The Skybus smallcraft is designed to be the ideal lifter for travel in and out of gravity wells. There are significant rules-motivated variations relative to the
Spacebus design which has a similar role for deep space operations.
Thrust: 3/5 is the effective minimum for exiting the atmosphere which means an engine 3x larger than the Spacebus's. This significantly cuts into lift capacity relative to the Spacebus---120 tons vs. 160 tons.
Shape: Using an aerodyne shape allows us to avoid a vertical liftoff control roll. The vertical liftoff control roll is particularly nasty, because failure results in damage to the landing gear. With a control roll of 5 on a an airfield (-1 modifier), there is a 1-in-12 chance of landing gear damage, implying that over 8 cycles there is fairly high chance of landing gear damage. Repairing landing gear is hard (+3 modifier, 2 hours) so a tech team would like to take extra time (+1 modifier, 8 hours), implying spheroids are hanger queens under routine heavy use. Furthermore, there are good odds of a partial repair which further worsens landing control rolls. Overall, an aerodyne appears to be a 'must' for heavy use.
As a consequence of the aerodyne shape, only 2 doors are available rather than 4. With only 3/4 of the available tonnage, this makes loading and offloading take about 50% longer. For containerized cargo with a squad using
Strongarm exoskeletons that's about 5 minutes in zero G to offload and reload rather than 3 minutes for the Spacebus. On the ground in a 1g environment it requires just 4 minutes. For a 120 ton bulk cargo item in space (the worst case), 23 minutes are required to offload or load.
Armor: There are 2 control rolls required in a liftoff, orbit, deorbit, land cycle. Deorbit requires entering the space/atmosphere interface where a straight control roll is required with MoF*5 damage applied to the nose. Landing also requires a control roll with a -2 bonus for a friendly airfield to avoid MoF*10 damage to the nose. Armor is easily repaired (at 5 minutes/point), so a typical day with a control roll of 5 over 8 cycles suffers an expected 18 damage to the nose, requiring 90 minutes of repair along with the standard 90 minutes of maintenance. Incidentally, a base control roll of 4 generates a radical improvement: only 4 damage to the nose requiring 20 minutes of repair. A control roll of 3 is even better---about 1 damage to the nose requiring 5 minutes to repair. And control roll 2 (i.e. elite) never sees damage on the cycle. Since we are aiming for functionality with a control roll of 5, significantly more nose armor is required to deal with bad days.
Skybus smallcraft
Type: Civilian Aerodyne
Mass: 200 tons
Technology Base: Inner Sphere (Standard)
Introduced: 3025
Mass: 200
Battle Value: 680
Tech Rating/Availability: D/X-E-D-D
Cost: 6,693,060 C-bills
Fuel: 3 tons (240)
Safe Thrust: 3
Maximum Thrust: 5
Heat Sinks: 0
Structural Integrity: 5
Armor
Nose: 141
Sides: 43/43
Aft: 41
Cargo
Bay 1: Cargo (120.5 tons) 2 Doors
Ammunition:
None
Escape Pods: 0
Life Boats: 0
Crew: 1 officer, 2 enlisted/non-rated
Notes: Mounts 15.5 tons of standard aerospace armor.
Weapons
and Ammo Location Tonnage Heat SRV MRV LRV ERV
None
An interesting note here is that the 120ton lift capacity is competitive with the lift capacity of large freight airplanes today. The
Mriya was the most capable, able to lift 250 tons, but it's destroyed now. The
747-400F can lift 124 tons while the
AN-124 can carry 120 or 150 tons depending on configuration.
Can you get by with just one design? The Skybus could be used in the Spacebus role, but the costs are significant: it's 43% more expensive while reducing overall cargo load/unload speed by at least 60% between 3/4 cargo capacity and 1/2 doors (while recovery & launch times stay constant).
What about a dropship instead? There are two utilities associated with Mammoth (Note that although I'm using the Mammoth for concreteness, an abstract optimally designed cargo dropship is similar enough.): the ability to transport over a jumpship->planet cycle and the ability to lift or land very large items. For the former, there are alternatives, but for the latter occasional dropship support may be necessary.
Given that there are other large craft, let's think about lift. A Mammoth's lift costs 15K/ton while the Skybus's lift costs 56K/ton making the Mammoth initially appear to be a winner. However, there are significant drawbacks associated with a Mammoth's lift.
- The lift capacity of a Mammoth is useless for the vast majority of a Jumpship->Planet->Jumpship cycle as the thrust will not normally exceed 1g. During that time, a Skybus can be doing useful work lifting cargo from point to point on a planet. Given this capability, a number of Skybuses (varying with the size of the colony) are economically useful for any colony and would consequently be available for unloading cargo from large craft in orbit during occasional visits.
- There is an element of cost approachability associated with the smallcraft approach. A small colony might have just 2 Skybuses with a number that then grows with the colony. Small dropships in contrast cost an order of magnitude more and start with no efficiency gains in cost/lift.
- The load/unload times for a Mammoth are much slower---even in a port with a heavy cargo platform it's 315 minutes during which time 70 Skybuses (with the same cost as a Mammoth) could do 5 ground-orbit-ground cycles shifting more cargo to&from orbit than the Mammoth.
- The ability of multiple Skybuses to transport to multiple locations is a significant logistics win over the Mammoth approach. This could be partially taken into account via the cost of equivalent transport. This is feasible via a slower fixed wing support vehicle which costs about 1/4 of a Skybus, implying that a portion of the Skybus cost is naturally defrayed.
Altogether, because of reasons 1&2, some Skybus-like vehicles appear essential while reasons 3&4 suggest that these remain useful even for quite well-settled worlds.
