Interpreting velocity from SL100 data

Saturday at Airfest I flew a 54mm MD to just under 30k. I had a PerfectFlite SL100 and PerfectFlight MT4 onboard for apogee deployment. The SL100 beeps out both altitude and speed and the MT4 beeps out acceleration and speed. Unfortunately, they were beeping at the same frequency after the flight, which made it impossible to clearly hear them apart from each other. As I was opening up the ebay, a couple wires were stretched which restarted both units. The MT4 doesn't store any data after power-down that I know of, so that was lost completely. I turned the altimeter back on, which reported an altitude of 29,472', and the downloaded data agrees with that.

Now I'm having trouble interpreting the velocity of this flight. The velocity graph shows a peak of ~ 4,600 ft/sec (Mach 4.1) which is impossible, and I'm really confused by the raw data reported.

I attached the raw data in a .txt file, the Perfectflite data file was too large. Does anyone have any ideas on how to find the peak velocity or is it a lost hope?

The motor burn was 1.95-2.00 seconds, and the simulated speed was right at Mach 3.

View attachment RawData.txt

You're probably not going to get a good peak velocity from this time/baro alt data. If you plot the data, you can clearly see the mach altitude dip, since this coincides with the mach transition you're not going to get the peak velocity from baro altitude. The velocity data doesn't really recover from the mach transition until about 4 seconds into the flight.

Baro-only altimeters are very good for deployments (assuming they can handle the mach dip, which any decent unit should nowadays), but they're really not good at getting velocity/acceleration data for mach+ flights. For that, you need a dedicated accelerometer chip. A Raven, Telemetrum, or other similar unit would be a better choice, but the SL100 would make a fine primary or backup unit to fly along with it.

[Full disclosure: I make and sell a baro-only flight computer, so I've seen this behavior a lot... it's a data issue, and shouldn't affect deployments if properly accounted for.]

Thanks for the reply. I kind of figured that would be the case, guess I'll just have to fly it again. The MT4 timer I used for backup is accelerometer based and reports peak acceleration and peak velocity after the flight, but unfortunately doesn't store any data after it's been turned off. I'll get a flight computer for next time.

I played with your data a bit and considered the launch site and V-max altitude and guesstimated that the v-max is not more than 3800 fps which would be about Mach 3.3 considering the launch site environmental. What I did was to look at the deceleration of the rocket I can fit the data to a limited type of fits to extrapolate what the velocity would be as a function of flight time. Using a 6th order polynomial I look at burnout time velocity and obtained 3850 fps at 3 seconds on the chart below. I also looked at the initial acceleration and fit the subsonic portion to a logarithmic fit (because it had the right shape factor) and looked for the velocity interception point. It was 3800 fps at 3.15 seconds which is close to the other value. Neither is the real value, however 3800 fps which is Mach 3.3 at the launch location and altitude ASL is what I would peg it as.

Now if you can sim your flight and match the time to apogee (35 seconds after liftoff with the apogee altitude of 29472' AGL using a 2 second motor burn time, the calculated velocity should be very close.



Bob

Thanks Bob, much appreciated. I was hoping something could be made of it.

I went back into the sim file and tweaked my drag settings and updated the ebay weight (never changed it before flight). With that I was able to match up the CG, altitude, time to apogee, and rocket weight with that of the actual rocket and flight from Saturday. The plotted sim data now shows a max acceleration of 83 G and max velocity of 3246 ft/sec at 4,500' AGL. The Argonia launch sight is approximately 1,250' ASL, meaning max velocity was achieved around 5,750' ASL, putting the mach number around 2.96.

I feel confident that this is very close to the actual velocity achieved.

That's exactly the right way to do it. If you look at your barometric data, you will find it is contaminated with transonic shocks, and lagging time constants on the actual external pressure. I did substantial filtering on the deceleration curve however I have no idea of the e-bay acoustic time constant which effectively causes a phase shift in the pressure data and suspected my estimated velocity would be too high which is why all I could be sure of was the peak velocity did not exceed 3,800 fps. There is an acoustic delay on the e-bay as my guestimate on altitude at burnout was between 3000' and 4000' AGL based on you state 2 second burn time.

The good altimeters run a Kalman filter in the altimeter as a way to prevent high speed deployment at mach transients. It would be interesting to see the actual altitude and velocity profiles based on the kalman filters.....

Bob

I'm impressed! 29K and mach 3 are very respectable numbers for a 54mm flight! Mind if I ask what the rocket length and weight were? And motor specs?

Tony

Thanks Tony, that's a big compliment from you!

The motor was a Loki Research demo ~4700Ns white load for the 54/4000 case. The case is around 44" long and has a tad under 42" of propellant I believe. The rocket isn't anything too special, all carbon fiber and as short as I could make it. It ended up at 62" long total and 11.75 lbs. on the pad (9 of that was motor). Electronics on top of the motor, recovery and tracker in the nose. It was quite a flight and I can't wait to do it again, this time higher and faster.

Yes, impressive flight with an impressive motor!


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Congratulations to both you and Scott. To Scott, for keeping that much/long of propellant in place for a 86 G flight. And to you, for designing/building a rocket to keep that beast contained!

I have been keeping an eye on Scott...well the motor really. I figured this motor was gonna turn into to the "N5600 Rocket Eater" of the 54mm's. And that it was going to be awhile before some one was able to build a rocket strong enough to handle it. I was wrong! You did it on the first flight!

I'm heading back to my seemingly inadequate rockets now..Thanks

Tony

Much appreciated. I've followed your builds for many years, and they are far from inadequate. I have learned so much from people like yourself and Jim Jarvis, who have great success pushing the limits and don't have a problem sharing your secrets. I'm very happy my rocket held together and I'm glad I can still say I've never shredded a rocket. My design leaves quite a bit to be improved, and I suspect there will be some much more impressive flights with this motor in the very near future. I'm already planning my next version of this rocket, but it may be another year before I can make another flight like this.

tfish said:

Congratulations to both you and Scott. To Scott, for keeping that much/long of propellant in place for a 86 G flight. And to you, for designing/building a rocket to keep that beast contained!

I have been keeping an eye on Scott...well the motor really. I figured this motor was gonna turn into to the "N5600 Rocket Eater" of the 54mm's. And that it was going to be awhile before some one was able to build a rocket strong enough to handle it. I was wrong! You did it on the first flight!

I'm heading back to my seemingly inadequate rockets now..Thanks

Tony

Click to expand...

Scott is an "Evil Genius" :dark: and just a little crazy too.

Bob