Interesting.

Are you guys trying the more unconventional approaches because you feel like the rockoon/solid rocket approaches aren't likely to be productive since so much effort has already been invested in them, or are you trying something different for the sake of variety and challenge?

It does seem like a few teams have considered (albeit none took to any stage of testing or construction AFAIK) something similar to your micro clustered liquid fuel rocket path.

It's a bit of both really - a lot of effort has gone into rockoon approaches and, although not trying for orbit, the ongoing The Register LOHAN endeavour is also a rockoon so there is a lot of work there which we don't really want to reprise.

To us the clustered liquid fuelled micro-rocket looks most likely to succeed and to provide an ongoing technology that is scalable and so that is going to be our focus of analysis. The use of MEMS as a technology baseline also fits well with providing femtosatellites with useful capabilities.

see Microlaunchers both an entrant here, and an attempt to do in space what microcomputers did (google the word).

we've always thought an off-the-ground 3 stage liquid rocket is the only way. For a 1 kg to LEO or 200 gram to escape
the rocket would have a gross mass (GLOW) of about 150 kg but a smaller one for just NP could be one discussed in 2009:
ASA meeting http://www.microlaunchers.com/7816/L3/sa09/sa09.html .

The rocket, described in an Amazon book MICROLAUNXCHERS will use liquid oxygen and propane or butane for fuel.

Out of curiosity, does anyone know the feasibility of ultra-high altitude flight using relatively small (large model) unpiloted aircraft?

The concept is feasible, but the aircraft would not likely resemble anything you’ve ever seen flying. The wings would have to be very, very long and slender (extremely high aspect ratio). Propellers would be ineffective in the vanishingly thin air at “ultra-high” altitude unless the blades were also very, very long and slender. With a conventional airplane configuration, just getting such a flimsy and ungainly contraption off the ground would be difficult, to say the least. Tiny (hobby-scale) jet engines are available, however, so there is still hope.

Quad-copters have become popular only because they are so much easier to control than conventional helicopters. Unfortunately, they are also far less efficient, both in terms of aerodynamic efficiency and power-to-weight ratio. The highest altitude achieved by a conventional helicopter specially designed to operate at such elevations is 40,820 feet (12,442 meters). In this case, 25,000 meters is definitely “ultra-high.” Even a quad-copter designed to function in such thin air is likely to run out of fuel (or electricity) long before reaching your goal.

One possibility is a torqueless “rotary flying wing” configuration, in other words a helicopter that is all rotor with little or no fuselage. Torqueless rotation, driven by small jet engines at or near the tips of the “wings” or rotor blades, eliminates the need for the multiple counter rotating rotors and multiple motors of a quad-copter as well as the need for the tail rotor and the complex, very expensive and comparatively heavy power train of conventional helicopters. Centrifugal tension in the rotating wing would provide stiffness that would otherwise be absent in such an extremely lightweight and elongated structure. The centrifugal effect would also deliver fuel from the hub to the engines at remarkably high pressure using only a small cheap conventional fuel pump. Such high pressure would be welcome in tiny high-performance high-altitude jet engines. I’ll leave the design of the unique flight controls up to you (and Rube Goldberg)!

The concept of torqueless tip-driven helicopter rotors has already been proven by major manufacturers and several variations have been produced, although with little commercial success. In the 1950s, using “cold pressure jets” (compressed air jets rather than jet engines), the Djinn by Sud-Ouest was the most successful. Small jet engines mounted on the rotors of other designs worked exceptionally well when the engines were running, but produced too much drag in autorotation, such as during an engine-out emergency, and so were abandoned as a risk to the safety of passengers. This ought not be a concern with a “telenautic” aircraft on the scale of a (very) large model.

Telenautic? Yes, I made that up (from the Ancient Greek tele = far away or remote, and nautes = sailor or pilot). What do you think? Will it fly?

Jay