Rapid-repeat re-entry vehicles

Question

In my current writing project, I'm working in a universe where mysterious alien beings travel not directly through space, but through planetary atmospheres via wormholes. These aliens shotgunned through our Solar System, networking our planets together among themselves and among other exoplanets around other star systems, and beyond.

These connections not only facilitate the aliens' travel through space, but ours as well. The wormholes function as the backbone of an interstellar economy and human civilization.

Spacecraft traveling along routes through the wormholes will need to re-enter atmospheres frequently. Consider this: a spacecraft re-enters and traverses a wormhole, exiting in the atmosphere of the other planet. The spacecraft queries a satellite network, which relays the position of the next wormhole along the spacecraft's intended route. The spacecraft then boosts back into orbital or suborbital flight, depending on where that next wormhole lies. (It may also boost into orbit to refuel or make repairs.)

(Suborbital flights would obviously be preferable for lower energy cost; however, depending on distance to the next wormhole and the spacecraft's time profile, exiting the atmosphere and assuming orbit might be preferred. In addition, the wormholes themselves migrate through the atmospheres, oscillating between higher and lower [dense & less dense] altitudes. While one side of a wormhole may be in the upper atmosphere, the other may be hundreds of kilometers deep in a jovian counterpart, unfit for entry or exit.)

Essentially, (suffice it to say) it is economically favorable in my universe for spacecraft to be timely and make re-entries at high frequencies. My question is: what re-entry technologies would likely be employed by my civilization with these rapid-repeat re-entries in mind? In essence, I'm asking for an extrapolation of a most efficient (durable) spacecraft design for this task.

Constraints: wormhole entry windows are on average open for 5-6 hours per 24 hours (so, a re-entry method shouldn't take much longer than this); it is also economically favorable for spacecraft to be "lightweight" (say, less than 800 tons, the majority of which isn't shielding, etc.¹). A spacecraft should need little repairs; for example, if a solution is some kind of ablative shielding, it should hypothetically withstand longer than its competitors.

Conditions: assume the average spacecraft cycles through re-entry, orbital boost-back, re-entry, etc. (occasionally stopping in a parking orbit for fuel, less so for repairs), and assume for simplicity that the atmospheric conditions around each wormhole are Earthlike at sea level. (It'll be simpler to work with what we presently know regarding re-entry vehicles on Earth, however, I may in the future ask a similar question for re-entries in other atmospheres.)

^{1 While a spacecraft comprised mostly of heat shielding would almost certainly require little in the way of repair, companies are in the business of transporting cargo and people, not heat shielding.}

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Clarification: it is human civilization using these wormholes. I am asking about how we may develop spacecraft to handle these stresses.

For additional information about this specific worldbuilding, see this question.

Answer 1

Firstly, suborbital flight will probably give you what you need, with a vastly reduced delta-V requirement and much easier re-entry heat loads. Boost-glide trajectories allow plenty of flexibility in range, direction of travel, choice of landing site, altitude and so on, and with the addition of actively powered flight they become even more versatile. If you don't actually need orbital speeds, bouncing off the atmosphere and reaching LEO altitudes is quite acheivable with relatively modest delta-V. Also, hypersonic boost-glide vehicles have a bunch of aerodynamic requirements that tends to make them look pretty bad-ass, by way of a bonus.

(Boeing X-51 Waverider)

When you're dealing with lower speed re-entries (say, 4kms instead of 8) there are a whole range of other options available to you, including "feathered" re-entry, the technique used by SpaceShipOne and Two and other older projects:

(Boeing X-20 Dyna-Soar)

WIth this sort of approach you need less (or no) ablative heatshielding, which means that a combination of a highly refractory hull and active cooling by forcing a coolant out through the shield into the air around you (water is always a good choice, though oxygen radicals released by high temperature disassociation are undesirable) may give you what you need. The coolant tanks will need to be topped up, but throwing water into a tank upon landing is not a very difficult task. The refractory tiles will still need to be replaced eventually, but not necessarily after every single re-entry.

If you had a common airframe design, a sort of thermal condom could be bulk fabricated and a fresh one slapped on in place of the old one upon landing, which could be a relatively quick operation compared to painstakingly re-assembling a large tiled heatshield. This might even work with orbital re-entry, especially if combined with active cooling to reduce the heatshield demands.

Other possibilities exist, like having multiple inflatable re-entry shields like a stack of pancakes... inflate the outermost one, decellerate and jettison it, then next time inflate the next one down on the stack. Engineering a powered aircraft using that sort of thing will be tricky.

It is remotely possible that some extra-fancy plasma shield could be made to work for re-entry from orbital speeds, where the plasma created by the heat of re-entry is manipulated by onboard superconducting magnets to manoever and reduce heat loads. You will need some powerful, compact and lightweight power source for this sort of thing (probably nuclear) and better superconductor technology than we have to day. If you have enough power, you might be able to generate a plasma buffer in front of a slower vehicle.

Answer 2

Nuclear ramjets.

https://en.wikipedia.org/wiki/Project_Pluto

The principle behind the nuclear ramjet was relatively simple: motion of the vehicle pushed air in through the front of the vehicle (ram effect), a nuclear reactor heated the air, and then the hot air expanded at high speed out through a nozzle at the back, providing thrust.

