Page 2 of 3 FirstFirst 1 2 3 LastLast
Results 31 to 60 of 77
  1. #31
    Join Date
    19th January 2009
    Location
    Stafford VA
    Posts
    6,649
    I have also never heard and explanation about why the upper button should be at the CG.
    Just off the top of the head at this late point, I would think putting the upper button at CG would put minimum stress on the upper button and maximum on the lower. If the upper button was above the CG, then I would expect the stress on the upper to be a little more, but still less then the lower button and that would be reduced.

    Handeman

    TRA #09903 L3 3/29/2015

    "If you don't use your head, you have to use your feet!" my Dad

    Tripoli Central Virginia #25 - BattlePark.org

  2. #32
    Join Date
    25th July 2012
    Location
    Sterling, Illinois
    Posts
    69
    I'm still waiting to see the link for the famous "pad hopper." Somebody has to have it. I was just at FB and there's no rocketry related Pad Hopper that an FB search to find.

    Where's my Pad Hopper video?


  3. #33
    Join Date
    7th February 2009
    Location
    Arlington, TX
    Posts
    1,173
    Quote Originally Posted by caveduck View Post
    3) There is a third torque arising from the fact that the center of thrust is offset by ~1/2 caliber from the rail guides, but there is not much we can do about that.
    There is a torque coming from the guides but it is the result of friction with the rail and being offset from the rocket CG. It has nothing to do with motor thrust. You can get a torque from thrust if it isn't aligned with the CG but the guides have nothing to do with that.

  4. #34
    Join Date
    7th February 2009
    Location
    Arlington, TX
    Posts
    1,173
    Quote Originally Posted by OverTheTop View Post
    I think that when launching, after the upper lug at the CG has disengaged, the lower lug still provides some guidance (just not quite 100% like when the both are engaged). The sideforce from any wind will be pushing at around the CG point (depends on your stability margin). This is trying to make the rocket translate sideways, but it is pinned at the rear. The rotational moment is applied to a lever that is effectively pivoting from one end. This makes the rotational inertia (second moment of area) quite high and provides just a bit more stability until the bird is off the rail.
    The most that a single rail button can do is constrain the motion of the rocket at that spot but the rocket is free to rotate around that point. Since the whole point of the guides is to keep the rocket pointed in a particular direction, this means a single button is useless.

    While the moment of inertia is increased with that aft button location, it also increases the torque (force times distance from button to CP) and the time that torque is applied. The result is that the rocket turns away from the wind which isn't what you want or expect.

  5. #35
    Join Date
    13th March 2010
    Location
    Bentonville, AR
    Posts
    649
    Maybe we should shift this discussion to the motor versus engine debate?
    Dwayne Shmel
    TRA 13137 / NAR 88172
    Tulsa Rocketry
    K.L.O.U.D. Busters
    L3

  6. #36
    Join Date
    6th September 2009
    Posts
    1,320
    Quote Originally Posted by OverTheTop View Post
    I think that when launching, after the upper lug at the CG has disengaged, the lower lug still provides some guidance (just not quite 100% like when the both are engaged). The sideforce from any wind will be pushing at around the CG point (depends on your stability margin).
    I would think the resultant wind is pushing against the CP (isn't that the definition of CP?) and putting a torque around the lower button still engaged in the rail.

    The only need for a button at the CG would be if the rocket is hanging upside down severely from an angled rail, to counter the -mg putting too much torque on the bottom button.

    Anyway, we are all over-thinking this. It probably doesn't matter. Just make sure your buttons give adequate launch guide length. Has anybody ripped off a rail button in the rail because of torque? Not likely.

  7. #37
    Join Date
    15th February 2009
    Location
    North Texas
    Posts
    383
    Quote Originally Posted by Incongruent View Post
    ....A related question, though not about rail buttons:
    If the CG was at CP90 (and front CP was behind the pair), would weathercocking be reduced? Weathercocking is a result of crosswinds, which should push at the side CP, so if the rocket pivots where the force is, rotatin should be lessened and the rocket should translate instead...
    Rockets can't rotate when both rail guides are engaged so it really doesn't matter where the upper button is in relation to CP (assuming reasonable locations). Rotation can occur only once the upper button is free of the rail. But at that point the rocket should be going fast enough that the lower button will be on the rail for such a short period of time that any rotation that occurs around it is minimal.

