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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)

#11

Post by has2k1 »

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.

Re: The Tyranny of the Rocket Equation (2012)

#12

Post by fpgaminer »

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…

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.

Re: The Tyranny of the Rocket Equation (2012)

#13

Post by ars »

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 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.

Re: The Tyranny of the Rocket Equation (2012)

#14

Post by ars »

fpgaminer wrote: post #12
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…

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

Re: The Tyranny of the Rocket Equation (2012)

#16

Post by claudius »

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 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.

Re: The Tyranny of the Rocket Equation (2012)

#17

Post by yoha »

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…

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.

Re: The Tyranny of the Rocket Equation (2012)

#18

Post by has2k1 »

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.

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).

Re: The Tyranny of the Rocket Equation (2012)

#19

Post by yoha »

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…

Nuclear engines are not practical for lift-off. They can, however, be used for inter-planetary travel for a big enough payload.

Re: The Tyranny of the Rocket Equation (2012)

#20

Post by SamReidHughes »

has2k1 wrote: post #18
ars wrote: post #13

Earlier quoted context omitted.

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.

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