Revisiting the Enzmann Starship Design: A Sixty-Year Perspective

[Strato Incendus]

The dumbbell design of my fictional generation ship Exodus, as I’ve recently discovered, bears some similarity to the Enzmann starships, proposed by Dr. Robert Enzmann in 1964. I’ve even gone so far as to label the ship part of the “Enzmann class” within my setting.

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

The main difference is that the Exodus, at a length of 3 km, with two spherical tanks 1 km in diameter, and with six ring habitats 500 m in diameter, is a lot larger than the Enzmann ship: That one would be 600 m long, with only one sphere at the front, with no rings, but a cylinder behind the front sphere, similar in diameter as the front sphere.

The cylinder would contain three habitat sections, equally long and wide — at just 91 m, the “corridors” would be wider than on board my ship (where the rings are just 64 m wide); however, at a diameter of just 91 m, these habitats would have to rotate much faster than the humanly tolerable 2 rotations per minute, in order to create 1 g of artificial gravity.

The second issue with the Enzmann design to me seems to be the lack of a protective sphere on the rear: When the ship turns around to brake, there would be no protection against radiation from the front. With the Enzmann ship being intended as an interstellar ark, and capable of travelling at relativistic speeds, such protection would be absolutely necessary.


Hence, I’m not thinking about using Enzmann-class ships in my setting as interstellar arks. However, it would make sense to me to use them within the solar system. This would also allow them to serve as a stepping stone between current-level space technology and the 3-km colossus that is my generation ship: There has to be some interim technology in order to make the development of the generation ship believable in the setting.

What I find interesting, and potentially relevant for the construction of the generation ship, too, is the construction of the spherical deuterium tank in space: By inflating a balloon and coating it with metal. On board my generation ship, there are several sub-spheres inside the larger spherical fore and aft tanks (sphere-packing in a sphere). I wonder if the same principle of inflating them would work here, too. The important part is that the sphere remains stable even once the tanks start getting emptier.
Crew members also need to be able to do repair work inside the tanks. As far as I understand the Enzmann design, the deuterium inside the spheres would be frozen, so I wonder how anyone would be able to do any maintenance work on the tanks if, say, a vent or pipe is blocked, preventing the fuel from where it has to go.

Thoughts on the Enzmann design in general, from a current-day perspective, sixty years after it was first invented?

 

[jedishrfu]

This sounds a lot like the Rama series of Arthur C Clarke. In the first book, Rendezvous with Rama, a mysterious cylindrical shaped artifact passes through the solar system and Earth explorers visit to discover what it is and why it’s here.

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

You might get some ideas from the story concerning the engineering and physics involved in the ships design. Clarke was a master of the hard science fiction style.

 

[Strato Incendus]

Yes, I've heard of Rendezvous with Rama before. However, the measurements cited for the Rama really make me think of it more like an O'Neill cylinder.

The property my ship Exodus has in common with the Enzmann design is not the cylinder - because the cylinder (called "central pipe") is much smaller on my ship, and then the ring habitats circle around it, in order to save mass, compared to creating an entire cylinder with a 500-m diameter.

Rather, what my ship design (originally suggested by @DaveC426913 ;) ) has in common with the Enzmann ships is the spherical tank at the front, which is large enough to cover all the habitats behind it, thereby protecting them from radiation.

While people on other forums (e.g., writing-specific forums) have indeed suggested to me to make my ship look more like an O'Neill cylinder again (which would make it pretty boring-looking from the outside, compared to the dumbbell), there's an inherent plot hole in that design. Or rather, a "setting hole". At least when combined with any kind of colony-ship plot: An O'Neill cylinder is already a self-sufficient habitat. There is no need to land on a new planet. The O'Neill cylinder is the new planet.

Sure, a generation ship has to be self-sufficient, too - but it's not necessarily built to last as long. It's also going to be smaller, because in contrast to the cylinder, it needs to be propelled to interstellar travelling speeds. The smaller size in turn implies less space for the crew, and as a result to that, probably fewer conveniences on board. Comfortable enough for them not to go insane over the duration of the journey - but not so comfortable that they might never want to leave the ship when the day of the landing arrives.

The only reason I could see to turn even an O'Neill cylinder into an interstellar spaceship would be if the carrying capacity of the O'Neill cylinder would still be too small, compared to the intended population of the destination planet. For example, if the eventual plan is to evacuate Earth, and/or to create a much larger population elsewhere, an O'Neill cylinder will not suffice.
Even though there seems to be no limit to gigantism when it comes to authors postulating the sizes of their interstellar arks, on the larger end, the population sizes are usually between 10,000 and 50,000. I can think of a few sovereign nations on Earth with such a population - but I highly doubt any of them are self-sufficient. Quite the opposite: Most of them probably have to import next to everything they need.



