My G-G-Generational Ship

Agreed fusion isn’t likely in 10 years- but it’s much closer than a millennium away.
A fusion powered propulsion device isn’t completely infeasible, although the details are beyond us right now.

Agreed also that time of flight is an issue.
If it take thousands of years to get anywhere, humans aren’t going to do it.
But something on the scale of 50-100 years, maybe 250 years, is not beyond the imagination and willpower of our species.
And that comes back to the propulsion method.

Hey, how many times can you gravity-assist?
Can you go back and forth between say, Jupiter and the Sun, gaining speed on each pass?
I’m sure there’s a point of limiting returns- but it’s something for nothing, just tacking around the Solar System.
The heat issue, that’s interesting. But really, it’s just engineering.
We’ve understood thermodynamics for a long time, we know what to do about heat.
First guess: pre-heat the reaction matter with waste heat from the rest of the ship.

Manned or unmanned, the problem with the propellant to mass ratio remains. There is just no feasible way, using conventional thermalized propulsion–even with confined nuclear fusion–is going to achieve sufficient I[SUP]sp[/SUB] to carry the requisite mass of propellant. As for gravity assist maneuvers like those used for probes to the outer planets, the primary benefit is not added speed but allowing for radical changes in direction. The limit is both the amount of momentum transfer possible from the planet being transited to the spacecraft and solar escape speed at that distance from the Sun; obviously, if the spacecraft achieves escape speed, it will not be returning for another pass. The Jupiter and Saturn swingby trajectories for the Voyager 1 are about the optimal for achieving maximum escape speed from the solar system without active propulsion. And note that it isn’t sufficient to just get the spacecraft going in some direction toward a target; it is then necessary to match velocity of the target once it is reached so that the craft doesn’t just fly past, which depending on the target may have a relative velocity delta of up to 150 km/s. Reaching another star system with a probe using conventional propulsion technology will take a period which well exceeds the life span of any civilization to date by at least an order of magnitude.

No one has yet mentioned Bussard-type interstellar ramjets, which use some method of collecting matter from the interstellar medium to use as fuel for nuclear fusion and propellant. The problem with this (aside from some of the major technical challenges of even constructing and managing such a system) are that you already have to get up to 1-2% of c to collect enough mass to achieve sufficient fusion to exceed drag from the interstellar medium, and thus, you end up with a bootstrapping problem.

But that is exactly the point; the “heat issue”, as you called, is not “just engineering”; it is a fundamental limitation of the thermodynamics of a closed system with a finite heat capacity. Even with a perfect blackbody radiator, the amount of radiative surface is going to have to be huge in order to reject all of the waste heat which would be produced by an active propulsion system and habitat into the cosmic background, and the other alternative. Your proposal to “pre-heat the reaction matter with waste heat” misses the point; the entire purpose of the propulsion system is to heat the propellant to as hot a temperature as possible in order to maximize propulsive efficiency. The inevitable residue of this–the deviation from perfectly adiabatic (fully insulated) heating contributes to heat on the vessel above and beyond that produced by maintaining a habitat, and trying to pump that heat up to a temperature that will further heat the propellant simply results in more waste heat to be rejected. Our everyday experience is living in an environment where natural cold temperature reservoirs are readily accessible and not having to worry about thermodynamics except in the limited confines of heat engines such as power plants and car engines, which can be easily transfered to the environment by convection to air and water. This experience fails to give conception to what it would be like to live in a truly closed environment.

The distances, speeds, and energies required for interstellar transit are literally astronomical, and everyday experience totally fails to give perspective on the requirements to accomplish this task. “Imagination and willpower” are by themselves not sufficient to overcome the fundamental physical limitations inherent in achieving the necessary velocity changes and sustain a habitable environment to transit interstellar distances on the order of human or even societal lifespans, notwithstanding building systems which will continue to function for millenia without replacement. There is simply no technology extant or in the foreseeable future that will mitigate these issues. There are theoretical possibilities which may someday allow for spacecraft that can generate sufficient propulsive power or create shortcuts to other arbitrary points in space, but we simply have no idea how or even if these can actually be made to be workable.


We’re obviously not sapient enough yet. Duh.

We might be able to colonize Luna.

Is it not better that we just gracefully go extinct and fail to infect the galaxy with our particular brand of stupidity?

