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Old 10-21-2013, 12:32 PM   #41
vicky_molokh
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Default Re: EuroSpace E950 Minerva Executive Transatmospheric Vehicle

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Altitude.

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).
Hmm. But crafts suitable for reentry should already be capable of dealing with that problem, right?
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Old 10-21-2013, 12:48 PM   #42
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Default Re: EuroSpace E950 Minerva Executive Transatmospheric Vehicle

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Hmm. But crafts suitable for reentry should already be capable of dealing with that problem, right?
Assuming they can manage aerodynamic lift, use non-ablative heat shielding, and can withstand re-entry temperatures for a sufficient time period, sure.

We have exactly one example of a craft that can do this (the Space Shuttle), and it flies like a brick -- glide ratio at hypersonic speed is 1:1 (i.e. drag == lift), which is totally useless for flying into orbit (as in, it would take less fuel to not use the wings).
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Old 10-21-2013, 01:04 PM   #43
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Default Re: EuroSpace E950 Minerva Executive Transatmospheric Vehicle

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Assuming they can manage aerodynamic lift, use non-ablative heat shielding, and can withstand re-entry temperatures for a sufficient time period, sure.

We have exactly one example of a craft that can do this (the Space Shuttle), and it flies like a brick -- glide ratio at hypersonic speed is 1:1 (i.e. drag == lift), which is totally useless for flying into orbit (as in, it would take less fuel to not use the wings).
Well, non-ablative radiative temperature protection systems seem to be TL7 but heavy and expensive (which seems to be reflected by most spaceships taking three systems worth of armour instead of one soft landing system).

The uniqueness of the shuttle though is indeed a problem. As in, we're basically TL6 in the TAV tech path: in the 'spacecraft’s acceleration must also exceed gravity (...), or it must be winged' choice, we are very firmly in the first tech path camp. And yes, I know that SS glosses over the TL of aerodynamics most of the time; due to this, stuff that's probably plausible for 2100-era Responsive hulls isn't any more difficult (system-wise) with TL7 hulls. I suppose it could be seen both as a bug ('hey, humans cannot build TAVs at TL7!') or a feature ('no reason to push a TL6+1 design up to TL9!'). (Come to think of it, having more retro-research into divergent tech paths would be nice. Some of the stuff there might be worthwhile.)
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Old 01-21-2014, 03:03 AM   #44
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Default Re: EuroSpace E950 Minerva Executive Transatmospheric Vehicle

Minor necro:
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… for a hypersonic airframe with a best L/D ratio of 5, a 1G ship needs 25% extra delta-V, a 0.5G ship needs 67% extra delta-V, and a 0.2G ship can't reach orbit at all.
Is there a more general formula for this? I'm looking at the Storm Bird-Class Helium-3 Shuttle (SS6, p. 22) and wondering if it can actually reach orbit. Its 1G acceleration exceeds Saturn's 0.92G gravity, but it is also winged, which initially seems redundant by the Spaceships RAW (being able to take out one of the ram-rockets would drop the price a lot), but might not be enough taking realistic drag into account.
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Old 01-21-2014, 11:12 AM   #45
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Default Re: EuroSpace E950 Minerva Executive Transatmospheric Vehicle

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Minor necro:

Is there a more general formula for this?
Real accel = (base accel) - (gravity / (L/D)) = (base thrust) * (1 - (gravity / ((L/D) * base accel))
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I'm looking at the Storm Bird-Class Helium-3 Shuttle (SS6, p. 22) and wondering if it can actually reach orbit.
It's got an air/ram, which basically has infinite delta-V, from what I recall.
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Old 01-21-2014, 12:22 PM   #46
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It's got an air/ram, which basically has infinite delta-V, from what I recall.
Infinite delta-V for atmospheric flight, but to boost from maximum atmospheric speed to orbital velocity it needs to go to rocket mode and burn reaction mass.

Having wings and also removes any minimum thrust requirement, by the rules, though I doubt the realism of a jet plane with an ion engine added in as an orbital launcher.
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Old 01-21-2014, 12:36 PM   #47
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Default Re: EuroSpace E950 Minerva Executive Transatmospheric Vehicle

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Real accel = (base accel) - (gravity / (L/D)) = (base thrust) * (1 - (gravity / ((L/D) * base accel))
How does that change the mps requirements for a winged spacecraft?

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It's got an air/ram, which basically has infinite delta-V, from what I recall.
Then why does it need 15 Fuel Tank systems for its ram-rockets? Not for orbital maneuvering or re-insertion.

The text seems to imply that the ramjets are only used to add the equatorial velocity to the spacecraft's velocity, though that doesn't make sense to me (even an unpowered balloon at the equator would get the eq.v. for free).
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Old 01-21-2014, 01:38 PM   #48
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Default Re: EuroSpace E950 Minerva Executive Transatmospheric Vehicle

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Infinite delta-V for atmospheric flight, but to boost from maximum atmospheric speed to orbital velocity it needs to go to rocket mode and burn reaction mass.

Having wings and also removes any minimum thrust requirement, by the rules, though I doubt the realism of a jet plane with an ion engine added in as an orbital launcher.
You'd still have a stall speed, which the ion engine would not maintain.
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Old 01-21-2014, 02:43 PM   #49
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You'd still have a stall speed, which the ion engine would not maintain.
You aren't using the rocket for propulsion in the aerodynamic regime. That's what the jets are for. The rocket is to kick you up to orbital velocity afterwards.

It's probably unlikely for the jets to give you a high enough trajectory that you don't reenter long before you could get near orbital velocity with an ion drive.
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Old 01-21-2014, 07:12 PM   #50
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Default Re: EuroSpace E950 Minerva Executive Transatmospheric Vehicle

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You aren't using the rocket for propulsion in the aerodynamic regime. That's what the jets are for. The rocket is to kick you up to orbital velocity afterwards.

It's probably unlikely for the jets to give you a high enough trajectory that you don't reenter long before you could get near orbital velocity with an ion drive.
How do the jets could give you trajectory high enough to re-enter in the first place?
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