Boat Tails =advanced=

Discussion in 'Techniques' started by henry8minus1, Dec 9, 2004.

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  1. Dec 9, 2004 #1

    henry8minus1

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    I wanted to ask how much of a benefit boat tails have on the performance of rockets. As I understand it the boat tail makes the rocket to be more of a streamlined body and thus reduces the form drag. I remember reading that you can make a rocket with better aerodynamic properties (and thus better performance) if you increase the diameter and use a boat tail versus a minimum diameter rocket. Is this true? (This probably depends on other things like weight, construction, etc., but I am asking if we assume these factors are similar for both rockets.)

    I would assume for most rockets that the hassle of making a boat tail is not worth the benefits, otherwise there would be more rockets with them.

    Also, with respect to the end of the rocket, what are the effects of thrust? Does the pressures and turbulence created by the engine's thrust create drag also? If so is there ways to reduce this drag.

    Thanks for the help!
     
  2. Dec 9, 2004 #2

    Zack Lau

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    I figured out an easy way to make boat tails for contest rockets using my miniature metal lathe. First, I make a mandrel out of 6061-T6 aluminum. I cut a thinner section at one end for the boat tail. The tail has two wraps of balsa--the rest of the body just has one. After the wraps are securely glued, I put the mandrel in the balsa and carefully turn down the balsa to the desired thickness. I then turn the boat tail. This method has the advantage of light weight--I can build a bigger rocket with no more weight than a conventional minimum diameter rocket.
    This is particularly important in the lower power classes, where minimizing weight is the major factor in maximizing altitude.
     
  3. Dec 9, 2004 #3

    powderburner

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    Boat tail fairings can reduce drag, yes. But there are some tricks to the details of how you shape everything.

    First off, all comments are assuming that all external surfaces are smoothly finished (you can see yourself in the reflection) and that there are no gaps or steps (diameter mis-matches) between the main body tube and the boat tail.

    The boat tail (or tail cone, or aft fairing) needs to be held to a closing angle of not more than about 10 degrees. For angles of 20 or 30 degrees you are asking for the flow to separate at the transition and go turbulent (draggy) over the surface of the boat tail.

    In subsonic flow, the transition between the constant-diameter BT and the boat tail needs to be gradual, not the sharp corner that is usually seen, to retain as much of the 'good' airflow as possible and stick to the boat tail for low drag. Viewed from the side of the rocket, I am talking about a gentle (and smooth) curve between the body tube and the boat tail (boy, if you abbreviate *both* of those as 'BT' it gets confusing!). The longer the transition can be, the better. What you are doing is giving the external airflow more time to adjust pressure and other flow conditions from the body tube to the boat tail. The down side of this configuration feature is that the combined length of boat tail and curved transition may not fit on the rear of your rocket without interfering with the motor mount-----you do the best you can.

    If you really want to get cut-throat and go for the last percent of drag, you extend the boat tail past the rear of the motor case so the final boat tail diameter is smaller than the motor case. The boat tail should not overhang the nozzle by more than about 1/4 of the motor diam or else you will start to get the 'Krushnic' (spelling?) effect and kill part of your thrust. This boat tail design means the motor case cannot be removed toward the rear; your rocket will have to include a mid/aft-body break and provisions to change the motor through the front of the motor mount.
     
  4. Dec 10, 2004 #4

    JRThro

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    If I understand you correctly, you're saying that a boat tail should "transition" from the body tube diameter down to the motor diameter at a "slope" of no more than 10 degrees (ignoring your mention of a smooth transition between the body tube and the boat tail itself).

