March Question Roundup

[avatar (Alistair Young)]

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@John D. Bell <jdb@systemsartisans.com> on 2017-04-20 14:23:09 wrote

[A]chieving orbital altitude is only half the problem. To stay up there (bearing in mind that orbit is essentially falling around and around the planet), you also need orbital velocity sufficient to ensure that you keep missing the ground.

And the converse is - this is why reentry is hard. Dissipating the energy due to your orbital altitude is bad enough, but dissipating the energy due to your orbital momentum is a stone cold bitch.

(Or rather, a red-hot bitch, as she screams and ablates through the atmosphere…)

@AI <ai0867@gmail.com> on 2017-04-21 06:03:11 wrote

I’m going completely off the information contained in the post above, so I may be way off here, but:

If you make it all the way up to GEO, you’re okay because the horizontal velocity required for staying above your launch site at that altitude is, in fact, orbital velocity. Staying inside the cylindrical area defined by the two plates might be a problem though.

To me, it sounds like this is a design for a space elevator that sidesteps the materials strength and durability issues and introduces a couple new ones.

@Alistair Young <athanasius.skytower@arkane-systems.net> on 2017-04-21 13:14:19 wrote

You won’t get that far.

As soon as you get out of alignment (which will happen very quickly once you’re out of thick air, from drift), you plummet. Even if you turned up the intensity of the paragravity (and thus your ascent velocity) to the point at which you reach geosync before drift moves you out from between the plates, your “orbital” velocity is still that of the planet’s surface - for Earth, 465 m/s - which is about 2,500 m/s less than it would need to be to sustain geosynchronous orbit and to stay in the paragravity zone.

(While the angular velocity is the same, it’s the tangential linear velocity that matters.)

Remember, there’s no coupling that gives you the horizontal velocity of the orbiting unit when you reach its height. Even if it was built into a station and you were pulled into a bay, you’d still just crash into the trailing bay wall at 2,500 m/s. Messy, to say the least.

@Ru <shearwater@gmail.com> on 2017-04-22 07:44:47 wrote

The thust and delta-V required to stay in the column would be quite a lot less than that required to reach orbit via reaction drive alone, of course. Keeping on station wouldn’t be a particularly challenging engineering problem, especially not to anyone capable of engineering a ~35000km long zone of personalized physics, which sounds a little less straightforward than a nice space elevator

@Alistair Young <athanasius.skytower@arkane-systems.net> on 2017-04-22 12:22:47 wrote

Said thrust and delta-v is at best (GEO) exactly the same as the horizontal (and rather larger) component of what you’d need to achieve that orbit by reaction drive; at any other altitude, you’re trying to move in a forced orbit, and it’s worse. (Meanwhile, of course, you’re spending the energy you would otherwise have expended on the vertical component powering the paragravity system.)

The problem with this system indeed isn’t, strictly speaking, one of engineering - any starship with a torch drive could pull the required maneuvers off or hold up the upper gravity rotor: it’s “why would you want to build this thing in the first place?”.

The standard response to such questions is, of course, “because I could”, but the “we do what we can, because we must” school of thought - while largely immune to economic counterarguments - does depend on the concept in question possessing awesomeness, coolness, radicalness, and a certain degree of elegance. A less-efficient space elevator that requires the loads that it’s lifting to have enough drive that they could make orbit on their own anyway doesn’t exactly scream that; it screams “kluge, megakluge to the max”.

(As a side note, you can obviously get around the drift/horizontal component problem by running the paragravitically lifted objects along a cable, like a regular space elevator. Then you just have to come up with a good explanation for why you didn’t just use the ridiculously-more-efficient solution of just using an off-the-shelf linear induction motor to haul them up there…)

@Ru <shearwater@gmail.com> on 2017-04-23 04:14:51 wrote

You only need a powerful enough engine to maintain synchronous speed throughout your ascent, which for most of the trip is of course much lower than orbital speed. You’re not in a forced orbit, because you’re being held up by the magical antigravity column.

This means you need a delta-V of ~2-3km/s for an earth-equivalent planet, with negligible atmospheric drag (because you’re only in air for <0.3% of the trip) and no gravitational drag. You could do it with a solid rocket. This compare favorably with the surface-to-geo delta-v of ~15km/s, or the LEO-to-GEO delta-v of ~5km/s.

But yes, I’m aware of the general pointlessness of the whole exercise (see below post, etc).

