Joel Levinson
Member
- Joined
- Nov 12, 2018
- Messages
- 6
- Reaction score
- 11
How high was "not the altitude we wanted"?
142K. We might have made 275K. It will take some additional analysis to determine what happened.How high was "not the altitude we wanted"?
Yes, I was using it for all flight events, except for stage separation, which was done from below. I haven't lined up every event to confirm the timing, but so far as I can tell, Kate 2 did everything correctly. I found it very easy to work with. When it turns out that most flight events will be done via gps, you don't have to think as hard about "how" the altimeter will calculate the various quantities, and the programming is easy. With other altimeters, you have to worry about whether the accelerometer is accurate, how things are filtered, is the altitude spoofed, etc. This is what makes the programming hard. With Kate, it is easy by comparison. I'm not sure this is 100% good, but it is my experience.Hi Jim,
so do I recall correctly that you were using Kate2.0 with the pyro board for all your flight events? Any feedback on how that was to work with? Happy with staging, etc? Mine had more packet loss than I expected, but obviously there is no post flight analysis for mine...
br/
Tony
My opinion may change over time (with more time and expertise applied), but I believe we had a little lunar coning going on during the sustainer motor burn. Although there is IMU (tilt) data suggesting this coning, the picture at 10:18 into the video is the most compelling evidence. And, the video shows one revolution of the rocket for each coning revolution (thus, "lunar" coning). There are several potential reasons for this, but one consequence is that this motion apparently translated into tumbling rather early in the flight - around 110K I believe, but I haven't verified this precisely. In my 175K flight, tumbling from 140K cost about 10K in altitude. Therefore, I suspect we lost more altitude than that in this flight.Very cool flight. It looks like the rocket was tumbling for the last 20,000ish of flight. Am I interpreting that correctly?
I don't think the spin up is a bad as it appears in the figure. Around 2 Hz on the average (the initial rate was higher) but definately "lunar" coning from the video. Lots of possible factors, including being quite overstable, uneven ablative, passing through Mach 1 at the start of the sustainer burn, not particularly dynamic balanced - the usual suspects. I think our experience on the packets is similar. I know Vern is looking at it.Hi Jim,
That thing really spun up during the second burn, any theories on it? Also how did your flight do on lost packets? Ours had a big drop out on the way up and reconnected near apogee.
I see the big difference between your predicted motor burns and the actuals. It looks like the classic r=a*P^n model is not sufficient. I'm glad it didn't come apart.
br/
Tony
I see the big difference between your predicted motor burns and the actuals. It looks like the classic r=a*P^n model is not sufficient. I'm glad it didn't come apart.
From the link: "For composite motors, a spin induced acceleration of at least 10g's is required before appreciable burn rate augmentation results. Is this a concern for spin-stabilized amateur rockets, then? A simple calculation shows that for a motor with a diameter of 4 inches (10 cm), a spin of 420 RPM is required to develop a 10g acceleration normal to the motor axis. Such a high spin rate is well beyond that required for stabilizing, so for amateur rockets, acceleration augmented burn rate is not a concern."Spin rate is well known to enhance motor burn rate.
"lunar" coning
https://apps.dtic.mil/dtic/tr/fulltext/u2/1020435.pdfJim what the heck is "Luna coning"?
I think the sharp rise in the gps data is just bad data. The rise in the inertial tilt might just be a slight weathercocking. I see a little bend in the trajectory in the video.on the tilt plot, right at launch there's a sharp rise, then drop, then a slowly rising line until sustainer ignition. Is this the rocket coming off the rail and then the fins doing their job and straightening out the rocket?
edit: what figure would indicate spin rate?
https://apps.dtic.mil/dtic/tr/fulltext/u2/1020435.pdf
"Lunar coning is the motion whereby the projectile flies at a constant angle, αα, with
respect to the freestream velocity vector while undergoing a constant angular
rotation, Ω, about a line coincident to the freestream velocity vector passing through
the center of gravity of the projectile."
This is my dog Luna, with a cone, i.e., Luna coning:Jim what the heck is "Luna coning"? Google was not my friend on this one.
I have seen, as well as read about, coning, roll pitch coupling, inertia coupling, inertia roll coupling, but "Luna" is a new one!
So, the oscillation rate in the tilt graph is also the roll rate. If it just rolled and didn't also cone, then the tilt value would remain constant, in theory. The graph would suggest a constant angle of attack of 3 degrees, but we are not convinced that this is actually what happened.
Jim
Vern can see the roll rate in the data file, and he says it was around 800 degrees/sec (2.2 Hz) until the rocket started slowing down. The gyro range is to 2000 degrees/sec.The frequency probably is a good estimate of the roll and cone rate, however the amplitude(s) of the tilt are not consistent to your video. Especially in the 3rd regime of the data up to apogee.
The frequency is very very high at sustainer ignition. I can tell from the graph but is it possible that the roll rate exceeded the limit of the gyro there?
What do you mean about the amplitude of the tilt? Just looking at the video, the first time the rocket goes to 90 degrees is at 63 seconds into the flight. That agrees with the inertial tilt. By the way, the speed was still above Mach 2 at that point.
Jim
Please keep asking questions. We might give this another try at some point, and it would be nice not to just repeat it.Ok, you are right. Nevermind.
We considered spinning this rocket. The sustainer fin can could have been made to do that. The reasons that we didn't go that way include the difficulty of spin balancing a rocket, no way to un-spin it, and Kate objected. On the spin balancing, I heard that this was difficult to do. Imagine trying to spin a rocket 400 rpm, measure that it's out of balance and then add weight appropriately to correct it. I don't know how to do that.Maybe you need to either purposely impart a spin or need larger fins in order to stay stable at the high altitudes? I've not done any of this extreme stuff but am trying to learn and understand it.
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