Calculating Nose Cone Drag Separation?

jderimig said:

Fsep IS an actual force acting on the bodies. It is the inter-stage coupling force. As drawn in the free body diagram a positive Fsep means the coupling is in compression which means the sections have the same acceleration which means they are not drag separating.

If Fsep calculates to be (-) then conditions for drag separation exist and yes in that case each section will not have the same a. But that point is moot, the purpose of the derivation is to indicate if the conditions for drag separation exist, not to determine the dynamics in that drag separation condition.

In other words, equal acceleration at each section is the condition where no drag separation occurs. Fsep is the intercoupler force required enforce this condition. If Fsep calculates (+), then this force can be supplied by the butt joint at the coupler. If Fsep calculates (-), then you have to supply this force either through coupler friction or shear pins that can support the force of Fsep.

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I follow you now. I guess I missed the conditions/assumptions because I was looking at it from a different angle. I would like to know the actual force required to prevent drag separation. In the case of no separation this equation does seem to give the correct sign (compression or tension) but I'm still not convinced that the calculated Fsep is correct. Take my example from above where the drag forces are 1N sustainer and 2N booster. At a moderate deceleration rate of 1.5G it gives Fsep of almost 4.5N. It seems to me the separation force should equal the difference in drag force when the masses of the bodies are equal.

micro said:

I follow you now. I guess I missed the conditions/assumptions because I was looking at it from a different angle. I would like to know the actual force required to prevent drag separation. In the case of no separation this equation does seem to give the correct sign (compression or tension) but I'm still not convinced that the calculated Fsep is correct. Take my example from above where the drag forces are 1N sustainer and 2N booster. At a moderate deceleration rate of 1.5G it gives Fsep of almost 4.5N. It seems to me the separation force should equal the difference in drag force when the masses of the bodies are equal.

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Using your example I do not get 4.5N, I get 0.5N, which is correct.

a = F/m = (1N+2N)/(1kg+1kg) = 3/2 = 1.5 m/s^2 (not Gs)
R = Fd2/Fd1 = 1/2 = .5
Fsep = a[m1- M/(R+1)] = 1.5[1 - 2/(.5+1)] = 1.5[1 - 2/1.5] = 1.5[1 - 1.33333] = 1.5[-.33333] = -0.5N

Intuition might suggest that the force should be 1N which is the difference in forces when in reality that is not the case. This is because Fsep works on both bodies in opposite directions. When you include it in the net force on the bodies the force on the sustainer becomes 1+0.5 = 1.5N and the force on the booster becomes 2-0.5 = 1.5N. With the bodies each weighing 1kg the acceleration of each body is a = 1.5N / 1kg = 1.5 m/s^2.

Ravenex said:

a = F/m = (1N+2N)/(1kg+1kg) = 3/2 = 1.5 m/s^2 (not Gs)
R = Fd2/Fd1 = 1/2 = .5
Fsep = a[m1- M/(R+1)] = 1.5[1 - 2/(.5+1)] = 1.5[1 - 2/1.5] = 1.5[1 - 1.33333] = 1.5[-.33333] = -0.5N

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1.5G= 1.5*9.8 = 14.7m/s2
14.7(-.333) = -4.9N

micro said:

1.5G= 1.5*9.8 = 14.7m/s2
14.7(-.333) = -4.9N

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Why are you multiplying an already calculated acceleration (1.5 m/s2) by G again? Stop doing that.

Ravenex said:

Intuition might suggest that the force should be 1N which is the difference in forces when in reality that is not the case. This is because Fsep works on both bodies in opposite directions.

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This AND this is not a static situation. By Brian's calculation a non drag separating rocket will decelerate at 1.5 m/s2.

Take the top section. It's mass is 1kg with a 1N drag force on it. If that was the only force on it would decelerate at 1 m/s2 (1N/1kg). So we need another 0.5N of force to get it to decelerate at 1.5m/s2. That force must come from friction or shear pins.

jderimig said:

Why are you multiplying an already calculated acceleration (1.5 m/s2) by G again? Stop doing that.

