Question about maximum liftoff weight

Discussion in 'Low Power Rocketry (LPR)' started by Bill S, Sep 13, 2019.

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  1. Sep 13, 2019 #1

    Bill S

    Bill S

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    I've built an Estes Interceptor, and noticed that the nominal weight of the rocket is 3.9oz. That doesn't include the weight of the engine. Both the B6-4 and C6-5 list maximum liftoff weight of 4oz. So it seems to me that the rocket with an engine in it is already close to 5 oz, maybe more if one went too heavy on the paintjob. I THINK mine is 5 oz, perhaps a tad over, but my scale isn't that accurate - I need a proper digital scale, not the old scale I have.

    Yet I don't see anyone complaining about their Interceptors flying strangely or crashing, so I'm wondering if there is more to it than that.

    I do also have some Quest Q-Jets (D16-6) to try out, and their max liftoff weight is 3.49 oz. I had hoped to use them in this rocket

    Thoughts?
     
  2. Sep 13, 2019 #2

    Nytrunner

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    Max liftoff weight listed on motors is a funny metric that I find dubious. A skinny rocket and a fat draggy rocket of the same weight will behave differently.

    Long launch rod and low to no wind are your friend here
     
  3. Sep 14, 2019 #3

    BABAR

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    I looked up max lift off weight on the quest site and you are right, that’s what they say. Seems kind of low for D MOTOR.
    https://www.questaerospace.com/2-Pack_D16-6_Q-Jet_Model_Rocket_Motor/p4193205_18980172.aspx
    99 grams


    Estes D12-5 is 10 oz

    https://estesrockets.com/wp-content/uploads/Educator/Estes_Engine_Chart.pdf
     
  4. Sep 14, 2019 #4

    neil_w

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    The D16-4 is max liftoff weight of 128g, which would put it very close to OK for the Interceptor. Delay on the D16-6 is probably too long. Thrustcurve.org has the rocket at 45 ft/s off a 3 foot rod, which fails their criteria but I would fly without worry (with 4 second delay).

    I agree with Nytrunner that max liftoff weight is a very odd spec. I'm never quite sure what to do with it myself.

    In any case, you're usually OK with Estes' recommended engines, and the Interceptor has a long history of flying well. It is probably a slow liftoff with a C6.
     
  5. Sep 14, 2019 #5

    Nytrunner

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    Look at the first 1/4 second of the thrust curve, and your rod exit speed.
     
  6. Sep 14, 2019 #6

    Matt_The_RocketMan

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    What would be the lightest rocket you can use a 29mm engine? I am curious as to what would happen....
     
  7. Sep 14, 2019 #7

    neil_w

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    You could build a minimum diameter 29mm rocket *very* light (I dunno exactly, but I figure well south of 5 oz, using carbon fiber). Requires careful construction to avoid shredding, and a tracker if you have any desire to get it back.
     
  8. Sep 14, 2019 #8

    Bill S

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    As I don't have a Rocksim file for the Interceptor (and one would be hard to make due to the wing pods, etc), I don't know how I would determine the rod exit speed.

    I just checked the package for the Q-Jet D16-6s I have, and the chart on the back of the package says D16-4 max liftoff is 340g, the D16-6 is 227, and the D16-8 is 170. Versus their website, which says 128g for the D16-4, 99g for the D16-6, and 71 for the D16-8.

    I wonder why the major discrepancy. Strange...
     
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  9. Sep 14, 2019 #9

    Evolvant

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    There is a general rule of thumb if you don't have it modeled. You want 5x the thrust to weight ratio to be "safe". Motor advertise average thrust in newtons. A C6-5 is 4.7 average newtons. http://www.thrustcurve.org/motorsearch.jsp?id=21. To convert newtons to pounds (lbs) you divide by 4.448 (4.45 is what I do in the field). The 4.7 average thrust is then 1.05 pounds of thrust. Divide this number by 5. 16oz/5 = 3.2oz lift off weight. 3.2oz = about 91 grams. Note, the weight you're looking for in the rocket is TOTAL WEIGHT INCLUDING MOTOR. So, for a C6 you don't want it to launch more than 3.2oz including the motor. For your 3.9oz rocket, without motor, a C6 wouldn't be enough. Again, modeling is better than these "field estimates". Ultimately it's better to model and just insure safe speed off the rod.
     
