From f359ad7b0b9231c0ac5b4b14797cbb088a476f38 Mon Sep 17 00:00:00 2001 From: Bdale Garbee Date: Thu, 27 Apr 2023 12:45:58 -0600 Subject: [PATCH] add another answer --- FAQs/apogee-above-100k.mdwn | 88 +++++++++++++++++++++++++++++++++++++ 1 file changed, 88 insertions(+) create mode 100644 FAQs/apogee-above-100k.mdwn diff --git a/FAQs/apogee-above-100k.mdwn b/FAQs/apogee-above-100k.mdwn new file mode 100644 index 0000000..b4fba3b --- /dev/null +++ b/FAQs/apogee-above-100k.mdwn @@ -0,0 +1,88 @@ +# Can I fly your products to an apogee over 100k feet? + +Our flight computers use a Kalman sensor-fusing filter to estimate the +flight state, which consists of three values: + + 1. Height above ground + 2. Vertical speed + 3. Vertical acceleration + +Apogee is assumed to be where vertical speed crosses zero. + +Below 30km altitude (about 100k'), we use both the barometer and the +accelerometer to update the flight state, along with a basic Newtonian +model of motion. That works super well, pegging apogee within a few +sensor samples essentially every time. + +Above 30km, the barometric sensor doesn't provide useful data, so we +can't use it to update the flight state. Instead, the Kalman filter +falls back to a single sensor mode, using only the accelerometer. + +At all altitudes, we de-sense the barometric data when we estimate the +speed is near or above mach as the sensor is often subjected to +significant transients, which would otherwise push the flight state +estimates too fast and could trigger a false apogee event. + +That means the filter is no longer getting the benefit of two sensors, +and relies on just the accelerometer. The trouble with accelerometers is +they're measuring the derivative of speed, so you have to integrate +their values to compute speed. Any offset error in acceleration +measurement gets constantly added to that speed. + +In addition, we assume the axial acceleration is actually vertical +acceleration; our tilt measurements have enough integration error during +coast that we can't usefully use that to get vertical +acceleration. Because we don't live in an inertial frame, that means +we're mis-computing the total acceleration acting on the airframe as we +have to add gravity into the mix, and simply adding that to the axial +acceleration value doesn't generate the right value. + +The effect of this is to under-estimate apogee when you base the +computation purely on acceleration as the rocket flies a parabolic path. + +For flights *near* 100k', all of this works pretty well - you've got the +flight state estimates adjusted using the barometric sensor up to 30km, +then you're flying on inertial data to apogee. + +For flights well above 100k', it's not great; you're usually going fast +enough through 100k' that the baro sensor is still de-sensed through the +end of its useful range, so the flight state estimates are not as +close. After that, as you're flying purely on accelerometer data, +there's no way to re-correct the state, so the apogee estimates can be +off by quite a bit. + +In the worst cases that I've seen, the baro sensor data was wildly +incorrect above mach due to poor static port design, leaving the state +estimate of speed across the 30km boundary way off and causing the +apogee detection to happen far from the correct time. + +The good news is that correctly determining apogee is not really all +that important at those altitudes; there's so little density that a +drogue will have almost no drag anyways. The data I've seen shows a very +parabolic path down to about 50k'-60k', even with a recovery system +deployed... + +So, what I've been recommending is to set up two apogee plans: + + 1. Use the built-in apogee detection, but add a significant delay (as + much as 30 seconds). This will probably fire near enough to apogee + to not have a significant impact on the maximum height achieved. + + 2. Add a back-up apogee which fires after apogee *when the height is + below about 20-25km*. This way, if the flight isn't nominal, and the + sustainer ends up reaching apogee in dense air, you aren't hoping + the chutes come out before it gets going too fast. And, you get a + second pyro channel firing at that altitude even if it reached a + higher altitude before. + +You can wire these two pyro channels to the same pyro device; you just +need to make sure they're wired + to + and - to - (the manual shows +which screw terminals are which). + +All of this is why we're encouraging people flying way high (like 300k') +to find a deployment mechanism which doesn't solely rely on altimeters +(like ours) which are designed for modest altitude rocketry. Besides, +flights like that probably need active stabilization to make sure they +follow the prescribed trajectory so that they don't end up outside the +waiver, but that's a whole 'nother adventure... + -- 2.30.2