Hacker News Reader

Comment by topmonk | Hacker News Reader

I know this is probably a very stupid idea, but I was thinking what if we shot ourselves out of rail guns into the air into a bullet shaped craft with fins?

You could be shot out of one rail gun in San Francisco, glide for awhile, fall to the earth and caught by a “reverse” railgun in San Jose.

It'd be cheaper and safer than planes since no need to carry fuel or an engine on board, and could have an emergency parachute incase something went wrong.

Or maybe I've been dreaming about Kerbal Space Program projects too much lately. (I'm fully expecting to be flamed and jided for this)

View on HN

That's not a stupid idea! Just keep the g forces to less than 2. https://en.m.wikipedia.org/wiki/Mass_driver#On_Earth

For personal drones, we could consider using railgun tubes to accelerate vertically and then glide. Without considering drag, a 100m vertical railgun could lift a craft to about 300m. With a 50-1 glide ratio, you could get to 15 km from launch without further propulsion.

If shot from a Burj Khalifa height at 2g, you'd accelerate for 9 seconds to 176m/s and reach a height of over 2000m in less than 30 seconds... allowing a glide of over 100km.

Another possibility is using aerostatic forces to support a giant platform in the sky, anchored to the tops of buildings. Would need to support an additional 200kg of weight climbing up the anchor line, to heights as high as 20km. From there, could rocket launch, fly, etc. https://en.m.wikipedia.org/wiki/Atmospheric_satellite#High-a....

In addition to nanotube cables, we'd also need to develop ways to mitigate the acoustic and electrical vibrations in the cable. Might be a way to generate electricity and dampen at the same time.

Not so sure about the "safer than planes" - planes are pretty safe really. Also I think the initial acceleration would be uncomfortable for passengers if they somehow retained consciousness.

Edit - the US navy rail gun apparently produces 60,000 G, humans black out around 10.

You might just about be able to do SF to San Jose with a glider and winch launch if they improved the tech 3x or so. Longer cable etc.

Time is a major component of g force tolerance. Healthily people can take 15g’s for short periods without issue, but extend that to a minute and it can prove deadly.

In ideal conditions linear acceleration to Mach 2 at 5, 10, even 20g is fine. But, if you want to go much faster the limits are reduced significantly. The Soyuz for example can hit 8.5g’s for about 60 seconds in some situations which adds up to ~Mach 14.6 and is a rather extreme situation.

Not only is this a great idea, it's done in the real world. Once you realize you don't need the whole aircraft inside the rail gun, but only a hook, this is how planes take off from the newest US aircraft carrier USS Gerald Ford. The system is called Electromagnetic Aircraft Launch System, EMALS [1].

If one day we start using this in the civilian aviation, this is probably one of the lowest hanging fruit when it comes to lowering aviation emissions.

[1] https://en.wikipedia.org/wiki/Electromagnetic_Aircraft_Launc...

This sounds like a failure mode for Hyperloop.

Edit: Also reminds me of a rather entertaining failure mode for UK AGR nuclear plants - but that's another story.

Heh, this is a really fun idea, actually, ridiculous, but fun. It would probably be quite inefficient, since you need a high enough speed to be worth it since the trajectory is ballistic. The higher the speed needed to get where you want, the greater the energy losses, since air friction does not scale linearly with speed, but exponentially. This is the reason I think it would be inefficient energy-wise.

I actually like the Hyperoop model for transportaion the best, since the my understanding you would have the least energy losses.

It's not the stupidest idea ever: I want to build a mega-slingshot for commuter purposes (and squirrel hunting). At least your idea has some possibility of controlling acceleration forces, and having the subject/ammunition survive the actual launch.

My idea is still more economical to prototype; which means I'll be first to the important step of courting investors; and of course after that who cares if the idea ever happens or was even possible to begin with.

Humans are comfortable with accelerations of maybe 1 m/s/s. So to accelerate comfortably to a plane-like speed of say 300 m/s, your railgun would have to be 600m long - comparable to the tallest skyscraper in the world - and likely pointing at 45 degrees up.

On the way down the vehicle would be going the same speed it went up at - so you're talking about plummeting ballistically towards the ground at 300 m/s. Even assuming you can steer perfectly, what happens if something goes wrong with the receiving railgun? You've got no way to abort and go around, and no time to do... well, anything, really.

Parachutes for vehicles the size of a passenger aeroplane are not practical. A few very small planes have emergency whole-aeroplane parachutes, but they're not to be relied upon. The German army experimented with parachuting a light buggy with two soldiers in and gave up after several failures. And even if you had a working parachute, it still requires a skilled operator and a safe landing zone - what if you hit power lines, or trees, or buildings?

Uh... 60mph is ~27m/s. Are you saying that drivers need to accelerate to 60mph in half a minute in order to be comfortable?

Just gravity is more than 1m/s^2, around 9.8m/s^2

Is the implication that most humans are comfortable with freefall?

No, the implication is that humans are perfectly comfortable with way more than 1m/s^2, particularly if it is linear and you are comfortably seated.

As an example of Fermi estimation, it's pretty reasonable. 10 m/s is too much, given that most people wouldn't want to spend a lot of time accelerating at the rate of a Corvette on a drag strip. So the correct number is between 1 and 10.

First, we’re talking about a short burst of acceleration, not a multi-day burn on the way to Mars. 1g of lateral acceleration is not a big deal for a few seconds. Most people think it’s fun.

Unfortunately that was actually the least wrong thing about lmm’s comment. We’re not launching a rock, it will not come down as fast as it goes up. And I don’t know what the mixup is around a “receiving railgun”.

Perhaps, though not really. Commercial aircraft generally accelerate at takeoff at around 2-3m/s. For most people, that is quite enough, and that only gets you to 140kt. By comparison, doing 1g for 5 seconds (being generous) only gets you to 95kt. So you aren't really going to be able to get much flight out of that. More realistically, a gun would have to accelerate you to nearly the speed of sound or more. Mannned rocket ships have a (throttled) peak acceleration of 3G, so that sets a realistic upper bound and probably way over what might be considered reasonable. I've read that the Willis tower elevators accelerate downward at 8m/s and that is uncomfortable and just for a short time. It's likely the case that such acceleration would feel better if one were lying down. It's a curious question.

Not as stupid as you say. This type of idea has been considered for satellites, to get them most of the way to where they need to be: https://en.wikipedia.org/wiki/Space_gun

The deviation from your initial trajectory is 1/2at^2 where "t" is time of flight, and "a" is net unexpected accelerations. A 30 minute flight time is 1800s, meaning that if you have a target with 20m of give, you can only tolerate an error in your predicted net acceleration of 0.000001g. This is not even close to achievable at launch time.

Long story short: make sure you have corrective gliding maneuvers as part of your imaginary system;)

For one, a wayward gust of wind would be enough to send people careening into buildings.

It's a fun idea, but G-Force makes it unusable for people. The speed to which you'd have to get, over the length of the barrel would make it unsurvivable, or at least very unpleasant.

Probably drag from the air will be too high on high speeds so you won't glide that far? Maybe it can work on Mars? :)

Certified skydiver here. Mid air collisions would be tough to avoid. Altitude and course management for this at scale is a nightmare.

Changing weather patterns could also do serious damage, and you would need pretty big landing areas all over the place as well.

Good idea though!

Reply on news.ycombinator.com