[AR] Re: SpaceX Single Stage to Orbit
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Sun, 26 May 2019 15:48:01 -0400 (EDT)
On Sun, 26 May 2019, Craig Fink wrote:
Yeah, I agree, NASP most definitely had the wrong approach if they
thought flying low and fast with a dynamic pressure of 2700 psi was the
way to Orbit.
They didn't. They were flying as high as they could.
Better to fly high with a low "Equivalent Airspeed" that
continues to decrease all the way to Orbit.
Then you can't airbreathe. At constant air density, air mass flow
intercepted by an intake of a given size grows linearly with speed, while
dynamic pressure grows with the square of speed, and heating rate grows
roughly with the cube of speed. Limiting dynamic pressure requires
climbing to make air density drop with the square of speed (or faster),
but then you quickly starve your engines of air, and have to switch to
rocket propulsion.
Whereas if you try to maintain the air mass flow through your engines by
having air density drop only linearly with speed, then dynamic pressure
(and with it, drag) keeps growing, and heating escalates horribly. This
is why NASP had such high dynamic pressure, terrible drag losses, and
nasty heating problems -- it's *inherent* in trying to airbreathe all the
way to orbit.
Almost universally, research in this area is
more about weapons development and not developing a cargo vehicle.
That's because airbreathing basically doesn't work at near-orbital speeds,
and Mach 10 is much more interesting for weapons than for cargo.
Henry
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