Author Topic: Vibrations during launch?  (Read 311 times)

Offline bknight

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Vibrations during launch?
« on: May 11, 2020, 02:16:08 PM »
This may be directed toward Jay primarily, but of course anyone may add relevant information
I was reading Astronaut Doug Hurley story about leaving a flag at the ISS during the last Shuttle mission.  In the print he described the launch of Shuttle versus a non SRB launch.  He described that there was a lot of vibration with the shuttle but he expected a smoother ride on the falcon.

My question is why an SRB launch has more vibrations?  I don't disagree that the vibration exist but I'm wondering why?
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Offline Obviousman

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Re: Vibrations during launch?
« Reply #1 on: May 11, 2020, 04:17:58 PM »
He's not talking about the 'twang' is he?

Offline raven

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Re: Vibrations during launch?
« Reply #2 on: May 11, 2020, 08:02:25 PM »
Hmm, maybe the difference in the distance from the large engines? The Dragon will have the first and second stages between it and the first stage engines, but on the shuttle the crew compartment was more than 100 feet closer to the main engines and over 80 feet closer to the SRB.

Offline JayUtah

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Re: Vibrations during launch?
« Reply #3 on: May 11, 2020, 09:50:37 PM »
He's probably talking about the longitudinal vibrations common to solid fuel rockets.  The STS SRBs exhibit most of their vibrational energy at about 4 Hz.  This is much lower than the typical 20 Hz product in smaller rockets.  The SRB would be felt as a definite vibration, while the higher-frequency oscillations in smaller solid fuel motors would be perceived more as a very low-pitched sound.

As you may know, the cavity in the middle of the fuel grain in which combustion takes place changes shape during flight as fuel burns.  This changes the dynamics of combustion and creates opportunities for different kinds of vortices to arise.  The technical term is "vortex shedding," and it covers many of the ways moving fluids interact with themselves and with obstacles.  Just the act of gaseous combustion products separating from the wall of the propellant grain and rolling down the wall toward the nozzle can create a vortex that expands as it descends.  The nozzle area inside the motor features lots of convoluted surfaces against which vortices of all kinds can impinge.  This then can cause a back-pressure wave which then becomes a standing wave inside the "pipe" of the motor.  It's actually not unlike the standing wave in an organ pipe.
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Offline Peter B

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Re: Vibrations during launch?
« Reply #4 on: May 11, 2020, 10:27:42 PM »
He's probably talking about the longitudinal vibrations common to solid fuel rockets.  The STS SRBs exhibit most of their vibrational energy at about 4 Hz.  This is much lower than the typical 20 Hz product in smaller rockets.  The SRB would be felt as a definite vibration, while the higher-frequency oscillations in smaller solid fuel motors would be perceived more as a very low-pitched sound.

As you may know, the cavity in the middle of the fuel grain in which combustion takes place changes shape during flight as fuel burns.  This changes the dynamics of combustion and creates opportunities for different kinds of vortices to arise.  The technical term is "vortex shedding," and it covers many of the ways moving fluids interact with themselves and with obstacles.  Just the act of gaseous combustion products separating from the wall of the propellant grain and rolling down the wall toward the nozzle can create a vortex that expands as it descends.  The nozzle area inside the motor features lots of convoluted surfaces against which vortices of all kinds can impinge.  This then can cause a back-pressure wave which then becomes a standing wave inside the "pipe" of the motor.  It's actually not unlike the standing wave in an organ pipe.

Hi Jay

Just a couple of questions a little off-topic: what was the shape of the cavity in the SRBs? And will it be different for the SLS SRBs?

I understand that the thrust of the SRB was proportional to the surface area exposed for burning, and this could be manipulated by changing the initial shape of the cavity, allowing for thrust to increase and decrease during the burn (something I once saw in one of those coffee table books about rockets in a library).

So, if the cavity was only in the centre of the SRB and was round, the thrust would steadily increase during the burn and then cut out; alternatively, if the cavity was in the centre was star-shaped, the thrust would steadily increase during the burn but then tail off at the end.

Offline smartcooky

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Re: Vibrations during launch?
« Reply #5 on: May 12, 2020, 03:31:21 AM »
Hi Jay

Just a couple of questions a little off-topic: what was the shape of the cavity in the SRBs? And will it be different for the SLS SRBs?

