What would a laser-powered interstellar cargo ship look like?

Question

Blurb

What would a ship with a 100 thousand tonne budged that needs to carry cryo-frozen cargo through 20 lightyears of interstellar space at 0.35C coasting velocity, using laser sails at both ends of the voyage, and putting the safety of the cargo above all else. (See "The Problem... Mk-2?" and "Edits")

The Setting

Side-note, I absolutely love this format. Let's say we use that big tractor ship I ought to make another post to check the math on, which can reach 0.7C. Let's call her the ISV Not So Venture Star. It's like it, but bigger. A lot bigger. And it carries a small team of colonists and the required materials for colonizing a star system.

The 3D printers at their disposal cant make advanced microprocessors, sensors and other advanced machines, but they can print out the bulk of drones, vehicles and such, bringing the important bits from earth. They will stay in cryostasis for most of the voyage, a form of hibernation that slows metabolism to almost stopping, letting them sleep for decades.

With the equipment they have, they build a dyson swarm- Well no. That isnt the right word. Dyson swarm evokes the idea of an advanced civilisation. This isnt even close. It's just a bunch of orbital solar panels and mirrors in a nice, even ring around the sun-like star.

But theres a problem, they have a machine powered by suspension of disbelief, called a phase-gate. It works by some magical phenomena we have yet to discover and creates a region of spacetime that are linked. Go into one kilometer-wide door, come out of the other.

For some reason, it runs on a flow of virtual particles which are directed by mass-energy, which in turn is held in suspension by some kind of magnet-like system, never discussed in the book. The point is that it needs a lot of cooling, and energy. Like a lot of energy. And it's extremely advanced, thus needing to be imported. But it's huge.

The Problem

How do we move this honking huge machine from earth, while being strangled to a wall by the tyranny of the rocket equation? We alleviated this with massive petawatt-power lasers to accelerate our ship, but we need to decelerate on our own when we arrive, which is a hairs-width from impossible, even when we use the most absurdly, stupidly dangerous fuel we possibly can. Anti-matter.

The Solution

Simple! We don't move them at the same time! Who said we need to ship the colonists and all the frozen cryo-samples of earth-life on the same ship? We can ship it on a bigger, much slower cargo ship, lets say at half-speed. Sure, it will be hammered by radiation and we can't ship people because they couldn't survive cryostasis and cosmic rays for that long, but a bunch of frozen seeds and eggs definitely won't care as much.

But then we'ed still need engines to slow down... right? WRONG! If we got the ship up to moving at 0.35C with a massive laser-sail array powered by solar collectors around the sun, then we can do the same thing around the colony star, which we can call Ilus.

The new flight plan would be as follows. Launch the slow ship, Aurea, using the laser batteries. Then collect antimatter for the next 2 decades. Then launch the fast ship, Theia, on route to Ilus. After 3 decades transit, the Theia arrives at Ilus and starts a (not really) dyson swarm. After a decade, the Aurea opens its sail and prepares to end its 6 decade journey.

The small crew awaken in Trenton city and launch center, just to discover that they have 2 weeks left to fix a cascade of failures that have gotten progressively worse over their 9 years in cryosleep, waiting for the autonomous systems to finish building the dyson swarm, and with the one controller that was supposed to stand guard for their 6 month shift having vanished without a trace.

And they have to repair and engage the laser, lest the Aurea and their ticket back to earth dash past at over 100,000 kilometers per second. Fantastic drama setting, like The Martian but theres an entire crew and a planet to work with, and at stake.

The Problem... Mk-2?

This is fantastic and all, but what would the Aurea even look like? Since it would be moving slower, it would need a much less extreme shield, possibly a hybrid ablative-whipple type, directly mounted to the bow of the ship.

It would need cryogenic systems, which would make heat, and thus need large radiators. It would also need fusion reactors to power it, and thus fuel for it. It would need a small main hall-effect engine to keep at cruise speed, and some maneuvering engines and an onboard AI, and thus computers.

The phase-gate ring would be disassembled into 6 sections and folded into the core of the ship, with a rigid truss from the shield to the reactor at the rear, fuel tanks behind the reactor and cargo modules ahead of the phase-gate. Along the entire length would be radiator panels which could double as Whipple shields around the phase-gate.

I imagine something a little like the Aion or the Kronos.

What would be the best configuration for such a ship? (and is there any other considerations I need to make?)

Credits

This isn't strictly part of the question, but I want to say it. Thank you Sphennings, and you're getting me to drop the need for a scientific explanation, which would wreck any illusion of suspension-of-disbelief. To Starfish and Nosajimiki for their over-the-top math, and to BMF and many, many others for working out the technical details.