  10. Sep 14, 2019 #10

    georgegassaway

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    The D16 is a WEAK D motor. 12.4 N-sec. Sorta like a C6 and and two 1/2A3's clustered.

    D12 has about 50% more N-sec.

    Also for heaven's sake do NOT use a SIX second delay with an Interceptor!!!

    That being said, a D16-4 probably would be good for a bit heavy Interceptor.
     
  11. Sep 14, 2019 #11

    Nytrunner

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    Try one of Neil's OpenRocket installers and set the fins to rough to fudge the fin pods (simulations are just guesses anyway, but they're better than shooting in the dark)

    Also Thrustcurve,org

    Enter your weight and boy diameter, set to complex type rocket (that'll increase the Cd of the batch sim)

    Thrustcurve will warn you if its under 50 ft/s on rod exit, but you can fly at 40 ft/s in calm/no wind
     
  12. Sep 14, 2019 #12

    Bill S

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    I've asked here, and asked the seller of the engines (ASP Rocketry), and the concensus was that a six second delay for the D16 probably would be okay. The Interceptor flies normally on a C6-5 (five second delay), and the D16 ought to get it up there substantially higher, so why wouldn't a six second delay be workable? It would be REALLY nice if Quest would give some guidance on appropriate motors for common models, or at least some guidelines.

    I just sent an email to Quest asking for guidance. I normally wouldn't do that, but in this case...

    The research I did online suggests a 4xwindspeed velocity at the end of the launch rod at a minimum, so in THEORY as long as I launch only when the wind is 10mph or under, it should be okay. Yes, there will be unexpected windgusts, etc, but due to the launch field I have available, I tend to be conservative and only launch when the wind is low. If its too windy, I likely will just not launch the Interceptor.
     
    Last edited: Sep 14, 2019
  13. Sep 14, 2019 #13

    BABAR

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    I wish I was as wise as you! Since I don’t get out that often to launch, I have fallen to the temptation to risk wind conditions which in hindsight were foolish.
     
  14. Sep 14, 2019 #14

    BRS Hobbies

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    On the front of each Q-Jet motor package they do compare it to an Estes engine. The D16-6 is compared to a D12-5 and I would also use it in place of a C6-5. The C12-6 is compared to the Estes C6-5. The max lift off weight on the package seems more reasonable given that it's a D16 motor.

    I'm still not sure what the deal is with maximum lift off weight. I have seen these discrepancies before and the rocket flies fine. I'm guessing these are just general guidelines with a small amount of leeway.
     
    Last edited: Sep 14, 2019
  15. Sep 14, 2019 #15

    Bill S

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    I joined a rocket club that has monthly launches, but its an hour drive away. So far, I haven't given in to the temptation to launch in higher winds, though I have downgraded my intended engine choice (example: At the start of the session, winds were pretty light, so I planned to use an C6-5 in a rocket that should go about 900 feet, launched it, and it drifted a reasonable distance, then the wind starts to pick up, and I say to myself "I'm not doing that, down grade to a B6-4".). :) I don't mind the monetary cost of replacing a new rocket, for me, its the wasted assembly and painting time - even if I enjoy doing those things, its not efficient to needlessly risk the rocket.
     
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  16. Sep 14, 2019 #16

    Alan15578

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    Ya know, as experienced contest rocketeers, etc., we don't need no stinkin MFGR recommendations or limitations. OTOH, if you are a BTC at a contest and the wind is 19 MPH and they will not cancel, postpone, or delay the contest, you are compelled to fly and make other adjustments. Its fly or die, maybe both. Downgrading a heavy PMC model from an E45 to a D12...well, it deployed just before it rekitted itself on a parking lot.
     
