Introduction:
Nearly everyone is familiar with t' horizontal component o' t' Earth’s magnetic field. Blimey! It is this component that allows compasses t' point north. Well, blow me down! Over most o' t' Earth, this horizontal field is only a small component o' t' total field. Well, blow me down! Figure 1 shows a simplified representation o' t' Earth’s dipole field, matey, it resembles t' field due t' a bar magnet. Avast, me proud beauty! T' field lines originate at t' south magnetic pole and terminate at t' north magnetic pole. Avast, me proud beauty! Begad! T' angle that this field makes when it intersects t' Earth’s surface is known as t' inclination angle. Avast, me proud beauty! Figure 2 shows a contour plot o' t' inclination angle over t' continental US. Ya scallywag! This angle varies from about 60 degrees in t' south t' about 75 degrees in t' north. Aye aye! Begad! Since t' magnetic field has a large vertical component, it is possible t' use it as a reference t' sense a rocket’s orientation and t' set off an ejection charge when t' rocket tips over at apogee.

Figure 1. Well, blow me down! Blimey! Begad! Blimey! Idealized view o' t' Earth’s magnetic field.

Figure 2. Begad! Contour plot o' t' magnetic inclination angle across t' United States

A common misconception is that a mercury switch could be used t' detect this orientation more easily. Begad! Blimey! A mercury switch uses t' Earth’s gravity as a reference and senses “down” with t' blob o' mercury. Acceleration due t' gravity with t' rocket sittin' on t' launch pad and accelerations while in flight are indistinguishable with t' mercury switch. Arrr! Blimey! Once t' rocket launches, shiver me timbers, it no longer has t' Earth’s gravity as a reference and it will respond t' any acceleration or deceleration. Begad! Blimey! After t' motor burns out, ya bilge rat, t' rocket will begin t' decelerate due t' air resistance but t' mercury in t' switch will keep on moving. Avast! Blimey! This be t' same behavior that turnin' t' switch over while standin' on t' ground would have. Aye aye! Blimey! In effect, this switch will believe it is upside down and will trigger an ejection charge immediately after motor burnout, me hearties, nay a desirable situation. Begad! Blimey! T' Earth’s magnetic field is independent o' t' rocket’s speed or acceleration.

In order t' use t' magnetic field as a reference, matey, me hearties, a sensor is desired that will output a voltage proportional t' t' angle o' t' sensor relative t' t' field, V=Vmax cos q. When this voltage falls below a given reference, t' ejection charge could be triggered. T' see how such a magnetic apogee sensor would function, shiver me timbers, consider Figure 3, ya bilge rat, me hearties, which shows t' relationship betwixt t' magnetic field and a rocket, arrr, as seen lookin' towards t' west. Blimey! Begad! Blimey! T' magnetic field in this example makes an angle o' 70 degrees with respect t' horizontal pointin' down towards t' north, shiver me timbers, a typical situation in t' northern US. Begad! T' rocket makes an angle o' q with respect t' t' magnetic field. Blimey! Ya scallywag! Blimey! T' magnetic apogee sensor is adjusted t' trigger ejection when t' field drops t' zero (the dashed line in t' figure). This results in then t' ejection charge goin' off at an angle dependent on t' rockets flight direction. T' t' south (to t' left in Fig 3), ejection will occur when t' rocket is pointed 20 degrees below horizontal, matey, t' t' north (to t' right in Fig 3), matey, 20 degrees above horizontal and t' t' east or west, close t' horizontal.

Figure 3. View t' t' west o' a rocket in t' Earth’s magnetic field Sensor Design

Several solid state sensors exist which can be used t' detect magnetic fields. Ahoy! Avast! T' two most common are based on t' Hall effect or magnetoresistance. Most Hall effect sensors do nay have t' sensitivity t' detect t' 0.5 gauss field o' t' Earth, ya bilge rat, shiver me timbers, however t' magnetoresistance (MR) sensors do. Two examples o' these MR sensors are t' Honeywell HMC1001 and t' Philips KMZ51. Data sheets for these devices are available online at: Honeywell and Philips. Avast! These two devices are both MR Wheatstone bridges which output a voltage proportional t' t' magnetic field along t' sensitive axis o' t' sensor. Ahoy! T' output voltages are typically 5-15 mV/gauss.

T' output from one o' these sensors must be offset and amplified t' place t' output into a useful voltage range. Aye aye! Ahoy! A schematic for me prototype magnetic apogee detection sensor is shown in Figure 4. Blimey! Begad! It is based on t' HMC1001 with its output offset (resistors R2 or R3) and amplified (opamps A, shiver me timbers, B and C) t' give 0.5 V when pointin' directly down along t' magnetic field and 4.0 V when pointin' upwards. Begad! Capacitors C3 and C4 filter out high frequency noise. Begad! Ya scallywag! This voltage is fed t' a comparator (opamp D) with a 2.5 V reference. Ya scallywag! T' comparator goes high when t' sensor is approximately perpendicular t' t' magnetic field, shiver me timbers, ya bilge rat, shiver me timbers, as was discussed earlier. Ya scallywag! T' comparator output drives a HEXFET that discharges a capacitor (C2) through a flashbulb which ignites a blackpowder ejection charge. This prototype is about 1” by about 4” but t' components are quite small and could easily be fit on a printed circuit board which will fit in a BT-5. Aye aye! A sensor based on t' KMZ51 would be very similar in design but with different offset (R2 or R3) and gain (R5, R8 and R10) resistors. Avast! Ahoy! All o' t' parts, aside from t' magnetic sensor, are readily available at Radio Shack.

