SubSonex: The Second Coming of the Microjet

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Pawing back through the archives, I can confidently declare that 2002 was the summer of the VLJ—the very light jet. Some were practically swooning over the notion of sub-$1 million jets and there was a thundering herd of them, led by the rock star of VLJs, the Eclipse.

During a round of interviewing that year and in 2003, I asked how these companies were going to deliver these breathtakingly low price points. Eclipse’s intro price was $837,000; the Safire S-26 was $800,000. We were assured that these prices would be met by a convergence of efficient, automated manufacturing technology similar to that used in auto plants and that would, in turn, instigate automotive volumes, making the price points both realistic and profitable.

Didn’t happen. There are a host of reasons why these airplane projects cratered, but one of them is that the companies made flawed assumptions about scale economy and typed in sticker numbers before they had anything beyond the vaguest idea of what the airplanes would actually cost to build. Williams tanked on development of the EJ22 for Eclipse and although Pratt & Whitney had a predictably sane development program for the PW600 series engine borne of years of experience, that yielded simpler and cheaper engines, but that’s not the same as simple and cheap in the absolute.

That this history was written in a slurry of blood and charred dollar bills isn’t because the people proposing these projects were stupid, dishonest or inexperienced. I think it’s because, as I have said before, that when certain people (including me) get within 50 feet of airplanes, their normally functioning brains turn to a mass of quivering mush incapable of anything resembling rational thought. I’m putting this idea upfront as a sort of warning as I segue into the main course of today’s blog: microjets.

Nearly 80 years after the turbojet engine first took flight, these remarkably simple engines remain all but inaccessible to light general aviation. This is so because despite their simplicity, the vaunted advent of CNC machinery hasn’t made them cheap to build and they aren’t especially fuel efficient, either. Not for nothing are Airbus and Siemens looking at electrically driven ducted fans as the powerplant for the future. But since we never give up in aviation, the small jet engine floats out there just within reach and that’s what makes the SubSonex JSX-2 such an intriguing project, if you’re willing to build it yourself.

Last month, I spent three days in the desert shooting the jet and covering my colleague, KITPLANES editor Paul Dye, learning to fly it. You can see the video on that coverage here and a full report in the August 2015 issue of KITPLANES. Veteran airshow performer Bob Carleton was instrumental in helping John Monnett develop the SubSonex and provided oversight for the flight training, which results in a kind of mashed up type rating, since jets, regardless of size, still require that.

I like to think I retain a small sliver of analytical capability when I’m around airplanes as appealing as the SubSonex and I find the numbers intriguing. The kit with engine costs about $130,000, so by the time you throw in the avionics, paint and other extras, call it $150,000 to $160,000. For a jet, even a single-seat jet, that’s not a lot of money in the context of upper strata experimentals or even LSAs. In fact, it strikes me as a decent value for a would-be builder or owner who passionately wants to fly a jet airplane. I could actually afford it myself.

It’s also not difficult to fly, but does require the somewhat cumbersome training evolution described in the video. I doubt if that would put many owners off; it certainly wouldn’t faze me. How would you use such a thing? I think it’s primarily a VFR sport flyer similar to other experimentals or LSAs. Take off of a Saturday morning and burn up 30 gallons of Jet A beating up the local area and the pattern. Great fun. (You’ll need a longish runway, however. The SubSonex isn’t exactly a STOL airplane and the brakes need improving.)  

Sonex gives the still air range as 480 miles at a 240-MPH cruise, so you could use the airplane for light travel, although it would take planning and effort. It burns about 34 GPH during takeoff and typically half that in cruise with a 40-gallon capacity. I wonder if a creative builder might find room for a little more fuel.

