# How can a space station prevent docked ships from irradiating each other or the station?

I've been puzzling over the logistics of orbital spaceports, and this has been the one sticking point I keep coming to: radiation control between docked, or even neighboring, ships. Mass limitations on ships without "torch" engines, which are engines without a high enough thrust and specific impulse to overcome the limits of the rocket equation, would prevent a designer from efficiently using anything more than a shadow shield that only shields the direction of the ship itself from radiation. Anything covering more of the reactor would require more propellant to gain the same total delta-v, which adds mass and reduces the thrust-weight ratio, requiring a larger engine to offset in a very delicate balance. That's all well and good until you need to dock; said shield would only protect a small fraction of the surrounding space, while pouring out radiation on any nearby ships.

I'd like to figure out a way to build an orbital spaceport that doesn't require everyone besides an incoming ship to live in a radiation-proof bunker, but I'm not seeing any good solutions short of "don't use nuclear power". The radiation concern in this case is the reactor of the ship, which would be providing the majority of the electrical power necessary to operate ship systems. Main engines wouldn't be used for docking proper, but short of battery power, the reactor would have to run to sustain ship systems such as life support while not directly docked, and my understanding of radiation tells me that even a shut-down reactor is radioactive to a degree.

While extending the shadow shield to cover the entire reactor is mostly possible (a direct cone behind the reactor doesn't work as well since the reactor has to feed into the engine somehow, and pointing the thinnest point for piping/etc straight back is safest), it also negatively impacts a ship's performance to a degree I don't find workable—lead and water are heavy, not to mention any other materials that could be involved.

My setting's tech level is fairly near-future; the main breakthrough relevant to this question is the development of working nuclear salt-water rockets, which allow for much faster interplanetary travel but are, as implied, very radioactive thanks to the reactor used to power them (and to power the rest of a ship). No energy shields or anything of the sort; if a solution is possible with current real-world technology or something achievable from ongoing research and development, that would be preferable. I'm trying to stick as close to that general idea as possible.

Please let me know if I have some fundamental misunderstanding!

(First time posting, so I apologize if this isn't structured well. I've tried to use as much real world terminology as possible; I'm still in the early stages of building my setting, so I don't have the clearest grasp on any of my own potential quirks yet.)

For a visual idea of the "shadow shield" I'm referring to, courtesy of Winchell Chung's "Atomic Rockets" site:

• What's a torch engine? What's overall tech level of your world? – Mołot Jul 14 '17 at 14:46
• OK, are these engines radiating when not in use? How hard? – Mołot Jul 14 '17 at 15:04
• Why exactly does your shield that protects the ship itself not work? I do not understand that part. Also why do you dock your ships with your main engines turned on? I think you expect people to have a lot of knowledge about your world already. If you are working on a well-known project, it might be nice to state its name. In any case, an explanation of most of what you just said would be nice to have. – Raditz_35 Jul 14 '17 at 15:04
• Also, please edit your question with all clarifications you provide. Will save time of future readers. Avoid "Edit1: ___" style, make your post one coherent piece, just as if you wrote everything you meant to from the beginning. – Mołot Jul 14 '17 at 15:06
• @SkyeAuroline "Torch" is a well known description of any rocket engine that produces a flame out the back. Because it looks like a torch. The term was coined by Robert Heinlein in 1953 for his short story "Sky Lift." – Draco18s no longer trusts SE Jul 14 '17 at 15:52

I'd go with non-radioactive tugs. If main engine is only radiating when in use, then require ships to turn it off on parking orbit. If it is radioactive all the time, make it detachable, and require leaving it in parking orbit. Then tugs, using regular chemical drives would tug that ship to the station and dock it.

Of course this would make docking and undocking expensive, but I think that's a nice plot point. Detachable engines may also be stolen. On the other hand, in near future there will be very little market for stolen parts like that... There are plot possibilities in this. And it does not require any breakthrough except the one you described.