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I was going by the 1.5G I suggested in my example above which was arbitrary, not calculated. I see that arbitrary number doesn't jive with the forces I gave and the true a would be coincidentally 1.5m/s2. I suppose mine is flying through molasses.

bobkrech said:

Note to Troy

Drag Separation and Internal Pressurization Separation are 2 different processes that are caused by very different mechanisms.

1. Drag separation is due to differential retained momentum. It does not require internal pressurization.
2. Internal pressurization is due to the pressure differential between the interior and exterior of the rocket. Drag is not relevant. There is no difference between the differential pressure created by an ejection charge or altitude induced internal pressurization as the net forces are the same.

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Bob,
Please read my explanation again (in whole) and consider your response - please excuse the typos.

Regards,

Troy

rocket_troy said:

Bob,
Please read my explanation again (in whole) and consider your response - please excuse the typos.

Regards,

Troy

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I read your post several times before I made my response.

1. Drag separation is an aerodynamic process and internal pressurization separation is a propulsive process.
2. You get drag separation of a forward section of a rocket if the sectional density of the forward end of the rocket had a higher sectional density than the unfastened aft end of the rocket even when the aft end is vented to ambient external pressure.
3. You get pressure separation of the forward section of a rocket if the unfastened aft section is pressurized, even if the aft section is not moving and is bolted to a test stand, no drag is involved.
4. Internal pressurization is not a requirement for drag separation, nor is drag necessary for pressure separation. They are two independent processes.
5. In a shot gun, the shot load held in a shot shell wad, is accelerated down the barrel by internal pressurization created by the combustion of propellant charge in the shell casing pushing against the base of the shot shell wad as, and after, it separates from the shell casing. Once beyond the barrel, aerodynamic drag separation rapidly decelerates the shot shell wad while the shot continues downrange. The sectional density of the shot is more than a order of magnitude higher than the shot shell wand and has a much lower Cd. That's a classic example of the differences between internal pressurization separation and drag separation.

bobkrech said:

I read your post several times before I made my response.

1. Drag separation is an aerodynamic process and internal pressurization separation is a propulsive process.
2. You get drag separation of a forward section of a rocket if the sectional density of the forward end of the rocket had a higher sectional density than the unfastened aft end of the rocket even when the aft end is vented to ambient external pressure.
3. You get pressure separation of the forward section of a rocket if the unfastened aft section is pressurized, even if the aft section is not moving and is bolted to a test stand, no drag is involved.
4. Internal pressurization is not a requirement for drag separation, nor is drag necessary for pressure separation. They are two independent processes.
5. In a shot gun, the shot load held in a shot shell wad, is accelerated down the barrel by internal pressurization created by the combustion of propellant charge in the shell casing pushing against the base of the shot shell wad as, and after, it separates from the shell casing. Once beyond the barrel, aerodynamic drag separation rapidly decelerates the shot shell wad while the shot continues downrange. The sectional density of the shot is more than a order of magnitude higher than the shot shell wand and has a much lower Cd. That's a classic example of the differences between internal pressurization separation and drag separation.

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1. Drag separation is an aerodynamic process and internal pressurization separation is a propulsive process.

What I was describing *was* an aerodynamic process and that’s a critical point. Internal pressurization separation (strictly as per static pressure differential) is not a propulsive process, it’s strictly an aerostatic process.

2. You get drag separation of a forward section of a rocket if the sectional density of the forward end of the rocket had a higher sectional density than the unfastened aft end of the rocket even when the aft end is vented to ambient external pressure.

Well, no, not necessarily. Chances are (*IF* you ignore base drag and that would be a big fat mistake) your forward section will be experiencing significantly more dynamic force than your lower section so the difference in sectional density is just one aspect to be accounted for as has been illustrated in many of the equations presented by other posters. Remember, the fundamental mechanism that initiates a drag separation event is *drag* ie. An aerodynamic mechanism. The difference in sectional density is irrelevant if there’s no atmospheric interaction with the rocket eg. If operating in a vacuum.

