An Atmospheric Corps is a branch of a nation's military devoted to air combat within a planet's atmosphere. These planes tend to be capable of entering orbit via the atmosphere, and have extremely fuel-efficient engines. They are very small craft, often with no more than one or two pilots. They are not very versatile and can only be used in certain situations or circumstances. They are never used to engage enemy ships - this is considered suicide and a waste of an extremely expensive plane, and rarely do they have fuel to engage enemy ships at a good distance and be able to return.
Atmospheric Corps may be deployed to other worlds via a airplane tender. Most tenders are classified as Star Frigates, but some may be large enough to be considered Star Galleons. They serve a variety of purposes, and are extremely expensive to produce in large numbers, as they require technology allowing them to travel in space as well as highly fuel-efficient engines for the atmosphere. Their best uses are in providing short-ranged patrols in asteroid fields, leading them to be tied to Star Frigate-sized tenders.
The low (if existent) atmosphere of asteroids and low gravity enables these planes to escape the gravity wells very easily, making them very useful. Planets with atmospheres and gravity similar to Earth will severely impact the performance of the planes. Also of importance is the density or composition of the atmosphere which can effect how well the engines run - planets like Venus would be extremely difficult to navigate. This means that atmospheric corps' assets may only be usable in certain circumstances, as with smaller planets or in asteroid fields.
Capabilities
Where they can be used, though, they are extremely important. During the Polaris Wars, the Ishamshuk Empire deployed 1,000 atmospheric craft from several tenders they had brought for this purpose, which were on the very edge of the atmosphere. The Kingdom of Polaris had a miniscule air force, which was annihilated, establishing air supremacy.
After airfields and other installations needed for atmospheric craft were taken, the planes began to perform their roles from the ground rather than needing to refuel in orbit. Air supremacy in planetary invasions is considered to be extremely important, as it gave the Ishamshuk a massive edge over the Polaris military, as they were able to quickly scout, raid, and interdict Polaris forces on the move, as well as inflict damage on supply caches and infrastructure with impunity.
As such, most planets keep their own atmospheric corps as part of the 'garrison' of the planet, although in these cases it is referred to as an Air Force rather than an Atmospheric Corps. The planes that form an Air Force tend to be less expensive and easier to mass-produce than ones designated as an atmospheric corps, as they do not need to be space-capable and do not need to be as fuel-efficient. They do, however, require more infrastructure to be effective - airports dotting the planet, adequately supplied and supported with ground crews.
All atmospheric craft, including those acting as a garrison, have VTOL-capability, meaning they can take off and land without a runway. Helicopters are rarely used as most atmospheric craft are also capable of hovering, although less tend to be able to carry and transport troops for quick insertion - this is considered redundant due to orbital deployment capabilities.
Armament
While the craft are equipped with guns (typically two heavy rapid-fire coilguns) they are considered a last resort, or for strafing of ground troops during an attack of opportunity in such cases where air-to-ground missiles are not available. The vast majority of weapon systems loaded onto atmospheric craft are long-range air-to-air missiles, which, in combination with advanced tracking and targeting systems are capable of accurate hits on aircraft as far as 175 kilometers. These air-to-air missiles can be divided into 3 categories - "dogfight" missiles, medirum-ranged missiles, and long-ranged missiles.
Although missiles are designated by maximum range, this is misleading, as the effective range is the point where there is no escaping - it is impossible to outrun the missiles in this range once they are launched. Poorly-trained pilots are known to fire their missiles as soon as a target is within maximum range, which gives the enemy a lot of time to maneuver or destroy the missile.
At certain ranges, however, there is an event horizon in which there is no way planes can escape the missiles - this is the effective range. However, various factors can contribute to reducing the effective range of a weapon. Altitude, speed, and direction of the target, along with those of the attacking plane, along with any evasive actions being taken, can greatly reduce the effective range.
"Dogfight" Missiles
Dogfight missiles are called so as they are designed to focus on the agility of the missile rather than range. They are also called "visual-range" missiles, as they have a maximum range of about 30 km which is within visual perception at high altitudes. It is very difficult to shake these missiles off as they are extremely agile and capable of keeping up with planes. Their guidance systems are also very efficient, tending to be radar-based or heat-seeking, meaning that only limited methods of countermeasures are available.
Their effective ranges are nearly as long as their maximum range, and this makes dogfighting missiles extremely lethal at close ranges, even with beginner pilots. However, the times in which planes will be within this range of engagement are few and far inbetween except in some special cases.
Mid-range missiles
Mid-range missiles try to retain the advantages of dogfight missiles while being longer ranged. In practice, mid-ranged missiles give the enemy less time to respond or to take any kind of action than long-ranged missiles and retain the agility of dogfighters. However mid-ranged missiles can be more easily fooled by countermeasures or jamming systems.
Mid-range missiles tend to have a maximum range of about 250 kilometers, with effective ranges reaching out to around 110 kilometers on a given missile type. They are thus more difficult for inexperienced pilots to utilize effectively, due to the tendency of pilots to fire as soon as an enemy is within range.
Long-range missiles
Long-range missiles have large engines and rocket motors that allow them to reach out great distances. Accuracy at these distances can vary wildly and this also gives the enemy time to intercept or avoid missiles, provided they can detect them.
Carver Missiles
Carver missiles are equipped with warp drives. This makes them capable of interstellar ranges - but they generally lack the endurance or fuel capacity to do so. Carver missiles are sophisticated devices which require proper timing to utilize effectively. One advantage of using warp in missiles is that they do not require explosive warheads, their sheer kinetic power is enough to shatter even the largest starships. They also cannot be intercepted or destroyed, and will hit their targets near instantaneously.
They are however difficult to manufacture, and can't be built on just any world, requiring a significant technological and resource base to build in large numbers. Carver missiles are often used to give planets a zone of control in their solar system, able to fire at anything within astronomical distances. The only issue is timing the fuze appropriately and ensuring the missiles have enough fuel to reach their targets.