Oshkosh, Wisconsin, 2008, was the launch pad for a world first in personal flight today when Martin Aircraft Company unveiled the ultimate personal flying machine – the Martin Jetpack. Glenn Martin, Inventor of the Martin Jetpack and Managing Director of Martin Aircraft Company, showed the world that sustainable personal flight is now possible.
Glenn has devoted almost 30 years to the research and development of the Martin Jetpack. Glenn and a group of avionic, technical, design and production experts at Martin Aircraft Company have created a jetpack that flies 100 times longer than its predecessor the Bell Rocket Belt.
The Martin Jetpack has a patented fan jet technology, uses regular gasoline, complies with Ultralight regulations and is easy to fly after completion of a unique training program.
In 2005, Prototype 9 achieved sustained flight times, laying the foundation for a viable and successful pre-production prototype to be developed. In 2008 the Martin Jetpack was launched.
The core of this machine is the fan, its duct and the flow straighteners. These have cost the team countless hours as they pushed the boundaries of the science of ducted fans in a low speed environment. Mr Martin found that while many of the 'facts' older designs were based on were as solid as the day they were first committed to the text book, others were due either to the originators trying to simplify the calculations because of a lack of modem processing power, or had a fudge factor to cover steps in the process that didn't become clear until high quality CFD systems were available.
A long series of experiments with Solidworks 3D CAD-drawn concepts run through CFD (a computer based simulation of a wind tunnel) brought about a series of promising steps forward in efficiency that were checked out on the workshop test rig. The team now have a fan blade that is not only light at roughly 100gm per blade, it also runs at 92 percent efficiency. The best Mr Martin can find elsewhere is the lift fans on the JSF which are published at 87 percent. The team have also worked hard to get the whole ducted fan package efficient across a broad speed range. The 2011 setup has a relatively flat efficiency curve with a small peak at around 60 km/h forward speed. Each fan unit is considered to be torque neutral which means the airflow from the duct is so close to straight that the twist is almost immeasurable, giving gains in thrust.
The fans needed a light, unobtrusive and reliable drive method which led the team to the modem synchronous belt. The latest designs have carbon fibre tensile cord inside the polyurethane belt with nylon facing on the teeth. The power transfer for width is better than for chain and challenges many gear-driven alternatives especially on weight. The major requirement for belts is to keep an alignment of better than half a degree and constant tension. Most belt drives use aluminium housings which shrink and grow with ambient temperatures let alone engine temperatures, so the decision was taken to go to a carbon fibre structure to all but eliminate temperature related dimension changes. The carbon structure also greatly assisted in keeping the alignment within spec.. The final touch was to make sure the drive and driven pulleys were fully supported by bearings on either side: no poor quality over hung shafts would be allowed in this design.
Once the horsepower requirements of the fans were known, a search of all known production engines was made. Power density, brake specific fuel consumption, package size and reliability were recorded, then checked against the requirements. No production engine had the power to weightt ratio needed allied with the high reliability the team demanded. Martin Aircraft sought advice from various companies and found that marine applications, such as outboards, shared many of the operational needs of the aviation world. Both users tend to run at three settings: idle; around 75% continuous; and 100% for extended periods. With this in mind, questions were asked of one specialist, Mercury Marine, to ascertain the level of stress an engine could be placed under and still achieve high reliability. The end result was four gentlemen For reliability and packaging reasons a water cooled V4 two-stroke was chosen. This gives a power pulse every 90 degrees which brings the peak and mean torque at the crankshaft nice and close together, resulting in a smooth output to the fans. To further smooth the power delivery, there is a 'centre flex’ type rubber coupling before the drive enters the c!rive pulley. The cylinders are from Honda (CR 500 motor crosser), modified by replacing the steel sleeves with aluminium coated With nikasil. The change gives better heat transfer and reduces the engine weight by over 3kg. New heads have are been made which have a carbon fibre top cover to keep the water in and the weight down.
The exhaust system is a simple yet brilliant piece of work which through its various interconnections and pipe lengths gives a lesser improvement at peak rpm compared to a full expansion chamber system, but functions well over a very wide band. The torque curve is one of the flattest I've seen, which means the horsepower line has no real humps or hollows as the rpm rise. Reliability is vital in a machine that has no wings (just a BRS 'chute fitted) so the engine team have incorporated all the hallmarks of a reliable engine such as mean piston speeds below 15m/s, bore and stroke ratio kept below 1.15 / 1, and porting size and shape designed to protect the rings as they pass. All based on hard-won data from the marine industry where after tens of thousands of engines they have learnt a thing or two.
The cooling system has a neat (and patented) method of pumping air through the radiator. To understand it, think about blowing air across the top of a milk bottle. The airflow from your breath across the top creates a draw on the air in the milk bottle. The horizontally mounted radiator on thej etpack has ducts that sit on the side of the entry to the ducted fans so that the rush of air going into the fans pulls air through the radiator. The more horsepower applied the more airflow through the fans and therefore more flow through the cooling system. Another simple yet brilliant design.
The first prototype Jetpacks were manually controlled and apparently not hard to fly, but the team have their eye on a market where a high level of automation seems preferable. The fully fly-by-wire system uses the same unit as the Predator UAV for its air speed, gyro, magnometer, GPS and other critical inputs, but the processing and control is done by a dual redundant Martin Aircraft system. When creating it the team were helped immensely by a professor from Bremen University who joined for six months to apply his experience in control systems. He also helped with the creation of a proper flight simulator suitable for training and checking out software changes to the real thing. The fly-by-wire approach allows the operator to simply command the machine to rise or fall, turn left or right, leaving the control units to move the duct vanes and the cruciform, surfaces in the jetstrearn to create directional change while changing the engine output to control height and speed.
Release the controls at any time and the system will maintain height and gradually bring you back to stable hover.
Martin Aircraft has created a concept, not just a one-off machine. The current version suits US Part 103 rules, so it is limited in overall weight, speed and fuel capacity. Accepting these limitations opens up a large private market for the Jetpack. A slightly bigger 'unregulated ' version on the same basic configuration can lift 200kg-plus payloads versus the current 115 kg and stay aloft for over 90 minutes. Still bigger versions exist on paper.
The 12th prototype featured a 200 hp V-4 engine driving two ducted fans. It had been flown to more than 3000 ft and 74 kph. The planned price at that stage was US$150,000 to US$250,000.
Flight and Engine displays
Energy absorbing undercarriage.
Height: 5 ft
Width: 5.5 ft
Length: 5 ft
Structure: Carbon fibre composite
Empty weight: 250 lbs (excluding safety equipment)
Gross weight: 535 lbs
Useful (Pilot) Load: 280 lbs+
Maximum thrust: 600 lbs+
Fuel Capacity: 5 US gallons (as required by FAA Part 103,Ultralight Regulations)
Fuel burn: 10.0 gph
Engine: Martin Aircraft 2.0 L V4 2 stroke, rated at 200 hp (150 kw). Max 6000 rpm.
Electrical system: 12 V DC Battery, starter, 360 w alternator.
Rotor: Carbon / Kevlar composite diameter 1.7 ft
Max: 7058 rpm
Range: 31.5 miles (at max speed of 63 mph as required by FAA part 103).
Hover in ground effect: 8000 ft (estimated)
Hover above ground effect: 8000 ft (estimated)