from The Expanse. Noting the donnager's (only ship with a cannonical mass) quarter-million tonnes and 400-odd meter length, lets say our any-ship is 200-odd meters long and 90 thousand tonnes. Probably the biggest ship that could be brought down.

Bringing the ship down the gravity well should be relatively strait-forward. Turn on fusion engines and slow down from LEO-speed to hanging over the ocean with no relative velocity, drop strait through the atmosphere, and then using chemical booster skiffs attached (somehow) to the ship, slow it right down before splashdown off the coast of wherever the spaceport is, and use a tugboat to tow the ship home.

But how do you do this the other way around? A (practical) chemical booster, or hundreds of them, couldn't get this thing anywhere near the Karman line, where you can just kick on your fusion drive and burn the rest of the way to orbital speed.

Is there a way around this? Something better than chemical rockets, but not as powerful as LFDs?

Would the ship need to get into space to safely fire its LFD or could the boosters get it as far as the tropopause, where the drive might be able run for a precious few seconds without melting itself. (Probably out in the middle of the sea, as the shockwaves and noise would still be insane. Stand next to Starship IFT-2 at point-blank and then multiply by a few orders of magnitude.)

Could the booster skiffs use some modified fusion thermal rockets or airbreathing-something? Less powerful than an LFD but powerful enough to get to space? Maybe some kind of fusion drive designed to run in-atmosphere, at least partially? Perhaps a combination of the above?

If all else fails I guess we can ditch the entire idea, because as far as my understanding of physics goes this is burning up some of this book's precious supply of suspension-of-disbelief-ium.

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    $\begingroup$ I'm sorry but I do not see the asymmetry. If the chemical rockets can slow down the ship from its terminal velocity down to zero, they can very obviously also accelerate the ship from zero to the exact same velocity, in the hundred to two hundred meters per second range. To reach the Karman line all you must do is arrange for the chemical rockets to be able to burn their engines for ten minutes or so. (Please also note than with any reasonable average density, a 400 meters long quarter-million tonnes ship will be wider than it is long...) $\endgroup$
    – AlexP
    Nov 19, 2023 at 19:25
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    $\begingroup$ @AlexP That assumes a terminal velocity high enough not to be accounted for, which would not be the case. Decent has a lot more in its favor, notably being able to aerobrake to some degree, while the tyrannical rocket equation works backwards for the boosters. There is practically no penalty for fuel brought down from space. On top of that the vehicle gets lighter as it burns through its fuel. $\endgroup$ Nov 19, 2023 at 19:36
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    $\begingroup$ Oh, one more thing... If you can control a descent and land safely, then you have the ability to take off. If you don't have the ability to take off, then you crashed. The exceptions to this are gliders, parachutes and autorotation... but that's not what we're talking about. $\endgroup$
    – JBH
    Nov 19, 2023 at 19:55
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    $\begingroup$ A 250,000 ton space battleship has capabilities suited for waging war in vacuum at millions of kms of remove. What use could it possibly have in atmosphere, on the ground? Talk about a fish out of water... That's a job for another type of vehicle, or a different strategy altogether. $\endgroup$
    – BMF
    Nov 19, 2023 at 21:38
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    $\begingroup$ I should note that weather you use Fusion engines or chemical ones to lift a 1/4 million ton spaceship really dosnt matter for all the non feasibility is concerned. A thruster array capable of lifting 1/4 million tons will destroy the ship inside the atmosphere, regardless of what the thruster actually is. Let alone that no landing pad could survive this. $\endgroup$
    – ErikHall
    Nov 20, 2023 at 2:00

3 Answers 3

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Obviously, bringing something that big down to Earth is just asking for trouble. It was built in space and operates in space... subjecting it to gravity and turbulence and weather and an atmosphere that's 20% corrosive gas seems like a terrible idea. Making something that could survive such a trip seems likely to make it vastly more expensive to build, and heavier than you really want (because that just wastes energy once you're in space).

Anyway. Clearly, that's what you want, and landing your huge spaceship on Earth looks cool, so here we are.

Turn on fusion engines and slow down from LEO-speed to hanging over the ocean with no relative velocity, drop strait through the atmosphere

So, problem one: dropping straight down is quite energetically expensive. You need to quickly eliminate your orbital speed, and then prevent your ship from going too fast when it reaches the thicker bits of the atmosphere.

