As I'm planning to use the APM2 to pilot a glider back from near space there are quite a few unusual questions etc that I need to clarify - so apologies for the many questions!
Originally I had thought about using my Lassen iQ, as this works about 18km (i.e. actually implements the CoCom limits properly). However, after reading the advice about avoiding NMEA if possible I've had a think and should be able to fly with the stock gps on board the APM2.
Firstly there isn't much "air" above 18km so it would probably be in free fall anyway therefore there would be no need to use gps. So - if I was to use the stock gps on the APM2 with the APM code - is there any software altitude limitations that I need to be aware of?
Also, a side question if any knows the answer - would I be able to fly the APM2 with GPS ON throughout the duration of my flight (up to ~30km) and then it would just work when it's below 18km? Or would I need to reboot the GPS below 18km for it to work again?
Many thanks :)
Fire up Xplane and HIL simulation for its Shuttle model and test out the altitude limits.
Ardupilot should keep running if it loses a GPS fix, which it will above that altitude, so you don't need any sort of reboot... if it gets a fix back it should start using it.
As you think about design... a couple of concerns. One - it's going to be falling FAST initially... as the atmosphere thickens a stable and balanced airframe will pull it's nose up... potentially WAY above the airframe's VNE... potentially ripping your wings right off?
Also, gps has trouble establishing a fix when it's in a dive.. and this will be compounded by the fact that the antenna will probably be oriented to point up towards the satellites when the aircraft is in level flight...
So.... as you think about solving the first of those issues (parachute that cuts away at a certain altitude... software designed to keep the plane in a dive until a certain altitude and then safely pull out... etc...) the second issue becomes a real issue.... your plane will lose lock when it crosses the 18km line, and may not get it back when you'd expect it to... so it may not know to cut the chute loose and fly, or when to pull out, etc.
Plus you want to know how high the balloon goes, right? I'd stick with a gps that can handle the whole flight and deal with the NMEA parsing issues... and then I'd design my vehicle for a descent profile that will do its best to keep the gps working and/or make command descisions based on when the barometer starts working again and/or make it stable and balanced and design it to not tear apart when it suddenly pulls out of a miles-long dive....
It's a big challenge!
We're talking about a small drogue parachute to keep the aircraft nose down and at a reasonable speed... a co2 supply seems like a pretty heavy thing to haul all the way to the edge of space just for stabilization.
There are a lot of aerodynamics factors involved... and I'm hypothesizing myself here, and would love someone with more aerodynamics knowledge to check me, but... If the aircraft is stably balanced... it should come nose down and then try to pull up and fly on its own... but at a high rate of speed. So while the aerodynamic forces of travelling at 200miles per hour in the upper atmosphere may be similar to the aerodynamic forces of travelling at 50 miles per hour at a lower altitude... Now, due to Reynolds numbers and the way they affect parasitic drag and turbulence (in a way that is beyond my understanding at the moment) flying a plane at one speed at a high altitude isn't exactly the same as flying the same plane at a lower speed at a lower altitude- there will be limits to how high up the wing can work... but as long as CG is forward of CP, the thing will turn around and fly straight pretty early in the fall. I'm less worried about the aerodynamic forces and more concerned with the g-forces.... as the air thickens the inertia of the fuselage and payload rocketing downward and the wings being slowed as the terminal velocity lowers is a real concern...
So we're looking for a solution that slows the craft at the same time it stabilizes it. All off this is just me talking theory! There may well be factors I don't understand... Somebody needs to throw some airframes off a balloon and watch what happens! :)
I agree, currently I'm moving my project forward with 2 vehicle designs:
1) is a standard glider made out of styrofoam - that I'm building
2) Flying wing - that a company is building for me
I think the airframes will be able to handle the G's given appropriate strengthening - I may fibreglass over them (if so they certainly will be able to handle re-entry :P).
I'm not planning to use a drogue as that adds to weight and would require extra cut down etc..
I've been trying to figure out how to get the APM to operate some cold gas thrusters (CO2) to help stabilize the free fall so the plane stays in the right attitude at high altitude.
Wow that's sound pretty ambitious. How about using solenoid valves operating with the PPM signals going to the all the control surface servos?
400 miles!? the space station is at 250 miles. im also designing a high altitude glider and i am using the balloon approach. i had thought of a rocket to give it height but didnt know what to use. which one are you planning on using???