The Cold Equations Of Spaceflight

Space Daily.com ^ | 9/9/05 | Jeffrey F. Bell

[SampleMan]

The pressure of what? The atmosphere? I don't think so.

Well then, imagine this. You have a room that is a total vacuum. You open a window on one side of the room. If you have pressure sensors all the way across the room, do think they would show the psi snapping to full atmospheric pressure in a wave, or do you think they would show a the lowest pressure at the farthest point, increasing as the distance to the window decreased?

[SampleMan]

plasma cuts through steel, Plenty to worry about. Your explosive charge might work if it were perfect.

[The Red Zone]

"Plasma cuts through steel" but is the ship just going to sit there while the busted-glass plasma eats into it?

[general_re]

Imagine you have a total vacuum in your room, but instead of opening a window, you remove an entire wall. Now, at t₀, you have a room at 0 atmospheres, with an "outside" pressure of 1 atmosphere, but that's not going to last. The atmosphere will move from high pressure to low pressure, and it's not going to slow down three inches inside the room and decide to take a leisurely pace the rest of the way - your leading edge is going to keep on trucking right on into the rest of the room, because the rest of the room, minus the first three inches, is lower pressure than everything else. And in a tube 100 miles (160,000 meters) long, there's pretty much only one direction for it to go - down the tube, and tout suite at that. ;)

[citizen]

The way I heard it was that it was a JATOB, Jet Assist Take-Off Bottle, straped to the trunk of a car. The bottles are used to get military planes up to speed on short runways.

Long before the D.A. winner left the road at a curve, the brakes had been smoked by the no doubt terrified guy standing on them cause he was really moving.

Somebody estimated 300MPH, who knows.

[SampleMan]

Fascinating, can you fix the water pressure in my neighborhood? Because I live on earth, where a gas or liquid filling vacuum or area of lower pressure decreases in pressure until it completely fills the area and meets itself trying to come back out.

Your assumption is that air molecules would all be traveling perfectly straight down the opening and not interacting with each other. This interaction is what prevents the rising water scenario you describe.

However, this could be even more distributed by putting an inverted cone muzzle on the tube, when the small end opened the entering air would decrease even more greatly.

As any method is just a delaying action anyway though, why not do as I suggested and allow pressure into the tubes terminus from its sides just before vessel passage. I'm sure you would agree that this could be done gradually, as air cannot possibly reach equilibrium within seconds, let alone the thousands of second we're talking about over a length of several miles.

[SampleMan]

You have to get a grip of the time frame.

Plasma has many different meanings in different sciences. For the sake of this discussion I'm talking about super heated gas which is under such enormous pressure it is actually solid again, but not in a "pick it up and look at it" sort of way.

An example of such plasma is a shaped charge. The Monroe effect describes this. When a shaped charge (RPG) detonates, the power of the explosion turns the concave copper into a slug of plasma. If atmospherically released it would turn to gas, but it is under tens of thousand of psi from the explosion, this explosion holds the plasma to the armor, which turns out to be the path of least resistance. In micro seconds (before the explosion shock wave can move 1 foot, the plasma turns the adjacent metal into plasma and burns a hole right through.

Some relatively small explosives (size of you fist) can thus penetrate 12 inches of steel. The hole is less than an inch in diameter, but when the plasma reaches the essentially unpressurized interior it turns back into gas (an explosion) and turns the inside of a tank to an instant 4-5000 degrees.

I take you back to the bird strike analogy. When you hit an object at 6000 miles an hour, it doesn't bounce off. It doesn't matter if its steel or feathers, all that matters is mass.

[general_re]

Because I live on earth, where a gas or liquid filling vacuum or area of lower pressure decreases in pressure until it completely fills the area and meets itself trying to come back out.

Oh, okay. Well, here on earth, let's say the atmosphere is about 30 km thick. Based on that, and the earth having a diameter of 12,750 km, the volume of the atmosphere is approximately 15,393,367,321,000,000,000 m³. When you open your vacuum tube, you're increasing the volume of the atmosphere by the volume of the tube, which, at 160,000 meters long, and let's say 20 meters wide, is a volume of 50,265,482 m³. Finally, since our opening is on top of Mt. Everest, the air up there will average around 228 mm Hg, or about 0.3 atmospheres.

Now you've got all the info you need to plug into P₁V₁ = P₂V₂, so why don't you solve for P₂ and get back to me with the Earth's newly decreased air pressure, eh?

[Phsstpok]

The approach that seems to make the most sense to me is using a progressive rail gun built into an equatorial mountain slope, to shoot a vehicle into space. Thus allowing a theoretical 0% usage of on board fuel to attain orbit.

Read Dean Ing's The Heavy Lifters. He uses a mag-lev rail "service line" as his initial launch track for his spaceships. The heros got the service vehicle contract for the mag-lev line from Los Angeles to Las Vegas and tricked the main contractors into letting them build the service line up the side of mountain. They designed the service vehicle so that it could be used as a launch cradle for their spaceship running at speed up that ramp. They then use a big ground based laser built for "Star Wars" tests to finish the boost to orbit against a bell shaped "nozzle" at the back of the vehicle.

