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Old 08-22-2021, 05:30 AM   #31
dcarson
 
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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
On Titan you carry oxygen and use the ramjet mode to scoop in the methane atmosphere as fuel.
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Old 08-23-2021, 08:40 AM   #32
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Default Re: [Spaceships] getting into orbit without superscience?

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Originally Posted by dcarson View Post
On Titan you carry oxygen and use the ramjet mode to scoop in the methane atmosphere as fuel.
As noted, that doesn't help a lot. Just looking at combustion, carrying methane and using environmental oxygen gets you 55.6 MJ per kg of fuel. Carrying oxygen and using environmental methane gets you 13.9 MJ per kg of fuel, while carrying both methane and oxygen gets you 11.12 MJ per kg of fuel. By contrast, carrying hydrogen and using environmental oxygen gets you 120 MJ per kg of fuel - but carrying oxygen and using environmental hydrogen gets you only 15 MJ per kg of fuel, and carrying both is 13.33 MJ per kg of fuel. So carrying oxygen around on Titan lets you generate a little more than 10% of the energy carrying hydrogen around on Earth would, which is only a little more (less than +5%) than what using rocket fuel (LH+LOx) without an air-breathing mode would generate, and it's possible water (the only product of hydrogen combustion) is a better reaction mass than carbon dioxide (which makes up 55% of the reaction mass with methane combustion; the other 45% is water). Methane certainly isn't useless - it's cheaper and easier to store than liquid hydrogen, which is probably why SpaceX uses it - but if you're trying to get into orbit with minimal "fuel," it's probably not the way to go.

For the math on these, you need one molecule of methane for every two molecules of oxygen, and O2 has twice the molecular mass of methane. So, that's one unit mass of methane and four units mass of oxygen - carrying just methane divides methane's heat of combustion by 1, carrying just oxygen divides by 4, and carrying both divides by 5. For hydrogen, it's instead two molecules of hydrogen for every molecule of oxygen, but oxygen has 16x the molecular mass of hydrogen, for divisors of 1, 8 (only need half a mol of oxygen per mol of hydrogen), and 9, respectively.
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Old 08-23-2021, 11:17 AM   #33
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Default Re: [Spaceships] getting into orbit without superscience?

At low velocity and altitude it can be useful to scoop atmosphere even if it doesn't provide any energy, because it acts as reaction mass, but this loses effectiveness fast and requires you to carry a lot of additional hardware. On Titan, you can get from surface to orbit with a conventional rocket and a 40% fuel fraction so I doubt anyone will bother.
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Old 08-23-2021, 09:16 PM   #34
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Default Re: [Spaceships] getting into orbit without superscience?

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Originally Posted by RogerBW View Post
It's not in Spaceships; it's in Ultra-Tech, p. 224. "Space Elevator (TL9)".

(SNIP)
Also, the Solar System has only one planet where humans live -- or might want to live -- in large numbers, which also has a gravity well deep enough to require exotic material for the beanstalk.

On Mars, with only 38 percent of Earth's gravity, you could probably build the elevator cable out of Kevlar or current technology carbon fiber, or something.

Every other place would only need steel cables.
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Old 08-24-2021, 05:52 AM   #35
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Default Re: [Spaceships] getting into orbit without superscience?

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Originally Posted by Stormcrow View Post
GURPS clearly defines what it means by superscience, and it marks all equipment as to whether it is considered superscience or not.

"'Superscience' technologies violate physical laws... as we currently understand them" (page B513).

Superscience equipment has a "^" for its TL or, if the writer decides that it appears at a certain TL, the "^" appears after the TL number.

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.)
I did not realize this but I did wonder about the ^ on some items.
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Old 08-24-2021, 02:06 PM   #36
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Default Re: [Spaceships] getting into orbit without superscience?

There are lots of proposals for cheap reusable ground-to-orbit systems. The catch is that virtually all of them require enormous infrastructure. Many of them could be with modern tech, they're just enormously expensive. Others are borderline superscience.

For all practical purposes, a ground-to-space flight will probably not be possible with just what a realistic ship carries. Some sort of facility on or near a planet will be necessary.

Ther's a reference here.
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Last edited by Mysterious Dark Lord v3.2; 08-24-2021 at 04:15 PM.
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Old 08-25-2021, 08:24 AM   #37
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Default Re: [Spaceships] getting into orbit without superscience?

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Originally Posted by Emerikol View Post
I did not realize this but I did wonder about the ^ on some items.
It could be noted that superscience ranges from "It's magic, don't question it" to "The math works out but we don't pretend to know how it works" to "Completely realistic except for the part where it doesn't instantly melt itself at this sort of energy density."
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Old 08-25-2021, 09:26 AM   #38
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Default Re: [Spaceships] getting into orbit without superscience?

I know I already responded, but let me put it in a different way:
Without super science or magic, getting to orbit from an Earth-sized planet is incredibly expensive and requires a truly staggering amount of infrastructure.

If you want a future society to have regular space travel, your options are
  • Super-science, of some flavor and some degree of reality-bending
  • A truly stunning amount of rocket launches world-wide
  • Established space-plane technology and all the infrastructure to support it
  • A non-rocket launch method, which requires far more upfront infrastructure and political will, but will ultimately result in a much low per-ton launch cost, which would be passed-on to the end-consumer

Each of these has follow-on implications for the setting you’re working up.
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Old 08-27-2021, 04:58 PM   #39
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Default Re: [Spaceships] getting into orbit without superscience?

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Each of these has follow-on implications for the setting you’re working up.
True. Let us look at those, shall we?
  • Super-science, of some flavor and some degree of reality-bending
PRO: Such settings usually lend themselves well to adventuring and sensational scenarios. Including personal spaceships.
CON: Lack of reality, unforseen consequences.
  • A truly stunning amount of rocket launches world-wide
PRO: Closer to realistic than most scenarios.
CON: Requires either a wealthier and more peaceful world, or a world running the razor's edge of World War Last.
  • Established space-plane technology and all the infrastructure to support it

PRO: More advanced world, realistic, and has the possibility of private spacecraft.
CON: Realistic space-plane tech would be Rich Folk's Toys, which would mean the mandatory presence of wealthy PCs or powerful Patrons.
  • A non-rocket launch method, which requires far more upfront infrastructure and political will, but will ultimately result in a much low per-ton launch cost, which would be passed-on to the end-consumer
PRO and CON: The sheer scale and power of most non-rocket launch facilities would require a World Government for it to exist at all. That has ramifications ranging from planetary dictatorship to a peaceful world where adventures are Simply Not Done, and anything in-between.
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Old 08-28-2021, 08:55 PM   #40
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Default Re: [Spaceships] getting into orbit without superscience?

In a context in which metallic hydrogen is the main fuel source, might there still be applications for which chemical engines are preferred for safety reasons?

The downside to metallic hydrogen is that it is metastable, which means that while it would work in the sense of allowing for easier rocket design, it seems like it would be much higher risk in the event of damage to your craft. Mechanically it has similar dangers to antimatter, even if to a much lesser scale.
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