If force fields haven't been invented and ships must be able to deploy and return too fast for doors, how might a capital ship be able to protect its hangar from space debris and opposing fighters
The most sensible way would be to avoid having an internal dock altogether. IF a capital ship needs auxiliary ships like lighters, sensor drones or cutters to carry landing parties, they should be held in external cradles.
This provides several advantages.
Firstly, the ships can be deployed and recovered much more quickly than through a hanger or landing bay.
Secondly, there is less risk of having an accident contaminate the living spaces of the ship if the auxiliary ships are held externally rather than internally. As well, of there is an accident, the ship could be ejected quickly from the cradle.
Finally, and this should be greatly stressed, the elimination of a hanger space inside the ship eliminates a potential weak point in the structure. This could be very important if you are going to postulate a high thrust/ high ISP drive. Manoeuvres which cause the ship to pull more than a standard "G" would have to be avoided, since the ship could buckle under the stress. Weapons impact could also cause stresses to accumulate around the hanger so the ship buckles or breaks at that point. If your ship is using physical armour, then you also eliminate weak points in the protected volume.
So having auxiliary ships attached to the outside of the ships hull on cradles seems to be the best means to solve your issue without the use of forcefields.
Future-tech Capital/Carrier ships will have to weigh their options when it comes to speed, versatility, and protection. The fastest option may be severely under-protected, and the most versatile options may be excessively slow, etc. To decide on the best possible options it is necessary to know not only what you need, but exactly why you need it.
What is a Hangar?
A hangar is an enclosed storage area for aircraft/spacecraft. The hangar is where most repair and maintenance occurs and depending on the particulars of the vehicles in question it may also be where a pilot actually enters/exits their aircraft.
What is a Runway?
On Earth, all heavier-than-air fixed-wing aircraft require a minimum amount of forward airspeed to generate the lift needed to fly. Runways provide a clear path for these vehicles to accelerate from a stopped state up to the speeds required for liftoff. They also provide a safe area for that same craft to perform a controlled deceleration when returning to a non-flying state.
Does a future-tech Capital Ship want or need both (or either)?
The specifics of future-tech may be hard to speculate but general engineering principles still apply the same now as they will in the future. Without some big game-changer new technologies or tactics severely altering the landscape, extrapolating currently used ideas and allowing for some best-guess technological improvements may be the easiest way to evaluate possible options.
Hangar - Pros and Cons of enclosed Hangars.
- Protection - Full enclosure offers some level of baseline protection. At a minimum it will prevent visual identification by hostiles, and beyond that minimum it could potentially be a well-fortified defensive position for all "grounded" craft. In non-combat situations it would likely offer a level of environmental protection (cosmic radiation, high-velocity space debris, etc.).
- Versatility - An enclosed hangar can offer repair services, maintenance, etc. in a variety of ways that might be otherwise impossible to externally-docked aircraft. With an airlock system, maintenance can be done without the need for all maintenance personnel to don spacesuits; while some time (and/or air) may be consumed in the pressurization and depressurization procedures, the overall time saved in manhours may more than make up for that shortcoming. Even if repair is largely automated and requires no pressurization procedure for human mechanics, repair robots can be made smaller and more agile if they operate in an enclosed space where they will need less radiation shielding, etc.
- Specialization - Depending on the airlock procedures or docking systems used, the entry/exit from an individual craft may involve quite a bit of hardware. The more capable/flexible the Carrier is the less hardware will be required on the smaller craft which allows room for more fuel, payload, etc. If pilot extraction can be carried out assembly-line style (as each pilot enters/exits) for an entire multi-ship hangar by one or two machines (rather than needing one extractor per ship) then overall weight of the Carrier can also be reduced.
- Space - Large open (unobstructed) spaces take up a lot of physical volume and the strength of the structure must be carried entirely by its shell since interior supports would obstruct the passages. Some of this can be mitigated by having the hangar built alongside the more rigid superstructure of the ship but the hangar space will still be a weak point.
- Containment - The fact that the individual craft are brought into an interior space of the ship can be bad for containment. Explosions and fuel leaks (which can lead to either fires or explosions) can become significantly more damaging when occurring in an enclosed area. Furthermore, embarkation ships (rather than the dogfighter-type scrambled aircraft generally described when talking about Carrier-type craft) which land on alien terra firma may introduce harmful contaminants, either chemical and pathological, that would be best kept outside.