@Alistair Young <athanasius.skytower@arkane-systems.net> on 2017-04-23 04:29:17 wrote

That was badly phrased, yeah. But the essential point is: whatever altitude you want to get off at, you’re going to need to develop the orbital velocity for that altitude. There are no overall delta-v savings to be had, here. If you don’t spend the relevant dV on achieving that, you get to slam into the GEO station at ludicrous speed, then fall back to the planet.

(And even if you just wanted to hop up and down, it would still cost you the same in pre-waste energetic terms as doing so with a regular rocket. The conservation laws will not be mocked.)

As a side note: there still is gravitational drag; it just shows up at a different place in the equations. In this case, it shows up in the energy you have to spend to get from 1G to a net 0G before you can even begin making things fall upwards. Such is life using paragravity in an existing gravitational field.

@nweismuller <nweismuller@hotmail.com> on 2017-04-21 13:23:41 wrote

As the person who asked about the Phidi, and as somebody who has played the game and followed development, I think I have additional evidence against the ‘cronyist nightmare’ scenario vis-a-vis the Phidi Combine:

1. In Jim Francis’ posting of the concept art of the Phidi science advisor on his Deviantart page, the following sentence shows up in the capsule description of the Phidi: ‘Their government is libertarian in the extreme, consisting of a loose federation of trade princes that is run like a corporation.’
2. In-game, Phidi populations gain additional bonus morale for markets over and above the standard morale boost they provide, and, given that ‘morale’ reflects broad-based satisfaction, this implies that they’re not routinely getting screwed over.
3. Phidi bonuses to Sociology research in-game make them uniquely well-suited in-game to establishing a star nation that both has smoothly-operating commercial networks that deliver higher morale to its people and greater revenues for investment, and has developed an effective legal base that lets them administer alien populations justly and without unrest- other playable factions find it harder to research into Sociology, and thus tend to cause a lot more unrest in alien populations they amalgamate into their star nation.

@Alistair Young <athanasius.skytower@arkane-systems.net> on 2017-04-22 12:34:14 wrote

Favorite cousins from another universe: confirmed!

@nweismuller <nweismuller@hotmail.com> on 2017-04-22 23:07:17 wrote

I present then, for your consideration: ‘My Little Seahorse: Friendship is Profit’. grins

@Specialist290 <carpentereli@outlook.com> on 2017-04-21 17:10:21 wrote

It does strike me that – in line somewhat with AI’s train of thought above – while paragravity might not be able to get you into a proper orbit all on its own, it might come in handy for giving your launch craft a boost up to where it can achieve orbit under its own power while avoiding (or at least minimizing, on the craft’s end) the drag losses from boosting in atmosphere. That might allow you to shed some weight and / or add some delta-v from the design requirements.

@Alistair Young <athanasius.skytower@arkane-systems.net> on 2017-04-21 20:30:12 wrote

The thing is, you’re still boosting in atmosphere - you’re just doing it by paragravitically falling upwards, and paragravity is inconveniently energy-conserving.

The gravity drag comes out of your energy budget inasmuch as you have to run the paragravity equipment at -2G to get 1G upwards on a 1G planet, which increases the total PGPE across the envelope. The aerodynamic drag still slows your ascent and saps your kinetic energy, so you have to pump more energy into the gravity envelope to balance the KE/PGPE equation. So you’re still stuck with those, alas.

(On a side note, there’s also the related issue of conservation of momentum, as half of the momentum you apply to pull yourself up is applied to the orbiting station as momentum downward, which you also have to boost it to compensate for. Lifts and counterweights…)

@Ru <shearwater@gmail.com> on 2017-04-22 08:12:45 wrote

Surely if you wanted a handy boost into orbit, you’d make use of one of any number of cheap, cheerful, ancient (and therefore well tested and understood) means of rocket-free space launch, such as space elevator or a magnetic catapult of some kind. If that’s not to your taste, you could always use a ground-based energy source and do laser thermal propulsion, or even a ground-based reaction-mass source like a pellet stream. A giant reactionless elevator seems like a solution without a problem (unless you could “how can i make something that looks more awesome than the neighbour’s space elevator” as a problem).

The more interesting question is perhaps, what (if any) megastructural engineering problems can you solve with vector control that you can’t do much more simply using conventional tecniques? Can you use it to reinforce materials to operate beyond their breaking lengths, and therefore build orbitals or even ringworlds without access to handwavium ultrastrong matter?

@Ru <shearwater@gmail.com> on 2017-04-23 03:58:49 wrote

(after a bit of reflection it seems unlikely that using vector control would be a sensible or even possible way to build an orbital, but the fact that the space elves would appear to have some ability to modulate the weak force there’s another possibility involving macroscale atoms, but that’s a different topic altogether)