Scramjets !!!! It works

> > 2/ to reduce gravity costs on a non-lifting body.
>
> Not true. Rockets 'reduce their weight' through orbital speed, not height.
> You have to thousands of km up before such a reduction is even noticable.
>

I think the gravity equation uses r-squared as the divisor, so that if you
assume that Earth's surface is 6400 km from the "center of gravity" then at
about 1440 km (900 miles) altitude you've lost half of your "weight", which
I think would be pretty noticeable.

Or have I screwed up my math? ... 30 years of writing computer
programs that summarize invoices have rotted my brain. :-)

Steve