[Space / Spaceships] Society limited to fission?

[JackFalcon13]

So, I ran a brief adventure in a setting where we have colonized the solar system due to a little bit of an apocalypse, but got rid of most of the sci-fi trappings - no smart AI, no fusion power, no nanomachines. Basically, if we were forced into space with near-future tech, followed by a few decades of refinement but no rapid improvement.

So, I've got most of basic tech down - most guns are basically just slugthrowers. Armor is mostly highly-customizable spacesuits (picture the RIG from Dead Space, without telekinesis). Drones exist, but they're more like modern UAVs - you can set them to automatically do a task, but fine control requires a human, and they aren't smart enough to trust with automatic weapon control. Most people live and work in space stations with spin gravity, so the vast majority of workers are involved in asteroid mining or shipping.

So, a few issues. One, no fusion power. So how do ships relatively quickly and efficiently transit around? And what would be an ideal "standard drive type" that ships mostly are limited to their "neighborhood" - the area around a single celestial body, with enough fuel that jumping to another orbit is possible but either expensive or time consuming. Is there some fission-type equivalent to the fusion pulse drive? Or some concept in between that and the NTR? I don't want to use saltwater drives, because they're overdone and too effective.

The other issue, but not key. I would really like to justify space fighters. Maybe not in a conventional sense of X-Wings, and probably either relegated to station defense or an escort duty where they can be tugged by whatever they're protecting. Is there any sensible limitation? Maybe they have to use chemical rockets because nuclear drives would be too big?

I kind of want a mix of chemical fueled fighters, nuclear powered "free traders" that are fast enough over short distances to be interesting but can't really just decide to pop off to Mars for an afternoon, and then large ships with ion drives or some similar high-efficiency propellant for if people decide that working in the Mars/Belt Zone is boring and they'd rather take a trip to Uranus. The problem is, limiting myself to the stuff in Spaceships, I either wind up with ships that are too high tech or that there's no reason a smaller ship can't just cruise to Pluto by gassing up first.

Any advice on "deteching" some of the stuff in Spaceships or any similar settings I can mine for ideas?

Edit: Another question. What kind of fuels would ships be able to use? My understanding is U-235 is kind of scarce off of Earth, and if it's too expensive to acquire, it doesn't really make sense to use as a propellant.

Edit Again: Thought I'd put this in -

Nuclear Thermal Rockets - Seem too high thrust. I don't want the big ships to be able to handle like fighters, and they have too limited of delta-v. That and the fuel is too cheap.

Orion Drives - Just no. I can't envision any universe where these would be accessible to private citizens.

Fusion Pulse Drive - No fusion, for starters. Also too fuel-efficient. Could a fission based alternative exist and be feasible?

Chemical Drives - Probably good for small short-ranged ships and shuttles, but don't really have the legs to even transit meaningful distances around the Earth neighborhood without having to justify huge fuel uses, if not multiple stages. Also, burn through their fuel tanks in minutes, which is just boring, because there's only so many times you can do Apollo 13 to add excitement to a trip that's basically ballistic.

Really, the ideal drive systems would be something that can sustain tenths of Gs, uses a fuel efficient to make either a long slow trip from, say, Earth to Mars or a fast trip from, say, the Moon to some space station a few tenths of AUs away in a stellar orbit, and most importantly, is something that exists on a drawing board or at least uses a concept that won't get torn to pieces, because I have players that include a few That Guys who will literally learn rocket science just to poke holes in things.

[AlexanderHowl]

U-235 is never a propellent, it is the fission fuel that heats the propellent, and drives that release U-235 are weapons of mass destruction rather than propulsion systems... I would suggest looking at Atomic Rockets, it has a whole bunch of fission designs, and most of them are realistic.

When it comes to drives, 216 MJ/kg of reaction mass gives you a performance similar to HEDM drives. U-235 fission produces around 0.86 MeV per amu, which translates to ~83 GJ/gram (meaning that every metric ton of reaction mass require 2.6 g of U-235). Without fusion or antimatter though, you cannot have small scale fission, so you need around 10 kg to create sufficient fusion to burn 2.6 g per second, though that will turn into nuclear detonation if you are not careful. A 10,000 metric ton spacecraft lifting from the Earth would require an average of 10 metric tons per second of reaction mass, meaning that you could have to burn 26 g per second (generating an average of around 2.16 TW of energy).

[a humble lich]

I would look at a couple options (some of which you already mentioned, but I thought I'd mention them too for completeness):

• Nuclear Thermal Rockets --- These are the most plausible, given that such engines were actually built tested in the 1960s. If you think they have too much thrust for your setting, perhaps they could be used for fighter craft?
• Open Gas Core Reactor --- The efficiency of solid core nuclear rockets is limited because the exhaust velocity is limited by the temperature of the propellent, which is limited by the temperature of the reactor core. If the reactor get to hot, the core melts. This is generally considered to be a bad thing. Gas core designs can allow much higher efficiencies then. If you want more expensive fuel, an open core design would loose nuclear fuel out the back of the rocket. This would make the exhaust very radioactive and the fuel more expensive. A closed cycle design could also work for you but they also have the disadvantage of cheap reaction mass. Both types have the disadvantage of being advanced technology which has never actually been built.
• Ion --- Instead of nuclear rocket you could use some form of ion/plasma rocket powered by a fission reactor. These wouldn't have the thrust you need, but like the nuclear thermal rockets they are very plausible because they have been actually been built.
• Chemical Rockets --- Chemical rockets could be used for fighter craft. They could also have a niche because they are much cheaper then nuclear rockets.
• Fusion Pulse Drive --- I wouldn't worry too much about these being fusion rockets, because the basic technology to make something like this exists at the National Ignition Facility. Now the lasers at NIF use more energy then is produced with fusion and NIF is a long way for being an efficient rocket, but the fusion part can be done. The stats given in spaceships seem very generous for a realistic near future version though.