Your ship, being designed for use in space, is unlikely to be particularly aerodynamic. Making it so would be unnecessary expense and a waste of mass. This means it couldn't go as fast as an regular atmosphere-capable vessel before bits of it start getting knocked off, or Max Q causing the whole thing to fold up in a really embarassing way.

What you need, therefore, is some kind of recovery aeroshell. If your ships are built with standardized hulls (at least with particular types or classes) then you can have prebuilt aeroshells parked in orbit. A given aeroshell design is specific to a model of spacecraft and atmosphere... you won't use an Earth-optimized aeroshell on Mars. Not more than once, anyway.

Your spacecraft makes a rendevous with an aeroshell, which is then clamped around it. This may be totally enclosing, or sufficiently enclosing to prevent the ship from using its own engines (which might be drained of fuel/reaction mass before hand), and the aeroshell could have its own engines optimized for the purpose, which your spacecraft's own propulsion is very unlikely to be. The aeroshell then does a brief retro-burn and re-enters the atmosphere below. It uses standard re-entry techniques, with a combination of aerobraking and thrusting as necessary to get down to sensible speeds, and then it lands in some suitable fashion. This might just be a splashdown (simplest and safest, perhaps) or a gliding landing at a gigantic runway, or a powered vertical or near-vertical landing.

You can then do the needful to your ship, which can remain inside the aeroshell hiddem from prying eyes if that's what you wanted.

But how do you do this the other way around? A (practical) chemical booster, or hundreds of them, couldn't get this thing anywhere near the Karman line, where you can just kick on your fusion drive and burn the rest of the way to orbital speed.

Is there a way around this? Something better than chemical rockets, but not as powerful as LFDs?

Well, the aeroshell you came down in well certainly help you get back up again, and protect you against atmospheric buffeting and heating, so that's a good start.

What you need is enough thrust, and enough specific impulse (or Isp) to do the job. Too much thrust causes excessive forces on your ship, and you'll break it. Too much Isp means your rocket exhaust is dangerously energetic, and that's the point at which you do Bad Things to your surroundings (see also, Project Orion). Too little thrust means you'll waste too much fuel trying to get up, even if you can get up at all. Too little Isp means you burn too much fuel and require huge amounts of the stuff and multiple stages which will be of gargantuan proportions.

Pick an engine with a decent thrust-to-weight and an Isp of 1000-2000 seconds max, and take it from there. Project Rho can help with this. Possible rockets include closed-cycle gas-core fission (AKA. nuclear lightbulbs) or ablative laser propulsion (ground-based laser deathrays, turned down a bit).

Could the booster skiffs use some modified fusion thermal rockets or airbreathing-something?

Bussard proposed a series of fusion rockets he called the Quiet Electric Discharge. There used to be some papers about this freely available online, but they seem to have been mostly removed, alas. The low-ISP variant used a fusion reactor to drive some relativistic electron beams which were used to heat propellant... this could be almost anything, such as hydrogen for high Isp, or water or even regolith for cheapness. It could also be air breathing, if the atmosphere were dense enough, or the rocket was travelling fast enough.

Myrabo's Lightcraft used air-breathing, laser-driven pulsed plasma propulsion to get up through the thicker bits of the atmosphere, before transitioning to using some other fuel when the air was too thin. This uses ground-based deathrays again, which keeps the cost, weight and complexity of the launch aeroshells down.

There are plenty of options available to you, in any case.

