Advice on spacecraft design sequence

[Omnivore]

Use a denser fuel - ion engines using xenon give quite a high impulse IIRC.

Denser fuels help with thrust levels but not with Isp which is another way of saying exhaust velocity. That's why you see the highest Isp figures with the electric drives (ion/hall effect/MPD/VASIMR/etc) coming from the lightest fuels and the highest thrust with those same drives coming from the heaviest fuels. Certain potential fuels are a bad idea because of corrosive effects or rarity.

With respect to theoretical drive systems we know about today that don't cause a physicist to have a mental breakdown, there are basically four categories:

1. Chemical fuel
2. Electric thruster
3. Thermal rocket
4. Reflected energy

I include metastable fuels in the chemical fuel camp, reflected energy includes everything from Orion to solar sail including inertial containment fusion.

For interface operations we're stuck with 1 for the near future though possibly some forms of 3 may give us a way out. From LEO to 10AU, advanced forms of 2 look nice though 3 and 4 might be competitive.

With the Dark Stars setting, I'm mainly concerned with the LEO to 10AU range since that would cover the majority of non-FTL commercial traffic in the setting. I'd expect in that setting to see robotic deep space bulk cargo haulers using some form of 2 (likely fission power driven VASIMR) or perhaps 3 (advanced NTR variants). Passenger traffic may end up using the same though at higher thrust levels. At some point IC-Fusion versions of 4 (advanced ICAN/Daedelus) might be a viable choice for high priority freight, including passengers. Trouble is most forms of 3 and 4 tend to have nasty radioactive exhaust trails.

Interface operations in civilized areas probably will use a combination of lift strategies depending upon cargo type, tech level, and economic level. Some form of space elevator or perhaps ferris wheel would be an option, otherwise advanced forms of 1 such as a SSTO multimode scramjet (TNE's AZHRAE for example) would probably be the choice for passenger traffic. There are some other interesting options for freight.

The 'fun' (fun for those of us who have no hair left to pull out) part is addressing wilderness operations such as player characters in a Dark Stars Traveller RPG campaign might well choose to undertake. In that scenario what we ideally want is a single craft that goes from surface to 10AU, and that is a big rabbit to pull out of a small hat.

The list of problems faced in the wilderness scenario are staggering. It is tempting to simply combine a LEO to 10AU choice with a carried craft to handle the interface but that fails to address some key issues. Combine the need for wilderness refueling with the need to be ecologically friendly (meaning burning up the landscape is ok, just don't turn it into a radioactive desert) and the problem seems insurmountable on any reasonable timescale.

There is one possible wilderness scenario solution I've identified, though I'd be the first to admit it's near the edge of possibilities given physics as we know it. In designing the Dark Stars setting, I'm trying to be strict on physics (except FTL ofc but even there I'm trying to stay within the edges of fringe physics), but I'm being much looser on engineering. This stands to reason since Dark Stars is a hard science fiction setting, not a hard science setting.

There is a form of 3 that we know will work. It's been prototyped and tested. Namely nuclear thermal reactor engines. Variations of this type of engine have been extensively studied, some of those variations are very interesting. First there's the idea of eliminating excessive cold start wear by using the engine as a nuclear reactor producing electricity when not needed for thrust (the Bimodal NTR design). Second there's the idea of boosting the thrust at expense of Isp with the LANTR design - by injecting liquid oxygen into the engine nozzle. Stretching that a bit, it may be possible to create a form of scramjet by replacing liquid oxygen with compressed air. Another design by Pratt & Whitney, the Triton design, combines Bimodal and LANTR into a Trimodal design.

The Trimodal NTR idea solves everything except Isp and the little matter of radioactive fission fragments in the exhaust. Higher Isp can be achieved by increasing the temperature. So liquid and gaseous NTR's were proposed, designed, and studied. The problem with these is that you tend to loose some hot uranium out the tailpipe. Not good for a bunch of reasons.

