Tripping the boundary layer: shuttle's re-entry experiment

[drwayne]

In an experiment that could lead to improved heat shield designs for future spacecraft - along with insights into shuttle aerodynamics - temperature data and infrared imagery confirm a modified tile on the underside of the shuttle Discovery's left wing caused air rushing over the belly of the orbiter to transition from smooth to turbulent flow as expected.

Rest of the story:

http://spaceflightnow.com/shuttle/sts119/090329blt/

 

[DrRocket]

In an experiment that could lead to improved heat shield designs for future spacecraft - along with insights into shuttle aerodynamics - temperature data and infrared imagery confirm a modified tile on the underside of the shuttle Discovery's left wing caused air rushing over the belly of the orbiter to transition from smooth to turbulent flow as expected.

Rest of the story:

http://spaceflightnow.com/shuttle/sts119/090329blt/

I must be missing something, or maybe the reporter did. What was reported strikes me as a no-brainer. OF COURSE you will see high temperarures if you stick a protuberance into the flow at those velocities and trip the flow, and OF COURSE yoiu will trip the flow with a geometricl discontinuity at those high mach numbers. The stagnation temperture of the gas at those velocities is quite high, I seem to recall it is a few thousand degrees F. Anythig that gets you nearer the stagnation temperature is going to result in very high temperatures. There has go to be more to the story that what is being reported

 

[tanstaafl76]

From the earlier article:

http://spaceflightnow.com/shuttle/sts11 ... index.html

"Once you understand when you are tripping that flow and getting the higher heat rate, now you can use that, you can put different coatings on tiles, you can put different types of tiles downstream from that and mimic a lunar return or higher heat than a normal shuttle return would be and you can do materials testing on the underside of the vehicle in a real flight environment," [Shuttle Program Manager John] Shannon said.

Sounds like it's not a matter of them knowing that it will create turbulence, but rather using it as a test section to find out when it trips, as well as using the higher temps for material testing (why they can't do that on the ground I don't know).

 

[Testing]

Simulating the velocities and thermal of a rentry is kinda hard to do on the ground.
They have several further experimental tiles for future flights planned.
From Wayne Hales blog:
So why is all of this important? Exactly at this time there is a large effort by many companies and government agencies to develop hypersonic aircraft. NASA has even sponsored a couple of test flights. The problem, as it is for all types of aircraft flight, is drag and heating. When the boundary layer over the wings or in the engine is laminar, there is low drag and low heating; and when the boundary layer is turbulent, drag and heating increase dramatically. All boundary layers can be "tripped" or transition from laminar to turbulent flow.

In some of these experimental aircraft the engines [called SCRAM jets for Supersonic Combustion Ram jet engines] have only operated for a fraction of a second or a very few seconds. Why? Because the designers do not know how to cool them; they don't understand when or whether the boundary layer inside the engine is turbulent or laminar.

In some of these experimental aircraft, the engine begins to melt as soon as it is turned on; hence the extremely short operating times.

This is no good for a hypersonic passenger aircraft which might carry a hundred people from New York to Tokyo in a couple of hours.

Why do we not understand this phenomenon? Because it cannot be recreated in a wind tunnel or other experimental apparatus. The wind tunnels that have long enough flow durations to study this phenomenon run only up to about Mach 6. These hypersonic engines need to perform at Mach 8 or 10 or 12. There are "wind tunnels" that operate at high Mach numbers but only for fractions of a second; not long enough to understand the way in which a boundary layer works.

No aircraft fly that fast, missiles can achieve it briefly, but there is one platform that spends a serious amount of time flying through the atmosphere at speeds above Mach 6:

http://blogs.nasa.gov/cm/blog/waynehale ... 16008.html

 

[Testing]

from Wayne Hales latest blog

The shuttle had an outstanding test last flight looking into the arcane science of how air closest to a flying vehicle (called the boundary layer) transitions from laminar to turbulent flow. Early results indicate we had a controlled transition around Mach 16 versus the usual Mach 8. This type of data is invaluable to the designers of future hypersonic aircraft. And it can't be gathered in wind tunnels or any other ground test.

 

[newsartist]

One of Doc's posts here linked to a piece about 'Hoot' Gibson's near disaster had a photo that showed them looking at the damage. That showed the downstream impact quite clearly.