Basically, if you want something between a nuclear thermal rocket and Fusion Pulse drive I'd use a gas core nuclear rocket.

I also second AlexanderHowl's suggestion to look at Atomic Rockets.

[Fred Brackin]

Originally Posted by JackFalcon13

S

So, a few issues. One, no fusion power. So how do ships relatively quickly and efficiently transit around? .

They don't. They don't even get around quickly and efficiently in the Transhuman Space setting which is significantly higher tech than what you appear to have in mind. You may be setting the tech too low for your concept to be even possible. If we had to launch into space today and build even a bare minimum of self-supporting space stations or die we'd just die.

Oh, and your reason for not allowing nuclear saltwater should be that they would blow up and kill anybody who tried to assemble enough fuel before they even had a chance to blow up and kill anyone who tried to use one.

[AlexanderHowl]

Even if they worked, they would irradiate an area the size of Delaware with every liftoff.

[a humble lich]

Originally Posted by AlexanderHowl

Even if they worked, they would irradiate an area the size of Delaware with every liftoff.

At minimum, the nuclear saltwater rocket is basically a marriage between an orion drive and a firehose. There aren't too many designs that make an orion drive seem like the practical and safe alternative.

[AlexanderHowl]

Originally Posted by a humble lich

At minimum, the nuclear saltwater rocket is basically a marriage between an orion drive and a firehose. There aren't too many designs that make an orion drive seem like the practical and safe alternative.

Pure fusion orion drives are actually not that bad (depending on the fuel). A pure fusion orion that used the fusion of a DD fuel pellet surrounded by iron would actually have minimal environmental effects. Even fission ones are not insanely dangerous and are probably less dangerous than any other fission design (we have plenty of practice blowing up bombs).

[JackFalcon13]

Thank you both. I should have been a bit more clear - the "no fusion" means that there's no viable fusion reactors, so energy is still either big solar panels, fission reactors, or RTGs. The adventure I ran actually involved extracting a scientist who had made a step towards a viable fusion reactor (and ended in the players executing him for reasons I'm sure made sense at the time, but hey, they had fun). I'm just trying to avoid "pure fusion" anything, be it reactors or missiles. It let's me keep things like beam weapons and railguns as cool exotic plot devices and not everyday things.

I did skim Atomic Rockets a few times, but neither browser I use makes it exactly a fun reading experience, and I'm still a few math courses short of any of their "stat blocks" being intuitive to me. I'm learning, though, mostly thanks to keeping a delta-v calculator open.

I really like the open gas core design. It actually fits with something in the setting, that being there's a "no nuke" zone around space stations, thus giving jobs to shuttle pilots. Me being terrible at math, can you guesstimate what kind of thrust this thing would provide? I'm trying to avoid ships that will basically be in microgravity during transit, mostly because it's boring as hell for space combat unless you run it like a boardgame where each turn is "Well, anyone want to train their gunnery skills for the week it will take for the enemy to catch up?"

It also seems to create a cool scenario where some fuels can make it smaller and more efficient, and if I'm reading it right, Americium 241 can only expensively created but would be a great power source. I kind of like that as a plot point. "Based on the signature of that drive system, the super warship that killed your parents/wife/hamster could only be powered by one fuel source. And there's only two places that make it!"

I've made mention of "Ferries", which got heard as Fairy's by the guy who takes notes. Basically, a giant ion drive with docking points for smaller ships and spin gravity habitation areas. If you want to change neighborhoods, you pay to ride on one of these and enjoy the cruise rather than spend three months in a tin can designed for a one month trip.

Part of what I'm aiming for is a "sweet spot" where a short trip between stations that might be may be tenths of an AU apart actually benefits from acceleration rather than huge delta-v, but interplanetary travel requires the latter rather than the former. I realize it's a tall order, but I like to be as internally consistent as possible. If for no other reason than showing off if somebody questions something.

[Anthony]

The first question has to be how fast you really need people to be going; you can actually reach anywhere in the solar system with a NTR, though a nuclear lightbulb is somewhat more convenient. Nuclear pulse drives can be quite easily fission based, they're mostly modeled on ICAN-II and AIMSTAR, both of which have a fission stage.

[JackFalcon13]

So, setting wise, the solar system is in rings of space stations. The stations themselves are broken into spirals - so if you imagine a five legged octopus (a quatropus?) moving from one leg to another would be changing spirals, moving up or down a leg would be changing rings. A given ring/spiral combination is a "neighborhood". Basically, chemical is enough to travel around a neighborhood, but transiting neighborhoods is more for nuclear drives, and transiting up or down a ring is more difficult than going from one neighborhood to another. Going multiple steps at once is more an ion drive issue and requires spin gravity rather than thrust gravity.

Basically, ships need a mix of delta-v and thrust that means they stop to refuel at each "neighborhood" along a spiral if they want to move up or down it, but at the fastest rate possible. So, the nuke drive should make each hop faster than a chemical engine, but not be so efficient as to being able to skip steps without a lot of coasting.

I'm not entirely clear on how realistic this is, and I realize this is very much trying to force science into a setting, so I'm sure things need to be massaged a bit. And for the record, if someone wanted to make an Orion drive launch site somewhere far from people, I'd be fine with it. I probably get more radiation a year from the fact my walls are stone. I'm not okay with nuclear saltwater existing anywhere.

Edit: The other reason I want decent acceleration - it makes combat more interesting. Shooting at a ship with an acceleration of an ion drive is also known as plotting a Moon mission with a slide rule. Combat is pretty boring if it's just broadsides at 1 AU.