[Spaceships] Homebrew: Alternate Spin Tethers

[DaltonS]

Another way to look at spin tethers (GURPS Spaceships 7 p.22) involves its tensile strength and two ships of (possibly) unequal masses. Given that Mn and Gn are the respective mass (in tons) and gravities of ship "n" and T is the tensile strength of the tether in tons load:

T/2 = G1*M1 = G2*M2

The mass of the tether would be T/20,000 tons (possibly adjusted by TL to reflect changes in materials), the cost $10*T and the minimum length would be (G1+G2)*50 yards†. It may be longer and thinner and spin more slowly as long as the stress didn't exceed its tensile strength. Please note that

1. mass and cost of this design feature only requires knowing the data for one ship,
2. the external cradle should be installed on the larger ship,
3. the center of spin would be Gn/(G1+G2) times the tether's total length from ship "n", and
4. since it is carried as cargo in a hold or hangar bay, its mass can be ignored.

† GURPS Vehicles Expansion 1 from 3E recommends a minimum 100 yard/G spin radius (twice the 4E value) with three times that considered nominal. Also, the cost there was $10 per pound of total loaded mass, or $20K/ton.

Example: A 0.4 G spin tether for an SM+6 ship heading to Mars (local gravity .38 G) would have a tensile strength of 80 tons, cost $800 and mass 8 lbs. If attached to another SM+6 ship, its minimum length would be 40 yards (80 yards by 3E rules). If an SM+5 ship was attached, the g-force in the second ship would be 0.4*100/30=4/3 Gs and the minimum tether length would be 173 1/3 yards or 520 feet.

Dalton “who loves putting a new spin on things” Spence

[DaltonS]

As an alternative to spin tethers, I would like to suggest a Spin Gravity System (as opposed to Feature). It would consist of a freely rotating spin armature and retractable arms that would extend two modular systems beyond the hull proper and rotate them for spin gravity. Use the Spin Gravity Table (SS1 p.30) for ships 3 SMs larger to get Max G and cost. The Handling penalty should be the same (-2). Like Wings and Sails, this system should be limited to ships SM+12 and smaller.

Dalton “who can see this as a Smaller SM system too” Spence

[Varyon]

Ever since I saw the Spin Tether option, I wondered if it would be possible to have different size ships use them. I see this being used in two main ways. In both, you have a smaller ship that you carry in the main one. Once you need spin gravity, you drop that ship out of the hangar with the tether attached, then start spinning. You can then either have the smaller ship move at an extremely high rate to give the main ship some spin gravity or, more likely, locate your Habitats in the smaller ship and have it working at only 1G or so.

An SM+7 "spin ship" would have 19 Habitats and 1 Control System*. With the Habitats as Cabins, that means it could hold up to 76 people. Carried around in an SM+10 ship, you could manage 1G (main ship experiences on 0.03G) with a 52 yard tether. Crew can move between the ship proper and their cabins either through EVA or, more likely, a small "shuttle" that climbs the tether to move between airlocks.


*You could justify getting rid of the Control System, as the main ship should be able to do the spinning by itself, and have the full 20 systems dedicated to Habitats, in which case the "ship" holds up to 80 people.

[DaltonS]

FWIW: Slowly Spinning to Mars

One of the protocols of the Mars Direct scenario involves tethering a Transhab module to its nearly expended booster and rotating the assembly at one RPM to provide the habitat with 0.38 Gs artificial gravity. Using the formula

r = 975.723 × a × ω²

where

a = acceleration in Gs (Earth standard gravities)
ω = angular velocity in RPM (rotations per minute)
r = radius of rotation in yards

the spin radius of the module would be 370.77 yards. Given that as an upper stage the module would be one SM smaller than its booster, the booster's spin radius would be dependant on how many fuel tanks were expended.

Example: An SM+6 upper stage would use an SM+7 booster. Fully fuelled, the 14 remaining systems of the booster would mass 210 tons. At least one system must be the reaction engine. If 12 tanks were emptied, the mass would be 30 tons compared to the 100 tons of the upper stage. The booster's spin radius where therefore be 370.77×100/30=1,235.9 yards, making the total tether length 1,606.7 yards. Note that this would be true for any upper stage with an even SM number; for those with an odd SM number, the total tether length would be 4×370.77=1,483.08 yards.

Dalton “for my next physics lecture, I'll need an apple tree and a hard hat” Spence