Hacker News Reader

Comment by ars | Hacker News Reader

Lest you despair of ever making space flight routine, a rocket is not the only way to get into orbit.

Virtually all of the needed velocity is tangent to the surface, not away from it. So you can accelerate the vehicle along the ground at least part of the way, and only then turn heavenward and burn fuel to get into orbit. With this boost you significantly reduce the amount of fuel needed.

There are many way of doing this. You can have magnetic propulsion (very futuristic and powerful), you can have a simple motor on the vehicle, powered by contact with rails on the ground (but motors have a limit of how fast they can work). You could have fuel "guns" fired into the back of the vehicle as it passes them, which would then capture the container and burn them as a normal rocket would. This has the benefit of not requiring a large rail, just fuel stations that the rocket would pass over.

These are just some basic ideas, there are many more.

View on HN

Starting from a ground propulsion is a priori a bad idea due to friction forces in the atmosphere. An almost plausible design though, would be to have a first stage taking advantage of the atmosphere (i.e. using jet engines with oxygen from the environment).

To build one's intuition about space exploration, Kerbal Space Program is an excellent start.

It's amazing how quickly and clearly KSP makes this line from the article quite evident:

>> The giant leap for mankind is not the first step on the Moon, but in attaining Earth orbit.

I wonder if you could build a particle accelerator of sorts. A toroidal tube in which the vehicle is placed, and magnetically accelerated to launch velocity. Would just need some way to let the vehicle out; some kind of track switch. Such a setup would not require a large area to bring the spacecraft up to speed, and could be vacuum sealed to negate air resistance.

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

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

A long time ago I did the math for putting one of these on Mt Kilimanjaro, with another few thousand or so feet of height magically added. If you build up any significant speed in an evacuated tunnel, even at those very high heights you still face an incredible pressure wave as you exit.

Given that earth is a ball, aren't you always pointed heavenward if you are between 0-180 degrees. So you might as well start of by pointing directly heavenward, i.e 90 degrees. Plus, turning requires acceleration you cannot just capitalise on the velocity you have built up otherwise you would violate the 1st law of thermodynamics.

You don't want to turn. To orbit you need to have velocity tangent to the earth (i.e. along the ground), not directly up.

You only turn ever so slightly heavenward, mainly to avoid air resistance.

I'm still lost. Isn't being tangential along the equator less advantageous than being perpendicular at the poles. When you start tangential, as soon as you cover a distance equivalent to the radius of the earth, you are then perpendicular (maybe not directly so but there is no practical difference as far as the gravity well is concerned).

When going tangentially, you'll maintain tangentiality until you're going so fast that gravity isn't pulling you down fast enough to keep you pressed against the track you're on (because the Earth's surface curves downwards like the widdle spheroid it is). At that point, you're in low earth orbit (though you might want to fire the rockets a little bit to give yourself an orbit that doesn't intersect the planet). The whole point is that you gain your velocity while you're pushing off the earth, instead of your own propellant. That way, you don't have to carry so much propellant.

> When you start tangential, as soon as you cover a distance equivalent to the radius of the earth, you are then perpendicular (maybe not directly so but there is no practical difference as far as the gravity well is concerned).

If you were assuming the tangential take-off would continue going a straight line instead of curving in an orbit, I still don't get what you mean -- after covering one Earth radius, you'd be traveling at a 45-degree inclination relative to the surface of the planet, not perpendicularly.

I see it this way. You have to accelerate the mass to the escape velocity. You also have to achieve a net effect of being in orbit (some distance x above surface of earth). The most direct vector to that distance is perpendicular. The two combined should give you the minimal energy requirement. Any engineering (and aerodynamics) creativity cannot give you anything better.

> being in orbit (some distance x above surface of earth). The most direct vector to that distance is perpendicular.

That's not what being in orbit is. In fact that's the opposite of being in orbit. To be in orbit you need to move parallel (tangent) to the surface of the earth, not perpendicular.

The distance about the surface is entirely for air resistance, and has nothing to do with being in orbit.

i'll spare everybody else the snark here

Zero feelings involved, just analysis before numerical computation. The concepts need to make sense first.

Okay. In this scenario where you accelerate upward then turn 90 degrees and accelerate horizontally, are you talking about this being powered by a rocket or by a track?

@SamReidHughes

> ... accelerate upward then turn 90 degrees and accelerate horizontally

I am not for such a scenario at all. The point of the upward (upward only and not considering the atmosphere) acceleration to the desired location is to give the lower bound of the energy requirements. This is the baseline (baseline-1) and the rocket equation is as simple as possible.

In reality with an atmosphere and to put the object in orbit, the aerodynamics change and the energy requirement increases beyond the above baseline-1.

If you "accelerate upward then turn 90 degrees and accelerate horizontally", you can calculate an energy requirement for that and it is easy. Only two vectors involved. That should give some limit (call it baseline-2). We should expect to do better than baseline-2, how better? A calculation using the diagonal of the vectors involved in baseline-2 should give us baseline-3.

