[AR] Re: Hypersonics have finally arrived
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Thu, 15 Oct 2020 15:01:48 -0400 (EDT)
On Thu, 15 Oct 2020, Anthony Cesaroni wrote:
the payload vehicle simmed out at Mach 4.92. The PM went nuts insisting
that it's not hypersonic unless it goes Mach 5. I asked everyone in the
room for a scientific citation that says that. I got crickets. Does
anybody know or is it like nano-particles?
"Because of the nonlinear nature of the governing equations, no absolutely
precise definition can be stated for when a supersonic flow becomes
hypersonic... the onset of hypersonic-flow characteristics is gradual and
varies with the vehicle geometry as well as with the flight speed and
properties of the ambient atmosphere." -- Rasmussen, "Hypersonic Flow",
1994.
He explains: Simple supersonic-flow theory uses linearized versions of
the flow equations, assuming that flow disturbances due to the presence of
the vehicle are small enough that higher-order terms can be neglected.
How good this assumption is, depends on how slender the vehicle is.
Also, when speeds get sufficiently high, even disturbances that are small
with respect to the flow velocity are no longer small with respect to the
local speed of sound, and again linearity breaks down. For vehicles of
practical slenderness, unavoidable nonlinearity typically starts to show
up *somewhere around* Mach 5.
Henry
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