3 hours ago, swansont said:

Further, I think one can distinguish between arguments that might be missing information and those that violate principles we have very good reason to conclude are true. You can’t engineer your way around e.g. a violation of conservation of energy, or momentum

Or the actual money available. I once had a struggle with someone who told me it was feasible to move Earth's present population to other planets (making Earth a sort of wilderness park). Other aspects aside, I pointed out the math on amounts of propellant, technology and resources needed just to keep pace with net population growth of 190,000 per day, i.e. just hold population at 8.3 billion.

The lowest price currently to get a mass to LEO is $1000/lb. Say through various engineering miracles it was cut tenfold to $100/lb. So, just to get to LEO (let alone suitable locations for space habitats or other planets) would in that reckoning cost an average adult person (140 lb) $14,000. A journey to a viable habitat and the cost of building it would easily be a hundred times that.* So, $1.4 million per person, an extremely conservative estimate. The median net worth of a US citizen is $190,000. One may safely assume it's lower for the global population.

This kind of disconnect with actual costs of anything space travel related is probably due in part to the human tendency to fill in gaps in knowledge with "a miracle occurs here," - in my example there are both engineering and economic miracles.

*(for a sense of the scale on this, consider the cost of moving somewhere on Earth to the cost of the home you're moving to - now think on how that ratio might compare to moving to Mars or an O'Neill colony or whatever)

3 hours ago, Moontanman said:

I agree, but how do these things prohibit star travel? There are ways to star travel without violating the laws of physics. Slow boats are an example, we might not be able to do it now but it's mostly engineering problems not the laws of nature.

And there’s no science here to back this up.

But there could be. You still have to have fuel and reaction mass to travel, to slow down at the destination, to do a controlled entry of a gravity well and to leave. None of that requires knowing the specific energy source, but the analysis could tell you how efficient of a method you’d have to use, which rules out certain (perhaps all) approaches

But a blanket statement? Not scientific.

5 hours ago, dimreepr said:

Really?

Intelligent life has to evole somewhere, that retains an atmosphere to live in, long enough to develope an intelligent approach to escaping a gravity well that retains an atmosphere, but prohibits an escape,,,

Seems like a fine balance...

I agree the balance may indeed be fine, maybe there are planets that require nuclear power to escape... my idea of colonizing space happens after the the beings escape their planet. In fact you have hit upon why a space faring civilization might want to avoid planets. coming and going from a Earth sized planet require enormous amounts of energy, it would be much easier to travel between space colonies than between planets.

Once a civilization begins to colonize via space habitats the game changes from going in and out of deep gravity wells to using things like ion drive or hall thrusters or maybe things we do not see happening yet.

As the integrity of space colonies gets better and better at total recycling, probably won't ever be 100%, the idea of spanning space in the home you already live in becomes much more attractive to certain types of people at least. It also make choosing a goal much easier since you are not looking for habitable planets.

8 hours ago, swansont said:

think one can distinguish between arguments that might be missing information and those that violate principles we have very good reason to conclude are true

And sometimes we think we have good reason to believe principles are true because we might be missing information ...

2 hours ago, swansont said:

And there’s no science here to back this up.

But there could be. You still have to have fuel and reaction mass to travel, to slow down at the destination, to do a controlled entry of a gravity well and to leave. None of that requires knowing the specific energy source, but the analysis could tell you how efficient of a method you’d have to use, which rules out certain (perhaps all) approaches

But a blanket statement? Not scientific.

Swanson, I honestly do not understand what you are talking about, your statement seem to be a blanket statement without any nuance. Please elaborate.

48 minutes ago, Moontanman said:

Swanson, I honestly do not understand what you are talking about, your statement seem to be a blanket statement without any nuance. Please elaborate.

Calculating the minimum energy it takes to launch a payload to orbit, or to interstellar space, doesn’t depend on the method of propulsion. Same for getting up to a certain speed. The detail of propulsion matters for how much energy is wasted, i.e. how much more you need than the minimum, and you can calculate that for different scenarios.

None of that analysis is included in saying “slow boat” or “we’ll refuel along the way”

The science is in the details.

2 hours ago, Moontanman said:

Swanson, I honestly do not understand what you are talking about, your statement seem to be a blanket statement without any nuance. Please elaborate.

Every change in velocity (speed and/or direction) requires energy. Each single change takes a piece of your finite energy budget and efficiency is never 100%.

19 hours ago, MigL said:

22 hours ago, dimreepr said:

Isn't there a maximum size of planet from which it's impossible to escape the planet with known propellant's?

A 'reaction' motor provides thrust by ejecting energized mass rearward, at velocity, to provide forward motion according to Newtonian mechanics ( essentially conservation of momentum ).
Escape velocity is proportional to the square root of the mass you are attempting 'escape' from ( assuming a fixed radius of departure ).

I don't see how your statement is valid.

But dim is right in this case: I think the key in his statement is "with known propellants". It is the ratio of chemical energy and mass of the propellant, and the gravity of the planet that makes escape from a 'too heavy planet' impossible. Of course you can add more propellant, but if you need more propellant to lift the portion you added just before, you'll never get very far.

Escape velocity has not much to do with that: this is rocket science (really, this time!), not ballistic mechanics. Only when the rocket engines are turned off escape velocity (from the point where the rocket is at that moment) is relevant.

Edited 19 hours ago19 hr by Eise

15 hours ago, Moontanman said:

I agree the balance may indeed be fine, maybe there are planets that require nuclear power to escape... my idea of colonizing space happens after the the beings escape their planet. In fact you have hit upon why a space faring civilization might want to avoid planets. coming and going from a Earth sized planet require enormous amounts of energy, it would be much easier to travel between space colonies than between planets.