Going back to my setting, as I've stated, I thought of using actual Enzmann-like ships (600 m in length, only one spherical tank at the front, habitat cylinder etc.) for travel within the solar system. The reduced gravity of the 91 m cylinders would be less of a problem if the people don't spend as much time on those ships as the crew of a generation ship, who spend their entire lives on board.

In fact, the 91 m diameter of the Enzmann cylinder is pretty close to the diameter of the Exodus's central pipe (100 m). The hubs of the ring habitats, at one fifth of the diameter of the ring habitats themselves, consequently create one fifth of the gravity, which is 0.21 g. That's comparable to gravity on the moon. So for astronauts, it should be "fine for the moment, but not long-term".

Speaking of astronauts who stay in space long-term, the obvious first use for rotating ring habitats would be space stations. Here, building a ring large enough to create 1 g at 2 RPM or less (requiring a diameter of at least 450 m) would be desirable.

Integrating the Enzmann ships into my setting would therefore greatly increase the believability of how this society is capable of building a generation ship like mine: Even though it's a first attempt, the individual components are already in place:

• You just take the trunk of an Enzmann ship (91 m in diameter, 600 m long)
• and wrap several ring habitats around it (450 m in diameter)
• for the spherical tanks at the front (and now also back), you use the same inflation-in-space method
• then all you need to do is upscale all of the components a little (central pipe: 100 m in diameter, 1 km long; ring habitats: 500 m in diameter; spherical tanks: 1 km in diameter) to arrive at the Exodus's measurements

The fun part is that I get to detail all of this explicitly in my prequel, when the ship's architects first sit down together. It gets a little info-dumpy, but I try to make it more entertaining, by having the three architects' egos fight with each other throughout the conversation. After all, if you ask three experts, you get at least four opinions.

To get back to the crucial questions of implementing this in my story:

What I find interesting, and potentially relevant for the construction of the generation ship, too, is the construction of the spherical deuterium tank in space: By inflating a balloon and coating it with metal. On board my generation ship, there are several sub-spheres inside the larger spherical fore and aft tanks (sphere-packing in a sphere). I wonder if the same principle of inflating them would work here, too. The important part is that the sphere remains stable even once the tanks start getting emptier.
Crew members also need to be able to do repair work inside the tanks. As far as I understand the Enzmann design, the deuterium inside the spheres would be frozen, so I wonder how anyone would be able to do any maintenance work on the tanks if, say, a vent or pipe is blocked, preventing the fuel from where it has to go.

I guess the practical application to the "sphere packing in a sphere" problem is now: Can you inflate a balloon inside a balloon? I'm pretty sure I remember experiments from some kids' science shows which demonstrated exactly that. But of course, that was on Earth - not in space. Plus, in my case, inflating a single balloon inside one larger balloon would not be enough: We're talking about a bunch of balloons. Currently, I've settled for 21 smaller spheres inside each of the two large 1-km spheres.

 

[DaveC426913]

Currently, I've settled for 21 smaller spheres inside each of the two large 1-km spheres.

What kind of sphere packing arrangment are you using? Have you checked to see if it provides sufficient axial shielding? It seems to me 21 spheres inside a large sphere might leave a lot of zones where there is little or possibly zero shielding, but I can't tell without a model.

(21? Funny number... An 20-side icosahedral arrangment with a centre sphere?)

 

[Strato Incendus]

I went with 21 in order to have three sub-spheres behind each other on the axis of the central pipe (marked here in pink):

The reason being that the central sphere contains the nuclear-fusion reactor, and the on-axis sphere behind it contains the engines. The third on-axis sphere now not only protects the central pipe from space radiation, but also from the radiation of the reactor. For this reason, the third on-axis sphere is the last one from which the fuel will be drained over the course of the mission.

The double-protection by this sphere is especially important during braking - when the ship's engines are facing forward.

Of course, if I'm missing something here (or misunderstanding something), please let me know.

 

[DaveC426913]

Well, that simulator is what I went searching for. I neded up faking it in Blender.

 

[DaveC426913]

Of course, if I'm missing something here (or misunderstanding something), please let me know.

I think you will find a very wide difference in shielding at different locations. Sure, technically, there are no paths through the spheres that don't intersect but I'll bet there are a lot of places where it's extremely thin.

If there were a way to render the spheres as semi-transparent, you'd be able to depict an area map showing where there are almost clear paths through.

But I may be nit-picking / over-engineering / bifurcating bunnies.