I recommend changing your NSWR reactor from a traditional nuclear reactor to a series of LENR reactors.

Traditional reactors use radioactive elements, which is exactly why you're running into radiation issues, but what if you remove the radiation all together?

LENR reactors utilize the process in which a slow moving neutron is moved into an element; since the element is now unstable, the neutron is split into an electron and a proton so that the element regains stability, thus releasing clean energy. Now that the element has an extra proton, it actually becomes a stable new element; in the case of Nickel, Copper. Or if you used Carbon, Nitrogen.

NASA researchers are already working with Nickel and this LENR type idea, so it's not implausible to use these to power your engines instead.

Here's a library of research papers on LENR if you'd like to read more.

Here's a source from NASA Langley Chief Scientist Dennis M. Bushnell, talking about LENR's being better than conventional rockets.

• Once I'm at home I'll take a look at the papers; while I've always heard of cold fusion as a goal to strive for, I wasn't aware it was actually physically doable yet, even in theory. Thanks for the heads up. Are the byproducts able to later be processed into usable materials? Not directly related to the engine, but a nice extra touch if so. – SkyeAuroline Jul 14 '17 at 17:51
• Can you provide a source on 'even LENR reactions can produce huge amounts of power'? My understanding of the subject is that LENR is so low-powered that it's not clear whether or not it actually exists. Given that LENR is a power source rather than a propulsion system, it sounds like what you're saying is 'ditch the NSWR and use electric propulsion instead', which isn't really answering the question. – Catgut Jul 14 '17 at 19:51
• @Catgut Except the problem is to solve radiation (which using LENR does) and claiming a breakthrough in LENR technology isn't a big change compared to claiming a breakthrough in NSWR's. It is a fictional story, after all. – Aify Jul 14 '17 at 20:45
• @SkyeAuroline Cold fusion is pseudoscientific nonsense. Although muon-catalyzed fusion at lower temperatures than standard thermonuclear fusion is theoretically possible. Arthur Clarke used the concept in, IIRC, 2010: Odyssey Two. – a4android Jul 15 '17 at 5:35
• I may have missed it in my quick look at the links. But what is the source of the slow neutrons? This low-energy nuclear reactions (LENR) approach looks promising for interplanetary travel. Delighted to find something new. – a4android Jul 15 '17 at 5:45

Why not put a cap on the open end of the "torch"?

Instead of extending the shadowshield to cover the entire reactor, build it in two pieces. One piece stays on the ship and protects it and the crew from the reactor's radiation. The other piece stays on the spaceport. When a ship wants to dock, a heavily shielded/remotely operated tug seals the other half of the shield on the ship, either before it docks or just after. That way, the ship doesn't have to carry a full shield around with it all the time.

Edit: Visual courtesy of Phillip. I was picturing something not quite as big, but it captures the concept.

• This is the simplest way. Good thinking. – Mad Physicist Jul 14 '17 at 18:49
• I wrote an answer and then realized that it is the exact same solution as yours, so I deleted it. But I made a picture for visualization which you can add if you want to: i.stack.imgur.com/POIp2.jpg – Philipp Jul 16 '17 at 15:52

To preface this, be aware that nuclear saltwater rockets don't involve nuclear reactors. The nuclear fuels are kept diluted as propellant and insulated in a neutron moderator, and the nuclear reaction that provides thrust occurs externally to the ship. The ship still needs an onboard power source, but if that source is non-nuclear (like solar panels), then when the engine is shut down the only significant radiation source would be due to transmutation of the engine components.

So with that said, if you can tolerate the radioactivity of an inactive NSWR, then the solution is:

## Tugs.

Tugs aren't just a solution for nuclear propulsion, they're a necessity for any setting involving large spacecraft which by virtue of their size will likely be little better at maneuvering near a space station than a supertanker is at maneuvering in port.

