thinking about spacecraft design

[whswhs]

Originally Posted by thrash

What logistical arrangements have you considered? Antimatter production in orbit, with hydrogen cracked from ice or hydrated minerals on the surface and shuttled up?

I haven't yet tried to resolve that. The existing modules for spacecraft include mining modules, for rocks and metals, and chemical refineries, for water, hydrogen, and perhaps organics, but it's not clear that either of those can produce antimatter. Nor does it seem that a factory can produce antimatter. So far I've just handwaved the issue. But I would want to have facilities for producing antimatter, perhaps on the huge starships that brought the colonists to the system. Any thoughts?

[Anthony]

Originally Posted by whswhs

I haven't yet tried to resolve that. The existing modules for spacecraft include mining modules, for rocks and metals, and chemical refineries, for water, hydrogen, and perhaps organics, but it's not clear that either of those can produce antimatter. Nor does it seem that a factory can produce antimatter. So far I've just handwaved the issue. But I would want to have facilities for producing antimatter, perhaps on the huge starships that brought the colonists to the system. Any thoughts?

Honestly, if you're using antimatter plasma drives handwaving is probably the right decision (bear in mind they use antimatter-boosted fuel, which is priced at $12 million per ton. Given that nuclear pellets cost $50k per ton and have almost as high performance, I can't actually think of any reason to use antimatter plasma).

[Varyon]

Originally Posted by whswhs

I haven't yet tried to resolve that. The existing modules for spacecraft include mining modules, for rocks and metals, and chemical refineries, for water, hydrogen, and perhaps organics, but it's not clear that either of those can produce antimatter. Nor does it seem that a factory can produce antimatter. So far I've just handwaved the issue. But I would want to have facilities for producing antimatter, perhaps on the huge starships that brought the colonists to the system. Any thoughts?

The only part of Ultra-Tech that discusses the creation of antimatter is under Nanofacs (UT91), where it explicitly states they cannot make antimatter, but they can make the particle accelerators that can. Offhand, I'd say a Factory System can, at 5x cost (so a bit more than the cost of a nanofactory), instead be a particle accelerator (possibly using the Accelerator Tube Limits Design Switch from SS7) that produces antimatter from hydrogen at the stated rate (which is in $/hour). I assume you'd have a series of stations orbiting relatively-close to each star, consisting of little more than Solar Panels, Antimatter Factories, and tanks for holding the hydrogen and generated antimatter, with ships regularly delivering fresh hydrogen and bringing back antimatter; the ships themselves would probably be using magnetic sails... or maybe you'd actually use Aldrin Cyclers, with smaller docked ships that drop off to actually do the exchange and then return to the cycler to go home, where they do the exchange in reverse.

[whswhs]

Originally Posted by Anthony

Honestly, if you're using antimatter plasma drives handwaving is probably the right decision (bear in mind they use antimatter-boosted fuel, which is priced at $12 million per ton. Given that nuclear pellets cost $50k per ton and have almost as high performance, I can't actually think of any reason to use antimatter plasma).

Hmmm. I hadn't thought of that option. I see that the delta-V is essentially the same. The acceleration is halved, but my analysis of flight from Omicron Polypi A to Omicron Polypi B had only about four days of acceleration at the start of a flight of several months; reducing the acceleration isn't going to be a big deal. And it would avoid the question of where they get the antimatter; I kind of like Varyon's proposal, but I hesitate to suppose they'd want to take it on this early, when the human population is less than a million. That might be a proposal for the future . . .

[Varyon]

Originally Posted by whswhs

Hmmm. I hadn't thought of that option. I see that the delta-V is essentially the same. The acceleration is halved, but my analysis of flight from Omicron Polypi A to Omicron Polypi B had only about four days of acceleration at the start of a flight of several months; reducing the acceleration isn't going to be a big deal. And it would avoid the question of where they get the antimatter; I kind of like Varyon's proposal, but I hesitate to suppose they'd want to take it on this early, when the human population is less than a million. That might be a proposal for the future . . .

