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The Tyranny of the Rocket Equation (2012)

nasa.gov

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Re: The Tyranny of the Rocket Equation (2012)

#21

Post by SamReidHughes »

ars wrote: post #13
has2k1 wrote: post #11

Earlier quoted context omitted.

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.

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

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

Re: The Tyranny of the Rocket Equation (2012)

#22

Post by has2k1 »

claudius wrote: post #16
has2k1 wrote: post #11

Earlier quoted context omitted.

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 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 h…

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.

Re: The Tyranny of the Rocket Equation (2012)

#24

Post by avoid3d »

has2k1 wrote: post #22
claudius wrote: post #16

Earlier quoted context omitted.

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 h…

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.

Re: The Tyranny of the Rocket Equation (2012)

#25

Post by has2k1 »

SamReidHughes wrote: post #20
has2k1 wrote: post #18

Earlier quoted context omitted.

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…

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.

Re: The Tyranny of the Rocket Equation (2012)

#26

Post by eterm »

has2k1 wrote: post #11
ars wrote: post #6

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.

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.

Re: The Tyranny of the Rocket Equation (2012)

#27

Post by ars »

has2k1 wrote: post #25
SamReidHughes wrote: post #20

Earlier quoted context omitted.

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…

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.

Re: The Tyranny of the Rocket Equation (2012)

#28

Post by SamReidHughes »

has2k1 wrote: post #25
SamReidHughes wrote: post #20

Earlier quoted context omitted.

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…

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.

i'll spare everybody else the snark here

Re: The Tyranny of the Rocket Equation (2012)

#29

Post by pjc50 »

daeken wrote: post #10
tempestn wrote: post #3

> In the 1970’s, an experimental nuclear thermal rocket engine gave an energy equivalent of 8.3 km/s. This engine used a nuclear reactor as the source of energy and hydrogen as the propellant. That was intriguing, but didn't go into detail on why a nuclear thermal rocket hasn't been tried since. The obvious explanation is that there could be serious consequences if such a rocket exploded, spreading radioactive materi…

> And rockets tend to explode sometimes. Chemical rockets -- the only ones we've ever actually used -- explode because that's what they're intended to do. The only difference between a successful rocket firing and a catastrophic rocket failure is the speed at which the explosion happens. A nuclear rocket engine has basically no risk of explosion; tearing itself apart at speed maybe, if the aerodynamics aren't done pr…

Normal rocket operation is a burn, not a detonation, and there's a clear difference between those that's not just about speed.

Re: The Tyranny of the Rocket Equation (2012)

#30

Post by pjc50 »

ytturbed wrote: post #2

>Currently, all our human rated rocket engines use chemical reactions (combustion of a fuel and oxidizer) to produce the energy. Yes, however, for completeness: an explanation of why we must limit designs to chemical rockets ought to include an explanation of why the dozen or so fusion projects underway around the world will all fail, i.e. let's inject some rational optimism. Note that the Apollo programme began befo…

The fusion projects have been running for a long time with little success. Apollo was built on scaling tech that already worked (1940s rocketry could reach space although not achieve orbit)
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