SPITBALL-1 Postflight Analysis

This is the analysis of the flight SPITBall-1 from Novemeber 20, 2011.

Overall Flight Summary

SPITBall-1 met the primary science objective of establishing a safe operating pressure level for designing a mechanical zero-pressure balloon pressure relief valve.

  • RESULT: DESIGN A VALVE TO OPEN BELOW 597 PASCALS

SPITBall-1 met its secondary objectives as well:

  • Comfirm that inversion cutdown works and balloon can take the stress
    • RESULT: FUNCTIONED PROPERLY
  • Document ascent aerodynamic motion of asymmetric envelope design
    • RESULT: HIGH RATE OF ROTATION ~10 RPMs

Detailed Analyisis

Burst pressure measurement

The balloon appears to have not burst according to all data collected and a rough initial inspection of the balloon envelope. The dual pressure sensors measured the same pressure fluctuations throughout the flight, and thanks to our internal heated package, both stayed above 10C for the critical measurements. They were connected to an external pressure manifold, and also connected together with a manifold for internal pressure measurement to ensure we could capture the same pressure data on two different ranges of sensor.

There was significant icing of the balloon ropes seen in the videos, which indicate that there was a risk of the pressure port icing over. We can confirm that this did not occur in any part of the flight based on how the pressure readings changed over the flight.

An ice-plug in the external measurement port when there was actually zero balloon pressurization would have produced a negative trend down below zero as the internal sensor port would be reading ambient air pressure dropping relative to the captured air pressure in the plugged external port.

An ice-plug in the external measurement port when there was positive pressure already measured in the balloon would have produced approximately a flat-line steady pressure segment of the graph at the pressure value that the ice plugged the external port. If the ice then cleared, the pressure reading would then jump to recording normal pressure. There was no straight flat line seen such as this, so this has been ruled out.

Maximum differential pressure seen was 597 Pascals around 910 seconds Mission Elapsed Time.

Rotation rate of balloon envelope

Observed a rate of ~10 revolutions per minute in the first 30 seconds of flight before cloud entry. This was before the inversion rope took up part of the load of the payload, reflecting real-flight dynamics. When the balloon filled fully, the balloon was tilted by the weight of the payload being supported partially by the inversion rope.

Inversion rope

Inversion rope was about 0.5m too short, which caused the inversion rope to partially support the load when the balloon fully inflated.

Cutdown timer

The cutdown was specified as having a 61 minute timer by the manufacturer, however the timer was not measured before flight, and post-flight calibration shows an actual firing time of 56 minutes, 23 seconds. This time lines up with the halt of the climb and beginning of descent seen in the altitude telemetry.

Inversion Cutdown method performance

The inversion method worked exactly as planned. The cutdown fired, released the connection to the bottom of the load ring, and the balloon did invert succesfully. This was determined by the fact that there is no obvious rupture of the balloon in the post-flight inspection, and in the recovery video the balloon is only about 1/4 full of helium, indicating that the helium must have vented through the valve.

Vent valve performance

The vent valve was determined to be successful for the purpose of initiating descent. The valve did not dump all the helium as planned, which most likely was due to the extra weight of data logger, cell phone and valve plate sandbag caused the load ring to fall to the side after a portion of helium was released, reclosing or descending below the helium level in the balloon.

The vent valve was only partially successful in the purpose of preventing helium release before burst. The pressure data matches with the modelled effect of the valve partially opening after pressurization, then closing, and reopening until the inversion cutdown.

Most Likely Pressure Loss Scenario

There were several contributing factors to this incomplete success. Several things added up together to cause the valve to open on ascent while the balloon was pressurized.

  • Turbulence far more severe than anticipated due to active cumulonimbus clouds from 300m AGL to ~7000m AGL altitude.
  • The plate was held in place by extra mass added, and also a bungee cord which was tensioned by the payload train. When the payloads were pulled to the side at extreme angles in turbulence the plate was seen to visible slide horizontally on its seat, presumably assisted by the side force applied by the bungee.
  • The too-short inversion rope caused the payload to be pulled to the side of the load ring by approximately 30 degrees from vertical.
  • The silicone caulk used for adhering the silicone valve seat pad to the acrylic ring was not evenly distributed, leaving certain positions that the plate could slide to where it would remaining apparently flat against the seat, but the seat would be unsupported, allowing pressurized helium to escape under the plate.
Pressure Loss Scenarios ruled insignificant

The silicone caulk sealant used to fill the gap between the acrylic ring was found after flight to have not adhered to the polished acrylic. A different sealant or plastic should be found for future valve construction.

The supports for the valve ring were four screws at 90° intervals. This may have been insufficient support and allowed the ring to flex slightly under the load of 25N (23.8CM diameter valve circle with 600 Pa pressure) and tear the silicone sealant off the wet cardboard. The cardboard was significantly waterlogged upon recovery, and likely was so in flight due to the rainy prelaunch conditions. However the amount of gap this would open if the plate merely flexed outward should not allow enough helium to escape to affect the rapid rise in pressure as much as was seen in the graphs.

It is plausible that the waterlogged cardboard load could pull free of the silicone caulk radially if it was distorted by the rapidly shifting side-forces exerted by the turbulence on the payload line. However the direction of force continually, severely changed for the period before and during of the pressure trough as seen in the video, ruling this out as a significant contributor to the loss of pressure.

Launch

Launch technique was perfected in 10-15 knot wind as follows:

  • Restrain top of balloon or choke point
  • Release top of balloon or choke point
  • As helium travels up balloon and reaches top, let the balloon pull out of the hands of the load ring restraint person
  • Payload crew starts running downwind
  • Balloon stands up and lifts payloads up one by one under vertical balloon

Flight Data used for the Analysis

Timeline

Official takeoff time: 11/20/11 15:26:12 EST

Cutdown

  • Initialized at 15:02:48 EST
  • Post-flight measured to fire after 56m 23s
  • Fired at 15:59:11, MET: 1979

(Video timings taken from cam: FACING UP, Go Pro Hero HD)

FILE: GOPR0043.mp4

  • Cutdown Initialize Clip 1:53 (15:02:48 EST)
  • LAUNCH MET: 0 Clip 25:17 (15:26:12 EST)
  • Begin of pressure rise: MET: ~550 Clip 34:25
  • Clip End MET second: 565 Clip 34:42

FILE: GO010043.MP4

  • Clip Start: MET: ~566 Clip 00:00
  • Highest pressure peak: MET: ~910 Clip 5:45
  • Bottom of pressure trough: MET: ~1235 Clip 11:10
  • 2nd pressure peak: MET: ~1491 Clip 15:26
  • Return to zero pressure: MET: ~1685 Clip 18:40
  • Clip End MET: 1739 Clip 19:34

Data Files

Pressure sensor names used in data files

  • P1
    • Part: MPXV5004DP
    • Range: 0 to 3920Pa
    • Compensated Temperature Range: +10C to +60C
  • P2
    • Part: MPXV5010DP
    • Range: 0 to 10000Pa
    • Compensated Temperture Range: -40C to +125C
analysis/spitball/1.txt · Last modified: 2011/11/28 20:41 by steamfire
 
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