RE: max alititude: boattails and minimum diameter rockets...

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RE: max alititude: boattails and minimum diameter rockets...

Postby Resuscitative » Tue Jan 17, 2012 6:45 am

Hi Forum,

I thought that the best place to ask this (maths) question was in this forum.

Background:

I understand that a boat-tail decreases drag.

In "Modern Exterior Ballistics" by Bob McCoy he talks about the benefits of boat-tails. He says that a boat-tail angle of 6 - 8 degrees is best and the amount of drag is continuously reduced as boat-tail length increases, indefinitely. (I'm paraphrasing from here: http://www.appliedballisticsllc.com/ind ... Design.htm).

Now, is it possible to design a rocket that will outperform the same engine class' minimum diameter rocket because it has a boat-tail?

Please consider the below:

Note 1: Rocket is optimised for maximum altitude.

Note 2: Please consider this from a HPR perspective.

Note 3: I use the below engines as examples purely because I know their diameter.

1. Say you are using a 24mm E9 engine in a rocket with a boat-tail. Accordingly, the diameter of the rocket at the end of the boat-tail is 24mm, and the diameter over the length of the fuselage is 27mm (essentially, the fuselage is a larger diameter than the engine requires because of the boat-tail).

2. In comparison, you have a minimum diameter 24mm E9 rocket. This rocket has NO boat-tail.

I would love to know the answer to the above question and the following questions:

1. How can you calculate the relative drag coefficients of these two rockets?
2. Which one would have the lower cd?
3. a) Is it possible that the boat-tailed rocket design trumps the minimum diameter rocket up unto a certain point, and if so what is that point? (perhaps in terms of a fineness ratio ratio?)
3. b) Or does the minimum diameter rocket straight up trump the boat-tailed rocket?

:-)

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Re: RE: max alititude: boattails and minimum diameter rocket

Postby strud » Tue Jan 17, 2012 7:04 am

One thing to remember is that drag is proportional to frontal area also, hence increases with diameter ^2.

Obviously the boat tail is only going to have an effect on the base drag, there is still body/skin and frontal drag to consider.

All other things being the same, the rocket you've described with the boat tail should have the lower drag co-efficient BUT likely higher drag for a given velocity since it is 3mm larger in diameter.

There is no good reason you couldn't have a boat tail on the 24mm dia rocket since the exit plume diameter of the motor is likely to be rather small, say 10mm.

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Re: RE: max alititude: boattails and minimum diameter rocket

Postby ROCKet STAR » Tue Jan 17, 2012 10:37 am

That is an interesting question and one that I do not know the answer to. It is also one that I have heard debated before without any definite conclusion being drawn, with the discussion going into debating all of the variables such as the geometry and efficiency of the boat tail, the velocity and the % increase in diameter of the airframe etc.

Personally I think that really if any improvements in performance were to be gained, they would be slight, the increase in diameter you could get away with would be VERY small, and would only be seen in a subsonic flow regime. As Strud has also pointed out, minimimum diameter with a boat tail will trump everything! :)
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Re: RE: max alititude: boattails and minimum diameter rocket

Postby rocket_troy » Tue Jan 17, 2012 1:05 pm

the increase in diameter you could get away with would be VERY small, and would only be seen in a subsonic flow regime.

Minor quibble - the theoretical maximum reduction in base drag from ultilising a boat tail should be around the transonic region... probably marginally into the supersonic regime M1 - M1.2.

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Re: RE: max alititude: boattails and minimum diameter rocket

Postby ROCKet STAR » Tue Jan 17, 2012 2:04 pm

I may be wrong, but my train of thought was that as the rocket entered transonic/supersonic flow regimes, the increase in form drag along with the increased skin friction drag and any other parasitic drag that there may be, would begin to outweigh the gains from the boat tail. :)
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Re: RE: max alititude: boattails and minimum diameter rocket

Postby air.command » Tue Jan 17, 2012 2:24 pm

Here is an article I came across a while back about reducing base drag in projectiles.

http://www.globalsecurity.org/military/systems/munitions/base-bleed.htm

Does anyone know how much does base drag get reduced for a regular rocket, (after burnout) while you still have the delay charge burning and residual hot gas coming out of the nozzle? I'd imagine it would behave similarly to the base-bleed concept above. Would this likely reduce base drag by any appreciable amount?
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Re: RE: max alititude: boattails and minimum diameter rocket

Postby Resuscitative » Tue Jan 17, 2012 4:25 pm

ROCKet STAR wrote:the increase in form drag along with the increased skin friction drag and any other parasitic drag that there may be, would begin to outweigh the gains from the boat tail. :)


In an effort to simplify this:

Please assume:

1. Both rocket 1. and rocket 2. have the same overall length.

2. They have the 'same' nosecone (for example rocket 1 has an x^1/2 power nosecone@27mm, and rocket 2 has an x^1/2 power nosecone@24mm.

