This weekly roundup thread is intended for all culture war posts. 'Culture war' is vaguely defined, but it basically means controversial issues that fall along set tribal lines. Arguments over culture war issues generate a lot of heat and little light, and few deeply entrenched people ever change their minds. This thread is for voicing opinions and analyzing the state of the discussion while trying to optimize for light over heat.
Optimistically, we think that engaging with people you disagree with is worth your time, and so is being nice! Pessimistically, there are many dynamics that can lead discussions on Culture War topics to become unproductive. There's a human tendency to divide along tribal lines, praising your ingroup and vilifying your outgroup - and if you think you find it easy to criticize your ingroup, then it may be that your outgroup is not who you think it is. Extremists with opposing positions can feed off each other, highlighting each other's worst points to justify their own angry rhetoric, which becomes in turn a new example of bad behavior for the other side to highlight.
We would like to avoid these negative dynamics. Accordingly, we ask that you do not use this thread for waging the Culture War. Examples of waging the Culture War:
-
Shaming.
-
Attempting to 'build consensus' or enforce ideological conformity.
-
Making sweeping generalizations to vilify a group you dislike.
-
Recruiting for a cause.
-
Posting links that could be summarized as 'Boo outgroup!' Basically, if your content is 'Can you believe what Those People did this week?' then you should either refrain from posting, or do some very patient work to contextualize and/or steel-man the relevant viewpoint.
In general, you should argue to understand, not to win. This thread is not territory to be claimed by one group or another; indeed, the aim is to have many different viewpoints represented here. Thus, we also ask that you follow some guidelines:
-
Speak plainly. Avoid sarcasm and mockery. When disagreeing with someone, state your objections explicitly.
-
Be as precise and charitable as you can. Don't paraphrase unflatteringly.
-
Don't imply that someone said something they did not say, even if you think it follows from what they said.
-
Write like everyone is reading and you want them to be included in the discussion.
On an ad hoc basis, the mods will try to compile a list of the best posts/comments from the previous week, posted in Quality Contribution threads and archived at /r/TheThread. You may nominate a comment for this list by clicking on 'report' at the bottom of the post and typing 'Actually a quality contribution' as the report reason.
Jump in the discussion.
No email address required.
Notes -
I think the idea of biological humans colonizing Mars is silly. It's very likely easier to make a strong AI than colonize Mars, certainly more profitable. Send robots and develop Mars. Or move inwards, there is lots of solar power closer to the Sun.
Likewise I've always been suspicious of chemical rockets. If it's not nuclear, why bother leaving Earth's gravity well? Chemical rockets are just too wimpy for serious space travel. Develop fusion first, then move out.
Now, obviously I love my LV-N as much as anyone. But fundamentally, the tyranny of the rocket equation can not be escaped by providing amazing energy density, because it is mostly not about energy density.
Fundamentally, rocket engines are characterized by two quantities: their thrust -- how much force they can deliver -- and their effective exhaust velocity (or specific impulse), which basically states how efficient they are at converting reaction mass into thrust. v_e is one of the factors which goes into the rocket equation for the delta v budget, the other being the logarithm of the initial and final mass quotient. If you want more delta v, trying to get to a higher v_e seems obvious.
For vanilla chemical rockets, the energy for accelerating the exhaust gas comes from having a fuel and an oxidizer react thermally. This puts some limitations on the exhaust velocity because chemical reactions will only yield so much energy per unit mass (especially as more exotic reagents like FOOF would have their own problems).
The nuclear thermal rocket avoids this energy bottleneck. The problem is that with hot gasses, the next bottleneck is right around the corner: you need materials to withstand the temperature. Energetically, you could just heat your reactor to 10000K (if your reactor can use fast neutrons, at least) and get an amazing exhaust velocity. [^1] Too bad your tungsten pipes will melt at 3700K, though.
The other thing you might do is just to forsake using thermal exhaust. You simply build an ion accelerator open at the downstream end (at least vaccum will not be a problem) and point it into the direction you do not want to go. If you have energy, there is no limit on how high your v_e can go.
Of course, the downside of ion drives is that their thrust is very tiny. This is a problem if you want to get somewhere before you die of old age. [^2] And using nuclear power over solar is not going to fix that -- even particle accelerators which we build on Earth, where we can just take power from a socket have abyssal thrust to weight ratios.
In conclusion, the use of nuclear energy (either fusion or fission) might help a bit, but it will not help you to escape from the tyranny of the rocket equation.