    If that's the case, then here are some calculated boat tail lengths:
    Body Tube --------------- Motor Mount --------------- Boat Tail Length
    BT-50 (0.976" OD) -------- 18 mm (0.736" OD) ---------- 0.68" (25% of motor within boat tail)
    BT-55 (1.325" OD) -------- 18 mm (0.736" OD) ---------- 1.67" (60% of motor within boat tail)

    Actually, these lengths are slightly more reasonable than I expected them to be. :)

    Do you have any idea if the sharp corner between the body tube and the boat tail, and/or the greater than 10 degree angles of most boat tails, makes them useless as far as drag reduction is concerned? Or are they still better enough than nothing to make them worth the trouble?
     
  5. Dec 10, 2004 #5

    powderburner

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    While I don't have any hard data at my disposal (here is yet another great spot for someone to do a research project), I can tell you that the benefits would be substantially reduced for sharp corners or steeper aft fairing angles. Maybe not completely useless, probably somewhere in between like half useful?

    Subsonic airflow does not like sharp corners and will readily transition from smooth to turbulent flow. Even if you must limit your transition length to something relatively abrupt like 1/4 inch or less, if you can round off that corner and keep it smooth you will get a much greater benefit in smooth flow and reduced drag. You don't have to use a 50-foot-long transition but every bit that you can work in will help.
     
  6. Dec 10, 2004 #6

    Jerry Irvine

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    Jerry Irvine boattail rules of thumb

    For subsonic rockets the opitimum angle is 7 degrees per side.

    For super sonic rockets the optimum angle is 30 degrees per side.

    For egglofters magazine paper is an excellent shroud material (preference toward Playboy).

    For HPR strong shrouds such as U.S. Rockets "Ace Fugue Shroud method" or "Integrated Tube™" method is best for combined cost, efficiency, strength, and ease of use.

    Jerry
     
  7. Dec 10, 2004 #7

    JRThro

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    It may be worth noting that all other things being equal, a longer rocket experiences higher drag forces than a shorter one. Or at least I think it does. So a gradual 50-foot transition from body tube to tail cone would very probably add enough drag to offset the benefit of both the tail cone and the fact that you used a gradual transition.

    I know you were exaggerating for effect, btw. ;)

    But there's clearly a point of optimum drag reduction for the gradualness of the transition, just as there is a point of optimum angle of taper for the tail cone itself. Both of those optimum points probably depend on (at least) the body tube length, the body tube diameter, and the motor diameter.
     
  8. Dec 10, 2004 #8

    powderburner

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    JRThro,
    I would strongly recommend keeping your boat tail within the original length of your rocket design and not adding it to the rear end.
    The whole idea is to convert the length of main body tube that covers the MMT to reduce base area. You can put both centering rings at the front of the MMT if you get nervous about thrust loads. I would recommend installing the MMT in the full-length body tube and use your eyeball to adjust the alignment for best fit, then cutting off the unwanted body tube and adding the boat tail.
    Jerry's notes about subsonic rockets match my own info, that the optimum closing angle is about 8 degrees. Point is, you don't want to use a 20-30-40 degree boat tail, it won't save you much drag and it will cost a lot more work to fabricate parts and to work up a smooth exterior finish.
     
  9. Dec 10, 2004 #9

    DynaSoar

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    Degrees per side of what, Jerry? I'm not following.
     
  10. Dec 10, 2004 #10

    DynaSoar

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    Zackly. The alternative to any amount of boat tail is big ole flat butt "base drag", worse yet on those birds that have a hollow base with the bottom centering ring set in a way from the aft enbd of the tube (yuck). This is what the rear transition is supposed to reduce.

    The factors that go into a good nose design for a given rocket design should apply. For subsonic, a 4:1 to 5:1 parabolic/ogive (minus the cut-off tip) would probably prove to be best. For supersonic, where number of/amount of transition is more important, a long (10:1) conical would probably be best. Of course for supersonic, you'd have to have a real great need to have a body larger than minimum diameter.

    I've used PML tail cones where a long boat tail wasn't feasible (see my Mirage mod on EMRR), based on the intuitive "it's GOT to be better than not". The sharp transition is less than optimal certainly, but it's less than a perpendicular plate would be.