    The main thing is safe takeoff speed when the top button leaves the rail for the wind conditions at the time of the launch.

    It seems to me the rocket should never really be in a crosswind while on the rail - I always align things so the rocket is downwind of the rail, never at an angle to the wind. In Texas it's almost always windy and that strategy as served me well.

    And the simple solution to having a rail button that is far forward is to use a longer rail. Same as if you are using a low thrust to weight ratio. Using a 8' rail instead of 6' should solve most issues. However as rail length increases you may have to use a T-bar and guys with heavier rockets to keep the top of the rail steady. But the same is true with very high thrust motors unless you have a rock solid base.

    The biggest mistake most folks make is not measuring the distance between the fore button and the top of the rail. That's the distance that matters. Too short and you need a longer rail.


    Tony
    why do people put so much stuff in their sigs?

  8. #38
    Join Date
    19th January 2009
    Location
    Portland, OR
    Posts
    1,403
    It seems to me the rocket should never really be in a crosswind while on the rail - I always align things so the rocket is downwind of the rail, never at an angle to the wind.

    The ability to do this is very dependent on the pad design.
    If you're using something like a QuadPod with it's rotating head, it is easy to do this.
    However a lot of clubs have pads that can't spin...so you are stuck unless the pad is setup for the prevailing wind and the wind direction is constant.

    Planning on crosswinds would be the prudent thing to do.
    Having a rail button at or above the Cp would limit side-torque on the buttons while waiting for launch.
    Not the end of the world if not possible, but since the OP asked....that's what I would target as the goal in placement.

    And yes, rail-whip is real. Full-length rail support using a tower (or equivalent) will provide a more vertical flight.
    Last edited by FredA; 11th February 2017 at 09:14 PM.
    Fred Azinger

  9. #39
    Join Date
    15th February 2009
    Location
    North Texas
    Posts
    383
    Quote Originally Posted by FredA View Post
    ..If you're using something like a QuadPod with it's rotating head, it is easy to do this.
    However a lot of clubs have pads that can't spin...so you are stuck unless the pad is setup for the prevailing wind and the wind direction is constant...

    And yes, rail-whip is real. Full-length rail support using a tower (or equivalent) will provide a more vertical flight.
    Wait a minute, rails have 4 sides, so you should be able to pick the right side. Pads can be rotated, rails can be removed and rotated if a rail stop is an issue. I can't imagine why it wouldn't be possible to adjust the pad to match the prevailing winds.

    'Rail-whip' is exactly they I mentioned using a t-bar and guides. But a full length support also works well. Most of my rails are backed by a nearly full length steel pipe and it is the pipe that is retained by the pad, not the rail itself. The rail is then very secure and much more stable under load. If you look at my avatar you can see that we use guys even on rails that are backed by aluminum lighting truss, which is very rigid. The guys keep the pad itself from rocking back and forth. With O and P motors, that becomes a huge issue.


    Tony
    why do people put so much stuff in their sigs?

  10. #40
    Join Date
    19th January 2009
    Location
    Portland, OR
    Posts
    1,403
    Wait a minute, rails have 4 sides, so you should be able to pick the right side. Pads can be rotated, rails can be removed and rotated if a rail stop is an issue. I can't imagine why it wouldn't be possible to adjust the pad to match the prevailing winds.

    Clearly you've not seen many pad designs.
    Many use the back edge as part of the support....which then interferes with fins.

    Your own pad looks/sounds like it doesn't work either...can't use the backside as the steel pipe is there....and fins may not clear if you try to use the sides.....
    With the T-Bar at the top, the only the front side is available.

    So what do you do when the wind shifts beyond pick up the whole pad and rotate it then redo the stakes and guide wires? All because you didn't place your rail buttons correctly?
    Last edited by FredA; 11th February 2017 at 10:23 PM.
    Fred Azinger

  11. #41
    Join Date
    15th February 2009
    Location
    North Texas
    Posts
    383
    Quote Originally Posted by FredA View Post
    Wait a minute, rails have 4 sides, so you should be able to pick the right side. Pads can be rotated, rails can be removed and rotated if a rail stop is an issue. I can't imagine why it wouldn't be possible to adjust the pad to match the prevailing winds.

