> Does anyone have any energy/money cost savings estimates for launch
> from an balloon platform 30+ km up?  I'd imagine it would be signficant,
> since drag increases as the cube(?) of speed, and rockets tend to be
> quite fast even in the thick lower atmosphere.

For larger (typical sized) launchers the predominant energy loss is "gravity 
loss" - your launcher must support itself against gravity until it has 
achieved a substantial fraction of orbital speed. Drag losses are about 10% 
and gravity loss about 90%.

As the launcher gets smaller drag losses rise, and come to predominate for 
very small launchers. This is due to the square-cube law. Surface area rises 
as the square of linear size but volume rises as the cube of linear size. As 
drag losses are approximately related to area this means that the drag loss 
per mass decreases linearly with increasing size.

So high altitude launch makes sense for small launchers.

Platform stability is not liable to be a major issue with dynamic control, 
and essentially all modern launchers use this. Very early launches used some 
or all of aerodynamic control, spin and careful timing. (The US's first 
satellites final stage sat in a "bucket" which was spun up at high speed on 
the ground to impart stability to the final unguided stage). Note the loss 
of major fins, which were a hallmark of early rockets. You still have some 
fins on some rockets for aerodynamic stability purposes, but guidance is 
almost universally achieved by thrust vectoring in some form or other 
(usually by nozzle gimballing or by deflection of the thrust by asymmetrical 
injection of a vectoring fluid into the exhaust stream as it transits the 
nozzle .)

Something like SS1 would make a good launch platform for a very very small 
orbital launcher at apogee. Ground level drag losses would be very greatly 
reduced and the square cube law effect would be largely overcome.



        Russell McMahon 

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