You need small, maneuverable, powerful spacecraft to help guide the bigger ships into port anyways, which calls for a (non-radioactive) chemical rocket. You can shut down the nuclear reactor and run on battery power while the tugs bring you in to dock. Keep personnel away from the still-hot engine, but it will be much easier to shield against than a live engine.

But if your engine is powerful enough to render its inactive components dangerously radioactive, then you'll need another solution.

## Launches.

In the 17th-19th centuries, it was common for ships to stay at anchor outside of port, and use small boats (called launches) to ferry personnel and supplies between the ship and the port.

Following this example, keep the ships at a safe distance whenever possible. Since radiation follows an inverse-cube falloff at distance, they shouldn't have to stay too far to keep bystanders within acceptable dosage limits. Use shuttles to convey personnel, and tugs to carry supplies, cargo, and equipment. If you need to work on the ship, bring a maintenance vessel (or mobile drydock to it).

Alternatively:

## Modularity.

As a third option, keep the radioactive drive unit separate from the crew habitat. This concept by artist William Black depicts an example, where a nuclear propulsion system and its propellant is a detachable module. The nuclear propulsion module is left a safe distance from the destination, while the chemical propulsion module completes the journey.

This gives you the best of both worlds. A small chemical propulsion module can come in to dock with much greater maneuverability than the whole, without any danger of radioactive contamination. The ability to jettison the nuclear propulsion system might also be highly desirable when dealing with a nuclear saltwater rocket, which is almost literally a flying bomb and something you would want to get away from quickly if there were signs of imminent failure.

On the flipside, modularity is expensive, complicated, and typically not as durable as a fixed design. This solution could conceivably coexist with one of the above methods, offering greater convenience and less logistical coordination in exchange for higher operational costs.

Depending upon the details of your propulsion system and technological assumptions, all three of these solutions might coexist to different degrees. You can likely carve out different ecological niches for each approach within the same setting.

• Launches have a number of security and safety advantages. Only station-owned equipment and station-trained crew are ever used for station docking, questionable cargo can go through customs before it enters the station, nuclear material can be stored off-station, and it's much easier to enforce efficient quarantine procedures to prevent biological contamination issues. – Tobias Hagge Jul 15 '17 at 4:00
• "Their boat is turning into more boats!" - in space. – corsiKa Jul 16 '17 at 0:07

You only need shields on the sides of the ship that are in danger of irradiating someone. For example, on nuclear aircraft carriers, there is no shielding (of the lead/poly/water type, there is still structural steel) on the bottom of the reactor, because no one cares if neutrons go that direction. There are no potable water or fuel tanks below the reactor, so the only thing in that direction are ballast tanks and the ocean. In the specific case of ships (and submarines) there is no concern that fish will get irradiated because the water is an excellent neutron shield, and a reasonably good gamma shield. In space, there are no fish and everything is very far away, so released radiation is generally harmless.

You could have a space-ship that is only shielded in one direction. If you imagine a ship with a reactor that is an $x$ meter radius sphere; then an $x$ meter radius shield provides a 90 degree arc of radiation protection. The reactor can be installed in the back of the ship with the reactor immediately forward of it. The rest of the ship forward of the shield is thus completely shielded. I am terrible at drawing so imagine the following picture.

What is labeled as mission module would be the reactor (and propulsion unit). What is labeled as the thermal shield would be a radiation shield (and also a thermal shield, incidentally). A good bit of separation, and then you put all the important things like people and cargo-you-don't-want-irradiated in the front.

This design drops the shielding area by a factor of 4 (the surface area of a sphere is $4\pi r^2$ vs $\pi r^2$ for a circle of the same radius) and saves you mass.

# Reactors and people compartments can have different shielding

Nuclear reactor shields on Earth are designed to bring radiation levels down to that of the Earth, or less. In my years as a Navy nuke I received less than 10 mrem per year, despite doing some work in the reactor compartment itself. By comparison, the average dose for a person on the Earth's surface is around 300 mrem per year (radiation data on page 17 of this NASA report).