    Clearly you've not seen many pad designs.
    Many use the back edge as part of the support....which then interferes with fins.

    Your own pad looks/sounds like it doesn't work either...can't use the backside as the steel pipe is there....and fins may not clear if you try to use the sides.....
    With the T-Bar at the top, the only the front side is available.

    So what do you do when the wind shifts beyond pick up the whole pad and rotate it then redo the stakes and guide wires? All because you didn't place your rail buttons correctly?
    I've been flying for more than 15 years and have helped run many launches. I've seen a lot of pad designs. The idea that it's too much work to simply adjust the pad is crazy. The steel pipe design is super simple to rotate, you loosen the bolts that hold the pipe in place, rotate it, and then tighten. The bigger the rocket the more important the issue becomes so the more likely we are to rotate the pad. And it has nothing to do with rail button placement, it's keeping the rocket downwind of the rail to reduce crosswise stress on the buttons, end of story.

    That big pad you see in my avatar? Even with the guy ropes in place we can rotate that whole thing in less than 10 minutes if needed. Really, to argue it's too much work to move a pad is ridiculous.


    Tony

    try this simple experiment: use gravity as a substitute for wind force. Slide a rocket onto a rail and hold the rail horizontal. Which way would you rather have the rocket face? Up, to either side, or hanging down? That's the point I'm trying to make.
    Last edited by manixFan; 11th February 2017 at 11:31 PM.
    why do people put so much stuff in their sigs?

  12. #42
    Join Date
    19th January 2009
    Location
    Portland, OR
    Posts
    1,403
    I like to load on top of the rail, not hanging from it....
    Rockets we build would pull the rail button off if we tried to hang all the weight off of a few little screws.
    So for your question of rocket up or down-wind, I choose upwind.

    Not that they are the same thing/same loading....but your example, not mine.

    You can only launch in winds <20MPH.
    Not worth rotating the pad....side loading isn't that high.
    The only reason to rotate the pad is if you need to launch at an angle other than 90-degrees and your pad only leans one direction.
    Fred Azinger

  13. #43
    Join Date
    20th April 2012
    Location
    Fredericksburg VA
    Posts
    1,258
    I'm going to submit the upper rail button vs CG question to Ask-the-physicist at physicscentral.com. I'll let you know if a get an answer.
    NAR Level 2 #96210 🚀 Tripoli Central VA #17 🚀 MDRA #95

  14. #44
    Join Date
    30th January 2016
    Location
    US > OK > NE
    Posts
    1,998
    Please do! It'll save me having to build a windbox to set up with my leafblower. :-)

  15. #45
    Join Date
    15th February 2009
    Location
    North Texas
    Posts
    383
    Quote Originally Posted by FredA View Post
    I like to load on top of the rail, not hanging from it....
    Rockets we build would pull the rail button off if we tried to hang all the weight off of a few little screws.
    So for your question of rocket up or down-wind, I choose upwind.

    Not that they are the same thing/same loading....but your example, not mine.

    You can only launch in winds <20MPH.
    Not worth rotating the pad....side loading isn't that high.
    The only reason to rotate the pad is if you need to launch at an angle other than 90-degrees and your pad only leans one direction.
    For larger rockets you really should be using a tower, not guides of any kind. That's what we do with our 100 lb+ P motor rockets. But even large rockets can use rail buttons if properly backed up. I've seen huge rockets at BALLS hanging from 2 buttons with no issues. The shear strength of a single stainless steel 1/4" screw (which fit the 1515 guides) is well north of 3000 pounds. So it's unlikely you'd shear the screws unless you are doing it wrong. And if you load from the top of the rail and the rocket doesn't stay centered over the rail that will create issues, including bending the rail guide. But if it's hanging from the rail it can't go sideways. Gravity makes certain of that. I've loaded big rockets both ways and loading with the rocket hanging is far easier and less likely to go badly.

    There's no winning this argument. But to me using common sense says a rocket downwind of the rail is less likely to experience issues than one placed elsewhere. Why do you choose upwind? What benefit do you see from that vs. downwind?