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  • $\begingroup$ I. LOVE. THIS. This is fan-frigging-tastic! I want to wait before accepting but the aero-shell idea is just GENIUS. I had an epic diorama in my head of a spaceship being brought down by hundreds of little skiffs like how the LDSS Nauvoo was aimed at Eros in the Expanse. But this is even better! Also, I need to do more reading on engines in the lower energy brackets, as I really only paid attention to the absurdly powerful ones I could use for torch-ships. I guess in the end it's a problem of turning energy from fusion into fast moving gas, but not so fast and hot that it's a plasma cutter. $\endgroup$ Nov 21, 2023 at 3:45
  • $\begingroup$ I'll also add that for the most part this kind of tech will be used for smaller ships, and this is like the worst-case-scenario. If this doesn't burn through too much precious suspension-of-disbelief-ium. $\endgroup$ Nov 21, 2023 at 4:51
  • $\begingroup$ If you did perhaps know, what would you guesstimate the lowest altitude you could safely (and probably legally) flip on your fusion drive without cooking off the atmosphere, the engine, or the ship. I feel like I could get away with intentionally lamp-shading using real small fusion engines, like the ones on (space-optimized) shuttlecraft (about the size of a 20 foot shipping container) in an atmosphere. Sure, you need to launch from the sea, and the government probably doesn't like people doing it, but its possible to do if it's nessesary. $\endgroup$ Nov 21, 2023 at 5:14
  • $\begingroup$ Of course atmospheric-rated shuttles exist and are preferred, but if you only have a space-rated shuttle, you could get planet-side in a pinch. $\endgroup$ Nov 21, 2023 at 5:16
  • $\begingroup$ @Shift_register honestly, it is much too hard to say, and so to a certain extent you get to decide for yourself. It depends a lot on drive performance and technology and fuels and so on. At least 10-15km, depending on the nature of your drive nozzles, though honestly there's good arguments that you shouldn't light up your nuclear death cannon anywhere below MEO (or even higher, depending on how busy GEO space is!), and even then you'd better be pointing it in a safe direction. $\endgroup$ Nov 21, 2023 at 14:14
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The most likely answer, as has been pointed out in the comments, is that nobody would try to land such a large ship in the first place. Not with any expectation of launching it again. Or surviving the impact, for that matter. Keep the ship in orbit, and use vehicles designed to operate in an atmosphere (shuttles, space elevators, etc.) to carry cargo and people to and from the ship.

If you're determined to get the ship to the ground and then back into space again, your best bet will be to use some large external structure designed for the purpose of moving large cargoes into and out of the atmosphere- a space elevator, or a skyhook, or something of similar scale.

If you don't want to use these (as you've also mentioned in the comments), then the only solution I can see would be to use a type of rocket engine whose exhaust does not interact with the Earth's atmosphere or crust. And there are a few particles that, in principle, could be used for this purpose.

Probably the best candidate in hard sci-fi would be neutrinos. Neutrinos can be produced artificially with nuclear reactions, but they interact with regular matter very rarely otherwise. The exhaust from a neutrino rocket would pass all the way through the Earth almost completely unaffected. You won't have to worry about your exhaust crashing into the atmosphere and incinerating your ship (or annoying the neighbors with sonic booms).

I'm not sure how much of a radiation hazard standing behind a neutrino thruster would be. I don't know how to estimate that. It surely depends on how many neutrinos you're producing and how energetic they are. Using fewer, more energetic neutrinos might (might- I'm guessing here) reduce the radiation hazard. Doing this would definitely also improve the engine's specific impulse, so accelerating those neutrinos as fast as possible may be a win-win.

Anyhow, if your engineers can manage to construct a neutrino engine with enough thrust to lift your ship, it would be a good choice for an engine to use in atmosphere. It'd also be a good engine for a warship in space- it could be placed inside your ship's hull, protected from enemy fire, and would still work just fine. And if you're trying to hide in space, lighting up a neutrino engine wouldn't produce a shining beacon of hot plasma visible from across the solar system.

If neutrino rockets aren't exotic enough for you, you could consider a graviton thruster: a rocket engine that, instead of spraying a jet of matter, instead emits a (very intense) beam of gravitational waves. Like photons, gravitational waves have no mass, but they do still carry energy and momentum. And like neutrinos, they don't interact with regular matter very strongly.

Another possibility for weakly-interacting reaction mass is dark matter. Of course, nobody IRL knows what dark matter is made of or if there's any way to use it in a rocket engine, but in a slightly softer sci-fi setting, your engineers may be able to figure it out. And the result would be very similar to the neutrino thruster described above: an engine that works equally well in space, in an atmosphere, or inside your hull.

