GURPS Spaceships TL 8 SM+9/+12/+15 orbital lifter

[Peter Knutsen]

Just to see if it - a two-stage-to-orbit design at TL 8 - can be done after all, here is an SM+9 ship, plus brief mods for increasing it by 3 or 6 SMs.

The trick behind this design is that it doesn't use a normal upper stage, nor a small upper stage, but a tiny upper stage, one 3 size classes smaller than the ship itself. Thus the SM+9 version can lift a single SM+6 ship to orbit. This ship takes up 2/3 of one module (3 times less than a small upper stage, and 10 times less than a normal upper stage), with the remainder of that module being used for fuel tankage.

Front:
[1] Tiny upper stage, plus 1/3 size fuel tank (0.05 mps dV)
[2 to 6] Fuel tanks, rocket fuel, 0.15 mps base dV per tank, 1M$ per fuel tank

Mid:
[C] 1/3 SL streamlined metal laminate armour, dDR 3, 3M$, plus 2/3 size fuel tank (0.10 mps dV)
[1] Control room, 6 stations, 0 workspaces, 6M$
[2 to 6] Fuel tanks, rocket fuel, 0.15 mps base dV per tank, 1M$ per fuel tank

Rear:
[c] Fuel tank, 0.15 mps dV
[1 to 5] Fuel tanks, rocket fuel, 0.15 mps base dV per tank, 1M$ per fuel tank
[6] Rocket engine, 3G thrust, 0 workspaces, 6M$

Total number of tanks: 17
Delta-V modifier for 17 fuel tanks: x2.2
Final delta-V: 5.61 miles per second
(5.6 mps is needed to get from surface and into orbit, so this one just barely makes it!)

A fragile ship (low dDR), but at least the fuel is much less volatile than HEDM.

Costs:
17M$ for 17 fuel tanks
(not including 2.04 M$ to fill tanks with rocket fuel)
1M$ for partial armour, dDR 3 on one hull section
6M$ for control room

Total empty cost: 24M$
Cost per launch: 2M$ (often a bit less, since the ship has more delta-V than needed to launch its payload into low orbit)

Crew: 6 in control room
Payload: One SM+6 tiny upper stage


Scaling up (crudely):
To launch an SM+9 tiny upper stage, multiply cost by 30 (getting 720M$). The control room would require 30 times as many crew, but the rocket engine now also requires 10 workers, so a total of 190 crew. Fuel cost per launch would be 60M$.

To launch an SM+12 tiny upper stage, multiply cost by 1000 (getting 24G$). The control room requirs 1000 times as many crew, and the rocket engine now requires 300 workers, for a total of 6300 crew. Fuel cost per launch would be 2G$ (much cheaper than the TL9 HEDM-using design, but then again you also only get to launch SM+12 instead of SM+14).


The design could probably be made somewhat more robust by giving it more armour, or giving it overall armour. Overall armour would give it dDR 1/1/1 instead of 0/3/0. More armour (which can be combined with overall armour) means reducing the number of fuel tanks from 17 to, perhaps, 16 2/3 or 16 1/2 or 16 1/3.

[thtraveller]

Note that 5.6 mps includes about 1 mps of efficiency losses due to gravity drag and air-resistance. it isn't a precise figure.

I also suspect this design only works because the fuel tanks are 100% fuel, with no structural mass included. The design looks about only 2% structural mass. EDIT: Although I note that only the upper stage is required to have any armor in any case.