So some bright folks came up with the idea of a closed cycle gas core NTR, see the details here. There are a few stumbling blocks in that idea, but, and I stress this, they are engineering stumbling blocks not violations of physics. There are some very interesting features of nuclear lightbulb engines, they have their own built in fuel reprocessing for one, meaning they don't need refilled with radioactive fuels that often. The fuel of choice is liquid hydrogen and we all know what that means for frontier refueling opportunities. Unfortunately the NASA studies were done quite a long time ago and it hasn't been politically correct to revisit the idea.

A later series of design studies, restricted to the use of NTRs in the deep space propulsion role, called MITEE, had one variant that raises some interesting implications. Hybrid Electro-Thermal MITEE converted some of the hydrogen fuel into monatomic hydrogen, increasing the Isp and eliminating much of the need for radiators.

A Frankensteinian hybrid of Electro-Thermal MITEE, trimodal Triton, and a nuclear lightbulb NTR would nearly solve all of our wilderness needs, though about half the Isp gained in increasing the temperature with a gas core is lost due to isolating the uranium gas from the propellant. It looks like Isp would top out at around 3600 or so for a realistically achievable nuclear lightbulb engine sometime in the future. There may be a way around that though.

Since we're already in Frankenstein engineering land, one of the later nuclear lightbulb studies seemed to imply the use of ionized hydrogen. Well... if we have ionized hydrogen plasma and electrical power.. why not turn the nozzle of the Nuclear Frankenstein Lightbulb into the tail end of a magnetoplasmadynamic drive, turning the electrical power output of the engine into more Isp?

Now those last couple steps might have taken me off the deep end from theoretical engineering into violations of physics, answering some of the questions is beyond my pay grade. But it is interesting.

 

[drkem99]

I went through a similar process as Omnivore over two years ago when I developed some alternate maneuver and jump drive technologies for my campaign. He is quite correct that the plausible high-end fusion drive Isp values under discussion will allow reasonable transit times to the 1 AU jump points he has planned (I believe he is planning a 1 month transit time each way to his jump points. BTW, it's a credit to him to see he did his homework on the rocket science involved; not everyone does.) I think it is important, however, to consider the effects of such a drive selection on his ATU before proceeding.

I've already mentioned the mass ratio issue for reaction drives; if insystem jumps are not allowed, interplanetary missions to worlds not close to the system jump point(s) will require a significant fraction of the ship to be allocated to reaction mass (similar to the fuel requirements for high-jump OTU Traveller vessels - or even worse.) The ship design system will need to take this into account; it might be best if his jump drive technology did not require fuel but just power to operate. Even so, there's a lot less useable space for the players, their cargo, etc.; it becomes really hard to pay off a tramp freighter's mortgage with the smaller cargo bays. Gas giant refueling becomes really hazardous with low-thrust drives unless your ships also have powerful secondary gravitic or other type of drives; also, you need a secondary drive if your ships are to be capable of planetary landings. Fusion drives are hell on ecosystems. (The engineering spaces are getting really large...)

In addition, if more realism is the goal than the entire concept of needing radiators for waste heat rears its ugly head, especially if we are discussing the large amount of waste heat from many of the fusion drive designs in question.

Combat between starships becomes an issue as well. As a general rule, with realistic drives you can have high thrust or high Isp but not both. Without a high thrust drive, there are real issues with the interaction between closing velocities, maneuver, and weapon ranges and rates of fire. It's very easy to get a situation where the two ships moving towards each other can't appreciably adjust their course or velocity enough to really affect the combat. The two ships end up unloading on each other as soon as the ranges permit until one ship or the other is destroyed (sort of the starship version of an Irish stand-down.) AV-T shows just how delicate the balance is between these factors; the designer had to introduce engines capable of combat thrust values several times higher than cruise thrust (used for interplanetary distances) to allow enough maneuver to make the game interesting. The starship combat in your ATU could become more 'realistic' at the price of being less entertaining for your players.

