Each of the stages has four fins, which won't do much on the accent, since we are hopefully starting very high from a balloon, but after they are jettisoned, and the stage starts falling back, a drogue chute is deployed out of the top of the rocket as is usual, but the system that ignites the ejection charge also unlocks the fins from their vertical position. Inside each of the fins is an embedded 2.5Ghz (802.11B wifi freq) yagi antenna. Basically several electrically isolated strands of wire that guide RF frequencies towards a single antenna to create a directional antenna. Old TV antennas on roof tops are yagis, but since these are antennas for a much higher frequency, the elements are much smaller. The elements are approximately half a wavelength of the frequency or in this case about 1.5 inches (3.74cm). Each of these four directional antennas are hooked up to a crystal tuner where the incoming beacon signal from the ground transmitter is compared and then used to slightly turn the fins perpendicular to the best signal to steer the rocket towards the signal. When a predefined power level is received, meaning that the returning stage is very close to the transmitter, the second chute is deployed to provide a soft landing.
All that you need on the ground is a powerful transmitter that you can use to guide the returning stages back to your retrieval point. All you need on the stage is a watch battery, four crystal tuners, twin comparator circuits and piezo deflectors for the fins.
I’d rather not use this frequency because of all the background noise, so it could be redesigned for 23cm ham bands by using tiny coils of wire instead of straight strands, but I think if you are pointing a Cantenna or parabolic dish type antenna powered by a legal limit transmitter, you should down out all the background noise. Also you could go up to 10GHz ham bands and make really small antennas, but that might have problems with clouds.-- MisterQED, Apr 04 2008 GPS limits https://en.wikipedi...ral_Export_ControlsGPS limits on altitude and/or speed [MisterQED, Feb 26 2019] Golf?-- normzone, Apr 04 2008 The homing system is very much that of guided missiles nowadays.
Wouldn't four fins do a lot to any accent?-- loonquawl, Feb 25 2009 I guess it's a choice between guiding the return to a target zone, and using a tracker to find where it lands. According to Ed from CUSF (on the N-Prize Google group), he can sell you a teeny tiny GPS transponder which will text coordinates from the rocket to your mobile.-- MaxwellBuchanan, Feb 25 2009 //Wouldn't four fins do a lot to any accent?// What, you mean like make it sound less Geordie and more like a Scouser?-- coprocephalous, Feb 25 2009 I think he meant "assent".-- MaxwellBuchanan, Feb 25 2009 //I think he meant "assent".// You mean he wouldn't get permission to launch?-- AbsintheWithoutLeave, Feb 25 2009 Probably not. It's an environmental thing to do with the odour left behind by the burning fuel. So it's basically a problem with a scent.-- MaxwellBuchanan, Feb 25 2009 Hey wow, someone read this!
[MB] Yes, SparkFun sells both a mini-GPS and a mini-GSM boards that will allow you to get texts of the rockets position once it is below the max altitude. And although they are very light, they are no where near this light and every gram counts when you are going ultra cheap.
This system is for recovering the first stage rocket where the stage will launch almost vertically. It is also handy if you are doing a water based launch where landings could be damp and irrecoverable like Prometheus' could be launching over the Gulf. Steering a chute from high altitude might get you all the way back to the launch site as long as the winds are low.-- MisterQED, Feb 26 2009 If you're willing to either a) give your LV radio homing-based descent guidance, or b) put a GPS receiver on it for tracking, why not combine the two and just use GPS-based descent guidance? It should be more reliable than (a), it would be trivial to implement if you're already doing (a), and it would probably be lighter than (a). And rather than using fins for descent, I'd use a steerable parachute/parafoil, to reduce drag during ascent and to have a gentler descent with more horizontal range to return to the launch site.
Just make sure you label your stuff as "not for weapons use" (not that any competent terrorist would have trouble developing such a system on their own, but authorities would worry otherwise).
P.S. This time I left this annotation typed but unsubmitted for a day, though that was partly because I was tired yesterday and just wanted to go home and sleep.-- notexactly, Feb 26 2019 //If you're willing to either a) give your LV radio homing- based descent guidance, or b) put a GPS receiver on it for tracking, why not combine the two and just use GPS-based descent guidance? It should be more reliable than (a), it would be trivial to implement if you're already doing (a), and it would probably be lighter than (a). And rather than using fins for descent, I'd use a steerable parachute/parafoil, to reduce drag during ascent and to have a gentler descent with more horizontal range to return to the launch site. //
I haven't looked at this stuff for about a decade, but I think there is an altitude limitation on GPS altitude. Okay, found it, GPS is limited to 59k ft. See link.-- MisterQED, Feb 26 2019 GPS receivers are supposed to stop giving output when over a certain altitude AND over a certain speed, so as to not be useful for ballistic missiles. A lot of them have this implemented improperly, where they stop giving output when over a certain altitude OR over a certain speed, which is inconvenient for amateur high- altitude balloon projects, which exceed the altitude limit but not the speed limit.
But if you get a good one, it's fine for that and should be fine for this too. A rocket stage descending under a steerable parachute is unlikely to exceed the speed limit.
I'm pretty sure you can also easily find GPS receivers that don't have the limitation at all. You can also build your own GPS receiver from scratch, though it might not be as miniaturized as a commercial one.-- notexactly, Feb 27 2019 random, halfbakery