Again, Stranger points out what people have a hard time understanding. Rockets–even ion rockets, even nuclear rockets, even fusion powered rockets, even total matter conversion rockets–suck.

It doesn’t matter that you’ve got a fusion reactor or a total conversion reactor onboard. You still have to heat up reaction mass and shove it out the back. And you’ve only got X reaction mass. And that means you have a limited amount of specific impulse. You can’t just turn the hyperefficient exotically powered rocket engines on full and blast away, because you can’t carry enough reaction mass onboard to blast for very long.

This is why Heinlein style torchships are impossible. Yeah, total conversion of matter to energy would be pretty sweet, and make for a kick-ass rocket, and total conversion doesn’t require any violation of physical law, you just make a bunch of antimatter. But you can’t just point your torchship at Tau Ceti, blast at 1 gravity until halfway there, flip around and blast at 1 gravity to slow down, because you can’t carry enough fuel. It cannot be done with any sort of rocket. This is not an engineering problem, this is a problem with the laws of physics.

If tomorrow we discover new laws of physics that enable us to warp space and time and dimension and god-knows-what-else, then hey, free lunch! We can head for Tau Ceti the very next day. But that requires a totally new understanding of how the universe works. And it seems pretty likely that we’re not going to rewrite the laws of physics any time soon, so if your starship has a dependency on new laws of physics then maybe you should hold off on designing it just yet.

Even if the mission where to build an airtight colony on Baffin Island, I wouldn’t bet on it.

Agreed and understood.
But let’s say we’ve got the fusion rocket mentioned upthread, with hydrogen as reaction mass.
That’s as efficient as we can get in the near future, using plausible technology.

But can we take enough fuel to thrust for a year? Or maybe five years?
And how fast does that get us going? On top of gravity slingshotting?
I think 0.1 c might be fast enough to get the job done- how impossibly out of reach is that?

(Been a long time since I’ve done this sort of math, so any help is appreciated!)


It’s not a closed system, you can dump or pump heat out at any time in any direction.


The interstellar medium is nowhere near as rich as Dr Bussard assumed.
It’s not practical as a fuel, you can’t scoop it fast enough.
You can and should use the idea to power cosmic ray deflector shields though.

You seem to either be not reading or not understanding what I am writing. Short of any sort of technomagical breakthrough in propulsion (e.g. being able to convert matter directly to directional photons) we simple cannot achieve rocket exhaust velocities to achieve a specific impulse which would make it feasible to reach even a significant fraction of a percent of c. Getting to 0.1c is not remotely possible with any extand or forseeable propulsion technology. Carrying more fuel does not help this problem, as this is just more mass that has to be accelerated until the time at which it is used, requiring more tankage, propusion system, et cetera. Momentum transfer via gravity swingby maneuvers simply cannot contribute significantly to the spacecraft speed compared to speeds required for interstellar transit as previously discussed. The few km/s that can potentially be gained before escaping from the system are dwarfed by speeds on the order of 10[SUP]4[/SUP] km/s to achieve even a small fraction of c.

A spacecraft may be treated as a closed system for all practical purposes pertaining to internal heat generation. It is insulated by vacuum with no direct medium or cold temperature reservoir to transfer heat by convection or conduction. Most real world materials out of which one would construct a spacecraft are not optimal for radiating heat, and unless the spacecraft is designed as a gigantic hollow sphere with all of the waste heat generating activities on the outer skin it will not naturally be able to radiate away heat. This will require radiators, like those on the interior of the Shuttle Orbiter payload bay doors, except instead of having to radiate away the modest heat from low power fuel cells and a handful of occupants for a couple of weeks, the vessel would have to radiate the waste heat generated by a powerful propulsion system and a closed-cycle habitat for millenia. I realize this seems like an arcane problem that should be solvable just by building a better HVAC system or throwing a bunch of heat pumps somewhere in the system, but it actually turns out to be a critical architecture problem even for small relatively low powered satellites and spacecraft that we launch today, and there are fundamental limitations on how much heat can be rejected into the cosmic background for a given amount of radiative surface area.

The problem with a fusion ramjet is not that the interstellar medium provides insufficient material; it is that in order for a ramjet to work, it has to be moving at a minimum threshold speed relative to the interstellar medium in order to compress the collected mass to fusion conditions. This threshold speed depends somewhat on the design and efficiency of the system, but it will be at least 1-2% of c, which again, is signficantly faster than the speed achievable by any practicable propulsion system.

