Altitude Target Overshoot Analysis

November 30th, 2010

IntroductionOn April 17th, 2010, the Madison West SLI2010 Senior

Team exceeded SLI target altitude of 5,280ft by 34% (barometric altimeter recorded flight apogee of 7,070ft).

Purpose of this investigation:Re evaluate ‐ data and procedures that led to the overshootDetermine the failure point in data analysisDevise safety measures that will prevent a repeat of such or

similar incident

Vehicle

Body Diameter

[in]Length [in]

AerotechMotor

Total Impulse

[Ns]

Liftoff Weight

[kg]

Stability Margin

[calibers]Cd Apogee [ft]

6.155/4.024 99

J800T 1229 13.73 3.93 0.65 1,100

L850W 3646 12.60 3.24 0.49 6,100

L1390G 3949 11.70 3.13 0.48 7,070

Investigation

FOCUS - possible absence of red flags in apogee prediction

Rocket reassembledAll parts weighted and compared to records in PDR, CDR, and FRRNo significant discrepancies were found

Existing RockSIM model verified for accuracy

All calculations and data analysis of flight data verified

Scale Model FlightSimulation

Flight Data

Barometric altitude data compared with simulated altitude and velocity. Model fitting provided Cd of 0.49. The motor used was AT H250G.

Cd = 0.49

1350ft

Full Scale Low Flight - AT J800T

The Simulated vs. Actual flight data. Cd of 0.65 was calculated by model fitting using flight data and RockSIM.

Cd = 0.65

1110ft

Weather concerns (low visibility) forced the team to limit the flight to 1,200ft AGL.

Motor Selection Deadline At this point, motor selection for SLI launch had to finalized. Based on

the measured Cd = 0.65, team selected AMW L1080BB motor, with apogee prediction being 5543ft. Because of AMW L1080BB and AT L850W shortage, the team ended up choosing AT 1390G motor.

AMW L1088BB, predicted apogee 5,543ft AT L1390G, predicted apogee 5,843ft

Full Scale High Flight - AT L850W

Barometric altitude data from flight. Model fitting provided Cd of 0.48 (this value should have been carried over to further simulations)

Cd = 0.48

6,100ft

Comparison: Cd = 0.48 vs. 0.65

At this moment, the inaccurate Cd = 0.65 prediction should have been replaced with the new value Cd = 0.48.

Data and simulation indicate that the switch never occurred and the team continued using the incorrect Cd = 0.65 which resulted in 34% overshoot.

Cd = 0.48

Cd = 0.65

Motor: AT L1390G

Full Scale SLI Flight – AT 1390G

Barometric altitude data from final flight at Braggs Farm, AL. Model fitting in RockSIM provided Cd of 0.46

Cd = 0.46

7070ft

MotorTotal

Impulse [Ns]

Liftoff Weight

[kg]Apogee [ft] Time to

Apogee [s]Cd Fitted to Flight Data

Scale Model

AT H148R 206 2.10N/A

(flight was

unstable)

N/A (flight was

unstable)

N/A (flight was

unstable)

AT H250R 231 2.30 1,350 9.3 0.49

Full Scale Vehicle

AT J800T 1229 13.73 1,110 8.5 0.65

AT L850W 3646 12.60 6,100 20.0 0.48

AT L1390G 3949 11.70 7,070 20.0 0.46

Flight Tests

ConclusionsThe multiphase apogee prediction procedure is reliable

however:

SLI2010 senior team missed the indication of severe overshoot because they switched two data analysis results.

Cd from low (AT-J800T) full-scale test flight Cd = 0.65Cd from high (AT-L850W) full scale test flight Cd = 0.48

Had the correct Cd been carried to further rounds of simulations, the indication of overshoot would have been clear (simulation predicts 6,900ft)

Incident Repeat PreventionBetter management of project files,

data and documentationMore detailed labor division among

team membersCloser initial estimates of flight apogeeUsing the same motor for final test flight

and SLI launch flight

Questions?