[eloisius]

A full tank of gas is a heavier load. The first half of the trip, you’re flying a full tank of gas just so that half of it can be used for the second half of the trip. If you stop over in the middle, you can fly two half-trips with a half tank of gas each time.

[perl4ever]

I've thought for a while that people don't seem to be nearly excited enough about the prospect of refueling spacecraft in orbit as SpaceX hopes to.

[paulmd]

People "get it" when it's the rocket equation for space, but from prior discussions, they don't want to accept the implications as far as electric aircraft.

Electric aircraft you are always "flying with a full load of fuel" which means every joule of energy is "the most expensive energy" as someone here put it, analogous to the last mile of range for a liquid-fueled aircraft. You never have that ability to "take off with a half tank" and trade fuel for extra payload, or operate more efficiently by tailoring fuel to actual needed range, or benefit from higher efficiency at the tail end of a long flight.

Unless electric power becomes significantly more energy-dense, it's always going to be a problem that dogs electric-powered aircraft. And it's hard to see that happening since it's very much the opposite - liquid fuel is significantly more energy-dense than batteries anyway, plus the implications of the rocket equation.

[KptMarchewa]

Pluggable battery packs might be a thing for electric planes. It might be even easier than for cars - for example, there is finite, and relatively small number of start and destination points.

[paulmd]

No, the (aeromotive) problem with electric packs in planes is that the battery weighs as much when you land as it does when you take off, while with a fuel plane you've burned all your fuel and your plane is tens of thousands of pounds lighter. It's like dragging every single stage along with you to space - every drop of fuel is "the most expensive" because you're moving the maximum amount of weight all the time, and you're repeatedly incurring the most energy-intensive phase of flight (takeoff).

To beat (or at least, not completely be ruined by) the rocket equation what you need is droppable battery packs - completely use them up in sequence, then drop them in midair as you fly to lighten the load. (Well, you need much higher energy density than batteries can currently deliver, too, but let's just talk rocket equation.)

Using "droppable" packs also has the implication that each individual pack (or at least a "core" pack) has to be able to support enough current for at least cruising power (and you probably do not want to design an aircraft with the known implication that full takeoff power will be unavailable after a certain duration of the mission - or that taking off with a "partial fuel load" means you do not have full takeoff power available - the FAA is not going to look kindly on either of those approaches, because if you miss a landing, going around is now very difficult, and that is one of the most likely places for crashes to happen already).

As far as pluggable battery packs - major airports already are critically short on available landing/takeoff slots. Airlines bid fiercely to get them already, and would love to add more slots for real aircraft and not gimmick 200-mile-range electric aircraft. And a large aircraft needs a large runway, so you can't just trivially "add more airports", nor do you want too many planes interacting in the same vicinity - two or maybe three active runways is about the practical limit. So that's not a very good idea at all, there's already a critical resource constraint that building an aircraft around high-takeoff-landing-cycle flight models would make much worse. The "meta" of airport landing slot economics already runs heavily to running bigger aircraft so you can fit more fares/cargo per landing slot - the A380 and 787 and other super-jumbo aircraft are basically designed for this situation where you simply cannot land any more aircraft so you have to land bigger ones.

Leaving aside runway slots, takeoffs and landings are also the most dangerous part of flight, more flight cycles means much more danger for the aircraft and its surroundings (and passenger airports tend to be near people, although perhaps this is not a constraint for cargo airports as much). It's also the hardest on the airplane - pressurization/depressurization cycles and wing loadings are what busts up an aircraft to a much greater extent than mere flight hours, there are only so many cycles you can make an aircraft take regardless of flight hours. So if you land and take off a lot more, you have to spend the energy (and financial) cost of aircraft manufacture much more frequently, which likely neutralizes a lot of the environmental benefits. It's also the most energy intensive phase of flight - if you are taking off 5x as much to make the same trip, are you even more efficient within a single flight, let alone when we amortize that the airplane (and it's "build-time emissions") now lasts 1/5th as long due to increased flight cycles?

It's a bad idea for a lot of reasons. Just make more railways, and high-speed railways, we'd benefit hugely from moving as much traffic as possible (obviously not overseas!) to trains and (for cargo) ships. Significantly more environmentally friendly than air travel will ever be.

It's fun from a tech development perspective though. Obviously pushing tech at the edge of capabilities benefits everything - motor technology, manufacturing, battery technology. And those do go into some useful things.

[dylan604]

I'll be more excited about that when the fuel is created in space so that you're not just launching fuel for another launch to take advantage.

[inglor_cz]

Well, depends on what you mean by "space".

The orbit around the Earth is the closest and travel there is the cheapest, but manufacturing fuel out of nothing (vacuum) is pretty much impossible. Technically, what you have on lower Earth orbits isn't vacuum, but a very, very thin gas. It is still probably too thin to be used as a source of stuff for industrial processes.

The closest solid body we have as of now is the Moon. A nice source of minerals, where fuel will be almost certainly produced in situ in the future, but then you still need to fly from Earth to Moon orbit and get the produced fuel to the Moon orbit to tank the ship. (Possibly using a space elevator.) Still quite a lot of delta v.

The best solution could be to intercept a comet full of volatile materials and drag it somehow to the Earth orbit, say, 1000 km above the planet. Of course, if you mess up and hit the Earth instead, you will cause a nice extinction event down there...

[noduerme]

I love HN just for the fact that this conversation started with shipping a laptop and ended up with the dangers of space refueling.

[dylan604]

By "space", I meant not from Earth. Extra-terrestrial might have been a better choice. Clearly, I wasn't propsing hoovering up parts of the atmo or from the vacuum of space itself (do the Brits call it the hoover of space???).

>Of course, if you mess up and hit the Earth instead, you will cause a nice extinction event down there...

Why does it need to be in Earth's orbit. If we have the ability to capture a comet and place it in a more friendly orbit, we'd surely have the ability to park it in the moon's orbit negating a SimCity-esque ELE.

[jhgb]

> but then you still need to fly from Earth to Moon orbit and get the produced fuel to the Moon orbit to tank the ship. (Possibly using a space elevator.

You mean a space elevator from the Moon (since on Earth it seems to be infeasible)? It's probably doable but I suspect that an electromagnetic accelerator would be far more affordable and technologically less demanding.

[inglor_cz]

From the Moon, yes. The cost and technological demand would certainly be significant, but unlike elmg accelerator, a space elevator would be suitable for transport of people as well.

[jhgb]

There's no reason why you couldn't use an accelerator for transporting people. The velocities are fairly low so acceleration can be moderate as well. Also there's no air to complicate things with aerodynamics, so all you have to do is to design a high-speed maglev with auxiliary thrusters and RCS that could lift from the tracks at orbital speed and then land back on them again (which is much easier in the complete absence of atmosphere which turns it into an orbital mechanics problem).

[rocqua]

Would a properly counter-weighted space elevator not have much lower power requirements than an electromagnetic accelerator?

[jhgb]

If you can't realistically build an elevator, the question is meaningless. As for if you could build one, I don't know. Maybe, but chances are that an electromagnetic accelerator working in reverse could recover a lot of the energy. Also, it wouldn't be constrained by having to keep the upmass and downmass ratio bounded. And third, energy is really easy to come by on the surface of the Moon, so it's not like it would be a severe limitation.