We shouldn't do better than baseline-3. Our launch designs and ingenuity should have an energy requirement between baseline-2 (this is bad, we are not thinking) and baseline-3 (this is maybe closer to ideal).

The rockets and shuttles do "pitch-over manoeuvres" to turn the straight upward acceleration into an elliptical acceleration.

* Note, I have not addressed the complications of the variations in atmospheric drag, but if it varies close to linearly along the vertical cross-sectional then how I think about it above does not change unless there is some other oversight.

I don't get what idea you're communicating here. You seem to be saying that the best option for rockets is to accelerate upwards, out of the atmosphere, gradually transitioning to accelerating horizontally (if this is what you mean by baseline-3). Well okay, that's what we do, when using rockets.

Doing that with a track would be expensive because the track would have to be built hundreds of miles high over all of its length. It would be cheaper to build most of it lower, and maybe accept that we'll have to handle the air resistance somehow. If we build a track that doesn't go out of the atmosphere, we could still use it to build up a lot of speed and then turn the rocket upwards before the thing is self-powered. If we do build a track that goes out of the atmosphere, we'd still want to get as much ground-level acceleration as we can.

Maybe it's more practical to build the track on the Moon, where there's no atmosphere.

The scenario for baseline-3 is just a conceptual tool(pythogras theorem) to establish a bound. It still doesn't get you into orbit.

What I was grappling with, is. I presumed @ars (parent) knew he was talking and in expressing my contention it would get addressed with a little bit more information in what I was missing. I now see some emphasis in his explanation and additional links.

The key is was that tangential acceleration opens up none fuel based acceleration mechanisms i.e change to the type of energy and the quantity (you accelerate less fuel to burn up the fuel).

It's escape velocity, not escape speed - and it's orbital velocity that matters here, we're not escaping entirely. If you're in the same place, travelling at the same speed, but pointed down, you're not in orbit, right? It's the same if you're pointed straight up. You need to be at an orbital altitude and travelling at the right speed in the right direction.

(well, any combination of speed, direction, and position will be an 'orbit' in some sense, in that there's a conic section you're on that you would follow if you were in freefall. But if you're sitting still over a planet then it's the degenerate ellipse that's a straight line down to the planet's core).

So in the absence of atmosphere the ideal ascent trajectory would look pretty much like a Hohmann transfer orbit: you'd accelerate horizontally until you were in orbit at surface level, and then you'd do the minimum energy transfer from that orbit to a higher orbit. In reality it's worth getting to altitude where the air is thinner before turning horizontal, but even so, the vast majority of a rocket's acceleration is horizontal, not vertical.

You can do the maths, but if you want to really understand these things, play Kerbal Space Program. Seriously.

escape speed is technically correct. you need to go fast enough, direction doesn't really matter.

Some directions may require more speed than others.

Or perhaps you are having a bad problem and will not go to space today.

> You only turn ever so slightly heavenward, mainly to avoid air resistance.

And then you turn Earthward, to avoid crashing into the Earth.

You are always pointed "up", but as long as you don’t accelerate too much, earth’s gravity keeps you on a circular trajectory. This is especially true if you fix the vehicle to the ground using e.g. rails. It would then be conceivable to accelerate to a high speed on ground-level rails (which is considered “free” as you can get energy delivered through the rails) until your speed is such that gravity alone does not hold you down any more and then unlock from the rails and fly off.

I did not do the maths.

You would have to build up a lot more speed if all you are going to do is decelerate once you "take-off". Also noteworthy, once you get to the escape velocity you would require an additional force just to keep you circular.

Staitght up is also wrong if you want a circular orbit.

You can still have a rocket on your vehicle to accelerate further after take-off, but you certainly need to accelerate less if you already at a certain velocity than if you are stationary. Since the “certain velocity” is free by assumption (i.e. supplied by ground-based electricty generators or something similar), I don’t see how you could be off worse. But I still didn’t do the maths fully.

Air friction, perhaps? There is more air in a tangent line.

You don't get to orbit by going up far enough, if you did you'd just end up falling back to earth.

You get to orbit by going fast enough, so that acceleration due to gravity acts perpendicular to your velocity and so acts to change your direction and pulls you around the earth.

What really helped me understand why you do not want to go up is the following thought experiment:

Imagine you are standing on Mt Everest (and the atmosphere has magically disappeared), and you would like to shoot a bullet once all the way around the earth, how would you have to fire it? It seems pretty intuitive that shooting it up into the air won't do the trick. If you want to shoot around the earth, you need to fire in parallel to the earths surface, with a sufficiently high muzzle velocity. And really an orbit is nothing else than a shot around the earth.

Yep, you just need the ground to fall away from the bullet as quickly as the bullet is falling towards the ground.

Turning requires acceleration, but you can convert velocity direction with reasonable efficiency by pushing on something (like rails or magnetic field in a tube or just road).

https://what-if.xkcd.com/58/

If you go straight up, you can never attain orbit.

Reply on news.ycombinator.com