You also don't know that. Why would they reveal themselves to us if they did?

The US considers anything past 50 miles in altitude to be space, and, in a manner of definition, they're not necessarily wrong. You can do more than one orbit at this altitude. "Space" is a human construct, so any definition is really going to fit human needs. In any case, 60,000 feet is not "space" by any reasonable definition.

What about a balloon with rocket boosters attached to it?

>The kármán line isn't even in space

Earth's exosphere extends out past the moon. Any definition of "space" will be squishy. Satellites in low Earth orbit experience drag, too. Objects at 100km can complete multiple orbits around the Earth before drag pulls them down into the atmosphere. I'd be willing to call that space if someone wanted to argue about it.

If you don't understand why Ganymede isn't a good general purpose gas station for space travel to other places in the solar system, then you really need to do a lot more reading than simply in this comment section.

You don't have the privileged position to be making such condescending comments, and you still don't seem to understand the context of my and another person's comments. You should work on your own reading comprehension because you still don't seem to get it.

I think it was kind of unclear how you meant that last paragraph in the context of the broader discussion, but clarification noted.

Someone mentioned using Ganymede as a gas station. Try and keep track. 👎

The exhaust from a rocket is basically unusable as a source of water for astronauts. Every drop of water you take from the exhaust, too, would be reducing the rocket's efficiency. In order for that rocket to work, you need all those combustion products to fly out the back of the nozzle at high speed. Anything you put in the exhaust stream that is attached to the ship is going to reduce the effective thrust of the rocket (assuming it doesn't just melt first). You'd be better off just already having a water storage tank on the spacecraft, but then we get back to the original problem.

Because anything you do in space needs to account for the fact that you're floating in a void with essentially nothing around you but radiation. The materials have to come from somewhere, and you need to consider orbital mechanics to get from one body to another in space.

There's no water you can't purify. The question is, how much energy is it going to take to do that?

Not sure if I didn't explain well enough or you just don't understand, but, anyway, the point I was trying to make is that the aerospike operates at lower efficiency at every altitude than a bell nozzle tailor-made for each altitude would. Over a range of widely varying altitudes, though, the aerospike nozzle, is, cumulatively much more efficient than a bell nozzle that only has one exact design altitude at which it is neither over- or under-expanded. Neither a bell nozzle nor an aerospike exhibit "optimal" expansion in any real-world scenario.

Nozzle under-expansion: There is energy left in the exhaust that could have been converted into momentum with a longer/more expanded nozzle. This energy goes into expanding the plume outward into the air aft of the vehicle that could have been used to push the vehicle forward. You get a certain amount of "pressure thrust", which is the pressure of the exhaust relative to the ambient air pressure pushing against the vehicle, but this is small compared to "momentum thrust".

Nozzle over-expansion: You have expanded the exhaust gases past the point where they are at the same pressure as the ambient air, and the atmosphere is actively pushing back against your thrust stream. Extreme over-expansion will cause the exhaust plume to creep back into the nozzle, detaching from the nozzle wall and eventually leading to combustion instabilities.

A vacuum nozzle can never be over-expanded, since the static pressure of vacuum is essentially zero and the exhaust plume will always have positive total pressure. Vacuum nozzles' sizes are limited by other considerations such as mass and structural integrity.

>Not just different temperatures

It has nothing to do with temperature.

>Different atmospheric pressures require different nozzle shapes to properly burn the fuel for the most efficient thrust.

It has nothing to do with properly burning the fuel. It has to do with the exhaust stream's static pressure at the nozzle's exit.

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Edit: a very poorly sized nozzle will cause combustion instabilities, so in a sense, properly burning is kind of a thing, but before that happens, just having the incorrect exhaust pressure at the nozzle exit will make for a less efficient expansion of gases than optimal thrust independent of any combustion issues.

Maybe I didn't explain well enough. Ion engines produce thrust equivalent to the weight of a piece of paper. The lightest breeze would render your engine unusable, or the slightest amount of drag assuming still air. Even in perfectly still air, you would never get anywhere because drag is proportional to the square of velocity, so your top speed would make walking a much more attractive option.

If you're talking about rockets that are currently out there, the only ones that would really be suitable are liquid or hybrid chemical rockets. Liquid chemical rockets use fuel and oxidizer that is in liquid form. A hybrid rocket has typically a fuel that is solid and an oxidizer that is liquid. Solid chemical rockets also work in and out of atmosphere but aren't really throttleable.

Ion engines, as you said, produce too little thrust to be useful in atmosphere and even require different mission planning in space than chemical rockets due to their incredibly low thrust. It's conceivable that with enough electric power, you could make ion engines produce thrust closer to chemical rockets, but we're talking about obscene levels of power required to achieve this, which would currently be in the realm of sci-fi.

Nuclear rockets are also a thing that have been tested and use the heat of a nuclear fission reaction to accelerate a fuel such as liquid hydrogen to produce thrust. Due to the low molecular mass of hydrogen, they produce less thrust than typical chemical rockets but are more efficient than chemical rockets and are also throttleable.

Ion engines, at least in their current state, aren't really suitable for atmospheric propulsion. They produce so little thrust that a light breeze would be a big problem.

Eh, not really. A lot of electric rockets (aka ion) produce so little thrust, that they effectively don't work in atmosphere.

Different altitudes / ambient pressures, not temperatures, and no, they're not "optimal". You pay a penalty for using an aerospike over a standard nozzle, but unlike standard bell nozzles, they're less bad at a wider range of altitudes.