101–110 of 163 posts
Earlier quoted context omitted.
The rocket equation applies to all momentum machines which carry its own "fuel", that is, the mass to eject. Orion gets its momentum transfer from the plasma debris of the nuclear explosion, and the plasma speed comes from the explosion itself. Orion carries the source of the plasma, hence it is limited by the rocket equation.
is there a form of transport that is _not_ limited by the rocket equation?
Right. This has been known for a long time. It's why rockets aren't much better than they were 40 years ago. Chemical fuels are as good as they can get. Space travel with chemical fuels is just barely feasible. In the 1960s, it was assumed that nuclear power would be necessary for space flight. Everybody involved knew the rocket equation. The original plan for Apollo included a nuclear upper stage. The engine (NERVA)…
I happened to be reading up on rocket efficiency and I was surprised to learn that rockets are fairly efficient for launching stuff into orbit. Wikipedia uses the example of the Space Shuttle, where 16% of the energy in the propellants ends up in the kinetic and potential energy of the orbiter. That's pretty good!
The problem is that you use a rocket once and then throw it away. Imagine if your car was one-time-use. How often would you drive somewhere? How often would anyone drive anywhere? It wouldn't matter how efficient they are and it wouldn't matter if they didn't even require fuel at all.
Now, the rocket equation still comes into play here, because it means you need a lot of rocket for a little bit of payload. But the main problem is the one-time-use thing. A nuclear disposable rocket wouldn't improve things much. A reusable nuclear rocket would be great, but then so would a reusable chemical rocket.
This is the genius of SpaceX. For decades, rocket designers have looked at the rocket equation and tried their hardest to save fuel. SpaceX looked at the economics of rocketry and realized that fuel costs more or less don't matter, and instead concentrated on building their machines cheaply, and on making them reusable. We'll see how it works out, but if they succeed in making reusable rockets then they'll cut the cost of launches by an order of magnitude or more.
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Whatever happened to transmitted power designs, like using a ground-based laser to lift a payload? If you can leave the powerplant on the ground and only send the power, you no longer have to lift the fuel, just reaction mass.
I don't think that gets you much, as the fuel normally doubles as the reaction mass. So take hydro-lox. The output is water and heat, which equates to steam, which equates to propulsion. Now you could just fill a tank with water and use ground based lasers to heat it into steam, and save the complexity of handling cryogenic materials. But you need a laser powerful enough to convert a rocket full of water to steam ove…
Earlier quoted context omitted.
Whatever happened to transmitted power designs, like using a ground-based laser to lift a payload? If you can leave the powerplant on the ground and only send the power, you no longer have to lift the fuel, just reaction mass.
I don't think that gets you much, as the fuel normally doubles as the reaction mass. So take hydro-lox. The output is water and heat, which equates to steam, which equates to propulsion. Now you could just fill a tank with water and use ground based lasers to heat it into steam, and save the complexity of handling cryogenic materials. But you need a laser powerful enough to convert a rocket full of water to steam ove…
To move from the 85% propellant of rockets to 15% (somewhere between a fighter jet and a train, according to the article) we have to increase exhaust velocity of whatever we're pushing down to push us up by 12x.
Earlier quoted context omitted.
Whatever happened to transmitted power designs, like using a ground-based laser to lift a payload? If you can leave the powerplant on the ground and only send the power, you no longer have to lift the fuel, just reaction mass.
I don't think that gets you much, as the fuel normally doubles as the reaction mass. So take hydro-lox. The output is water and heat, which equates to steam, which equates to propulsion. Now you could just fill a tank with water and use ground based lasers to heat it into steam, and save the complexity of handling cryogenic materials. But you need a laser powerful enough to convert a rocket full of water to steam ove…
Earlier quoted context omitted.
I don't think that gets you much, as the fuel normally doubles as the reaction mass. So take hydro-lox. The output is water and heat, which equates to steam, which equates to propulsion. Now you could just fill a tank with water and use ground based lasers to heat it into steam, and save the complexity of handling cryogenic materials. But you need a laser powerful enough to convert a rocket full of water to steam ove…
The Lightcraft design didn't carry it's own fuel: http://en.wikipedia.org/wiki/Lightcraft
Right. This has been known for a long time. It's why rockets aren't much better than they were 40 years ago. Chemical fuels are as good as they can get. Space travel with chemical fuels is just barely feasible. In the 1960s, it was assumed that nuclear power would be necessary for space flight. Everybody involved knew the rocket equation. The original plan for Apollo included a nuclear upper stage. The engine (NERVA)…
Everyone Knows™ that putting stuff into space is expensive. Then Everyone Assumes™ that it's because of all the fuel. But no, fuel is cheap, hardware is what's expensive. If you look at the costs involved in putting something into orbit, the cost of fuel is a trivial detail, on the order of 1% of the total costs. Compare that to an airliner, where fuel is around 1/3rd of the total costs, or a car, where fuel can easi…
Earlier quoted context omitted.
I wouldn't dismiss the space elevator out of hand. It requires carbon nanotubes of a few meters length to achieve the required strength, and you wouldn't need to lift it pre-built - you could build it with a guideline and cable laying cars traveling up and down, adding to the cable, much like they do with suspension bridges. Long term, it seems like far and away the best approach if we can solve the materials science…
I would certainly dismiss elevators because there is a much more practical alternative that doesn't require materials that don't exist in sufficient quantities to reach a quarter of the way to the moon. The Lofstrom Loop ( http://en.wikipedia.org/wiki/Launch_loop ) could be built with materials we have today, although it requires sufficient amounts of money and land that only large countries or multibillionaires coul…
Earlier quoted context omitted.
Everyone Knows™ that putting stuff into space is expensive. Then Everyone Assumes™ that it's because of all the fuel. But no, fuel is cheap, hardware is what's expensive. If you look at the costs involved in putting something into orbit, the cost of fuel is a trivial detail, on the order of 1% of the total costs. Compare that to an airliner, where fuel is around 1/3rd of the total costs, or a car, where fuel can easi…
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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…
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.