Verifying Mach 10+ Shocks in Shuttle Launch Exhaust

[DaveC426913]

My friend (whom I normally trust) told me something I did not know and would like to verify.

We were watching a shuttle launch, talking about the main engines which, while much fainter than the boosters, can be seen in the right light.

http://fascinatingly.com/wp-content...-space-shuttle/lift-off-shuttle-wallpaper.jpg

He said that the pulses seen in the blue hydrazine flame from the main engines are actually supersonic shockwaves. He went on to say that you can count how many Machs the exhaust speed is by counting the pulses visible. The attached pic is cut off but in the vdieo we were watching we counted 10+ shocks, the implication being that the exhaust was in excess of Mach 10.

True?

(I had seen the pulses but assumed they were simply ... pulses, generated by some function of the engine.)

 

[DaveC426913]

Followup:

Well, the math works out at least...

According to Wiki, the engines can generate an exhaust velocity of 4440m/s at sea level which is ~ Mach 13.

 

[lonton]

Seriously I wished I had a place in the spaceship. When can I fly ?

 

[Astronuc]

He said that the pulses seen in the blue hydrazine flame from the main engines are actually supersonic shockwaves. He went on to say that you can count how many Machs the exhaust speed is by counting the pulses visible. The attached pic is cut off but in the vdieo we were watching we counted 10+ shocks, the implication being that the exhaust was in excess of Mach 10.

True?

The SSME's are fueled with liquid hydrogen (H₂) and oxygen with the mixture rich in H₂. The engines do not pulse, but what appear to be pulses are indeed reflected shock waves due to the fact that the flow is supersonic. The numbers seem reasonable, but I would recommend confirming with NASA or Pratt & Whitney Rocketdyne.

FYI -
http://www.pwrengineering.com/articles/nozzledesign.htm
http://www.pwrengineering.com/data.htm

One can see a similar pattern with the exhaust of an SR-71 when it takes off with max thrust.

 

[Mk]

This is amazing

 

[wildman]

Seriously I wished I had a place in the spaceship. When can I fly ?

You have an extra $30 million laying around? The Russians will let you fly with them for that fee.

 

[DaveC426913]

He also said that the distinctive crackling sound that the engines make is sonic boom after sonic boom.

 

[Astronuc]

He also said that the distinctive crackling sound that the engines make is sonic boom after sonic boom.

That is essentially what is happening. The air around the exhaust plume is displaced at the speed of sound in the air. There are tremedous pressure gradients in the plume, particularly at the boundary of plume and air.

 

[LowlyPion]

The sound pressure from miles away even is most impressive. I saw a launch a couple of years ago. No sub-woofer can reproduce the thundering sound that pounds at you. Dumbfoundingly awesome is about the the only way to describe the sensation of power.

 

[LURCH]

From what I have heard, the number of shock diamonds depends not on the mach speed of the exhaust, but rather the pressure difference between the exhaust gasses and the sarounding atmosphere.

As (merely anecdotal) support, the same exhaust will produce a different number of shock diamonds at different altitudes. Also, I've seen pics of the X-1 with about 8 or 10 shock diamonds behind it, and I'm pretty sure that the exhaust from that vehicle does not exit at Mach 10.

Let's keep researching, I'm quite keen to know the truth on this one.

 

[Astronuc]

Mach number, pressure (and pressure differential), and exhaust velocity are related.

http://exploration.grc.nasa.gov/education/rocket/rktthsum.html