I think the rocket plume causes the air ahead of the rocket to expand out of the way, it is sort of like a standing explosion. the rocket also slows the rocket relative to the atmosphere so there is less frictional and compressive heating after the rocket burn.
The first burn takes the stage upward and starts killing some of downrange velocity, which is around 3 to 3.5 km/s (Mach 10-ish if there were any sensible air). From there it accelerates downward under gravity toward the sensible air. This event takes place at orbital altitude (160+ km).
The second burn reduces vertical speed downward that was gained by falling under acceleration of gravity from orbital altitude, or even more, depending upon how high up the first burn took it. At these speeds, the sensible air interface is somewhere in the vicinity of 150,000 feet. It's moving well above the nominal shown Mach 3 at the grid fin deployment point, which would be somewhere around 100,000 feet. They call it "hypersonic", which for blunt objects is at least Mach 3-ish, and for not-blunt objects (like this) is at or above Mach 5-ish.
The third burn is the landing burn, which takes them from supersonic to nothing at touchdown. Don't forget, it accelerates downward under gravity between burns. But apparently they don't want it over Mach 3-ish with the grid fins deployed. It has to be a whole lot slower than that to deploy the landing legs, or the wind pressure will rip them right off.
What I'm showing from compressible flow calculations is that the ideal gas model works well enough up to about 1.8 km/s speeds (about Mach 6 at 100,000 feet), and above that, the old reentry rule-of-thumb ("effective temperature in K is numerically equal to speed in m/s") gives you a better estimate of effective gas temperature. That's because there is increasing dissociation above that speed. The energy that would have raised total and recovery temperatures (and those two are not much different, really) goes into ionization instead.
At 100,000 feet, the ambient static air temperature is 227 K. At the 1.8 km/s / Mach 6 point, total/recovery temperatures are about 1830 K. Engine chamber gas temperatures are about 3000 K, which is the stagnation temperature in the engine gas stream. At 3 km/s (Mach 10) speeds, the rule-of-thumb oncoming air effective temperature is also about 3000 K.
The retropropulsion plume from the engine shocks down subsonic just behind the vehicle bow shock wave, so it can turn downstream alongside the stage. It reverts to pretty near its chamber temperature once subsonic like that, and its recovery temperature still drives heat transfer rates, even after it reaccelerates to supersonic alongside the stage tankage.
Shocks are compression events, not expansion events, by the way. Post shock turns back streamwise are expansion events, but these are localized. That is what reaccelerates it supersonic alongside the tankage.
The oncoming air stream shocks down subsonic at the bow shock wave. It goes to its effective temperature once subsonic, and that temperature drives its heat transfer, even after it reaccelerates supersonic alongside the tankage.
So there are two very hot flows mixing alongside the tankage. The gas is largely rocket plume gas right adjacent to the tankage. Mixing is less than perfect, to be sure.
I don't see any cooling effects or low speed effects here. During the second burn, they're trying to decelerate down to Mach 3, so they can deploy the grid fins. That means they are moving somewhere well above Mach 3 when they start that burn. Mach 6? Mach 10? I don't know. But very fast. Both the local airstream and the plume gases are in the 3000 K class during this event, all around the engines and tankage.
I know these engines run slightly fuel rich at normal mixture, but that doesn't explain the heavy sooting seen all over the bottom and sides of the stage. As I said above, I'd hazard the guess they're running a little kerosene through the inactive engines to keep their bells from overheating. They look bad enough as it is, with all the soot all over them, but I don't see melt damage, which should be evident if those bells were not protected in some way. The meltpoint of carbon steel is only 1886 K, by the way, and stainless is a little lower than that, actually.
That kerosene flow would burn very inefficiently in the mixing slipstream, which would put gobs of sooting all over everything exposed to that shocked flow, but without adding much heat. Spontaneous ignition of that kerosene is extremely like in the localized zones where recovery temperatures are high, not so likely outside the boundary layers where things aren't so hot. Vaporization absorbs some heat, so there a localized cooling where that takes place.
High speed aerodynamics is a complicated bitch, ain't it? So is maneuvering flight dynamics. I did this crap for a living for over 20 years directly, and over 40 years indirectly. I might know just a little about the subject, after all.
Last edited by GW Johnson (2016-06-09 18:29:28)
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
We have heard the recent wishes to change the EM-1 and EM-2 mission timelines to being a crewed mission1 and an actual landing on 2 which on paper would save about 15 billion out of the Nasa budgets but is it realistic.
Do this change the view point that sls is to expensive by just changing the time line?
I would say no as the price really has not changed but only the goal posts.....
The propulsion system that will give the Orion spacecraft the in-space push needed to travel thousands of miles beyond the moon and back has completed major assembly at United Launch Alliance in Decatur, Alabama. The Boeing-designed interim cryogenic propulsion stage is a liquid oxygen/liquid hydrogen-based system that will give Orion an extra punch of power on the first, uncrewed flight of the spacecraft with NASA's new rocket, the Space Launch System, in late 2018.
But even once we test this we will be looking at a total redo of the EDS stage for use going anywhere else with this very expensive rocket....