Rail Exit Speed, how slow is too slow?

Discussion in 'Ground Support' started by shoebox9, Mar 25, 2020 at 10:53 AM.

Help Support The Rocketry Forum by donating:

Tags:
  1. Mar 25, 2020 at 10:53 AM #1

    shoebox9

    shoebox9

    shoebox9

    Member

    Joined:
    Mar 25, 2013
    Messages:
    17
    Likes Received:
    0
    Hi Folks,

    I'm simming a collection of mid power rockets of interest, and while OR hasn't given me any warnings yet, how slow is too slow when looking at velocity off the rod?

    5 m/s? 6 m/s? etc.

    Thanks!
     
  2. Mar 25, 2020 at 11:15 AM #2

    timbucktoo

    timbucktoo

    timbucktoo

    Well-Known Member Staff Member Global Mod

    Joined:
    Jun 13, 2014
    Messages:
    6,727
    Likes Received:
    736
    Gender:
    Male
    Location:
    Cocoa Beach
    Me I keep all mine at 50 FPS or better, especially if you have any wind.
     
    o1d_dude likes this.
  3. Mar 25, 2020 at 11:21 AM #3

    shoebox9

    shoebox9

    shoebox9

    Member

    Joined:
    Mar 25, 2013
    Messages:
    17
    Likes Received:
    0
    Woha, that's fast. Really?

    Is this a mid/high power thing, cause low power is often only 2/3rds that speed, some even less.
     
  4. Mar 25, 2020 at 11:31 AM #4

    ThirstyBarbarian

    ThirstyBarbarian

    ThirstyBarbarian

    Well-Known Member

    Joined:
    Feb 11, 2013
    Messages:
    8,031
    Likes Received:
    465
    I usually aim for 50 ft/s or more, so about 15 m/s or more. I don’t start getting concerned until it’s below 45 ft/s.

    A lot depends on conditions. If it is windy, or the rocket is over-stable, then higher speed is more important to avoid weathercocking into the wind.

    If the speed is too low, you can use a faster motor, or you can increase the length of the launch guide. You mentioned speed off the rod. Midpower is often where people start putting rail buttons on rockets and launching off a rail instead of a rod. If you increase the speed of the rocket with a faster motor, or you make the rod too long, you increase the chances of rod whip sending your rocket off course. Rails don’t flex as much as rods, so rod whip is not much of a problem. Rails can be much longer than rods.

    I like slow flights, so I often look for extra long rails at launches — 8, 10, or even 12 feet. The rocket might sim only 35 ft/s off a 6 foot rail with the motor I want to use, which is dangerously slow, but it’s plenty fast off a 10 foot rail. I know someone with a 12-foot rail I’ve been able to borrow for extra slow flights.
     
    timbucktoo likes this.
  5. Mar 25, 2020 at 11:55 AM #5

    shoebox9

    shoebox9

    shoebox9

    Member

    Joined:
    Mar 25, 2013
    Messages:
    17
    Likes Received:
    0
    Thanks.

    I'm simming for F15 motors, as I like a long burn I can see. By switching to a 12' rail I'm getting 40f/s. (500g, 2.6" rocket.)
    What wind speed is that fine below?

    I'm surprised there isn't more readily available data around for this. Wind speed meters (anemometers) are quite cheap, it would make sense to me to be able to know your preferred combo is good up to X wind speed, rather than an all or nothing approach.
     
  6. Mar 25, 2020 at 12:00 PM #6

    timbucktoo

    timbucktoo

    timbucktoo

    Well-Known Member Staff Member Global Mod

    Joined:
    Jun 13, 2014
    Messages:
    6,727
    Likes Received:
    736
    Gender:
    Male
    Location:
    Cocoa Beach
    These are really rules of thumb. Some people like 45 fps but with any wind, forget it. ThrustCurve uses 45 fps as a guideline. Try it.
    Here is a link. Pretty simple.
    http://www.thrustcurve.org/guidepage.jsp
     
  7. Mar 25, 2020 at 12:15 PM #7

    shoebox9

    shoebox9

    shoebox9

    Member

    Joined:
    Mar 25, 2013
    Messages:
    17
    Likes Received:
    0
    I found this article. Has a larger list (bottom of the page) been calculated beyond Estes LP motors?

    Something in the article I hadn't thought of was that in addition to moving your CG forward, adding nose weight increases your polar moment of inertia (resistance to tumbling).

    The further weight is from your CG (center of rotation) the more effort that is required to move it. There's a UTube vid with a professor rolling various objects down a slope, and regardless of weight or size, all roll with virtually identical speed. In contrast a hollow metal tube is more resistant to rolling because the weight is distributed further from the center of rotation (CG). Until attempting to read the linked article, I'd never considered this applied to rocket nose weight.
     