    For most rockets I've simply used plastic nose cones. I applied the slotted-tube idea to the cone, making the fins go through a slot in the cone roughly equal to half the fin chord, and slotted the body to fit the other half. Here's one constructed along those lines.

    (Note the slot for the Estes style motor hook. The cone was a plastic Bertha style. The tip was blunter than an optimal subsonic shape, but of course that was cut off. What remained is nicely parabolic with very little curvature at first. There's a smaller centering ring (BT55 size; motor tube is BT50, main is BT60) on the end of the motor tube for gluing into the end of the tail cone. It's far enough up the tube that the engine hook is able to flex sufficiently.)

    I worked with a second assumption here, and I'd be glad to hear if others think it's valid: fins set forward from the base reduce drag. Even though here they're along the length of the boat tail, it just seemed to me that having some "flat" surface for relaminarizing the flow would be beneficial. It might be less relaminarizing, but still there. Again, intuitively, it just seemd the that low pressure being pulled by the decreasing diameter would pull the airflow into laminar more so along the boat tail than it would on a straight tube. Such a phenomenon would be part of the "equal area" body/fin design, would it not?
     
  11. Dec 10, 2004 #11

    ELBRAZ

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    Remember that the drag form factor of a smaller diameter rocket will be less than a larger diameter bird, no matter how optimum the boattail. For efficiency, always go for the smaller diameter. Boattails help the efficiency of large diameter rockets, but they don't make them less draggy than the minimum diameter rocket.
     
  12. Dec 10, 2004 #12

    bobkrech

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    I strongly recommend you look at Dr. Derek Brays aerodynamics courses at http://www.rmcs.cranfield.ac.uk/aeroxtra/

    They are excellent summaries of all aspects of rocketry in an understandable form.

    Boattails are addressed on pages 31-33 of http://www.rmcs.cranfield.ac.uk/aeroxtra/e338drag.ppt

    The optimum angle for subsonic flow is 9 degrees, for supersonic flow 6.5 degrees.

    Base drag is reduced by 1/2 if the boattail is 1/2 caliber long. You don't gain much by going beyond 1 -1 1/2 cailbers in length.

    Bob Krech
     
  13. Dec 10, 2004 #13

    r1dermon

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    i figured that the supersonic optimum angle would be less. what kind of performance gains can be expected. say i take a rocket that flies straight and high to 1500 feet on a G80, at a subsonic speed, then i take the same rocket and fly it on the same motor, how MUCH of a difference will be noticed...will it be significant? or just maybe a hundred feet and a couple mph? im looking for noticeable difference.
     
  14. Dec 10, 2004 #14

    illini

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    First, I've never run across the website Bob posted before. He's right...it is an incredible resource of info presented in easy to understand terms. From what I've seen so far, this is highly recommended.

    Now, go take a look at the last slide of the powerpoint brief he linked to. Note the sizable differences in Cd's with and without boattail.
     
  15. Dec 10, 2004 #15

    DynaSoar

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    From the presentation it would appear that applies to a finless projectile, for base drag only. A longer boat tail may be better on a finned rocket, according to the "equal area" principle, would it not?
     
  16. Dec 10, 2004 #16

    JRThro

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    Wow, that's a great resource!

    On the last page of that Powerpoint presentation, it looks like a good boattail can reduce the total drag on a finless projectile by 40 - 50% at subsonic speeds. I never realized that base drag made that big a contribution to the total drag on a rocket.

    We need to bear in mind (or at least I do) that that presentation doesn't address drag from the fins. Has anyone found where he addresses that?
     
  17. Dec 10, 2004 #17

    JRThro

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    I hadn't really thought of it, but having the bottom centering ring recessed somewhat into the end of the tube definitely would increase the base drag, wouldn't it? And of course, that's what I usually do: follow the instructions and recess that ring.