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  • $\begingroup$ Biggest problem I see with the neutrino drive: How are you going to make the neutrios go in the direction you want them to? A nozzle isn't going to cut it. You would have to control the reaction itself so that they are emitted in the correct direction in the first place. I don't know if any plausible physics for that exist. $\endgroup$
    – LazyLizard
    Nov 20, 2023 at 13:11
  • $\begingroup$ @LazyLizard Afaik in the DUNE experiment they're creating a tightly focused pion beam, and the neutrinos are emitted during the decay of the pion preferentially in the direction of the beam. How efficient this is, I don't know. $\endgroup$
    – biziclop
    Nov 20, 2023 at 15:12
  • $\begingroup$ A neutrino drive powerful enough to lift a ship is going to do to whatever's underneath far worse than than a mere fusion rocket or Orion nuke propulsion. I can't do the math, but strongly suspect matter itself will melt through IBD. Anything that dissolves the neutrons you are made of is bad mojo of the Lovecraftian variety, and I think you're a good fraction of the way there with this technology. $\endgroup$
    – John O
    Nov 20, 2023 at 16:04
  • $\begingroup$ @biziclop thanks, so there definitely is a way of controlling the direction, interesting. I assumed it would be random. JohnO What du you mean with IBD? Probably not Inflammatory bowel disease ;) $\endgroup$
    – LazyLizard
    Nov 21, 2023 at 11:05
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Well, first of all, they wouldn't. Bringing something this ludicrously big (90,000 tons) down to the surface on anything atmospheric just wouldn't be done with the hard-scifi level you describe, just how today's Aircraft Carriers don't regularly spend downtime in Kansas.

That said, if you are looking for solutions to transport large mass objects from LEO to Surface, there are several options:

Infrastructure

Space Elevators have myriad issues, but it's not absolutely inconceivable that a space elevator or similar "tower to orbit" could provide an easy way to move massive ships around. Simply park your 200m ship on a corresponding elevator platform, queue the music, and wait. Other, more esoteric infrastructure systems like skyhook systems or lofstrom loop-esque actively supported structures could also be used.

If your focus is on ground-to-space, megaproject scale launch infrastructure could also do the trick. For example, a magnetic accelerator tube that's a couple hundred kilometers long might be able to shoot your 90kT ship into a suborbital trajectory that gets it high enough to where it can use the nuclear torch to finish getting to orbit.

Aerodynamic Lift + efficient propulsion

There have been concepts for air-breathing single-stage-to-orbit spaceplanes since forever. The most notable example is probably the currently-in-development SABRE engine. The idea behind this class of engines is that they use a pre-chiller to bring superheated hypersonic inlet atmosphere down to very cold temperatures where it can be used by the engine and have two stages of operation which allows them to seamlessly switch from air-breathing to "rocket" mode. This allows it to have very good ISP, as it can achieve a large percentage of necessary orbital speed within the upper reaches of the atmosphere. Similarly, advanced aerospike engines could also achieve high enough efficiencies that SSTO becomes feasible.

I think a very large 200-meter long SSTO spaceplane would be feasible in a hard-scifi setting, however this would be a craft engineered around this specific purpose: it wouldn't be a general spaceship that can just happen to land and take off from Earth, but rather it would be a dedicated super-large transport shuttle essentially only optimized for this one journey (even with an efficient SSTO, the overwhelming majority of the craft's mass would still need to be fuel).

Modular Design

Maybe you don't need to bring your 90kT 200m ship down and up in one piece. Instead, it is designed to consist of modules that can link together but still operate independently (to a degree). Whatever reason you have to bring these megaships down to the surface, maybe you can get away with detaching a surface-going module, or just those modules that need to be brought to the surface.

Stupidly big launch vehicles

There is no fundamental reason why you can't just put your 200m large, 90kT ship on the tip of a stupidly large chemical rocket. In fact, chemical rockets get more efficient with scale due to the square-cube law. Likely this rocket would need to be launched from the middle of the ocean like the Sea Dragon concept, because it would be so ludicrously powerful that, like a nuclear weapon, it has a large kill-radius. Environmentalists would not be happy with you though, and it would be very expensive.

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    $\begingroup$ I think the Modular design here is such an underated idea and the best solution here. $\endgroup$ Nov 20, 2023 at 22:04
  • $\begingroup$ I like the modularity idea, but what about that giant glider? That seems even more badass than a propulsive landing. How big would it need to be to carry a ship under it or on its back up to a high enough altitude to fire its fusion engines. $\endgroup$ Nov 21, 2023 at 4:54
  • $\begingroup$ @Shift_register It would need to be absolutely enormous, however concepts like this have been explored: see the Stratolaunch Plane, which was designed to act as a flying launchpad for rockets. One of these at an absolutely massive scale and with more advanced propulsion might be able to boost your empty and nearly fuel-less megaship onto a suborbital trajectory. The problem is that stratolaunch, one of the largest planes ever with a wingspan of 117m, only has a payload of about 250 Tons. $\endgroup$
    – Dragongeek
    Nov 21, 2023 at 8:24

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