The engine is a 40-pound jewel of a creation from PBS Velká Bíteš, a Czech Republic company formerly in the Soviet orbit. The TJ-100’s origins are in APU applications, but it finds use in small UAVs today. Rated thrust is 250 pounds, which Carleton says is a perfect match for the airplane. When we were shooting the airplane, he explained how a jet engine’s simplicity percolates down through the airplane itself. With no heavy structure to support a piston engine and no linkages for throttles and props, the SubSonex is an exceptionally simple build. The engine is FADEC controlled and throttle by wire, so the only connections are the fuel line and a few wires. Carleton told me when he tours with his Salto glider for airshows—same engine—he unpins the engine and takes it into his hotel room at night. Try that with an IO-320.

Of course, the piper always demands his pay and that’s the other side of this jet’s coin. It has a short TBO— 300 hours. What this means exactly in operating costs is a little murky. Cost of the engine is nominally about $46,000 at current exchange rates or $153 per engine operating hour. Ouch! But Carleton told me the engines are overhauled on condition so he expects they’ll be shipped off to the Czech Republic and brought back to fresh condition for some percentage of the full cost, perhaps 15 percent as an educated guess. At the moment, there just aren’t enough engines out there turning hours to have reliably repeatable data. As the engine gains more fleet experience, the overhaul times will probably increase, so at least the cost trend is in a favorable direction.

So is this really a second coming of the microjet or just the sui generis of lucky timing? Hard to say, but my guess is that it has better legs than the BD-5 did which was, after all, somewhat of an accidental jet. It got the TR-18 turbojet when the planned piston engine development ran off the rails. The BD-5, don’t forget, was a huge seller with some 5000 kits ordered. The demand was even stronger for a factory certified version that I’m not sure could have been certified then (1972) or now, even if Bede hadn’t run out of money and folded. The kit price was about $3000 or about $17,000 in 2015 dollars. So the SubSonex costs nearly 10 times more, but it’s also intended to be a jet. There’s a lot of wealth out there looking for distinctive rides like this one.

No, it’ll never be everyman’s jet. But it’s at least accessible for those who are just hot to burn up a bunch of Jet A. If you gotta have it, you can get it. Personally, I think that makes it a uniquely cool airplane. I hope they sell hundreds.

Comments (13)

Nice video and write up. Thanks.

Posted by: Rafael Sierra | May 10, 2015 7:05 PM    Report this comment

I have a picture of a mucho younger -- and less 'cranky' -- Larry sitting in the cockpit of a BD-5 at Edwards AFB getting ready to taxi it over to the thrust stand to measure its thrust circa 40 years ago. Jim Bede had brought three of them to Edwards in his DC-3 and we actually carried them out of the Gooney Bird and then assembled them for test by USAF Test Pilot school students. Wow ... what a memory!

More recently, I was on a tour of the Sonex factory's "BeeHive" area where they were assembling -- I think -- 10 of 'em. That thing could turn MY head into mashed potatoes easily! Can you imagine what would happen if production numbers could bring that down to the sub $100K cost. Even at $130K, it's tempting ... as you say.

Actually, I think the OneX is a pretty interesting airplane. Your juxtaposition of the BD-5 to the JSX-2 is appropriate, Paul. I even know where a BD-5 airframe is being used as a wind direction indicator on a pole.

HEY ... I wasn't cranky this time !! :-))

Posted by: Larry Stencel | May 10, 2015 10:07 PM    Report this comment

Very interesting--and a most enjoyable video. I don't think I'll trade off my P172D very soon, but I have an idea this will be a successful venture for Sonex. Wish I was younger!


Posted by: Cary Alburn | May 10, 2015 10:11 PM    Report this comment

Boy ... you caught MY interest, Cary. I never heard of a P172D ... had to look it up. Interesting. How many of them were built?

Posted by: Larry Stencel | May 10, 2015 10:30 PM    Report this comment

Thanks to the FAA's assumption that *all* jets will be at least as complex and challenging to fly as a Korean War-era fighter or a business jet, we have the rule requiring a type rating (or equivalent LOA) for all turbojet-powered aircraft, plus the rule requiring an FAA-approved maintenance program, plus annual pilot proficiency checks.

