EuroSpace E950 Minerva Executive Transatmospheric Vehicle

[vicky_molokh]

Originally Posted by Anthony

Since Cd*A for any given design tends to scale as Ws^2 * Form Factor, that works out to L/D = 0.5 * sqrt( (pi*e) / Form Factor ), meaning you can just list standard values based on airframe type (with adjustments for mach number).

The (lack of) adjustments for mach number seem to be what you disliked about Spaceships rules, I'm not sure where's the improvement in that - SS seems to assume the best form factor with possibly adjustable shape (this being THS).

Originally Posted by Anthony

Note that it would be fairly simple to produce a table of expected values for spacecraft; it would take some research on supersonic drag coefficients, but then you just come up with a table, and note that air density can be treated as adjusting SM: a SM +6 craft at 1/10 atmospheric density flies just like a SM +12 craft at normal atmospheric density.

Again, how is that plugged into what we've got? An SM+12 craft has the same top speed for a given thrust and streamlining as an SM+6 one. SM has no intrinsic effect on air travel by RAW. If you want SM-adjustment, the SM-dependency has to be implemented first.

[Anthony]

Originally Posted by vicky_molokh

The (lack of) adjustments for mach number seem to be what you disliked about Spaceships rules

Yeah, but you can to a large degree cover this by just using a different drag coefficient for a supersonic airframe and ignoring its subsonic performance.

Originally Posted by vicky_molokh

Again, how is that plugged into what we've got?

By dropping what we've got into a deep hole?

[vicky_molokh]

Originally Posted by Anthony

By dropping what we've got into a deep hole?

And replacing an imprecise method of building spaceships with no method of building spaceships is going to help us design the most optimal spaceships (particularly TAVs) for THS precisely how? Constructive criticism is an awesome thing, but 'destroy everything without any alternative replacement' is not constructive.

[Anthony]

Originally Posted by vicky_molokh

And replacing an imprecise method of building spaceships with no method of building spaceships.

Huh? No, you just toss the existing table and put in a new table. Your max speed in atmosphere is the speed at which your drag exceeds your thrust.

[vicky_molokh]

Originally Posted by Anthony

Huh? No, you just toss the existing table and put in a new table. Your max speed in atmosphere is the speed at which your drag exceeds your thrust.

Ah. When I posted 'what we got', I meant Spaceships. I didn't realise you were limiting your comment to only the speed calculations. Sorry. Though admittedly so far I'm not seeing the whole procedure for calculating it the way you do from beginning to completion as clearly as you are seeing it.

[Anthony]

Okay, here's an example; it's crude, and I'm ignoring supersonic effects (which would modify drag in the 700 mph and higher columns), but it's something like this (---- means below stall, numbers are drag in Gs):

Speed(mph) 100   150  200  300  450  700 1000 1500 2000 3000 4500 7000 10000
SM +5      0.33 0.18 0.14 0.18 0.33 0.75 1.00  2.2  4.6   10   21   46   100
SM +6      ---- 0.23 0.15 0.15 0.23 0.47 0.70  1.5  3.2  6.8   15   32    70
SM +7      ---- 0.33 0.18 0.14 0.18 0.33 0.75 1.00  2.2  4.6   10   21    45
SM +8      ---- ---- 0.23 0.15 0.15 0.23 0.47 0.70  1.5  3.2  6.8   15    32
SM +9      ---- ---- 0.33 0.18 0.14 0.18 0.33 0.75 1.00  2.2  4.6   10    21
SM +10     ---- ---- ---- 0.23 0.15 0.15 0.23 0.47 0.70  1.5  3.2  6.8    10
SM +11     ---- ---- ---- 0.33 0.18 0.14 0.18 0.33 0.75 1.00  2.2  4.6     7
SM +12     ---- ---- ---- ---- 0.23 0.15 0.15 0.23 0.47 0.70  1.5  3.2   4.5

[vicky_molokh]

Originally Posted by Anthony

Okay, here's an example; it's crude, and I'm ignoring supersonic effects (which would modify drag in the 700 mph and higher columns), but it's something like this (---- means below stall, numbers are drag in Gs):
[snip code]

Hmm. First, you seem to have calculated stall speeds based on Spaceships data somehow. Is there a way to calculate it more precisely than to the nearest 50mph? (Very short runways are a recurring potential hazard in another, non-THS campaign, and knowing what the pilots can get away with would be nice. So far they played it face.)

Second:
I just realised that total drags for level flight at normal conditions can be back-calculated from the crafts' top speeds (since this is the point where thrust is insufficient to further increase speed without going into a dive, but sufficient for attaining said speed). So for a craft with a 2,500mph top speed, drag would equal its thrust at 2,500 mph. Nice. Hmm, seems like the PDF is more optimistic about aerodynamic properties / drag coefficients of the spaceships (e.g. this calculates to 1G as neutralised at 2,500 mph).

