Motor Comparison?

[Conan4480]

I have a question about the physical effects of additional motor burn time on a rocket. This is kindof something I was thinking because that is just how my mind works.

This is a hypothetical question. I do not have a specific manufacturer in mind for the question.

The comparison is between a J415W and a K185W on a rocket that is 3" in diameter, designed by the manufacturer to fly on J impulse motors. The materials for the rocket are the standard cardboard and ply, built per the directions provided by the manufacturer. Both motors put out roughly 100lbs of thrust. The burn time for the J415 is two seconds and the burn time for the K185 is eight seconds.

What would the effects of the additional burn time of the K185 have on the components of the rocket? Could I expect the additional burn time of the motor cause extra fin flutter and potentially shred the fins? Would there be adverse effects on the body tube?

Ok, so this was more like three questions. Sorry about that.

Thanks
Conan

 

[troj]

There are pros and cons to the two motor choices.

The J415 is going to accelerate the rocket a lot harder, which definitely puts some stress on things. The K185 won't hit it as hard in terms of acceleration, but depending on the particular rocket, you may find that you get a higher max velocity, which comes with its own additional stresses.

Regardless, the point of greatest stress on a lot of rockets isn't the up part -- it's the recovery system deployment. Take a look at the spikes in accelerometer data at the points of liftoff and recovery deployment.

-Kevin

 

[dixontj93060]

I cannot comment on all the areas of stress, but in looking at flutter for my L3 rocket, it is simply a function of velocity, fin geometry (span, chord, thickness) and fin material. A rocket powered by a K185 in general would have a lower velocity than one powered by a J415 (a vote for the longer burn motor). The key is if the flight profile exceeds the critical flutter velocity and, in turn, the probability of damage by flutter would go up the longer the rocket spends above this critical velocity (a vote against the longer burn motor). Beyond that, I don't think you can make any other blanket statements regarding flutter.

 

[bobkrech]

I have a question about the physical effects of additional motor burn time on a rocket. This is kindof something I was thinking because that is just how my mind works.

This is a hypothetical question. I do not have a specific manufacturer in mind for the question.

The comparison is between a J415W and a K185W on a rocket that is 3" in diameter, designed by the manufacturer to fly on J impulse motors. The materials for the rocket are the standard cardboard and ply, built per the directions provided by the manufacturer. Both motors put out roughly 100lbs of thrust. The burn time for the J415 is two seconds and the burn time for the K185 is eight seconds.

What would the effects of the additional burn time of the K185 have on the components of the rocket? Could I expect the additional burn time of the motor cause extra fin flutter and potentially shred the fins? Would there be adverse effects on the body tube?

Ok, so this was more like three questions. Sorry about that.

Thanks
Conan

The motors almost have the same total impulse, 1232 Ns for the J415 versus 1417 Ns for the K185, but the average thrust of the J415 is more than twice that of the K185.

The rocket on the J415 will accelerate at more than twice the rate of the K185. Both rockets will reach approximately the same peak velocity, but the rocket on the K185 go about 20% higher than the J415.

A quick and dirt on-line simulator is webalt. https://www.markworld.com/

Fin flutter is velocity dependent. Both motors will reach the same peak velocities, so you can expect the same amount of flutter.

Bob

 

[dixontj93060]

I ran Rocksim runs of both motors in a few stock 3" and 4" kits from LOC, Binder, etc. and the simulations consistently had the J415 peak velocity 100 to 150 ft/s above the K185.

Both motors will reach the same peak velocities, so you can expect the same amount of flutter.

 

[AstronMike]

Since we all know that flutter is velocity dependant, lets look at the other side of the same equation: Weight.

Obviously, if you fly two of the same type rocket (or in my case, glider) on the same motors, but one of them is appreciably heavier, then the heavier one will tend to have less flutter, even though it is made of the same materials (fin or wingwise) while being subjected to the same total and average impulse.

Case in point:

Back in '95 I had a glider that flew just great on a J90, with no sign of flutter, and this was with a dork-ply wing with NO reinforcement. The J350, even though it actually has a bit LESS total impulse, wouldve been a nonstarter here.

The J90 flight had a liftoff weight of 7lbs, which is pretty much the maximum for this motor, as it came off the pad cleanly and then 'chugged' its way up, never getting TOO fast.

An even larger glider flown around the same time that always did solid on I211 loads was then shot on a J415, and had a liftoff weight of 12lbs. This fluttered like mad right off the pad but HELD, albeit barely, and did well enough, but obviously, any more speed/velocity wouldve been curtains, so I never tried a K550.

As to a 3FNC rocket versus a long burn motor, liftoff weight is paramount, as in my J90 glider example, if you are near max wt, then the rocket will not accelerate too much even over the 7 or 8 sec burn. Now, lets say I would have flown the lighter version of the same glider on the same load, but with a 5lb liftoff weight. I have no doubt that this wouldve accelerated to the point of failure. Obviously, a rocket doesnt have those tight of boost tolerances (or is that intolerances :eyepop::y.

If I had my pick in your example, Id go with the K185, as I have seen rockets up to 12lbs on these, and you get to see a realistic boost instead of BANG its gone. That is, assuming your intended rocket is on the fatter side but not overly so. 12lbs or so is probably max for that load.

