Lowering Rocket noise 14dB using 0.5% of the flow

[john smith 19]

Found this little project to look at ways to reduce the damage (on the ground and the aircraft) of VTOL jets.

http://www.dtic.mil/dtic/tr/fulltext/u2/a458295.pdf

The goal was less about making such a landing "stealthy" as to reduce acoustic damage to the plane, any surrounding structures, jet re-ingestion and foreign object damage.

However a vertical jet fired at the ground is essential a rocket on a launch pad.

Historically people have reduced jet noise by trying to put a layer of intermediate-speed air around it that it will mix with. This is basically the turbofan principle.

The rather counter intuitive results of the project were that adding more supersonic flows (in the right places) could reduce noise a lot (IIRC -14dB is roughly 1/32 as  loud) for a side flow of 0.5% of main. The flow even seems to work better if it's intermittent, which they found cut the flow rate in half.

Keep in mind that a GG cycle typically uses 2-5% of main flow to run the pumps and Musks comments about how much trouble FH has taken partly due to severe noise issues with 27 engines and this starts to look like a very interesting piece of research. I'm kind of amazed it hasn't been done earlier.

[Jim]

Not applicable to rockets

[john smith 19]

Not applicable to rockets

Why not?

[Pete]

... could reduce noise a lot (IIRC -14dB is roughly 1/32 as  loud) ...

Firstly: -14dB is NOT 1/32 as loud, it is about 1/3 as loud.

Secondly: With rockets, the bulk of the noise comes from the hypersonic airflow of the exhaust gasses slamming into the ambient air.
To ameliorate the noise of the rocket, you would need to fiddle with the boundary layer of the *whole* exhaust plume, where it exceeds the local speed-of-sound. This volume is typically many thousands of times bigger than the volume of the engine itself, indeed it is usually many times larger than the ENTIRE rocket being propelled.

[S.Paulissen]

I believe Jim, but not the above explanation as stated.

Jets also have supersonic flow in their exhaust plumes that cause the same shear noise.  That alone cannot be the difference as to why it's not applicable to rockets.

[HMXHMX]

After Don Hart, who used to run the USAF Rocket Propulsion Lab (RPL) back in the 1980s, told me about some work he'd been a part of at the USN's Point Mugu facility decades earlier, I tried his suggestion of spraying water into the shear layer of a firing rocket engine.  The flow is normal to the jet and typically uses 50-150% of the m-dot of the rocket engine flow.  The approach is different – more targeted – from merely flooding water into the boat-tail, as is done on conventional pads.

We found more than 30 dB of noise reduction from this technique.  Since you only have to use it for a few seconds while close to the ground (for vertical liftoff) the penalty turns out to be fairly insignificant, but the reduction in acoustic damage to a pad might be worth it. 

[john smith 19]

Firstly: -14dB is NOT 1/32 as loud, it is about 1/3 as loud.

And you know this because??

Let me suggest a few places which would disagree.

http://www.sounddeadsteel.com/what-is-a-decibel.html

https://pulsarinstruments.com/blog/how-to-understand-the-3db-rule/

I will note that for humans 10dB seems to be needed for humans to perceive a reduction but that's not the actual figure.

Secondly: With rockets, the bulk of the noise comes from the hypersonic airflow of the exhaust gasses slamming into the ambient air.

Did you read my OP? If you did you'd know I'm aware of the usual thinking about noise abatement, which is why I found the report so counter intuitive.

To ameliorate the noise of the rocket, you would need to fiddle with the boundary layer of the *whole* exhaust plume, where it exceeds the local speed-of-sound. This volume is typically many thousands of times bigger than the volume of the engine itself, indeed it is usually many times larger than the ENTIRE rocket being propelled.

Perhaps you should read the report?  You might find out something you don't already think you know?

[aboughen]

If a methane-oxygen rocket possibly called BFR conducted a 90% propellant flight, & was appropriately modified, could drop the 10% excess propellant around the outside of the engines once it is a safe distance from the tower to form a 'liquid droplet in flight muffler'? The propellant would mostly combust once evaporated & you could achieve a 5 to 10 sec of sound reduction. Does anyone want to try to calculate the droplet cloud density & the possible noise reduction? Every decibel reduced gets your launch pad 100m closer to a populated centre.