[scythe]

>Even in flight, large aircraft will burn a lot of their fuel during ascent.

I've been wondering if this offers any escape. Suppose that you have a power supply from the ground during the initial acceleration, and the final cruising velocity is not much higher. Or just build a huge ramp.

It sounds like a joke at first, but it might not be impossible. You just need some kind of reverse linear induction motor that doesn't require much weight on the plane side. Perhaps the fuselage is the magnet? If the takeoff acceleration is 2g, you need a 1 km ramp. The varying lift of the wings will be an obstacle, though this might be manageable with flaps. Of course, a 2g takeoff would be a dramatic experience for the passengers.

[acoard]

This principle is already in use in "Ski-jump" aircraft carriers[0] like the British and Chinese use, compared to the catapult operated American carriers. The problem is it isn't remotely high enough. It does have an effect on take off distance, so for that short amount would help for fuel efficiency, but then you still have +30,000ft to climb. 737's often cruise at 30-40k feet, as the air is thinner up there so there's less drag and you have better fuel efficiency. Even if you launched airplanes off the tallest structure ever built (Burj Khalifa, 2,700ft), you'd still have the majority of the climb ahead of you. Planes go high.

My non-credible idea would be to just use an Apple-style magsafe charger on the back of the airplane that disconnects midair at 30,000ft and falls on the helpless people below.

[0] https://en.wikipedia.org/wiki/Ski-jump_(aviation)

[scythe]

Yeah, I didn't check to consider that the specific potential energy (10 km times g = 100 kJ/kg) is actually much larger than the specific kinetic energy (200 m/s squared over two = 20 kJ/kg) so my idea of pre-charging the velocity doesn't pencil out.

I do like the magsafe charger idea. However, I anticipate some regulatory issues.

[Manuel_D]

It's not takeoff that consumes a lot of fuel, it's climbing to cruising altitude. You're not going to get much savings with a launch catapult. A catapult really only helps with shortening the runway distances required to take off (e.g. off an aircraft carrier).

[Sohcahtoa82]

You just need a fast enough catapult mounted on a slope. ;-)

An object dropped at 30,000 feet would be traveling at about 3,000 mph when it impacted the earth, ignoring the atmosphere.

Maybe launch at 4,000 mph to overcome drag to throw something into cruising altitude? We'll just wear some noise canceling headphones to block out the OVERSPEED alarms.

[Arelius]

I mean, isn't ignoring the atmosphere ignoring like 90% of the domain?

[bhandziuk]

Just for perspective this is about mach 5. But I guess there is no speed of sound if you're ignoring the atmosphere :)

[Night_Thastus]

A catapult that launches a commercial aircraft with enough force (over a typical runway length) to get it to cruising altitude with no other power source would also turn all the passengers into raspberry jam.

[Ekaros]

I wonder how much energy would be stored in that catapult and what would happen if something went slightly wrong. Like it got stuck midway... Will we have a plane left? Or how many pieces?

[vidarh]

Brush up on your Jules Verne - clearly the solution is (perhaps not so much) a giant cannon....

[njarboe]

So the ramp needs to be about 35,000 feet high?

[kirrent]

If you're going to try and get an aircraft to cruising altitude without using its own energy, surely the easiest concept is with a tug aircraft towing to altitude. Hell, there's even fairly speculative concepts like Magpie envisaging a series of tows.

[matt_heimer]

So if I understand correctly you'd like to shoot planes into the sky with a giant railgun?

[standardUser]

This is how they launch some jets on aircraft carriers. It sounds ridiculous to apply that to a passenger jet, but not really any more ridiculous than the fact that we routinely fly through the air for thousands of miles in the first place.

[Kye]

I'm no expert, but catapulting a sturdy ~30 ton fighter plane seems like a fundamentally different engineering challenge to catapulting a ~400 ton aluminum can.

[standardUser]

The E-2 Hawkeye is one of the largest plane that regularly uses catapult takeoff and it has a wingspan of 92' and weighs about 43,000 lbs.

A220 and 737 both carry roughly 100-150 passengers and have wingspans of 115-120' and the lightest versions weigh around 130,000 lbs.

Seems doable if the jet and catapult system were specifically designed for this purpose. Maybe less plausible for jumbo jets.

[greedo]

Catapults just shorten the runway needed. The plane still needs a ton of fuel to climb to altitude. Plus, I doubt you'd ever get a lot of civilians to fly off a catapult...

[standardUser]

The goal is to reduce the onboard fuel needed to achieve flight. I thought that was obvious from the parent comment.

> Plus, I doubt you'd ever get a lot of civilians to fly off a catapult...

I'm sure many short-sighted people said that about passenger air travel in general. Plus, if you actually watch a video of a modern catapult launch, you will see that it would be mostly invisible to passengers.

[greedo]

It sounds ridiculous because it is ridiculous.

[baz00]

I think getting the passengers to flap has a similar physics outcome.

[hef19898]

Sometimes passengers clap already, we are one letter away from harvesting that energy!