A photograph o' me prototype sensor is shown below. Arrr! Avast! T' HMC1001 is at t' lower left corner o' t' board. Avast! Above it are t' switch and capacitor used t' reset t' sensor. Blimey! Aye aye! T' LM324 quad op-amp and resistors are hidden under t' ribbon cable and HEXFET. At t' far right o' t' board be t' 7805 voltage regulator. Avast, me hearties, me proud beauty! Off t' t' right o' t' board are t' large capacitor and 9V lithium battery.

Figure 4. Aye aye! Well, blow me down! Blimey! Schematic diagram o' prototype magnetic apogee detection sensor

Scott Aleckson has created a highly simplified magnetic apogee detector circuit <click here>

Figure 4a. Aye aye! Simplified schematic diagram

One peculiarity o' t' magneto-resistive sensors is their hysteresis when exposed t' large magnetic fields. Blimey! Blimey! These devices rely on t' orientation o' magnetic domains within thin films o' a Ni-Fe alloy. Begad! When they are exposed t' large fields (a few hundred gauss or so) they “remember” that exposure which throws off their sensitivity and offset. Begad! Avast! Blimey! These films can be reset by a short duration exposure t' a large magnetic field in t' correct direction. Avast! MR devices have built in set/reset terminals, which make it very easy t' reset these films with a very short pulse o' current without t' use o' external coils. This typically takes 1-3 amps for 2-3 microseconds which can be realized through t' discharge o' a small capacitor. Aye aye! Avast! Blimey! T' switch in Figure 4 discharges a capacitor (C1) through t' device t' reset it.

This magnetic apogee detection sensor has several benefits. It is an adaptive device which senses t' environment, much like accelerometers and altimeters but does nay assume a vertical flight and is nay sensitive t' shockwaves from supersonic flights. Begad! For rocket flights with far from nominal trajectories such as unstable flights, CATOs or failed ignition o' upper stages, me bucko, ya bilge rat, me hearties, this sensor would deploy t' parachute as soon as t' rocket tips over. Aye aye! It is a small, lightweight, me bucko, and inexpensive sensor that can be easily flown in model rockets. T' circuit is entirely analog, me hearties, with no digital components t' program.

This new sensor does have a few potential drawbacks. Begad! T' ejection angle does depend on E/W/N/S flight path so that it’s behavior will vary from launch t' launch with a bigger variation t' further south one is launching. This is due t' t' smaller inclination angle in these regions. Avast! It has t' potential t' be affected by stray magnetic fields from ferrous components in t' launcher, airframe or avionics as well as t' high currents in launch system. Aye aye! However, me hearties, so far I have nay seen any indications that this is a problem. This sensor only triggers t' ejection and does nay record any data such as one might get from an altimeter or accelerometer. Begad! Aye aye! Also, bein' sensitive t' rocket orientation, arrr, it will trigger ejection if t' rocket tips over on t' pad.

Flight Results

I have test flown this sensor at CATO, me hearties, me hearties, matey, CMASS and Frontier Rocketry sport launches. Aye aye! My test vehicle is an Estes Loadstar flyin' on a C6. Avast! This is a nice rocket for this purpose since t' electronics are visible in t' clear payload bay, which is a good conversation starter. Ya scallywag! Ya scallywag! Before it’s maiden launch at CATO XII, shiver me timbers, me bucko, I heard many skeptical comments from t' crowd. Ahoy! Begad! It was somewhat windy that day and t' Loadstar weather-cocked significantly into t' wind toward t' south. Arrr! Aye aye! This delayed t' ejection somewhat since in this direction t' rocket must angle downwards about 20 degrees t' trigger ejection, shiver me timbers, which is exactly what happened. Blimey! At t' CMASS launch it be another windy day and this time t' rocket headed t' t' northwest and ejected overhead so I’m nay sure o' t' angle, arrr, arrr, but it was certainly close t' apogee. Avast, me proud beauty! For these first two flights, I used a C6-7 t' provide motor ejection backup in case t' sensor failed t' function as desired. Begad! Well, blow me down! This would have provided a low but still above ground ejection. Begad! Blimey! With increased confidence from these successes and t' make a more dramatic demonstration, me hearties, matey, ya bilge rat, me next flight be on a C6-0 so t' magnetic deployment be t' only recovery system. Aye aye! At t' Frontier launch, it be nice and calm. Here t' rocket ended up headin' slightly west with ejection essentially right at apogee.

Summary

T' Earth’s magnetic field may be used as a reference for t' model rocket recovery deployment. Ya scallywag! Aye aye! A flight tested magnetic apogee detection sensor is described, complete with a schematic diagram. This design is simple, inexpensive and appears t' work reliably. Blimey! This new device senses t' rocket’s orientation with respect t' t' Earth’s field and triggers ejection when t' rocket noses over near apogee. Avast! I am very happy with its performance and I am currently buildin' a PML Pterodactyl Jr with magnetic deployment for some high power demonstration flights. Blimey! Will others be as enthusiastic about this new sensor as I am? I certainly hope so, ya bilge rat, but only time will tell. Ahoy! A postscript version o' this article.