Here's a 3025 tech solution.
Marines provide much denser marine points than battle armor, so instead of 24 tons of battlearmor bays, we have 10 tons of infantry compartments (storing up to 100 marines and providing up to 200 marine points) and the extra tonnage is used to mount an AC/20 making the design capable of assault as well as boarding. The move is also 7/11 providing much more overtake than a 6/9 design. When used as a boarding system it's short duration only since the marines are using suit air.
Interdictor Boardingcraft
Type: Military Spheriod
Mass: 200 tons
Technology Base: Inner Sphere (Standard)
Introduced: 3025
Mass: 200
Battle Value: 2,244
Tech Rating/Availability: D/X-E-D-D
Cost: 13,376,750 C-bills
Fuel: 10 tons (800)
Safe Thrust: 7
Maximum Thrust: 11
Heat Sinks: 12
Structural Integrity: 11
Armor
Nose: 201
Sides: 169/169
Aft: 137
Cargo
Bay 1: Cargo (2.5 tons) 1 Door
Bay 2: Infantry Compartments (10 tons) 3 Doors
Ammunition:
20 rounds of AC/20 ammunition (4 tons)
Escape Pods: 0
Life Boats: 0
Crew: 1 officer, 2 enlisted/non-rated, 1 gunner
Notes: Mounts 39.5 tons of standard aerospace armor.
Weapons
and Ammo Location Tonnage Heat SRV MRV LRV ERV
AC/20 Nose 14.0 7 20 0 0 0
5 Small Laser Nose 2.5 1 3 0 0 0
Edit: I'll add some notes.
Overtake is a key concept for hostile boarding ops. It's how much faster the boarding craft is compared to the boarded. Comparing speeds, we can form an overtake table as:
| | Boarder | Speed | |
| | 5/8 | 6/9 | 7/11 |
| Boarded | 3/5 | 3 | 4 | 6 |
| Speed | 4/6 | 2 | 3 | 5 |
| 5/8 | 0 | 1 | 3 |
The amount of time that a boarding target has to apply fire to boarding craft is inverse in the overtake. So, in an extreme case a 5/8 boarding target may deliver 3x the fire against a 6/9 boarding craft relative to a 7/11 boarding craft.
W.r.t. making aerospace more popular, there's a fair bit of complexity in the SO rules. There's some lack of canon units for many roles, addressed here. But, my expectation is that popularity relative to mechs is actually driven by (lack of)identification. Relative to mechs:
- Lack of humanoid shape & actions for ASF => lack of identification.
- Lack of stacking limits rewards quantity in ASF more making combat outcome more of a mathematical exercise => lack of identification.
- Limited weapon complexity in ASF provides less rewards to tinkering design to mission => lack of identification.
There's nothing that can really be done about the first one within the limits of "realism". For the second, I primarily wonder about screen launchers---they do AoE damage in space hexes and are underutilized in that role. Theoretically, a sufficient quantity would strongly discourage stacking near a large craft. For the third, of course you could just remove the limit on standard-only munitions for ASF.
I wonder about a 'drone first' strategy, where the pilots stay aboard a relatively-protected large craft in almost all cases and you pilot drones to achieve effects. That at least allows a pilot to develop more (providing more identification).