[^1]: Of course, what you get is the exhaust velocity, but what you pay for in energy is proportional to the exhaust velocity squared. This unfair business practice lead to a class-action suit against physics which was settled in 1905. Now as your exhaust velocities approach c, you the energy costs of marginal momentum get constant. Consumer advocates claim that this is a bad compromise because most households have exhaust velocities much lower than c and do not benefit from this at all. Attempts to lower c have been met by resistance of both high school students (who prefer Newtonian physics for real world problems) and gamers (who fear their ping times will increase).
[^2]: Additionally, in KSP, you can not speed up the simulation while you are accelerating. This makes missions which rely on ion drives rather cumbersome. (Though they would be even more cumbersome if the devs had not increased the thrust of the Dawn engine by a factor of 2000 over their Earthly counterparts.)
Why would we need to escape the rocket equation? It's like going from horses to cars. Both need fuel and both have limits in speed and transport capacity. But cars are so much better than horses. The range of a car is much greater than the range of a horse.
I know that fusion rocketry is very difficult, getting a magnetic nozzle or similar advancements has not been achieved. Fusion (besides Orion-style) has not been achieved. But that should be what we aim for.
Neither horses nor cars are propelled by the physics of the rocket equation. The rocket equation is an exponential (or a logarithm, depending on which way you arrange it). It provides a hard limit on performance that cannot be hand-waved away. You say, rightly, that future technologies can perform better. This is true. How much better? What are the numbers that we can plug into the rocket equation in order to compare to the other numbers that we can plug into the rocket equation? It is only then that we can really get a sense for the scale of how much better future technologies can be.
Ironically enough quiet_NaN doesn't actually do that, he just gives general exposition about the difficulties of spaceflight. Misleading, in my view, since energy density is vital, that's the fundamental essence of the entirety of rocketry. Nuclear fusion based rocketry would not just 'help a bit' but provide enormously greater capabilities.
We all agree that nuclear rockets are far more effective. Trying to plug in numbers to the equation is useless at this phase because we don't know how heavy a fusion rocket will be, nor what kind of exhaust velocity can be achieved. We don't have any such rockets. But we do know that chemical rockets are extremely slow and inefficient. They're unsuitable for serious space colonization (as are human bodies in my view).
Stepping back and taking a very broad view, there are several steps to the research, development, and engineering of a system. Generally, one begins with physical principles. With those physical principles, one can compute theoretical limits. One can also sketch a concept of operation based on those physical principles. Often times, at that point, one can still handwave away many practical concerns and compute how close a concept could, in theory, get to the raw theoretical limits. As one progresses, one may include an increasing number of more real-world difficulties.
For nuclear rocketry, we are not building on a blank slate, as though no one has ever started down this path at all, as though we simply have no idea what the theoretical limits are or what the concept-based performance could look like (still handwaving away many practical considerations). People have been doing this work and publishing it for half a century.
Do you agree or disagree with this general picture?
I'm not agreeing or disagreeing, it's just not relevant.
I am saying these things:
Quiet_Nan was saying
Neither point matters in relation to my argument. I think we all know that fusion rocketry is difficult.
You are asking in response:
But that also doesn't really matter to my point. As long as it's significantly better than chemical rocketry, which it is, then that makes it a better option for long-range spaceflight, since it can do the work and chemical rockets can't.
I don't understand your somewhat patronizing approach of asking about concept-based performance. I don't need to cite a specific fusion design to know that fusion designs can provide much more capable rocketry. That's inherent given the nature of fusion vs chemical rocketry. We already know this. There is plenty of variance between designs and some may just not end up being workable.
Trying to explain specific impulse, thrust vs delta v to me is wholly irrelevant to the substance of what I'm saying!
Lets use another one of your examples. Cars are much better than horses. Does that imply that cars are a better option for long-range spaceflight? If you think this statement doesn't quite make sense, try to explain without reference to any first principles, conceptual designs, concepts like specific impulse, thrust, and delta-v, etc.
Alternatively, to hone in really narrow:
How do you know that it can do the particular type of work you're asking it to do? Wouldn't it be nice if you had some reasoning, from first principles and/or conceptually, which could inform you as to whether it is plausibly up to the type of task you're asking of it? Some sort of check to see if you're accidentally expecting a car to go to the moon, just because it's better than a horse?
You know what's even better than first principles reasoning? Already knowing the answer. I already know that fusion rocketry designs can perform long-range spaceflight, to the edge of the solar system at least without being restricted to tiny payloads. You can do a quick internet search if this is new information to you.
More options
Context Copy link
More options
Context Copy link
More options
Context Copy link
More options
Context Copy link
More options
Context Copy link
More options
Context Copy link
More options
Context Copy link
More options
Context Copy link
More options
Context Copy link