28mm aluminum engine
3-grain Bates configuration
"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)
Altitude at apogee:
Kn / PSI
3 outside-inhibited Bates grains
1.85 inch (each grain)
216 / 650
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|
|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.