Third:
as far as I found, drag is directly proportional to atmospheric density, while stall speed is proportional to 1/sqrt(density). So at ½ density, stall speed increases by 41%, but drag is halved. If I understand correctly, the reduced drag can be treated as increased thrust, so a 1G craft with -50% drag will have a top speed equivalent to a 1½G craft. Which is a +22% increase in speed. So yeah, stall is catching up, and precise calculations of stall speeds might be necessary to figure the top speed attainable 'on wings'.

This probably kills the Nuclear Lightbulb-powered TAVs, but Nuclear Thermal Rocket-powered ones might just be good enough for the cost (maybe).

Edit: Again, if I didn't screw up anything, a ½G NTR needs get to an altitude at which ½G of drag corresponds to a speed of 5mps = 18,000mph. Normally, it takes 52G of thrust to keep such a speed (i.e. at 1 atm.). I.e. it needs to get to an altitude with slightly less than 0.01atm. pressure, reducing drag below ½G. With a 100-fold reduction of pressure, stall speed increases to 1/sqrt(0.01) = 10× the normal value. Hmm.

[Anthony]

Originally Posted by vicky_molokh

Hmm. First, you seem to have calculated stall speeds based on Spaceships data somehow.

No, I calculated it based on standard aerodynamics and some assumptions about the shape of a winged craft.

Originally Posted by vicky_molokh

Is there a way to calculate it more precisely than to the nearest 50mph?

You're limited by the quality of your data input, which isn't very good.

Originally Posted by vicky_molokh

I just realised that total drags for level flight at normal conditions can be back-calculated from the crafts' top speeds

Assuming you have accurate figures for top speed, temperature and pressure where top speed is achieved, and thrust at top speed, sure. None of these things apply to top speeds as given in Spaceships (you should basically ignore any atmospheric performance numbers in Spaceships).

Originally Posted by vicky_molokh

Third:
as far as I found, drag is directly proportional to atmospheric density, while stall speed proportional to 1/sqrt(density).

Parasitic drag is proportional to atmospheric density * velocity ^ 2, so at stall speed, half atmospheric density means 41% higher velocity, and parasitic drag is x0.5 * 1.41^2 or x1. Assuming you have enough thrust to fly, and your airframe can survive mach 20+ in atmosphere, you can fly into orbit.

[vicky_molokh]

Originally Posted by Anthony

Assuming you have accurate figures for top speed, temperature and pressure where top speed is achieved, and thrust at top speed, sure. None of these things apply to top speeds as given in Spaceships (you should basically ignore any atmospheric performance numbers in Spaceships).

Why would the default temperature and pressure be anything other than standard ambient temperature and pressure? I do admit that variable thrusts are beyond SS specifications, but apparently that way lies the much-hated VE2e. -_-

Originally Posted by Anthony

Parasitic drag is proportional to atmospheric density * velocity ^ 2, so at stall speed, half atmospheric density means 41% higher velocity, and parasitic drag is x0.5 * 1.41^2 or x1. Assuming you have enough thrust to fly, and your airframe can survive mach 20+ in atmosphere, you can fly into orbit.

Hmm. That sounds as if the unmodified stall speed is reasonably low, and the airframe is good enough (e.g. the THS dynamic Responsive hull), then the TAV works just fine.

BTW, why Mach 20+? The stress should be significantly lower in lesser atmospheric pressure, right? Anyway, wonder how the stressability of the hull is identified; GURPS seems to be operating under the assumption that at a TL sufficient for making TAVs, TAVs will be made with sufficiently sturdy frames (if they meet other criteria).

[Anthony]

Originally Posted by vicky_molokh

Why would the default temperature and pressure be anything other than standard ambient temperature and pressure?

Altitude.

Originally Posted by vicky_molokh

BTW, why Mach 20+? The stress should be significantly lower in lesser atmospheric pressure, right?

The issue isn't stress per se (other than a sharp increase in stress right as you cross the speed of sound, that's not a big deal). The issue is the stagnation temperature of gas. Looking at a calculator:

• At 1 km/sec, stagnation temperature is ~700K, which is too high for aluminum.
• At 2 km/sec, stagnation temperature is ~2,000K, which is too high for steel.
• At 3 km/sec, stagnation temperature is ~3,900K, which is too high for any known material.

Very high temperatures force creative hull design to avoid actually contacting the hot gas, and/or prevent the gas from reaching its full stagnation temperature.

Note that the flight airspeed record for manned air-breathing aircraft is under 1 km/sec, though the space shuttle has gone much faster, as have some unmanned scramjet tests (though the HTV-2 falcon did not survive testing).