What do people use to measure high speeds? The barometric readings shouldn't work due to an increase in pressure at mach+ and the accelerometers I have seen don't handle 100+ G's. So what do you use?
Steve
Steve
How do you measure speeds past mach 1?
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EeebeeE
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Unless you are going real extreme, it is fairly hard to get to 100 G's. Not saying it isn't possible with N or O motors. I have a Mad Cow FG Arcas. I put CtI J530 and J600 motors in it. It will pull 25-30 G's and fly to Mach 1.2 as measured by my Perfect Flite MT-4, which also serves as my redundant recovery. I do not think the MT-4 will work for a Mach 2 project I am working on, but other accelerometers should.
I have a 38mm minimum diameter that will hit mach 1.2 and 169 Gs on an H999. At least that is what it sims to. What could I use on that? (other then radar) 
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I have a 38mm minimum diameter that will hit mach 1.2 and 169 Gs on an H999. At least that is what it sims to. What could I use on that? (other then radar)
A raven with the high G accelerometer option?
https://www.shop.featherweightaltim...58532E1B380E06ABF.m1plqscsfapp05?productId=18
The Raven 3 with the 250 G option looks good. Only problem is they are out of stock......
watermelonman
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I have a 38mm minimum diameter that will hit mach 1.2 and 169 Gs on an H999. At least that is what it sims to. What could I use on that? (other then radar)
I went faster, though with less acceleration, with an Eggtimer. It performed beautifully and I would not hesitate to try it with your described rocket.
Ahhhhhh, Aside from measurement, a decent baro unit like W.melonman used is perfectly acceptable for the deployment chores. If the G's get really stupid high, one needs to be worrying about whether or not some electronic components would be
ripped off the board. Folks here have done it so it's not unheard of. Kurt
ripped off the board. Folks here have done it so it's not unheard of. Kurt
DennisMYeh
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From an experimental standpoint, accelerometers can easily be integrated to find your velocity, although integrating noise adds a significant amount of error in your velocity calculations. Also, I've heard that GPS is inaccurate in the vertical direction (not sure why, but it does require 4+ satellites to resolve a 3D position mathematically).
From a theoretical standpoint, it should be possible to estimate your altitude at supersonic speeds using only a barometric altimeter. Assuming you have a blunt nose cone, resulting in a normal shock in front of your rocket, the pressure increase across the shock wave depends on the flight mach number (https://www.grc.nasa.gov/WWW/k-12/airplane/normal.html or https://en.wikipedia.org/wiki/Normal_shock_tables).
Unfortunately, the increase in pressure depends on the flight mach number (which we're trying to find by differentiating the altitude w.r.t. time) and the ambient pressure (which we need to find the altitude) depends on the altitude. This means we have more unknowns than equations, so estimation or some other mathematical tricks would be needed to determine your velocity.
Unknown Variables: ambient pressure and temperature (before the shock), altitude, velocity
Known Variables: Pressure behind the shock, temperature behind the shock, time
One way to solve this could be looking at the data to see when we hit supersonic speeds (which is inaccurate, because buffeting occurs from Mach ~0.8 to ~1.2 and purely supersonic flow doesn't really happen until past Mach 1.2). At this data point, during assumed purely subsonic flow, we have the altitude and velocity. We can use these two values to find the approximate altitude for the next data point, during assumed purely supersonic flow. The approximated altitude gives us the ambient pressure, which we use with our actual measured pressure and temperature to find the flight mach number and velocity. This approximation scheme relies on approximated altitude, and will become more and more inaccurate as time goes on. To fix this, accelerometer or GPS data can be used to correct the altitude or velocity every few time steps.
If we remove the normal shock assumption, since most amateur rockets don't have blunt nose cones, the equations for the change in pressure get a bit more complicated (https://en.wikipedia.org/wiki/Oblique_shock#the_.CE.B8-.CE.B2-M_equation or https://www.grc.nasa.gov/WWW/k-12/airplane/oblique.html) but can still be easily solved.
From a theoretical standpoint, it should be possible to estimate your altitude at supersonic speeds using only a barometric altimeter. Assuming you have a blunt nose cone, resulting in a normal shock in front of your rocket, the pressure increase across the shock wave depends on the flight mach number (https://www.grc.nasa.gov/WWW/k-12/airplane/normal.html or https://en.wikipedia.org/wiki/Normal_shock_tables).
Unfortunately, the increase in pressure depends on the flight mach number (which we're trying to find by differentiating the altitude w.r.t. time) and the ambient pressure (which we need to find the altitude) depends on the altitude. This means we have more unknowns than equations, so estimation or some other mathematical tricks would be needed to determine your velocity.
Unknown Variables: ambient pressure and temperature (before the shock), altitude, velocity
Known Variables: Pressure behind the shock, temperature behind the shock, time
One way to solve this could be looking at the data to see when we hit supersonic speeds (which is inaccurate, because buffeting occurs from Mach ~0.8 to ~1.2 and purely supersonic flow doesn't really happen until past Mach 1.2). At this data point, during assumed purely subsonic flow, we have the altitude and velocity. We can use these two values to find the approximate altitude for the next data point, during assumed purely supersonic flow. The approximated altitude gives us the ambient pressure, which we use with our actual measured pressure and temperature to find the flight mach number and velocity. This approximation scheme relies on approximated altitude, and will become more and more inaccurate as time goes on. To fix this, accelerometer or GPS data can be used to correct the altitude or velocity every few time steps.
If we remove the normal shock assumption, since most amateur rockets don't have blunt nose cones, the equations for the change in pressure get a bit more complicated (https://en.wikipedia.org/wiki/Oblique_shock#the_.CE.B8-.CE.B2-M_equation or https://www.grc.nasa.gov/WWW/k-12/airplane/oblique.html) but can still be easily solved.
I agree that a good baro unit can work just fine. My Stratologger SL100 said I flew to 849 mph on a flight that simmed to almost exactly that.
As for ripping components off the board, I would be shock if any motor available to a hobbyist could generate enough Gs to rip components off a circuit board, even the biggest Warp9. I have a HiAlt45 that was in a rocket that lawn darted. The disintegrating battery was on the other side of the board an the board didn't break under the altimeter, so it survived a +300G stop (300 mph to 0 mph in 14") in the ground with no damage. I did a lot of vacuum chamber testing on it before flying it again, but its had almost a dozen flight now and works as well as it did before. If a lawn dart isn't going to rip components off the altimeter, I really doubt any motor is capable of doing that.
Talk to Mike Fisher and perhaps Robert DeHate. Sure, remember I said, "Stupid High G" and not everyone flies N5800's. It's large capacitors that "could" be ripped off a board if not secured. You're right, sport fliers needn't worry though but I do recall it being mentioned in Raven docs and perhaps in the MAWD manual that it might be prudent to stick a dab of epoxy underneath the large capacitor on the board. Kurt
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