dkomo wrote:

Robin Levett wrote:


dkomo wrote:



Dale wrote:



"dkomo" wrote in message
...



Dale wrote:




"dkomo" wrote in message
...




Thomas H. Faller wrote:

[...]




So trying to escape the earth's gravity at less than escape velocity

requires




very large power sources.


Good grief, haven't we heard about conservation of energy? It takes
the same amount of energy to escape the earth's gravity whether you do
it at
1 m/s or 25,000 mph. That means you use the same amount of fuel either

way.




Look at this way. If you go up slowly you burn fuel more slowly over a
longer time period. If you go up fast you burn fuel rapidly over a
shorter time period. Either way, you end burning the same amount of

fuel.



No, if you go up slowly, you burn fuel at pretty much the same rate as

if



you go up rapidly, but it takes more time, therefore you burn more fuel.

What? If you burn fuel at a slow rate, you generate less thrust,
produce less acceleration, and therefore you go up more slowly. If you
burn fuel at a fast rate, you generate more thrust, produce more
acceleration, and therefore you go up faster.


As long as you're fighting gravity, you have to burn fuel at a certain
rate just to hover. It's pretty complicated to explain fully, but just
take a look at the limits. If you have just enough thrust to hover, you
could be burning fuel at some given rate forever and never get anywhere.
If you don't have enough thrust even to hover, you'll come to a rest on
the Earth's surface.

If you burned a little extra, say enough to accelerate at 1 m/s^2 you
could achieve orbital velocity in 123 seconds. Well, actually, that's
against a constant gravity, but let's just leave it at that for now. It's
also assuming a constant mass, when of course, the mass would decrease as
fuel was expended, and thus the acceleration would increase over time.

But anyway, if you burned enough to accelerate at 10 m/s^2, you'd achieve
orbital velocity in 39 seconds. Now let's say that accelerating at 1
m/s^2 against Earth's gravity costs 1.1 times as much fuel as hovering,
and that accelerating at 10 m/s^2 takes 2 times as much fuel as hovering.

Where do you get this stuff? Accelerating at 10 m/s^2 will take 10
times the thrust as accelerating at 1 m/s^2. Therefore you'll burn fuel
at ten times the rate.


He said accelerating at 10m/s^2 *against gravity*; you keep fogetting that
thrust is required simply to counteract gravity. The thrust which provides
acceleration relative to the ground is the marginal thrust, after deducting
what is counteracting gravity.



In a sense I have "forgotten" it because the acceleration of gravity has
already been included. I'm reading his post as saying that the *net*
acceleration is 10 m/s^2 with gravity already factored in. And the
*net* accleration is the only thing that matters when computing the
amount of fuel consumed.

That is absolutely not true. If my rocket is hovering in mid-air are
you contending that it is using NO fuel because it has no net
acceleration? Bull****! It is using A LOT of fuel!