The Apollo 14 astronauts picked up about 1140 mrem in nine days of travel to the moon and back; on the ISS the expected dose is around 12000 mrem for six months; about 80 times the radiation level of earth. There is more radiation in space, in general. Therefore, crew compartments must already have radiation shielding built into them already.

Therefore, reactor shielding does not have to be as thick as it is on Earth to bring radiation levels down to the general level in space. If your shielding only needs to be 1/100th that of Earth, you can calculate the weight savings by using the tenth-thickness. The tenth-thickness of lead for a 1 MeV gamma is 1.5 inches. Therefore, if you require only 1/100 as much radiation protection, you save on 3 inches of lead on your shield (assuming your shield is solid lead). Since a lead shield is only around 8 inches in total (on Earth, that is), you get almost another factor of two mass savings from this.

# Spherical station; docked bow in

Put these factors together, and you have saved almost an order of magnitude of mass on your shield. You now have a lot of long skinny ships that have a radiation safe zone that extends about 45 degrees from the bow.

The solution for docking them is to have them all moor to the station bow facing inwards. In order for their safe zones to overlap, the space station needs to curve away from each ship in all directions: a sphere.

So the solution is directionally shielded spacecraft parked bow in on a spherical space station.

• But how do you protect ships when docking and undocking? Suppose every docking port is full and one ship needs to leave, won't it pass through the unshielded directions of every ship docked nearby? – ShadSterling Jul 16 '17 at 16:39
• depends on how far apart they are and how wide your safe arc is. Plus your ship needs to have some shielding to deal with cosmic radiation. – John Jul 17 '17 at 17:50

First off space is full of hard radiation anyway so a space station that you're going to inhabit for any length of time at a stretch has to be a radiation bunker, especially if it's outside a planetary magnetic field. That's the simplest answer; ships and stations are going to get hit with a LOT of radiation all the time so shielding isn't the issue you seem to think it is.

But to address the question anyway 1. start with traffic control, ships may not fire their main drives on a particular set of vectors within this area on pain of really stiff fines, this creates a series of "approach corridors" (actually cones) where radiation emission is legal and shields the station and any docked ships from an extra dose 2. still a traffic control measure really, require ships to leave their drive modules parked at a point of closest approach and use docking tugs 3. more traffic control, ships don't really need to get all that close to a station anyway, send out cargo tenders and supply vessels that have a shorter range and heavier payload to resupply the "docked" vessels and exchange cargo pods without long-range high-emission ships coming too close to the station.

All of which ignores the fact that the real answer is in fact you can't. If someone decides to use the particles or radiation from a reaction engine as a weapon there is nothing you can do about it, by the time you can see that they have turned their exhaust on you they're already pumping out hard radiation in your direction and depending how far away they are they've been doing so for quite some time. Always remember that at seventh and last a space ship is a weapon in the wrong hands, and with a reaction drive you don't even need to hit someone with the ship directly.

Have the engine module or the ship itself on a long tether. That keeps them away from each other and the station. Crew and cargo will ride the tether like a zipline to transit between ship and station.

• This has another possible bonus. You can use the tether like a string on a bolo while in transit, rotate the ship and tether around each other, and create gravity. – AndyD273 Jul 14 '17 at 15:49

There are a few options here, one would be the development of anti-radiation drugs or treatments that everyone in space is taking. You can handwave the details but they allow people to survive much more radiation. This also opens up various plot points such as if the meds start running low as well as helping to explain how they cope with events such as solar flares. There is a lot of radiation in space, not just from the reactors.

