Asklemmy @lemmy.ml frightful_hobgoblin @lemmy.ml 3 days ago

Who will win the technology race: VacMagLev, or hypersonic planes?

I want to travel around the world in three hours, who is gonna get me there first?

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VacMegLev would be the correct answer. It's just the best way to move things, energetically speaking, and by conceptual simplicity. It might be hypersonic planes, though, because all you need is the plane and a blatant disregard for pollution or expense. I really hope not.

Suborbital deserves a mention as a sort of in-between option. It coasts through vacuum most of the way, so between antipodes it might actually beat conventional passenger service, for theoretical cost. I should do some napkin math sometime.
- What about rocket slingshots and space tethers? Couldn't they work the same as a suborbital, but cheaper?
  
  I'd split the napkin calculations into "passenger ICBM" and a more efficient scenario where the rocket just gets you to altitude, and the rest happens by skyhooks that recycle all their energy.
  
  Ah shit, I'm doing this, aren't I? Stay tuned for the edit.
  
  Edit:
  
  Alright, to get vertically up, it's around 2 km/s delta v. It would make sense to launch from a jet in the stratosphere, Virgin Galactic-style, so we'll neglect drag, and assume this is all lost on re-entry. (Maybe you could use the heat for manufacturing somehow? Another time)
  
  To get to an antipode suborbitally on your own, it looks like it's the same as for low-earth orbit, which is kind of obvious in hindsight. So, 8 km/s. Distances in between are going to be in between, and we'll measure by fuel used.
  
  Now we pull out the rocket equation, and assume an (effective) exhaust velocity of 2 km/s, for a really pessimistic case, or 20 km/s for an optimistic scramjet-type case. Let's assume each passenger counts for 200kg of dry mass, including luggage and the craft itself.
  
  (e^2km/s ^/ ^2km/s-1) * 200kg ~= 444kg propellant.
  
  (e^8km/s ^/ ^2km/s-1) * 200kg ~= 10,700kg propellant.
  
  (e^2km/s ^/ ^20km/s-1) * 200kg ~= 21kg propellant.
  
  (e^8km/s ^/ ^20km/s-1) * 200kg ~= 98kg propellant.
  
  Meanwhile, for a conventional airline, we'll assume 2L/100km per passenger to make room for modest future improvements in efficiency. For an antipodal trip, that's 400L * the density of jet fuel, so roughly 320kg.
  
  Scramjet cases all win out, and the bad rocket case is in the same ballpark with skyhooks (but takes only a couple hours and you get to be weightless in space). The trick, of course, is that none of these run on pure jet fuel. Conventional rockets will probably will run on some hydrocarbon and some LOX, and a scramjet is very chemistry sensitive and will need something special like LH2. LOX looks like it comes at around 1USD/kg in bulk, while A1 is going for 0.81USD/kg at the moment, so that's not a huge issue. Hydrogen is easy enough to make, but LH2 is god-awful to handle to the point where even SpaceX doesn't want to bother.
  
  There's also still a lot of unknowns about the expense of maintaining hypersonic airbreathing craft - it might well dwarf fuel costs in even the most advanced, refined case. I don't know how easy a few hundred kg per passenger is to achieve with conventional rockets going into near-orbital trajectories. There's also the issue of upper-atmosphere pollution, which could hurt the ozone layer.
  
  I think the conventional + skyhooks case is most interesting scenario, looking at the issues and advantages. Virgin Galactic is taking a b-line for profitability and mass access, and there's startups working on the kevlar skyhook. Who knows, maybe this will be a thing starting in the 2040s?
  
  What are skyhooks?
  
  It's like a shorter, less demanding space elevator that spins with a counterweight. It would enable you to reversibly fling things in and out of orbits just using mechanical force.
  
  Wikipedia has an article complete with a nice gif of how it would move in this exact scenario, to connect with a craft at 0 groundspeed.
  
  From that gif it looks like the sky hook has to be orbiting. But then its release point is giving objects twice the velocity required for orbit.
  
  Depends where you release. I haven't actually done the orbital calculations for this, but I'm assuming there's some setup that would work for juggling scheduled flights around the globe. If not, it's a better propulsion technique or bust, basically.
  
  Yeah, there must be some point on the arc that gets you back to some point on Earth
  
  Several. Not getting on the skyhook sends you straight down, getting off at the top puts you into a solar orbit (escape velocity from Earth is ~11km/s, local escape velocity from Sol is ~42km/s, and we end up with 15km/s or so). In between releases should somehow do in between things.
  
  You also need to have a skyhook in sync at the destination as you land, though, and they need to switch out between scheduled flights to keep a reasonable momentum, so it gets complicated. I realised after writing this you probably want to be able to survivably "crash land" at orbital speed if you miss the tether coming down, so that adds weight as well.
  
  I'd still guess it's viable.
  
  Done.

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