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Old 08-15-2013, 02:50 AM   #11
vicky_molokh
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Default Re: EuroSpace E950 Minerva Executive Transatmospheric Vehicle

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The point of reaching orbital velocity is so that can go round and round! Before you are at orbital velocity you expend energy to avoid falling back to earth.
Well, yes and no. You can still go round and round at a suborbital velocity while attaining orbital velocity.

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If you are boosting into orbit with a high thrust craft we just call that energy "gravity drag". If you are planing to fly around the planet slower then orbital velocity you will end up spending huge amounts of energy just keeping your altitude up
No you're not. Notice that the aAccel is the aAccel the craft maintains in addition to maintaining its current speed. And the light bulb has more energy available than the rocket thruster, though it can only dump it at a smaller rate. You still get more total Δv out of a light bulb thruster, and it's cheap.

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(especially at the start when there is a large difference between your speed and proper free falling speed).
What is a proper free falling speed? If you're talking about terminal velocity for a free fall, then I'm not sure how it's relevant: the craft is not being in a free fall unless you failed to reach stall velocity at the runway (in which case you've got a totally different problem on your hands).

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I doubt that you would even be able to keep yourself up with 0.05 G of thrust.
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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Old 08-15-2013, 02:58 AM   #12
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Default 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.
There is a trade-off with the TL6 high-lift wings also being high drag. It's acceptable at TL6 velocities, but not at TL10.
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Old 08-15-2013, 03:51 AM   #13
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Default Re: EuroSpace E950 Minerva Executive Transatmospheric Vehicle

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There is a trade-off with the TL6 high-lift wings also being high drag. It's acceptable at TL6 velocities, but not at TL10.
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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Old 08-15-2013, 06:22 PM   #14
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Default Re: EuroSpace E950 Minerva Executive Transatmospheric Vehicle

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Well, yes and no. You can still go round and round at a suborbital velocity while attaining orbital velocity.
Only by continuously expending energy to maintain altitude.

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No you're not. Notice that the aAccel is the aAccel the craft maintains in addition to maintaining its current speed.
That is only true in "space" (whatever that means). If you out of the atmosphere but not yet in orbit (or passed escape velocity) then you are on a suborbital trajectory. If you are trying to go up with 0.05 G of thrust and being pulled down at ~0.8 G you are going to fall. One way to get around this is to "jump" out of the atmosphere (using, for example a combination of rockets and air breathers) and then accelerate to orbital velocity before you fall back into the atmosphere. That is not going to work if the accelerate-to-orbital-velocity part takes 6 hours. Well, I guess it all depends on how high you jump...but yeah I really don't think that would work. :/

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What is a proper free falling speed? If you're talking about terminal velocity for a free fall, then I'm not sure how it's relevant: the craft is not being in a free fall unless you failed to reach stall velocity at the runway (in which case you've got a totally different problem on your hands).
No sorry my bad, I just meant "orbital velocity".

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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.
And here I'm talking about keeping your self out of the atmosphere while you accelerate to orbital velocity.
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Old 08-15-2013, 07:13 PM   #15
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Default Re: EuroSpace E950 Minerva Executive Transatmospheric Vehicle

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Well, yes and no. You can still go round and round at a suborbital velocity while attaining orbital velocity.
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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Old 08-16-2013, 03:10 AM   #16
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Default Re: EuroSpace E950 Minerva Executive Transatmospheric Vehicle

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Originally Posted by MatthewVilter View Post
Only by continuously expending energy to maintain altitude.
Which is already factored into the stats.

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That is only true in "space" (whatever that means). If you out of the atmosphere but not yet in orbit (or passed escape velocity) then you are on a suborbital trajectory.
Well, of course you need to time your ascent such that you're going from Very Thin to Trace, you've already attained orbital velocity.