• Radiation causes damage at the molecular level to your body. There is no drug that can keep your ionized water molecules from turning into hydroxide and then reacting with other components of your cells. There is no science-based way to have a drug that protects you from neutron or gamma radiation. – kingledion Jul 14 '17 at 17:58
• @kingledion Not quite true. The drug could prevent or neutralize the hydroxide somehow. More likely though would be improving the bodies repair and recovery process to the point that anything less than an immediately lethal dose can be repaired. They would probably also need a cure for cancer since I expect that would be a side effect of such a process and the drug itself may even involve some nano machinery or genetic engineering. That's why I suggested hand-waving the details though since it's a complicated subject and by definition we don't know how to do it with current tech. – Tim B Jul 15 '17 at 8:24

Eight 45-degree shield sections that can be rotated to adjust the unshielded angle on demand between 0 and 270 degrees.

The minimum opening is given by the current reactor power level -- if your reactor is running, all that energy needs to go somewhere, and the only way to not convert energy to motion is to vent it off to the side so it cancels out (that's why you need more than 180 degrees opening radius).

Stations expect you to power down well in advance and drift in, with a set of tugboats catching you and bringing you to the station.

Of course, that requires a lot of advance planning -- all these heavy ships drifting with no reactor power and no way to spool up quickly need to be on safe trajectories.

This means that you can only dock at friendly stations that actually do send their tugs out.

This gives a good reason for the military to build large carrier vessels and huge support fleets. The reactor for a carrier is extra large, so it takes extra long to start and stop, and the carrier is a either burning through fuel at an insane rate, or it's a sitting duck that needs to be defended by slightly smaller ships.

You don't have a problem of spatial proximity, you have a problem with logistics and management, which is trivial to solve. The key is logistics.

Imagine all your ships parked or docked side to side, passengers and noses to the station and nuclear engines facing outwards. When docked, the ships switch to power from the station's own power system (solar/nuclear in its own separate module) and turn off their own power. Result - if only one ship at a time is under power, then all (or all but one at a time) of all those engines are off or quiesced. So for all those parked ships, there is only a lower level radiation in the fuel and engine interior and no (or no significant) emission of radiation into vacuum. So all that's left when parked is residual radiation from the engine's interior, much like a hot car engine still gives of a lower level of heat, and even that is mostly going into space.

Also if needed, make all ships in a given dock be of similar length classes. As the ships are long, the forward facing 45 degree angle isn't a problem. Any radiation from the engine exit that doesn't exit rearwards, is minimal anyway, shadowed by your 45 degree shields from any crew compartment or the dock itself, primarily affects the rear of ships, and falls off rapidly anyhow with distance or curvature of the dock. (In space, ships can be long and more fragile to accentuate this effect).

The ships can if needed, also be either empty of people, or emptied of people and fragile materials, when parked. You park your ship, you leave it (taking anything with you in a cargo pod if needed for security or safety), and only when the dock is safe and empty again does the next ship move.

Your problem is now reduced to having a load of stationary parked ships present (which could be made empty) while one ship parks or departs only.

The time taken to travel from say 3 miles away to docked is minimal and for any given ship doesn't happen all the time (perhaps not often), so any exposure on arrival/departure due to being exposed to the rear of these ships is probably reasonably small and doesn't need extra shielding.

There are many ways to work with this. For example the docks themselves could also be in small "clusters" around a "parking module" a bit away from the main station and transfer to/from there to the main station (think cruise ships).

The ships can leave the dock by a simple sprung pressure (air or other) - they get pushed off, and a few hours later are 20 - 100 miles away and can fire up safely. Think air traffic control.

As long as only one ship at a time is moving, its easy to make safe. So your problem in space becomes a much easier problem in time/logistics.

Avoid having outbound spaceships fire directly towards the station. When they undock, a small amount of thrust sends them directly away from the station. The spaceship fires a side thruster to move the station out of direct line of sight with the main engines. Depending on the design of the spaceship, they might also be able to rotate the whole ship to achieve the same effect. Once the space station is out of the danger area, the spaceship can fire its main engine to move in a direction perpendicular to the station.

Modular ships. Have the dangerous nuclear engines in a pod that can be detached and left in a parking orbit some safe distance away. Ships would then approach for docking using their non-nuclear maneuvering thrusters.