[Spaceships] Drive economics

[AlexanderHowl]

A slow boat works only if the underlying economics supports the idea. It might instead make more sense to send 'dumb' barges on Hoffman trajectories (or even ITN trajectories) and keep the spaceships close to the source and destination. The way, each spaceship is capable of a faster launch/capture rate, allowing for a higher rate of return on investment.

For example, the Hoffman trajectory for Earth-Mars costs 3.6 mps in delta-v. A SM+10 spacecraft with 4 fusion engines could push a SM+11 barge at 0.005g, requiring 33 hours of acceleration. It would then burn 16.5 hours of acceleration to return to its position, so a total of 50 hours of effort, easily allowing two such trips per week (a 39% utilization rate). If it traveled with the barge though, it would be only used for 1.33 days out of the 237 day trip, which is only a 0.6% utilization rate. So, by leaving the fusion engines at home, you improve utilization rates by 70x.

[DaltonS]

The main problem I've always with the Hohmann trajectory (think Santa) is the long wait time between launch windows.

The chief drawback to Hohmann orbits is that launch windows come only every 2 years. A crew on Mars has to wait more than 500 days before the planets are properly aligned to leave for home.

GURPS Classic: Mars page 32 (sidebar)

Of course there is no reason an unmanned barge has to go home again. It might be more economical to let the recipients retain the empty barge and use it to build orbital infrastructure. I'm actually more interested in the tugs you mentioned. They would have to designed not only to boost the barge but get home again as well. And you'd need one at both ends: one to boost the barge and one to catch it. The orbital mechanics are doable, the logistics and management are a bit more tricky.

Dalton “who wouldn't want to be a passenger on that barge” Spence

[Anthony]

Originally Posted by DaltonS

The main problem I've always with the Hohmann trajectory (think Santa) is the long wait time between launch windows.

All drive paths have a wait time between launch periods of period1 * period2 / abs(period1-period2) (synodic period), because that's how often the two bodies are in the same relative position (you can use more than one drive path, of course, but there's a wide range of efficiency). This is an unavoidable feature of orbital mechanics. The hohmann transfer is just an optimum of a porkchop plot.

However, the wait time isn't as long as it sounds, because that waiting occurs while you are traveling. For a craft that makes the transit every time its window comes up (2.15 years), it spends 1.41 years in travel time and 0.74 years waiting (not sure exactly how that time is split between the ends).

[AlexanderHowl]

Originally Posted by DaltonS

The main problem I've always with the Hohmann trajectory (think Santa) is the long wait time between launch windows.
Of course there is no reason an unmanned barge has to go home again. It might be more economical to let the recipients retain the empty barge and use it to build orbital infrastructure. I'm actually more interested in the tugs you mentioned. They would have to designed not only to boost the barge but get home again as well. And you'd need one at both ends: one to boost the barge and one to catch it. The orbital mechanics are doable, the logistics and management are a bit more tricky.

Dalton “who wouldn't want to be a passenger on that barge” Spence

Hoffman trajectories work because of the differences in orbital velocities, so you do not necessarily need someone on the other end. It is a good idea to have a receiving tug, especially if you have anything that you want to keep intact, but you can use chemical rockets for the minor amount of delta-v (around 10% of the delta-v of the Hoffman trajectories).

Hoffman trajectories work regardless of timing, it is just that the optimal trajectories (the shortest time traveled) occur infrequently. The timing matters for living cargo and passengers, but it does not matter for nonliving cargo. Barges are likely to only transport nonliving cargo.

As for the fusion tugs, the required delta-v is relatively small. In the case of the tugs, two tanks would be sufficient, as that is 24 mps. When pushing the barge, the tug burns 14.4 mps of delta-v to accelerate the tug and the barge to 3.6 mps. After detaching, the tug burns 3.6 mps to negate the previous delta-v difference and another 3.6 mps to return to its original point (leaving 2.4 mps for manuevering). A tug would probably have 8 fuel tanks, meaning that it would need refueling every month.

[Jinumon]

It should also be pointed out that at TL 10 or higher, High Automation reduces the overall cost of running a ship, assuming that each workspace is filled by Joe Schmo who's making the standard G$5,600 for his TL. As a matter of fact, the break even point for High Automation is just over G$3,700* on a per workspace basis, meaning that even TL 9 schlubs are likely to cost more, assuming they don't make the bare minimum. Not to mention they don't come with pesky things like human error, death benefits, etc.

