08-15-2013, 01:50 AM | #11 | |||
GURPS FAQ Keeper
Join Date: Mar 2006
Location: Kyïv, Ukraine
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Re: EuroSpace E950 Minerva Executive Transatmospheric Vehicle
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Why? Even the silly TL6 aerodynamic knowledge is enough to keep stall speed at 40mph. With a TL10 design that is optimised for a low-thrust launch, this shouldn't be a problem at all. Well yeah, you need a 400m runway to lift off the ground with that sort of thrust. Big deal. |
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08-15-2013, 01:58 AM | #12 | |
Computer Scientist
Join Date: Aug 2004
Location: Dallas, Texas
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Re: EuroSpace E950 Minerva Executive Transatmospheric Vehicle
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08-15-2013, 02:51 AM | #13 |
GURPS FAQ Keeper
Join Date: Mar 2006
Location: Kyïv, Ukraine
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Re: EuroSpace E950 Minerva Executive Transatmospheric Vehicle
Then again, Spaceships doesn't go into such details as variable wings designed for lowering stall speed at the cost of decreasing top speed during the runway phase. They're probably the implied reason behind the 'needs >=1G or wings' requirement.
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08-15-2013, 05:22 PM | #14 | ||||
Join Date: Jan 2010
Location: Los Angeles County
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Re: EuroSpace E950 Minerva Executive Transatmospheric Vehicle
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08-15-2013, 06:13 PM | #15 |
Join Date: Feb 2005
Location: Berkeley, CA
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Re: EuroSpace E950 Minerva Executive Transatmospheric Vehicle
Only if you can fly at orbital velocity (17,600 mph) in atmosphere, or you have enough thrust to keep yourself out of atmosphere (requires >1G). Not sure which atmospheric flight rules you're using, but SS1 certainly won't let you do that with a nuclear light bulb. The SS1 rules on atmospheric performance aren't particularly realistic, but at a minimum you need thrust that exceeds your best 1/(L/D ratio) and materials that let you survive mach 25 atmospheric flight for long enough to reach orbit; the first probably requires 0.1g or better, the second is a severe engineering challenge though it may be doable.
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08-16-2013, 02:10 AM | #16 | |||
GURPS FAQ Keeper
Join Date: Mar 2006
Location: Kyïv, Ukraine
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Re: EuroSpace E950 Minerva Executive Transatmospheric Vehicle
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You attain the velocity gradually as you leave the atmosphere gradually. |
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08-16-2013, 02:24 AM | #17 | |||
GURPS FAQ Keeper
Join Date: Mar 2006
Location: Kyïv, Ukraine
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Re: EuroSpace E950 Minerva Executive Transatmospheric Vehicle
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In fact, the rule for transatmospheric flight was roughly the same in the original TS book, it wasn't dumbed down in Spaceships (though perhaps the coefficient has changed): Quote:
(Also, where's what's the page for the 1/(L/D) rule? I can't find it) |
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08-16-2013, 10:31 AM | #18 | ||
Join Date: Feb 2005
Location: Berkeley, CA
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Re: EuroSpace E950 Minerva Executive Transatmospheric Vehicle
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Basically, orbit is about moving around a planet fast enough that falling and the curvature of the planet keep up with one another. If you're moving below orbital velocity, you fall faster than that, and so you have to do something so you don't hit the ground. Your choices on that are 'downwards rocket thrust matching gravity', 'aerodynamic lift', and 'get a temporary large upwards velocity, and accelerate while you're in free-fall'. The third option allows you to have short periods where acceleration is less than 1G, but your average still has to be pretty high -- for example, if we figure a velocity of 6,000 mph at a 45 degree angle near the top of the atmosphere, velocity is 1,900 m/s vertical, 1,900 m/s horizontal. It will take us 190s to stop moving vertically, 380s to hit the atmosphere again. If we can accelerate to 7,900 m/s before that happens, we achieve orbit. That requires 1.5G of acceleration... Quote:
It comes from Real Physics, not GURPS. Basically, if GURPS is letting you do this, GURPS is wrong. Last edited by Anthony; 08-16-2013 at 10:35 AM. |
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08-16-2013, 11:04 AM | #19 | |||
GURPS FAQ Keeper
Join Date: Mar 2006
Location: Kyïv, Ukraine
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Re: EuroSpace E950 Minerva Executive Transatmospheric Vehicle
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I see some similar-looking calculations in TS. I'm guessing SS craft are assumed to be built with the correct values of surface area drag, and whatever I'm forgetting from VE2/TS. |
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08-24-2013, 05:34 PM | #20 |
Join Date: Jan 2010
Location: Los Angeles County
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Re: EuroSpace E950 Minerva Executive Transatmospheric Vehicle
I'm Back! (From my camping trip.)
OKAY. Okay. ok... ;) First off I want to apologize. I did not do my homework in regards to ThS/GURPS Spaceships rules for transatmospheric vehicles. *shame* Okay so, I've looked over the formulas on TS 52 and SS 37. You are clearly intended to be able to reach orbit in a winged ship with less than 1 G of thrust. This is reasonable and realistic AFAIK. The problem here is that the model is too simple allowing unrealistic designs to slip through the cracks. The problem of attaining orbit (outside of logistics, navigation, etc) breaks down into needing speed and altitude and needing to overcome gravity drag and air resistance while getting the first two. Speed is the basis for both formulas and in both books transatmospheric craft get to subtract (start with) their airspeed from the required delta-V. Good. Altitude seems to be ignored in GURPS altogether. This is not as big of a problem as it may seem. See this explanation. Gravity Drag is addressed in SS by requiring the spacecraft to have acceleration > 1 G or wings. This is weak. Ideally conventional spacecraft have ~2-3 G thrust at launch so that they can accelerate up at ~1-2 G (fast to minimize gravity drag but not fighting air resistance too much) and may have higher thrust as the air thins out. Even at this ideal gravity drag costs you some energy and the farther from the ideal you are the more it costs you (exponentially, I think). In ThS Time to Orbit for conventional spacecraft (which determines delta-V used) is calculated with (thrust - gravity). This seems a bit harsh to me. I don’t know the real math but (while gravity drag is a significant concern) in a normal launch you do spend most of your energy gaining sideways speed and it seems wrong to me to apply the full effect of gravity to the whole launch. In both SS and ThS winged craft get a completely free pass on gravity drag. This may or may not be a big problem idk. Air Resistance is not addressed in either book. This is the big problem in the case at hand! During any kind of launch the air is going to slow you down. This is going to require energy to overcome which translates into some kind of delta-V cost (even though this energy is being used to overcome resistance, not to change your velocity). One way to sidestep this cost is to use air breathing engines to get up to get up to the “edge” of the atmosphere. OTOH if you intend to use lift to maintain altitude while you accelerate to orbital speed you will need to stay inside the atmosphere, fighting air resistance the whole way! |
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spaceships, spaceships 7, ths, ths spaceship, transhuman space |
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