This was all done on the cheap and on the sly but some enthusiasts who have sneaked it in under their Bill Gates or Ted Turner like mega rich bosses nose. The supposed main plot of the book, involving that bosses hatred for long haul truckers and plan to put them out of business with a new type of rigid airship is OK, but clearly is eclipsed by the secret plans of his underlings. I actually don't think Ing intended the book to turn out this way. The best books are sometimes like that.

I put this book into my "trilogy" of entrepreneurial space novels. The other two are Fallen Angels by Niven/Pournelle/Flynn (stealing the Delta Clipper manned prototype from a museum to rescue astronauts from an "Al Gore in charge" greenie nightmare government) and Privateers by Ben Bova (private enterprise space venture wresting control of space from a UN bureaucracy).

We will go to the stars, riding a fire in the sky!

[SampleMan]

Big numbers, but no sense. The outside weather shows that air is elastic not solid. Movement is not instantaneous. Objects in motion and all of that. Your statement doesn't work on water (or hold it) let alone on air. Indeed, when you set off an explosion, it is followed by a vacuum. It doesn't just push out evenly in all directions dissipating to zero. Explosions are in fact a wonderful example of the kind of pressurization we're talking about.

Your instantaneously equalizing world simply doesn't exist.

Your theory would also lead one to believe that pouring a bucket of water into the ocean would instantly raise the worlds coastal waters a molecule or two.

But your numbers are nice.

[SampleMan]

Well, I'd say density.

Negative. At 60 mph density matters, not at 14000 mph. Density only matters when the lack of it provides cushioning as the structure compacts. At unbelievable high speeds, this doesn't happen. It stops acting as an object and starts being a mass of molecules. The first part of the feather would turn to plasma before the last part was even affected. That plasma would merge with the offending object coming at it and create more plasma, it would not press back against the rest of the feather. This is not like a bug hitting a windshield.

[general_re]

Nice strawman - you build that yourself? The point is not that it somehow happens "instantaneously", but that the air is going to flow from high pressure to low pressure. In your tube, that means flowing down the tube from the open end to the closed end. Whether it takes 1 second, 10 seconds, ten minutes, or an hour to equalize the pressure with the outside atmosphere is neither here nor there - the point is that it's going to happen, and that flow is going to move down the tube in the opposite direction of your payload and meet it at some point.

[SampleMan]

Whether it takes 1 second, 10 seconds, ten minutes, or an hour to equalize the pressure with the outside atmosphere is neither here nor there

Beg to differ. From beginning of depressurization to vehicle away would be less than maybe 1/8 of a second. No let's say I have 12 miles of tube. At mile marker one I place one baloon, at mile marker two I place two baloons, and so on, I pop all the balloons at once. The pressure readings are going to differ for some time after. No matter what scale you use this is the way it works. Been fun, but I gotta go coach soccer, I'll see what you have when I get back.

[SampleMan]

Ah, I have five more minutes. In you model, how does the front of the wave "know" what the final equalized pressure will be. In fact, as air enters the tunnel it creates a bit of a vacuum compared to the surrounding area. The air molecules bounce around, vice proceed on a direct path, thus, some of that leading wave is always turning back. If the pressure behind them isn't as great, as it was at the beginning, a forgone conclusion, then not as many bounce back in the initial direction. As time progresses this leads to a diminishing pressure. This effect is constant trough out the air mass, and thus you have the effect of slowing increasing pressure in the path of propagation.
Despite the subject, this isn't really rocket science, but rather freshman (in HS) chemistry.

[null and void]

The approach that seems to make the most sense to me is using a progressive rail gun built into an equatorial mountain slope,

Mona Loa comes to mind.

Might have to sacrifice a few virgins to Pele once in a while, though...

[RightWhale]

Has anyone figured out how they are going to get through the Van Allen Radiation belt alive?

No. They will all die just like last time.

[SampleMan]

With my spelling being what it is, I should perhaps refrain from referencing grade levels ;)

[RightWhale]

the first man made object to truly leave the earth's bounds was a man hole cover

Hadn't heard about that. The a-test high speed cameras are amazing.

[SampleMan]

Oops, I'm so smart I posted this to myself ;)

Now for you. In your model, how does the front of the wave "know" what the final equalized pressure will be?

In fact, as air enters the tunnel it creates a bit of a vacuum at the entry compared to the surrounding area. The air molecules at the front of the wave bounce around, vice proceed on a direct path, thus, some of that leading wave is always turning back. If the pressure behind them isn't as great, as it was at the beginning, a forgone conclusion, then not as many bounce back in the initial direction. As time progresses this leads to a diminishing pressure. This effect is constant trough out the air mass, and thus you have the effect of slowly (relatively speaking), or perhaps better put, lineally increasing pressure in the path of propagation.
Despite the subject, this isn't really rocket science, but rather freshman (in HS) chemistry.