I know we’ve had nearly a century of science fiction books, television, and movies that make it seem like going to other star systems is just a slightly bigger challenge than going to the Moon and that if we just put our backs into and launched on a decade-long effort to improve propulsion technology it the galaxy would be at our feet, but once you start to look at just the magnitudes of distance, time, speed, and energy–which requires nothing more complicated than basic exponential arithmetic and simple algebra–you begin to understand that the scale of the problem is many orders of magnitude greater than anything we can do with conventional technology, even without delving down into the specific technical issues and fundamental physical limitations of materials and power-generating and propulsion processes. I’d like to imagine going to distant worlds and exploring new star systems, too, but the reality is that without some fundamental breakthrough in physics, physically going to other star systems, or even sending unmanned probes within anything like a human lifetime, is beyond feasibility.


Stranger on a Train, just expressing my appreciation for the knowledge you are bringing to this thread. I’ve always felt in these long distance manned space exploration threads that there was a lot of handwaving and glossing over technical problems, and I appreciate your effort to keep this discussion grounded in real world problems.

Same here, we do much appreciate the input.

Getting to 0.1c is not remotely possible with any extand or forseeable propulsion technology.

Lightsaill? NASA and the JAXA have both sent them into space, the Japanese one being interplanetary even. It wouldn’t work for generation ships, but perhaps a probe?

The problem with the lightsail is a thing called the “inverse square law”: from your light source (presumably the sun), the effective pressure on the sail drops rapidly with increasing distance. You would have to keep spreading out the sail to maintain a linear (non-diminishing) acceleration.

Oh, I’m understanding what you are saying. And appreciate you taking the time to say it. :slight_smile:
You believe it’s impossible. And you might well be correct,you’ve got science on your side.
But I think maybe I’m asking the wrong question…

How fast can current/near future tech get a thing going?
10% of light speed is out, accepted. Can we get up to 1% of c?
Let’s assume fusion power, hydrogen fuel, but nothing really exotic.
Ion drives won’t be powering a star ship in the next few decades, though they may one day.

(Aside, could someone point me at an acceleration calculator?
I used to know how to do it on paper, and could re-learn. But I’m sure there’s a calculator.
Say you want to reach 1/10 c in five years, how fast do you need to accelerate? Like that.)

Velocity from acceleration is just v = v[SUB]0[/SUB] + 1/2at[SUP]2[SUP]. (Technically at speeds approaching c you have to account for relativistic effects, but even at a final velocity of 0.1c those effects can be neglected in a first order calculation as they will be lost in significant figures.) If you want to know how much propellant mass that will take, you have to go to the Tsiolkovsky rocket equation (scroll down about a third of the way, just below the first integral). You can see that for a rocket with an I[SUB]sp[/SUB]=10,000 seconds, the propellant mass to final mass ratio ends up being in the trillions to get to even 0.01c. Even at I[SUB]sp[/SUB]=20,000 s, the mass ratio is in the millions. You have to get to somewhere around I[SUB]sp[/SUB]=80,000 s before you get to a mass ratio (~45) that are even plausible to accelerate to this speed, and this doesn’t consider having to also carry enough propellant to decelerate at the other end, which for a first order approximation squares the ratio, e.g. a ratio of 45 to accelerate is ~2000 to decelerate. (Again, this is regardless of actual mass or thrust of the spaecraft; scaling it up to add more propellant or more engines doesn’t help.)

A photon or magnetic particle sail doesn’t help for the reasons already described, although as a craft using one doesn’t have to carry onboard propellant it does make it propulsively infinitely efficient provided you don’t mind taking years or decades to make a significant course change in a relatively small payload. However, an electrostatic field could be used to slow the ship via drag with the interstellar medium with minimal energy expendature (akin to letting your car slow itself to a stop via friction rather than using the brakes), albeit at the expense of making the trip significantly longer.

But all of this is radically beyond what could be feasibly done in the next few years given our current state of the art in which it is still challenging to even send a few people to our nearest natural satellite or a modest mobile probe to Mars. I think we will eventually send probes to explore beyond the solar system, but they won’t be manned (at least, not in the manner depicted in science fiction) and they won’t look anything like the relatively crude machines we send now. They will likely be something like Freeman Dyson’s “Astrochicken” concept or von Newmann machines; small, light, capable of self-repair and propagation. Unless we discover a way to manufacture and stabilize wormholes, develop some drive capable of warping the fundamental plenum of spacetime, or otherwise subvert the laws of physics as they are currently understood (for which there are theoretical possibilities but no way to effect them that we currently know of) we’re stuck with reaction engines and transit times on the scale of thousands of years.


