New! Unique! Almost worth the free download! It's the one and only Rocketpunk Manifesto Travel Planner, available in all flavors of plain vanilla Excel. I did this workbook for my own amusement, inspired by this recent post, then decided to offer it to an indifferently awaiting world.
It is tailored for a high specific impulse drive, and trips across planetary distances conveniently measured in AU. If the ship pops through an FTL rabbit hole at the midpoint, that is between you and Albert Einstein. It has no effect on this worksheet, except that the given travel distance is misleading.
Each worksheet is laid out as three columns: mission characteristics, ship characteristics, and (inevitably) cost estimates. User entry values are across the top, in light blue.
The mission column models a steady acceleration, coasting phase, and steady deceleration, all in flat space. The ships I was modeling are fast enough, with transfer speeds in the dozens of km/s, that solar space beyond 1 AU is fairly flat for them. Real ships don't have constant acceleration, either - acceleration increases as propellant is burned off - but this gives a decent first approximation of travel time.
Providing a coasting phase is more efficient than a brachistochrone orbit, because you aren't putting on speed only to almost immediately take it off again. A coasting distance of zero corresponds to a brachistochrone. (If you change the distance, reduce the coasting phase first - Excel will freak out if you make the trip distance less than the coasting phase.)
Note that I round off 1 g to 10 m/s/s, versus ~9.8 m/s/s, just as I round off 1 AU to 150 million km. Acceleration is in milligees; drive specific impulse is in seconds (not exhaust velocity in km/s). The worksheet computes travel time, burn time (accelerating and decelerating), mission delta v, peak travel speed, and initial and final acceleration.
The second column is for basic ship characteristics. The first user entry is departure mass - the ship, fully loaded and fueled. The second is drive fraction - the percentage of the ship's arrival mass (what reaches the destination) that is given over to the drive engine, including shielding, radiators, etc.
The third user entry is keel fraction - the portion of all-up departure mass given over to the framework that holds drive engine, tankage, and payload together, along with bells & whistles such as the navigation and comms equipment. Since these are low acceleration deep space ships I used a modest 5 percent for this figure. (Each section of the ship would also have its own internal bracing.)
The final user entry here is tankage fraction, as a percentage of propellant load. l used a generous 10 percent, since for liquid hydrogen fuel the tankage must include a cryogenics plant.
There is no user entry for the payload. It is calculated (and shown on the right hand column, just below the user cost entries). The payload is simply whatever mass allowance remains once necessary mass has been assigned for the drive engine, keel, tankage, and of course propellant.
I also don't give a figure for the dry mass of the whole ship, because this depends on the payload. The payload figure given is gross payload - the whole payload section, including any hab compartment, cargo bays, the cargo itself, whatever. I do give a figure (also over on the right) for the mass of the bus, the ship structure minus the payload. So the dry mass is bus mass plus whatever part of the payload is 'fixed.'
Finally, the right side of the worksheet permits some cost estimates. The first is fuel energy cost (not propellant cost), measured in US cents per kilowatt hour. The figure I used, 2 cents per kW/h, is roughly the current cost of nuclear electric power, approximating the case of having to breed fusion fuel. This is the price to beat if you're mining He-3 from Out There somewhere.
The next user entry is hab mass per passenger berth, probably several tons. I used 5 tons per berth for trips of a few weeks, up to 25 tons per berth for an 18 month mission requiring long term life support.
The next entry is ship cost, per ton of dry mass, with a unit value of $1 million, which is about what a jumbo jet costs. If you want cheaper ships, just enter something less, like 0.3 for $300,000 per ton. The final figure, 'annually,' is what the ship costs you each year, for paying off the building loan plus operating costs, as a percentage of building cost. I use 20 percent.
All of this lets me calculate, i.e. guesstimate, the cost of a passenger ticket, helpfully broken down into berth charge ('rental' of the berth space, for duration of the trip), plus a carriage charge, the energy cost of actually making the trip.