This principle is really simple, and should work in any atmosphere. Take it in the front, heat it hot, throw it out the back and go.

For getting into orbit: same principle. Achieve supra-orbital speed while in the upper atmosphere and then coast upwards, shedding speed. It is like achieving orbit using a rocket: most of the acceleration happens in the atmosphere.

For reentry: turn your rocket nozzle 180 degrees. Take it in the front, heat it hot, throw it out the front and slow. Rather than shed kinetic energy as friction use your engine to slow you down just like a plane after landing uses its engines to slow itself down.

The nuclear ramjets are small and really fast. The ramjet has no moving parts except for the rotating rocket nozzle. The heat element is ceramic and so impervious to damage from exotic atmospheres.

The nuclear ramjets are for going in atmospheres wormhole to wormhole. They can traverse some interplanetary distances by accruing great momentum while in the atmosphere and slinging out. A more predictable method is a different ferryboat like ship which picks up one or more ramjets and ferries them to the next planet using rockets.

Answer 3

There are a couple things that aren't fitting together great (specifically: why go to orbit at all?), and I think we can cut through some of that by thinking about the problem of interplanetary atmospheres:

If we open an in-atmosphere wormhole between Earth and Venus, won't that be catastrophic for everyone living on Earth?

I think there's a tidy solution to this:
Wormholes "require" negative gravitational energy.

Think about that within the "rubber sheet" metaphor of gravity: While all the planets res in their gravity wells, each end of the wormhole is a gravity pillar. If the wormhole is low in Earth's atmosphere, you could fly a plane above it, but even approaching from above you would still be flying "up" in gravitational terms: If you're not running your engines, you'll decelerate and probably fall back out of the wormhole. The air will get thinner as you "climb" until your in the same near-vacuum as outer space (or LEO).

That means that we don't need to worry about the planets' atmospheres mixing. It also gives us some really cool options for flight trajectories.

Let's ignore the atmospheres for a moment.

Getting into orbit around a planet is hard. We're not going to do that! Instead of going sideways we'll just go up on a tight parabolic trajectory about as high out of planet A's gravity well as the wormhole's gravity pillar is "tall". Then we fall back toward the wormhole with just enough energy to jump up it's gravity tower (and fall back the far side of the wormhole).

(Why not just approach the wormhole from a standing start? Because then we'd have to accelerate really hard all at once. I'm not actually sure how the efficiency-math works out, but if the gravity pillar is 1000Gs steep, it'll be gentler on our passengers to climb it in "freefall".)

As we pass through the wormhole, we're still maneuvering in (a) space: we want to set up our exit trajectory so that we fall down the far side of the gravity pillar and get shot up out of planet B's gravity well on just the right trajectory to fall down into the next wormhole in our journey. Going through the second wormhole doesn't take any more fuel because we haven't lost any of the energy we used to get through the first one. Of course we'll need to spend fuel to decelerate eventually, but going through a chain of twenty wormholes takes only slightly more energy than going through a single wormhole.

Does this still work with an atmosphere?

If the atmosphere is dense the way you describe then that elegant efficiency is gone: projectiles falling toward earth loose most of their speed before they're 30km from sea-level. But we can still see that going through a wormhole is just like "flying to space", and the craft you'd use wouldn't be much more advanced that existing rocket-powered planes. In order for the wormhole to be navigable by a human this way its gravity pillar probably can't be much steeper than 5Gs, which suggests a "distance" from end-to-end within the wormhole of about 40km. If you're aiming for really hard science then you'll need to do some hard math, but so far it sounds like it will work.

Answer 4

Main difficulty aliens should encounter with this wormhole jumping approach is this: - different planets has different atmospheres. Atmospheres can be quite dense and with oxygen line Earth one, so internal combustion engines should work, or atmospheres can be oxygen-less and thin, like Mars one, or oxygen-less and very dense, like Venus one. So, aliens spaceship propulsion system should work in all possible atmospheres. Also it should be working in vacuum too, so spaceship can leave atmosphere and fly in space. With nearly future tech level, it can be achieved with different propulsion systems.

First propulsion system to work like this are electric engine powered propellers, with changing geometry blades to work properly with any atmospheric pressure. They should work in lower, denser part of atmospheres. Also this propellers should work, even if device is submerged in water or other fluid

Second system (to fly high, in thin atmosphere with any contents) - is Nuclear thermal rocket engine. It can work in atmosphere of any contents.

Third system - its superconductors to make vehicle levitate using https://en.wikipedia.org/wiki/Meissner_effect, if planet has strong magnetic field. Probably, presence of wormholes correlates with magnetic field of planet?

Forth system is used to fly between planets in vacuum - it can be either Nuclear Rocket Engine or Ion Thruster

So, i assume aliens has quite complicated vehicles, because they should have a lof of different propulsion systems that should work during travel. As result, more than 50% of vehicles are consumed by different propulsion systems.

Other problem - is to make vehicle aerodynamics work ok in every environment - Mars, Venus, Earth, Earch ocean... Probably, vehicle can change it geometry? Probably, vehicle looks like big bird, squid or something organic...

Also its worth to mention, that planets can have dramatic environments on them - consider few miles deep on Earth ocean, acidic and hot surface of Venus, depth of Jupiter with terrible pressure... Vehicle had to be very tough.