Actually the first problem I think you will encounter is even being able to get an EDF jet to 30Kft. A jet turbine, if you can get it to run at that altitude which will require compressor mods most likely, would allow one to carry a fuel which has a much higher energy density than batteries carry. Altering the design to something more reminiscent of the U-2 would allow increased carrying capacity, plus greatly increase the efficiency of the wing (via increased aspect ratio reducing induced drag).
FWIW, I do some engineering in low Reynolds numbers aerodynamics. I've designed the airfoils (and in some cases the wings) for perhaps half the competitive discus launch gliders in use around the world. My work has influenced most of the rest. It's one of my hobbies but I've thousands of hours into research and development in that area, which I've mostly given away to everyone.
FWIW, I'm also looking at high altitude rocket projects. I've spent a couple years now doing a lot of research and burning several 50# buckets of AP developing propellant and motors up into the O range to get to the point where I'm seriously developing the motor for single stage to potentially > 100Kft. One of my quality of work tests is that I not only make motors which work, I can predict with a high degree of accuracy the resulting altitude given the initial conditions. Some of the predictions have been within 1%, amazingly enough. I'm happy with 5%. Essentially if I can't tell with considerable accuracy what something is going to do, how can I design something for a specific application?
I might be able to make a project similar to what you want to achieve work. But I have about 3000 hours research and development on where you are now, beyond degrees in physics and computer science. Plus I have the capability of fabricating quite a diverse range of materials into useful parts!
Anyway, as an example of the sort of things to think about, EDF setups use air flow for cooling the electric motor... At the lower air density at 30Kft ASL, there may not be enough cooling without appropriate mods to prevent the motor overheating. Just because something might work at 3Kft doesn't mean it will at 30Kft!
At 30Kft, what RPM will be achieved by the EDF unit? Air density is lower... What will be the power draw? What will be the thrust? How much heat need be dissipated? What is the equilibrium temperature of the motor?
At 30Kft, what is the atmospheric temperature? How do the batteries of choice perform at that temperature? What are their discharge characteristics? Usable capacity? During discharge, how much self-heating is available to offset heat losses to surrounding structure and air? Are the cells going to be getting hotter, stay near constant temperature, or get cooler?
To get to 30Kft, one needs to carry a lot of potential energy (batteries, or fuel). One can climb on motor power, or one can climb on wing lift. The thrust profiles will be quite different for these two cases, and there will be considerably different engineering considerations. Sketch up some crude designs for each approach, and compute out the required energy budget for each to achieve an ascent profile to 30Kft. See which approach, if either, can be made to carry that much energy (fuel, charged batteries, solar cells, fuel cells, technology of choice...) in a design of tolerable size. Look up cubic loading...
At higher altitudes due to reduced air density, a wing needs to fly faster to generate the same lift. You lose about 30% of your air density just getting to 10Kft... Assuming one stays away from transonic flow, the lift generated by a wing is roughly proportional to the speed squared. So at 10Kft a wing would have to fly sqrt(1/0.7) faster to maintain the same lift. The greater the altitude, the faster it has to fly. With an EDF unit one might find that one is moving out of the efficient RPM range of the unit to achieve the required thrust to cancel airframe drag losses and provide enough excess power to achieve a climb rate.
Anyway sorry for rambling. I'm just pointing out a few of the things one would need to carefully consider on such a project on the technical side.
There is also the regulatory and insurance side. This won't fit what would be allowed under AMA for the RC portion, nor under NAR or TRA for the rocket portion. What this means is that you have a lot more work ahead of yourself to get the FAA approval for flight(s), to get permission to launch the project from a site, to get insurance coverage for the launch which is likely required by the site owner, and there may well be additional regs that need to be met and approvals that need to be acquired. That side by itself is a project.
Depending on the technology used for the RF control link, there could also be FCC regulations to contend with. Do you have an amateur radio Technician's license? That might simplify some of those issues, plus you'd be familiar with them.
To have good odds of success in a project of this magnitude, it helps to map out each step of the project in detail. For each step, detail how each subsystem will perform under the conditions present during that step. Verify that each subsystem will perform nominally under those conditions to the extent possible (design work and/or testing). Determine possible failure modes, and contingency plans for those failure modes which cannot be eliminated or ameliorated through design or procedure.
In the situation where you are now, you do not yet have enough background knowledge to know how much of this endeavor there might be for which you do not even know the appropriate questions to ask. That is the normal starting point for any truly ambitious project!
I do not know your background. What I suggest is the following:
Getting through L2 certification including 2 stage project work. After all, what you are attempting is a 2 stage project, where the first stage is a plane. So some knowledge and experience will transfer.
Study up on physics specifically. You'll have a greater appreciation of the task at hand, and be developing the cognitive and analytic tools needed to tackle various aspects of the project.
Become good at making things. There will be plenty to make on this project!
Get current with FPV technology if you are not already current! Get experience with it as well, but stay legal!
Consider getting a HAM Technician's licence. At 30Kft you may be playing with antennas and desire higher power output, and possibly working on different RF bands than the hobby FPV systems use. A Technician's license will open up more possibilities. Study up on the RF propagation and electronics side of things beyond the license requirements so you have a better theoretical and practical knowledge of issues which might surround the bidirectional control system you need to have working extremely reliably for this project.
Etc.
Gerald
PS - I only put some of my background in this post to indicate I have an above average clue about some of the issues with a project such as yours, rather than talking completely out of my @#$. Background is what you acquire if you keep an inquisitive mind and keep attacking projects! The project you contemplate will take you a number of years to achieve. Break it down to areas of expertise and experience required. Tackle each in the order which it pleases you. When you have sufficient expertise over a sufficient portion of the areas required for the project, then you can consider starting to take a serious stab at it. Good luck!
PPS - The other approach is to become expert on at least one of the areas required, and team with other experts until the project is covered.