Thank you everyone! Im writing and working on my book every day, and this is special because I am only just starting the book, and I know what the book is about, but I'm not locked in yet, so I can change and add the things that you guys worked out. >(^-^)<

Edits and Tech Specs

For future reference, the distance it will cross is around 20 light-years, give or take a few. The phase-gate unit is a 0.7km-wide ring, split into 6 even sections and totaling in at 3k tonnes. It also has a power ballast that slowly accumulates energy and releases it quickly, weighing around 2k tonnes in shipping containers of machinery.

The weight budget of the starship is around 70k to 100k tonnes. The safety of the gate is the most important part of the mission, and the cryo-cargo is just a side-note.

The ship needs to have some mechanism to expose a reflector shield at the front and rear of the ship during the laser-sail driven phases, and hide them away to to minimize risk of being damaged. The ship should be resilient as possible and carry as much and the most efficient shielding it can.

The phase-gate will need a few months to upwards of a year be set up and the power system will also need time to charge and fire, after which more materials can be shipped in from earth, in the case that the cryo-cargo is damaged or lost. A loss nonetheless but not a big one, compared to loosing the phase-gate.

The ship should be designed to be as safe and redundant as possible. Thus, maneuvering engines and a backup main engine would be best, as the ship might need to do slingshots around planets and the star to loose speed.

There is almost 7 decades of work and preparation all hinging on this one ship, so it should be built like it.

Answer 1

TL;DR: a couple of flat plate-like shields cut to match the ship's cross-sectional silhouette at the front (a thin shiny one foremost, a thicker grey or black one some distance behind it), engines behind that, and then the cargo, fuel, power plants, reaction mass, control systems etc. a looooong way behind that strung out on a bunch of cables. Not rigid. Doesn't do a flipover.

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Starship design is going to be driven by two main things:

• minimizing mass
• not getting wasted by debris and radiation en route
• not getting incinerated by your own propulsion system

Your spacecraft has no significant engines (because boost and brake are done by lightsail) so you don't need to worry about huge tanks of fuel and reaction mass and nuclear rocket shielding and heat sinks and so on, so that's a good start.

Lets consider your laundry list of stuff though:

It would need cryogenic systems, which would make heat, and thus need large radiators.

Cryogenic systems for what? You don't have any meatbags to keep cool, and you don't have huge tanks of hydrogen (or worse, antihydrogen) to keep frosty.

It would also need fusion reactors to power it, and thus fuel for it.

It is obviously fairly hard to work out how much power you need, and hence how much fuel you need for your reactors, but as you're not running life support, artificial gravity, enormous rockets or mag brakes it suggests you don't actually need very much.

It would need a small main hall-effect engine to keep at cruise speed, and some maneuvering engines

The drag will be negligible in interstellar space. You don't need to maintain cruising speed because there's nothing to reduce it, though you almost certainly want to be able to do a bit of mid-source correction. You've got decades to line yourself up though, so the engines can be teeny tiny. You do still need to run engines during the flight though, because:

a rigid truss from the shield to the reactor at the rear, fuel tanks behind the reactor and cargo modules ahead of the phase-gate.

I don't think the truss needs to be rigid. A flexible spacecraft has much less strict engineering constraints regarding stiffness and compressive loading (of which there will be none, unless something went very wrong), and probably weighs less into the bargain. The Project Valkyrie design was like this, and Charles Pellegrino (one of the people who worked on that design) was also a technical advisor to Avatar and contributed some of the ideas behind the Venture Star.

Some very small engines at the front of the vessel can run at very low thrusts (even a tenth of a milligee might be excessive) during the flight to maintain tension on the rigging whilst the laser sail is not in action. Nuclear electric is fine for these... Hall effect, etc. Their exhaust is benign enough that you don't need lots of shielding, so long as the engines are angled outwards a teeny bit so as not to hose down the payload.

Along the entire length would be radiator panels which could double as Whipple shields around the phase-gate.

It isn't clear that you need that much heat rejection, because your spacecraft isn't really doing a whole lot... running some small ion drives of some kind, and twiddling its virtual thumbs.

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So much for nit-picking. Lets move on to my next two constraints.

At .35c, little things like hydrogen and helium nuclei are hitting you with a kinetic energy of 19.2MeV per nucleon, and big things like specs of dust are hitting you with the equivalent of 1.45 kilotonnes of TNT per gram.

The local interstellar medium has a density of ~0.3 atoms per cubic centimeter, so if your whole 20ly journey is at a similar density and all the atoms are neutral hydrogen (they mostly will be), you'll sweep about 0.1g of matter per meter squared of the cross-sectional area of your spacecraft, which adds up to ~58 MJ of absorbed energy per meter squared (~14kg TNT equivalent)

The radiation hazard from individual small nucleons, at least, isn't so bad. The particle energies are high, but solar particle events can produce even higher energies and if you can build a laser array around the sun then you've obviously developed some ability to protect yourself from that sort of thing. At .35c, the Lorentz factor is only ~1.06 so relativistic aberration is minimal (~20 degrees maximum for things perpendicular to you). This means that you still need to care about radiation hitting you from the sides, eg. galactic cosmic rays. You can't easily shield against those but again: you're talking about a mature spacefaring society, so they'll have good radiation hardened electronics and magnetic shielding.