UPDATE:

This is what I mean:

I replaced the solid discs with graded colours approximately like the thickness being viewed through them. It's very rough, but it makes the point that there are areas where there is very little shielding.

 

[DaveC426913]

It would surely be a wiser configuration (though not prettier for your story) to put them in a cylinder. That would make it easier to distribute the shielding more evenly on the cross-section. The constraint of a sphere along the axial direction is not helping matters.

 

[Vanadium 50]

How exactly are you planning on keeping your deuterium cold?

How exactly do you plan on getting that much material in orbit? Scaling up from a Nimitz carroer, thus will weigh 100 million tons. That's 3 million space shuttle launches: one launch every minute for six years.

 

[jedishrfu]

Wow @Vanadium 50 that's cool. I hadn't thought of the logistical nightmare of transporting raw material up to an assembly area. The spacestation must be considerably smaller than the OP's ship.

I wonder if the space elevator concept could mitigate the logistics. It almost seems like a potential story in itself.

I recall the ring world concept that had incredible dimensions and yet needed an earth sized planet for raw materials. Sadly, the books on ring world never addressed how the engineers could build such a structure.

 

[Vanadium 50]

Logistics will get you every time.

 

[DaveC426913]

I recall the ring world concept that had incredible dimensions and yet needed an earth sized planet for raw materials. Sadly, the books on ring world never addressed how the engineers could build such a ststructure.

Jupiter-sized planet.
And their godlike intelligence was superceded only by their drive to build a safe home for their offspring.

 

[Strato Incendus]

Thanks for your ideas, as always!

How exactly are you planning on keeping your deuterium cold?

Since the tanks must also act as a radiation shield, the first question is: Does the deuterium have to be stored as deuterium, or can it be stored as heavy water? In the latter case, it would freeze at 3.8 °C.

How exactly do you plan on getting that much material in orbit? Scaling up from a Nimitz carroer, thus will weigh 100 million tons. That's 3 million space shuttle launches: one launch every minute for six years.

Thanks for doing the math on the launches! This however assumes all the material will come from Earth. Weren't there plans to produce deuterium on the moon, turning the moon into the first "tank stop" for any ship once in orbit?

For metals, what about mining in the asteroid belt?

I wonder if the space elevator concept could mitigate the logistics. It almost seems like a potential story in itself.

Good thing I've already postulated one in my setting. Though related to the previous question, the primary working space elevator is on Ceres, due to its much lower gravity. The one for Earth is still in construction.

 

[DaveC426913]

the primary working space elevator is on Ceres, due to its much lower gravity.

Why would you even need a magic beanpole on Ceres? It gravity is 1/40th of a g. A person could jump 50 feet high!

 

[Strato Incendus]

As a proof of concept. Start out with what works, then upscale the elevator concept to the moon, eventually Mars, and so on.

 

[Vanadium 50]

Does the deuterium have to be stored as deuterium, or can it be stored as heavy water

It's your story - you can do whatever you want. Just remember that heavy water is 20% deuterium, so you need 5x as much fuel. And the fuel to move that extra oxygen. And the fuel to move the extra fuel. And so on..

what about mining in the asteroid belt?

Metal ore is not a starship.

Lets start with something simple. A pencil. How does your crew get pencils? A pencil is made of wood, graphite, a brass (copper/zinc) ferule, an eraser (synthetic rubber or natural, your choice) and paint, ideally yellow. That's it - what could be simpler? How do you make pencils out of asteroids?`

Repeat for all items your crew needs.

And if you have all that industry in space, why are you screwing around with interstellar travel?

 

[Strato Incendus]

Pencils are a good example: A while ago, we discussed whether wood would be a precious resource on board an interstellar spaceship.

Trees in general could be grown in greenhouses on various rocky planets and moons in the solar system. Regarding metals, sure, the ore is not a starship, but at least we’re talking about metals in general. The ship hull itself would most likely be primarily out of aluminium.

And if you have all that industry in space, why are you screwing around with interstellar travel?

That’s the crucial question with all interstellar slowboats: On the one hand, the need to be self-sufficient enough to survive the journey; on the other hand, it must not be eternally self-sufficient, lest the crew would never want to leave the ship and land on the surface of the destination planet.

This is why I’ve always pushed back against anyone suggesting to turn my ship into something resembling an O’Neill cylinder: An O’Neill cylinder is a self-sufficient permanent habitat.

In my case, my answer is that the ship wasn’t built to last several centuries: It’s self-sufficient, but the structure won’t withstand natural wear and tear for millennia. Of course, the question is what kind of wear and tear the ship would be exposed to in the vacuum of space, if it just remained in orbit of the destination planet forever.