- Bottlenecks/Bandwidth - Although large hangars can operate efficiently using a few pieces of equipment over-and-over assembly-line style, those limited capabilities may create bottlenecks when something goes wrong. If two pilot-extractor machines can typically service an entire hangar wing, then one extractor going offline instantly halves the hangar's throughput. Individual external docks offer the maximum possible bandwidth, being able to launch all craft in the time it takes to launch one.
Runways - Pros and Cons of Runways in space.
- Space - The space taken up by the defined acceleration/deceleration laneway can potentially take up a lot of physical volume. If this area is even partially enclosed this can add up to a lot of material.
- Traffic - If everyone uses the same (or one of a few) designated entry/exit points then there will be an accumulation of traffic around these points, leading to more accidents. It also means that these hotspots are easy targets for enemy combatants.
- Purpose - Spacecraft operating in space have no minimum-airspeed requirement to "fly" the way modern aircraft do. The designation of a specific "takeoff" area is therefore somewhat archaic- just a potentially vestigial procedure no longer warranted in the zero-g landscape.
- Movement - Although individually-docked ships can all scramble and detach simultaneously, they are all equally exposed until they're all successfully detached and cleared from the main ship. Starting from a stationary position they are sitting ducks until they come up to speed. An enclosed runway, though offering less bandwidth, is able to offer a visually-concealed area for scrambling ships to reach appreciable speeds. Though enemies may know the vector the ships are launching from, the actual time-frame allowed for them to line up a shot may be significantly shorter than the timeframe for shooting low-starting-speed individual externally-docked ships. Runways also provide opportunities to install assisted-launch devices.
- Deceleration - Landing on a modern water-based aircraft carrier is difficult from what I understand. Docking on a Capital ship likely has similar difficulties. Although in space there is no fear of "falling off the edge" if your approach is too slow, there may be many instances where trading precious seconds of landing time for precision can cost lives. Runways funneling traffic into per-specified approach vectors can severely aid recovery by including rapid-deceleration gear using tailhooks and similar-concept devices.
- Defense - Although these entry/exit hotspots will likely be noticeable to enemy combatants, they are also more easily defended by the home team (compared to having a bunch of decentralized landing positions). Concentrating firepower at these locations will make it very difficult for enemies to line up the sort of "sitting duck" shots they'd ideally be getting by zeroing in on a docking (and therefore slow) aircraft.
Though others may read the same above pros/cons and see different areas to optimize or design around I think the following ideas offer at least one reasonable interpertation:
Have a runway (primarily) for assisted-deceleration and landing.
Individual docking procedures are generally rather precise operations requiring a low-speed final approach; this makes them an extremely dangerous maneuver in a combat situation because the low-speed and fixed-location creates an easy target over a relatively long timeframe. The use of a deceleration runway can alleviate some of these issues by making the position more easily defensible while decreasing the timeframe where "landing" pilots are prone to attack.
Using a runway funnels pilots into a focused approach vector, which makes it easy to concentrate defensive countermeasures along this corridor, while also making it easy to install automated deceleration and recovery systems. Pilots can enter at appreciable speed, and begin a mild deceleration while automated systems line up the approach and latch on with a deceleration arm/anchor which can then decelerate the craft much more quickly without further precise input from the pilot. The anchor can then hand the ship off to an automated taxiing system to tow the ship to the most expedient hangar bay while the anchor resets for the next approach.
Divert craft to different hangars as needed.
Once a ship has been recovered it should be taxied to an appropriate hangar or bay. Depending on the situation the ship may need extreme repairs, a quick refuel and rearm and redeployment, or just a parking space. Ideally, some amount of hangar space should be optimized for these different sorts of roles. Refuel/rearm/redeploy ideally should be extremely automated and should stay completely outside of airlock. Repair would likely follow a tiered triage system, placing high-explosive-risk ships in bays where they pose least structural risk to the Carrier (and probably with highly-automated repair systems to minimize crew casualties), while pushing more average repairs to various other bays either mechanized or manned depending on the extent and complications of the damage. Tailoring different hangar bays can help improve automation procedures as well as minimize the number of compression/decompression events necessary (each of which require time, energy, and some amount of air resources).
Additionally, since the taxiing is an automated towing process, pilot extraction could potentially occur during that time. This would be particularly useful for medical intervention, but would also make hot-swapping pilots during a refuel/rearm pit-stop very easy.