    Now, since I'm an engineer, once I'm convinced that drag is [EDIT] DEcreased substantially by building my rockets that way, I'll try to optimize things at least a little and will henceforth mount the bottom centering ring flush with the end of the body tube.


    You've mentioned the "equal area" principle a couple of times. What is it?
     
  18. Dec 10, 2004 #18

    Zack Lau

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    If I understand the Powerpoint presentation, an optimum boat tail for a 0.51" diameter motor requires a main body tube diameter of 0.59 inches--the boat tail goes from 0.59 to 0.51 inches over a length of 0.26 inches. Thus, the optimum design, neglecting the differences between a finned rocket with tracking smoke and a shell, is actually a little bigger than the standard minimum diameter rocket. It might even be close to my two layer balsa technique. :cool:
     
  19. Dec 10, 2004 #19

    JRThro

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    Is this true, or do you still get less total drag with a minimum-diameter rocket? The drag is essentially proportional to the cross-sectional area presented to the air flow, so then:

    Area (0.59" BT) = 0.273 sq. in.
    Area (0.51" BT) = 0.204 sq. in.

    So the 0.59" BT rocket would have 0.273 / 0.204 = 1.34 times the drag of the 0.51" BT rocket.

    I suppose that this ignores base drag and all of the other contributors to drag, though. Or more likely, all of those other factors can be combined and give us an "effective" cross-sectional area.
     
  20. Dec 10, 2004 #20

    JRThro

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    What I've gathered from all this, and I'm veering dangerously close to the topic of techniques here, is that I should buy TWO nose cones whenever I build a non-minimum-diameter rocket, and convert the second one into a boat tail.

    That would seem to be a very simple way to get a reasonably optimum boat tail shape, without having to build one up from cardstock or something. PLUS it would avoid the dreaded sharp angle between the body tube and the boat tail.

    DynaSoar's description of how he did it is what I have in mind here.
     
  21. Dec 10, 2004 #21

    polaris

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    This may shed some light on the topic.
     
  22. Dec 10, 2004 #22

    illini

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    Not quite correct. Its not an "effective cross-sectional area" but the drag coefficient that takes these factors into account. You are right that drag is proportional to the cross-sectional area and that this is 34% higher for the .59" BT than the .51" BT. However, drag is also proportional to the drag coefficient, and if this will be be lower for the .59" BT rocket with boat tail than the .51" rocket without boat tail. The question is then, does the drag coefficient lower enough to make up for the increased area (i.e., by more than the 34% area increase)? My guess is that it does not. i.e., min diameter is still better. Polaris just posted a link that might answer that question.
     
  23. Dec 10, 2004 #23

    DynaSoar

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    That's what you're seeing when you see "wasp-waist" shaped aircraft (F5, B58, B70, F106). Also just called the "(Whitcomb) Area Rule". It mostly matters in transonic and supersonic speeds.

    Basically, when something sticks out, it makes more drag, and that drag can be reduced if something else sticks in in the same region of air flow. Where the wings start, front to back, they start adding more surface area. The body is starts being reduced there, and gets reduced until the point of the greatest surface area added by the wings, then goes back to the original size (or so) as the amount of added surface area decreases. The pressure rises around the fin, and by hollowing out the body surface enough to give it someplace to go, it doesn't build up to as much pressure and thus greater drag. It's the cross sectional area being added by the one that you want to reduce by that amount on the other, hence "equal area".

    Rockets don't often have wings or fins sticking out in the middle. Or when they do they're for control, and you don't want to rob them of their airflow by channeling it elsewhere. Rockets do have big fins on the bottom, and narrowing the tail can reduce the drag. I'm just guessing here, but I *think* it might also help keep the airflow from spilling out over the end of the fins into the tip vorticies and thereby improve the lift/control characteristics.

    Whether that last is actually the case (and if it is, significant enough to matter), as well as whether this makes a difference in the effective length of boat tails, I'll defer to someone who can be more precise about it.