    Last edited: Mar 25, 2020 at 12:42 PM
  8. Mar 25, 2020 at 12:34 PM #8

    ThirstyBarbarian

    ThirstyBarbarian

    ThirstyBarbarian

    Well-Known Member

    Joined:
    Feb 11, 2013
    Messages:
    8,031
    Likes Received:
    465
    I think you might need a bigger motor. Try an Aerotech F20-4 in your sim. I have a comparable size rocket, and I like that motor in it. That’s a single-use motor with a 2.5 second burn you might enjoy. That one comes in 2-packs and ships without hazmat. It’s part of the Aerotech Econojet line and has a white propellant. Probably any of the Econojet motors would work— F23 black smoke, F27 red flame, F42 fast but no smoke.

    An F25-4 has a 3.1 second burn and would probably work well too for a higher flight. That’s also single-use, but not Econojet.

    Here’s a great tool for screening motors. http://www.thrustcurve.org/index.shtml

    C
    lick on motor guide. Put in the parameters of your rocket, and pick a motor manufacturer and type (like single use), and it will pop up a list of suggested motors that fit in the rocket and will give a 15 m/s launch speed using some default assumptions. It does a rough sim of each motor too. That will give you a good starting point for picking motors.

    I know what you are looking for with the F15, but I think it is underpowered for that size rocket. You might get a very nice slow flight, but there is a significant risk the rocket will go sideways in a long arc, and the chute will deploy with the rocket going extremely fast. It might hit the ground before the ejection charge fires.
     
  9. Mar 25, 2020 at 1:22 PM #9

    shoebox9

    shoebox9

    shoebox9

    Member

    Joined:
    Mar 25, 2013
    Messages:
    17
    Likes Received:
    0
    Thanks Thirsty.

    The reason for favoring the F15 is I live down under, where virtually no composite 29mm motors are available (for years) and there's no guarantee of future availability. So, sticking to pre-L1 I have to pick rockets that fit F15, G74, or G80. There is an F32 (1.6 sec burn) but @US$28 it's virtually the same price as the G80 here.

    So, the only slow burn option is the F15, unless I move up to 38mm rockets. In my state (which is the size of Alaska & Texas combined) there are only 2 MP/HP launch sites, so, with farming country all around me it's a limit of <L1 for me.

    We have an awesome event every 3 years that a special shipment of whatever-motors-you-like is ordered for (9m's ahead), but other than that, it's a tough place to be into lower mid powered rockets. Sorry for the whine, I'm sure there are worse places!

    I'll update my spreadsheet. Thanks.
     
  10. Mar 25, 2020 at 2:21 PM #10

    ThirstyBarbarian

    ThirstyBarbarian

    ThirstyBarbarian

    Well-Known Member

    Joined:
    Feb 11, 2013
    Messages:
    8,031
    Likes Received:
    465
    That makes sense.

    Can you fly clusters? In the US we have limits on the total grams of propellant above which it is considered a high-power flight. I think two F15 motors exceeds the limit here, but I’m sure rules vary around the world. Or you could try some smaller high-thrust motors in a cluster combined with the longer burning F15 as a sustainer?

    Another option for larger sized rockets that fly slowly on low-thrust, long-burn motors is to use ultra-lightweight materials. I’ve built a couple. The most recent was made from rigid foam insulation material. It was 48” tall and 7” in diameter and weighed less than 1500 grams, including the motor. That’s the one I needed the 12’ rail for. 1500 grams is the total loaded weight limit, and above that it requires a high-power launch with a waiver. I built that one specifically to fly the largest physical size rocket possible at a non-HPR launch. It flew great on most G motors, including long-burn motors like CTI G54. Majestic slow boosts to around 500 to 700 feet. Very fun! You could probably make a 4” diameter foam rocket that would fly on an F15.

    And it is possible you could get a successful flight with your existing 500 gram rocket on an F15, but it would break a few rules of thumb and would be considered a bit risky. Use the extra-long rail. Aim it perfectly straight up. And only try it in zero wind.
     
  11. Mar 25, 2020 at 2:51 PM #11

    ThirstyBarbarian

    ThirstyBarbarian

    ThirstyBarbarian

    Well-Known Member

    Joined:
    Feb 11, 2013
    Messages:
    8,031
    Likes Received:
    465
    According to the Estes description of their F15-6 motor, they say the maximum recommended liftoff weight is 17oz, or 482 grams. That should include the weight of the motor too. The motor is 3.66 ounces, so the rocket should probably be closer to 13oz. I think they are probably using a more generous thrust to weight ratio than a more cautious person would recommend. So most likely you should be looking for rockets that are around 11 or 12 ounces, so maybe 300 to 350 grams. Maybe a bit more under great conditions with a long rail.