The dust hazard is obviously greater. Dust Grain Damage to Interstellar Vehicles and Lightsails (no free online source that I'm aware of, legitimate or otherwise) suggests that the first line of defense is actually a very thin foil shield, perhaps less than a micrometer thick, made of something like gold or beryllium. Like the outer layer of a Whipple shield its job isn't to stop incoming crud, but it liberates enough energy to at least partially ionize whatever hits it. Some distance behind the foil (and the further the better... at least 10m) you have a layer of something tough and refractory. Graphite is a simple option (and has some real-discussion and papers), but woven carbon nanotubes in a diamond matrix might be ideal. This probably needs to be a layer a few centimeters thick. It needs to be refractory so it doesn't melt too easily (splashes and vaporisation will wear down the shield over time) and it needs to be tough so impact shocks and gas buildup under the surface doesn't crack it.

At just .35c, you might not need additional layers of armor. A paper that is readily available and free is The interaction of relativistic spacecrafts with the interstellar medium which might be worth a read. The authors suggest it is exceedingly unlikely for dust grains to be bigger than 3μm, and the authors of Evaluation of the Hazard of Dust Impacts on Interstellar Spacecraft make similar assumptions, so the threat of the big bad kilotonne-nuke-dust-grain is probably minimal. You can scale the shielding depending on your paranoia and aesthetic requirements.

The Kronos ship design you linked has this big curved shields much wider than the ship out in front which look very nice (if you like the sort of jellyfish aesthetic), but the outsize shield is wasted mass and the curvature is not necessary for a starship (and maybe not even for the Kronos). The Aion (or Ourea) has a big round shield, but at least it is there to protect the spinning habitat, although it would make more sense to save mass and shield the habitat modules instead. In any case, you don't have habitats or spun gravity. Your shields might not even be round (see the diamond/squished hexagon shape of the Venture Star shields, for example), but cut precisely to protect the silhouette of the ship behind them with no superfluous mass at all.

The shields probably do have some rigid spars holding them in place and in shape. Behind them will be the propulsion bus, which holds your course correction and tensioning engines. Everything else dangles off that on a tensile spine... probably a bunch (at least 3, but probably more for redundancy) of woven nanotube cables if you can make them, spaced out a bit rather than as a single fat cord which can suffer from undesirable twisting and bending.

At either end of the ship you'll have mount points to which can be fastened the lightsail or a laser shield. Everyone talks about spin-flips where your ship rotates 180 before entering the brake phase of its flight, but that's a) tricky with a flexible ship and b) exposes you to a horrendous amout of risk in the form of high-energy particles (which will wreck your electronics, even if radiation hardened... read Radiation Hazard of Relativistic Interstellar Flight for more information) and dust which will threaten potentially serious damage if you're unlucky. Flipping is fine for little interplanetary craft, but not big starships. After boost, you fold up your lightsail (mounted at the bow) and your protective shield (probably made of the same stuff, mounted at the stern) and tuck them all away neatly behind the shielding. For your brake phase, you remount the sail at the stern, and the laser shield at the bow. The shield will probably get battered by the ISM, but it is less important now and the regular shielding should be able to soak up stray laser illumination without serious damage.

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Now that I've answered your question, more or less, lets consider this bit of your mission design:

But then we'd still need engines to slow down... right? WRONG! If we got the ship up to moving at 0.35C with a massive laser-sail array powered by solar collectors around the sun, then we can do the same thing around the colony star

I suspect the best way to park a relativistic starship is using magnetic brakes, which is using a large magnetic sail to initially brake against the interstellar medium (the sort of drag that killed the dream of the Bussard ramjet put to good use here) and then against the solar wind of your destination star. From The Magnetic Sail Final Report to the NASA Institute of Advanced Concepts,

[a magnetic sail] could decelerate a starship traveling at 95 % lightspeed to about 4 % lightspeed in ~320 days, 1% in ~ 620 days, and 0.56% in 800 days Deceleration would start at 5.47 g’s but would later fall to 0.0058 gees.

This is perhaps a little optimistic, but it'd be an excellent way to slow down and even manoever in the destination system without the need for any infrastructure there, or the need to carry fuel and reaction mass for rockets (avoiding the issues of all that pesky antimatter).

Obviously this spoils the drama of your setup somewhat, but no-one said that the mag brake of the Aurea had to work as planned...