Doesn't that explain the Fermi paradox?

You may as well be talking about the teapot orbiting Jupiter...

16 hours ago, swansont said:

Calculating the minimum energy it takes to launch a payload to orbit, or to interstellar space, doesn’t depend on the method of propulsion. Same for getting up to a certain speed. The detail of propulsion matters for how much energy is wasted, i.e. how much more you need than the minimum, and you can calculate that for different scenarios.

None of that analysis is included in saying “slow boat” or “we’ll refuel along the way”

The science is in the details.

Wouldn't that depend in part on how fast your slow boat was going? To people who live inside space habitats the trip would just be life as usual, so the speed could be quite low. The time of travel would be very long of course but if you used the same amount of energy to slow down as you used to speed up...

Then again this could seem trivial to beings with much longer life spans than ours, or technology that allows them to spend most of the time in hibernation, combine that with long life spans... Technology is a wonderful thing.

Also i can see a system similar to staged rockets, the engine could be dropped after the habitat hits a certain speed, things like magnetic sails could be used to decelerate, or carry a second stage capable of slowing down in conjunction with mag sails.

I am sure that thousands of years in the future technology could be somewhat more advanced than today and be capable of things like antimatter drives or fusion drives or more likely something we haven't thought of yet.

1 hour ago, dimreepr said:

Doesn't that explain the Fermi paradox?

You may as well be talking about the teapot orbiting Jupiter...

I'm not sure how that could explain the Fermi paradox, are you suggesting that most planets are too big to escape from?

Teapot orbiting jupiter? I have not suggested such a thing, only logical extensions of current technology not physics breaking Clark Tech!

16 hours ago, swansont said:

Calculating the minimum energy it takes to launch a payload to orbit, or to interstellar space, doesn’t depend on the method of propulsion. Same for getting up to a certain speed. The detail of propulsion matters for how much energy is wasted, i.e. how much more you need than the minimum, and you can calculate that for different scenarios.

I'm not sure I understand this, things like Ion drives use much less fuel to do the same thing as a chemical rocket, other methods like fusion or antimatter could hypothetically be even more efficient, wouldn't not requiring high speeds be another way to requre less fuel?

8 minutes ago, Moontanman said:

Then again this could seem trivial to beings with much longer life spans than ours, or technology that allows them to spend most of the time in hibernation, combine that with long life spans... Technology is a wonderful thing.

Also i can see a system similar to staged rockets, the engine could be dropped after the habitat hits a certain speed, things like magnetic sails could be used to decelerate, or carry a second stage capable of slowing down in conjunction with mag sails.

How do magnetic sails work, or provide the ∆v required for something which was accelerated with a rocket?

Also, this slow boat speculation shifts the topic from aliens visiting Earth to aliens who dwell in space and are willing to move their city to the Sol system over several millennia (and be separated from a handy radiant energy source for a very long time). Hardly a "visit."

2 hours ago, Moontanman said:

I'm not sure I understand this, things like Ion drives use much less fuel to do the same thing as a chemical rocket, other methods like fusion or antimatter could hypothetically be even more efficient, wouldn't not requiring high speeds be another way to requre less fuel?

The energy to move from one place to another in a gravitational field is not affected by the method,

A 1 kg object moving up 1 meter on earth requires 9.8 Joules because that’s the change in potential energy. You can devise methods that use different forces acting for different distances or times, but the minimum energy you need to supply is the same.

The method affects how much energy is wasted on whatever it is you’re throwing out of the back of the rocket. That dictates the efficiency.

9 hours ago, Eise said:

But dim is right in this case: I think the key in his statement is "with known propellants".

Maybe not.
There are various ways to get to a higher radius, where less energy is required to break free.
The solid rocket booster with Hydrogen/Oxygen combustion has been more convenient for Earth, but other methods have been studied ( mainly to increase payload capacity ).
The air breathing first stage, to get to a height of about 70000 ft, has been considered since the 70s, with the most advanced concept being the British HOTOL single stage to orbit system

British Aerospace HOTOL - Wikipedia

Recently Germany is examining a similar system for payload launching.

Germany's POLARIS Spaceplanes Develops Hypersonic Test Vehicle for Defense and Space Research | Mahir Zeynalov posted on the topic | LinkedIn

The point is, that if you can 'gather' propellant as you go, you only need one of the propellants ( Hydrogen in HOTOL's case ).
Even in the case of gas giants like Jupiter, the amount of combustible methane available in its atmosphere is a resource that shouldn't be ignored ( necessitating only oxidizer, no teapots required ).

Contextual Efficiency

Contextual efficiency seems to be a potential key.

Which energy source is more efficient (and safe & clean) for a motion of things (e.g. like a spacecraft) with specified speeds (e.g. slow, average, and fast; Pros and cons for each speed)?

14 minutes ago, tylers100 said:

Contextual Efficiency

Contextual efficiency seems to be a potential key.

Which energy source is more efficient (and safe & clean) for a motion of things (e.g. like a spacecraft) with specified speeds (e.g. slow, average, and fast; Pros and cons for each speed)?

It also a matter where you are; you aren’t going to accelerate upward unless your thrust or lift exceeds the gravitational force. It’s why ion drives aren’t used until you get away from the planet. NASA’s best gives you about 100 milliNewtons of thrust

https://science.nasa.gov/mission/dawn/technology/ion-propulsion/

(similar argument for using photons)