Q:high impulse motors-short burn. As compared to moderate impulse-long burn

I am trying to understand the effects on a rocket between two motor types. CTI White Thunders 54mm 5, 6 or 6gxl grains. As compared to other motors of same grain number and diameter. The white thunders burn really short, 1.7 seconds or so as compared to a similar motor, up to 4 seconds. Both take a large 7.5", 9' tall rocket to about the same height on rocksim. But the white thunder must put an incredible amount of shock on the rocket. I haven't used a white thunder and am a little concerned that the white thunder could cause damage somehow. Examples: https://www.pro38.com/products/pro54/motor/MotorData.php?prodid=2372K1440-17A
and https://www.pro38.com/products/pro54/motor/MotorData.php?prodid=2377K711-18A
Am I comparing maximum thrust to compare how hard these are on a rocket? I have seen videos where rockets kind of disintegrate on ignition.

NattyDread said:

I am trying to understand the effects on a rocket between two motor types. CTI White Thunders 54mm 5, 6 or 6gxl grains. As compared to other motors of same grain number and diameter. The white thunders burn really short, 1.7 seconds or so as compared to a similar motor, up to 4 seconds. Both take a large 7.5", 9' tall rocket to about the same height on rocksim. But the white thunder must put an incredible amount of shock on the rocket. I haven't used a white thunder and am a little concerned that the white thunder could cause damage somehow. Examples: https://www.pro38.com/products/pro54/motor/MotorData.php?prodid=2372K1440-17A
and https://www.pro38.com/products/pro54/motor/MotorData.php?prodid=2377K711-18A
Am I comparing maximum thrust to compare how hard these are on a rocket? I have seen videos where rockets kind of disintegrate on ignition.

Click to expand...

Aero forces on the fins, shock cord loads, and landing forces tend to be significantly greater than the forces applied by motors themselves. Build the rocket to handle the speed, and it'll handle the force.

High acceleration (>50 g) is another story, and you have to start taking care to secure your electronics.

NattyDread said:

Am I comparing maximum thrust to compare how hard these are on a rocket? I have seen videos where rockets kind of disintegrate on ignition.

Click to expand...

If the rocket "disintegrates on ignition", that's a motor failure (CATO). If it disintegrates in flight, that's structural failure (shred).

Shreds are caused by air friction tearing the rocket apart once it loses its aerodynamic shape. Losing the aerodynamic shape usually occurs due to premature ejection or fin failure.

Premature ejection can be solved by drilling a small vent hole and using shear pins. Note that despite the name, it's not caused by the ejection charge going off early, but instead by the pressure inside the rocket (loaded at ground level) pushing out the nose as the rocket ascends to lower-pressure air at altitude.

Fin failure is a more complex and contentious topic. As I understand it, unless the fins a perfectly symmetric and perfectly positioned the air flows over it irregularly, causing small forces in various directions. These forces can cause waves that propagate along the length and breadth of the fin. Depending on the fin material and shape, the waves can reflect and reinforce themselves (standing waves) and eventually destroy the bonds at the root.

IMO, fiberglass is the worst material for fins because it has no damping and is not stiff. Wood and aluminum have natural damping ability and carbon fiber has better stiffness. The best construction is a core with good damping characteristics and a stiff skin (i.e., carbon fiber over a wood core). For single-material fins, aluminum and carbon fiber plate have the best track record.

Again, this is a contentious topic and poorly understood in the hobby community.

If you think the White Thunder has kick, take a look at the VMAX (or Warp 9 by AT). I haven’t used 54mm yet, but they dump all their thrust in 0.7 seconds. The 38mm lasts about half a second.

The advantage to the quick burn motors is if you have a rocket you need to get moving quickly off the pad (like a Phoenix with lots of fins), they do that.

As long as you build your rocket out of good materials (the cardboard and wood kits vendors provide are sufficient) and use good adhesive, the rocket should be fine. I feed my Madcow Phoenix H400 VMAX and have never had damage from the motor. It’s cardboard and wood and 5 minute epoxy.



If you want to see something really wicked, watch Gerald Meux’s record altitude launch with a O-25000 VMAX. It burns in less than 2 seconds. There’s a camera mounted on the rocket. When he pushes the button at about 0:53 you can hear the O coming up to pressure for millisecond. It’s a scary awesome sound, and then his rocket is gone. Sort of a whooOOOMP PSHHHHHHHHHHH!

[video=youtube;Jl9JIsEkrlc]https://www.youtube.com/watch?v=Jl9JIsEkrlc[/video]

I launched my L1 rocket several times on the AT I1299N-P Warp-9 motors. 1/3 second burn time. In that 1/3 second it was 138 ft in the air and doing 520 mph. It pulled 80G.

It was a scratch built made from 1/8" wall shipping tubing, craft store plywood fins, a foam nose cone, and homemade coupler tube for the av-bay. It held up just fine. I calculated the weight on the tubes at 80G. The lower tube had the biggest load, but the upper tube had very little because of the light nose cone. Do a rough calculation and you'll probably find that the rocket will hold up to the thrust. As other have said, it's the speed and aerodynamic forces that usually cause that shred.