Duane spent a few hours this afternoon giving a couple of new TARC teams much needed practical experience in launching mid power rockets. Naturally, he had to fly the rocket that propelled him to Geezer TARC glory. It was configured as before, loaded with the standard Mayer TARC motor - an Aerotech F32. I have no doubt he expected to show the assembled teams how close he could come to the TARC goals of 835 feet max altitude and 41-44 seconds duration.
But it didn’t happen.
Much to his surprise, the rocket, which was only 5 feet off the mark in October, soared much higher, to 955 feet!
While talking to him on the phone, I flashed back to the many instances TARC teams have complained that they couldn’t repeat the previous practice’s performance. Everyone acts puzzled, but they shouldn’t be. One should not expect the altimeter in a TARC rocket configured exactly the same to always give the same altitude. If you read my blog post from March of last year, you know one reason why.
It has to do with temperature - model rocket altimeters compute altitude assuming the temperature is 59 degrees. If it isn’t, the altimeter will not spit out the true altitude, but what it thinks the altitude is. This altitude will be low or high, depending on the temperature. If it is colder than 59 degrees, the altimeter will think the rocket went higher; warmer temperatures will give a lower reading. In NAR competition, flights made in altitude events have the altimeter heights corrected for the temperature, but in TARC, you are stuck with whatever the little beastie beeps or flashes out. No corrections allowed.
So let’s look at the performance of Duane’s rocket on its two flights. After his October 8 Geezer TARC flight, the Perfectflite APRA beeped out 840 feet. The temperature on the field at that time was 81 degrees - hot for October! Today’s flight went to 955 feet; the temperature at Pegasus field was a cool 36 degrees. If we apply the formula in my March blog post to get the “true” altitudes, we obtain values of 876 and 913 feet, respectively.
This means that:
1) Duane’s rocket is performing fairly consistently, with the actual difference in altitudes being 37 feet - not the 115 feet given by the altimeters!
2) His rocket is also “hot”, as the corrected altitudes are above the mark. Need to add some weight.
So temperature can be a big deal! The graph below shows the altimeter readings as a function of temperature for a rocket that hits exactly 835 feet when the temperature is 59 degrees. This means that TARC teams must add or subtract weight depending on temperature. If the day is cool, weight must be added. If the temp is 60 or higher, weight must be reduced. If the weight is left the same, the altimeter altitudes will vary from practice to practice, and much frustration will arise.
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This is just one factor influencing altitude, but it is one that is often neglected. Many teams adjust for wind speed; very few take into account temperature.
Reckon they oughta?
Most barometric sensors have temperature compensation built-in (see https://www.rocketryforum.com/threads/temperature-relation-to-electronics.170732/#post-2235264). The sensitivity isn't ideal for a rocket flight so, I would expect _some_ compensation in the altimeter rather than _none_.
ReplyDeleteFactors that change the standard atmosphere—temperature, starting altitude, barometric pressure, etc.—will change both the altitude measured by an altimeter and the aerodynamic forces on the launch vehicle. Thankfully, some simulation software factors this in, and, overall, it isn't as complicated for TARC teams as it gets for high altitude (>25K) flights.
Rocket motor performance is another source of variation between flights. Certified rocket motors are only required to perform within +/- 10% of the recorded thrust data.
While it is true many altimeters can sense the temperature of the silicon, the standard NAR competition procedure requires correcting the altitude to the ambient temperature as measured on the ground, i.e. assuming the altimeter does not make any temperature corrections. In this blog post from last year (http://billsrockets.blogspot.com/2021/03/learning-more-about-altimeters.html), I demonstrated that the readings from a Flightsketch altimeter were exactly what you would expect from the standard atmosphere model at 59 degrees Fahrenheit. So at least in this case, and I am pretty sure it is true for all altimeters certified for NAR competition, there was no temperature correction made by the altimeter. It assumes the temp is 59 degrees regardless of what the temp sensor measures.
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