[Spaceships] Drive economics

[the-red-scare]

Suppose I have a list of plausible space drives for my setting. I can do some basic math and come up with trip times and ticket prices for each variant and number of drive to a specific destination. Is there a rule of thumb for weighing ticket price vs trip time to determine which of the drives will actually end up being used?

I'm thinking of air travel -- we could technologically all fly supersonic everywhere, but the cost isn't worth the saved time (and its loud, etc). We could also take zeppelins everywhere for probably pretty cheap, but that's too slow. The market decided "subsonic jet travel" is the sweet spot.

When you have a lot of options and times vary from three weeks to nine months and tickets range from $10K to $10M, what factors determine the "main" drive and (perhaps even more importantly) which of the faster/expensive or slower/cheaper options would remain available for special adventuring cases? Or is it just going to have to be GM fiat?

I could see perhaps taking the crossover point of possible times versus possible costs and saying that's the "standard," then using some exponential formula to estimate demand of alternatives and any that are out of balance get dropped as infeasible, but I'd have no idea what that would really look like.

[Fred Brackin]

Originally Posted by the-red-scare

I'm thinking of air travel -- we could technologically all fly supersonic everywhere, but the cost isn't worth the saved time (and its loud, etc). We could also take zeppelins everywhere for probably pretty cheap, but that's too slow. The market decided "subsonic jet travel" is the sweet spot.
.

Actually, I've never found any mix of LTA technology that makes it competive with conventional airplanes even for the infinitely patient. The "killer app" for LTA is staying airborne without expending energy. This might make it usable for long-term surveleelance like the early Cold War era radar blimps.

However, as soon as you try and go from point A to point B the greater drag of the gasbag compared to wings kicks in and LTA craft jsut don't compete even for fuel efficiency on a per mile basis.

Simiiarly, you will find some drives in Spaceships just can't compete with other drives. They won't be found in the same setting (except possibly with one beign a failed tech paradigm). The list is so long for completeness and matching sources in fiction.

[the-red-scare]

Originally Posted by Fred Brackin

Simiiarly, you will find some drives in Spaceships just can't compete with other drives. They won't be found in the same setting (except possibly with one beign a failed tech paradigm). The list is so long for completeness and matching sources in fiction.

In a sense, I guess I’m trying to figure out how (other than GM fiat) to decide which drives failed due to never having a chance to be competitive. And since it isn’t just “more expensive” or “too slow,” how to balance those factors.

[Fred Brackin]

Originally Posted by the-red-scare

In a sense, I guess I’m trying to figure out how (other than GM fiat) to decide which drives failed due to never having a chance to be competitive. And since it isn’t just “more expensive” or “too slow,” how to balance those factors.

You simply build some ships and see which one works best. With Speceships this can actually be much easier than it sounds.

You estalish parameters about what the ship must be able to do. We'l say it will be SM+10 so it doesn't need an Engine Room. We'll also say it needs at least 1 armor per section for meteorite and radiation protection. It does have to have a Control Room. Depending on the Drive it may need a Power Plant. If so that gets tossed in with the Drive.

So you have 16 slots to play with. You rule that it must carry so many Habitats for crew and passengers and a given amount of Carg spaces. The rest you allocate to Drive and Fuel. Then you compare these.

[AlexanderHowl]

It also depends on other factors, including technological assumptions. For example, magsails are either static (operating according to inverse square law, up to a maximum velocity of 375 mps) or dynamic (allowing expansion or contraction according to solar wind pressure, allowing for constant thrust within the Sol System, up to a maximum velocity of 375 mps). The former is implied in Spaceships, but the latter is the actual assumption of the research IRL.

So, if we are using the real life assumptions of the research when comparing magsails to other TL9 drives, they tend to win out for interplanetary passenger travel in realistic settings. A SM+9 spacecraft with 10 magsails could maintain 0.01g thrust out to 75 AU, meaning that it could get to Saturn in 13 weeks. If we assume three armor, one control room, one engine room, one habitat, one hanger bay, and one reactor, that leaves only two cargo holds (300 tons). The spacecraft would likely cost $400M, so it needs to earn $6M a month to break even. Since each leg of the Earth-Saturn run is an average of 13 weeks, that means it needs to charge $80k per ton to break even ($100k per ton to make a profit).

So, with just that analysis, we have an idea of the baseline for the economics of magsail. If we want to compare TL9 fusion engines, we would do a similar analysis, though we would include reaction mass as well. For example, a SM+10 fusion spacecraft with two engines and ten fuel tanks could have a maximum delta-v of 168 mps, allowing 80 mps for acceleration. The same trip with the fusion spacecraft would take around a year, so the overall cost would be equivalent to a magsail, even though it takes four times as long. As people would likely not want to pay the same amount for an inferior service, fusion would not be competitive.