Finally, the increased transit time has implications for your players and game universe. Just what are they doing on the ship during those months in transit to and from the jump point? Are you going to put them in cold sleep for transits? Consider that the week spent in jump frequently is wasted downtime for characters except for the occasional unruly or interesting passenger or cargo; my players used to hate jumps for just that reason. A number of science fiction writers and bloggers have observed that starships should travel at the speed of plot.

Even if your jumps are instantaneous, the increase in transit time to the jump points (one month each way instead of a couple of days maximum) effectively has moved the worlds in your ATU apart by a factor of six to eight times or more in travel time compared to the OTU. A freighter captain can only move a fraction as much cargo in a year. For a given amount of travel, your crew ages six to eight times as fast. Imagine trying to rule an Imperium six to eight times as large as the one in the OTU; by increasing the transit time, you have essentially created that situation. The only way I can see any interstellar government maintaining control over more than a few dozen worlds spread many months apart is if your ATU has some form of near-instantaneous FTL communication. (Look how much trouble the imperial powers of Europe had controlling their distant colonies during the Age of Sail on Earth.)

None of these things are intrinsically bad or good for your ATU, Omnivore, but you need to be sure when you consider these issues (and the many others I have not discussed) and their effect on your ATU that you end up with the background you need to tell the stories you and your players want to roleplay. Sometimes good logical and well thought-out hard SF backgrounds don't make the best setting for space opera like Traveller. I wish you luck with your design project; all this hard work and thought you are doing will really pay off for your players and you are to be commended for it.

Kem

 

[Omnivore]

None of these things are intrinsically bad or good for your ATU, Omnivore, but you need to be sure when you consider these issues (and the many others I have not discussed) and their effect on your ATU that you end up with the background you need to tell the stories you and your players want to roleplay. Sometimes good logical and well thought-out hard SF backgrounds don't make the best setting for space opera like Traveller. I wish you luck with your design project; all this hard work and thought you are doing will really pay off for your players and you are to be commended for it.

Thank you. Undoubtedly both interstellar economics and politics (including warfare) will be profoundly different in a Dark Stars setting than in the OTU or even a more normal ATU. In fact one of the driving forces in creating the Dark Stars setting has been to try and find the emergent behaviors - rather than trying to pound a preconceived notion of 'how things work' into the setting.

The nature of interstellar trade and commerce in Dark Stars seems to be headed towards more of a courier, negotiator, high value density item trade carried by manned spacecraft, with actual shipment following through by slower, largely automated, craft. Infrastructure in space seems to have a much higher value in a Dark Stars setting I believe. Transhipment points would likely be space stations constructed near the FTL 'highway' and high ports orbiting worlds in each system.

Interstellar communications in the Dark Star setting are probably on average the same as in the OTU, though communications between adjacent systems is much faster. Overall, the Dark Stars setting seems much more early steam era than the age of sail.

Warfare is another form of politics, and politics is about control. Control of what? Resources, people, and trade routes. In other words we don't have warfare to fight battles, we have warfare to enforce policies. The objects of worth in the game are (in Dark Stars) just as much the space transport infrastructure as the worlds themselves. Or at least so it seems to me.

However I think we can easily get too sidetracked about warfare in space, in most RPG groups large scale warfare is off stage - just a part of the setting mostly. What is important to the players is the small scale combat.

Still, I'm trying not to paint a picture and then force the setting to accommodate, once I work out the base technologies and ship design (first draft stage), I plan to solo game a bit in the setting in order to flesh it out and discover what makes sense and what doesn't.

Now I do admit to pushing the limits of engineering and physics as far as I feel I can get away with to see if I can make single ship wilderness operations a possibility. I believe I have the shape of a possibility in the Frankenstein Lightbulb drive - I do need a better name

I wish now I'd have taken up the US Navy's offer to become a nuclear engineer - it'd make it easier to figure out what is remotely possible and what is totally ridiculous. My background in electronics and computer programming isn't helping much here.

Dark Stars is turning out to have nearly as much Gurps:Transhuman Space as it does Traveller.

 

[drkem99]

If you plan to have orbital habitats at your jump points, the jump points will need to orbit a world or the star (not clear on the exact mechanism you are planning to use) so that your stations can orbit the same gravitational anchor. A static jump point (one that is fixed with relation to the star) will require the stations to use continuous or near-continuous thrust to hold their position with relation to the jump point. Gravity may be a weak force but it is a relentless one. (You can generate enough thrust using solar sails to hold such fixed positions but it does make the infrastructure a little fragile. Maybe Age of Sail was right after all. )

I think you should stick to dedicated interface vessels for your ATU. It lets you avoid using dangerous, powerful starship drives within a fragile ecosystem and it also makes planetary exploration more interesting from a story standpoint; not having an 800 dT mobile base with laser cannon and massdrivers on the ground with you makes life a little more interesting for the explorers. Plus, most starships in a more realistic ATU like yours where ship mass is at a premium can be lighter and unstreamlined (like Transhuman Space) if they do not need to enter atmosphere. Put automated refueling stations near your gas giant jump points that use robotic vessels to skim for hydrogen and your FTL superhighway even has gas stations.

I would not reject the AV-T drives out of hand for your purposes. Their primary technological magic revolves around their ability to not melt from their own waste heat; in a universe with FTL, that is a small handwave in the scheme of things. AV-T drives do not require you to have a vast infrastructure for antimatter production in quantity, they give you a variable thrust/Isp fusion drive with both combat and long-term cruise modes as well as a 3-D combat system you can 'borrow' for use in your ATU games, and finally, by selecting them, you can stop worrying about the details of the engines in your spacecraft and starships and move on to more interesting ATU design decisions.

 

[Omnivore]

I would not reject the AV-T drives out of hand for your purposes. Their primary technological magic revolves around their ability to not melt from their own waste heat; in a universe with FTL, that is a small handwave in the scheme of things.

And that is the precise reason I reject their use - FTL is a necessary handwavium plot device to have any sort of space opera - magical heat management in a setting that is trying to treat heat problems realistically is throwing the baby out with the bath water to fit a preconceived idea.

To me, hard science fiction (or perhaps better hard science speculative fiction) has at most one piece of handwavium in a setting, adding more than that and you might as well add in magical anti-gravity or whatever. I enjoy Traveller OTU, but I enjoy it as what it is, soft science soap opera. Dark Stars ATU is an attempt to go as far as possible in the other direction while still having interstellar civilization that is even remotely recognizable.

The Lares drive idea as initially conceived had a problem that, with the Lares regions orbiting the star, the pathway idea didn't make a whole lot of sense. When I changed that to stationary (relatively) regions, I introduced some other problems - throwing a bit of a wrench into the transit station idea by needed some form of station-keeping along with even more delta-V. It is highly likely to undergo another change, as since the FTL mechanism is the single piece of handwavium, changing it is far preferable to changing reaction drive physics.

Update: after a couple more hours of thinking and researching, I've made a minor change to the Lares idea - establishing a gravity threshold where the center points of the regions can be found. This threshold is where a = 0.015 m/s^2 or the surface of a hollow sphere with approximate radius 0.628 AU for our solar system. I think the transit stations might be viable with that level of station keeping - probably a combination of solar sails and solar powered electric thrusters. With that change, the travel distances between Lares regions and other Lares regions or habitable worlds comes out to under 3 AU. Some form of hibernation might be desirable, but not completely necessary with transit times of under six months.

Further Update: another option might be a slow cycle stutterwarp, something like a primitive J-drive with no fuel requirement and short transit times but a minimum of 1 light day and maximum of 1 light month per jump with a recycling and recharging period in between. Maximum distance per jump would be inversely proportional to local gravity, effectively dropping to lower than light speed inside solar systems. Add in easily detectable entry and exit signatures and that gets rid of the surprise attack factor.

 

[CliffBates]

STL solar sailers

Does anyone have any designs for a solar sailer, small I sh. I remember there was an article in some game journal about solar sailer in Traveller. I think the journal was White Dwarf.