(spaceships) rocket (super)science

[johndallman]

Originally Posted by jacobmuller

In a space opera setting - a "drive" system that "alters inertia" such that your 3G drive effectively gives you 3,000G but you still only feel the 3G, what does this do for delta-v?

As an alternative, you might want to adopt the "Doc" Smith inertialess drive, the "Bergenholm". That gets rid of your inertia entirely, so any thrust you apply immediately accelerates you to a velocity where the resistance of the medium you're passing through (which is rather low in space) equals your thrust.

Smith did not go into details of just how this allowed travelling faster than light, but it certainly did in the Lensman series of space-operas.

[jacobmuller]

Inertia is telling me I should just accept that space travel is going to take a while - excepting the "jump drive" option.

[ericbsmith]

Originally Posted by jacobmuller

Inertia is telling me I should just accept that space travel is going to take a while - excepting the "jump drive" option.

"Space is big. Really big. You just won't believe how vastly hugely mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space." - Douglas Adams, Hitchhiker's Guide to the Galaxy

Any realistic propulsion system is going to take a long time to travel to other planets, let alone to the stars. The quickest travel times to Mars right now are on the order of 9 months each way when the planets are in correct alignment to use Hohmann transfer orbits. An advanced realistic propulsion system might get that down to a couple months. A superscience rocket/reactionless drive with 1G thrust can go from Earth to Mars in 2-4 days using constant acceleration, which is quite fast all things considered.

The 3000G acceleration you were thinking of using just means that a ship can get to light speed in a few minutes, making travel times between planets on the order of minutes for the inner solar system of a few hours for the outer solar system. If you're going to go that far just use a lightspeed drive of some wort, whether it's warp, jump, or hyperspace.

[malloyd]

Originally Posted by ericbsmith

The 3000G acceleration you were thinking of using just means that a ship can get to light speed in a few minutes,

Hours. The speed of light is about 8500 G-hours, so 2.8 hours, and for interplanetary distances you still need a few - at constant acceleration with a midcourse flip you'll need 1.2 hours to cover an AU. Even with insane levels of acceleration like this, space travel is still an actual trip, not a daily commute.

[Anthony]

Originally Posted by malloyd

Hours. The speed of light is about 8500 G-hours.

Conveniently, very close to a G-year. For less cinematic spacedrives that's actually a sorta useful measure.

[ericbsmith]

Originally Posted by malloyd

Hours. The speed of light is about 8500 G-hours, so 2.8 hours,

Yup, you're right. When i was doing my back of the envelope calculation I'm pretty sure I either dropped or added a multiplier or divisor by 60 (for 60 sec or 60 min) which turns hours into minutes.

[Johnny1A.2]

Originally Posted by jacobmuller

Inertia is telling me I should just accept that space travel is going to take a while - excepting the "jump drive" option.

It depends on your assumptions.

As far as we know now (emboldened because humility is important), the iron limit on travel time is the velocity of light. You just can't get there any faster than that (assuming causality is maintained and that relativity is the last word).

For interstellar travel, that means years, decades and centuries or more. No way around it (as far as we know).*

For interplanetary travel, the speed of light is not that big an issue. It only limits you to interplanetary trips in hours (or months if you want to go out into the cometary halo).

There is no law of physics that says you can't travel from, say, Earth to Saturn, in well under one day. It only requires energy sources and propulsion systems in excess of anything we know how to build.

The issue with postulating one-day trips across the Solar System is that if you have the energy resources to do that, then you can do a whole bunch of other things, too, that you might not want to deal with in a setting.

* I keep emphasizing the 'as far as know' because you can't know what you don't know. Imagine a group of Bronze Age thinkers trying to imagine travelling around the world in a few hours. It would seem as self-evidently impossible to them as FTL travel does to us. Many of them would not even be conceptualizing the world properly as they considered the matter (flat vs. round). Our own understanding of the universe might be equally erroneous, we can't know.

[malloyd]

Originally Posted by Johnny1A.2

* I keep emphasizing the 'as far as know' because you can't know what you don't know. Imagine a group of Bronze Age thinkers trying to imagine travelling around the world in a few hours. It would seem as self-evidently impossible to them as FTL travel does to us.

This is a popular take on this that is actually sort of false.

Bronze age guys don't know *how* to move that fast, but they have no reason to think it is impossible, and indeed have excellent evidence it probably *is*, in that the sun manages to circle the Earth in a day, and you can see things apparently about as high in the sky (like clouds) outrace the sun all the time.