A host of details are ignored, such as a reserve propellant margin, that you don't want to ignore in practice, along with other details such as necessary downtime between missions. And you know what resides in the details.
It is not exactly a 'ship design' tool - it doesn't even give the official mass and cost of a ship, but it is a way to suss out some ship characteristics required for long space trips with high specific impulse drives, whether Realistic [TM] midfuture drives or more advanced torchlike drives.
Naturally I took it for a test drive, with the results that you'll see. Drive engine characteristics (except for specific impulse) are calculated, not user input, but I fiddled each sim to represent a ship with a hefty 1000 ton main drive engine. For the first two sims this drive puts out 1 gigawatt of effective thrust power, the performance level of a nuke electric or solar electric drive that we could probably develop now if we put Pentagon money into it.
With this drive a ship with a mass of some 5500 tons should be able to reach Mars in three months, burning off about 2000 tons of propellant and delivering a 900 ton payload section. As a transport it could carry about 120 passengers, each paying about $1.2 million for the one way trip. Cargo rides cheaper, about $130,000 per ton.
The same drive plant, with a different specific impulse setting, will send a 1700 ton payload to Saturn in 18 months, one way - a passenger ticket costs $17.5 million, but a measly half million to ship a ton of cargo.
The next few sims step up drive performance to 10 gigawatts from a 1000 ton unit. With this drive a 3500 ton ship can just barely reach Mars in 21 days (at opposition), carrying a nominal 35 ton payload, enough for a handful of passengers. But a slightly slower trip, 30 days, allows a vastly more economical ship. For this mission our ship has a departure mass of 6400 tons and delivers a 1700 ton payload, carrying more than 300 passenger. A one way ticket to Mars now costs $440,000; a ton of cargo can be shipped for $80,000.
A longer and slower trip, three months, turns out to cost more for passengers, due to (assumed) greater life support requirements, but cargo cost falls to $40,000 per ton. Notice the enormous size of this ship: Departure mass of 35,000 tons, and a 16,000 ton payload section.
Then I tried the drive out for going from Earth orbit to Moon orbit, which turns out to be approximately a 38 hour one way trip. And for once the ticket prices are not exorbitant - $9000 for a cozy passenger berth, or $3000 per ton for cargo.
There is an important lesson here. What makes my other ticket prices so high isn't that spaceships are expensive, though they are, but that they are so unproductive, only delivering a few passenger and cargo loads per year, if that. For short trips with rapid turnaround the cost of space transport (under these assumptions) falls to levels comparable to global first class air travel.
This is another argument for settings that involve local space travel, such as Earth orbital space, or for that matter Saturn orbital space.
Moving right along, a few more sims. A 4000 ton ship fitted with a 1000 ton, 10 GW drive is just barely able to reach Saturn in 180 days, carrying only a nominal 5 ton payload. Extending travel time to 200 days yields a more practical ship, though a ticket still costs $25 million. If you're willing to take a year to get there, Saturn can be yours for less than $10 million, and the freight charge is a mere $300,000 per ton.
Finally I stepped the techlevel up another jump, to 100 gigawatts - verging on the low end of the torch range here. This is sufficient to reach Saturn in 90 days, with a peak speed above 270 km/s - nearly 0.1 percent of the speed of light, nothing to a photon but a lot for you and me. (I cut the energy cost by a factor of 10 too; the energy bill for that kind of speed was still most of the cost of the ticket.)
So there you have it. Go play with it, and see what results you come up with. Here's the link again: Rocketpunk Manifesto Travel Planner. And a recent post on getting to Saturn, among other places.
Image of the ISS from Astronomy Picture of the Day.
Update: The download is being a bit glitchy. I've tried renaming the file and re-uploading - let me know in comments if you're able to download it now.
Update II: Link fixed, thanks to Winch of Atomic Rockets. (See comments for brief, embarrassing explanation.)