Have a number of mini-runway launch-tubes for faster scrambling.
Although spaceflight requires no minimum-airspeed with regards to takeoff, there is a slight tactical advantage to having a non-zero launch speed. The use of an automated landing and hangar system rather than multiple individual docks means that the remainder of the system is already strongly inclined toward using runways, however, the traditional long runway isn't a necessity. Assisted-launch systems are far simpler in design and principle than assisted-recovery systems, so these take-off runways do not need to be nearly as large as the landing runway. A large number of these launch-tubes could be used, partially recreating some of the high-throughput bonuses of individual-exterior-dock systems, while retaining the bonuses of concealed deployment and non-zero launch speeds.
One approach is to have a shield wall in front of the opening that requires one or more turns to get into the bay properly. This would prevent anything coming directly into the bay. This is similar to how breakwaters function to reduce the impact of waves on docks.
You could also have a friend-or-foe system with a point defense that automatically shoots anything that comes toward the shield wall. Assuming public key infrastructure (PKI) is valid, ships would have to have a valid encryption key to disable the system. Having the shield wall form a gauntlet (multiple turns) would give the point defense plenty of time to shoot enemy ships that actually go around the wall before they hit the bay.
Having a PKI and automated docking systems would let the computers take over for docking the ship and allow the system to have more precise entry vectors and turns ("maintenance tunnels are too small to pilot" paraphrased from Matrix 3) for the automated system, which would add another lay of security. In this case, once the docking system engages, it can pilot the ship precisely through the multiple turns of the shield without sacrificing significant speed ("wait until you are a thousand meters and give them a little shock" paraphrased from Alien 4). If a computer knows the exact turning ability and dimensions of the ship, they could probably get the ship through a precisely small opening or around the corners with very little tolerance.
You could also use automated docking robots to guide through the shield or gauntlet. This would also mean that the individual ships require less fuel to maneuver since that is handled by the docking "tugs".
Even if you don't have a shield wall, using PKI to identify friend-or-foe would allow you to have a point defense system (shoot all unknown ships). Of course, that would let into overriding the system for unknown or stolen ships, but that might use a secondary hanger that is well protected with a door.
How do they leave their smaller ship once inside the larger one? What about gravity?
A cradle system might work well, but you won't be able to do a variety of sizes and shapes of ships, and will have the embarkation debarkation issues. I liked the system in the first Capt. America movie where the small fighter jets were in the belly of the vehicle. But in space, well, there would have to be tubes or something so they could breathe on their way in--or they wear space suits, even inside their planes. This really depends on how large the fighter ships are and how much time people actually spend in them.
As for it being "too fast" any one who flies a ship can come in as fast or slow as needed. And they can keep doors open if need be, I suppose.
As the posters above have noted, if you aren't going to have passive defenses, things that shoot any ship not one of yours might be the way to go. But there has to be time to process that. There can be an automatic key, but having something the pilots also say to get in, might be good. And it should be unique to their ship.
Generally speaking, fighters/drones shouldn't be docking while mid-battle and if a fighter can't slow down it can't maneuver so this doesn't really make sense...
Though if this is something you need you can drop the whole hanger out of the ship so all the docking is exterior to the ship, attached through some elevator system to the main bulk of the ship. This would make it so the hanger can enter and exit the ship and the ship can close the armor over that area and reinforce it's structure by having it so any time it drops out beams extend into the empty space. Generally speaking this isn't all that smart and it pretty much leaves all your support gone and your fighters vulnerable and if it gets wrecked you've just lost all your fighters even if you're fighters are fine. In fact if someone tried this I'd just wait for them to deploy and then target the hanger... then fly away. They either are going to sit there till supplies are used up or leave and then I can attack them without engaging their fighters...
Another option is similar to the above is to not have a hanger at all. Instead, have individual hanger bays that stay out while in battle, but aren't physically connected to an internal hanger save through a mechanism that can pull them from the bay to a place where they can be worked on. The bays would each have their own air locks and you could probably carry a lot more of them, but it would also be a pain to work on them.
Another option is to use torpedo like tubes where you shoot out the fighter and the fighter can enter the incoming tube and then you can deal with an internal air lock and such after it has landed. Of course this slows deployment and pick up, makes it hard to dock because it have to aim for a tube, and also creates a target to just destroy the tube and you cripple the ship pretty good.