    Tutorial on the subject: http://www.aerospaceweb.org/question/aerodynamics/q0104.shtml

    NACA history on its development: http://history.nasa.gov/SP-4219/Chapter5.html

    In fact, now *I'd* like to know if since this matters most to trans/supersonic flight, would high speed rockets benefit from conical rather than ogive boat tails?
     
  24. Dec 10, 2004 #24

    JRThro

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    Thanks for that correction.

    In my post above, I almost said what you just did - that I would guess that the minimum diameter rocket would have less drag than the slightly larger one with the boat tail, but that I didn't really know.

    What *really* surprised me from the link in Polaris' post was how incredibly much drag is added by the launch lug. 29% more! I am happy to see that a launch lug typically adds even more drag than not finishing a model does, since I often fly "naked" models.

    [EDIT} Of course, not only do I often fly naked models, I also use launch lugs, so I'm adding something like 50% or even more to the drag on my rockets. Sounds like I need to look into using pop lugs.
     
  25. Dec 10, 2004 #25

    illini

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    Regarding the area rule, DynaSoar's explanation is pretty good. Basically what it comes down to is the strength of the shock waves caused by the something that is sticking out. If the area suddenly changes, then you get a single strong shock at that point. If area gradually changes, then weaker shocks will result. Can't remember if it was this thread or another, but similar theme to what I posted before: multiple weak shocks - good; one strong shock - bad.

    One request for clarification from DynaSoar: When you mention the area rule in your posts above are you referring strictly to supersonic rockets, or are you thinking that there's some benefit to the area rule for subsonic rockets as well?
     
  26. Dec 10, 2004 #26

    Zack Lau

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    According to my interpretation of section 2.7 in John DeMar's paper, the boat tail (10) actually has less drag than the minimum diameter model (2). At 80m/s he measured 3.3g for the minimum diameter and 2.3g for the boat tail. At 120m/s, the difference narrowed to 10g for the minimum diameter and 7.7g for the boat tail. But, these are just a few measured numbers--hardly proof one way or another. :rolleyes:
     
  27. Dec 10, 2004 #27

    illini

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    Yes, I think you do.

    I think you've basically got it, but I'll try to repeat/reword it to make sure *I've* got what you're thinking. True that during thrust base drag will be less, but it is not clear that it will be less by the same amount for both with boat tail and without. Also, keep in mind that thrust is a relatively short period of a typical flight. During coast, the boat tail rocket will have a lower Cd, but will it have a lower overall drag that allows it to out-perform min diameter? That's open for debate, but my guess is min diameter is better.
     
  28. Dec 10, 2004 #28

    illini

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    One thing I just realized...model 2 in the DeMar paper is not a min diameter model - or at least, it is not the min diameter of the boat tail. It is 0.75". The boat tail model tapers from there to 0.55". So, while the drag coefficient numbers might be interesting (he shows 20% improvement with boat tail), the drag force numbers themselves don't allow us to draw any conclusions regarding whether increasing diameter in order to get a lower Cd with a boat tail would have any benefit. I still think min diameter is better.
     
  29. Dec 10, 2004 #29

    powderburner

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    Launch lugs are very draggy, there is almost no way to get around it. You can cut the front at a swept angle, you can mount your LL in a fin root, you can even do a 'disappearing' trick (I saw a design once that had a little flap of paper that was supposed to cover the LL after it cleared the rod) but it WILL cause drag, often as much as the entire rest of the rocket. That is why you see the serious competitors going to such trouble to eliminate them, using towers, pistons (with guides), pop-lugs, etc.

    Keep in mind: for sport models, the drag of a launch lug doesn't matter as much, especially considering the value of being able to use a simple launch rod or rail.
     
  30. Dec 10, 2004 #30

    illini

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    That's true, and in practice I would expect the boat tail rocket to weigh more due to larger main BT, nose cone, and added boat tail. So you'd actually be *adding* weight to the min diameter model just to make the comparison fair.
     

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