A smaller-scale version of my proposal should be able to provide sufficient antimatter for antimatter-catalyzed fusion, as seen with the TL 11 Portable Fusion Reactor and suggested as part of how the Fusion Pulse Drive (and thus Advanced Fusion Pulse Drive) function. So if you like it, you could still have a version of it, with the colonists working their way toward a larger setup.

Also, I could have sworn somebody suggested this in the thread, but I don't see it now - this has stats for an antimatter factory, derived from the Industrial Antimatter Factory from UT's Designer's Notes (which I keep forgetting about) rather than my off-the-cuff stats, so I'd suggest using that if you do opt for some orbital antimatter production.

I was going to talk about scaling down the drives by 1 SM to fit more fuel tanks (to make up for the reduced amount of delta-V you get when using Advanced Fusion Pulse instead of an Antimatter Plasma Rocket, but then I noticed that you had the latter giving 0.1G acceleration in your example. But, unless the copy I'm looking at is outdated, it appears the APR only gets 0.01G acceleration (AFP is still half that, at 0.005G). Using a High-Thrust variant with water as the reaction mass boosts that to 0.06G; are you assuming roughly two drives? Note AFP doesn't have the option of using water - you can double acceleration with the High-Thrust option (halving delta-v in the process), but water isn't listed as an option. It probably should be - the description notes the pulse units are either adjacent to or surrounded by inert reaction mass, and having this mass be water (for higher thrust and lower delta-v) should be an option, even if it's not quite enough for x3 acceleration.

[whswhs]

Originally Posted by Varyon

A smaller-scale version of my proposal should be able to provide sufficient antimatter for antimatter-catalyzed fusion, as seen with the TL 11 Portable Fusion Reactor and suggested as part of how the Fusion Pulse Drive (and thus Advanced Fusion Pulse Drive) function. So if you like it, you could still have a version of it, with the colonists working their way toward a larger setup.

Also, I could have sworn somebody suggested this in the thread, but I don't see it now - this has stats for an antimatter factory, derived from the Industrial Antimatter Factory from UT's Designer's Notes (which I keep forgetting about) rather than my off-the-cuff stats, so I'd suggest using that if you do opt for some orbital antimatter production.

I was going to talk about scaling down the drives by 1 SM to fit more fuel tanks (to make up for the reduced amount of delta-V you get when using Advanced Fusion Pulse instead of an Antimatter Plasma Rocket, but then I noticed that you had the latter giving 0.1G acceleration in your example. But, unless the copy I'm looking at is outdated, it appears the APR only gets 0.01G acceleration (AFP is still half that, at 0.005G). Using a High-Thrust variant with water as the reaction mass boosts that to 0.06G; are you assuming roughly two drives? Note AFP doesn't have the option of using water - you can double acceleration with the High-Thrust option (halving delta-v in the process), but water isn't listed as an option. It probably should be - the description notes the pulse units are either adjacent to or surrounded by inert reaction mass, and having this mass be water (for higher thrust and lower delta-v) should be an option, even if it's not quite enough for x3 acceleration.

I'll have to recheck that. I may have misread a number, or mistyped it.

[Varyon]

Originally Posted by whswhs

I'll have to recheck that. I may have misread a number, or mistyped it.

Doing some calculations myself (away from books, so I just figured out how many miles per second per second 1g corresponded to, checked how long that would take to accelerate to 225 mps, and converted to days, then adjusted that by dividing by actual acceleration), 4.26 days to reach 225 mps would be consistent with 0.1G. 0.01G would instead call for 42.6 days, during which time you would cover 414,072,000 miles, or around 4.45 AU. That's 8.9 AU in 85.2 days, leaving between 41.1 AU and 16.1 AU (depending on if it's a 50 AU or 25 AU travel) to travel. 225 mps is roughly 0.21 AU per day, for either 195.7 days or 76.7 days, total travel time between 280.9 days and 161.9 days (with the latter spending more time accelerating and decelerating than coasting). Dropping to 0.005G, you need 170.4 days for acceleration and deceleration, during which time you would cover 1,656,288,000 miles, or 17.8 AU, leaving between 32.2 AU and 7.2 AU. At 0.21 AU per day, those take 153.3 days and 34.3 days, for a total travel time between 323.7 days and 204.7 days. That's roughly +40 days in both cases (a month, a week, and some change), which is probably alright. Assuming I did all the calculations right, anyway.