Along comes the SubSonex, a jet that is smaller, lighter, simpler, and stalls slower than, say, a Harmon Rocket or a Glasair. It's almost certainly less challenging to fly than the original Nemesis. And yet, the FAA still requires that it be treated like an F-86.

I've dreamed about taking a small glider like the Sparrowhawk (a Part-103 legal glider that weighs less than I do) and fitting it with a retractable jet engine installation, using one or two of the R/C-sized turbines. It would be lighter and simpler than a folding prop and piston engine and have less drag when deployed, albeit very loud when running. But even something like that, an aircraft whose gross weight would be less than half of a C150's empty weight, with a better L/D, better stall speed, and similar climb rate... would be considered as demanding and complex to fly as a MiG-17. Why? "Because jet.".

Let's address the rules in 8130.2H and the other regulations, and drop the "jets are special" language when we're talking about jets that operate at high-end piston speeds and have stall and field performance in line with traditional piston singles.

Posted by: Bob Martin | May 11, 2015 5:49 AM    Report this comment

Technology - but not the FAA - has evolved to a place where the nature of an aircraft powerplant has little to do with the complexity of its operation. Time for some certification sanity? To quote myself.....

"For human-occupied aircraft, the current methodology of certification standards based on the gross weight of the vehicle makes no sense - and it hasn't for a very long time, if it ever did. I've proposed a system that focuses on two metrics: the maximum number of occupants, and the atmospheric operating environment (essentially, whether or not the vehicle is pressurized). I've proposed five certification groups based on maximum occupant count and pressurization:
- Trainers (2 occupants or fewer)
- Personal (3 to 8 occupants) non-pressurized
- Personal (3 to 8 occupants) pressurized
- Business/commercial (9 to 19 occupants)
- Transport (20 or more occupants)
Experimentals still would be permitted.

The basic approach would be that the first three groups are intended for occupancy by informed volunteers, so airliner standards are inappropriate and unnecessary. Part-135 operations still would be permitted with all except the trainers. Regardless of group, the type of powerplant(s) would be irrelevant. The empty and gross weights would be irrelevant. An instrument rating and a type rating would be required to act as PIC of pressurized personal vehicles, B/C vehicles, and transport vehicles. Those vehicles would have to be equipped with FIKI-quality anti/de-icing apparatus, and would have to meet lightning-strike-tolerance standards. Balanced field length requirements would apply to all operations of B/C and transport vehicles."

But rules is rules, as Paul has pointed out. With that in mind, I'm very much looking forward to the certification of Cirrus' personal jet. The Sonex looks like fun; the SF-50 looks like fun transportation, which to me makes it practical.

I've had some issues with the SF-50 design. Some were addressed; some remain open:
1. The recent addition of active control surfaces on the ventral fins deeply concerns me. There must have been some serious flight-behavior issue to justify the added complexity, weight, and cost. This may or may not be related to a pronounced dutch roll behavior that was visible in the early videos of the "production configuration" articles. Oddly, this behavior was not apparent in videos of pre-production article, which employed "first guess" ventral fins. I find that to be troubling.
2. The main gear doors extend too close to the ground. This will be an issue in snow operations.
3. The BRS (parachute) still is a major program risk, and adversely affects the airframe structure and weight.
4. The fit & finish of removable panels and fairings has left a lot to be desired.

But I still expect this plane to be a true game-changer for light GA - the first practical personal jet.

Posted by: Tom Yarsley | May 11, 2015 7:14 AM    Report this comment

Just imagine if your ideas applied to LSA, Yars. Instead of the absolutely STUPID 600kg/1320lb MGTOW (tell me, Mr FAA, what does THAT do for us?), specify just a reasonable stall speed and a maximum horsepower. With a MGTOW of something reasonable -- lets say a nice round 2000lb -- and a max engine power of say 160hp or 180hp, imagine the airplane that could be produced. The price would be only marginally higher than today's LSA yet the performance would justify those numbers. Even if it was just 1800lb and 160hp, an LSA would become justifiable.