    I agree that my example isn't a perfect analogy but it's not irrelevant either. Lets try a different one. Put a rocket on a rail vertically and get a big fan that can produce a 20 mph blast. (Like a those big warehouse cooling fans.) Turn the rocket on the rail so it faces all four directions - towards the fan, either side, or away from the fan. Which direction is less likely to cause interference between the rocket, rail buttons, and rail? How many reading this thread would say that the rocket upwind of the rail is the best position?

    I just try and do what seems to have worked out over time. Pretty much everything I do is based on empirical evidence and personal experience and observations. Plus I have many hours of launch video that I have analyzed over the years. But, that's just what works for me, as always, YMMV.


    Tony
    Last edited by manixFan; 12th February 2017 at 01:12 AM.
    why do people put so much stuff in their sigs?

  16. #46
    Join Date
    15th February 2009
    Location
    North Texas
    Posts
    383
    Quote Originally Posted by Nathan View Post
    I'm going to submit the upper rail button vs CG question to Ask-the-physicist at physicscentral.com. I'll let you know if a get an answer.
    Here's a link to how much a rocket can rotate due to wind while on the rail:

    http://tripolinorthtexas.org/how-muc...e-due-to-wind/

    It would be interesting to have it reviewed for accuracy. The reality is that the rocket is on the rail for such a short period of time once the front button is free that it rotates very little. Dave argued that the angular momentum that the small amount of rotation built up was still significant, but I have no way of figuring that out. I know that Tim Van Milligan wrote several articles about it - he may be able to shed some light on it.

    In the end I don't think it really matters much as long as the rocket doesn't bind and it's going fast enough once the top button is free.


    Tony
    why do people put so much stuff in their sigs?

  17. #47
    Join Date
    19th January 2009
    Location
    Portland, OR
    Posts
    1,403
    Talk about making a mountain out of a molehill....
    Give it a rest....
    Fred Azinger

  18. #48
    Join Date
    18th January 2009
    Location
    Urff
    Posts
    6,615
    I think two buttons is the easiest method. From math: Two points define a straight line.

    Three buttons can be done but require more care in drilling the holes. Misaligned holes can be corrected but that's extra work.
    Cranky Kong, Certified Rocket Monkey
    "We were promised jet packs!"

    Total Impulse in 2016: 1,318.99 Ns (Equivalent to a 3% K motor)
    Total Impulse since 2010: 12,338.57 Ns (Equivalent to a 20% N motor)

  19. #49
    Join Date
    13th March 2010
    Location
    Bentonville, AR
    Posts
    649
    Quote Originally Posted by FredA View Post
    Talk about making a mountain out of a molehill....
    Give it a rest....
    Like I said earlier, engines or motors?
    Dwayne Shmel
    TRA 13137 / NAR 88172
    Tulsa Rocketry
    K.L.O.U.D. Busters
    L3

  20. #50
    Join Date
    6th September 2009
    Posts
    1,320
    Quote Originally Posted by FredA View Post
    Talk about making a mountain out of a molehill....
    Give it a rest....
    Nah, keep it going. This is a good thread. I like any discussion that blows up "rules of thumb" with real science and data.

    Oh, and every club I have visited (about 10) has rails that can be rotated, angled, removed, and adjusted.

  21. #51
    Join Date
    19th January 2009
    Location
    Portland, OR
    Posts
    1,403
    2:1 epoxy is better than 3:1 but only on days when the wind swirls.
    Fred Azinger

  22. #52
    Join Date
    25th October 2016
    Location
    Texas, United States
    Posts
    1,318
    Quote Originally Posted by dshmel View Post
    Like I said earlier, engines or motors?
    RASP files are (.eng). There is no format that is (.mot) or (.mtr) or something.

    *rubs hands together* *glances at popcorn*

  23. #53
    Join Date
    25th October 2016
    Location
    Texas, United States
    Posts
    1,318
    Quote Originally Posted by FredA View Post
    2:1 epoxy is better than 3:1 but only on days when the wind swirls.
    The wind swirls every day. Unless you mean wind over the epoxy?

  24. #54
    Join Date
    19th January 2009
    Location
    Portland, OR
    Posts
    1,403
    The wind swirls every day.