Jinumon

*EDIT: That's based on a ship life of 25 years, which is the standard for commercial naval vessels launched after 1946.

[AlexanderHowl]

If you assume 1.5% of capital costs per month for repayment/replacement and operations and maintenance, the effective minimum monthly cost of High Automation is $15,000 per required technician. Now, there is some economic benefit to not having to use cabin space or not having to supply provisions, but it is not quite as good as you are suggesting for a realistic setting. In general though, it probably only makes sense for long duration trips, as the reduced provisions allow for extra cargo or reaction mass.

[Jinumon]

Originally Posted by AlexanderHowl

If you assume 1.5% of capital costs per month for repayment/replacement and operations and maintenance, the effective minimum monthly cost of High Automation is $15,000 per required technician. Now, there is some economic benefit to not having to use cabin space or not having to supply provisions, but it is not quite as good as you are suggesting for a realistic setting. In general though, it probably only makes sense for long duration trips, as the reduced provisions allow for extra cargo or reaction mass.

Isn't that sort of thing only for cheap or very cheap ships though? At least in RAW?

Jinumon

[AlexanderHowl]

Originally Posted by Jinumon

Isn't that sort of thing only for cheap or very cheap ships though? At least in RAW?

Jinumon

No, that is the simplified assumption for costs (Spaceships 2, p. 31). The maintenance costs for cheap and very cheap ships are in addition to the 1.5% of the cost per month. With cheap spacecraft, this ends up being 2.5% of its cost per month. With very cheap spacecraft, this ends up being 5.5% of its cost per month.

[Rupert]

Originally Posted by Jinumon

Isn't that sort of thing only for cheap or very cheap ships though? At least in RAW?

It is. The maintenance cost of a ship that's not cheap or very cheap is the cost of paying the maintenance crew. What's more, the monthly or weekly cost of parts for (very) cheap ships isn't reduced by automation, and so is a separate issue.

[DaltonS]

Originally Posted by AlexanderHowl

Hoffman trajectories work because of the differences in orbital velocities, so you do not necessarily need someone on the other end. It is a good idea to have a receiving tug, especially if you have anything that you want to keep intact, but you can use chemical rockets for the minor amount of delta-v (around 10% of the delta-v of the Hoffman trajectories).

Hoffman trajectories work regardless of timing, it is just that the optimal trajectories (the shortest time traveled) occur infrequently. The timing matters for living cargo and passengers, but it does not matter for nonliving cargo. Barges are likely to only transport nonliving cargo.

As for the fusion tugs, the required delta-v is relatively small. In the case of the tugs, two tanks would be sufficient, as that is 24 mps. When pushing the barge, the tug burns 14.4 mps of delta-v to accelerate the tug and the barge to 3.6 mps. After detaching, the tug burns 3.6 mps to negate the previous delta-v difference and another 3.6 mps to return to its original point (leaving 2.4 mps for manuevering). A tug would probably have 8 fuel tanks, meaning that it would need refueling every month.

A Hohmann orbit is an elliptical orbit that permits a transfer from one circular orbit to another. It requires one boost at the beginning to enter the elliptical orbit and another at the end to match the destination orbit. It takes a fixed amount of time to travel from one orbit to the other so you can only launch at particular times if you want your target planet to be where your transfer orbit ends when you get there. In an Earth-to-Mars case you need 2.1 mps to break Earth orbit and 1.8 mps to enter the transfer ellipse. At the Mars end you need 1.6 mps to match the planet's orbital speed around the sun and 0.93 mps to acquire a circular orbit around the planet (the last bit may be done by aerobraking). There may be other types of transfer orbits but this is the one that requires the least ΔV.

(BTW, I've never heard of Hoffman trajectories.)

As for using a tug, the ΔV of the tug/barge combo equals the ΔV of the tug * its LWT / the LWT of the tug/barge combo. Thus if you want a tug to push a barge that is 2 SM larger (10 × its mass) from Earth orbit to the transfer ellipse it would need to have a total ΔV of at least 3.9 mps × 11 = 42.9 mps in the tug's tanks not counting what it would need to get home (another 3.9 mps at least). A Quarterhorse-Class Deep Space Tug (TL9) (Spaceships 6 p.15) could manage launching a SM+11 barge (barely) if you put collapsible tanks in 3 of the 4 cargo holds. (It could manage an SM+10 barge on its own.)

Dalton “who likes playing with numbers” Spence