  17. Sep 14, 2019 #17

    Bill S

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    :)

    Fortunately I am just a casual flier, so I don't have to worry about that. But I see your point. :)
     
  18. Sep 15, 2019 #18

    georgegassaway

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    BTW - unless it's a really windy day, a C6-3 would probably be fine for a heavy-ish Interceptor.
     
  19. Sep 15, 2019 #19

    BEC

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    The only reason I’d not go Q-Jet D16-6 on an Interceptor that is OK with an Estes C6-5 is that the Q-Jet’s thrust starts immediately and increases as it burns. Since drag goes up with the square of velocity, the drag on a D16-powered model goes up in a hurry. I can’t imagine it really coasting another second over a C6 for that reason, even with the extra 3.6 N-s over an Estes C6.

    Having flown a bunch of Q-Jets by now, usually with an altimeter in a model, I can say that Q-Jet Ds don’t take a model as much higher as you’d expect with its 2.6 extra N-s over a Q-Jet C.

    That said, a D12-4 will probably be a bit short unless C6-3 is the right Estes motor for your particular model. The much discussed Estes RTF 1/200 Saturn V does well on C12-4 and D12-4 Q-Jets. The -6 would be too long. I don’t have an Interceptor so I can’t speak from direct experience there.
     
  20. Sep 15, 2019 #20

    Bill S

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    I finally went to the trouble to work up a Rocksim file for the Interceptor, borrowing an Interceptor-E file I found online, and modifying it. Wasn't that bad to do the wings/pods as I thought, once I figured out how to do it. I was not able to weigh each part since the rocket is assembled, and I don't have a scale that precise anyways. I don't know where Estes gets the idea the rocket will go 525 feet as claimed, but it is certainly possible there is some flaw in the design file, as I am not an Rocksim expert. :)

    Strangest part was at certain weights (mainly below the nominal 3.9oz Estes claims for the Interceptor), the rocket was overstable and in simulations it would go up and loop in place in the air, and then recover and go up. This was particularly bad with the Q-Jet D164/6. Once I entered the actual weight (accounted for paint weight) and kept the same CD it flew normally. I'm more confident it'll work okay now.

    The rocket is definitely draggy. According to Rocksim, a C6-5 should send it 380-390 feet, whereas a Q-Jet D16-6 sends it up to 685. As for delay, the D16-4 pops the parachute .4 seconds before apogee, while the rocket is moving 54.6 fps. Not great. A D16-6 pops it .7 seconds after apogee, while the rocket is moving downward at 27.8 fps.

    My hacked-up Rocksim file for anyone who wants to look it over:
     

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  21. Sep 15, 2019 #21

    mikec

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    FWIW, my stock Interceptor weighs a little over 4 oz dry with an altimeter and got to 274 feet on a C6-5 (a little weathercocking) and 840 feet on a D10-5. The C was a little late and the D was a little early, as you might expect.

    Within reason, I'd rather have a late than an early ejection. Note that delay values have a fair amount of slop.

    As to stability, if you have a built rocket you can actually measure the CG and override the predicted sim values for more accuracy.
     
    Last edited: Sep 15, 2019
  22. Sep 15, 2019 #22

    Bill S

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    I probably ought to do that as a double-check, thanks for reminding me. :)
     
  23. Sep 15, 2019 #23

    Scott_650

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    Not that we shouldn’t do our best to fly safely, not that the software analysis and simulation isn’t interesting and informative, and not that we shouldn’t take every advantage to make improvements to advance our hobby - but Estes Interceptors have been successfully flying on Estes BP C6-5s since 1971 (and before that counting research and development by Estes). That’s 48 years of launches and counting.

    Though I’ll be the first to admit, after the initial flight on a C6-5, my Interceptor will fly over a QJet D - just to be safe ;)
     
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  24. Sep 15, 2019 #24

    neil_w

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    I have to say I've had some weird experience with the QuestJet C12 and D16 in OpenRocket. Most of my rockets tend towards the very draggy, probably not too far off something like the Interceptor. Frequently, OR is showing ideal delays withe QuestJets that are not too different from the C6.