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If you are trying to go up with 0.05 G of thrust and being pulled down at ~0.8 G you are going to fall. One way to get around this is to "jump" out of the atmosphere (using, for example a combination of rockets and air breathers) and then accelerate to orbital velocity before you fall back into the atmosphere. That is not going to work if the accelerate-to-orbital-velocity part takes 6 hours. Well, I guess it all depends on how high you jump...but yeah I really don't think that would work. :/
Why would I ever thrust up? You can't fly a TAV up unless it can do >1G. In fact, flying up is totally against the whole point of building a TAV.

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And here I'm talking about keeping your self out of the atmosphere while you accelerate to orbital velocity.
You attain the velocity gradually as you leave the atmosphere gradually.
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Old 08-16-2013, 03:24 AM   #17
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Default Re: EuroSpace E950 Minerva Executive Transatmospheric Vehicle

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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).
No TAVs are built in the hope of having an air speed that high. The whole point of a TAV is that you don't need either of the two. The Molniya does 6k mph. The Pegasus does 4k. The Saturnian SATV does up to 3k.

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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.
Uh, no. You never try to exceed high machs like those. You reach your stall speed, lift off the runway, attain your cruise speed (or maybe top speed), and then you gain altitude gradually, which reduces atmospheric density (well, you fly to where it's lower), increases both your top speed and your stall speed at the same time. Somewhere between Very Thin and Vacuum atmosphere, you get your orbital speed.

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:
Originally Posted by TS52
Spaceplanes: A streamlined delta lifting body or
other flight-capable TAV may be able to fly into orbit
on worlds with very thin or denser atmospheres. It
takes 0.046 (O - A) / (sAccel) hours, where O [orbital speed] and
sAccel are as above, and A is maximum air speed in
mps. The spacecraft must have sufficient Burn
Endurance for the flight.
The whole point of TAVs is that wings allow you to ignore the >1G requirement and have a sub-orbital airspeed. Notice that if your airspeed exceeds orbital speed (like you claim it has to), you get negative time-to-orbit.

(Also, where's what's the page for the 1/(L/D) rule? I can't find it)
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Old 08-16-2013, 11:31 AM   #18
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Default Re: EuroSpace E950 Minerva Executive Transatmospheric Vehicle

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No TAVs are built in the hope of having an air speed that high. The whole point of a TAV is that you don't need either of the two.
Yes you do. I don't have the stats for the TAVs in front of me, but all the ones I remember have >1G.

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...
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Uh, no. You never try to exceed high machs like those. You reach your stall speed, lift off the runway, attain your cruise speed (or maybe top speed), and then you gain altitude gradually, which reduces atmospheric density (well, you fly to where it's lower), increases both your top speed and your stall speed at the same time.
What you're missing is that your mach number continues to climb, because speed of sound does not increase at high altitude, and the result is that the stagnation temperature of air at your leading edge also goes up. At mach 25, stagnation temperature of air is on the order of 30,000K.
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(Also, where's what's the page for the 1/(L/D) rule? I can't find it)
It comes from Real Physics, not GURPS. Basically, if GURPS is letting you do this, GURPS is wrong.
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Old 08-16-2013, 12:04 PM   #19
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Default Re: EuroSpace E950 Minerva Executive Transatmospheric Vehicle

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Yes you do. I don't have the stats for the TAVs in front of me, but all the ones I remember have >1G.
I think the Saturnian TAV has worse acceleration than Saturn's gravity. Anyway the formula for TAV launches makes it very clear that aAccel>1G is not required, nor is aSpeed>oSpeed required. I doubt Pulver would've kept it throughout the years if it were wrong.

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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...
Nobody's going for the third option. I don't see any place claiming that TAVs work by #3. TAVs are about option #2.

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What you're missing is that your mach number continues to climb, because speed of sound does not increase at high altitude, and the result is that the stagnation temperature of air at your leading edge also goes up. At mach 25, stagnation temperature of air is on the order of 30,000K.
I must admit that I'd rather leave this paragraph for an actual physicist/engineer to comment, not me.

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It comes from Real Physics, not GURPS. Basically, if GURPS is letting you do this, GURPS is wrong.
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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Old 08-24-2013, 06:34 PM   #20
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Default 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 dont 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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