3. You get pressure separation of the forward section of a rocket if the unfastened aft section is pressurized, even if the aft section is not moving and is bolted to a test stand, no drag is involved.

Yes, I’d agree with that (with the assumption that you're strictly referring to pressure separation), but hey, I wasn’t describing pressure separation – I was strictly explaining *drag* separation and as I explicitly highlighted, the mechanism I was describing has only a loose correlation to the static pressure differential which is the fundamental criterion to pressure separation.

4. Internal pressurization is not a requirement for drag separation, nor is drag necessary for pressure separation. They are two independent processes.

Internal pressurization is not a requirement for drag separation, but the point is it *can* be a cause of drag separation and the math says it’s likely to be the most frequent cause (or at least the most frequent contributor) of drag separation events.


5. In a shot gun, the shot load held in a shot shell wad, is accelerated down the barrel by internal pressurization created by the combustion of propellant charge in the shell casing pushing against the base of the shot shell wad as, and after, it separates from the shell casing. Once beyond the barrel, aerodynamic drag separation rapidly decelerates the shot shell wad while the shot continues downrange. The sectional density of the shot is more than a order of magnitude higher than the shot shell wand and has a much lower Cd. That's a classic example of the differences between internal pressurization separation and drag separation

Yes, it’s a clear and it's an extreme example of drag separation via the mechanism you describe (although with fundamental technical flaws as an analogy to rocket drag separation noted below) and just to be clear, that’s the hobby’s general understanding of what drag separation is.
The problem is, you and the hobby in general are under the erroneous impression that this mechanism is the only cause of drag separation and that’s a naïve and stubbornly ignorant understanding of the dynamics and will bite some flyers in the bum – especially those who like really fast aggressive flight profiles and utilising minimal venting and don’t take the necessary measures to fasten the sections together.

Note above: – the shots start dispersing quickly after leaving the barrel thereby exposing the wad to some of the frontal dynamic pressure of the atmosphere. In the case of rocket drag separation, the lower section is still attached to the upper section before separation, so only the fins and body (well, and base of course) are exposed so it’s not the ideal example to illustrate the mechanism but I suppose it’s clear in describing the general point.

Troy

rocket_troy said:

... – especially those who like really fast aggressive flight profiles and utilising minimal venting and don’t take the necessary measures to fasten the sections together.

Troy

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I just realised that I might have provided a bit of a false impression that enough venting will eliminate or at least significantly reduce the likelihood of a drag separation event from base drag causation. However, (again) for really fast aggressive flight profiles ie. high acceleration, there are some more concerning issues to be wary of. (1) the venting will unlikely be of a sufficient throughput capacity to equalise the inside:base pressure differential in time (for burnout) but (2) of even greater concern is that even if the static pressure differential was zero (the inside static pressure = outside static pressure), the *absolute* pressure of such could be sufficiently high in comparison to the base pressure thereby opening up the susceptibility of a drag separation event.
So, if you’re going fast enough, quickly enough so you're still in a dense atmosphere, you will see a near vacuum pressure condition on a flat base (regardless of altitude) at burnout and the only way to equalise your internal pressure to that level is to vent your internal pressure out the base (venting out the side will only equalise your internal pressure to the static pressure of the outside).
Of course, the easiest solution is to positively retain your sections together and (the impression I’m under) that’s pretty common practice for such flight profiles anyway.

Troy

rocket_troy said:

The problem is, you and the hobby in general are under the erroneous impression that this mechanism is the only cause of drag separation and that’s a naïve and stubbornly ignorant understanding of the dynamics and will bite some flyers in the bum – especially those who like really fast aggressive flight profiles and utilising minimal venting and don’t take the necessary measures to fasten the sections together.

Troy

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I've been employed as a professional researcher/consultant in the aerospace/defense industry for 45 year, so I'm hardly ignorant of dynamics. What's your experience?

bobkrech said:

I've been employed as a professional researcher/consultant in the aerospace/defense industry for 45 year, so I'm hardly ignorant of dynamics. What's your experience?

