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Do liquid propellant rocket engines experience thrust oscillation?

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2

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I know SRBs have a quite bit of thrust oscillation, and it's pretty much part and parcel of burning solid propellant. Do liquid-propellant engines experience something similar albeit at a much smaller magnitude? If so what is the main cause of this constant small variation in thrust level? Is it because of the combustion process inside the bell? In Falcon 9 launches you often see photos like this

The streak patterns of the exhaust plume indicate there is at least some non-uniformity of the pressure and temperature field inside. I assume every liquid engine actually behaves like this, but it's only apparent on LOX/RP-1 engine because of soot acting like temperature indicator through the intensity of the glow for the pattern.

rockets exhaust

share|improve this question

asked 9 hours ago

Meatball PrincessMeatball Princess

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  1
  

  
  
  
  
  
  

  
  $begingroup$
  I think the vertical streaking is closely related to the physical arrangement of the fuel/oxidizer injectors and resulting flow patterns, but I'm not sure why I think that.
  $endgroup$
  – Russell Borogove
  7 hours ago
  
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  Agreed, and if it was perfectly mixed, you likely wouldn't see the streaking.
  $endgroup$
  – Organic Marble
  6 hours ago
  

  
  
  
  

add a comment
 | 

2

$begingroup$

I know SRBs have a quite bit of thrust oscillation, and it's pretty much part and parcel of burning solid propellant. Do liquid-propellant engines experience something similar albeit at a much smaller magnitude? If so what is the main cause of this constant small variation in thrust level? Is it because of the combustion process inside the bell? In Falcon 9 launches you often see photos like this

The streak patterns of the exhaust plume indicate there is at least some non-uniformity of the pressure and temperature field inside. I assume every liquid engine actually behaves like this, but it's only apparent on LOX/RP-1 engine because of soot acting like temperature indicator through the intensity of the glow for the pattern.

rockets exhaust

share|improve this question

asked 9 hours ago

Meatball PrincessMeatball Princess

49011 silver badge1010 bronze badges

$endgroup$

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  I think the vertical streaking is closely related to the physical arrangement of the fuel/oxidizer injectors and resulting flow patterns, but I'm not sure why I think that.
  $endgroup$
  – Russell Borogove
  7 hours ago
  
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  Agreed, and if it was perfectly mixed, you likely wouldn't see the streaking.
  $endgroup$
  – Organic Marble
  6 hours ago
  

  
  
  
  

add a comment
 | 

$begingroup$

I know SRBs have a quite bit of thrust oscillation, and it's pretty much part and parcel of burning solid propellant. Do liquid-propellant engines experience something similar albeit at a much smaller magnitude? If so what is the main cause of this constant small variation in thrust level? Is it because of the combustion process inside the bell? In Falcon 9 launches you often see photos like this

The streak patterns of the exhaust plume indicate there is at least some non-uniformity of the pressure and temperature field inside. I assume every liquid engine actually behaves like this, but it's only apparent on LOX/RP-1 engine because of soot acting like temperature indicator through the intensity of the glow for the pattern.

rockets exhaust

share|improve this question

asked 9 hours ago

Meatball PrincessMeatball Princess

49011 silver badge1010 bronze badges

$endgroup$

share|improve this question

asked 9 hours ago

Meatball PrincessMeatball Princess

49011 silver badge1010 bronze badges

share|improve this question

asked 9 hours ago

Meatball PrincessMeatball Princess

49011 silver badge1010 bronze badges

asked 9 hours ago

Meatball PrincessMeatball Princess

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asked 9 hours ago

Meatball PrincessMeatball Princess

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1 Answer
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Yes, in liquid engines this phenomenon is called "rough combustion".

Sutton (4th edition) says

Combustion that gives pressure fluctuations greater than about +/- 5%
of the mean pressure at a chamber wall location and which occur at
completely random intervals is called rough combustion.

It's different from "combustion instability" defined as activity which

displays organized oscillations ocurring at well-defined intervals
with a pressure peak that may be maintained, may increase, or may die
out.

pp. 257 - 259

The streak patterns you mention are more likely due to non-uniform mixing resulting in slight variations in local mixture ratio, than due to rough combustion, IMHO.

share|improve this answer

edited 8 hours ago

answered 8 hours ago

Organic MarbleOrganic Marble

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add a comment
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6

$begingroup$

Yes, in liquid engines this phenomenon is called "rough combustion".

Sutton (4th edition) says

Combustion that gives pressure fluctuations greater than about +/- 5%
of the mean pressure at a chamber wall location and which occur at
completely random intervals is called rough combustion.

It's different from "combustion instability" defined as activity which

displays organized oscillations ocurring at well-defined intervals
with a pressure peak that may be maintained, may increase, or may die
out.

pp. 257 - 259

The streak patterns you mention are more likely due to non-uniform mixing resulting in slight variations in local mixture ratio, than due to rough combustion, IMHO.

share|improve this answer

edited 8 hours ago

answered 8 hours ago

Organic MarbleOrganic Marble

79.2k44 gold badges235235 silver badges338338 bronze badges

$endgroup$

add a comment
 | 

6

$begingroup$

Yes, in liquid engines this phenomenon is called "rough combustion".

Sutton (4th edition) says

Combustion that gives pressure fluctuations greater than about +/- 5%
of the mean pressure at a chamber wall location and which occur at
completely random intervals is called rough combustion.

It's different from "combustion instability" defined as activity which

displays organized oscillations ocurring at well-defined intervals
with a pressure peak that may be maintained, may increase, or may die
out.

pp. 257 - 259

The streak patterns you mention are more likely due to non-uniform mixing resulting in slight variations in local mixture ratio, than due to rough combustion, IMHO.

share|improve this answer

edited 8 hours ago

answered 8 hours ago

Organic MarbleOrganic Marble

79.2k44 gold badges235235 silver badges338338 bronze badges

$endgroup$

add a comment
 | 

$begingroup$

Yes, in liquid engines this phenomenon is called "rough combustion".

Sutton (4th edition) says

Combustion that gives pressure fluctuations greater than about +/- 5%
of the mean pressure at a chamber wall location and which occur at
completely random intervals is called rough combustion.

It's different from "combustion instability" defined as activity which

displays organized oscillations ocurring at well-defined intervals
with a pressure peak that may be maintained, may increase, or may die
out.

pp. 257 - 259

The streak patterns you mention are more likely due to non-uniform mixing resulting in slight variations in local mixture ratio, than due to rough combustion, IMHO.

share|improve this answer

edited 8 hours ago

answered 8 hours ago

Organic MarbleOrganic Marble

79.2k44 gold badges235235 silver badges338338 bronze badges

$endgroup$

share|improve this answer

edited 8 hours ago

answered 8 hours ago

Organic MarbleOrganic Marble

79.2k44 gold badges235235 silver badges338338 bronze badges

answered 8 hours ago

Organic MarbleOrganic Marble

79.2k44 gold badges235235 silver badges338338 bronze badges

answered 8 hours ago

Organic MarbleOrganic Marble

79.2k44 gold badges235235 silver badges338338 bronze badges