--- /dev/null
+[appendix]
+== Example Pyro Channel Configurations
+
+ Programming configurable pyro channels on Altus Metrum products that
+ include them isn't difficult, but in an attempt to aid understanding
+ of the configuration interface and help "keep simple things simple",
+ we offer the following examples of the simplest configurations for
+ common situations, along with some hints on avoiding unexpected
+ results.
+
+ The rich set of conditions provided can be used to configure almost
+ any pyro event you can imagine, for a wide variety of objectives.
+ But don't be fooled! Typical events need only one or a few simple
+ conditions to be configured for success. A key thing to remember is
+ that *all* configured conditions must be true to allow a pyro channel
+ to fire. Trying to include too many conditions often results in
+ conflicting rules that never allow a channel to fire. The most
+ important advice we can offer is, therefore, to try and find the
+ simplest set of conditions that will do what you need for a given
+ project.
+
+ === Two-Stage Flights
+
+ Successful completion of a two-stage flight often involves
+ programming of two events. The first is firing a separation
+ charge, the second is igniting the sustainer's (primary)
+ motor.
+
+ Separation charges are best fired as soon as possible after
+ the previous stage has completed providing acceleration, to
+ minimize drag of the sustainer's coast phase before ignition.
+ Recovery, whether the remainder of the flight is nominal or
+ not, usually works best when the states are separated. So,
+ the "best" way to configure a pyro channel for a separation
+ charge is to just set "after motor number". For a 2-stage
+ project, set this to "1". This will cause the pyro channel
+ to fire as soon as the firmware's flight state machine
+ determines the first motor has burned out.
+
+ Safe ignition of a sustainer (primary) motor requires that
+ it happen after the previous stage burns out, while the
+ airframe remains mostly vertical, and typically after the
+ sustainer has coasted away from the booster a bit. A good
+ starting point is thus "after motor number" set the same as
+ the separation charge, which is "1" for a 2-stage rocket.
+ Then "angle from vertical less than" set to some
+ reasonably vertical amount, perhaps 20 degrees. Then "delay
+ after other conditions" set for the desired duration of coast.
+ Use simulations to figure out what a reasonable value here is,
+ but for typical high power rocketry sport flights that aren't
+ trying to set records, something like 2 seconds is usually a
+ good place to start.
+
+ === Triggered Clusters and Air Starts
+
+ When an airframe has a cluster of motors, one of which is
+ "primary" and centered, surrounding by a ring of "secondary"
+ motors, you may want to use the launch control system to fire the primary motor and use onboard electronics to light
+ the rest of the cluster as soon as launch is detected. This
+ is particularly true if the primary motor is significantly
+ different in geometry and may take longer to come up to
+ pressure than the secondary motors. In this case, a simple
+ configuration to light secondary motors is is "time since
+ boost greater than" enabled and set to "0". There's
+ really no point in setting an angle limit since no time has
+ transpired for the airframe to change orientation.
+
+ Air starts can use the same simple configuration, but with
+ the time set to a non-zero value. However, if air starts
+ are going to light after the airframe leaves the launch rail
+ or tower, add an "angle from vertical less than"
+ condition just you would for a 2-stage sustainer to stay safe.
+
+ === Redundant Apogee
+
+ When flying a board like TeleMega or EasyMega, it's easy to
+ configure a programmable channel to fire a redundant apogee
+ charge. This is of course not *fully* redundant, since it's
+ always possible that the board itself or its battery could
+ the the failure source, but far more often, pyro events fail
+ due to broken wires, bad connectors, or bad e-matches... so
+ firing two charges from one board can add useful redundancy.
+
+ The simplest configuration for redundant apogee is "flight
+ state after" set to "drogue", and then "delay after other
+ conditions" set to a second or two.
+
+ === Redundant Main
+
+ Similarly to apogee, configuring a redundant main charge can
+ provide useful redundancy. What we want is to configure an
+ altitude for deployment lower than the primary main deploy
+ altitude, and then ensure we only trigger on that condition
+ while descending.
+
+ The simplest configuration for redundant main is "flight
+ state after" set to "drogue", which will ensure we're in to
+ the descent phase, then "height less than" set to a number
+ lower than you've chosen for the primary main channel
+ deployment height.
+
+ === Apogee Above Baro Sensor Limit
+
+ A question we've seen increasingly often is "How does the
+ Telemega/Easymega detect apogee for flights above 100,000
+ feet?" Flights above that height are a bit outside
+ our original design envelope, but can be made to work...
+ This is *not* a simple flight, and the configuration for it
+ is also not simple, but we think including this information
+ is important for anyone contemplating such a project with our
+ electronics!
+
+ 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 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 we have 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 high altitudes; there's so little
+ density that a drogue will have almost no drag anyways. Data
+ from customer flights shows a very parabolic path down to
+ about 50-60k feet, even with a recovery system deployed.
+
+ So, what we recommend 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).
+
+ The bottom line is that flights to altitudes modestly above
+ the range of the baro sensor with Altus Metrum products can
+ be accomplished safely, but flying "way high" (like 300k')
+ demands a deployment mechanism which doesn't solely rely on
+ altimeters (like ours) which are designed for modest altitude
+ rocketry. Flights to those altitudes also probably need
+ active stabilization to make sure they follow the prescribed
+ trajectory and stay inside their waiver.
projects / fw / altos / commitdiff