    Yep - that's why it is silly to think of pulling up stakes and rotating pads to chase the wind....
    Fred Azinger

  25. #55
    Join Date
    25th October 2016
    Location
    Texas, United States
    Posts
    1,318
    Quote Originally Posted by FredA View Post
    The wind swirls every day.

    Yep - that's why it is silly to think of pulling up stakes and rotating pads to chase the wind....
    Ideally, wouldn't no wind be optimal for launching? Why would you chase it?

    By "the wind swirls every day", I was referring to convection currents in the atmosphere. Not a personal tornado on your epoxy/launch site.

  26. #56
    Join Date
    19th January 2009
    Location
    Portland, OR
    Posts
    1,403
    Ideally, wouldn't no wind be optimal for launching? Why would you chase it?

    Of course we all want zero-wind for a launch.
    But if you read the thread, people want to reposition the pads to keep the rocket down-wind while it sits on the rail.
    Fred Azinger

  27. #57
    Join Date
    25th October 2016
    Location
    Texas, United States
    Posts
    1,318
    Quote Originally Posted by FredA View Post
    But if you read the thread, people want to reposition the pads to keep the rocket down-wind while it sits on the rail.
    Yes, I understand that people want to have the rocket downwind. The thing is, usually, wind isn't swirling around the launch pad, it's in one general direction.

  28. #58
    Join Date
    10th July 2007
    Location
    Melbourne, Australia
    Posts
    474
    Talk about making a mountain out of a molehill....
    Give it a rest....
    As tempted as I am to say something similar, threads like this can provide enlightenment and things I had not previously thought about. As such, I'll keep listening...
    TRA 13430, Level 3

    "Everybody's simulation model is guilty until proven innocent" (Thomas H. Lawrence 1994)

  29. #59
    Join Date
    15th February 2009
    Location
    North Texas
    Posts
    383
    Quote Originally Posted by Buckeye View Post
    Nah, keep it going. This is a good thread. I like any discussion that blows up "rules of thumb" with real science and data.

    Oh, and every club I have visited (about 10) has rails that can be rotated, angled, removed, and adjusted.
    As I said, there is no winning this argument. Fred thinks it's silly to try and position the rocket in such a fashion that I feel minimizes interference from the wind. I tried to present a reasoned approach to my thoughts, nothing more. Not really a mountain but not flippant either.

    My questions to Fred in post #45 are still open. But everyone that reads this thread can come to their own conclusions. But I agree, time to move along...


    Tony
    why do people put so much stuff in their sigs?

  30. #60
    Join Date
    19th January 2009
    Location
    Portland, OR
    Posts
    1,403
    My questions to Fred in post #45 are still open. But everyone that reads this thread can come to their own conclusions. But I agree, time to move along...

    I guess I can't read but the only question I see is "why put your rocket upwind" which I didn't think was really a question.

    My response is I like to put the rocket on the front of the pad facing launch control and the spectators....period.
    If that's upwind, downwind or sideways I don't care.
    I want to see and photograph the rocket taking off.

    Your rocket should be strong enough to sit there in any wind and not excessively torque the railbuttons.
    If you need to sit out a strong gust and wait for sub-20MPH winds then it needs to survive.

    Is that a sufficient answer????

    Fred Azinger

Similar Threads

  1. Rail button spacing and rail length Qs
    By BeerorKid in forum The Watering Hole
    Replies: 13
    Last Post: 1st June 2013, 08:44 AM
  2. Rail Button Question
    By Ford Prefect in forum High Power Rocketry (HPR)
    Replies: 6
    Last Post: 29th May 2010, 05:33 AM
  3. Big Rocket Rail Button spacing
    By droopy in forum High Power Rocketry (HPR)
    Replies: 6
    Last Post: 20th April 2008, 05:54 AM
  4. Rail Button spacing on 4" rocket
    By TeenRocketNerd in forum Scratch Built
    Replies: 3
    Last Post: 27th August 2007, 04:28 PM
  5. rail button question
    By irishwebber01 in forum High Power Rocketry (HPR)
    Replies: 10
    Last Post: 6th April 2007, 11:26 PM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •