Comment by msandford | Hacker News Reader

Comment by msandford | Hacker News Reader

It's an interesting idea to be sure. And it's definitely one that seems kinda crazy, which probably means it could work. But there's one crucial detail missing: calculations on the centripedal/centrifugal force necessary to continue acceleration and/or direct the payload from horizontal to partially/completely vertical.

http://en.wikipedia.org/wiki/Centrifugal_force

a = v^2/r

so let's go with 1km/sec and a diameter of 5m. a = 1,000 x 1,000 / 2.5 = 400,000 m/s^2 or roughly 40,000 g

Now let's assume that it's a 1/4lb weight as described or 0.1kg.

F = m x a = 0.1 x 400,000 = 40,000 newtons.

Next let's assume that the piece of metal is steel and roughly cubic. It's density is around 8g/cm^3 (http://hypertextbook.com/facts/2004/KarenSutherland.shtml).

Mass = density x volume => volume = mass / density = 100g / 8g/cm^3 = 12cm^3

cube root(12) ~= 2.3 so we've got a cube with faces around 2.3cm on a side.

They've said that they're going to encase it in plastic so let's neglect the strength of the plastic and call it 2x the size of the cube. That brings this to 5cm x 5cm.

Now let's translate that into pressure.

P = F / a = 40,000 / (.05 x .05) = 16 MPa

The tensile yield strength for a regular, boring steel (A36) is 250MPa and the ultimate tensile strength (the max it can hold prior to breaking but after deforming) is 400MPa so this is fine for now.

http://en.wikipedia.org/wiki/Ultimate_tensile_strength

At 2km/sec you get 64MPa needed and at 7km/sec you need 784MPa all of which are within the realm of possible but rapidly heading towards the limits of material strength.

The 784MPa number would go down if you made the diameter bigger, too.

Ultimately they aren't building a thing which will disintegrate the slug (my initial thought) but it's going to have to be extremely well engineered and precisely made for balance in order to ensure that it doesn't tear itself apart. The technical combination of a steel rolling mill and a swiss watch. And due to balance issues definitely harder from a technical perspective than even the really tough portions of a rocket.

EDIT: didn't realize that "*" did formatting so I fixed it.

EDIT2: Screwed up on the calcs, assumed 10,000m/sec instead of 1,000/msec. Fixing those.

This is so funny, I was just watching "Modern Marvels - Weird Weapons of WWII (The Axis)". Turns out, a similar approach was discussed by Werner von Braun (the father of modern rocket science, basically the most important guy other than Goddard in that whole field) to create a "space plane" with the capability of quickly dropping a (radioactive, but not atomic) bomb on a target anywhere in the world, but more specifically major US cities. One major difference was that the track to launch the space plane was straight, and not curved. Given it was curved, would they have still been able to launch the lightweight space plane with just enough fuel it needs to get up to LEO, or would they still have run into the issue that they can't get the plane to go fast enough and point upward enough to break the atmosphere?

Source: http://youtu.be/EHZC9Hab05s?t=2m26s

In the book "The Moon is a Harsh Mistress" (1966), an electromagnetic catapult is used to deliver goods from the Moon to Earth. It is similar to what you described -- just a really long tube (or rail) at the highest elevation possible.

my bet is that the next important guy in this field - Musk - will build Hyperloop v2.0 as a launch system (the Hyperloop 1.0 supposedly being a supersonic electromagnetically driven mini-Concorde in a tunnel seems like a nice prototype)

When I first got the space bug as a young man (was going to be an astronaut like many of my peers) I realized that if you ran around the equator fast enough you could put yourself "in orbit" at ground level. There is an orbital velocity where your angular momentum is in balance as Galileo figured out.

Of course you burn up because you are moving so fast. But imagine that you put a big tube around the planet and pulled a vacuum in that tube. Then you could be in orbit just off the ground, how fun would that be?

Just as impractical as this idea however as you'd have to hold the vacuum in a very very long tube.

Also, if I'm reading Wikipedia right, delta v to LEO is 10km/sec, and that's not counting the insane atmospheric drag on something travelling that speed at sea level. Good luck.

So assume 1000 meters in diameter and 15km/sec for giggles.

a = v^2 / r = 15,000 x 15,000 / 500 = 450,000 m/sec^2 or 45,000g. It's on the same order of magnitude as the previous calcs.

That's obviously also a big engineering challenge because it's freaking HUGE, especially for the precision required.

The really great thing about going super-super fast is that at 15km/sec you're in space in ~10 seconds. Obviously you're going to start scrubbing speed really fast since drag is proportional to velocity squared. But you can evacuate the whole launch assembly and put some kind of an explosively opened door at the exit point and maintain your speed up until the last second.

The amount of drag during the acceleration phase is going to be inconsequential compared to tearing through the entire atmosphere at an angle.

what if you launched two vehiucles, but the first vehicle is more like a bullet that disintegrates and causes a wake in front for the second vehicle to pass thru with less drag?

That still involves sending the first at the requisite speed, and any drafting you might get is limited at best. It'd also leave a highly unstable wake trail, as any object ripping through the air at 7-8km/s is going to tear things up.

As soon as the first one hits the air it starts to slow down. The second one, as it's hauling ass through the wake, doesn't slow down. The difference in speeds gets very big, very quickly. As such the second one, the payload, would find itself smashing into the first one in short order.

Since this thing can launch stuff continously maybe you could lunch few hundreds punchers to push air aside before the actual payload.

It doesn't matter how many you send. If your scenario relies on the fact that the last in line must be the fastest of the group, it's always going to run in to the object directly in front of it.

I was imagining air behaving like water. If you send first projectile it'll slow down but move some water to the sides. Second will slow down less so it'll catch up with the first one but it will also push some water to the sides. Third also will catch up with the previous ones but each subsequent one will get farther as it travels through space punched out by previous ones.

I don't think it's practical in any manner but I think such brute-forcing stream of projectiles out of atmosphere by literally punching hole in it might be nice to look at.

Since it's such a crazy idea, why not build a gigantic dirigible to take it up to high altitude? No problem, right?

I love it!

Because what matters for getting into space / orbit is velocity to achieve orbit. An object without that velocity will simply fall down to earth instead of orbiting.

https://en.wikipedia.org/wiki/Orbit

I believe his suggestion is to use the dirigible to lift the Slingatron into the high atmosphere where the tremendous atmospheric drag will be substantially reduced, thus making a crazy idea slightly less crazy and at the same time, more crazy.

All I can say is I love the kind of crazy going on in this thread and all of you should buy kerbal space program.

Newton's third law gets in the way of that proposal.

When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction to that of the first body.

If you plan to generate acceleration using mechanical force like the reference design, you'd need a massive counterweight. An equal and opposite energy would be transferred to the counterweight, so you need a lot of mass, which would require an even more massive dirigible. You also need somewhere to dissipate that energy. In the reference design, the Earth is used as the counterweight.

Alternatively, you could generate the circular motion using something like a rocket engine, in which case you might as well just use a rocket for linear acceleration and ditch the gyration mechanism.

You would, i assume, build the real thing on an equatorial mountain.

In the video, they address that you would still need a rocket both to circularize your orbit, and possibly recover kinetic energy lost in the atmosphere.

"rapidly heading towards the limits of material strength"

indeed. tho, we should also note that we are looking at a weakest link thing. can that one bolt sustain that much pressure? or maybe that one weld? or any of the concrete parts that attach to the earth itself?

shrug

m3mnoch.

The first thing that struck me was that curved ramp at the end of the track. So, that curve is going to deflect masses going at 7km/s. Really. (And this thing is 100m-1km in diameter and built to the tolerances of a watch...)

It could use quantum locking to keep payload on track. No friction and some people say that the locking effect is pretty strong.

1) how about carbon fiber as material? 2) what about internal friction during acceleration?

The thing with the tightly curved track idea is it introduces many difficult problem while solving only a few less pressing problems - the problem I can think of would getting a long track of land and having to around that long track. But there's the centrifugal force, there's it's effect on any delicate machinery inside the capsule and there's the potential for electromagnetic weirdness from any charge in the capsule. Also, the curve track would mean that a capsule going off the rails could go in a totally unpredictable direction.

Also, you'd durability as well as raw strength of materials will come in here since they'll be trying to use the thing repeatedly.

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