Originally published at: Neither Fish Nor Fowl | The Eldraeverse
And next in our review of less conventional starship types, we come to that odd duck, the aerospace cruiser. (And many of these remarks, naturally, also apply to its larger cousin, the aerospace carrier.) Ever since the early Imperial Navy absorbed the old air forces into its Close Orbit and Atmospheric Command (CLATMOCOM, under the Second Space Lord), these specialized classes and their equally specialist crewers have existed in something of a limbo, engaging in practices often deemed unnatural among decent, right-thinking spacers. Such as, if I may write in hushed tones for a moment, streamlining. In short, while normally…
Some people say “Well, guess we’ll have to do without air support until we establish the orbithead.” Others say “get me a dozen good engineers and a barrel of whiskey”.
Given the Navy budget, rule of cool/shock & awesome makes this reasonable. But I do see a couple questions from more brutally practical engineers.
Why does the carrier need to be in the air at all? That gives more volume with line of sight to it, so more potential ground-based AA. And you say the altitude requires putting a bit extra mass on the ACV. Why not land on the water and turn into a naval aircraft carrier, or find a nice large area, flatness and forestation optional, use the engine and weapons, and mass of vehicle to make a place to land. My first guess is this suffers from backblast, irradiation, sinking too much to get back out, and all the other typical reasons why starships don’t land and interface vehicles use prepared fields.
But, why not take advantage of operating in an atmosphere? At least one of your fuel, remass, or coolant should be low molecular weight. Add some lightweight, low permeability cloth, probably divided into many little cells to prevent explosions or leakage from incoming weapons. Use pump. Instant dirigible. Alternatively, suck in ambient atmosphere and dump some waste heat into it for a hot air dirigible.
Even if you lose all the gaseous element in your balloon, and even if you can’t completely offset the ship’s mass, you will be drastically cutting down on fuel usage in the station keeping burn, and in fuel burned by oilers coming to refuel the ship.
Spacefaring dirigible with nuclear missiles and Top Gun memes.
In my understanding, hovering above the AO offers much greater mobility than landing in the sea or becoming a blimp, since you can just relocate laterally by tilting the ship left and right. I will note that neither blimps nor wet ships are known for their blistering maneuverability and/or agility.
Besides, this method also allows the ship to be built with standard spacefaring sensibilities in mind, without having to dedicate additional engineering to making the ship seaworthy / lighter-than-air.
Ah, that’s the easy one. Because if they did that , planets would locate all their deliciously attackable facilities in the local equivalent of Mongolia, or some other mid-continental location that doesn’t have to concern itself much with naval attack. One of the few advantages you have in attacking from orbit is that you can hit anywhere on the planet, and it’d be a real shame to give that up.
Now this idea, I’ve got to tell you first off, I kind of love. In fact, I’m going to award you an official Thinking Like An Eldrae Engineer award for coming up with that one. 
But here’s the problem. If you look at the operating altitudes specified - 65 km/211,000’ at the low end - those are well above the sort of operating altitudes achievable for balloons. (Current record altitudes for them are 136,000’ for a crewed balloon and 174,000’ for uncrewed; it’s also around 124,000’ for an non-rocket-propelled aeroplane, and only around 86,000’ in sustained flight rather than a zoom climb.) Since the lift force you can get from a balloon depends on the density difference between the lift medium and the surrounding air –
– well, the density of the atmosphere at 211,000’ is 1.8e-4 kg/m³. Gas-phase hydrogen at equivalent pressure (0.12 mbar), a density of 9.7e-6 kg/m³ will still provide some lift - but when you actually run that through the balloon lift equation, you get a skosh under 1.7 millinewtons of lift per cubic meter of envelope. Even a pure vacuum dirigible won’t do you much better on that front - and while you might be able to make an envelope that it can lift, bearing in mind you need it to weigh no more than 180 milligrams, it’s for sure not going to be lifting anything else .
This all gets easier if you’re willing to dip lower in the atmosphere, of course, but then you take on other problems like needing to provide for greater structural stress and thermal shock on entry (which adds to your mass), and being reachable by much cheaper anti-air than would be needed to reach >200 kft, along with a proportionally less effective laser pd due to greater atmospheric dispersion/absorption, etc.
All of which is to say, alas, that much as I love the idea, I don’t think the numbers work on balance.
Something else that’s occurred to me here - aren’t ACV’s basically the atmosphere-rated equivalents of AKV’s? And as I understand it AKV’s have delta-v budgets comparable to actual space warships which can pull off interplanetary brachistochrones. If this is the case they could probably make full orbit thousands of times over, so then why are ACV’s struggling to even make an 80km hypersonic trajectory, or rather why is designing for hypersonic / orbital capability a severe impact on the ACV’s performance?
Unfortunately –
– not so much. Short legs are the tradeoff for being smol. That’s the reason carriers tend to send them into the battlespace with mass drivers and/or discardable thruster packs, and why in the event of a sufficiently lost battle, sacrificing the AKVs is a normal tactic - because while you might plan on 'em being able to get back to the carrier to rearm/refuel if things go smooth, things don’t always go smooth.
But also:
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Space drives aren’t optimized for atmo. ACVs don’t want to use space drives for that reason; they want to use atmo drives, which perform better and have hilariously better fuel efficiency, up to and including functionally infinite endurance. But if you do that - and you really want to do that - then lugging around a space drive too is just adding dead weight that hurts your performance envelope where it counts, and relative to ground-based ACVs.
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And also, AKV space-drives are usually the sort of high-performance torch that it would be very bad form to go around using in atmo, largely because firing them up would be a fast ticket to a war crimes tribunal and potentially more dangerous to you than the enemy. Do you want close - or even distant, really - air support from an NSWR or XK-PLUTO?
Ah, so the performance impacts here are to kinematic performance, not to endurance / range. That makes sense.
Also yeah, had not considered the environmental impacts of AKV drives. In fact, had not really considered what drives these AKVs even! Any chance you’d do an AKV breakdown in a later post?
Where AKV drives are concerned, the key thing to bear in mind is that the chief determinants of AKV survivability and effectiveness are velocity and maneuverability.
As such, if you want to know what the AKV drives of any given era are, all you have to do is think of is which maximally torchy drive system is technologically possible but no-one outside an utter lunatic would actually use on a regular starship, and you’re probably in the right ballpark. At different historical periods, it’s been metastable metallic hydrogen rockets, NSWRs, pion drives with maybe a tenth of the regular shielding applied, etc., etc.
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