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I don't want to get into a pissing contest Bob, so I'll just have to respectfully disagree with you. I'm genuinely sorry if I caused any offence.

Kind Regards,

Troy

I was just about to get out the popcorn as this was getting good.

Unknown Newbie with a "unique" view on physics -V- Seasoned Pro

FredA said:

I was just about to get out the popcorn as this was getting good.

Unknown Newbie with a "unique" view on physics -V- Seasoned Pro

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Ah, Fred, I'll have to commend you on your selection of bait. You got me hook-line-and-sinker.

I’m not quite a newbie to rocketry. I’m not terribly active on this forum, but I am active on other forums more geared to experimental rocketry. I’ve been a TRA member for 20 years (TRA #6012) and have been flying and building motors for years prior to that. Over the years I’ve designed, constructed and either flown or tested over 1000 motors (mostly solid) using about 6-10 different oxidizers. I’ve also built and constructed over 50 hybrids (in fact, it’s probably over 100) and participated in liquid projects. I’ve built many solids (including sizable ones) from both homebrew AP and homebrew polymer binder and I’ve achieved making a propellant with a zero exponent in doing so – photo of such in flight www.propulsionlabs.com.au/Oct16_11_Launch/Solid3.JPG

Over the decades, I’ve developed many many spreadsheets and software applications relating to rocketry, like detailed tools for high quality AP manufacture, solid motor simulators, general rocket motor performance simulators, chemistry tools and other stuff. Some of this stuff has been publicly available for free download for literally decades. Some of it has been utilized by various universities all over the world as tools to assist their propulsion work. For example:
www.propulsionlabs.com.au/pdf/IPT2006_04_Phase3_TeamH_BAFO_Rev02[1].pdf
www.propulsionlabs.com.au/pdf/IPT2003_Phase3_Final_Report_Team3.pdf
Do a document search for “Grains2”

I’ve spent thousands of hours (Yes literally!) on rocket equations over the past 20+ years. Mostly geared around propulsion but I’ve also had a special interest for nozzle behaviour and the interaction with the aft end of a rocket with the surroundings. I derived and programmed every equation and line of code in my GrainsCAD application – some 50,000 lines of code with thousands of complex geometric equations. There’s none of that 3rd party module stuff – it was all done by me (other than the DirectX libraries). I produced all the geometric equations for my other solid propellant simulators including the equations to simulate the more complex grain geometries. Greg Deputy asked to use these for his BurnsSim software which I happily agreed for no cost. It took me hundreds of head throbbing hours to produce and verify these incredibly complex equations.

I’m a contributor to the Encyclopedic Dictionary of Pyrotechnics https://www.jpyro.com/ref-series/encyclopedic-dict-pyro/
I’ve manufactured the parts for an electrically driven liquid propellant turbo pump www.propulsionlabs.com.au/pump2
https://plus.google.com/photos/114181838665529935795/albums/5848809994338746673
[video=youtube;TMMMMaVVAVM]https://www.youtube.com/watch?v=TMMMMaVVAVM[/video]

I fly all my rockets from parts that I make and design myself including electronics, nose cones, body tubes, parachutes, pyroless deployment devices, pyroless release (detachment) devices, motor casings, combustion chambers, nozzles, liners, propellant, sometimes oxidizer and fuel, tanks, injectors, check valves, remote hose disconnect devices, pistons, couplers, boattails, fins etc.
I was involved in the 1:1 scale V2 project where I had the challenging job of developing the separation system – had to be pyroless, had to ensure a 450Kg rocket separated and deployed that was simulated to reach an apogee of a few hundred feet (!), it was a challenge and we pulled it off. This included designing and constructing remote hose disconnects that provided the safe filling of the deployment devices. [video=youtube;y7HT6eOhUOU]https://www.youtube.com/watch?v=y7HT6eOhUOU[/video]

As I’ve previously mentioned here, I’ve also designed and constructed my HPR pyroless deployment device which I’ve been solely relying on since 2010 https://www.propulsionlabs.com.au/Pyroless_Release/