[Agemegos]

Originally Posted by johndallman

The usual answer is to carry landing vehicles on board a larger ship. The rocket equation makes having two different drive systems on a single ship hard: the unused one is always parasitic weight.

Lighters aboard and their propellant are also parasitic weight, though a lighter that can make several ground-to-orbital round trips (restocking with some propellant such as water on the surface), and thus land and load the cargo and passengers in several small batches can be much less massive that an SSTO engine and propellant to land and lift the whole ship including its long-endurance engines, their propellant, and the long-term habitats of the passengers and crew. But that approach can mean long unloading and re-loading times.

I reckon that there are four categories of spaceport facilities corresponding to different solutions to the logistical problem of servicing the ground-to-orbit leg.

N The destination can supply Nothing of significant use, not even inexpensive propellant. This means that spaceships have to carry propellant, spare parts for maintenance, and maintenance technicians for orbital lighters. If traffic at the destination os very thin, then ships might carry lighters as deadweight on their interplanetary or interstellar trips. If it is a bit thicker it might be economical to stash lighters in orbit about each destination world for use when the ships visit.

G The destination has intermodal Ground facilities near railheads and ocean ports, where lighters can refill their propellant tanks with e.g. water, and where there are technicians and supplies of parts for maintenance. Lighters, their crews, and their maintenance facilities and staff are resident at the destination, meaning that ships can serve the destination without any deadweight to serve the ground-to-orbit leg.

O The destination has cargo-handling facilities and passenger assembly lounges in Orbit. Cargos and then passenger complements can be assembled and mustered gradually by continual operations of a local fleet of lighters, allowing a spaceship to dock, unloaded passengers and cargo to the orbital port for gradually landing, re-load with new passengers and cargo, and then depart without waiting on the lighters to make repeated trips.

L The destination has some sort of non-rocket Launch facility, such as a rotorvator, launch loop, space fountain, or even a beanstalk. Propellant for lighters is not an issue.


A while ago in another place I did a bunch of specification searches to minimise the full-system cost of delivering cargo and passengers to orbit, using GURPS Spaceship costs and vehicle performance. For service between the surface and low orbit a planet with the characteristics of Earth TL10 limited-superscience fusion "torch" rockets with water propellant were far and away the cheapest.

• HEDM chemical is optimised for launch with 3 engine systems and 10 tanks. Cost of launch is $147,700 per payload system (15 tons, 20 passengers).
• Fusion torch is optimised for launch with 1 engine system and 3 tanks of water. Cost of launch is $140 per payload system (15 tons, 20 passengers).
• AM thermal is optimised for launch with 5 high-thrust engine systems and 8 tanks of antimatter-catalysed hydrogen. Cost of launch is $463,000 per payload system (15 tons, 20 passengers).
• AM plasma torch is optimised for launch with 3 engine systems and 1 tank of antimatter-boosted hydrogen. Cost of launch is $741,500 per payload system (15 tons, 20 passengers).

Those figures are for a O-type spaceport, with propellant available on the surface and in orbit.

[Anthony]

Originally Posted by Agemegos

Lighters aboard and their propellant are also parasitic weight

Which is why you offload that task to the ports. There's no reason for ordinary commercial transport to ever visit a location that doesn't have its own launch facilities with their own launch vehicles; the first step to establishing a colony is probably "build a fuel refinery and launch facility, which will be used to re-orbit the shuttle originally used to land the colony".

[Agemegos]

Originally Posted by Anthony

Which is why you offload that task to the ports.

Yes? John Dallman described carrying landing vehicles on board a larger ship as "the usual answer". I explained why it isn't, and gave a list of alternatives, explaining how the task may be off-loaded to the ports for four different levels of port infrastructure.