We on the other hand have a pretty good reason for concluding that things can't go faster than light (essentially the same one the Bronze Age guys have for concluding things you do now can't change the past) and few even apparent counterexamples - none that hold up to close scrutiny - despite different strong theoretical reasons for thinking we should have [lots] from all the quantum mechanical processes that should explore all possible states.

A lot of the difference is just difference in how interconnected the things we know are. Bronze Age facts largely aren't very connected - being wrong about the speed of the sun doesn't imply you are wrong about anything else. But being wrong about relativity basically breaks all of modern science and engineering in very central ways, and there is a [lot] more evidence it is at least broadly right (among other things it implies everything we know about electromagnetism is wrong, and yet generators and radios do work....)

There's a reason FTL proposals are always way out on the edges of known energies or require material properties we can't prove exist - if it could happen in the range of materials or energies we can regularly observe, we should have already seen evidence. One important corollary to that which I think deserves more attention is that if FTL were somehow possible, it is likely to be useless. We've can already see something of an example - hydrogen fusion - that's blindingly common in the universe but almost useless to us because it's so far from achievable conditions, FTL would presumably be much worse.

[Varyon]

Originally Posted by malloyd

Bronze age guys don't know *how* to move that fast, but they have no reason to think it is impossible, and indeed have excellent evidence it probably *is*, in that the sun manages to circle the Earth in a day, and you can see things apparently about as high in the sky (like clouds) outrace the sun all the time.

I rather suspect Bronze Age thinkers recognized clouds were between them and the sun, given there would have been no known instances of the sun covering a cloud but plenty of clouds covering the sun, but certainly may have thought the difference in elevation was small enough that some clouds did indeed outpace the sun. There were plenty of myths involving gods, monsters, etc outrunning the sun at the same elevation as it, however, and many myths involved the sun being transported by (or itself being) a chariot, so the idea that it was possible to move that fast or faster was accepted (albeit considered rather exceptional). A man who had a machine that could do the same wouldn't be considered an impossibility - rather, it would likely be thought he must have received/stolen it - and/or the knowledge to create it - from the gods or similar entities.

[Johnny1A.2]

Originally Posted by malloyd

This is a popular take on this that is actually sort of false.

Bronze age guys don't know *how* to move that fast, but they have no reason to think it is impossible, and indeed have excellent evidence it probably *is*, in that the sun manages to circle the Earth in a day, and you can see things apparently about as high in the sky (like clouds) outrace the sun all the time.

But in their ways of conceptualizing the world, the movements of the sun and the clouds are irrelevant to what is possible for mortals.

We on the other hand have a pretty good reason for concluding that things can't go faster than light (essentially the same one the Bronze Age guys have for concluding things you do now can't change the past)

Which is loaded with overt and hidden assumptions that we take on faith, because that's all we can do. The most science can ever say, as far as it can legitimately go, is 'as far as know now'.

A lot of the difference is just difference in how interconnected the things we know are. Bronze Age facts largely aren't very connected - being wrong about the speed of the sun doesn't imply you are wrong about anything else. But being wrong about relativity basically breaks all of modern science and engineering in very central ways, and there is a [lot] more evidence it is at least broadly right (among other things it implies everything we know about electromagnetism is wrong, and yet generators and radios do work....)

Which proves nothing about what is possible or not possible. The impossible always seems clearly impossible until it isn't, and there's no way to know what will hold up and what won't. It really doesn't matter how interconnected what we think we know is, as far as assessing it's reliability. We cannot go beyond 'as far as we know'.

There's a reason FTL proposals are always way out on the edges of known energies or require material properties we can't prove exist - if it could happen in the range of materials or energies we can regularly observe, we should have already seen evidence. One important corollary to that which I think deserves more attention is that if FTL were somehow possible, it is likely to be useless. We've can already see something of an example - hydrogen fusion - that's blindingly common in the universe but almost useless to us because it's so far from achievable conditions, FTL would presumably be much worse.

non sequitur. If it's possible at all, it means there's something basically wrong with out view of How Things Work, which means we can't make any meaningful predictions about how useful it will be or won't be. We're in the same position there that predictions about the utility of atomic energy once were. Or someone in 1930 pondering the precision of engineering necessary to create something like a modern processor chip.

For that matter, we can't say that fusion will remain useless to us with any confidence. Only that we can't do much with it (other than weapons) right now. It was known that flying machines and horseless carriages were possible long before they were practical, there was a long, long legacy of failed attempts before it all finally came together and worked. Ditto practical submarines.