 

[stickershock23]

I wonder if some of the extra velocity on the J415 is due to the reduced weight? (the reload weighs 20% less or so)

the kit manufacturer may say "J" impulse because 99% of "K" motors are fast burning motors, it saves them the time of saying.. good for "J" motors plus k185 k250 and maybe k270's

My personal opinion about your hypothetical question. if you have flown it on the J415 you should be good to go on the K185. unless the rocket has some huge fins that might start flapping.

as an example, I have (ok had, don't ask) a polecat Aerospace Skeeter, Flew it dozens of times on 38mm and 54mm "I" motors then flew it on the J90 (similar lbs of thrust, just longer burn) it flew great, after a few of those I said WTH. and threw in a J135 similar thrust of the J90 well thats all it took, about 3/4 through the burn you could see the fins flapping (nice spiral smoke trail) and sure enough on landing I had split a fin. I don't think it was the amout of thrust, but the combination of high thrust for a long time added up and did its worst.

 

[bobkrech]

I ran Rocksim runs of both motors in a few stock 3" and 4" kits from LOC, Binder, etc. and the simulations consistently had the J415 peak velocity 100 to 150 ft/s above the K185.

Tim

For a 3 kg unloaded 76 mm rocket and adding the motor weights from thrustcurve.org, webalt predicted an 18% higher flight with the K185 but a peak velocity 2.5%-4% lower (cd 0.4-0.6) than that obtained with the J415. The claculated velocity differences were between 21 and 33 fps.

There must be a difference in the thrust curves used by webalt and rocksim. I rechecked webalt and thrustcurve.org and webalt uses a TI of 1201 Ns instead of 1232 Ns for the total impulse of the J415W and a TI of 1457 Ns instead of 1417 Ns for the K185 so this would explain the trend but probably not the magnitude of the difference.

Bob

 

[dixontj93060]

Yes, you know, it could also be my launch condition settings as a week ago I was doing strong wind, heavy shear simulations. I'll check. I may have not reset these.

Tim

For a 3 kg unloaded 76 mm rocket and adding the motor weights from thrustcurve.org, webalt predicted an 18% higher flight with the K185 but a peak velocity 2.5%-4% lower (cd 0.4-0.6) than that obtained with the J415. The claculated velocity differences were between 21 and 33 fps.

There must be a difference in the thrust curves used by webalt and rocksim. I rechecked webalt and thrustcurve.org and webalt uses a TI of 1201 Ns instead of 1232 Ns for the total impulse of the J415W and a TI of 1457 Ns instead of 1417 Ns for the K185 so this would explain the trend but probably not the magnitude of the difference.

Bob

 

[dixontj93060]

Well, no. Still seeing the same thing after going to a "no wind" setting.

Yes, you know, it could also be my launch condition settings as a week ago I was doing strong wind, heavy shear simulations. I'll check. I may have not reset these.

 

[bobkrech]

Well, no. Still seeing the same thing after going to a "no wind" setting.

You did a 4" rocket versus the 3" 3 kg unloaded rocket I simulated. That could make a difference.

Added: In loading the motors I accounted for the weight difference. Weights were around 4100 and 4400 g.

 

[dixontj93060]

Well, actually, no, I did a number of stock 3" and 4" rockets (attached is one 3") with all having the J415 peak velocity 100 to 150 fps more than the K185. Didn't do the math, but maybe it is the ~9oz difference in motor weight?

You did a 4" rocket versus the 3" 3 kg unload rocket I simulated. Taht could makie a difference.

 

[Handeman]

I ran sims on the 3" and 4" rockets with the same Cd. Weights at 4,5,6,7, & 8 lbs with the J415 and K185.

The J415 was consistantly faster and the K185 was consistantly higher. Although the actual numbers were different between the 3" and 4", the difference ranges were very similar for both.

 

[Conan4480]

Thank you for the replies on this. It has been helpful.

At what point would 3/16 birch ply flutter to the point of failure? Would it be helpful to put a layer or two of glass on them for strength?

 

[cjl]

Thank you for the replies on this. It has been helpful.

At what point would 3/16 birch ply flutter to the point of failure? Would it be helpful to put a layer or two of glass on them for strength?

That entirely depends on the shape and size of the fins. Glass would help, but you might not need it.

 

[FredA]

The slow push of the K185 will hurt the most -- IMHO.

TIME spent in the transonic region will kill you....
That's "Flutter time" -- the longer you stay there the more chance you have of ripping the fins off.

Fast transonic transitions are best.
The slow push of the K185 will take longer to push through mach....leaving your fins fluttering longer...and causing more potential damage.

My THEORY -- YMMV.
FredA

 

[cjl]

The slow push of the K185 will hurt the most -- IMHO.

TIME spent in the transonic region will kill you....
That's "Flutter time" -- the longer you stay there the more chance you have of ripping the fins off.

Fast transonic transitions are best.
The slow push of the K185 will take longer to push through mach....leaving your fins fluttering longer...and causing more potential damage.

My THEORY -- YMMV.
FredA

This is true on the assumption that the K185 is capable of getting the rocket to mach. In many cases, the 185 won't be capable of getting there in the first place, as it is a low thrust, regressive motor, and many 54mm rockets will stay solidly subsonic with that thrust profile (while with a J800 for example, they might attain significantly higher speed).