Super Heavy Duty pyroless release Latch www.propulsionlabs.com.au/Misc_Video_And_Images/HD_Latch.JPG

My Propellant mixer: www.propulsionlabs.com.au/Misc_Video_And_Images/Mixer2.jpg (Mixes and Casts under vacuum with 20kgf Vibrator attached and jacketed heater -not visible in photo)
Making Nosecones: https://www.ausrocketry.com/forum/viewtopic.php?f=10&t=2268
Miniature Hybrid Vent Valve (now superseded) www.propulsionlabs.com.au/Hybrid_Vent_Valve

My latest (but very old now) solids & and performance software : https://propulsionlabs.com.au/Software.htm

Probably the 1st person in Tripoli to make and use composite wound phenolic nozzles (nearly 20 years ago). Probably the 1st person to make both oxidizer and fuel and flew such on sizeable motors. Supplied numerous large (up to O designation) experimental motors for large projects. Produced hybrids with >18sec burn time.

dup

Care to give us a real name?
I don't see a "Troy" listed on any of those papers you reference...
Wish this forum didn't allow people to hide behind nicknames.

And what do you make in that mixer? A kg or two, maybe.... Seems rather small for all the effort....

FredA said:

Care to give us a real name?
I don't see a "Troy" listed on any of those papers you reference...
Wish this forum didn't allow people to hide behind nicknames.

And what do you make in that mixer? A kg or two, maybe.... Seems rather small for all the effort....

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Troy Prideaux. It's there all over the place in half the links I embedded .... geez...

Up to 3.5Kgs of propellant IIRC. Up, it's a lots of work, but I made a LOT of motors.

Troy

OK Troy - Nice to have you back in the hobby.
I looked at those Puff Adder (cheesy name) papers and didn't see any "Troy" as contributor.....hence the "who are you really" question.

And 3.5kg batches are small - do those on my little mixer which has about 3 years of dust on it.....
11kg are the norm, looking to move to 25kg.

rocket_troy said:

Troy Prideaux. It's there all over the place in half the links I embedded .... geez...

Up to 3.5Kgs of propellant IIRC. Up, it's a lots of work, but I made a LOT of motors.

Troy

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Nice to have such experienced contributors participating on TRF.

Now that we're into the stick measuring contest of comparing credentials, this is the time to go get the popcorn ready! :smile:op:

FredA said:

OK Troy - Nice to have you back in the hobby.
I looked at those Puff Adder (cheesy name) papers and didn't see any "Troy" as contributor.....hence the "who are you really" question.

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Fred,
Just to clear this up (coz you mentioned it a couple of times): I wasn't on those teams or participated directly in the proposals and nor did I ever claim this. They used my Grains2 spreadsheet as a tool to verify their grain design ideas as is noted throughout the propulsion section of the documents. The 1st doc does list my name as the developer of Grains2 which was a very ancient and rudimentary iteration of what is now Grains...whatever it is...4M I think.

Troy.

jderimig said:

Update to an old thread. Tim Dixon and I have been sharing thoughts on this derivation (which is nearly 7 years old) and a refresh was done.

Below is a rederivation of the problem. I switched the coordinate system + direction this time so the signs are different but the theory is the same.

Note that the Drag Force ratio R is really the ratio of Cd*A of the top and bottom airframe sections. Usually the fin area is negligible so the Cd ratio is sometimes a good approximation, but to be totally accurate the Cd*A should be used for R. Both easily obtained from Rocksim or OR.

Also "a" is the deceleration from drag only at motor burnout. So if your sim says 5g's of deceleration, 1g of that is from gravity. So subtract that 1g out and use 4g. Note in the derivation below + is pointing "down", so deceleration is entered as a positive value in the equation. Also in the below derivation Fsep is defined as a "compression" force. So that if Fsep is positive then drag separation will not occur. Sorry for flipping the sign convention this time.....

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Reposting the derivation since it looks to have gone missing/broken.

OK. Now can you post the derivation of the Lovelace effect?