Improving long range accuracy of rockets without active guidance in flat/minimum arc trajectory
I am using rockets as a main weapons platform for my vehicles. Currently they are fired from a railgun like apparatus. A rocket is at the center of three rails arranged in the fashion of a triangle. A sabot jacket surrounds the rocket, which is connected to the rails. The rocket motor ignites bringing it up to a maximum burn rate, after that the breaks on the rails disengage and the sabot jacket carries out it out of the rail system. The jacket separates and the rocket flies off to hit its target at an incredible speed. The rocket motor would still be active throughout the flight, stopping only if it runs out of fuel or hits its target. The sabot may rotate, as of now I'm not entirely sure if that would increase or decrease the accuracy of the weapon.
The weapon system will be firing both guided and unguided kinetic energy missiles. This question focuses strictly on the unguided solid rod penetrator rocket.
Kinetic energy missiles have a massive benefit in that they can accelerate to high speeds without using complex technologies that require advanced technologies seen in railguns, coilguns, combustion light gas guns etc. They also have a significantly higher upper velocity compared to the upper limit of chemical/gunpowder rounds. Outside of the initial acceleration phase, they greatly out speed even armor piercing fin stabilized discarding sabot rounds.
Interestingly enough the fuel needed to get such a weapon system to such speeds and velocities (1.5-2.23km/s) to kill in a small portable fashion was created during the days of the Future Combat System and 1980s-early 2000s. Several kinetic energy missiles were created, LOSAT, KEM, CKEM for reference. They observed excellent penetration powers during testing application against even ERA equipped T-72s. 898
The minimum range of CKEM was 200m whereas LOSAT was 400m.
All my targets will be engaged at ranges of 1Km+ (maximum ranges for older kinetic energy missiles range between 5-10km using technology from the late 90s to early 2000s), and the maximum speed will be much higher due to in setting rocket fuel sources. The issue I have with such a system is accuracy. In the real world the situation was solved with active guidance. However, my rockets can't use active guidance for multiple reasons. Once it leaves the barrel, whatever direction its pointing at, is the direction in which its going to hit its target. Ideally the rockets should fly in a level trajectory. They won't be doing any maneuvers or top attack of any kind or use any chemical effect warheads (HEAT/HEAT-FS/HE/HESH/HEDP).
What modifications would be needed to make such a rocket more accurate to be able to hit targets at long range (1-10km) without the use of active guidance? Active guidance in this case refers to something like beam riding, warhead seekers, or radar guidance from a command vehicle.
I am okay with some "passive" guidance systems so long as they aren't actively scanning data from outside the rocket itself. For example, keeping track of a rockets angle of attack or speed would be considered "passive" in this sense since an onboard computer in the rocket can do this without having to communicate with a command vehicle. But an image/heat seeker would be considered active since its scanning target data to make major course changes instead of course correcting to its initial launch vectors. Preloaded data onto the rocket is fine to set information such as range, elevation, angle, etc. The goal is to stabilize the rocket as much as possible so that it has a linear flight plan, based on the initial vector it left the barrel from. In this sense the "unguided" rocket rounds act as tank rounds.
While active guided missiles are obviously better, they're also significantly more expensive and complex to mass produce. A "dumb" KEM would cut heavily on costs and past certain speeds would benefit from not having seekers/fragile electronics as they can be damaged by shock/trauma which KEMs are more than capable of doing on launch or even main boost phase. 234