 

[Vanadium 50]

Why aluminum? It doesn't have to be light if you aren't going to lift it, and it is less common in asteroids than the earth's crust.

Pencils are just the beginning, of course. You need either a lot of industry already in space, or you need to lift what you need out of the earth's gravity well. In the former case, you don't really need a starship and in the latter, logistics kills you.

Your story needs some reason for people to move all the way to a star. For example, Litttle Green Men moving in to the solar system.

 

[Strato Incendus]

Why aluminum? It doesn't have to be light if you aren't going to lift it, and it is less common in asteroids than the earth's crust.

But lower mass would still be desired, given that the ship must be accelerated to interstellar speeds?

Your story needs some reason for people to move all the way to a star. For example, Litttle Green Men moving in to the solar system.

As I‘ve described in previous threads, the reason is the definite confirmation of Earth being in the line of fire of an impending gamma-ray burst from WR 104, which would affect the entire solar system. ;)

 

[Vanadium 50]

Earth being in the line of fire of an impending gamma-ray burst from WR 104,

And you think moving off-axis by a fraction of a degree is going to help?

 

[Strato Incendus]

This is why I posited the solar system to be on the outer edge of the gamma-ray burst's cone.

Of course, the other option would be to pick the one planet with a slightly higher Earth-Similarity Index than Teegarden b as a destination (KOI-4878.01). At 1075 light years from Earth, it should be more than far enough away. However, there's no way of telling this particular story with this destination planet without postulating an Alcubierre warp drive. Because even if the ship were travelling at light speed, if the journey took over 1,000 years (which of course is an option for generation-ship stories in principle), it would completely change the dynamics on board.

 

[Vanadium 50]

This is why I posited the solar system to be on the outer edge of the gamma-ray burst's cone.

So there is a sharp cutoff of a small fraction of a degree? OK...it's your story, but it will provoke a lot of questions of the form "why don't they just....".

 

[Strato Incendus]

So there is a sharp cutoff of a small fraction of a degree? OK...it's your story, but it will provoke a lot of questions of the form "why don't they just....".

Fair enough, but it would be helpful for me if you completed that sentence. ;)

“Why don’t they just…” …do what? Travel to a planet further away? If so, how? That’s a lot harder than going to the destination they’ve chosen, so “why don’t they just…?” seems like the wrong framing; rather, it would be “why don’t they do even more?”

 

[Vanadium 50]

A GRB is 2 seconds long. You could hide behind the sun. You could hide behind Jupiter. Even if you didn't know exactly when, your space asteroid-miners and pencil-wood-growers can orbit these bodies and have a 50-50 chance. Much better than sending 0.1% to the stars and consigning the rest to death.

What do you think the reaction will be to "Work harder, pencil-wood farmer. We have important people to save. People not you."

You want to tell a certain story. That's fair. But you also want that story to make scientific and logical sense. You might not be able to have both.

 

[Strato Incendus]

Much better than sending 0.1% to the stars and consigning the rest to death.

The rest aren’t consigned to death; the majority of people on Earth are facing the prospect of having to live underground. The danger does not just arise from the 2 seconds of the GRB itself, but from what it does to the atmosphere and the surface of Earth, i.e. the long-term effects of the aftermath.

The remaining uncertainty for all the already-existing underground places on Earth is: How deep will be deep enough to survive the burst, and which underground areas may be too shallow?
The Hoxha bunkers in Albania definitely won’t save you; will the Moscow Metro suffice? Finland has enough bunkers for all of its citizens, including underground hockey stadiums and swimming pools. Those bunkers were made for war times, though — not to survive a GRB. The lowest known place is the Veryovkina cave in Georgia / Abkhazia, but it’s questionable to me whether anyone could build settlements there. In my version of the future, those who really want to be on the safe side pay high amounts for a residence so far underground.

And then, there’s those who don’t want to take the gamble of “how deep will we have to go?” Some of these consider themselves (and their future children) safer on board a spaceship that leaves the solar system altogether.

In the first prequel, the debate about the threat from WR 104 is conducted in an analogous manner to how we talk about climate change today:
Some believe the GRB will be the end of the world and think we can’t possibly build enough underground settlements; others think the threat is overstated, or that the research regarding the angle of the star towards Earth can’t be trusted at all; some agree the star is dangerous, but question the methods commonly proposed to deal with this threat. Meaning, some question that a generation ship is a useful response to this threat — much like “escaping to Mars” won’t help people on Earth deal with climate change. Others prefer the generation ship, because they find the idea of living underground to be removed from human nature (to adapt a popular current-day meme, “I won’t live underground and eat rainworms”).