    Yesterday however I flew my IRIS-T (probably similar weight and drag to an Interceptor) on a D16-6 (after debating the delay for a while) and it used every bit of the 6 seconds of delay. Went pretty high, too, according to the altimeter in my eyeballs; quite a bit more so than I expected. So.... I dunno.

    One other thing: when I'm at a launch, it seems to me that in the vast majority of motor-eject flights (which includes pretty much all LPR), the delay is too long and the ejection occurs after the rocket has built up a good head of steam coming down. This is no doubt at least partially because so many LPR flights arc over at least somewhat, and so their coast phase reaches apogee more quickly than an "ideal" flight. That's probably why the IRIS-T used all of its delay... the flight was (surprisingly) *straight up*. Usually the chute and rocket hold together and everything is fine (and hey, it might even drift less since the chute opens at a lower altitude). But if I'm choosing between two delays, I'm inclined to err on the slightly shorter. If is pops slightly early, the rocket is decelerating, and by the time the chute actually opens the rocket might have hit apogee.

    The nice thing with a shorter delay is that you are reducing the chances that a severely weather-cocked flight will end up deploying close to the ground (I've witnessed that sort of thing plenty of times).

    Finally: my IRIS-T weighs 4.8 oz without motor, and again worked great with a D16-6 on a perfectly vertical flight. My guess is that the D16-6 would be OK for the Interceptor, but if it arcs over the deployment will be a little late. A D16-4 would probably be excellent unless the flight is perfectly vertical, in which case it might be a bit early.
     
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  25. Sep 16, 2019 #25

    Nytrunner

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    If the Iris is anything, it aint over stable. Great way to combat weathercocking
     
  26. Sep 16, 2019 #26

    neil_w

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    Ha ha, that is quite true.
     
  27. Sep 22, 2019 #27

    Bill S

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    Neil, your prediction turned out to be accurate. I got to to fly the Interceptor 3 times yesterday. Twice on a C6-5: , first launch it had a fair bit of arc, parachute got scorched a bit with some tangling. I think the nosecone was a little too tight, so I sanded the shoulder a bit. It broke off an wing pod upon landing, but it easily superglued back on. Second flight the rocket spun several times as it went up, then arced moderately, and recovery was fine. Third time I broke out the D16-6. The rocket really jumped off the launchpad, more arc than I would like to see, and the rocket definitely deployed late (I'd say 1.5 - 2 seconds after apogee), but it had a perfect deployment and recovery.

    This was with near zero wind, really nice conditions. Model amazingly weighed in at .05oz over what Estes puts on the package (3.9oz), which surprised me since I tend to go heavy with the paint.
     
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  28. Sep 22, 2019 #28

    BABAR

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    Something not commonly discussed with weather cocking, once the rocket’s trajectory is off vertical, the thrust is no long directly opposed to gravity. Therefore (I think) the rocket is likely to accelerate more (unfortunately not in the VERTICAL dimension.). So I am thinking that weather cocking birds are likely to reach a higher peak net velocity, likely to coast a greater distance between burnout and deployment, and FAR more likely to be at higher velocity (probably horizontal or arced downward) at ejection (unless the weather cocking is so severe that ejection occurs after landing, of course,)
     
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  29. Oct 20, 2019 #29

    Bill S

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    I've now flown the Interceptor 6 times now. The first launch, and the 5th and 6th, the kevlar shock-cord somehow got tangled on the wing-pod, and caused the rocket to land horizontal instead of vertical. On the 6th flight it broke one of the top fins, which I now have to fix.

    I've decided to try some C6-3 engines to see if parachute deployment is better. ALL of the flights have had a fair bit of arc to them, and I think the 5 second delay is too much for the Interceptor. I may pick up some of the Q-Jet D16-4 to try vice the D16-6s I have now.
     
  30. Nov 7, 2019 #30

    John Taylor

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    I have found with a rocket that always cocks into the wind, point the model slightly with the wind. The model should wind cock into the wind. If you get the angle right it will go straight up. Takes practice and a feel for it. Also depending how fast it's blowing.
     
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