I understand that the thrust of the SRB was proportional to the surface area exposed for burning, and this could be manipulated by changing the initial shape of the cavity, allowing for thrust to increase and decrease during the burn (something I once saw in one of those coffee table books about rockets in a library).

So, if the cavity was only in the centre of the SRB and was round, the thrust would steadily increase during the burn and then cut out; alternatively, if the cavity was in the centre was star-shaped, the thrust would steadily increase during the burn but then tail off at the end.


And I have an additional question about that.

As the fuel is burned, the surface area of the cavity increases, causing more pressure in the cavity, but the volume of the cavity also increases as the fuel is burned off which would itself lead to a drop in pressure.

I seem to remember from my aeronautical engineering training that

𝜋r²h gives the volume of a cylinder
2𝜋rh gives the surface area (+𝜋r² if you include one end and 2𝜋r² if you include both ends)

Do these two (the increase in volume and the increase in burnable surface area) cancel each other out to any degree?
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Offline bknight

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Re: Vibrations during launch?
« Reply #6 on: May 12, 2020, 09:16:13 AM »
He's probably talking about the longitudinal vibrations common to solid fuel rockets.  The STS SRBs exhibit most of their vibrational energy at about 4 Hz.  This is much lower than the typical 20 Hz product in smaller rockets.  The SRB would be felt as a definite vibration, while the higher-frequency oscillations in smaller solid fuel motors would be perceived more as a very low-pitched sound.

Ok, then what is the "average" vibrations of a liquid fueled engine?
Quote

As you may know, the cavity in the middle of the fuel grain in which combustion takes place changes shape during flight as fuel burns.  This changes the dynamics of combustion and creates opportunities for different kinds of vortices to arise.  The technical term is "vortex shedding," and it covers many of the ways moving fluids interact with themselves and with obstacles.  Just the act of gaseous combustion products separating from the wall of the propellant grain and rolling down the wall toward the nozzle can create a vortex that expands as it descends.  The nozzle area inside the motor features lots of convoluted surfaces against which vortices of all kinds can impinge.  This then can cause a back-pressure wave which then becomes a standing wave inside the "pipe" of the motor.  It's actually not unlike the standing wave in an organ pipe.

It seems to me that there would only be rather slight differences between a solid versus a liquid fueled engine. If so then there shouldn't be a vibration difference here.
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Offline JayUtah

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Re: Vibrations during launch?
« Reply #7 on: May 12, 2020, 09:42:38 AM »
Quick answer now, full answer later (hopefully with illustrations).

1. The cavity in solid propellant is generally star-shaped, but can be practically any shape you can mold.  For the shuttle SRBs the answer is complicated.

2. Yes, there is a carefully controlled relationship between the combustion surface area and the cavity volume.

3. Liquid-fueled rockets vibrate for entirely different reasons.  The pogo frequency for the Rocketdyne F-1 was about 16 Hz.
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Offline Jason Thompson

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Re: Vibrations during launch?
« Reply #8 on: May 12, 2020, 11:28:09 AM »
He's probably talking about the longitudinal vibrations common to solid fuel rockets.  The STS SRBs exhibit most of their vibrational energy at about 4 Hz.  This is much lower than the typical 20 Hz product in smaller rockets.  The SRB would be felt as a definite vibration, while the higher-frequency oscillations in smaller solid fuel motors would be perceived more as a very low-pitched sound.

Ok, then what is the "average" vibrations of a liquid fueled engine?
Quote

As you may know, the cavity in the middle of the fuel grain in which combustion takes place changes shape during flight as fuel burns.  This changes the dynamics of combustion and creates opportunities for different kinds of vortices to arise.  The technical term is "vortex shedding," and it covers many of the ways moving fluids interact with themselves and with obstacles.  Just the act of gaseous combustion products separating from the wall of the propellant grain and rolling down the wall toward the nozzle can create a vortex that expands as it descends.  The nozzle area inside the motor features lots of convoluted surfaces against which vortices of all kinds can impinge.  This then can cause a back-pressure wave which then becomes a standing wave inside the "pipe" of the motor.  It's actually not unlike the standing wave in an organ pipe.

It seems to me that there would only be rather slight differences between a solid versus a liquid fueled engine. If so then there shouldn't be a vibration difference here.

