[ExI] Power satellites

spike spike66 at att.net
Sat May 2 15:10:11 UTC 2009


> -----Original Message-----
> From: extropy-chat-bounces at lists.extropy.org 
> [mailto:extropy-chat-bounces at lists.extropy.org] On Behalf Of 
> Stefano Vaj
> Sent: Saturday, May 02, 2009 5:58 AM
> To: ExI chat list
> Subject: Re: [ExI] Power satellites
> On Sat, May 2, 2009 at 12:11 AM, spike <spike66 at att.net> wrote:
> >> Stefano Vaj:
> >
> > No.  Well, not exactly.  If the earth were more massive, 
> there is not 
> > a point where achieving escape becomes *impossible* but 
> rather it does 
> > quickly approach impractical.  The process is exponential, 
> but with no 
> > brick wall stopping the whole parade.
> Please forgive me if I am saying something stupid in terms of 
> elementary physics, but let us say that chemical reaction x 
> liberates energy y for any kilo of reagents...

Not stupid at all.  The same argument was used for a long time by British
physicists before the war to assure Churchill that the nazis couldn't use
rockets to drop bombs on London.  Then the nazis demonstrated the error of
their ways in a most dramatic fashion, thru the magic of multiple staging.

>... If you keep 
> increasing gravity, the work required to lift any given 
> quantity of reagents may well sooner or later exceed the work 
> that can be obtained from the same quantity thereof, or not?...

Keep in mind the *work* that is needed.  The overwhelming majority of the
fuel and oxidizer aboard the Saturn V on the launch pad doesn't actually go
all that high out of the gravity well.

> In other terms, when the gravity is strong enough, a Saturn V 
> does not take off at all, let alone gets to orbit, 
> irrespective of its payload, right?...Stefano Vaj

Right, but if the gravity well were stronger, we wouldn't use a Saturn V.
We would have a first stage that would spend itself sooner that the V does.
Think of a Saturn V first stage but imagine it with ten nozzles instead of
5, so it makes a lot more initial thrust but spends itself much more
quickly.  Then the second stage with four nozzles instead of 2, then a third
stage, which instead of making it to orbit only gets to 20 km up and 3 km
per second.  Then a fourth stage kicks in which takes you to 40 km and 5
km/sec, then a fifth stage about a quarter the mass of the fourth, and so
on, until you finally make it to orbit velocity and altitude, but with
perhaps a dozen stages and only a kg of payload.  

There is no absolute physical limit, but there are practical limits.  As
Eugen put it, the rocket equation is a cruel thing.  Yes this is a big
planet for chemical rockets.


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