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Old 08-20-2021, 08:32 PM   #21
Fred Brackin
 
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Default Re: [Spaceships] getting into orbit without superscience?

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Originally Posted by Stomoxys View Post
Another drawback is that it requires an atmosphere with oxygen (or possibly some other oxidizer?).
Places with significant atmospheres in our Solar System besides Earth comes out to Venus, Titan, Jupiter, Saturn, Uranus, Neptune and maybe Io. Of those Titan leads as a possibly desirable landing site by a wide margin.

Trying to guess about hypothetical planets/moons outside your Solar System with non-oxygen atmospheres and the biggest possiblity might be a sort of super-Mars.

So needing oxygen may not be that big a limitation
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Old 08-20-2021, 09:02 PM   #22
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Default Re: [Spaceships] getting into orbit without superscience?

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Another drawback is that it requires an atmosphere with oxygen (or possibly some other oxidizer?).
There isn't anywhere in the solar system that you're going to be taking off from (i.e. we're leaving out Venus and gas giants) with a gravity well anywhere near as significant as Earth. It takes about 8 km/s to reach Earth orbit, 3.6 km/s for Mars, 3 km/s for Mercury, and nothing else is above 2 km/s.
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Old 08-21-2021, 03:34 AM   #23
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Default Re: [Spaceships] getting into orbit without superscience?

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Originally Posted by Fred Brackin View Post
Places with significant atmospheres in our Solar System besides Earth comes out to Venus, Titan, Jupiter, Saturn, Uranus, Neptune and maybe Io. Of those Titan leads as a possibly desirable landing site by a wide margin.

Trying to guess about hypothetical planets/moons outside your Solar System with non-oxygen atmospheres and the biggest possiblity might be a sort of super-Mars.

So needing oxygen may not be that big a limitation
The ramjet mode of the SABRE engine is air breathing, burning fuel using oxygen from the surrounding air, so it is useless in an atmosphere with no oxygen.

Titan would be great for an electric propeller plane and it could work in the upper atmosphere of Venus too. XKCD Interplanetary Cessna
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Old 08-21-2021, 04:04 AM   #24
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Default Re: [Spaceships] getting into orbit without superscience?

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So the original question can be restated as: what is the most efficient way to get from the surface to orbit using only equipment that doesn't have a "^" on its TL? (And there is an implicit "also not magical" in there.)
It's not in Spaceships; it's in Ultra-Tech, p. 224. "Space Elevator (TL9)".

Seriously because of the problems with rockets that other people have already pointed out, any spacefaring society that can use beanstalks probably should. So spacecraft that can get to orbit may be quite specialised, not your general-purpose freight and passenger haulers.

That said, in GURPS Spaceships terms and avoiding speculation about real-world drives, assuming a vaguely earthlike planet, you want:
  • TL is not ^
  • number of fuel tanks needed to get to orbit is as low as possible

Spaceships 1 p. 37 says that there's no constraint on thrust if your vehicle has wings, so by a strict reading of the rules an antimatter pion drive will get the job done. But at 0.005G per drive I feel it would have some trouble overcoming air resistance (and even ignoring that, at the end of a 10,000 foot runway it's doing a mere 38 mph, not to mention the runway has been vaporised). Maybe you can loft it off a balloon or high-altitude carrier aircraft?

But let's also say we need at least 0.1G performance. TL11 high-thrust fusion pulse drive, then, with 20mps per tank of pellets. A bit marginal? External pulsed plasma will unambiguously get the job done, and you don't even need wings. Then you're down to antimatter thermal rockets, HEDM, nuclear thermal, and chemical.
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Old 08-21-2021, 07:41 AM   #25
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Default Re: [Spaceships] getting into orbit without superscience?

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TL11 high-thrust fusion pulse drive, then, with 20mps per tank of pellets.
I question whether fusion pulsed drives will even function in an atmosphere, which will interfere with the "laser beams, particle beams and/or miniscule amounts of antimatter" needed for ignition and with the control of the plasma that results.
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Old 08-21-2021, 08:29 AM   #26
Fred Brackin
 
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Default Re: [Spaceships] getting into orbit without superscience?

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The ramjet mode of the SABRE engine is air breathing, burning fuel using oxygen from the surrounding air, so it is useless in an atmosphere with no oxygen.
Yes, scramjets need oxygen atmospheres but our Solar System is an example of how few desirable places that rules out.

Even Titan is a sort of "There must be something worth landing there for". The upper atmosphere of Venus might be achievable but why are you going there?
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Old 08-21-2021, 02:29 PM   #27
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Default Re: [Spaceships] getting into orbit without superscience?

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Originally Posted by Stomoxys View Post
The ramjet mode of the SABRE engine is air breathing, burning fuel using oxygen from the surrounding air, so it is useless in an atmosphere with no oxygen.

Titan would be great for an electric propeller plane and it could work in the upper atmosphere of Venus too. XKCD Interplanetary Cessna
Titan's atmosphere is over 5% methane, so you might be able to build a methane-breathing engine with tanks of oxygen as fuel. Or could you? (Earth's atmosphere is about 20% oxygen.
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Old 08-21-2021, 03:27 PM   #28
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Default Re: [Spaceships] getting into orbit without superscience?

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Titan's atmosphere is over 5% methane, so you might be able to build a methane-breathing engine with tanks of oxygen as fuel. Or could you? (Earth's atmosphere is about 20% oxygen.
It wouldn't be terribly helpful. The big gain from atmospheric oxygen is that the oxygen is 89% of total fuel weight for hydrogen/oxygen and 75% for methane/oxygen. Using atmospheric methane with onboard hydrogen is only a 25% savings.
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Old 08-21-2021, 05:42 PM   #29
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Default Re: [Spaceships] getting into orbit without superscience?

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Originally Posted by Fred Brackin View Post
Trying to guess about hypothetical planets/moons outside your Solar System with non-oxygen atmospheres and the biggest possibility might be a sort of super-Mars.
That presumes that larger planets have enough life for a Great Oxygenation Event. Without that, there won't be significant free oxygen: it's just too reactive.
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Old 08-21-2021, 08:33 PM   #30
Fred Brackin
 
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Default Re: [Spaceships] getting into orbit without superscience?

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That presumes that larger planets have enough life for a Great Oxygenation Event. Without that, there won't be significant free oxygen: it's just too reactive.
Of course Earth-like atmospheres require life and probably photosynthesis.

The somewhat complicated thing I was tryintg to express was the concept of an exo-planet that did nto have an oxygen-bearing atmosphere yet was somehow practical and desirable for landing upon.

The universe is probably full of planets where scramjets won't work but we won't have any reason to visit them either.
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