Being able to accelerate without carrying reaction mass, even if your thrusters don't otherwise violate the laws of thermodynamics, allows access to space really easily.
Highport for a Tide-Locked World
- Thread starter Garnfellow
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Being able to accelerate without carrying reaction mass, even if your thrusters don't otherwise violate the laws of thermodynamics, allows access to space really easily.
AnotherDilbert
SOC-14 1K
The air/raft isn't quite that nippy. Depending on edition the performance differs.
In CT it is specified that it takes 8 h to achieve some unspecified orbit with what appears to be a nominally 1.1 g agrav drive (LBB3, CT Striker). I believe this would be called a G-drive in T5.
The Lifter equipped air/raft in T5 is more sluggish with a cruising speed of only 20 km/h, and very little surplus acceleration.
I believe this should be (starting at rest):
Acceleration is constant, so A = a [m/s²].
Velocity is integrated, so V = at [m/s].
Distance is integrated, so D = at²/2 [m].
To achieve 28000 km/s (over twice the escape velocity) at 0.1 g ≈ 1 m/s² would take 28000000 [m/s] ≈ 1 [m/s²] × t
Orbital speed at 1000 km should be about 27000 km/h ≈ 7500 m/s:
https://en.wikipedia.org/wiki/Orbit#Earth_orbits
I think the Air/Raft uses a Lifter (Z-Drive). The higher speed/performance of some air/rafts likely comes from stage effects due to higher TL of construction. A Speeder likely uses either a G-Drive or (more likely) a Z-Drive for lift with a secondary thrust agency for aerospace maneuvering speed.
The G-Drive is more of a short range maneuvering drive, and takes up a fair amount of space due to being self-powered (by FusionPlus with effectively very long fuel duration) and not requiring a dedicated fuel-guzzling Power Plant (P-Drive) with overclock to support its operation.
Compare the size of a G-Drive to the standard M-Drive. A Z-Drive is simply a hull-fitting.
AnotherDilbert
SOC-14 1K
Agreed, but in VehicleMaker the difference is just slightly higher TL and cost for G-drive, but much higher performance (T5.09, p259).
Speed 6 (100 km/h) is very high for Lifters, much easier to achieve with Grav drive.
The Enclosed Air/Raft on p265 appears Lifter-based, but is very slow; Speed 3 (20 km/h).
mike wightman
SOC-14 10K
Isn't 28,000 km/s just a tad high for Earth's escape velocity?
11.2 km/s is all you need...
11.2 km/s is all you need...
AnotherDilbert
SOC-14 1K
Agreed, which is why I pointed it out: it was used by Grav_Moped in his calculation:
mike wightman
SOC-14 10K
Yup, I was agreeing with you.
Hmm, is he getting feet per second and metres per second mixed up?
Ah ha, spotted it. He converted 28000 kph to 28,000,000 m/s instead of m/h - you have to divide by 3600 to get m/s.
Hmm, is he getting feet per second and metres per second mixed up?
Ah ha, spotted it. He converted 28000 kph to 28,000,000 m/s instead of m/h - you have to divide by 3600 to get m/s.
Thanks for the catch.
You can hit 7.778km/sec pretty quickly even at 0.1G. (13 minutes or so, unless I've made another unit conversion error...)
You can hit 7.778km/sec pretty quickly even at 0.1G. (13 minutes or so, unless I've made another unit conversion error...)
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I don't have that edition, alas.
I was working with the LBB3 ('81) stats cross-referenced against Striker's Design Sequence Tables to get 1.1G from the Air/Raft's 120kph absolute top speed. The LBB2 description pretty clearly indicates that the speed constraint is aerodynamic, rather than due to an arbitrary drive limitation. "An Air/Raft can cruise at 100kph (but is extremely subject to wind effects) with some capability of higher speed to 120kph." And since it can reach orbit (not just orbital altitude), the maximum speed in vacuum has to be no less than orbital velocity. I'm not assuming that there is more than 0.1G left over after cancelling the vehicle's weight, but do assume that the 0.1G can be applied in any direction.
Stated time to an unspecified orbit is equal to the planet's UPP size digit in hours. That's a quite convenient figure for game purposes, but based on the obvious flight profile it's a vast oversimplification. Factors that could increase time-to-orbit over the simple case I've been describing are wind at low and mid altitudes, and having to "get under" the desired orbit and if necessary wait for the appropriate time to begin the ascent in order to rendezvous with something in that orbit.
Another thing this suggests is that for long-distance Air/Raft travel in atmosphere, the quickest way to get somewhere is likely to go straight up until in near-vacuum, then take a great-circle route to the destination and descend almost vertically on arrival (with as much aerobraking as your Air/Raft skill permits...).
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AnotherDilbert
SOC-14 1K
Agree completely, except that in atmosphere the thrust giving the 0.1 g is used to maintain the 100 km/h speed. So in atmo, we are limited to 100 km/h horizontal speed with no extra acceleration.
Agreed, but any result we calculate has to be reasonably close, despite that number being presumably picked at random.
Agreed, unfortunately.
