Scene Description in my Book:
(You can skip this part if you want, I will put the details below)
You are an admiral for "army Red". You have just gained space superiority over the planet you plan to invade by wiping out the "army Green" space fleet. Both sides suffered heavy losses and the space around is littered with the debris of ships from the battle. You decide it’s time to launch your ground forces onto the planet, but the first wave is cut to ribbons by heavy orbital guns belonging to a Green army fortress lying just 5km away from the Green's capital. A Green army traitor gives you the blueprints for the Green's fortress, only for you to discover that taking it would be a more difficult task than you first thought. The fortress is fifteen stories high and eight stories deep in the centre of the complex; the entire compound itself is a two km in diameter. The fortress is built on a hill with 150m straight incline and only one road up. It's also providing overwatch on an air force base 400m away from the hill. The Red army’s admiral has more problems as well. Your fleet may have won, but a large EMP has taken out your long distance travel for several months. With no reinforcements and with the Green army building up man power by the minute, you can’t take or besiege the fortress in time. You have to destroy it, but how? Your ships can’t bomb it, the orbital guns would shoot them down in low orbit, and you can’t use WMDs. Your air force also suffered too many losses to be of use, not that it would matter. The vast network of AA-guns and missiles would cut them down, not to mention the air base. The Admiral is at a loss. There's no way to win this. He walks to the window looking down on the planet with his fist clenched. His view is disrupted by a floating piece of a vessel and the idea comes to him to use destroyed ships as weapons.
Important details: The planet is earth-like in nature but with 0.5 times more gravity. The orbital guns wouldn't be able to shoot down something that fast. There are plenty of different sized ships to choose from. WMDs are not allowed to be used, but dropping a battle ship is. The EMP has taken out all space monitoring devices the sole exception being the fortress itself, so no one else will see it coming (or report it); some of the ships are in low orbit around the planet so accidents happen (if you know what I mean). The fortress is fifteen stories high and eight stories deep in the centre of the complex; the entire compound itself is two km in diameter. The fortress is built on a hill with a 150m straight incline and only one road up. It's also providing overwatch on an air force base 400m away from the hill.
Rules of Combat:
- The Blast Radius and how Powerful would the Blast be.
- The Fortress must be ether Destroyed or Damaged Beyond Repair (the airport is a minor concern).
- The Blast Radius must NOT harm the Capital. The Sonic Boom would be big so Minor damage is fine (broken windows, missing roofs etc).
- Can only use one ship for the attack (multiple strikes against one target would raise suspicion).
The Question is this: What Size Ship Should I Launch at the Fortress to be the Most Effective?
You can use the names/types of real world vessels to answer this question. For any more details just ask.