    Here’s the motor page. Check the technical description. https://estesrockets.com/product/001652-f15-6-engines-29-mm/
     
  12. Mar 25, 2020 at 10:21 PM #12

    Stefan2k4

    Stefan2k4

    Stefan2k4

    Member

    Joined:
    Mar 31, 2019
    Messages:
    17
    Likes Received:
    5
    Gender:
    Male
    How about piston launching? The F15 is a black powder motor right? Pistons give a rocket a little extra push at the very beginning by extracting energy from what would be otherwise wasted heat in the exhaust. This gets the rocket up to speed quicker. Of course tip off can be an issue if the rocket separates from the piston prematurely and before the piston is fully extended.
     
  13. Mar 25, 2020 at 10:25 PM #13

    shoebox9

    shoebox9

    shoebox9

    Member

    Joined:
    Mar 25, 2013
    Messages:
    17
    Likes Received:
    0
    Hi Thirsty,

    Thanks again. I like clusters and it hasn't occurred to me a couple of 18mm's may also fit in the BT. I'll check this out.

    Also, one day I'll make a large <1500g foamie, I have been watching these build threads with a future quiet winter in mind. Meanwhile, I appreciate your suggestions. I see a 12' rail in my future.
     
  14. Mar 25, 2020 at 10:29 PM #14

    shoebox9

    shoebox9

    shoebox9

    Member

    Joined:
    Mar 25, 2013
    Messages:
    17
    Likes Received:
    0
    Hi Stefan,

    Interesting idea. Is there a formula, or even just a rule of thumb related to how much extra speed this gives at the end of the launch rod?
     
  15. Mar 25, 2020 at 10:35 PM #15

    Voyager1

    Voyager1

    Voyager1

    Forum Supporter TRF Supporter

    Joined:
    Oct 23, 2015
    Messages:
    408
    Likes Received:
    105
    Location:
    Oz
    15 m/s or more generally, particularly if there is wind. I never go below 10 m/s.
     
  16. Mar 25, 2020 at 10:49 PM #16

    shoebox9

    shoebox9

    shoebox9

    Member

    Joined:
    Mar 25, 2013
    Messages:
    17
    Likes Received:
    0
    But you would be comfortable launching at 10m/s in no wind?

    I've done this very often with long BT60-70 rockets without ever an issue, so the 15m/s goal surprised me.

    I guess a difference with park/farm flying is you only head out when the wind is low, rather than needing to build for flight days, come what may.

    Also, now that I think about it, our long LP scratch builds with their acrylic payload bays and heavish payloads would have a much greater polar moment of inertia than typical kits, I just never considered it.

    The family fav is a 4 cluster BT70 rocket that showers out Easter eggs with streamers at ejection. Depending on the amount of chocolate loaded in, it could be under 10m/s.
     
    Last edited: Mar 25, 2020 at 11:30 PM
  17. Mar 25, 2020 at 11:30 PM #17

    Voyager1

    Voyager1

    Voyager1

    Forum Supporter TRF Supporter

    Joined:
    Oct 23, 2015
    Messages:
    408
    Likes Received:
    105
    Location:
    Oz
    If I was launching a small low power rocket, then maybe I would be comfortable at 10 m/s, or lower. It's a very long time since I launched low power though. However, since I mostly launch mid and high power rockets, I prefer 15 m/s as a minimum.
     
    timbucktoo likes this.
  18. Mar 26, 2020 at 12:33 AM #18

    bill_s

    bill_s

    bill_s

    Well-Known Member

    Joined:
    Jul 29, 2012
    Messages:
    913
    Likes Received:
    6
    The Estes 2.5" PS II kits after dropping composite motors called for F15-4 only. I've flown F15-6's in rockets up to a pound unloaded and the ejection is very late and I finally broke a shock cord. Unfortunately the only F15-4's I've acquired were from Estes Warranty service. Fun motor with lots of smoke etc. even if altitude is low. The only way a single F15-6 would work for 500g is if it was all mass no drag.

    As to exit speed, when I started many said 30 fps off a rod, later 30 MPH (faster), I've definitely done 30 fps in substantial wind without problems. Some of my designs I consider low speed specialists. The only cruise missile flight was an E9 in an Estes Tomahawk that flew many times on the same motors.
     
  19. Mar 26, 2020 at 1:00 AM #19

    tfish

    tfish

    tfish

    Forum Supporter TRF Supporter

    Joined:
    Dec 8, 2009
    Messages:
    2,450
    Likes Received:
    400
    Estes max lift off.jpg

    Tony
     

Share This Page

Group Builder