Can I post a followup on teh thermodynamics question?
Because you’re right, I totally don’t get what you are saying.
First let me say I am imagining the ship is made from spherical modules on a central truss, not concentric cylinders like Heinlein’s Vanguard. (1)
But I understand that doesn’t help the heat issue- radiating into space is nearly impossible.
Even with 100x the surface area, passive radiation isn’t going to do the job.

But meanwhile, there’s a huge heat trail.
We are shoving heat and matter out the back of the thing.
Why can’t we include our waste heat in that heat trail, and leave it behind us?
That’s the part I don’t get- why do you consider the system closed?

Side thought, that trail will linger like a fart in an elevator.
Any alien race that stumbles across the trail has an unmistakeable beacon pointing at Earth.
In the scenario of this thread, Abandon Earth!, it’s ok, but might be a consideration for general exploration.

(1) The Vanguard is a sweet ship though, and the matter converter is a glorious handwave.

Well said! You we’re gonna pack in ice and send out to the stars as the best our planet has to offer. Ultimately of course, that means a rotting or mummified carcass.

It should be understood that heat is a form of energy that is statistically randomized; that is, it has no predetermined momentum (unlike a flying bullet) and will tend to flow toward the coldest area of the system. We measure hot and cold as temperature, but temperature is not actually a measure of energy or thermal capacity, but rather a measure of activity in the system which gives the preferred direction of heat flow (heat flows from high temperature to low temperature).

The exhausted propellant is as hot as the system can make it to maximize efficiency, i.e. the higher the termperature, the more thermal energy it has, and thus the more momentum can be transferred from the escaping and expanding exhaust. Because it will be at a higher temperature than anything else on the vessel (presumably) it will be transferring heat to the vessel before it can be ejected because the second law of thermodynamics wants everything to be in thermal equilibrium (the same temperature). The only way to take heat internal to the vessel and transfer it to the exhaust is to heat it to a higher termperature than the exhaust, but this requires more work and thus creates even more heat. We just can’t move heat wherever we would like it to go; we have to elevate the temperature somewhere within the system if the goal is to cool another part of it, which is the fundamental limitation. This is how a heat pump or refrigeration cycle works; it pumps thermal energy at low temperature to a higher temperature via compression processes, but in doing so it creates more heat, which is why the cooling coils on an air conditioner or refrigerator (which is just a heat pump cycle in reverse) are on the outside, using the ambient air as the rejection medium for both the heat that is pumped out and that produced by the work done by the compressor.

The boundaries of a “closed system” are always somewhat arbitrary as there are no true closed systems in nature but we can draw a boundary around the spacecraft outer mold line across which the only heat transfer will be the thermal energy of the exhaust plume and radiation to the cosmic background. As we can’t push our extra heat into the exhaust as explained above, all we can do is radiate it into space. However, the amount of heat that can be radiated is limited by the exposed outward-facing surface area (surfaces that radiate toward each other don’t help) and the emissivity (a fractlional quality of the effecacy of the radiating surface) of the vessel, plus whatever internal processes we need to deliver excess thermal energy from the interior of the vessel to the surface. For any significant work producing processes such as generating electrical power, distilling water, reducing carbon dioxide, or otherwise doing any of the other things necessary to sustain a habitat, excess heat is generated and has to be rejected lest the internal temperature become greater than can be endured by the inhabitants, but in order to pump heat out, we have to compress it and make it higher termperature, which creates more heat, ad nausum. In the end, you cannot sustain the production of more thermal energy than can be radiated away from the vessel, and this inevitably requires massive amounts of outward-facing surface area. Aside from making the emissivity of this surface as close to unity as possible, there is just nothing that can be done by technology to improve or avoid this situation.


Yep, I guess this is what I was missing, thanks.

Doesn’t really help if the engine down at the end of a long boom either, heat still transfers up.

Initial premise-busted.
Ongoing conversation about what can and can’t happen-better than my original op.

Which way is “up”?