3. They have the same effective laminar flow.

As ROCKet STAR indicated, the increase in wetted area because of the boat-tail (27mm fuselage will have a larger wetted area w.r.t a 24mm fuselage) will increase drag. As I understand it the increase in drag due to the increase in wetted area will become more and more pronounced as the rocket goes supersonic (mach 1, mach 2, et cetera).

Thus, perhaps,
ROCKet STAR wrote: the increase in diameter you could get away with would be VERY small
? :-)

Next one,
strud wrote:There is no good reason you couldn't have a boat tail on the 24mm dia rocket since the exit plume diameter of the motor is likely to be rather small, say 10mm.

What about if one was using an N-class motor?! I was asking Scoop about this on the weekend and he mentioned that having such motors not flush with the end of the rocket is a bad idea :-(

Thus, in the case of the N-class motor, there would have to be an increase in diameter to accommodate the boat-tail, thus we return to the original question, does the boat-tail on the slightly larger diameter rocket decrease absolute drag when compared to the minimum diameter rocket?! :-)

I'm asking all of the above questions in good faith! (hope that I don't tread on anyone's toes!!) :-)

Many thanks for your responses thus-far! :-)

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Re: RE: max alititude: boattails and minimum diameter rocket

Postby ROCKet STAR » Tue Jan 17, 2012 5:13 pm

Resuscitative wrote:
Thus, perhaps,
ROCKet STAR wrote: the increase in diameter you could get away with would be VERY small
? :-)


I wasn't specifically referring to your example, but pointing out that if there was to be any gain from increasing the diameter of a rocket as well as adding a boat tail, i believe the amount you could increase it by and still see the gain in performance would be very small.

Thus, in the case of the N-class motor, there would have to be an increase in diameter to accommodate the boat-tail


That would only be the case if the diameter of your nozzle's exit cone was the same as the diameter of your rocket, which is certainly never the case in commercial HPR motors. As such you can have your boat tail taper down to that diameter. I believe that CTI even manufacture aft closures that do just that.
Last edited by ROCKet STAR on Tue Jan 17, 2012 5:15 pm, edited 1 time in total.
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Re: RE: max alititude: boattails and minimum diameter rocket

Postby PK » Tue Jan 17, 2012 5:14 pm

Right so you're assuming every motor has a thrust ring that necessitates an increase in diameter so that the boat tail can be implemented as part of the airframe?

This doesn't address the theoretical question but:
a. The boat tail doesn't have to be part of the airframe. You could just glue it onto the nozzle/aft closure. I'm pretty confident this wouldn't constitute modifying the motor so your "assume HPR" assertion would hold.
b. I don't have an encyclopaedic knowledge of HPR casings, but I'd be surprised if you couldn't buy one that had either no thrust ring (ie a snap ring motor that you just don't fit the thrust snap ring too) or had a boat tail closure.


To answer your question more directly, I've just had a skim through some of my references and it looks like base drag accounts for somewhere between 15% and 20% of Cd in rockets with conventional aspect ratio's, both sub and super sonic (although there is less data for supersonic flight) So, if your increase in drag due to an increased diameter is <10% then a boat tail will definitley improve things.