[the-red-scare]

To avoid seeming excessive, I refrained from mentioning I actually have a spreadsheet with several hundred entries with every combination of drive, number of engines and reaction mass. If it’s capable of using a continuous acceleration it uses the brachistochrone travel time, otherwise the normal rules but ignoring delta-v for breaking orbit. Slap in a distance in AU and you can compare all the possibilities at once, with estimated costs. I did not keep the payload mass/number of passengers equal, assuming if a given trip needs more or less it will either be larger or smaller, or uses multiple ships.

So I have entries like this…

TL10 5x fusion rocket (high-thrust, H2): 16 days, $167,508/ticket (1 habitat).

…for every combination.

Now many are right out as not remotely affordable, or taking far longer than a Hohmann transfer. But, to pick a random example, compare the above to…

TL10 2x fusion rocket (standard, H2O): 35 days, $1,728 (9 habitats).

It’s not important at the moment if these specific example numbers are right, so let’s not get distracted by that, but how would a market decide if (in this example) double the time for a tenth of the price is worth it? What’s the cost of a travel day? Is it exponential as more time is lost?

I have a feeling it’s just “make up whatever the market decided in this universe,” but I thought I’d ask.

[AlexanderHowl]

Realistically, you should take into account the mortgage/replacement cost and associated expenses of the spacecraft (~1.5% of the capital cost per month) when calculating the cost of shipping (anyone who does not will likely not be in business for long). In addition, commercial enterprises realistically need to make money, meaning that they will likely charge 50% over their costs, to compensate for unused cargo capacity and to make a profit (at 80% capacity, that translates to a 20% profit). Those that do not charge that much will likely not be in business for long (established customers can be given up to a 20% discount if they agree to ship a lot of cargo over a long period of time).

[Ulzgoroth]

Getting a prescriptive answer out of some kind of economic model is probably over all of our heads - though I'd love to hear otherwise! But some analysis of that so that you can look at what you're considering and ask what it implies and whether that makes sense is probably useful!


One important consideration is that transport is naturally a very non-uniform market. Some people simply need to get places very fast. Some people have less demanding or more flexible speed requirements. Some people are intensely price-sensitive, some have severe budget ceilings, and some have money to burn. In some cases, more amenities and less speed might be more attractive. All of these also apply to non-sapient cargo.

Any given design isn't going to be optimal for all of these people. There may be several different viable builds that largely don't compete with each other. Like water vs. air freight or economy plane tickets vs. luxury cruise ships.


Basically, when you ask what a travel day costs, the answer is different for every possible payload. The real question is were the economically significant clumps are.

A not ridiculous place to start, though, would be the daily pay for the passenger's wealth level. Assuming they can't do their job in transit (which may vary depending on the job) that's a measure of the incremental value lost by spending more time in your flying box rather than on the ground.

I would mostly think severe non-linearity will kick in when a traveler has crucial deadlines, making speed strictly obligatory.

And of course one should consider what outright alternatives to travel travel has to compete with. Even across a few light seconds a lot of things can, after all, be done remotely. Spending several million dollars to attend an off-world conference rather than streaming the events for a relative pittance might be a hard sell for all but the best-funded potential attendees.

[the-red-scare]

Yeah, I don’t really expect a truly rigorous accounting mode here, just trying to ballpark something plausible. I basically want to end up with one standard, one cheap, and one fast drive that I can use as the basis of some typical ships for the setting, plus one expensive but fast as hell drive for military PC types to use.

When I started working the combinations and realized the scale of how many options there actually could be, I figured there’s got to be some formula I can throw in the next column to rank these things. If there just plain isn’t, alas, I’ll play it by ear.

[Ulzgoroth]

Originally Posted by the-red-scare

Yeah, I don’t really expect a truly rigorous accounting mode here, just trying to ballpark something plausible. I basically want to end up with one standard, one cheap, and one fast drive that I can use as the basis of some typical ships for the setting, plus one expensive but fast as hell drive for military PC types to use.

When I started working the combinations and realized the scale of how many options there actually could be, I figured there’s got to be some formula I can throw in the next column to rank these things. If there just plain isn’t, alas, I’ll play it by ear.

Well, what I'm suggesting is you need multiple rankings.

For example, you probably want to look at the ranks by pure best time and pure lowest cost. You might not take the absolute top ranking result for each of those, but seeing what is up there would be good.

Then the simplistic formula I sort of suggested was to rank by ticket price + travel time*income. You can plug in different values of income for different significant demographics in your setting, and see what looks good for each.