I would expect the opposite. A liquid fuelled engine has the fuel and oxidiser injected into the space in the engine bell and ignited, with combustion occurring effectively outside the rocket, while the solid rocket has the combustion happening inside the rocket body (in fact up the entire length of it) and the high pressure combustion exhaust being directed through a nozzle and into the engine bell. Those seem to be such different scenarios I would expect entirely different mechanical and vibrational effects to be going on.
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Offline JayUtah

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Re: Vibrations during launch?
« Reply #9 on: May 12, 2020, 12:44:49 PM »
Let me handwave past a lot of the interrelationship between burn rate, chamber pressure, chamber temperature, thrust, burn cavity, and time.  They're all complexly interrelated.  If we concentrate solely on the relationship between thrust and cavity, most of the questions can be answered.

We classify thrust profiles broadly as regressive, neutral, or progressive.  Regressive means a lot of thrust in the beginning, tapering off.  Neutral means roughly the same thrust throughout the burn.  Progressive means the thrust increases over time.  Different cavity geometries produce different profiles.

About two-thirds of the way down this page http://www.collectspace.com/ubb/Forum14/HTML/001062.html you can see a cross-section of the Apollo LES motor propellant casting.  That's the classic star-shaped cavity, which gives you roughly neutral thrust.  This is a great page https://vc.airvectors.net/tarokt_1.html . It has photos from the rocket park outside the factory where the SRBs are made.  (Thiokol, Morton-Thiokol, ATK, Orbital-ATK, and now Northrop-Grumman.  I don't care what they call themselves as long as the checks don't bounce.)  I added this one because this is a solid propellant casting you can actually touch, if you go there.  I've given this tour to people many times.  It's a two-hour drive or so from Salt Lake City.  Here you can see where someone cut out a chunk of the casting for testing.  (This was a casting process test, if memory serves.  The "propellant" is actually inert.)

So what about the shuttle?  Because the orbiter was so drag-sensitive, the whole STS stack had to throttle back between T+50 and T+70 seconds.  How do you throttle back a solid rocket motor and then throttle it back up again?  I'm sure you could come up with a convoluted cavity geometry to do that, but then it becomes a manufacturing problem.  It's hard enough to cast these giant segments with simple cavity patterns.  How it happens is that the forward segments are cast in a fin-style regressive pattern that peaks at T+50.  Then the aft segments are cast with a cylindrical-cavity grain that's progressive.  The progressive thrust picks up steam at T+70.  The sum of their thrust results in the desired profile. But don't think that varying combustion rates along the axis at different times doesn't pose some fun design challenges.

Here's a cross-section of the regressive forward segment of the space shuttle SRM grain.  https://www.pics4learning.com/details.php?img=srbcross-section.jpg  Yes, a blank is put in there to reserve the cavity, and then the goopy "solid" propellant is poured in and allowed to harden.  Then the blank is carefully removed.  It's a lot trickier than the hold-out blank for the cylindrical castings.

SLS will use the same hybrid thrust profile, only with an additional booster segment.  The regressive grain has been redesigned using better predictive models.  This is not because SLS is as drag-sensitive as the shuttle in the same way.  But the length and elasticity of the SLS stack means you need to temper your velocity through max-Q in case a lot of steering has to happen.  Turning the rocket sideways to the slipstream at maximum aerodynamic pressure is risky.

As far as the internal structural geometry goes, an SRB is nothing like a liquid-fueled motor.  The latter almost always integrates the thrust chamber and nozzle so that they are the same manufactured part.  In the classic Rocketdyne method, both are made up of tubes furnace-brazed together into a single assembly with very smooth inner walls.  Solid rocket boosters are almost always built as a motor casing with the other stuff attached later.  This is because of how the casings have to be made.  Many are filament-wound.  The shuttle SRBs had the design constraint that the casing segments had to be modular.  None of that allows for a nozzle to be manufactured integrally to the casing.  Further, the nozzles are made from entirely different materials, usually because they have to be ablatively cooled.  And there's the gimballing hardware, which has to go somewhere there.  So where the nozzle and skirt assemblies have to be mated to the casing, there's a lot of geometry to create joinery strong enough to keep the nozzle end of the booster from being blown off under pressure.

Okay, this was about vibration. So let's get back to it.

