So we are preparing for a testflight, but due to regulations we are not allowed to fly above 5k ft. The M2400 needs 3 grains fully loaded, but we will overshoot quite significantly, so I thought maybe just remove 1 of the grains and insert a placeholder (a metal cylinder or smth, the change in stability due to the shift of CG is not concerning). Has anyone done something similar? Does it even make sense to do so?
Flying AeroTech M2400 with only 2 grains? (It normally has 3)
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It would definitely be a research motor.
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Why waste the M2400
A successful motor must have the proper ratio of burning surface area to nozzle throat area (Kn). Too low and the motor will 'chuff' or not ignite at all. Too high and it will overpressurize and (usually) eject the nozzle or the forward closure.
There is some leeway in the value of Kn...but removing one grain in a three-grain motor will reduce Kn by a third; that is far too low. In a research motor the solution would be to make the nozzle throat smaller to compensate...but you can't decrease the diameter of a too-large nozzle throat.
There is some leeway in the value of Kn...but removing one grain in a three-grain motor will reduce Kn by a third; that is far too low. In a research motor the solution would be to make the nozzle throat smaller to compensate...but you can't decrease the diameter of a too-large nozzle throat.
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Adding weight to stay low results in needing more chute.
Adding drag to stay low usually requires a test flight or three.
Adding drag to stay low usually requires a test flight or three.
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Nozzle won't be correct.
Might not reach stable operating pressure.
Might not reach stable operating pressure.
DAllen
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As prfesser says your nozzle diameter will almost certainly not be correct for the amount of propellant being burned. It might work or maybe not. When you're spending that much on propellant why take the risk and just buy a smaller motor with the right thrust/weight you're looking for and do that instead? Besides what were you planning on doing with that leftover grain anyways if you do go through with this?
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You could just purchase a L1500 (2 grain reload) and save the M2400. IF you INSISTE on using two of the three grains , you must go thru RCS and get the correct nozzle. As far as a metal cylinder to take up the space of the missing grain, that is not a huge issue, you can leave the area empty.
Whether fast- or slow-burning, every propellant has a range of values of Kn within which it will ignite and sustain combustion. That range for Blue Thunder has lower values than White Lightning or Black Jack, but it's still a range. Drop below the low end and you're gonna have a bad time...
As you say, it might work...but expending an M motor for a test would be pretty expensive.
I completely forgot about the nozzle throat. I dont think we have time to play with it, so I guess just more weight should do the trick.
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Do your sims confirm that you are already at or above optimal mass?
If not, adding more weight will actually take you higher.
Do you have a larger chute handy?
If not, adding more weight virtually guarantees you'll take damage on landing.
Have you posted anything about your design here? There's a few fairly straightforward ways to add substantial drag.
Drag is much better than weight as an altitude limiter. Once you start adding weight you might increase the size of chute needed to bring it down safely. You definitely change the dynamics of drag separation and risk of damage.
Why not just pick a more appropriate motor?
Our goal is to fly the rocket the same as it will be at SAC, just with a smaller engine that we happen to have. Buying another just for the test in not an option. Changing nosecone/fins/boattail to add drag also doesn't work for us. It seems adding more weight will do the job actually.
Our goal is to fly the rocket the same as it will be at SAC, just with a smaller engine that we happen to have. Buying another just for the test in not an option. Changing nosecone/fins/boattail to add drag also doesn't work for us. It seems adding more weight will do the job actually.
mikec
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Why did you buy this motor in the first place? Did you just not do any simulations to see if it fit your constraints, or did you learn about the constraints too late?
Seems like this sort of thing is becoming a theme from what I assume are student teams.
Seems like this sort of thing is becoming a theme from what I assume are student teams.
mikec
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True, but many ways of adding drag (drag plate on the lower section, for example) also make drag separation more likely. Though perhaps not as much as adding a large amount of weight to the nose cone.
Doubling the NC weight increases all the shock loads by at least a factor of four.
How is adding more weight the same as flying it the same as it will be at SAC?
I would see if someone there has a motor they would trade for yours.
Concur
No lol, we just have it from a previous project. We are not that reckless
Fins, nosecone, body tubes stay the same.
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The only motor that we fired that way that worked was the H550ST. Our old production manager mistakenly made a batch of H550STs with the 2-grain 38mm DMS case instead of the 3-grain. Same nozzle and everything else. We thought they were trash until we tested them and found out they made a pretty nice motor. And that's how the H283ST came to be.
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I'm sure there would be a lot of people willing to trade a lower power motor for the one you have. Adding weight requires larger chute. A larger chute is probably more expensive than the L motor. Also need to factor in labor. I swear the things university students will go through to avoid purchasing something... Usually ends up in a busted up rocket.
rlboothbna
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Nice anecdote, glad to hear it!
How does it help this flyer make a sound and safe decision?
rharshberger
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Fly the damn motor in its intended configuration, they are certified in a certain configuration for a reason. Lots of good advice has been given in this thread and the best piece of all is USE A DIFFERENT MOTOR!
Maybe re-read what the manufacturer said. Does it sound like he would fly it in your situation?
What are you risking by trying it out?
What would a responsible engineer do?
rlboothbna
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YES!