LSA already HAS a pseudo CS23 operating environment operating under ASTM Standards. It already has a no medical pilot situation. It already allows non-TSO'ed yet equivalent performance requirements. LSA has no 51% rule. All that it lacks is sufficient MGTOW and has the nutty speed limitation. Minor changes to the LSA rules could be made almost overnight and would satisfy most everyone. I could imagine the plethora of LSA manufacturers complaining that such a rule change would put them out of business but -- if ya think about it -- they'd be MORE in demand because then the prices could be justified by higher levels of performance. Imagine if Vans/Synergy could sell a pre-made RV-9 at something around $150K and LSA pilots could fly them. Synergy would have to build a new factory! Same thing for the rest of 'em.

The Appendix G-4 to the ARC recommendations to FAA contain a new category of airworthiness called Primary Non Commercial. This would allow a person like myself who owns a C172 to relicense the airplane (bi-directionally) to that category. This would then allow installation of non-TSO'ed equipment such as the G3X but without 3rd class medical relief, it's just not enough. Assuming for a moment that relief DID come, this would be an alternate route for some.

For me, personally, I'd like to have a new LSA priced airplane but I cannot justify the designs as they are. I need and want more airplane ... but not MUCH more. Minor changes to the LSA rules could resolve many, many issues that I have with the LSA rules as they are.

It may well be that the best AND fastest way for the FAA to make many of us happy is to institute exactly these changes while the bureaucratic gear train operating at snail speeds grinds on.

With respect to the JSX-2, same story, as Bob M described above. Why do we need anything but certification by an instructor who HAS a LOA added to his CFI that we're capable of operating the machine safely? Looks to me as if the two highest areas of hazard are the requirement for a runway of sufficient length to operate from and slightly higher approach speeds plus maybe fam with a different type of engine. Anyone certificated to fly a complex airplane like a Mooney, et al, ought to qualify for that. The guy with a two place training glider that uses that engine should be capable of signing off reasonably qualified pilots.

So many ways to make things better, SO little time, SO much bureaucracy impeding our path forward!

Posted by: Larry Stencel | May 11, 2015 8:39 AM    Report this comment

"So many ways to make things better, SO little time,..,". I agree Larry.

The SubSonex cabin is small for me but the price for the package is good. I would support Sonex's endeavor - good luck to them on the project.

Posted by: Rafael Sierra | May 11, 2015 11:09 AM    Report this comment

Larry: Under my scheme, LSAs could be considered to be "Trainers," and would not be subject to power, weight, or speed constraints.

- Yars

Posted by: Tom Yarsley | May 11, 2015 5:43 PM    Report this comment

Amazing little machine and I share the sentiment of wising them lots of success with sales.

Posted by: Jason Baker | May 12, 2015 7:52 AM    Report this comment

Interesting sports plane concept, though the fuel efficiency is so low as to render it not useful for much of anything else, including cross country travel.

Turbojets are just not practical travel machines in very small sizes, unpressurized, and operated at low altitudes. Doesn't mean they can't be a lot of fun to fly, though. People don't buy jet skis to go on long offshore cruises - they buy them to go fast and zip around the shoreline and have fun, nor do jet ski owners worry a lot about their effective range. So the Sonex seems to be the aviation equivalent of a jet ski.

Posted by: Duane Truitt | May 12, 2015 8:45 AM    Report this comment

Back in the 80's I remember reading about a very small jet being offered by an Italian company (Caproni?) that was similar in concept to this except for having two engines and a bit more range. It sounded like a great little toy if you could afford one (with the operant word being "Toy"), but I don't think it ever really took off in the marketplace.

Posted by: Eric Gudorf | May 12, 2015 11:01 AM    Report this comment

"...the Sonex seems to be the aviation equivalent of a jet ski." Great perspective, Duane.

Posted by: Tom Yarsley | May 13, 2015 12:06 PM    Report this comment

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