Rather than taking the “Off the Rail Velocity Poll” (//www.rocketryforum.com/threads/off-the-rail-velocity-poll.175241) completely off the rails, I’ve started a new thread.
I did a few calculations from 480fps videos shot at Oregon Rocketry’s NXRS, but did not really dig into them at first – the above poll discussion drew me in.
First is the most basic, and most reliable. The speed measurements were very straight-forward, and the likely error is well within 10%. We’ll start with speed off the rail, grouped by motor class. Plotted are 81 launches. Not many low power – most low power launches were from a rack, the rods were not visible on camera (and if they were, it would have been hard to tell which a given rocket was on). One in six launches failed to reach 45fps off the rail.
Next up is speed at a standard distance, 10 feet, for 122 flights (including many more low power rockets). I also tracked average acceleration, which in this case is simply the speed divided by the time from liftoff to 10ft. This is a little less precise – a significant number of rockets did not just lift cleanly. It often took some time for the motor to reach a steady burn and motion before that ranged from a little to very erratic. Still, probably reasonable numbers.
From here, things get a little more scattered, but still interesting. For 65 flights, I had weight and motor type from the flight cards. These were self-reported and some weights were missing, a few probably incorrect (dry weight, etc.). I’ve graphed weights vs initial thrust reported in Thrust Curve. This is not ideal – the nominal 0.5 second interval averaged by Thrust Curve rarely if ever exactly matches the time interval of travel on the rail (which usually ranged from about 0.1 to 0.35 seconds), but it is the best we’ve got. Even a very detailed calculation (based on launch weight and the sim files) probably would not help – many, many motors did not ignite immediately, and quite a few were still smoothing out long after they hit 5% thrust. Short of a high rate accelerometer AND a calibrated thrust cell on board, we will probably have to be satisfied with a general idea of thrust/weight. Five launches were below 5:1 T:W, only one of these was a low power rocket.
Finally, I made a judgement call on stability. Most flights were (within the field of view of the fixed camera) arrow-straight. Most of the rest veered only slightly. I measured the deviation (from the camera POV) of the roughly 10% that were at least 10 degrees off course. I could have missed deviation in my line of sight, but I had a reasonably good vantage point relative to the wind, and the worst performers of the event were clearly apparent.
All of these rockets passed the 45fps and 5:1 thumb rules. Only two were scratch built, and most of the others were very common designs. Two rockets of one very common model made six flights between them, most of which went just fine. The overall pattern of the plot of 10ft(fps) to 0-10ft(g) pretty much matches that of low to moderate acceleration rockets that flew straight.
High T:W rockets were not immune (Acceleration in the graph above is a reasonable stand-in for T:W). Most ‘swervy’ rockets were low power, without a measured rail speed (but speed at 10ft did not indicate any issues there). The fastest veering rocket with a measured rail speed was 83fps off the rail - no reported weight, but it flew on a G64. The other large rockets that swerved were less than 60fps off the rail.
It should be noted the Brothers, Oregon desert launch site is prone to gusty winds, at times approaching the 20mph limit. There is no way of knowing the wind above a particular launch pad, but there were often perfect flights in visible wind immediately before and after questionable ones. Wind is certainly a factor but definitely not the only factor.
What does this all mean? It would take ten times this number of flights to permit meaningful statistics, but I think it is fair to claim that rockets that don’t meet the 45fps and 5:1 thumb rules will often fly straight, and rockets that don’t fly straight will often meet both rules. I don’t know if I can say anything definitive beyond that.
If I had a takeaway, it would be don’t take anything for granted with rockets that weigh more than a few ounces. Speed is good, but anything you can do to get a clean ignition probably also helps – your motor needs to be at its best in the first hundred milliseconds of flight and, in practice, quite a few are not. Standoff between the flight line and pad is your friend on windy days.
Be careful out there . . .
I did a few calculations from 480fps videos shot at Oregon Rocketry’s NXRS, but did not really dig into them at first – the above poll discussion drew me in.
First is the most basic, and most reliable. The speed measurements were very straight-forward, and the likely error is well within 10%. We’ll start with speed off the rail, grouped by motor class. Plotted are 81 launches. Not many low power – most low power launches were from a rack, the rods were not visible on camera (and if they were, it would have been hard to tell which a given rocket was on). One in six launches failed to reach 45fps off the rail.
Next up is speed at a standard distance, 10 feet, for 122 flights (including many more low power rockets). I also tracked average acceleration, which in this case is simply the speed divided by the time from liftoff to 10ft. This is a little less precise – a significant number of rockets did not just lift cleanly. It often took some time for the motor to reach a steady burn and motion before that ranged from a little to very erratic. Still, probably reasonable numbers.
From here, things get a little more scattered, but still interesting. For 65 flights, I had weight and motor type from the flight cards. These were self-reported and some weights were missing, a few probably incorrect (dry weight, etc.). I’ve graphed weights vs initial thrust reported in Thrust Curve. This is not ideal – the nominal 0.5 second interval averaged by Thrust Curve rarely if ever exactly matches the time interval of travel on the rail (which usually ranged from about 0.1 to 0.35 seconds), but it is the best we’ve got. Even a very detailed calculation (based on launch weight and the sim files) probably would not help – many, many motors did not ignite immediately, and quite a few were still smoothing out long after they hit 5% thrust. Short of a high rate accelerometer AND a calibrated thrust cell on board, we will probably have to be satisfied with a general idea of thrust/weight. Five launches were below 5:1 T:W, only one of these was a low power rocket.
Finally, I made a judgement call on stability. Most flights were (within the field of view of the fixed camera) arrow-straight. Most of the rest veered only slightly. I measured the deviation (from the camera POV) of the roughly 10% that were at least 10 degrees off course. I could have missed deviation in my line of sight, but I had a reasonably good vantage point relative to the wind, and the worst performers of the event were clearly apparent.
All of these rockets passed the 45fps and 5:1 thumb rules. Only two were scratch built, and most of the others were very common designs. Two rockets of one very common model made six flights between them, most of which went just fine. The overall pattern of the plot of 10ft(fps) to 0-10ft(g) pretty much matches that of low to moderate acceleration rockets that flew straight.
High T:W rockets were not immune (Acceleration in the graph above is a reasonable stand-in for T:W). Most ‘swervy’ rockets were low power, without a measured rail speed (but speed at 10ft did not indicate any issues there). The fastest veering rocket with a measured rail speed was 83fps off the rail - no reported weight, but it flew on a G64. The other large rockets that swerved were less than 60fps off the rail.
It should be noted the Brothers, Oregon desert launch site is prone to gusty winds, at times approaching the 20mph limit. There is no way of knowing the wind above a particular launch pad, but there were often perfect flights in visible wind immediately before and after questionable ones. Wind is certainly a factor but definitely not the only factor.
What does this all mean? It would take ten times this number of flights to permit meaningful statistics, but I think it is fair to claim that rockets that don’t meet the 45fps and 5:1 thumb rules will often fly straight, and rockets that don’t fly straight will often meet both rules. I don’t know if I can say anything definitive beyond that.
If I had a takeaway, it would be don’t take anything for granted with rockets that weigh more than a few ounces. Speed is good, but anything you can do to get a clean ignition probably also helps – your motor needs to be at its best in the first hundred milliseconds of flight and, in practice, quite a few are not. Standoff between the flight line and pad is your friend on windy days.
Be careful out there . . .
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