Verifying Mach 10+ Shocks in Shuttle Launch Exhaust

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Discussion Overview

The discussion revolves around the nature of the pulses observed in the blue hydrazine flame from the Space Shuttle main engines during launch, specifically whether these pulses represent supersonic shockwaves and how they relate to the exhaust speed, potentially exceeding Mach 10. The conversation touches on theoretical and observational aspects of rocket exhaust behavior.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether the pulses seen in the exhaust are indeed supersonic shockwaves and suggests that counting these pulses could indicate the Mach number of the exhaust.
  • Another participant provides a reference to the exhaust velocity of the engines, suggesting it could be around Mach 13 based on available data.
  • Some participants assert that the pulses are not actual engine pulses but rather reflected shock waves due to supersonic flow, recommending verification with authoritative sources like NASA.
  • There is mention of a distinctive crackling sound associated with the engines, attributed to sonic booms generated by the supersonic exhaust flow.
  • One participant argues that the number of shock diamonds is influenced more by the pressure difference between the exhaust gases and the surrounding atmosphere than by the Mach speed of the exhaust itself, citing anecdotal evidence from different altitudes.
  • Another participant notes the relationship between Mach number, pressure, and exhaust velocity, suggesting a complex interaction that warrants further investigation.

Areas of Agreement / Disagreement

Participants express differing views on the interpretation of the pulses and their relationship to Mach number, with some supporting the idea of shockwaves while others challenge it based on pressure differentials. The discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Participants highlight the dependence of shock diamond formation on various factors, including altitude and pressure differences, indicating that assumptions about exhaust behavior may vary under different conditions.

Who May Find This Useful

This discussion may be of interest to those studying rocket propulsion, fluid dynamics, or aerospace engineering, as well as enthusiasts of space exploration and launch phenomena.

DaveC426913
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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.)
 
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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.
 
Seriously I wished I had a place in the spaceship. When can I fly ? :frown:
:smile:
 
DaveC426913 said:
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.
 
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This is amazing :bugeye:
 
lonton said:
Seriously I wished I had a place in the spaceship. When can I fly ? :frown:
:smile:

You have an extra $30 million laying around? The Russians will let you fly with them for that fee.
 
He also said that the distinctive crackling sound that the engines make is sonic boom after sonic boom.
 
DaveC426913 said:
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.
 
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.
 
  • #10
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.
 

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