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Re: RE: max alititude: boattails and minimum diameter rocket

Postby rocket_troy » Tue Jan 17, 2012 5:23 pm

Thus, in the case of the N-class motor, there would have to be an increase in diameter to accommodate the boat-tail, thus we return to the original question, does the boat-tail on the slightly larger diameter rocket decrease absolute drag when compared to the minimum diameter rocket?! :-)


The answer to this more specific question is a simple "NO". Because the geometry of the motor's aft end will be producing the base drag in a minimum diameter motor, and guess what - that same geometry of the same motor will produce just as much base drag on a larger diameter rocket with an efficient boat tail. The difference is, the wider bodied rocket will see more drag due to more exposed frontal area and will also see less than 100% efficiency of the boat tail ie. <100% efficient boat tail will nearly always produce more drag than no rocket body at all (that difference in frontal/sectional area between the 2 bodies). The the Cd of the larger airframe will be lower, but that means nothing for this set of circumstances. The role of the boat tail is not to decrease the base drag offered by the motor's aft area, it's to decrease the base drag of the sectional area between the motor and the airframe's outside periphery or to be more precise - to decrease the base drag of the sectional area taken up by the boat tail only.
If the motor itself had a boat tail between its periphery and nozzle exit, then sure, that will improve the drag efficiency of the base, but again, that goes for both options, rendering the same answer.

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Re: RE: max alititude: boattails and minimum diameter rocket

Postby Crazy Jim » Tue Jan 17, 2012 5:24 pm

Speaking from experience not theory:

Tapered closures/boat tails work. CTI sell tapered closures for 29mm & 54mm. One can grind the plastic 38mm's or machine tapered closures for any size motor .

They work by reducing base drag significantly, so do long burn motors. The long tail off in burn profile supplies a small thrust factor that helps overcome/negate base drag. Full size smoke grains do the same but at the expense of substantial heat to the motor case.

My 54mm[minimum diam] Space Cowboy gained 800-1200 ft at 10,000 and 2,000 to 2,400 at 23,000ft by utilizing a tapered closure.

I suppose one could say, in my designs, that approximately 10% in altitude has been gained by using tapered closures, with no cost in rocket diameter size. This can easily be seen in simulations in Open Rocket & Rocksim.

A small BP motor such as you are describing can easily be converted to a boat tail with some ingenuity, with out gain in rocket diameter. By gluing a small section of BT to rear of the motor and using light weight filler such as Rock Hard, spackle, then shaping the cone. Insert this into body tube, no gain in diam, but now you have your boat tail.
Can also be done by using BT & couplers continually forming a reducer cut in small slices, then filled on the outside to shape. I have done these things & they will give you greater altitudes than the standard configuration.
I have never seen a need to go to a larger body diameter to gain any advantage in boat tails. They can be made to fit minimum diam. rockets & motors, so why do it?

This can be hard to evaluate with smaller motors considering they can fluctuate 15-20% in performance just due to batch #.
And at this small level of performance, the luck of the draw may outweigh a boat tail.
All things being = tapered on a minimum diam. wins every-time

How pertinent my above statements are to your quest for raw numbers & knowledge, I cannot tell. Suffice to say I let computers do the simulation work for me [then I go fly the darn thing and see the results] & hope maybe my information may serve you some small service.

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Re: RE: max alititude: boattails and minimum diameter rocket

Postby Scoop1261 » Tue Jan 17, 2012 6:18 pm

Resuscitative wrote:What about if one was using an N-class motor?! I was asking Scoop about this on the weekend and he mentioned that having such motors not flush with the end of the rocket is a bad idea :-(



Actually, what I said was, "Having a nozzle that far up a rocket airframe is bound to end in tears"
Not really going into specifics at the time, more just a generalization about burning the ass of a rocket as not being a desirable effect.

As others have noted, boatails can be fitted to minimum diameter rockets along with most any rocket depending on the width of the exhaust plume, distance of the nozzle from the aft end of the boatail, along with many variables.
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Re: RE: max alititude: boattails and minimum diameter rocket

Postby Resuscitative » Tue Jan 17, 2012 6:49 pm

:oops:

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Re: RE: max alititude: boattails and minimum diameter rocket

Postby PK » Tue Jan 17, 2012 7:19 pm

rocket_troy wrote:The answer to this more specific question is a simple "NO"


Nah, not so simple. It depends on whether the aft diameter of the boat tail is < the motor diameter. Eg a boat tail motor retainer.
If it isn't then the simple NO applies. If it is then it's marginally less simple.
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Re: RE: max alititude: boattails and minimum diameter rocket

Postby Scoop1261 » Tue Jan 17, 2012 8:04 pm

Resuscitative wrote::oops:


No problem....I just needed to quantify for the purpose of the discussion :D
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