SR-71 video... interesting! Warning, some real Einstein stuf in here

[danielhv]

http://www.wimp.com/lockheedblackbird/

[03trubluGT]

I've always wondered about the effects of air on speed and heat.

If the higher you go, the thinner the air, so it must be more difficult to produce enough power to go that fast.

Also, is the heat generated at 20,000 feet @ Mach2 the same heat generated at 1,000 feet at Mach2?

[Ratt]

I've always wondered about the effects of air on speed and heat.

If the higher you go, the thinner the air, so it must be more difficult to produce enough power to go that fast.

Also, is the heat generated at 20,000 feet @ Mach2 the same heat generated at 1,000 feet at Mach2?

No. Less air = less friction.

[03trubluGT]

No. Less air = less friction.

I wouldn't think so. So why do they hype the heat factor? I know they aren't doing Mach3 at 2,000 feet.

[Broncojohnny]

Even at the edge of space where the air is as thin as possible, if you are going fast enough you are going to generate a lot of friction. Thus the tiles on the shuttle.

[03trubluGT]

Even at the edge of space where the air is as thin as possible, if you are going fast enough you are going to generate a lot of friction. Thus the tiles on the shuttle.

I would think the tiles would be of more use the deeper into the atmosphere the shuttle gets.

Remember seeing the video of the capsules re entry? It looked like the closer to the surface it got, the hotter and bigger the flames.

[Hobie]

Any of you ever read excerpts from Sled Driver, or buy the whole thing?

http://www.sleddriver.com/

According to what some of the guys in that book claim, when you do the math from when they mention how far they flew in how much time, some of them did over 4000 mph.

Only $595 for the book!

http://galleryonepublishing.com/BlackbirdStores/product_info.php?cPath=22&products_id=29

[appelingogt]

Fuck yeah!!! great post...i like the part where Russia sold "us" the titanium to build this shit to spy on their ass...

[Strychnine]

So why do they hype the heat factor? I know they aren't doing Mach3 at 2,000 feet.

You're right, they aren't too far supersponic at low altitudes and the air is thinner up high, but...

Fd = 1/2Pv^2CdA

Fd = drag force aka. friction
P= density of the medium(air)
v = speed in the medium
Cd = Drag coefficient
A = area (frontal)



If you're looking 2,000mph at two altitudes then the only variable is density, so then the friction will change proportionally.

But generally they will speed up as they climb and cruise higher. Notice that v-squared parameter in there... as speed increases the friciton goes up exponentially not linearly. That's why even with thin air the friction is incredible at high speeds.

[03trubluGT]

You're right, they aren't too far supersponic at low altitudes and the air is thinner up high, but...

Fd = 1/2Pv^2CdA

Fd = drag force aka. friction
P= density of the medium(air)
v = speed in the medium
Cd = Drag coefficient
A = area (frontal)



If you're looking 2,000mph at two altitudes then the only variable is density, so then the friction will change proportionally.

But generally they will speed up as they climb and cruise higher. Notice that v-squared parameter in there... as speed increases the friciton goes up exponentially not linearly. That's why even with thin air the friction is incredible at high speeds.

Careful, here comes another one of those compliments......




I'm glad you answered, but can you kindly please tell me what the fuck you just said?????

:WTF:Thanks for the attempt, but I'm not that well with formulas.

[Strychnine]

Careful, here comes another one of those compliments......




I'm glad you answered, but can you kindly please tell me what the fuck you just said?????

:WTF:Thanks for the attempt, but I'm not that well with formulas.

In short: Speed plays a bigger role in friction than altitude, by quite a bit.

[03trubluGT]

In short: Speed plays a bigger role in friction than altitude, by quite a bit.

I would think that with the thinner air, it would play a bigger role. If there are less air molecules, then they would be farther apart, and........ well, you get my drift...

[danielhv]

I would think that with the thinner air, it would play a bigger role. If there are less air molecules, then they would be farther apart, and........ well, you get my drift...

Another angle to think about:

The higher they go the thinner the air, the thinner the air, the less resistance at the same speed as lower, thus allowing them to go faster, which means more air, which means more friction.

[TruestreetTim]

Even at the edge of space where the air is as thin as possible, if you are going fast enough you are going to generate a lot of friction. Thus the tiles on the shuttle.

I would think the tiles would be of more use the deeper into the atmosphere the shuttle gets.

Shuttle reentry speed is around 17,322 statute mph. No matter what part of the atmosphere your in, that's friction!

According to what some of the guys in that book claim, when you do the math from when they mention how far they flew in how much time, some of them did over 4000 mph.

SR-71 holds an absolute speed record of 2,454 mph. That doesn't take into consideration Earths rotation for distance traveled either though. For instance; the Earth rotates at 1,000mph. So as the sun rises in the west, the SR-71 can fly west across the Pacific Ocean 2x faster than the Earth can rotate away from it. Fun fact: can travel from US East coast to US West coast in 1 hr., 7 min., 54 sec.

[Strychnine]

I wouldn't think so. So why do they hype the heat factor? I know they aren't doing Mach3 at 2,000 feet.

I'm mentally done here at work but not ready to go home and open a beer yet, so im gonna throw something together in Excel... try this on for size.
I know you said you're not a fan of formulas and stuff, but pay attention to the blue highlighed boxes, they are the most telling part.


Generally for every 18,000 to 20,000 ft you go up in altitude, your air density is cut in half. 20,000 ft elevation has ~1/2 the air density of sea level 40,000 = 1/4, 60,000 = 1/8, 80,000 = 1/16...

I'm only throwing that out there as a reference for the next part

Just using generic numbers here take a look at what happens when you do two things:
1. stay at a constant speed and change altitude
2. stay at a constant altitude and change speed.

There is a 1:1 relationship with respect to altitude, but there is a squared (exponential) relationship with respect to speed.

Fd = drag = heat.




So you're right there are a LOT fewer molecules to run into (16 times fewer), but the speed at which your hitting them not only offsets the lower quantity but also makes you run into waaaaaay way more (56 times more, when going from 400mph to 3000mph) than you would if you were still going that slower speed.