Liquid-fuel engines suffer from three broad categories of vibration, in increasing order of (acoustic) frequency:  chugging, buzzing, and screaming.  Chugging is mostly caused by elasticity in the whole fuel piping system and rocket structures being excited by hydraulic fuel-flow and flight loads.  That causes combustion instability by varying propellant flow rate.  Pogo and chugging are the same thing.  Buzzing is usually a localized problem in the propellant feed system, but same effect on fuel flow.  Screaming is usually a harmonic resonance problem in the thrust chamber itself.  It will occur even when propellant flow is stable.  What these mostly have in common is the fact that the propellant is a fluid and is subject to many effects on its pressure and flow rate.  They arise from the need for pumps, tanks, pipes, and sometimes some innovative structural designs to hold them all.

Solid-fuel motors have fluid-flow problems, just in a different regime of the design and with a different fluid.  Yes, it's all a problem of fluids moving through a "pipe," but in the solid-fuel case the pipe is the entire rocket casing and the fluid is a torrent of combustion products driven by thermodynamics to do perplexing things.  Channel flow is a fun problem even before you add rocketry.  We can solve instability in propellant-feed systems by largely brute-and-klunky means like adding accumulators or thickening pipe walls.  Mitigating flow instability in solid-fuel motors is a whole-system problem and as such has a plethora of additional constraints.  (Don't get me started on the Ares-1 and those damn shock absorbers.)  Hence we do what we can, but we mostly just live with it.  As long as it stays below the acoustical loading limits of the payload, we just say it's going to be a bumpy ride as long as you ride SRBs.  Northrop-Grumman just has to stay below the Orion capsule's shake-apart limit.

Stepping back several steps, the broad model that covers both rocket species (and, in fact, any fluid-management engineering) is the acoustic-mechanical coupling.  Fluids flow.  As they flow, they encounter solid-ish surfaces and impart loads on them.  The solid parts behave according to ordinary mechanical harmonics.  That in turn imparts loads back into the fluids, which then react.  It's a feedback system.  In good designs, feedback damps out.  In bad designs, it doesn't.  In acceptable designs if may reach an tlerable equilibrium.  In all cases it requires attention both to fluid flow and to mechanical resonance.  But when you draw the diagram of what affects what, the diagram is different for solid-fuel motors than for liquid-fuel motors.  The phenomenon is there in all cases, but the boxes and arrows are different.  When you see Elon Musk's rocket nozzles fluctuating under exhaust flow loads, that's an acoustic-mechanical coupling that's no different in its basic mechanics principles than obstacle vortex shedding between the forward and middle segments of the SRB.  They're at the same time very different phenomena but also examples of the same general problem.
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Offline bknight

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Re: Vibrations during launch?
« Reply #10 on: May 12, 2020, 01:39:25 PM »
Thanks to all.  Jay, I never thought about different cross sectional patterns in the SRBs, but now realize that there needs to be a way of limiting the burn rate/thrust profile.  I never realized that the SRBs were throttled as the RL-25 to reduce the loads at the time Max Q on the rocket, but you aerospace engineers have a lot to design around.  No wonder there were so many failures early in the development!!

You mentioned the SLS SRBs have as one limiting factor, the Orion shake apart limit.  What is that approximate value?
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Offline JayUtah

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Re: Vibrations during launch?
« Reply #11 on: May 12, 2020, 07:58:18 PM »
Aerospace design is tedious. time-consuming, and expensive.  As for the "shake-apart" limit, that's just my playful term for the myriad of kinds of limits on all the mechanical loads the payload has to endure.  I have no idea what any of those values are off the top of my head.
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Offline smartcooky

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Re: Vibrations during launch?
« Reply #12 on: May 12, 2020, 10:53:12 PM »
Jay

Thank you very much for the comprehensive answers.

I knew that STS was throttled down at around MaxQ then back up, but I always thought that was entirely done with the RS25 engines. I never realised that they "throttled" the SRBs down as well.

Another new thing learned!
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Offline Kiwi

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Re: Vibrations during launch?
« Reply #13 on: May 13, 2020, 02:59:50 AM »
Another new thing learned!

Me too!

It's now 6:59 pm Wednesday 13 May 2020 NZST. And I've had a good day because I've just learned something new.

Many thanks, JayUtah.  Your ability to explain complex things to laypeople has now impressed me for 18 years. It's a valuable ability that not all high-IQ people have.
« Last Edit: May 13, 2020, 03:03:11 AM by Kiwi »
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