Yeah, I agree with both of your points with respect to X-30 and Skylon.
These are just two examples of vehicles that cannot fly air-breathing to
Orbit. Just looking at both of them, the wing(body) loading is way too high
on both. I wouldn't expect either to make it, and in the case of Skylon
they aren't even trying. Skylon punts, and essentially builds a new kind of
jet engine capable of running in rocket mode, but it's air-breathing mode
isn't any (or much) better than an SR-71. Choked flow at the intake is just
a problem of this particular designs, they should have a bigger intake.
X-30 would have come closer if it had ever flown.
https://en.wikipedia.org/wiki/NASA_X-43
As a matter of fact, the X-43 which actually did fly made it up to Mach 9.6
almost twice the speed that Skylon punts. The X-43 wing loading look very
high, so consequently will have to fly to low to go to Orbit. But that's
not what it was meant to do, it's was essentially just an engine test.
On Mon, May 27, 2019 at 11:51 AM Keith Henson <hkeithhenson@xxxxxxxxx>
wrote:
On Sun, May 26, 2019 at 10:07 PM Henry Spencer <hspencer@xxxxxxxxxxxxx
wrote:
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.
Which is exactly what the proposed Skylon does. I recreated their
performance spreadsheet and spent many hours going over it to see what
the secret sauce was. The basic thing they do is fly up a constant
dynamic pressure line. And, just as Henry says, they switch to rocket
propulsion at 26 km and Mach 5.5. Trying to go higher or faster in
air-breathing mode is not effective for that vehicle.
The not-so-obvious thing they do is using the wings to unload gravity
drag. They do this even after the switch to rocket mode. The wings
save an incredible amount of delta-v compared to a rocket that took as
long to get to orbit.
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.
As usual, Henry has this dead on correct. When you are thinking of
near-orbital speed, it's called reentry.
Keith
speeds,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
and Mach 10 is much more interesting for weapons than for cargo.