But it is difficult to ascend and descend vertically in a rotating system. Ovals and spirals are the natural shapes of movement in rotating system in a gravity field. As you ascend you have to increase speed to stay over the same spot on the ground as the angular speed is constant but the radius is increasing. But in atmo we have a top speed of ~100 km/h so we can't increase speed, hence the planet will rotate away from us.
AnotherDilbert
SOC-14 1K
V = at.
V = 7778 m/s, a = 0.1 g ≈ 1 m/s.
So 7778 m/s ≈ 1 m/s² × t ⇒ t ≈ 7778 s ≈ 2.16 hours.
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Sorry, this was a wall-of-text thread drift.
I moved it to here, where it's on-topic.
With thanks to AnotherDilbert for the math, Air/Raft performance:
Quick summary:
[Edit: it takes 1 hour to get to 30km (drag-limited maximum airspeed of 30kph since it's a flat plate as it goes straight up... After that the air's thin enough that it accelerates normally.)]
An Air/Raft takes 12 minutes to ascend above the mesosphere (100km) and into soft vacuum.
Up there, travel time to anywhere on the planet is essentially T=2*(sqrt(D/0.1G)), plus the 30 minutes spent on the ascent and descent. EXCEPT average travel speed is 12,500kph (maximum speed 25,000kph at midpoint) to keep from exceeding escape velocity. Aerobraking, if the craft has re-entry heat shielding, can cut about a third off that but the passengers experience 3G for a few minutes.
Wind has at most 12 minutes to affect the ascent path.
Geographic displacement from vertical ascent at 100km due to planetary rotation is under 10m/sec at the equator. It's far more significant at orbital altitudes.
Orbit:
Time to establish Low Earth Orbit (2000km altitude, 7.8km/sec): 5.66 hours [Edit: 4.66 hours is incorrect.]
Time to 2000km altitude: 3.5 hours [Edit: 2.5 hours ws incorrect.]
Time spent accelerating laterally at 0.1G: 2.16 hours, starting 1.25 hours after takeoff.
This is for a nonspecific orbit. Achieving a specific orbit (a particular orbital inclination, or rendezvous with an object in a specific orbit) may take substantially longer -- which may explain the nominal 8-hour trip time.
Descent takes similar time, especially if aiming for a specific destination from an arbitrary point in the orbit. Aerobraking can help; see above.
This excludes the contribution of the surface rotational velocity at the point of departure (460m/sec at the equator, 390m/sec at 45N or 45S)
I moved it to here, where it's on-topic.
With thanks to AnotherDilbert for the math, Air/Raft performance:
Quick summary:
[Edit: it takes 1 hour to get to 30km (drag-limited maximum airspeed of 30kph since it's a flat plate as it goes straight up... After that the air's thin enough that it accelerates normally.)]
An Air/Raft takes 12 minutes to ascend above the mesosphere (100km) and into soft vacuum.
Up there, travel time to anywhere on the planet is essentially T=2*(sqrt(D/0.1G)), plus the 30 minutes spent on the ascent and descent. EXCEPT average travel speed is 12,500kph (maximum speed 25,000kph at midpoint) to keep from exceeding escape velocity. Aerobraking, if the craft has re-entry heat shielding, can cut about a third off that but the passengers experience 3G for a few minutes.
Wind has at most 12 minutes to affect the ascent path.
Geographic displacement from vertical ascent at 100km due to planetary rotation is under 10m/sec at the equator. It's far more significant at orbital altitudes.
Orbit:
Time to establish Low Earth Orbit (2000km altitude, 7.8km/sec): 5.66 hours [Edit: 4.66 hours is incorrect.]
Time to 2000km altitude: 3.5 hours [Edit: 2.5 hours ws incorrect.]
Time spent accelerating laterally at 0.1G: 2.16 hours, starting 1.25 hours after takeoff.
This is for a nonspecific orbit. Achieving a specific orbit (a particular orbital inclination, or rendezvous with an object in a specific orbit) may take substantially longer -- which may explain the nominal 8-hour trip time.
Descent takes similar time, especially if aiming for a specific destination from an arbitrary point in the orbit. Aerobraking can help; see above.
This excludes the contribution of the surface rotational velocity at the point of departure (460m/sec at the equator, 390m/sec at 45N or 45S)
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AnotherDilbert
SOC-14 1K
Sorry, I didn't see the tread change, moving post...
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AnotherDilbert
SOC-14 1K
Escape velocity is ~11 km/s ≈ 40000 km/h (≈25000 mph I believe).
Condottiere
SOC-14 5K
I think that whatever anti gravity device that air/racfts and other such vehicles use operate on a bell curve.
The closer to the ground, the more performance is affected by the need to negate local gravity, and the higher us the faster they can go, until they reach some form of plateau where the anti gravity motors lose traction with the gravity field, and while they can climb, the engine performance declines increasingly as well, capping orbit height.
The closer to the ground, the more performance is affected by the need to negate local gravity, and the higher us the faster they can go, until they reach some form of plateau where the anti gravity motors lose traction with the gravity field, and while they can climb, the engine performance declines increasingly as well, capping orbit height.
