Flying 4x4
28mm aluminum engine
cast nozzle
3-grain Bates configuration
Flight Test
How To...

"Kurt" was my first 4x4 airframe.  It flew well three times, but was disabled by CATO on Christmas Day 2001.

The remnants sat around for awhile.  Then I realized a need for a slightly lighter airframe for use with these aluminum engines, somewhat smaller than the PVC engines for which the original 4x4 was designed.  So I cut off what was left of the engine bay,  glued the splinters back together, and made a shorter version.  With a new set of dorky fins and a really odd paint-job, "Kurt" is ready to fly again. He is about three pounds lighter, good for testing these "Low-G" class engines.


The funny green thing is a foam ear-plug.  I find these to be really good nozzle sealers.  Notice in the second photo how the nozzle protrudes a bit from its tubing.  This nozzle was a bit loose after casting.  I tried to drive it out with a rod and hammer and that was as far as it would go.  I static-tested it with a full charge - it stayed put.  So I figure it has found its spot

Click on the photo above to load a video of this flight 
(1.6 meg MGP file, 10 seconds of video)
Quick summary:
Flight duration: 
Thrust duration: 
Altitude at apogee: 
Grain configuration: 
Grain Dimensions 
Total fuel: 
Nozzle throat: 
Kn / PSI 
Launch weight: 
Total Impulse:

6.7 seconds 
0.76 seconds 
51 meters 
3 outside-inhibited Bates grains 

1.85 inch (each grain) 
1.0 inch 
0.375 inch 
101 grams KN/sucrose 
0.25 inch 

216 / 650 
256 / 850 
237 / 800 
3.29 kg 
108 N-sec

Calculated thrust curve is asymmetrical because the grains are a bit longer than the ideal Bates dimension for this diameter.


Note that the nozzle has shifted forward.  This is unexpected.  Apparently, the inertia of a sudden stop is significant.  I also suspect that the thin aluminum tube heats more quickly than the massive aluminum nozzle, thus expanding a bit and loosening the nozzle.  I have sometimes drilled and tapped small holes in the nozzle region and inserted small setscrews to lock the nozzle in place.  This worked OK, but then I found a better way.

Details:  Spreadsheet output:
LumberCalc Version 1d     Data for flight 5-14-02a "Kurt" - 7-poundish 4x4  
Measured:       Calculated:    
Distance * Feet Meters   Rise/Run at apogee: 0.653529 < ratio of height on pole at apogee / distance to pole
A. Camera to measuring pole 5.1 1.554404   Rise/Run at burnout: 0.211765 < height on pole at burnout / distance to pole
B. Camera to launcher 280 85.33984        
C. Camera to touchdown 231 70.40536   Height at apogee: 50.89203 < distance to apogee * rise/run ratio at apogee
D. Launcher to touchdown 59 17.98232   Height at burnout: 18.07197 < distance to burnout * rise/run ratio at burnout
E. Rise on pole at burnout 1.08 0.329168   Rise height after burnout: 32.82007 < height at apogee - height at burnout
F. Rise on pole at apogee 3.333 1.015849      
* Letters A-F refer to drawing on Sheet 2       Rise time, burnout to apogee: 2.588044 < calculated from rise height, deceleration by gravity
        Time, launch to apogee 3.348044 < boost time + rise time
Time Seconds     Fall time, apogee to ground 3.222752 < calculated from height at apogee & gravity
Thrust duration 0.76     Total Flight time (calculated) 6.570796 < calculated boost + rise + fall time)
Descent from apogee 3.3     %Error between flight times: 0.016201 < comparing measured vs. calculated times
Total flight time (measured) 6.679          
        Velocity at burnout (m/sec) 25.36283 < calculated from rise height, deceleration by gravity
Weight Kilograms     Acceleration, actual (m/s^2) 33.37215 < velocity at burnout / thrust duration
Fuel charge 0.1007     Acceleration, felt (m/s^2) 43.17215 < actual acceleration + gravity
Total launch weight 3.2925     Thrust (Newtons) 142.1443 < felt acceleration * total weight
        Total Impulse (N-Sec) 108.0297 < thrust * thrust duration
        Isp (N-Sec/kg) 1072.787 < total impulse / mass of fuel charge
Jimmy Yawn       Seconds  109.4681 < Isp / gravity
5/14/02       Height (of doubtful accuracy) 53.361 < calculated from measured fall time

Click here to download a copy of this spreadsheet, Excel format

Graph of wavefile extracted from video:

0.6 seconds, Ignition switch clicks
1.45 seconds, Thrust begins*
2.2 seconds, burnout (tapering-off noise is echo)
8.4 seconds, vehicle hits ground

*The ignition switch is right by the camera and the launcher is 280 feet away.  Thus the ignition delay is about 1/4 second less than the .wav file would suggest.